Qdestioas and Answers OpzitionaI 3 r d Management level
Mum bai 2.0'~August, 2001
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3fety, on boaria a ship, is critically dependent on the s & e ~ , &ir values and the environmenr in which they live and work. Basic safety is inmporated in a ship though regufations and pr es. As kGp7ss t r u a - e is exposed ro a highly corrosive environment, w d rriaiirte machinery is constantly subjected to very severe conditions, hazardous coi:rlii;iufis can develop, misss the ship and its machinery is properiy maiutained. Mariners need to get a sufficient theoretical howledge, to srrpplemtnt their own practical experience. 3 is nccessarj far them to be pmperiy rrairied, both by lectures ashore, as we11 2s by self skdy, w%k a; m. l o enabk stucle~tsto a d y , while at sea, the authors have prepared this text book in the form of a comprehensive s d of questions md answers, w5ic.h should supplement rfie riurnemrls standard textbooks already avai!abIe. MI. Vikram Gokhaie 22;ld 34. Nmda are b ~ t halready well known in ihi: marine field. They a: Chief Engjneei-s, with e lot of practical experience, both as ship-board engineers, as welt as senior facu!ty in the LBS Coliege of Ar_lwam;ed Maritime Studies m d Research, one of the premier maritime imtitu.tions in India. ,l'%sbook "Advanced Marine Engineehg Knowbrfedge- Volume 333 Qu&ions arsd A~swers"mitten by ?&. V i h m Gokhale and Mr. N. Nanda, iri1.s a co~omprehensive coverage of the t o p i ~ srequired at- an advanced level for . M F
~ >io t ~ their dedication and sincere effortLAll Mariners will find this book of coisiderable v&re arid guidance I sincerely +sh them the best of st.~;i:i;ss i;I this book.
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Deputy Chief Surveyor with The Govt. of India, ?&inistry of Suriace Transport, Directorate General of Shipping.
arine Engineerjag Knowaedge - Volume BX Questions 2nd Answers' cciiers *he following Functions / Subjects a: the Operatiand and
Functions : 1 . Marine hgincering ar Operariaad !Managemen? level. 2. Bedrical, Efectronjc and Control Engineering at Operationai !Managemerit kye!. 3. I./iainlenanse and Xepair at Operational i Management lwei. 4. Ccntroliing Operation cf the Ship and care for persons at Operational / Mmagement kvef.
Page Nos 5 - 28. 29 - 60. 61 - 95.
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Surveys, Reguiations and Environment Protection Describe the In-water survey, to classification requirements, of the external underwarer structnre of a VLCC-
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The Swvey planning should be worked otit well in advance of :he in-water stirvey, by the ship. in cooptratim 'virh Ihe classificaiion Society. Preparation :Following documentation should be cokcted and consulted wiih a view to seiectin~areas and structural elements lo be examined. Basic ship information; Documentation on board. ivfzin structural plans including information of higher strengh steels Relevmr previous survey and inspeciion reports. Infomratior: regarding corrosion protection level Location o f heated tanks Informalion regarding relevant maintenance levels To assist divers, coloar photographs should be provided. o f items s w h as ruddzr closing plates and wear-down gauge p!ugs. The design of the ship %mt facilitate in-water inspec:ior, m d repair e.g. Sea inlets must be capable of being blanked off and drained to biiges, shell gratings hinged, if practicable and the anodcs easily changed. The hull should be clean, to have meaningftil maintenance leve! during operation, besides h v i n s a heavy diity coating. This must be camerl out by approved diving company, in clear water, with good visibility. ~ ~ e r a t i b:n A self propelled, steerable survey vehicle fitted with a long range T V camera is used. To aid steering and to check for hull dis~oition,a d o s e up, high resvlution, TV coiour camera gives a true picture of the state of the coatings and we!d szams. In some cases, a 35 mm still camera is fitted. An ultrasonic probe :s provided to measure plate thicknesses and other equipment includes a depth meter and speed indicator. Power is supplied and information ieiayed by means of an unjbilical from the vehicle to ihe survey boat. Survey Boat equipment: Is usually housed in a console c0ntainingT.V. monitors, plate thickness print out. audio cassetze recorder, video recorder and play back unit, diver communication system, vehic!e control system and associated instrumentation.
The survey vehicle is taken to the staSing datum by a diver. With rhe aid o f one of the TV nionitors and using h e shell expansion plan as a map, the vehicle may
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be p,uided, from the control console, over the bottom and sides of the hull, by foilowing weld runs and by reference to other features, such as inlets and tank p l u g . Pictures and navigational information are relayed back and video films recorded, along with plate thickness, giving the surveyor an integrated picture of all the required and relevant information. In addition to plate thickness, print-out can be produced andlor an audio recording. The vehicle will also provide pictures of such items as Stem frame, Rudder, Fropcller, Bilse keels and hull ~penings.A diver may b s used. with a hanu held camera. for closer inspection of these items and also for inspection of plating on the tun? of the bilge. It should be ensured during this operation, that there is a 2 way co~mnunicationbctween diver a d attending surveyor.
Q.2. Describe toe h.;:l examination you would carry out on a ship in P r y dock, making special rderence to essential maintenance, that can be carried out in Dry dock.
Preparation shouid be to a sufficiefit extent, as to facilitate an examination to asccrt,~iir for an;r excessive corrosion, defomaticn, fractu~es,damzges and other s!ructucai deterioration. Examination a n d testing All sp-aces within the hull and superstructure are be examined. In certain circuiiistances, the internal examination of lubricating oil, fresh water, and oil fuel tank:, nlay be waived. in sp;ices iused for salt-water ballast, excluding double bottom tanks, where -
a protective coating is found in Poor condition,
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a protective coating is not renewed,
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where a soft coating has t e e n zpplied or
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where a protective coating was not applied, from the time of coiisil-iuction, Maintenance of class will be subject to the space in question being irgte.rr~;il ly examined and gauged, as necessary, at Annual surveys. .
Double bottom compartments, peak tanks and all other tanks are to be tested si.ifiicir,nt to give the maximum pressux that can be experienced in service. T;trl!ts may be tested afloat, provided that their internal examination is also carried o~.it afloat. Where repairs are effected to the shell plating or bulkheads, any tanks, in way, are to be tested to the Surveyor's satisfaction, on completion to these reIjairs. 1r1 cases where the inner surface of the bottom plating is covered with ccrrwi!, asphalt, or other composition, the removal of this covering may be with, provided that it is inspected, tested by beating or chipping, and c,.,.iperr:jetl ; fijl.iii
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decks or sheathing are to be examined - if decay o r rsr is fosnd or ri;e . . 2wood is excessiveiy worn, the wood is be renewed. %'her; a wood deck, ialc stringen and lies, has worn by 15 WXI or more, it is to b e icncwed. Alxen\ion is to be given 10 the condition of the plating under wood decks, s5eciqir;o --:'. ,or ,~;:i:r deck covering, i f i t is found that such coverings are broken, or are nct adhefizz closely to the plating, seciicns are to be remmed, as zecessary, to ascertain the iVooCs
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condition of the plating. Mechanically operated hatch covers zrs to be resed, to confimi sarisfac:oT .. operation, including stowage, proper 51 of seakng arrangements, operatjo1;al tesiing of power components, wire and chaias.
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The anchors are to be examine&. W.e%&e chain cab!es are rzngeii, :hey are to be examined, if any length of chain caSle is found io be r z h c e d in mean diameter, at its most worn pa?, by i2 F/o or more, from 2s nominal diamerer - i i so, it is to be renewed. The Windlass is to be examined.
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The chain cables are to ranged and examined on ail ships over five years aid. The Surveyor is to be satisfied that there are suitabi- mooring ropes .<,*he,; these are Rule requirement. !he hand parnps, s-oc?ions,warenight doors, air and sounding pipes are to be examined.
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Thickness measurement :
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The Surveys may reqliire to measure the thickness of the material in ally pxtion of the structure, where s i p s of wastage are evident or was:JSe is normally fo~md.Any parts of the structure, which are found defective or excessively reduced in . scantlings, are to be made good, by materiais of the approved scantlings and quality.
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Hail Pain:iry (Essential rnaintenanee) ?'he shell plating is examined for areas of paint work which must be repaired. The whole surface of the shell is cleaned and prepared for re-coating with paint. In some instances thz 'null mz;. be cleaned down to bar- metal and completely reccrated; in most situations, areas where paint is damaged and rus:ins has started. the shell p!ate is cleaned dcwn to bare metal and rest of the areas are cleaned and swfzce prepared for re-coating. Surface preparation is done by m a n u ~ Iwire brushing and scraping with steel scrape~.s,power driven wire Snlshins, or high pressure water jetting 91shot-blasting. Compete surface is washed with fresh water and surface allowed to dry, before coinmencemeni of painting. Any scuppers, discharges or overflows, which may direc: watiter on to the surface to be painted, should be biocked or diverted, before paintjag is &+fled.The paint to be used should be compaiible with the previous paint, un!ess the complete paint is being rcnewed. Paint sprrcifiiations are to be piovided to the ~ & / dPainting sub-contractor. This inciudps nu;-.be; o'cnzts, f o r touchup and complete coat, type of paint for each coal, thiclaess of t a r 5 coai for each section, i.e. bottom plating, boot top area and toasid:s.
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& Q.3. As Chief Engineer briefly discuss the procedures you will follow :-
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When taking over as Cm of a large vessel. When taking ow; as C/E of a new vessel from a shipyard.
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Ans. Procedure :
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Briefing at Office
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Superintendent I Tech. Manager will brief, as per ISM Cock practices. I I T ~ ~ ~ M td;th K ~ D PA Fill up appropriate check lists - wherever required. ,k,hci~o-3 Read correspondence file. ' '
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Staius of surveys/certificates Conditions of class, if any Take copirs and start planning for your stay on board vessel.
Read lzttcr prepared by outgoing Chief Engine?., giving all derails. nisciiss 1 Verify : i;uel oil /diesel oil / lube oil soundings - confirm actual figures match the logged figures, before acceping respmsibility from outgoing Chief Engineer.
* Voyage Requirements Bunkers expected
* Consumptionpattern
- a n y special instructions.
v b Oil record book. 9
Overdue Certificates I surveys, if any, and the company's action plan in respect o f this.
,Maintenance status o f Main I auxiliay machinery Spares. Stores Vessel's sailing programme. Random checks of alarms / instrumentation.< Special tools on hoard
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Briefing at ofiice and Tzking-over on vessel
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Readiness for Port slate inspecri ons-LSA/FFA. Check a]] files.
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Drawings list.
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M a i n t e n a n c e canied out by ship's stafffworkshops.
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Meet 2/E, Elect~calofficer and other engineers/staff
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to corfinn any mcxe known or spzcific problems. Siyrt and send a cornbind report, by incoming and outgoing Chief Engineers. to Head office. Taking w e r as Chief Engineer, for b r a d new vessel iron, tke E) Shipyard The Objeciive shouid be to ensure th'dt Efficiency of all systems is achieved b e f o r e a c c e p t a n c e . LiaLoil betwxn Vessei and shipyard is goad
Procedure : A)
Office : familiarize with the Vessel's contractual position
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Scrutinize progress reports Witness typical program
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Inspection afier t n a1
Taking over (Final)
Office Famiharise with vessels contractuai position, viz. : Specifications and capacity. Contract penalty clauses. Delivery date. Speed and fuel consumption.
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Status on class certificated. Guarantee period.
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Shipyard
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Obtain working specifications covering changes, interpretations, additions and anicndments in respect of: 0
Working drawings.
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Progress reports.
* Random inspections of installed
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including switch gear.
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{~djustment;for inte&ted bperations). . .~ ~
steering
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Biack-out test.
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Turning circle. .. Communication and navigation equipment to be demonstrated. Boiler safety vaive test. Hull vibration, including Accomodation
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and Machinery spaces, esp.ecialiy when ship is in tke light condition and - .:~. .,mo,,jng @tern; ..:................ - ;.. :. ..... .. . ~ . . .~ . #achin&y Vibration, over-speed & power ranges. .~ . .. . _,_/:<_: :,..-. ......................... ~ i r b o i now? k suri;ey of accommodat&n, Machinery spaces, Bridge *~t@ne control room. .~~ ;
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Ventilation flow rates. . . .
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Potable water ~ t m e npiant, i galley and pantry equipment. I n s ~ e c t i o nafter
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Certain eq6ipment may have to bc opened for inspection, if not satisfied with its ~. with respect to safety of the ship and personnel. performance, . . . . . . . . . . . ~. ...
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Electrical generator windings to be clean and h u l a t i o n readings recorded. Bilges to be cleaned and repainted. 1 3 1 ~L c - i n c c e io c h e c k / c i ~
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Ccmplete the sea trial data. Spare gear, Outfit. F!~elr:and Lbbiicar,ts. Cei;ificatcs of ciass Clezn Eilgc's repaicr if necessary The Master and Chief E ~ g i n e t will r sign the Foim of Acceptance, for the Vessel and the Owner's Representative wi!: accept responsibility of rhc ship, after satisfactory corny;!etlon FF ail the hzucial ziiangernents. 1 J'Q. 4.
With rcspzc! to Kegulstions and Pc!lction contrul, expfain Reguiatiens with respect ti, c o ~ t r o of l discharge of oif from machintry spaces o f at: ships
This regulation controls dumping of -11 victuals, don?estic and op~r3riozaiwaste oenerated by a ship and her crFw 1 passengers. Imposes a complete Ban on a dumping of Plastics e.g. Synthetic ropes, nets and garbaye bags. Deals \villi pcllittion from (i) Oil (ii) Noxious Liquids (iii) Eazardoiis packaged snbsrances.(iv) Sewage; (v) Garbas-. Eve13 stricter coti:rcls for 'Special Areas' e.g. Mediterranean, Baltic & Black Seas. Here dumping is completely banned - even food waste cannot be dumped within I 2 miles cf iand. Contracting parties to the com'ention are oblized to -provide facilities in ports for reception c f gaibage~ MARPOL, Anriex V
Garbage type
Outside Speeial Areas
Piastlcs
In Special Areas Disposal prohibiied.
Floating dunnage, lining and packing materiak.
> 25 miles off shore.
Disposal prohibited.
Paper, rags, glass, metal bottles. crockery and sin~ilarrefuse.
> 12 miles.
Disposal pi-ohibited.
A]! other garbage including paper, rags, glass coniniinuted or groirnd
> 3 miies.
Disposal prohibited.
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Food waste not comminuted or ground.
> 12 miles.
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I Comminuted or ground garbage must b e able to pass through a screen wi& mesh size no larger than 25 mm. Garbage disposal regulations for special a r e s shall take effect in accsrdance with regulation 5[4)(b) of Annex V.
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ships I Oil or oily mixture should nc? be discharged into the sea, unless the following conditions are satisfied :Control of discharge of oil from hhchinery spaces o:a!l
Ship is proceeding on a voyage, i.e. en-route and not at anchor. Not within a Special area The ship is more than i2 miles from the ne-rest cczst. The oil content is less thar I5 p.p.m.
The required oil separation, filterkg and monitonnz ~quipmeq!is in use.
Oi: Discharge Monitoring and Control systemSystem to come into operason when there is any discharge of effluent into the sea and automatically stop discharge when the oil content exceeds the permirtcd 'zvei. System to provide a continuos record ofoil content of the effluent. record io be identifiable as to time and date and retained for three years. Any failure of ~cjuipmentto be noted in the Oil record book and all discharse stopped. Defect to .he rectified before commencing next voyage.
0.5. With respect to Sewage treatment, discuss the foHowing terms : Biochemical Oxygen Demand.(B.O.D.), C d i f o r m count, Recommended lev& &purnping-oui solids and Bio-chemical digestion of seivage.
Ans. Rlochemical Oxygen Demand (B.O.D.) It is used to give a measure of the strength of sewage, i t identifies the iioiogicai decomposable substances and is a test that depends on the activity of bacteria, which in the presence of oxygen feed on and consume organic matter. Results o i i h e test are expressed as the amount of oxygen taken by a one illre sample (diluted with aerated water) when incubated at 200 for five days. Can be defined as the amount ofoxygen utlised by micro-organisms in the stabilisation of organic matter, B.O.D. of iaw sewage is 300 to 600 mgllitre. I.M.O. recommend a B.O.D. of 50 mdlitre aiter treatment.
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Coliform Count:Coliform organisms are recognised as the Indicator Organisms c f sewage pollution. The numbers presenl in sewage are large, each person contributinz between 125 billion, in winter 10 400 billion, in summer.
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Colifonn are present in the human inteGme and their presence in water taken as an indication of the pathogen count. The pathogen count are disease causing organisms, responsible for TyFhoid, Dysentery; Poliomyeiiris, Cholera LM.0. recommend a Coliform count of 250IlOO ml. of effluent after treatmert.
Recommenr!ed Iweis ofpumping-out soiids
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Solids:. Dizsolved - Solids which are in soluiion Suspended - Solids physically suspended in sewage, that can be removed by laboratory filmtion. Arc relatively Ngh in organic matter.
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Settleable--Suspended solids that wili subside in quiescent liquid in a reasonabii period (usua!ly taken as one hour). Suspended level ofraw sewage is 300 to 400 mg/litre. 1.M.O. recomrnen6s i! level, after treatment, of 50 mglitre.
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Afler treatment the residual disinfectant should be as low as poss~ble. Canadian iestfictior is between 0.5 and !.0 mgflilre. I.M.O. prefers thc use o i Ultra-Violet exposure to the method of Chlorinaiion.
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Biochemical digestion of sewage .
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Anaerobic Process :-
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Anaerobic bacteria can only multiply in the absence of free oxygt.;~,as tliey use chemically bound oxygen to survive: in the anaerobic process, the bacteria break down the organic matter into, water, carbon dioxid; methane, hydrogen sulphide and ammonia.
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This pro'cess is also called 2utrefaction. Theses-produced --are both noxious and toxic. The effluent produced is of poor quality and o&er by-producrs are .. .. -highveorrosive.
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Aerobic Proc255 :Aerobic bacteria require free oxygen to survive. In the aerobic process the bacteria Lie& down the organic matter safely. The Aerobic Process lias end products of fizO iC 0 2+ Inen Residue + Energy to synthesis new bacteria.
Advanced Marine Engineering Knowledge Vot. Ill
4 . 6 . With respect to Sewage pumping-our systems, describe briefly she Chemical sewage system and the Vacuum sewage system. Ans. Chemical recirculation or the Zero discharge system Sewage enters thz chemica! dosage tank, where it is mixed with chemicals, WLbreak dom the sewage and improve the.colour. It thcn passes through a conimirtulor (wbich is a grindzr or macerdtoi) that curs :he sewage into small paticies) and e n t m a chemicai treatment tank, in which a further chemical treaiment is added, to sierilise and deodourise the!iquid. e.-?-je A";..': r liuUiru iS+4cge + i i + - ' ~ A ) /' I-,m .- -wG C ~
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A circulating pump draws un-treated sewage from this tank and delivers back l o riw dosagr rank, where :he txeatment is agGn qplied. The sewage remains in this sction for about 5 minutes before passing to the senling tank. After senIing, It162 cleiu liqilid is dram through a filter to the Sanitary Hydrophore, which proviilcs the water supply to the toilet flushes. As the level in the settling rank rises,ihe sterile sludge may be removed to a Sewage holding rank or incinerated. V ~ C I Ysewage U I ~ system.
This system is based on a vacuum created by an Eductor, which is used to pull in t!ie sewage into the sewage tank. Calculations are based on a daily fiow oS say, 10 liircs per person - using 1.2 litres of water per flushing operation. The %>%%get a n k capacity varies h - o m 2.5 - 10 m3. The holding tank is at atmdsFheric pressure. ,A pccssure switch maintains vacuum in the line, from the toilets by auto stnrlislop of centrifugal pumps. The water in the sewage tank is used as driving walv i'nr tile ediictor. Float switches may be used to control the discharge from the 1ioItli11,r:lank to the sewage treatment plant, while still maintaining the vacuum in thc sysiero~
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Gnumerate the preventive measures you wot>ld take to avoid po!lution of the env;ii-,rnent, with respect to Bunkering. What instructions wil! you issue to the p c r s a n ~ e !under you, in this respect.
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Preventive measures to avoid pollu!ion, while bunkering The responsible officer should be famiiiar with ai: aspects of bunkering and the ship's bunkering system, and shouid personally supervise the operation. He must h shore 1 barge crew, especially with respect to stopping be in ciose contact ~ ? t she of bunkers, in case o f emergencies. Bunkering Operations : The vessei shouid be securely moored.
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Ail valves should be checked and those not to be used, must be securely
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closed. Scuppers should be sealed.
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Oil absorben: materiai (sawdust, sand) should be readily available
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Communication systems should be checked.
Ali hose connections should be frequently checked.
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Bunkering should commence a! the i ~ n i n l u r n pumping rate so [ h a ~ any problems can be detected early. Frequent sounding 1 &ages should be Taken 'Paflicular care taken when 90% filling, capacity of tank is strained, aud bunkering stopped.
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O n completion special care taken when disconnecting hoses, removing drip trays.
Note : A 'persistent' oil is one that wiil not disperse easily e.g. - Heavy Fuel Oii, Diesci oil, Lubricating oil - and will i q c i r s a solvent or emulsifier, to d i s ~ e r s ein case ail oil spili occurs. Volatile oils, like pztrol are not persistent. S e G m any ;ransfer of product is undertakeq the O S c e r must w n f i n the ibi!owing items, with the person in-charge of bargdterminal bunkering. Each will sign this f o m (sarnule). to acknowle6ge~ 1 akirig 05/erwztch
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personnel, will slso revie-# the subject matter as be:ow-
Pinrpiiig Data
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Q~murtityand typc of stock to bet:msferred initial trmsfer iatc
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Anticipated stoppages
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bfaximum transfer pressure
I'he method of communication bemeen b x g e a ~ ve~sellteminal d has been established (! ~%illbe understood that except for emergenci?~, a 15 i h i : for shutting d c w d transfer is required.
minute stand-by
Arc hoses in good condition? ;be connections between the barge and vessel/terminal properly secured 7
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A continuous deck watch will be kept by barge and vesseU terminal crews In the event of an oil spill, a clear mderstanding exists on steps to be tnken (conaainment, clean up, reports, etc) s o PCP
All unused manifold connections arc blanked O K Rotii prtics should cany out constant sun-eiilance o f adjacent waters to deted and pii:v:rtt ariy leakage / spillage of oil.
Advanced Marlne Engineering Knowiedgc Vol. !it
Q. 8. A s a Chief Engineer, what procedures will you foilow, ivhen accepti~);: bunkers, to ensure that tiley conform, With respect to the quantity and required fuel specification a n d how will you ensure this ?
Ans When accepting bunkers from a barge or a teimiaal, the Chief Engineci sho&~ always check the local supplier's C o c u m ~ to , make certain that the bunker conforms, in tenns of quailtity, as well as file1 specification, with what has been actuzllv ordere6. :'I
The Chiei Engineer, or his nominee, sh9uld al..vayz check that the bunkers, to of water contaniination be received, do net contain hn unacceptabie ?-age the maximum ailowa'uIe being 0.05 'A for 63s oil, 0.25% ibr-Diesel oil and i % for cavy oil (of IS0 Cst). In case of distillates, this can be established by rncaiis of a simpie test, involving water finding ;;ask applied on sounding tape. The tape should be smeared with paste (usually y e e n in colour), which will turn pink, i i any water is detected. The water depth should then be read off a!id iis volumc and weight calculated From the barge or storage tank calibration tables. 1~1iisrn~tk~od can a l s be ~ x e d %i Fue: Oli - but 11s accuracy may not always Sc 100% The o d y safe way of checking, in the case of Heavy fuel 011, is for a sample to be taken and the water coatert ttsted in a water tesz kit.
in additioii to checking the tightness of the hose couplings, it is important that rhe pumping rate between bunker ba:&lcmiina! and receiving Vessel is asrceii by the barge Master and the vessei's Chief Engineer. Excess pressure can cause the hoses.to burst with the harbour becoming polluted resulting in claims. fines ,$ and even vessel's arrest. Ensuring correct Quantity/L)elivery: Claims of insufficient volume delivery are the cause of many disputes between owners and bunker suppliers. It is the ship's s!affs responsibility to ensure t i n t they ectually receive the quantity ihat has been ordered. If not, then a lalcr of proicst mus! be niade 311t. The Chief Engineer or his represenLalive must always check the supplier's bar$ terminal tank soundings, before and after pumping. Barge soundings should be checked by using sctinding tape, which are to be used with tank caiibration tables to verify the actual quZtity, both before and after pumping, ia ensure thai the correc: qilantity is received. Due care must be taken-to correct for tempeature variations, if my. Note: In a quantity dispute, it is generally a mle with bunker suppliers, that ship's figures wi!l not be accepted. Measurements taken on tanks, barges and oil tmcks are accepted, as they are under the independent supervision of Csstoms. However, a format letter.of protest must be made out, to keep the ship's staff in the clear. Flow melen should be checked, both before and afier bbunker delivery Mcters only rccard volume (no! weight) - either in 'US barrels' or 'Litres' - both of
Advanced
Marine Engineering Knovrledge Vol. IN
wirkh c a n be convened into metric tons, by using the product's specific gravity and !lien adjuslins for lcmperature differences.
Tmt liils for checking fuel quality : Bunker quality on board can be checked wiih a fuel oil test kit. By carrying out a few simple tests on representative s:~mpi~?s, iiom the ship's bunker flange, and not the bunker barge or shore tank, the C h k f Engineer can satisfy himself, that the prodact is indeed upto specification ant! compatible with existing~bunkers.Samples of the loaded product must he taken jointly. 'Thi: sample must be representative of the total delivery, and ideally taken by drill feed at the discharge side of thz manifold, during the course of the pumpin!: process. Samples should not be taken at the start or 3~ h e completion oC bunkering, because then they will not be a re?resentative of the rota! t3nnage loaded. ALSOsamples should pot be from just one t a k on the barge. .
.
Sainpie bottles should be sealed, datzd andsigned hi - by boik the C h i d Tingineei- and the local supplier. Two identical samples should be taken. One silould :.hen be retained by the ship's staff, for about three months or at leas? until thc burtkeis loaded have been consr~meawithout prohiem. lit c;n;:: of any quaiiiy problem, a sealed sample must be sent by{ the Chief Lrigliieci asiinrc, so that a proper analysis can be carried out. In the event of :I gcrwiiii: &icy problem insins, it k imperative that the supplier is advised as soot? as posslbi~.The supplier may decline to accept liability, i f this is not done
witflin a specikd period. (The prribd for notification should he cie2ir:y stated ihr: suppliefs terns and eocditions)~
I::
lhm!ccrs are the rernsining products from varying world sources orcrude oil. 3f cracking, thus there is no 'standardised' heavy oil - each and v w y i ~ ~methods g om: is corismng of different hydrocarbons from varying sources. Thc delivcrcd proiluck may conform to the specifications, but however when mixed in the lank with a pmdl.ict refined from a different crude oil source, incompatibility can occur. I hi: lr~ixedproducts will layer and could re;u!t in an ~un-pumpable s i u a g ~ anct wrisequent ?nor combustion in the main engine. Altemaiiveiy, the product could hecrime completely un-pumpable or unbumable. .. ... ~~.
'1.0 ~tt no mixing occurs - although this is not always possible on smali
ships. With respect to regulations, discuss briefly :
Q.9. a)
<:ontrot of discharge of oil from cargo tank areas of Oil tankers.
b)
Con(!-01 of discharge with reference to Chemical Carriers.
Ans. /
Corittol o f Clisctlarge o f o i l from Cargo Tank Areas of Oil Tankers
Advanced Marrne Engineering Knowledge
Vol. tll -
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i
Sea areas
Discharge criteria
Wiihin a Special area
No Discharge segregated ballast
except
clean
No Discharge segregated ballast
except
cl&n
C
i I
50 Within nautical miles from land.
.-
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No Discharge except either : clean or segregated b;iliast; or when: tne tanker is en route; and the instantaneous m e of discharge of oi! doesnot exceed 30 litres per nautfcal mile. and
Outside a Special area
More than 50 nautical miles from lrnd
the total q-aiitity of 4 discharged does r -ders) sot exceed 1/30.000 ( f ~ new of the total quantity of cargo. which was camed on the previous voyage: and the tanker has in oprrarion : An oil discharge monitoring and coiiirol system and slop tank arrangements as required by Regulation 15 of Annex I of MARPOL 73/78.
.
'Clean ballast' is the ballast i~ a tank which has been so cleaned that the effluent from &ere does not create a visible sheen or the oil content exceed !5 3pm . Regarding chemical carriers, Amex I1 of MARPOL 73/78 deals with poliution by noxioils liquid substances.
3-10 With respect to Oily water separators, justify the statenrent : Separation of oif and water depends upon the density difference behvcen oil and c*ater. Aiso comment on the use of z cna!escing device and heating coits. Most designs of Oily water separators in use are of the gravity / coalescer type, i.e. rhe separation rakes place by gravity, and depends upon the density difference between oil and water. The coalescing device encourages the formation of large oil droplets from the dispersed pha$e. .eneral. bilge water contzins a mixture of oil in water i.e. a small amount of oil in a large amount of water. The water is know? as the contini~ousphase and the oil is the dispersed phase. The oily water enters the separator and is slowcd down (ideally lo laminar flow). Thus the larger giobules oroil arc :illc.:~:c.d 10 rise due to the density difference. in
Advanced
Marine Engineering Knowledge Vol. NI
-i '
P!ates cncouragc a laminar flow and act as coalescing suriaccs. Thc ratc or separa:ion depends upon the difference behveen the viscous drag at the oilfwater interface and the effect of gravity. As the size of the oil globules increases the viscou.; drag decreases and the gravity increases.
.
The fomiarion of larger gkbules is accelerated at the coalescing surfaces. Also as rhe rate of change in density, with respect to temperature, is geater for oil than - - twa!w, lire ---rale oiseparation will increase with ternzerature. BgL2nlp3:
Shouid be matched to the application {must not exceed the intenzed capacil] ~ i ! h eseparator). Vane or screw type are the n o s t suitable pumps. Provision f2r washirg out with sea water should be pl-ovidcd.
Heeiing Coils; Reciuce visco~ityfa: washins out. Ilcdiice viscosiry o i the oil water - thus aiding sepaiation. ICcduce viscosity of oil -thus aiding pumpins. lricrcase differential specific gravity.
'0.12 Oii poilution regulations require any transfer o r discharge o f o i l or oity mixiawes lo be recorded in :be ..
a. P o i i i ~ t i o nC o n t r o l record b , Wsidge log
,>A:. Oii Record Book 4.
tMvsters log
e, None of above Rrietly jristify your answer. h s . ?'tic correct choice is the option c).
All ?egistered merchant vessels must eany an Oil Record Book. .1 ' 1. ,~ ii~r;!udes s a11 Bilge transfer operations, Ballasting or cleaning of bunker fuel 8
!ail!is and the discharge of dirty ballast or cleaning water, Disposal o f oil residue, i>ischaiy oveiboa~dof purihed bilge water from machinery spaces.
.,. i ankers have additional entries to record - Loading, transfer during voyage and rlis<;har~;cof ori carso. Ballasting and cleaning of cargo tanks and the discharge ofdirty bi~llast. .
.
lf;iny vessc! fails to carry an approved Oil record book or to make proper entries, !!ic owner / Master are liable to a substantial fines and / or imprisonmeni.
.@
(1.13. How is discharge of oil monitored. Describe the general principles of measurement or the following: a) Ballast Monitor b) Bilge Monitor c) Turbidity meters d) Clean Oi! System e ) infra Red Absorption
r7 riitra violet
detector
g) Light Absorption & gas measurement. Ans. The oil pollution reguiaticns p t h i f a t i o n s on the quantity o f oil discharged into the sea. There is 3 requirement to monitor the overboard discharge from-.
A ranker bc:last line as i i discharges directly overboard. A tanker ballas; discharge afier an oiiy-water separator. ( s ~ o ! = ' < G b d P ? A bilge Sischai-ge from the mxhinery space. (i 5- p (J m ) ~~~
The equipmen! mxst be suitable for the marine environmeat (Xithough rilierc are many laborztory me:hods, which.are accurate; not all can be used on hoard shipj. The equipment must be suitable for reading both high and low levels of contamination 2nd tc respond quickly to sudden changes iit Lhose levels. There shoul8 be no appreciable loss of accuracy, due to the presence o f san;, rust and other debris, and must operate satisfactorily, irrespective of the rypc of oil used. The equipment must be easy to operate and maintain. Its working should be unaffected by considerable periods ofidleness. It must be accurate to i !0% .
Principles of Measurement : i . Infra Red Absorption.
2. Ultra Violct Absorption. 3. Visible Light Absorption. 4. Visible Light Scattering.
5. Ultra Violet Fluorescence
~
.
R e first four are al; poor with respect to sensitivity and would usually be used only to detect an oil-water interfaces (in an ci!y water Separator). Infra Red absorption is a useful method, as_most*bsorb in the 3.3 Prn waveleggth. The vari&ns, in absorption rates, between heavy oils through to the iighc diesels is approximately 10%.
Advanced Marine Engineering Knowledge
Vol
111
However, watcr also has a strong absorption at the same wave length and this makes detectors complex. It would be useful if the oil was extracted from the watcr with a suitable sclvent, the solvent having no absorption of tke infra re2 w e d s n g t h . However this would not allow a speedy response. Utra vioiet absorption does not encounter the water absorption problem as i t uses a wavelength of 0.25 Fm. but the requirement, for the opto-electronics to detect sniali c!~anzcs in a high light level, limits the low range capability. Absorptisn devices using any wavelength, on a system whcie the oil is present in tihc fonn of particles, suffers from the effects of sand and rust, distoning the accuracy. This is significant a i d adds geatly to the prsblen of inaccuracies. . . .. Devices using v r s ~ b h t ylight are usually cheaper, simpler and are nowspecific with rzspect to 31: types. However, they also detect, without distinguishing between, oil and non-oil particles of similar diameter. Of the two visible light teciiaiques, absorption and scattcikg, the most sensilive is light scattering. tlltra violet fluorescence suffers from a wide variation in respcnse !o different types of oil. .
-
Caliasi Monitor :
h icprcsentative sa7iple n u s t be extracted. This is achieved by a strengthened intrusion pipe in the ballast line and :he sample is ?her?con-eyed to the nlonitoi oy 2 puinp. 'Yo ensure a representative sample is obtained 2nd to encoiiiaze sood mixing, the sample point is usually ,in the middle of the ballast pipe, near !he discliarze pump. Care niust be taken to ensure the ballast line is a:ways Cull of sea water, so that no settling-out occurs. If the response time of the monitoring system is Ioii:> ~ n s i d e r a b i epollution can occur b e f ~ r ethe large discharge valves can be closed. Additionaily it is important, that the operation of the valves shouid not be iniriarcd by a false alarm, caused by a small spike of oil exceeding the alarm level. Geimaily the response d t h e m c n i t ~ is r instantaneous and most of the system respoilsc delay is in the sam.pling pipe-work. To reduce the delay, short length of samplc pipe with a minimum number of bends, utilizing a fast sample veIocity are iidopted. This pipe- work ofien becomes clogged during periods of inactivity and, when restarted, erroneous readings are obtained as oil, deposited during periods oi'idieness, strips off the pipe-work. Most monitors depend on an optical teciu~iqi,!i:and this leads to problems with the sealing and cleainp of the optical witldows. h fast sample flow rate helps in keeping the windows clean. . . .. ~ . ,
'Clilgz i\;lonitor : I iru~ailationand operational problems with a biige monitor are less than tt~os;: hi. the ballast monitor. The bilge monitor must provide an alarm at IS ~ . P . H I .7 ' t ~ alarm, being within the engine room, does not have the installation probletr~sof the ballast monitor, as very short sample pipes can be used. Wi!ii the bilge system the type of oil czn vary from fuel oils to lubricating oils, ilr:m:c the monitor should not be specific to an oil type. Additionally the wir~dow!)roblen> assumes greater importance ar the system may well have to operate with llic machinery space unattended.
ige
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~..
Advznced Marine Engineering Kncwledge
f
a E
Vol. ;;I
.;j$ Turbid@ Meter (Scatterqd Light Detector) :
:Q
:~E
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If an oiliwater mixture with a low oil content is heavily agitated, so that the oi! droplets become v e y small; the water will turn 'milky' to varyjng degrees, depending on the amount of oil present; the actual colour of the oil droplets is of no importance. This method can be used for indicating the oil content, provided the conditions for homogenizing thz sample are well contro!led. If a light beam 2 projected through a test cell containing sample water with well-homogenized droplets, pan of rht lighi passing through the czll will be scatiered. f i e intensity of light picked up by a photocell at the end o f a straight path through the cell will be reduced, whereas the intensity of scattered light sensed Ey a photuceli mounitd at an ang!e to the original path will increase. R 7-t c F . ~ [ $ i + - ~ ~ i ~ I me&w.i&ceil
-
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Mearming reif with
rwo P.E.celis
;Measuringci:.cuil
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Laser light nray be used to obtain a well defined Ik*t bean] and a selective light-scattering effect. This principle is used in the Ci: content meter, in which the light beam and the s i p p i c k e d up- by the photocelis are transmitted via optical -to the electronic measuring circuit in the engine room, through-ne fibres mom bulkhead, where the penetration is quite small. The Indicator, Alarm and controi panel are in the Engine room. A similar instrument, operating with infralight, is also inuse. Another Lnsmtment, based on theturbidity prixip!e, operates only on the direct transmitted light through the test cell, in which the heavily agitated water circulates. instrument of this type can measure oils ranging from heavy cmde oils to gasoline, but some changes in the calioration are required, to cover the extremes of the range. As the instrument measures the number of pmicles in the water, it is rather sensitive to other contaminants such as rust or air bubbles.
Advanced Marine Engineering Knowledge Val. ill
Infra-red absorption : The absorption o f infra-red !ight by oil can also be m e ~ u r e d .As infra-red absorption by the background water is aiso high, am oil-kee reference water of relevant quality must be obtained at all iimes; this is done by purifying a small part of water in a micro-filter. Tte inka-red absorption by the oily water and by oil-free water can then Se measured. The difference is caused by absorprioii by the oi! and, the s i p a l can be calibrated in oil content.
T h e Ultra Violet Detector : The Principle used here is that of Uiha Fluorescence. This is the phenomenon .
- of the emission of light from a molecule which has absorbed light. In the brief
period, before the emission can occur, some energy is dispersed and the emitted light is of a longer wave-length :ha? thr absorbed light. For a given oil-in-water concentration, the instrument response depends on a) the particie size and b) the florescent efficiency of the oil. The effect of particle sizc is minimized by the sample conditioning unit which reduces the oil particles to a uniform size. The fluorescent efficiency of the oil is based on the phenomenon that - molecules of "unsaturated" hydrocarbons become excited, when illuminated with ultraviolet li@t of a certain wavelength. They radiate light in the visible spectrum. Different oils contain different amounts of msaturated hydrocarbons, so that the instrument must bc calibrated each time, for the type of oil being monitored. The instrument is simple in dcsign, and has be& installed in tankers.
Advanced Marine Engineering Knowledge
Q.14. Sketch and describe a Sludge Incinerator ? effected.
Yo/.
How
NI
is the
waste
disposa:
Ans An Incinerator is capable of dealing with waste oil, oil and water mixtures u p to 25% content, rags, waste and soiid matter from sewage plants, if required. The figurc below shows a small combined water tube iype boiler cum incinerator plant which gives a compact unit with good economy.
Si"d~r/,,il
I;*::,$
c,7m-
Spinning cup bu
Incinerator Wasit: i oily-water mixtures, suitably homogenised, produce a well-dispersed emulsion. These are supplied to a rotsry cup burner. Solid waste f n m ?Ite galley and accommodation is collected in bags and placed in a chamber adjacent to thc main combustron chamber. There is a safety device, which prevents the doors being opened, if the burner is 'on'. Hydrocarbcn gases are formed, duz to the low air supply to this compartment, which pass throtigh a series cismall h ~ i e sin the furnace. Dry ash has to be removed pcriodicaliy through the ash pit door. Solid matter from sewage systems is also incinerated in this unit by homogenizing it with the oily-water mixture, befjre supp-;;ring thc rotary cup bfirner. The incinerator is capable of burning liquid waste or wet g a r b q e . Combustion o i solid paicles requires a considerable dwell time and this is usually achieved by angling the burner to give a 'cyclone' effect. One of the main problems is to dispose-off items like glass and metal containers, which tend to soften in the flame, but do not readily bum. It is necessary to prevent these agglomerating into a mass that is difficult to extract. For this reason many incinerators bum refuse on a grate. The burning process for liquid waste requires that there are no rapid changes in content. Hence it is necessary to ensure an efficient homogenising process in the sludge tank.
Cost 05 the incinerating process must be considered. Since incineration is iniriaied using diesel oil, lo sran with a stable flame, i t is using up fuel. In an effort io rccovcr t5is cost, combined boiler cum iccinerstor units are used, as shown abmve, which may not be economical on a cargo ship, with a small crew, but is a econnmica! unit on passenger ships, which incinerate a large quantity of garbage daiiy.
~,
Q.15. Discuss hriefly the methods used for the measoremeot of Noise levels and t h e recommended limits for noise levels.
Sour~dis generated by vibration o f surfaces or by turbulence in air streams, sciiing-up rapid prssure variations in the surrounding air. The nomal human ear is sensitivz to frequexies between 20 Hz and Z0,OOO Hz. The human ear is pai-;iciiIarly sensitij~eto noise in the 1000 - 4000 Hz [ 1 - 4 kHz) range, which is objeciionabie and m+y lead to hearins impaimcnt.
f?ieasilren,m; of Noise : P-loisc is measxed in terms cf thc 'sound pressure level' excressed in decibeis WG), 01 k e 'A - weightzd sound level' dB(A). Rwuoxriei~dedNoise Level timiis : Unrnarinzd machinery spaces (UMS)
1 l o dR(A)
Con!inuousiy manned machinery spaces
90 dB(A)
Ensine control room ( E C X )
75 dB(A)
Mess moms and public spaces in Accomrnodatio,l
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65 dB(A)
Day rooms, offices Cabins and hospital
75 dB(A)
.
60 dB(A)
?fk?hodo 02' controlling Noise exposure : S.?pi!a-:l?iou of Noise sources.
(i) W t w e practicable, noisy machines should be sited in spaces, thar do not r q w c c continuous attendance. (ii) Provision of suitable partitions, bulkheads to reduce the spread of noise. ?sovision oi'sound absorbing material in certain spaccs.
(i)
111llow and discharge ducts should be arranged, such rhrt !hey are remote Smru spixes frequented by personnel (such as Fan rooms) and be fitted witit i:fkc!ive silencers.
(ii)
Siicncec; shooid be reg-zlarly inspected and cleaned, to ensure sr~fficientair iniakc i:; possibie, with theminimum of noise levcl.
Advanced Marine Engineering .Knowledge Vd. 111
Machinei-y Enclosul-es. .
(i)
In continuously manned mschinery spaces, which contain machinery emitting noise above the prescribed leve!s and where i t is not practical to isolate this, consideration should be given to the fitting o f sound insulation enclosures (acoustic hoods).
Use of ear protectors Where noise ievels in any space are above the prescribed limits, signs should be posted, advisins the use of suitable ear prolective measures. Ear protectors
shauld be provided for personnel entzncg such spaces, viz. duty engineers making r~urineinspections. Manufacturers siiould supply i n f o r m a t h on expected noise lzvels and r e c o ~ m ~ e nappropriate d ins~allalionmetho&; to reduce theK, as f2r as practicable.
Q.16. Discuss what precautionary practical measures wouid you fo!lo~v oil baard vessel, as Chief Engineer, with rzference to lke ccatrol ana m;tnasemznt of the ship's Baffast water. Enumerate the basic safety precautions to be taken, in your opinion. Ans.
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o z n i s m s , =hogens Minimising iniake of harmful axtttiatic ~
a n d sediments :
When loading ballast, every effort should be made io avoid the intake of potentially harmful aquatic organisms, patbogens and sediment that may contain such organisms. Avoid baliasting, ifpossible, in a r e s and situations such as:
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in very shallow water;
ir, dadmess - when bottom-dweiiing or2amsns may rise up: where propellers may stir up sediment. &move baflast sediment on a timely basis : Where practicable, routine cleaning of the baI!asi iank to remove sediments should be canied out in mid-ocean, or under controlled arrangements in port or dry dock, in acwrdvlce vv3h the provisions o f the ship's ballast water management plan. If it is necessary to ballast or discharge ballast water in the same port to facilitate safe cargo operations, care should be taken to avoid unnecessary dischharge ofballast water, that has been taken up in another port. Ship's engaged in Ballast water exchange a t sea should be provided with procedures, which account for the following, as applicable: Avoidance of over and under pressurization of ballast tanks; Free surface effects on stability and sloshing loads in tanks ihat may be slack at any one time,
~ d v a n ; e d~ w i n Engineerhg e Knowledge
Vol. I11
To take account of weather conditions; W e a t h ~ rrouting in areas seasonably effected by cyclones, typhoons, humkanes, or heavy icing conditions; Maintenance of adequate 'intact stability' in accordance with an approved trim and stability bookiet; Pemissibie seagoing strength Iimits of shear forces and bending moments in accordance with an approved loading manual;
Torsions! Forces, where relevant; Minimum/maximurn forward and aft drailgk:~; Wave-induced hull vibration; Documented records of ballasting andlor de-ballasting; Contingency procedures for situations which may affect the ballast water exchange at sea, including deteriorating weather conditions. pump faliure, loss of power; Time to complete the ballast water exchange or an appropriate sequence thereof, takine into account that the ballast water may represent 50% of the t ~ t a cargc l capacity for some ships; and Monitoring and contro!iing the mount'of ballast water. I f the 'flow-though' method is used, caution shoukl be exercised, since:
Air pipes are not designed for conrinuous ballast water o v d o w ; Pumping of at !cast three full volumes of the tank capacity could be needed to be effective, when tilting clean water from the bottom and overflowing. from the top; and certain watertight and weather-tight closures (e.g. manholes) which may bc opened during ballast exchange should be re-sewred, Ballast water exchanges at sea should be avoided in freezing weather conditions; However, when it is deemed absolutely necessary, par;icular attention should be paid to the hazards associated with the freezing of overboard discharge an-angements, air pipes, ballat system valves together with their means of contro!, ar~dthe accretion of ice on deck. Some ships may need the fitting of a loading instrument t o perfom. calculations of shear forces and bending moments induced by water - -,exchange at sea and to compare with the permissible strength limits. An evaluation should be made of the safety margins for stabi!ity and strength contained in allowable seagoing conditions specified in the approved trim and stability booklet and the loading rnanual, relevant to individual types of ships and loading conditions. particular account should be taken o f Stability, which is to be maintained at all times, to values not less than those required by the Administration.
Fire and Ships Safety Q.1. With reference to C o n t r o l stations a n d F i r e p a r t i e s , discuss t h e importance of: a) Musterlist b) Fire control PIan Essentirrl R e q u i r e m e n t of F i r e Parties c) Ans.
.
The muster list shall be pemanentiy positioned and displayed throughout the vessel and shei! q e c i f y definite signais or, th: whistle or siren, for calling the crews to their emergency stations. The muster list shall also specify the means of indicating when !he vessci is bc aSandoned. The ~ . . muster !is: shall show the duties assigned to c.ew m e m b e r s i n rcspec: of a. :he d o s i n g of watertight dgors, fire cloijrs, side scuttles, valves and other ope~;in_gsin :he vessels superstructure. b. The equipping o f the lifeboais and other lire saving s?pliances. C . The iacnchir.2 o f lifeboats and liferafts. d. General preparations o f any other boats and life saving a~;.liances. e. The niuster o f passengers (if any). f. The sxtinctiun o f fire. Fire Controt Plan
:
For the extinction of fire, a -5re conlro! plan should be drawn u p and be p e r m m e n ~ l yon display, showing the following detaiis. a. Sections o f the vesse! enciosed by Oje resisting bulkheads. b. Section o f the vessel enclosed by f~e-retardingbulkheads. C . T h e fire cor.lro1 plans should be annotated, showing the fire alarm call points. sprinklers, fixed insiaiiations, poriabie extinguishers, equipment, breathing apparaius and fireman's outfits. At a glance, the complete fire a r r a n ~ e m e nand t distribution can be seen. @: Means of access to and escape from compartments and decks. e. ventitition systems, fan controls and dampers erc. f Location o f the international ship to shore ccnnectidn(s) g. Locations of all machinery stops, fuel oil remote shut o f f vaIvcs and e n ~ i n eroom skylight closure points. Fire Parties I Drills The essential requirement o f a good fire drill is that it is made ? s realistic as possible and nevcr al!owed lo become monotonous o r routine. Fire drills shouid be held in rotation to include :a. All crew members. in different parts of the vessel b.
C.
d.
All fire righting equipment to bcutilised. Fire drills to b e carriedbut a t different times and, on occasions, the drill should be carried out without an advance warning.
Fire drills shousd be carried out in the following way : i. One officer from each department, i.e. Engine and Dcck, should be put n s Fire Drill Officer, whose duly i s 1s submit typical fire drill situations relevant to his department. These to include details of extent of fire, scurce of ignition, equipment to be used, personne: to be involved and a full de-brief peiiod afterwards, which is equally important to the learning process. .. 11. Fire drills should be conrlucted in different areas oF:he vessel, s o a s to include accommoda:ion area, cergo, and machinery spaces. ... i ~ i . Use blacked out Breathing appaiatus face =asks o r safety smoke generators lo give B. A. wearers the benefit of experiencing zero visibility, as worild be expected in rzal life sitcations. . Fill up an old boiler suit v:ith rags, to simulate a 'body', for search an<: xescue Teams to get p:ac!ice, in evacuatins personnel. Apart from the need to conduct fire drills invo!ving the entire crew. ik:e is a strong case ibr ifivoiving speciali~edfire parties, hand picked men, >wiio have a particuiar aptilude, skil: and knowledge for fire fighting so a s to
mgendcr team spirit, confidence and communications as on efficient ream. and Iluring these drills, breathing apparatus should be - worn coii~mnnications/!ine signals difigenily practiced, until the B. A. teams can iiiiiy understand and be understcod. Fire drills should be varied every time and hypothetical fire situations weatcd to co.jer every possible contingency. Try to visua!isc a g i - e n fire and adopt boundary cooling accordingly . All equipment should be brought to a ::\ate o f rcadiness, i.e. fire pumps started, fire mains charged, hoses run out in position and charged. It is very important for the p-rsonnel to get the feel of itre equipment during practice, rather ihan in action for the first time. Before any fire drill is actually starled i t is mosr important that a roll call is taken and 211 persocnel accounted for. This is especially. significant when the case of an engine room fire, before the vessels ~SXST-W CO, fixed instal!ation can b e actuated into the space, the area must be fully evacuated. Many such fires have, in the past, been allowed to grow in intensity because of the confusion and delay caused by a lack of positive knowledge regarding the whereabouts of all the staff. The following ~ o i n t sare considered to be necessary to a good shipboard fire or-pnisation. I. The organisation should be simple to understand by all onboard. .. 11. It should be easily adaptable, to keep ilp-to-date. . .. 118. The system should, a s far as ~ o s s i b l e be , standard throughout the fleer.
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vili.
ix.
it should incorporate a simple but effective roll call procedure. i t should ensure :hat all personnel, with appropriate knowledge o f the vessel and fire fighting training, are used effectively i.e. deck crews for cargo and accommodation fires, engineers for machifiery space fires, pursers. stewards for first aid and support services. Fire fighting parties are we!! trained to operate in all situations, s o a s to become an effective learn. Alt other personnel, not directly engaged i n fire fighting operations, should remain at the muster point, arid must be given support tasks relevant to the tire sihation. Good communications should be set up and maintained, between the muster point, the bridge and the seat o f the tire. Realistic fire driilsltraining are czrried nut throughout the vessel 10 cover all eventualities.
The plan z!iould he scpervised by the senior cfficers on board. who wiii be picsent :o co-ordinate and control the proceedings. Bridge T e a m
- Overz!! in command, regarding the Ere and the vessel's s p e d , course manzu~erin!: and radio messages sent.
%aster
Third Officer Assisting Master in the above and responsib:r for shipboard cornmunicalicns between the control stationand the bridge. Helmsman, looking out for other ships acd as messenger between stations and in the event of a communications breakdown. .
Chief ~ n g i n e e r He is responsible !o the Master for the highly technical details, which should be fully utilised, especially regarding machinery space fires, Emergency fuel shut-off, ventilation, fuei oil bunker transfers. Engine Room Team SecondEngineer Officer-in-Charge o f engine room and maneuvering of the main engine. If the . . space(s) then he is aiso in charge o f fire fighting fire i s in thc machi&ry operations within. ~
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Duty Engineer As instructed by the second engineer, either in the engine controls and o r fire fighting operations. Engine ratings '4s directed by the Second engineer, either in starting the fire pumps, or, i f the engine room is involved in fire, as part of t h e fire fighting ieams.
Electi-ical Officel-
- to stand-by at the main switchboard, put on-line additional generators, or emergence lighting circuits. To stand-by and Re available for instructions from the secon8 engineer. A l l electrical requirements
F i r e Fighting Team Chief Officer Officer in Charge o f fire fighting operations for accommodation and cargo ,tiisn ?ce,x&~e ~f j,'k b e d -k spaces. Second Enginkeer M ~ i ~ r \ Cl (C ~ \~; < ~ rf Officer-in-charge of all machinery space fires. O f f W a t c h Engineer1 Deck Offrcers Assistin,o the Officer in Charge cf fire fighting operations in Ere c m t r o l and fire fighting operations as directed.
I I off watch crew members A s direcicd hy thc Officer in Chars:, cuuliiig arid orircr- C u t i c s .
for all fire fighiing cpcraiions. boii::tla:y
Galley persont?el A s Girected by the Officer in Charge in fite fighting operations, if the galley arcs is involved. I f no[, then in prouidinz support scrvicci lo lhc fire fiyl~ltng stretcher party tezms. Also to prepare the ship's hospital and-render -firs&d, and any other duties as directed. Second Officer H c is responsible to the Chief Officers a s regards cargo stowage and transfer. and also in charge of fire equipment - all hoses, exiinguishers. foam and breathing apparatus
/ JQ.2
Discuss recommendations on Safety Measures f o r periodically unattended Machinery spaces in addition t o tbose normaily considered necessary for attended Mlc. spaces. Base y w r assumptions that qualified personnel a r e available to a n s w e r alarms.
Ans. An unattended machinery space is one, where the provision o f automated alarm, control and instrumentation equipment compensates for the absence o f the machinery space watch-keeper. Sensors are used to detect the onset o f potentially hazardous conditions. ..
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Protection against Fire The fire detection system should be capable o f detecting the onskt o f z (a) -fire, and be self-monitoring for faults. Visual and audio alarms are relayed to the Accommodation -spaces, Navigation bridge and to the duty engineer officer's cabin. It should be possible to restricting !be fire to the space o f origin, by (b) ensurinz zdequate structursl design, and the elimination o f combustible materials near doors, casings, skylights and other openings. The remote starting of fire pumps, quick clitsing arrzngements, the (cj shutting dfi" of ventilation fans, fire extinguishing system controls, an6 shut-off arrangements for frtel pumps should be czntralised in the fire control station, together with at least one breathing apparatus a d ' a supply o f fire-fighting -equipment. The oossibilitv of the fornlation o f oil mist, can be dc:ected bv fd) , Oil mist detectors. - Fuel oil high p r e s s u r e ~ i p eleakare can b e detected b y using double walled pipes --and a leak-off tank with alarm. ----~~~
Protection against F!ooding: In mattended machinery spaces, an alarm s h ~ u l dwarn o f bilge water (a) or other iiquids accunulating ar an unusuzl rate or have reached an abnormal level in bilge wells. These w€lls should b e large enoush to hold mere than the norm'al drainagc expected during the lonzes! unattended operation, whi!e detccticg at normal ang!es o f trim and heel. In the chse o f bilge pumps srarting automatically, means shou!d be (b) provided to inEkate excessive running ('Long run' a&>) 2nd if the influx of :iquid.is greater than the.capacity o f the pump. (the usual arrangements apply to prevent oil pollution). The controls for sea inlet, bilge injection-and discharge valves below (c) the waterline should be sited to allow adequate time, in the case o f flooding, for these to be operated. Lsrger valves may require remoie control from above the bulkhead deck. Cancel l o A O PROC2AM R - 4 ,' Bridge control of main engine : The engine speed and direction of thrust o f the propeller (in case of controllable pitch propeller) should be f i ~ l l ycontrollable from the bridge, with means o f stopping the main engines, in an emergency. Remote automatic control system failure should give an alarm while the preset speed and direction o f thrust o f the propeller should be maintained, until under local control.
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Communication : A direct and independent means of communication should b e povi'ded belween the engine control room, the bridge and engineer officer's azio~snodation.
Rlachiraery space : . system of alarm, shvuld be provided, which should indicate any fault ihac r q , ; i i s zttention, additionally giving automatic shut-down i f nrc;.iss,,)-, but allowing for overriding in emergency situations. %lolorVessels : 1 . o ~lubricating oil pressure, high :emperaturc cooling water / rhrusr h e w i n g / exhaust gas, and crankcase oil mist detection. Turbine Vessels : o w lubricating oil pressure, reserve oil supply, and high temperarurs of hearings. Maiil propulsion boiiers: Hi:? and low water level, flame and air faiiure, high satinit)-. including i i i t s l a m if purgiag or re-ignition apparatus malfunctions. Oil supply arrangemenls: High and l o v ~alarms in daily service ianks and malfunction o f oil p ~ . ~fiws. ri illcrirical system : .- .c voltage or frequency variations, cperation o f load shedding ~,,s.,ive aimivgxnents and loss of po-aer in 'alarm syslems. :\!~xiliasy power units : C;i!ncrally make provisions as abobe.
3&/~tii reference t o h a z a r d s of enclosed spaces : W h a t oxygen content of air would you accept es -s_fe? 8) Discuss dangers involved d u e to the toxic effect of petroleum B) vapours a n d chemicals. >tare the iastructions a n d T r a i n i n g you would give to stnff. ct i .:garding entry into enclosed spares, in ships.
Aity s p w e that is not adequately vmtrlated, such a s cargolfuel oil ., dolibl~: bottom tanks, ballast tanks, cargo holds, pun& rooms, coiT*:rila!ns, d w t keels or even store rooms may c o n b i n toxic or flammable gasc:j o i m i i y be deficient in oxygen. Death has occurred when people have citicrcd ciicloscd spaces withoul checking for a dangerous almosphere. Thp da11gi:i.s ivilicli exist arc many and range from oxygen deficiency to toxic gases. Enainpli: are tanks which contain or have contained a toxic, corrosive or o:cygi:ii absorbing" cargo, refrigerated spaces from which the refrigerant riiay leuk, spacos i n which an internal combustion engine is insralled o r even c i i ! p i y Fuel taiiks.
Oxygen Deficiency
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Normal oxygen content o f air is approximately 21 O h by volume
Toxic Effcct o f Petroleum vapours and Chemicals ~. EV& very s m d l quantities of petrolecm qapcrurs, ivher?. i n h i i d , dull the sense of smell, and cause symptoms of diminished respor;sibi!ity and dizziness giving the i m p e s s i o n o f drunkenncss, headache and irritation o f the eyes.
Threshold Limit Value (TL'J) his is the highes< concentration, o f harmful substance in ai;, to which a person may be exposes [or eight hours per day, for an indefinite period, withgat danger to health. TLV for petroleum is not uniform, due to constituents varyins i n t h e i r proportions an2 various constituents having a greater toxic cffect r h a ~others. The main constituents are Methane, Propane, Ethane and Butane. A standard figure of 500 ppm is quoted for working i : ~a p e t r o l e ~ m atmcsphere but must not be taken a s applicable to vapours containing hydrogen, sulphide or benzene. The human body can tolerate somewhat higher concentrations for shoiter periods. The following are iypical effects from such higher co.&entiation :-
Hydrogen Sulphide, HIS Cmde oils may have the extra hazard of containing m c e quantities of Hydrogen Sulphide. Its presence as a vapour can be detected as low as I p.p.m. in air by its most offensive and pungent odour, somewhat simiiar to rotten eggs. .. . ;-s:. .' . Its toxic effect kowever, is one .. of . .paralysis . of the nervous systzm and one of tbz first senses tg b e rendered - ineffective i s that of smell. Concectritions of 200-30G p.p.ni. vaponr in air will produce such marked eye end respiratory trac: irritation that longer exposure than a few r n i n u t ~ scznnot willingly be tolerated, At a concentration o f 1,000. p.p.m. a few seconds exposure czn result in immediate unconsciousness and respiratory faiiure which unless quickly restored will be rapidly fatal. TLV is given as 10 p.p.m. but in an enclosed space a nii concentration should be achieved before entry isupemitted withoutthe use ofbreathing apparatus. ,
When entering an enclosed or confined space, the following principal points shsuid be observed :a) identifying the potential hazards. Instituting and adhering to a risid permit-ro-work system. Ensure thsr bi the space is secure against i-gress of injurious substznces. C) Freeing the atmosphere of'gas and removirig sludge zndlor ether sources of gas (a tank is not cansic3ered gas free if any siudge remains). d) Testing for the presezce of toxic gases and/or oxygen Geficiency e) Instructing or training personnel in the safe conduct of the operation. 9 Provide adequate safety eqilipment. g) Organising emergency rescue t-amsffirst aid.
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I f it is found-necessary to er.ter a confined space with breathing apparatus two supplies of air are required. On no account should a person stationed at the entrance attempt t o enter the space before additional-aid arrives, no attempt to enter must be made without breathing apparatus. The testing of the space should be carried out at different levels, and . . . h r t h e r tests to be carried out while space is occupied. Breathing apparatus must be worn, if any doubt exists about the possibility of vapour. ~ i f e l i n e sand safety harnesses should be worn. The lifeline must be capable of being easily detached by-the wearer, in c a s e o f entanglemefit. 4 c y . attempt to rescue a person from a n enclosed space should be based on a prearranged plan. Survival, after loss o f air supply, is time dependent and restoring the victim's oxygen supply is the first priority. Unless the person is gravely injured, e.g. broken back, any physical injury he has sustained is of secondary importance - the victinl must be brought out wirh the least delay. Restoration of the casualty's air supply at the earliest must always he the first priority. All ship-board personnel are already qualified to render first-aid, after attending suitable courses ashore. so detailed descriptiow'h ::'e not been provided here.
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'~.4
Explain in detail h o w a n Oxygen Analyzer w o r k s a n d h o w the m e t e r is zeroed.
Oxygen Analyzer Various types of meters may be used for measuring the oxygen content. A contir,uozs reading type is one in which platinum wire elements are m o ~ n t e din two chambers, one the rneasuling chamber and the other, the reference chamber. Oxygen is paramagetic, i.e. ii is attracted to magnetic fields. Thus one Elamen1 has a magnetic field, while the referewe filament bas no field, an: attracts only the air. The circuit forms parT o f a Wheatstone's Bridge. The filtered and dried gas is drawn across the elements and &e difference in thermal conductivity o f the O,, reiacive to air, causes -.temperature diff~rericein the wires. This changes the wire resistance, and unbalances the Wheatstone's bridge circuit, generating a resultant current, which is proportional to the percentage o i oxygen in the s m p i e . F a k e readings arc likely if tne gas sarn;.,le contains anorher paramagnetic gas such as NO,.
Zero position check : 'Zero' position setting can be done by using a pre-calibrated sample, and then setting the span of the instrument. Test with 100 % Nitrozen. [ COZ may be used in emergency.] Open control valve for 3 minutes, to obtain zero reading. Now test with atmospheric air to obtain 20.8 % reading for which spa11 control can be adjusted if necessary.
I.
~oaislrtCuerype
t'sed where ihel o r other combusfihle materid pmdur'cs~rnriou$ ~ r n d r ~ e lof s combvslion much b t b r t the appearance OF smokc or flame*.
A rediaactivz sm~rec.such as radium, ionizes Lhc armasphm i n lmtk open and closed cchrnbtrs. llnder norma! condilions. the circuil is elec~ricallyb.~lancrd W h n c o r n b i ~ s l i mproducts enlet thr: cpca chamber. 15c ion Raw is T t t a ~ d t d and thc e 1 c ~ t ~ : s l resistarrce i s inzrerscd. lhus cr,alin: i~:!salancc, whirl1 t r i g ~ e r sr i a u l o m i Tcsrinp, i 5 r a r r t d out 5y inject in^ a pre-Elid hydro carlmn gas Inlo thc d c i c c ~ whcnd, hy mranJ;ni a s+cidl spray czn provided.
k 34
Advanced Morine Enginering Knowledge
Vol. I l l
P h o t o Electric I S m o k e Detector type. These are used where smoke is produced much before any flame is visible e.g. insulation fires.
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Light source
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Phnto ilectric 4l
When there is no smoke in the head, the photo electric ccll csnnc: dzteci the flash tube signal. The zddition o f smoke causes light to fall on to the P. E. cell, which triggers the alarm. Testing is carried out by actually passing s ~ o k einio the dckctor hcact.
lnfra-red I Fiame sensor type Used where flames could occur in hot spaczs, wnere heat de.,"ctors would not work, such as in the machinery space cylinder head platfarm. The head is designedto sense radiation waves of 25 Sertz, which corresponds to that of naked names. A time delay mechanism reduces false alarms due to Iight reflecting offrotating machinay or similar cause.
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1NiRA E D ( F U M E ) D n E C I . O R
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Testing i s carried out by a naked flame at 5 m distance.
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H e a t sensor / r a t e of r:'se sensor These are u s e 6 where iherc may b e high ambienttemperatures e.g. machinery spaces, galleys. Two bi-metallic strips are placed in parallel to form an electrical circuit. C n e is insulated to the atmosphere and the other .is open. A sudden increase i n temperature would cause the open strip to bend quicker tkan t~e'insuiatedone, m d trigger the alarm circuit. However, gradual ambient temperature increase would cause both strips to move equally and n3t trip
RATE OF R I j E TYPE HEATDETECTOR
Tested by using a heat source.
'i'estiog of F i r e detectors : A n efficient fire detection system is required, when a vesscl is operaled . w ~ t h an unattended machinery. soace. G r o u ~ sof detectors are 2rranted in circuits, according to their posiiion in the spaces be!ng protected. The choice o f which type to use depends on the type of fire expecied, whether high level o i ambient heat is expected (e.g. Enginc room) and whether flame can be easity detected (open spaces, so direct line of sight), and are thus arranged, so as to detect an outbreak of fire, in any zone, as quickly a s possible. T h e an'ccted zone will be indicated on the alarm panel. Small indicator lamps are usually fitted to detector heads to show which head has operated. Most systems operate on 24V D.C. The emergency b;!itery must be capable o f operating the sysrem for 6 hrs. in the 'no alarm' siate, and !& hr. in rhe 'alarm' state. (This is for cargo ships - the requirements for passenger ships are different). When tke detector is ;ictiva!ed, by the method appropriate to the type o f detector head, the local irtdicator, as well a s the lamp on the main alarm panel, will indicate the .ivorking of the detector head, during testing.
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Detection of faults in cables a n d detector heads Detector heads must be checked on a regular basis and the cables lo them rwsr be checked, because a fire could damage the cables before the detectors have reacted. T w o methods are used :
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a) Loop monitormg b) Line termination monitoring Both systems rely on detectors being open circuit under normal condition^ L o o p Monitoring The alarms are activated by closure of rhe contacts i n the heads through (a) and (b) while continuity is checked rhrough (a) and (d) and (b) to (c) A
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st
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Alaz-m panel
Detector head
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Checking Lines The alarms operate when contacts acioss (a) and (h) close. Continuity is checked by monitoring the resistance at the ;iiJ o f the line. If the resistance of the line measurqd across (a) and (b) is less than the and end resistance - the alarm sounds. If the resistance across (a) and jb) increases ab0x.e a level eqtial to the resistance shown plus the cable resistance, a system fault alarm is acluated.
4.6. (a) (b) ...
Ans. (=)
(bf
State where information can be obtained with regard to the safe earriagz of hazardous substances as cargo. F o r the hazardous cargo of y o u r choice, discuss the following: i) Storage, transport and H a z a r d o u s properties. ii) Fire fighting and suppression techniques. iii) Medical effeets and treatment after physical contact.
International Maritime Dangerous Goods Code (I.M.D.G. code) gives (he requirements for carriage of dangerous s c a d s in small packages, bales and so on. A general search for any particular cargo is :-e.g. Nitric Acid.
i) In I.M.D.G. code, locate NITRIC ACID. The U.N. No. identifies rhe substance on a United Nations list and is tinique :o that substance avoidin? confusions due to different languages. I.M.D.G. Code lists. its propcrties a n d t h e inherent hazards. It also lists its packaging group (e.g. small packages) and stowage requirements - - Category D. Category D gives details of stowage limitations (on deck cnly). 1 1 ) In the Emergency Schedule, emergency equipment, procedures and emergency acti?ns arc reconlmendcd in case o f spillage and/or fire.
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iii) The Medical First Aid Guide (M.F.A.G. Tables) gives guidance into recognizing symptoms and their treatment, where someone irray have been in cantact wiih ..:he substance. The lnternatioual Chzrnber o f Skipping's Tanker Safety G1iiFI.e (Chemicals) gives recommendations for all the above c ~ i t e r i a ,when the subsiance is zarried in bulk.
Q.7
Wiiln r e g a r d to carriage of c r u d e Oil a n d associated p r o d u c t s a) Sketch a n d describe a n EipIosimeter suizabie f o r testing P u m p rooms o r Tanks. Calibration procedures you would fofl$w f o r s u c h meters. b
Ans.
Con8bustibk Gas Detector Thc principle of operation is that a samp!e mixture is drawn into the rn-ler b y an aspirator bulb. This sample is ignited by the catalytic action o f a heaicd filament, since the sample coming into contact with the hob filament will b u m . The b u r n i ~ gsample heats up only this section of the Wheatstone's bridv: and thus increzses its electrical resistance. This unbalances the bridge arid causes a resultant current to flow through the meter, which is proporlionat lo ihc tlcaiins cffcct, and thus thc co:iccntration of cxplosivc gnscs prcscnt i n the s;imple. Even 'too lean' concentrations are capable of being ignilcd and thlis de:ec!ed, due to the catalytic acticn of the filament. ?'hc meter is usually marked to read the gas concentration a s a pci-cci~la$eof the Lower Explosive Limit (L.E.L.) or a s parts per million (p!m). However, any deflection of the needle (above zero)-is a potentially h ~. .I .~ ~. I I ~ O I and J S , thus an unsafe condilion. it is compact and portable, being po~wcretlb y small batteries. The Explosimeier will not detect the presence o f Hydrogen gas. False readinss \\'ill hi: obtained if the sample gas contains a very lo, oxygen conrent. The meler indica!cs up to the L.F.L. and could thus read zero, when actually the mixture is i n 'loo rich' a condition, i.e. explosive. (To check for this possibility, purge the sarrlple with air. SO as to get a leaner sample, for analysis)
There are many types o f instrumenr, but the type most conlmonly found on ships is the resiskance type Explosimeter shown below :-
Calibration procedure T c s ~p s e s include
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50% L.E.L. penlane (0.75% pertane) 50% L.E.L. methane (2.5% methane)
F!ow control
Adaptor Test gas
Test ki:s for shipboard use are available for-this purpose, which provide a mixture of a hydrocarbon gas in air, such a s 50 % LFL Butane i n air. Leak testins may be achieved by pinching the sample line and squeezing the Aspirator bulb - the bulb must not expand, as long as the sampling line is kept pinched (i.e. a partial vacuum is maintained, indicating that there is no air ingress). fnstr~imentsused must have flash-back arrestors in the inlet and otrtlcr o f ti-: Detector filament chamber, so as to reduce the fire hazard.
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If the concentration o f gzs is about twice the LFL, there is insufficient Oxygen i n the mixture to burn the hydrocarbon gas complettly. The response o f the instrument, to such a concentration, i s that the needle initially deflects to rhe maximum scale reading, and then falls back to zero. Continuous observation is thus required to detect and identify this condition, and the operator needs to be alert to this. ~ r o i o n ~ operation sd with such a gas mixture causes the depositicn o f c a r b o n x e o u s matter gn the sensor ~. filament, which will affect the response o f the instrument. For the same reason, the instrument does not give a reliable reading with a deficiency o f oxygen in the gas sample, such a s what exists in inertpcd cargo tanks. This meter cannot, therefore. be used for inerted tanks. Attach flow controller, fit the adaptor and connect the tubing. Opcn the con:rol valvc for 15 seconds. The meter should indicate between 37% and 55% defection (adjust span control, if necessary?.
In the figure shown, a meter reading of 68 % to 92 % of the L.F.L., Lbr a precalibrated sample of 3 % methane, would help to check the accuracy o f the instrument. Factors that can influence the measurement are : large changes in the ambient temperature hcavy or large flow rates, which affect the filament temperature. To prevent any inaccuracy due to flow rate, a reading should be laken when there is no flow, i.e. between hvo successrve squeezes ofthe aspirator bulb. -
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Advanced Morinr Engineering KnmvI~dge VoL JII
Q.8
(a)
Sketch a n d describe a Bulk Carbon dioxide system, a n d state specifically where such extinguishing media c a n be efiectivelv used. Sketch and describe a Bulk Dry Powder Installation a s used on LPG & LNG carriers. ~
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Ans. Bulk Carbon dioxide systeni Carbon dioxide is stored in b d k , in a single container at -17' C. Two refrigeration systems are fitted, for keeping it cool. One is a l w q s in use, whjle the other one is on stand-by. In the cvznt of loss of power, the tanks are sufficiently well insulated, to maintain this temperature for a minimum of 24 hours, before any danger of "boil off' occurs.
Two sets of relief valves are fitted to the pressure vessel. Set A lifts at 24.5 bzr, to atmosphere. Set B lifts at 27 bar to the C02 room, in case of fire in this space. Each set has an isolating cock to enable one valve only of the set to be opened up for surveyfrepair. There are two means of indicating tank level :Remote electrical display of contents (capacitance bridge) a, A stand by indicator, consisting of a vertical, external un-insulated b. pipe. This can be filled with C02 to the vessel level, by opening one valve. Level is determined by frosting on the outside of the pipe (or by level detector).
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The remote operated valves in the system can all be operated manually, using 8 spanner provided at each valve. When the required amount o f COZ has been discharged, z signal f r o 2 the electrical contents system, closes the E. R. distributionvalve. If more C 0 2 is needed this valve can be re-3pened by there-release button. Alarms are fitted for 5% loss of contents and for over-fill o f vessel above 98%. A third relief valve C , set at 35 bar, protects the system pipe lincs. Storage vessel is specially fabricated from sophisticated steel, suitable for low temperature operation. The system is emptied and internally inspected every ten years. Pipe system is o f solid drawn galvanized steel pipe (as in the bottle system pipe work). Pipe blown through with compressed air periodically. System has lower filling costs than the bottle system, and result; in a saving in weight and space. Unlike the boitie system, this allows for re-release. Bulk Dry Pqwder Installation (for LPG I LNG Ca~riers) J3r-y pcwder, discharged as a free flowing cloud, eiitingcishes ti fire rapidly f'he action is simiiar to Flalon. Also, the powder gives some sniuliicrlng effect.
I Hose box on deck
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Bulk Dry Powder installstion
When the operating valve in the hose box is opened, nitrogen flows to tilad$-ypowder compartment, which : opens the appropriate direction valve. (I) activates the rr-in nitrogen release operating cylinder. (2)
Nitrogen flows into the dry powder container via a pressure regulating valve. When the pressure is about 16 bar, the main discharge valve opens and powder flows to the hose.bok. The in the container remains fairly constant, until it is nearly empty. If more dry powder is required, the stand by system can be operated. Nitrogen pressurz from this system holds a valve in the main system closed, preventing powder enterin3 the main container. Briefly describe the Physical characteristics of Liquid Nzturat G a s (LNG). What precautions and action will you take in case of an LNG fire?
Q.9
Liquefied Natilral Gas (LNG) Physical Characteristics: 'This is a clear odour less liquid, stored at - 160 'c. Less . yiscous .~ & !ig5ter rhan water (Relatiue density i s 0.5). Main constit~~ent is Methane > 90% . ., . Tla'sh point - 188 "C Auto fgnition 650 'C Density :Immediatelv on va~ourisation = : !.4 x air ' At - 104 OC = Air At 1 5 ' ~ = 0 5 5 x air ~~
Flammable Two ohase va-oourisation I" - htgh rate for about 30 secs znd- lower, steady rate due to thermal insulation of the already vapourised layer
w l d vapour forms at lower deck level forming visible condensation cloud which is in the explosive range. Generallv :- No visible cold vapour cloud then no risk of vapour ignition.
Hazards Causes 'Frost' burns on physical contact, . protective clothing reqxired. Can cause 'Brittle' fracture of steel work, wood c!adding & stainless steel drip trays give some protection.
I Spillage /
Stop source o f leak and contain spillage if possible Sound the alarm Avoid ghysical contact & protect steel deck. Speed up vapourisation by use a h e water spray or 'Fog' (reduces risk of -fire & o f brittle fracture).
LNG Fires Requires a HOT spark o r flame to ignite the cold Vapour Rapid vapourisation prevents ignition o f the liquid itself, even with its low flash point of - 188 O C . Protect Personnel & adjacent equipment etc. with a fine water spray or 'fog'
-Avoid 'Run Off; water e n t e r i ~ g the pool of liquid LEG, as this would serve to aggravate the fire by rncreased vapourisation
Flame size & heat release TzckIe Fire is similar t o other hydro- Use dry Powder with the carbons but there is :ittle maximum rate of smoke. application. Flame propagation is law Position down wind with resulting in a 'lazy the powder jzt slig!$ly flame' depressed, syecp back & forth over the e d r e area ACTION Isolate sourceof !eak Sound Alarm, Ensure adequate personnel u e a available to tackle fire with a minimum of d c l q
Avoid Jet impact o ~ t liquid o pool as this would aggravate the fire
Watch for Re-Ignition fiom hot surfaces, burning paint work etc
Large Fires (Conflagration) Co~isiderthe possibility of allowing the fire to burn itself out taking account of %herisk of the fire spreading and greater damage being caused. NOTE : . . ....
It may not be possible to deal with the fire with the available powder due to the contained radiated heat. Extinguishing the fire night run powder reserves so low. that ReIgnition could not be contained. Enclosed sbaces :- Use smothering system COz for engine room and Nitrogen for void spaces & vent pipes.
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Briefly discuss t h e S t a t u t o r y R e q u i r e m e n t s f o r a n I n e r t Gas System. DISCUSS t h e i m p o r t a n c e of a n y a l a r m s necessary.
inert Gas System Requirements (STATUTORY) The Inert gas system shall be capable of providing on demand, a gas or a mixture of gases; to th? cargo tanks, so deficient in oxygen that the armasphere within a tank may b e rendered inert, i.e incapable o f propagating a name. Qperariotlai Condition The system shall satisfy all the following conditions : 1. The need For frxs'n air to enter a tank during hormai operations shall be eliminated, except when preparing a rank for enlry by personnel. 2 . Empty tanks shall be capable o f being maintair.ed in a n inert atmospheie. 3. 7 he washing o i tanks shall be capable of h e m g carried out in a inert atmdsphcre. 4 . S u i t a b k means lor purging tanks with fresh air, as we]! a s with inert ?as shall be provided. T h e system shall be capable of supplying -insert gas at a rate o f at least 125% of the maximum rated capacity of the cargo pwnps. 6 . Under norrna! running conditions, whcii :acks are being filled or have been fillzd with inert gas, a positive pressure shall be capable o f bein2 maintained. 7. During cargo disrharge, the system shall be such a s to ensure that the volume o f gas (!25% of pump rated capaciiy) i s available. At other time sufficient gas to ensure compliance o f this regulation shall be available. 8. Exhaust gas outlets for purging shall be suitably located in t h e o p e n air a r d shall b e to the same general requirementc. a s prescribed for ventilating outlets o f t a n k s ~ 9. A scrubber shall be provided which will effectively cool the g a s and remove solids and sulphur combustiun products. 1 0 . ~ 1least two fans (blowers) shall be provided which together shall be capable of delivering at least the emount o f gas stipulated (125%). 11.The oxygen content in the ineri gas supply shall not normally exceed 5% by volumc. 12.Means shall be provided to present the return of hydrocarbon gasses or \,apours from the tanks to the machinery spaces and uptakes and prevent .. ... the development o f excessive pressure or vaculln1. ];.in addition, an effective water lock shall be installed. Branch piping tblinert gas shall be fitted with stop valves or equivalent means o f control at every tank. 1 4 . 7 h e system shall b e designed so as 10 rninirnise the risk of ignition from {he generation of static electricity.
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Advanced Marine Engineering Knowledge Vol. J I I
I S . lnstrumentation shall be fitted for continuously indicating and permanently recording, at all time when inert gas is being supplied, the pressure and oxygen content of the gas in the supply main on the discharge side o f the fan. L6.Such instrumentation shall be easily accessible to the officer in charge o f cargo operations. 17. Portnb!e instruments suitable for nneasuring oxygen and hydrocarbon gas and the necessary tank f i t t i ~ g sshall be provided for monitoring tank contents. 18. Means for indicating the Temperature acd pressure i r ~the inert gas main shall bz provided. Alarms shall be provided to indicate :a) b) c) d) e)
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Hieh oxygen content in the inert gas main. Low gas pressure in the inert gas main. L O W,pressure in the water supply to the deck water seal. High i-tmperzture 3fgas in the ineit gas main. L o w water pressure to the scrubber. High gas pressure in inert gas main.'
auromatic shutdown of the system shal! be an-anged at predetermirxd [ f ) above. The ship shall be provided with an inert gas system manual covering operational, safety and occupa:ional health requirements relevant to !he system. iiri
, jmirs . - in respect of (d), (e),
Q.11
Explain the principle o f Autonomous I n z r t G a s Generator
Autonornous Inert Gas Generator :-Snls I .. system does not draw the gas from a boiler uptake. It i s designed ro "stand alone". ~I'hetre are many variations o f this type of inert gas system. T h e one shoivn incorporates a gas turbine which generates electrical puwer. The eXhsust f'rorn the turbine (which always uses a very large amount o f excrss air) is Icd to a combined scrubbcrlafterburner arrangement. Fuel is burned in the exhaust, to reduce the oxygen content. The final exhaust is then scrubbed a r ~ dled to thc inert gas main.
This arrangerrrent show above could also be used a s an emergency generator and bulk tire extingsishing system (for cargo holds). When used in this way tile incrz 9 s rvould replace the Carbon Dioxide bottle sysrem. i t can not be used as a bulk fire extinguishing system for the engine room due to the slow speed at which the gas is generated (Note! For engine rooms the incrt xss has to have an 80% discharge in two minutes).
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In other autonomous systems the gas may be generated without using a gas turbine.
Q.12 Discuss Life Saving Appliances as required for Class VI1 ships. With a simple sketch esplairi the working of 1. H a n d Brake .. 2. Centrifugal Brake
Ans. The life-saving eqxipment required on board a ship is governed by its classificuion. The two ciasses which cover the majority of ocean-going vessels are Class I and Class VII. The fol!owing notes refer to the requirements for Class VII ships. Lifeboats In an emergency it may be necessary to disembark from one side and hence life boat accommodation must be provided on each side of the ship for all the ship's personnel. The boats must be at least 7.3 m long and must carry sufficient equipment and provisions to ensure a high degree of survival. including such items as buoyant oars, boat hook, hatchets, lampjs), compass, distress rockets, smoke signals. fi-?:-aid equipment, fishing lines, suitable rations and fresh water.
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One of the lifeboats must he fitted with a compression ignition engine and carry sufficient fuel for 24 hours continuous running at 6 knots. One function of the motor lifeboat is to two the remaining boats clear of the abandoned vessel. It is usua11~uscZ as a working boat, i.e. for ferrying the crcw ashore and transporting light stores. . In ~ i tankers ! having midship accommodation there is a risk that in the event of fire, or explosion, the two sections o f accommodation may be separated and hcnce ii is necessary t3 provide two lifeboatsamidships and . ~. .. . ~-;. .. .. two aft. . . ~. .. . . . . . .. . . . Davits . There are three baGc rypes of davit : a) radia! . b) luffing c) graviry For sma!! working boats not rsyuircd to act as lifeboats, radia! d z v i s :ire acceptable but seldom used on mo3ern vessels. They have advantage of having few mechanical parts but are awkward to handle. Luffing davits maybc uscd for boats under 2.25 :cnne in & cargo ships. Gravity davits are fitted on most modern ships and have the advanl?oz thet when released move automatic.al!y into position. ~
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The davits must be capable of Lowering the boats when the ship is heeled to 15" on either side. Should the heel exceed 15" It may be impossible ~to . . launch the high side lifeboats. 52
A \\-ire rope span is fitted to the stop o f the davits and knotted llfeljnes i d Ci-om i h e span into t h e boat, allowing embarkation to the lbwered boat
from the boat deck. The wire which allow the boat to be lowered arc termed "falls" and are controlled by a small winch. The boats are lowered by raising a weighted lever known as a "dead man's handle" which releases a brake in the winch. A separa:e centrifugal brake is fitted to restrict the speed o f descent to 56 mlmin. while the power supplied to the winch must be sufficient to raise the boat at 1S mlmin. minimum. Hand operated Brake
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-Lowering
Cent-ifugal Brake
Lifebuoys At least eight life-buoys must be carried on all but the smallest ships to assist crew members who have fallen overboard. In order to enable personnel to be more easily sighted a? night, self-igniting lights are fitted ta at least half of the number provided. These lights must be of the electric type on tankers. At least two o f the lifebuoys should have self-activating smoke signais for daylight recognition a d must he carried on the bridge, ready for quick release. For w e near the ship one lifebuoy is carried on each side and provided with 1 5 fathoms of buoyant line. The lifebuoys may be of cork or any other suitable buoyant material which can withstand the effects o f sea water, oil and variations in temperature and climatic conditions which are likely to be encountered on open sea -voyages. . ..
Lifejackets Each crew member must be provided with a lifejacket which may be m-2c from buoyant material such as kapok or (except in tankers) may be inflatable. The lifejackets are capable of being worn inside-on! and are
designed to turn the wearer to a safe floating position within 5 seconds so that an unconscious person w m l d float safely. Lifejackets must have lights and whistles attached. Liferafts Liferafts are provided on most ships and are required to have ssfficirnt capacity to carry 50% of the total number s f persans on board. The iiferafl:; are usually of the inflatable type stored in cylindrical fibre-glass conraiiirrs. Infiaiion cakes place automaticaliy when the life-raft is launched overboard. the container bursting open and the life-raft floating clear. The liferafts are extremely seaworthy, and, being fully enclosed, provide excellent protection frcm exposure. Buoyant Apparatus
This is required on sorce passenger vessels. They must have a rigid s:ructi.m, able to iloat [stable) either way up and not depend upon inflation fot. buoyancy. Must withsrand drop test. Fittcd with grab lines and painter. Q,13 1 Wjth reference to t h c >hip's strvctural fire protecticn, discuss : Classification of D i v i s i o x viz. *-Class, &Class, C-Class ) special czre is takert for passenger ships What b) cf H o w openiogs are protected.
Ans. A Class
Ihese are divisions formed by bulkheads a-3 decks which are constrtucted of steel or other. equivalent materials, suitably stiifeaed snd constructed so as to prevent the passage of smoke and 11ame up to the cnd of the 60 ininures standard fire test. T h e y must be insulated with non-combustible material such that the average temperature of the unexposed side will not rise more than 139"' C abovi. the original temperature nor will the temperature at any one point rise m o w ti~anI X O " C above the original temperature within the following times :
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Class A Cliiss A CLass A Class A
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60 30 15 0
60 minutes 30 minutes 15 minutes 0 minutes
B Class Thcsz are divisions formed by bulkheads, decks, ceilings or linings which arc cor~structedso as to prevent the passage of flame to the end of the fitst SO rniriutes of the standard fire test. They must. be constructed o f non cornbus~ibii: niaterial and to have a n insulation value such that the average tcnipcraturi: of the unexposed side will not rise more than 139 "C above the
original temperature nor will the temperature at any one point rise more than 225" C above the original temperature within the following times. Class B - 15 Class B - 0
15 minutes
0 minutes
C Class These are divisions zoristructed o f i n c o m b ~ s t i b i ematerials but need nor meet any of thc requirements of the stzndard fire test in relation :o passage of smoke or flame or temperature rise.
NOTE: ~
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An incombustible material is one which neither b v m s nor gives off nammable vapours in sufEciect q u s n t ~ t yfcr self ignition when heated to approximately 750 OC according to an esrablished re.! procedure.
Structuraf Fire Protection ?assengzr Ships These notes are based on the requirements for passenger ships carrying n ~ o r ckhan 35 passcngcrs. For ships carrying nGr more thzn 36 passengers, the requirements are slightly less stringent. The hull, qxis:ructures, srructura! bulkheads, decks and deck houses must be of steel or other equivalent material. The hull, superstructures and deck houses must be divided into main vertical zones, the mean length of which must not in general exceed 40 m. the bulkheads forming zone boundaries should if possible, be vertically in line with the watertighr sub-division bulkheads situated immediately below the bulkhead deck. Any steps and recesses must be kept to a minimum. The boundary balkheads ar,d decks of zones mxst be of A class standard with fire integrity standards ranging from A-60 to A-O a s laid down in the Rules depending upon rhe fire risk o f the spaces invoived. Similariy, bulkheads and decks within vertical zones may have fire integrity standards --.'. icnging from A-60 to C Class. Except in spaces having a very l o w fire r i s k , a l l linings, grounds, ceilings and insulations must be of non combustible materials. Within limits, facings, mouldings, veneers and decorations may be of combustible materials but most exposed surfaces in accommodation and service spaces must have low flame spread characteristics. A fixed fire detection and fire alarm system must be fitted through-out each separate zone whether vertical or horizontal, in ail accommodation and service spaces and also where necessary, control stations except spaces where there is little fire risk such a s void spaces, sanitary spaces etc. Alternatively an automatic sprinkler, fire de~ection and fire alarm system may he fitted i n these spaces and in addition a fixed fire detection and
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fire alarm system fitted to provide smoke detection in corridors, stairways and escape routes withir, accommcdation spaces. The tire integrity standards of divisions of spaces fitted with a sprinkler system may be reduced. .-..- . - .~. -. .. . - , ..~<:~-.,:..?~. . . . ~... .~. ~.. . , . - . . .~. Protection of Openings .: . . . . . .~. > .; . : . . . .. .~ .. ~ . -4 Class Divisions . .. . .....,.. .. Doors and f r b & in A-class divisions must beconstnicted t 3 provide resistance to fire 3s well as smoke and flaine a s f a r . as. is practicable ~. equivalent to that of the bulkheads in which t h e d o o r s are.sitilated. Doors must be constructed of steel o i other' equivalent material. Each d o o r must be able,\oipbe opened or closed by one person from each side of the bulkhead.. . g2Fire doors in main vertical zone bulkheads .and . . .stairway ~.~ . .encl&res must be of the self-closing type capable of closing against an incliniti6nof 3.5 degrees. All such doors except those that are normaliy'closed, must be capable of release from a control station either simultaneously or in groups and also indi~iduatlyat the door. The release mechanism must be designrd so that the door will automatically close in the event of disruption of the control system. Hold back hooks nor sxbject to control station release arenot permitted. ~
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B Class D i v i s i ~ n s Doors and door frames must have a resistance to fire a s far as practicable equivalent to rhe division in which they are fitted except that ventilation openings of limited area aiid fitted with a grill made of non be fitted. Doors are to be of non combustible combustible material L.:;I material. ~.
Protection o f stairways and Efts in Aeco,mmodatioo spa& Stairway and ladders must be provided to provide means of escape to the lifeboat and life-raft embarkation deck fromall pas&%@r and crew spaces , . . . . and from spaces in which the crew is normally employed. ~ . . . . . ..~. -. All stairways must b e of- steel constructioc unless an equivalent material i s specifically approved. Unless lying wholly within a space they are to be within~enclosuresformedby A Class divisions with positive means of closure at all openings except that a stairway connecting o n l y t w o decks need not . be,enclosed . . .~ . provided . ~. . that a . . . . bulkhead or door is fitted at oile level. Lift trunks have the same fire integrity standards as stairway enclosures. They must be fitted so as to prevent the passage ,of smoke or flame from one between deck to another and must be povided with means of closing so as to perniit the control of draught and smoke. "
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Ventilation T r u n k i n g Precautions must be taken to maintain the tire integrity of bulkheads and decks through which trunking passes and to reduce the likelihood o f smoke and hot gases passin2 from one space to another. This achieved by using suitable materials, in conjunctionwith sleeves and. dampers where ducts.... . pass through divisions. The following are some o f the reqcirements to illustrate the principals involved. Ducts h a v k g a sectional arra o f not less than 0.075 m' must be constructed of steel or equivalent, ducts of smaller area need not be o f steel but must bc of non combustible material, whilst ducts of cross sectional area not exceeding 0.02 m2 need not be o f non combustible material subject to limirarions o!i their length 2nd position. Ducts with a cross sectional area exceeding 0.02 m' passing through A or B class divisions rrlust be fitted with an insulated steel sleeve unless the duct is of steel in x a y o f thc div~zion. Ducts with a crosz sectional area exceeding 0.075 m' must be firt-d with fire dampers where they pars through A class divisions. The dampers must operate automatically but nmst be iapabl? of being closcd nianuhlly from b x h sides o f the division. The damper must be provided with an openclosed indicator.
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Dry Cargo ships The hull. superstructure, structural bulkheads decks and deck houses most be construcred o f steel. The fire integrity standard of boundary bulkheads and dccks separating adjacent spaces within the ship such i~~ acconi~tiodation. corridurs. conlrol srations. rnachiiwl-)- spaces and c:ir;c> spaces r a n y front 11-50 to C Class dependins upon the tire risk involi-eii. One of the followin_r methods clf protzctim must he adopted 1,-ithin ~ c c o m n ~ o d a t i oand n service spaces : (a) Method I C All bulkheads, cxcrpt houndary hulklicads p r c \ i w s i y incntioned. must be of non combustible B or C Class standard. .All linings. draught stops. ceilings and their associa~cd r o u n d s ~ilust be oi- nailsombustihlc material. ;\ lixcd lire drtcctio!l and lire xlilrm sysrun n ~ u s tbc installed anJ ~:r:mged to pl-ovidr smoke deieclion and manually oprratsd call poinix i n all corridors. stairways and escape routes within a c c t > ~ i ~ x l ~ , > d : ~ t i ~ > ~ ~ 5pcL-s.
and control stations, the ceilings, linings, draught stops and their associated grounds are to b e of non-combustible material. An automatic sprinkler fire detection and alarm system must be fiti
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Method 111 C Generally, nc restriction on the type o f internal bulkheads except that in no case musi !he area of any accommodation spacc bounded ;%y arl K Class or B Class division exceed 50 m2 alrhough this area may be increased for public spaces. The arrangemects for ceilings pic. is the same as for method I11 C. A fixed fire detection and fire alarm system must be inscatled and arranged to delect the presence of fire in all accommodation spaces and servicc spaces except where there is no substantial fire risk sdch a s void spaces and sanirary spaces.
in ail cases, combustible veneers o f limited thickness may be applied to ti~~r\.combus:iblebulkheads in accommodationand services also corridors. stairway enclostxes and control statidns. Ail exposed surfaces in corridors and stairway enclosures and surfaces
Oil. 'l'artkcrs Accommodation space must bc positioned aft o f all cargo tanks, slop tauks, cargo pump rooms and cofferdams which isolate cargo or slop tanks from i~iaziriricryspaces of category A, although, if necessary, acconimodation spacrrs inay be fitted forward of such spaces.
Esierior boundaries of superstruclures and deck houses enclosing accommodation and s e r v i c e spaces must be insulated to A-60 s:andard on all surfaces facing the carso tanks and for 3 n~ aft of the front boundary. Entrances, air inlets and openings must not face the cargo area and their distance from the iront o n the sides o f superstructure must be ai.leasr L160 b u t not less than i m althollgh t h i s ~ n z e dnot a p p l y to c a r g o control statidns a:so provisior~and store room5 having no a c c e s s ~ t oacconmoZation spaces, sprvice s p a c i s and.contro1 stations. The navigating bridge is also exempt so long 3s rapid gas and vapour. tightening o f doors and windows can bc o b t a i x d Tk,e fire i n t e-q r ~. t yslandard of boundary bulkheads separating- adlacen: . spaces within the accommodation range from A-60 to C &ss in a similar manncr ro dry cargo ships. The protection wi
A s Chief Eneineer of a vessel, discuss procedures you w o n l d -
initiate to ensure s a f e operations in T a n k i r s . If [he inert gas plant breaks d o w ~d&ng discharge and air enters the tank, no dippins, ullaging, sanpling or other equipment should be introduced into the tank for 50 minutes afttr the cessation of the injection of inert gas. After 30 minutes, equipmen; :in be introduced provided all metallic components are securely earthed. During the re-ineriing following a breakdown o f the inert gas sysrem, no dipping, ullaging, sampiing or other equipment should be inserted until it has bpen established that the tank is inert. This shouid be established by monitoring the efflux gas From the tank being inerted, w h e n i t is known the efflux gas is truly representative o f the gas condition in the tank. However, i f i t is necessaryto introduce 3 gas sampling system into rhe tank for this purpase, there s h o u l d b e a delay o f 3 0 minutes following the cessation of inert g a s injection before inser!ion o f the sampling system. Metallic components of rhe sampiin: sy&m i h o i ~ l dbe securely earthed. During the initial inerring of the non-gas free tank, the same precautions should be taken as when re-inerting after breakdown and repair of the inert gas system. .
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The earthing o f the introduced metallic conducting equipment a s recommended above should he maintained until 5 hours have elapsed from the cessation o f the inert gas injection.
YQ.~s What
is a n Internntionaf s h o r e coupling ? Briefly describe its purpose, use a n d components t h a t go with it. sketch the coupling f bolts a n d nuts. a n d s t a t e t h e materials - ~ couplings,
I n t e r n a t i o n a l s h o r e coupiing The purpose of the International shore cocnection is, to be zbie no connect the shore water supply to the ship's Fire line, or to inteiconnect t w o ship's Fire I ~ n e s for , the purpose o f fighting the fire.
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TIIF: International shore connection is shown, which is required to be
c i w i r d in the ship and has the~followingspecification - . Outside diameter : 178 mm. Inner- diameter 64 mm. h i t circle diameter : 1 ;2 mm. Holes 4 holes of 19 mm diameter equidistantly place slotted to the Zange periphery. Flange thickness 14.5 mm with eight washers. Flange surface flat face Four 16 mm bolts, 50 mm in len th p, - : Gasket any suited to I N per mm servlce When fixed on the vessel, the connection should be accessible from either side of the vessel and be plainly marked.
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Auxiliary Machinery ~~... . ,. . P u m p s and pumping systems. . -. Heat exchangers, drinking water systems ~, Deck machinery ,. .~ . . :. ~ .. . Boilers, boitcrlwater treatment ~
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With reference to Centrifugal pumps, juptify & discuss the following :a) Impeiler can 'convert shaft input to fluid kinetic energy'. b) Volute is responsible for 'conversion of kinetic energy to head' Pumps are not se:f priming. c) d) Clearances behveen impeller and ear rings are critical. -41~0, state the materials used. Ans. a) Fiuid enters the eye of.the impellei, and changes direction as it flows into the impeller. The fluid is given kinetic energy by the rapidly spinning impeiier, so that it exits the impeller at a high velocity. The impel!er, itself, turns due to the i:lput poker tc the shafifrom a prime mover, such as an electric mctoi. Thus, we can say that the Impeller caii 'convert shaft work input to fluid kinetic . energy'. ~
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b) The,fluid exiting the iinpeller has a high kinetic enerzy when it enters the volute casing. The 'volute' casing is bisically a duct having a smoo?hIy incrraing cross-sectional arca. Thus, we can say That Volute is responsible for the 'conversion or' kinetic energy to head', as the liquid ieaving (he volute casing has decreased in kinetic energy (or velocity), while increasing its potential energy (or presscre head).
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-c) Since the suction effect or 'lift' of the pump depends on flow of liquid though the-pump,.abm-ce of liquid ji.e. presence of air) will carise the pump to 'lose suction': Thus, centrifugal pumps are r,ot 'self priming', and need some externa-assistance, either in the foim of shaft drivm priming p m p s , or 3 -.connectionto a central priming system, in order to ensure that the pumps do not lose suction, while running. .
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d) The conversion of energy, from kinetic to potential (or pressure head) rakes place between the suction side and the discharge side. Consequently, if sealing is not proper, there will be a tendency for high pressure liquid to flow back to . the low pressure side, through any large clearances, such as the clearance . -:ljetweenthe impeller and the wear rings. The impeller rotates, while the wear rings provide a stationary sealing arrangement. If the clearance should be excessive, not only will there be a drop in pressure, but also a drop in esciency, besides creating other problems like excessive vibration, due to increased 'play' between the shaft and the wear rings. Materials : The cayin; is usually of gun-metal, the impeller of aluminium bronze, the shaft of stainless steei so as to prevent corrosion.
Q.2
in case of rs pumping system : a) Explain, with a simple sketch, the working of a %quid ring' priming ptimp, b3 Briefly describe e Central Priming System and state its advantages. Ans.
Liqtiid ring priming pump ad-
Ziquid ring pump
iri~pellzris concenmc with the drive shaFt and the impeller is fitted into an 'eccennric'pump casing. Due to the action of centrihgal force, 'here is a rotitling 'cing' of water, which must follow the shape o f the casing. XIUS,due !n the eccentric shape, there is alternate increasing and &x
C e n t r a l p r i m i n g system This is a central system for air handling, for a group of centrifugal pumps, and is consideied to be more economical and effkient, than to have individual shaft-driven priming pumps for each centiifugal pump. The system usually h ? two ~ priming pumps, o n e in use and the other stand-by. This pump is usually of the liquid-ring type and operates automatically between a cut-in pressure of 0.5 bar (absolute) and a cut-out pressure of 0.3 bar(abso1ute). The vacuum tank has a valve, which is float operated. When the air is exhausted (i.e. when the float rises, due to a rise in the- wzler Icvel), the connection betwsen the vacumn tank in: the pump suction is shut off, thus preventing the pump from drawing water (from the systza). Compared with individual priming units, there are certain advantages, a s well as disadvantages.
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Advantages (of the centld priming system) ji) Lower first cost (sharing of priming arrangement between a I x g e number sf pun7ps). ( i i ) Saving in poww, since only one priming pcmp is doing the J o b for all ceniiifugal pumps. (iii) Less maintenance, as only one priming pump is running Dis~dvantages No priming avzilable to ali centrifugal pumps in Engine room, if both pumps (i) fail. (Rarely occurs, if proper routing maintenance is done). (ii) Since one central !a&. is c o ~ e c t e dto many pumps in the Engine room, there are iong lengths of pipe1ines-fhn the vacuum tank to the individual pumps. Small pipe linei may be easily broken or blocked, thus rendering the priming effective. (iii) Failure of the float valve WGUMallow water to flood the central priming system. (2.3
z
l
a)
minimum'^.^.
$+
5 & E ! , k z: 2~
With reference to Gear pumps, briefly comment on the followiog : Tooth depth is to be increased to the m~xiznumpossible. a) b) Leakage in gear pumps. . Typical clearances. c) d) Materials of construction. Ans. To ensure maximum displacement (output per revolution) tooth depth is increased to the maximum practicable and the number of teeth reduced to the some high-throughout pumps have only eight or nine teeth. There are two main operating problems with these pumps, one of which is leakage, that can take place through t h e e main paths r(i) At the meshing point of the teeth. (ii) Between the tips and the cylinder walls. (iii)Betwecn the sides of the gears and the wear plates.
b)
63
The other operating problem is, that a gear pump is not hydrostaticaliy baiasced, as there are distinct suction and discharge pressure zones. The trilbalance'd forces, due to this, must be taken by the bearings. Additionally there is a further load on the bearings, due to the progressively increasing pressure imposed on the fluid trapped in the teeth, as &ey come into nxsh =d the space behveen them decreases, up to ihe dead cenhx or mesh point. c)
Typical Clearances & 190 mm For a distance between wheel centre lines of 60 Clearance on diameter .............................. 0.13 1.m- & 0.5 mm Axial Clearance.. .................................... -0.13 mm & 0.4 mrq
d)
Typical materials (For gear type pumps) Casings and end covers - cast iron, stzel, bronze or gunmetal. Gears - Hardened Steel or Stainless Steel.
Q.4
. ;\ !
Vliik! reference to Bilge pumping systems, briefly discuss : Rules for vessels (relating to zbove).
h) r)
ii)
What could be reasons for 'ine~fective'Biigepumping ? What are Mud boxes and ?'here are they fitted ? What is The importaace o i an ' ~ p p ~ o v eBilge d alarm system'? How is the iso!ation of Fuel acd Ballas: systzms done?
e) Ans. Ruies relating to Bilge pumping systems, for vcssels over 90 m in lxigti! :.1.
2.
A f~iplngsystem and pumping plant should he provided, to p x n p o u i watw and to drain any adjacent damaged watertight compartments (inclxling between decks) under all reasonable damage conditions. Ei:Ticient drainage should be provided, especially to unusual form o f compartments and the piping system design should not allow flooding imrier (Lamage conditions.
V e s s e l shall have at least four independently powered pumps connected to the main bilge line. Ba!last, sanitary are acceptable, also
en::ine driven pumps, provided they are of sufficient capacity and are conn.scted to the main line. 3.
One such pump should be of the remote controlled submersible type o; t1;e power pumps and controls should be so placed, that at least one pump is always available, under. all reasonable damage conditions. ,yncii pump should, where possible, be located in a separate watertight como;~rti~~enc.
4.
All pumps which are essential for bilge services are to be of the selfpriming type, unless an approved central priming system is p r o m ,~~ Ethesepumps. Cooling water pumps having bilge ejection connections need not be of self priming type.
5
Each pump should have a direct suction to the space in which i t is situated, such suction to be at least the same bore as the bilge main. Not more than two such suctiocs arc required and in the machinery space, such suctions should be arrrnged a n each side.
6.
Main engine circulsting pumps shall have a direct suction (with non return valves), draining the lowest level in the mzchinery space, such ~ the diameter of the main sea inlet. suction pipe bcing at least 2 1 3 ~of Eirect suctions on other suitable pumps o f equivalent capacity is accepted.
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,3/
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Bilge valves should be of the ndn-return type.
-<
Emergency bilge pumping systems, where provided, should be separate from the main bilge system.
d .L/
The bilge piping sysrem is ro be separate f r o n cargo and oil fuel systems. Spindles to all master valves, bilge injection, .shou!d be led above the engine rcom pfz-tform. Ail valves, extended spindles, to be clearly marked and accessible at all times.
Bilge pipes are to be provided with Mud boxes. Ends of suction pipes should be enclosed in pasily removable strum boxes, the holes through ~and their combined which should be approximately 10 mm diameter area not Less !>an twice the area of the suction pipe. Sounding pipes, where provided, are to be as straight a s possible, easily acces;itle, normally provided with closing plugs. Also, all machinery space pipes to have self-closing cocks. Problems relating lo Bilge pumping systems
(a) Air in System :Perforated pipe Perforated jointing. Lack of fluid in suction well. Air drawn in, especially at the pump gland. Damaged or badly made joints at valves, mud boxes. Air drawn in at valve glands.
(b) Loss of suction
- air
in the system
- foreign material in mud boxes. (c) No discharge (low or zero suction pressure)
- as
above.
- suction valvz shut or choked. or too many suction valves cpen.
(d) High discharge pressure.
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(e) Pump defect
-
3ischarge valve shut.
Loss of efficiency due to increased clearances, damage to parts or insufficient speed.
M u d boxes Each bilge suction pipe terminates in a 'mud-box', which is basically a device to remove large particles, waste or rags, from enietirig the pump. The ~ u box d has a 'zoarse' strainer, so that it will remove only larger particles. Too fine a strainer w ~ u l dresult in frequent chockage and loss of suction. Thus, Mud boxes are fitred to prcvent dcbriz (which collects ip the bilges) from passizg along the suc!io;i pipe to the p u z p and causing danage to ihe pump impeller, pistorx and valves.
'iigb Bilge level Alarm system l n case of Unmanned Machinery Spaces (UMS), where it is intciitied that the engine and/or boi!er rooms will not be continiicus!y manned at sea, an approved 'bilge high level' alarm systrm is tc be provided to give timely warning of flooding. T t ~ za!arm system is to operate audible and visib!e signals at the Engic~econtrol room, i.e. the station from which the machinery is controlled, and which shotild be in direct communication with the Navigation bridge. When the vessel i s under BrIdge control, the alarms mtrst opera?e on the Navigation bridge, the Engine control room (ECR) slation, in ali pub:ic spaces and in the Duty Engineer I Chief Engineer O f f i c d s accommodation. l'hc system gives warning that fluid level in the engine room bilgi::; has reached a pre-determined level. This level is to be sn?iicir:ntly low, to prevent liquid from overflowing from the bilge pits, onto the tank top. In ships above 2000 gross tons two independenr systcms of level detection are provided. Automatic starting of bilge pumps may be acceptable provided the following conditions are met :-
Advonceif
Mnrim Enginecring Knowledge Yo/I / /
1 . In case of heavy leakage of liquids, scch as from a burst water main or any
fuel I lube oil pipe, or even a hole in the ship's side, the heavy ingress into the bilges may detected. If the bilge pump starts auto~natically,and nms for too long a time, since it cannot cope up with the heavy ingress of liquid, a bilge pump '!ong run' alarm is activated, which will alert the Duty watch-keeper, who will then take necessary action. 2. To take of the problem of pollution of the sea. the automatic bilse pump is rot dircctiy ccmected over-board, bu: i-mtead, wiil pump the contaminated bilge water into the Bilge ho1dir.g tank. Isolaticn of Fuel and Ballast systems It is imperative that the Fuel oil system and the Ballast system are not only isolatd from each other, but segregated in such a way, that error in zpe~atingvalves will not permit the fuei fromfuel tanks to be accidentally discharged over-board, along with the ballast water. Also there must be no oppcflunity for inadveflent transfer of salt w a ~ e r into the foe1 sysiem, causing contaminztien of servicz fuel tanks with di:strous consequences &d a possible s k t down of main and asxiliaq engines, which could seriously jeopardize tbt safety of the vessel. The 3 i l / Water Ballast Chest has blanks fitted, which wi!l prevent any such inter-connection, and modem vessels have seg-gated ballast tanks, which are only used for sea water, ti~ush a v i ~ gseparate $pz-lines and pumps, having no inter-connecticn..vi:b thz cargo system.
Q.5 -
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Explain the salient features of a Central cooling system and discuss its advantages and disadvantages over a conventional cooling system. Ans. Central cooling system The Central cooling system uses a 'closed circuit' of fiesh 'chemical!y treated' water for !he coolant side of dl the primary hcat exchangers. This fresh water is itself cooled by Ceniml coolers, which are sea water cooled. The salt water is thus limited to one set of pumps, coolers, valves and filters, which considerably reduces the salt water corrosion problem, which is inherent in marin? systems, zs well as appreciably reduces maintenance costs and down-time d m to difficuities posed by repair / replacement of corroded pipes of conventionr?lsea water cooling systems. Since sea wa:er is limited to a small section of pipes and coolers, special materials may be used to limit corrosio~ problem there at a comparatively reduced cost, (as compared to the cost of doing this in a conventional system). Thus the few salt water pipes may be protected by rubber-lining, and the cooler plates made of special corrosion resistant metals like Titanium.
Ccnlnl C.W.
Centrnl Cooling System
Due to the differing temperature require men:^ of the main systems there would be various cooling water circuits :Sall wa:er for
Tbe S. W pumps take suction 'om both sides of the Engine room, through sea chests (High and low) and filters.. After passing through the Central coolers, the sea water is discharged directly overboard..lvlaterials for i his part of the system c a l be of high quality, as the actual S. W. system is small. Fresh 'rreateci' water for the circuit is circulated by its own Central coding pumps, with temperature control by means of a three way valve awangernent, which means that it can either pass through the Cooler or is bypassed. In steam ships, the main condenser is having an independent sea water circuil, while the central cooling system provides the coolant for all the other coolers in the Engine room.
Advantages (1) Less corrosion and hence less maintenance, with increased reliability as systems are not subjected to unscheduled maintznance, due to leaking tubes, or chockage, silting or marine growth on cooling surfaces. (2)Reduced initial cost of equipment ( as compared to using the same naterials for a conventional system) as the quantity of items having sea water is less. ( 3 ) Constant tzmperatur: of ioo!ant to various systems, since no change in the coolant tempera:urz, as would be the casewith sea water, whose temperature varies with the geographical location of the vessel. This gives a better and easier control and thus impoves the operating conditions fcr the running machinery. Disadvantages (1) Two separate heat exchmger systems means a greater overall temperature
difference, which reduces the thermal efficiency and thus increases the operating costs. ( 2 ) Greater initial first cost, as the design cost would add to tile ccst cf extra cquipmsnt for the Cmtral ccolers, purnps, valves and associated piping. (3) Plant is more complex and vulnerable to problems, such as break-do-rn of ihe Central cooling circuit, which would hanper operation of all other services.
0.6
a) -
With reference to pumping systems, discuss : a) Working principle of Cen:riftig-1 puings. b) Limited applications of Axial flow purnps. c) ~ h a ; a c t e r i f t i e s of a S c r e w purnpz. Ans. Centrifugal purups : These purnps are used for the maximum applications on board ship. They can be single stage (ore impeller) or multi-stage (with two or 'more impellers, in series, on a single shaft) end singie entry (from one side only) or double entry (which givesbetter balance). The pump's characteristic is decided at the design stage, whea the energy exchange from shaft work to fluid kinetic energy is fixed by pump speed and impeller diameter. I t is n3t good practice to have too large a head per stage. This would require either unnecessarily high speed or excessively large impeller diameters. Both these. factors result in high fluid velocities, which is also undesirabk
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Centrifugal pumps are not self-priming. When used on duties that requires a suction lift, tkey are fitred with external priming arrangemtmis, such as shaft driven priming pumps, or have connections to a Central priming system. When the suction head is low, or when the fluid temperature is near its saturation value, these pumps can suffer from severe erosion resulting from cavitation.
Advorrced Marine Engineering Knowledge
Vd. I11
Nnle : Some pumps are designed to mn with cavitation, as a means of level coiCrol, but they are made of very special materials. Erosion c m also result from operating away from designed speeds (shock), or with some duties, fiom the ef'fict of debris entrained with the fluid. P ~ m p scasinzs are often = r a s e d with the Prime mover (usually an electric motor) niounred vertically above, giving a small compact arrangement xnd covering very little floor space, which is important, when considering rhe number of pumps in a corifined space like the engine room. Fluid entry into the casing is also easier with this type, resulting in less friction loss in the suction head. There is a variety of vertical arrangements. In a! of them there is a need -?or a thrust m g e m s n t to support the mass of the rotating elements. The ikrmt may also hzve to cope with any hydraulic unbalance at the impeller. Eithlr siilgle or double entry impellers can used, although the double entry impeiizr has a reduced N.P.S.H. or Nett positive suction head, than the equivdent single entry type. The vayes c w be backward curving, which is the rmst common, s m i @ or forward curving. The choice is determined by the d ~ t yrequired. Forward ciurving vmes give a power curve that goes !bough a maximum, then reduces wi:h incl-eased throughput: The backward cuwifig vane has a power requirement increasing with thoughput.
b)
Axial flow pumps Axial flow pumps tiave the disadvantage that they develop a very low hex1 GO m) per stage. This is offset by their ability to give this small head increase ro a very large mass flow of fluid. They have a characteristic e~eiciencycun.e that is very steep. Hence they are economical if operated at a rate c!ose to the optimum design range. Also, the Hzad-Flow characteristic shows a region of instability at low flow rates and low flow operation is Gangerons. Serious vibrztion occurs in litis u:atable region and operztion in this range is not recommended. These plumps have a vety limited suction capacity, hence they are prone to cavitation. All of tiiese factors considerably limit their appiication. They are used for high volxne flow rates, requiring low heads. e.g. Circi~latingduties. In this respect they arc useful as, in a scoop system they cat! be left idling when not required, a s they offer very finle resistance to flow. .l'iri:y will also p m p in either direction, so can be used for heeling and trirn:ning applications.
c)
Screw punps S c r w pumps have a good low speed efficiency, which together with the low v.l,I ii.,b- . of induced fluid velocities, make them ideal for handling viscous fluids. They aperate with very tow noise levels. They are small and compact. They is no ~upswcptvolume in their operation hence, there is no problem with rc-cqiarrsion, which would reduce volumetric efficiency. P4ivery is free from p i s :A t'Lon.
Alluoneerl Marine Engirze~rirtgKnowledge
Q.7
a)
b)
With reference to carriage and pumping of liquefied cargo : a) Explain a saitable pumping system. (i) Why are submerged hydraulically-driven pumps not used for b) liquefied gas cargo? (ii) How is overheating of pump shaft hearing avoided? State how risk of fire azd explosion in cargo tanks is obviated, both C) in loaded and discharged condition. Am. Pumping system (liquefied gas cargo) Liquefied gas cargo is usually at very low tempea:urss. This means that hydraulic inems cannot be uuiised for the prime mover, sinc? the oil would be below its pour point. Instead, the pump is comected to a long shafi, which is made of a suitable material which is not affected by the cargo, e.g. stainless steel. There are suitable bearings, e.g. made of carSon, which w 9 u l ~ reduce the friction. The low temperature of the cargo is utilised for keeping the bearings cool, thus the shaft is usually located within the discharge pipe itself. If the flow of car20 is insufficient, this could lead to over-heating of &e hearings, which are ~rotectedby thermal cut-outs. The long shaft emerges from the cargo tank, and car. be driven by a prime mover, such as b flame proof electric motor iocated on the outside, at the deck level. Some amoun: of cargo is usually lei7 behind, which senres to keep the l a d cool.
(i)
(ii)
)
Vol. I/I
Pumps are usual!y electric and not hydraulic,Bs there is some difficulty in obtaining a hydraulic fluid, that can p d o r m a! ihc very low cargo temperatures.Als6 the use of hydraulic submerged pumps requires motor p$es io be triple cased, to avoid possibility of leaks contaminating the cargo. Overheating of pump shaft bearings is avoided, as the liquefird gas cargo passing over the bearing cools it. There are also thermal cut-outs to prevent the bearing from damage.
The risk of fire and explosion is always present, both in loaded and discharged condition. This needs to be reduced by : Cables being of special metal insulation. (i) (ii) External junction boxes being exp!osion proof .and packed to prevent gas transit along the conduit As liquid cargoes are near the Boiling point, the pumps will probably be (iii) fitted with Inducers. Since the cargo is under pressure, there is a mix of liquid and gas, with no possibility of air ingress. Thus the fire hazard is minimised, and there is no need of using inert gas, which could possibly spoil the cargo. With reference to centrifugal pumps : a) Sketch Typicai Discharge eharactcristics, showing variation of throughput as diseharge head and speeds are altered.
b)
.c)
Explain why this characteristic is desirable. .. Statc the releyaoce of dischwge characteristic, for selection of an emergency fire pump.
Ans. From a mathematical consideration of lhe action of a centrifuga! pump it can be shown that the :heoretical relatioxhip between head, H and throughput, Q is a straight line, wit! minimum th~oughoutoccuming w ~ e nthe head is muimum. Duz to shock and eddy losses caused by impeller blade thickness mn id orher mecha$ca! considerations, there will be some head loss. This loss increasing slightly with th~oughout.
Th-u+put,
Q
-
'Chese losses; together with ffiction ‘asses due to fluid contact with the pilmp casing and inlet and impact losses, result in zhe H/Q curve shown in the . ligure. The final shape of this c w e wili vary accordiig to the design of che pwn:), and depend on the WQ curves or, if required, the curve can be steep to give a relatively large shut-off head. From the Figure, it can be seen that the minimum power occurs when there is rro ilovi and when the discharge head is at its highest - in other words, when lilt: discharge valve is closed. Since throughput decreases E -the discharge head is increased, there is no P-ecessity to fit a relief v d v e to centrifugal pumps. It will also be noticed that the efficiency curve for the pump is convex, which means that maximum efficiency occurs at a point~somewherebetween rrlaxirr~umand minimum discharge head and throughput conditions. I'hc cinirgency fire pumps is to be positioned, so that it will be able to operate at the iightest draught to be encountered (also allowing for fall in pump perfomance). A means of air extraction, for priming purposes, must be provided, .,hen piimp is morc than 2 meters above this lightest draught, unless the pump i s of the positive displacement type (even wi& positive displacement pumps, suction- is Iimit~dto 4 5 m) the pump&& system should provide 2 jets of water, with a throw of 12.5 m, through 1215 mdiameter nozzles.
Q.9 Briefly describe the working of a Smash Plate pump. Explain with ;I block diagram, a Crane circuit using a variable-stroke pump. How is Braking carried out ? Discuss the importance of Override controls. Ans. The arrangement of the cylinders and pistons with their axes parallel to the shaft makes for a very compact design w?& small outside dimensions. The small radius of the rotating parts allows higher rot~tionalspeeds than the equiva!ent radial piston pump. The usual arrangement, in the "Swash Plate" pump, is to have either a rotating swash plate and a stationary cylinder block, or a rotating cylinder b!ock and a stationary swash p!ate. The latter is the mosr commor?, being more adaptable to aive an accurate i o n u d of thz flow. A typical example has the piston rod with a small ho!e :bough, 10 provide lubrication of the universa! joint (knuckle) in the plate. n e pistons are hollow to reduce inertia. Many models have pressure limitin5 amngements to reduce erTecrive piston stroke if peak pressures are attained. Note : Both Radial and Axial piston pumps al-e Gnly suitzble for fluids rha! have Lubricatiag properties.
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C r a n e circuit Thc block. diagram illustrates the use of a variable stroke pump and a fixed smke hydrdulic- motor arrangemerit. The'completesy$em has three such . . circuits, one each for the hoisting, luffing and slewing operations., . .. . . .. ~.
." i h e rzquirernent is to hold a !oad securely end allow a smooth changeow from the static position to mcvemect and still be 'fail-erc'. I.iomally, rill: brake is spring loaded, defaulting to the 'On' position, a hydmulic cylinder being used to rotate it. The signal to operate the brake comes fri>rc~ a micro-switch on the 'Swash Plate' control lever, openkg the salenoid. Override controls :These are needed to prevent damage or operation in unsafe areas e.g. 'sle~w'anglr, maximum and minimum 'luff'' angle and 'sf&ck r ~ p e protection. ' System presslire will 3epend on the position of the "Swash Plate" and the size oL ii~eload. A!! t i ) Tensioning Mooring Winch :Moor-ir~gwinches provide the facility for tensioning upto 15 times of the fiill load. Once i t is reached, load is held by the motor brake or by a barrel bmki:. i n a coiiventional Mooring winch, the winch cannot 'pay out' more when .m.: power is off, unless brake is off or unless it is manually operated.
Whi:rl the Winch has an 'auto tension' arrangement, there are features adrliriorral lo the manual controls, which allow the wire rope to be 'paid off so as t o restorc !he rope to a pre-determined 'tension'. Should the rope become
.slack', the winch would haul in the rope, to reach this pre-deternlined value of tension again. Load sensing devices arc used with auto mooring winches. E.g. fluid press sensing. Q.10 What are the various systems for Cargo Stripping systems, approved under
MARPOL ? Ans. There are three systems approved under MARPOL. Pressurising the tank and stripping from the bottom of the tank well, by a. means of a smail diameter pipe. This process is very s!ow and requires a large quantity of Nitrogen (or inen gas), which is used to pressurise the tank. Also, a caiehl set&g cf tank relief valves is required. b. Using a s:ripping box in the tank, the box being connected to the strippi~lgline. Thir, box is alternately put under vacuum and pressure, in such a way that it fills with up with the liqvid (cargo) and thus tlie cargo is displaced up the stripping line. Back flaw is prevented by non-return valves. This system is simple and does pot rely oil rotating machines. However the non-return valves can give rruuble if ?he cargo can crjstallise. Submerxib;e or deep-well~pmpsare used driven by a prime move:- such c. zs a hydraulic motor, which is located izside the tank. The hydraulic rnotar 1s driven by hydraulic pressure produced by hydraulic pumps located elsewhere. The suction of the pump is normally towards the tank bottom, to aid in the easy removal of cargo. The discharge line, however, would remain full of cargo and couId leak back into the tank, if the cargo pump were to be stopped. To take care of this, the centrifugal cargo pump is left running and the deck discharge valve is shut. Now purging connections are opened, a i d purging is carried out, e.g. by means of inert gas or compressed air, so that the cargo rcrnai;li&g inside the outlet line is blown out through a riser tube, directly into the deck manifold, after the deck discharge valve. After this has been done, the pump may be stopped, without any danger of leak of cargo back into the suction side. There is usuaily an empty space or cofferdam provided between the :lydraulic lines to the prime mover, and the contents of the cargo tank. This is to reduce the possibility 2f any mixing of the t w z The cofferdam is also ?ressurised before stopping the pump to check for any leakage. Q.11.
Identify with reasons, factors which contribute to : a) Failure of multi-tubular heat exchangers b) High water velocity in the tubes of heat exchangers. c) Steep temperature gradient across tube walls. Ans. Problem Areas in Heat Exchangers Impingement : Near mlet ends of tubes - oxide films, which norn~ally I. minimise corrosion, are broken down by the unnecessarily high water velocity.
The tubes are under continuous attack, thus thetubes will continue to corrode n t i . Partial blockage. can cause local increase in water velocity and twbulence, further aggravating the situation. 2. Acidic water can cause general wastage of the tubes, since this attacks . i h e protective oxide film. The tube is under continuous attack causing thinning ' a i d cventual perforation. 3. Anaerobic bacteria :- In polluted waters, Bacteria give off sulphurated hydrogen, which snacks rdbe material. . .. 4. Deposits :- of foreigi matter, on the metal surface, can creare con-osion problems, if the metal under the deposit~becomzsanodic, to tke rest of the surface m d electrolytic ection accurs. . 5. Erosion :- abrasive solids and high water velocitits can cause erosive ,Mdstage. ,.
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Corrosion Corrosion by :=a-water may occasionally cause perforation of heat transfir surfaces. 'fliis will cause ieakqe of one fluid iiito the other but this is not always easy to iktect i f ihe leakage is small, although substantial leaks may become evident thro:igh iapid loss of lubricating oi!; jacket vieter an< so on. Location oFa perforation is a streightforwai2 matter in the case of a tubular heat exchanger, whether this is of the shell-and-tube type or of other tubular constriic!ion. Having drained the heat exchanger oisea-water snd removed ;he covers or headers to exposethe tube ends, some flow of the liqaid on the other side of the surface will be apparent, in the case of clil and water coolers, from m y tabes which zre perforated. 0 Th tesi i'or leaks i n air coolers or drains coolers, c q out the following: each . rube in turn can be plugged at one end x.d pressuriszd with air; inability to hold - -prewge indicates a leak. . . l'o.aid rllc detection of leaks in a large cooler, in which it is difficult to get the . rirhrs dry enough to witness any seepage, it is usual to add a special fluorescent dye to tile shell side of the cooler. When an ultra-violet light is shone on to the tt.~besand tube plates, any seepage is seen, since the dye glows with a vivid green li.ghi. * Ln plate heat exchangers, the only way to locate leaks is by visual inspection of tire $ale surfaces. 0 On docking for any protracted period, such as for dry-dock, refitting or lay-up: i! is advisahie to drain the sea-water side of heat exchangers, clean and flush ti~rw~gii wit11 fresh water, after which the heat exchanger should be left drained, ifpossikble, until the ship re-enters service.
u
V e r ~ t i r and l ~ Draining I t is importantthat, in any heat exchanger, the coolant should run full. k t vi:rtically-mounted, single-pass heat exchangers o f the shell-and-tube or plate types, venting will be automatic, if the sea-water flow is upwards.
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This is also the case with heat exchangers mounted in the horizontal condition. with single or mclti-pass tube errangements, provided that the sea-water inlet tranch faces downwards and the omlet branch upwards. . With these arrangements, the water will drain substantially completely out of the heat exchanger, when the remainder of the system is drained. i With other arrangements, a vent cock fined at the highest point in the heat exchanger should be opened, when first introducing seawater into the heat exchanger and thereafter periodically to ensure full running. A drain plug a1 the lowest point should be provided. M a j w cooling defects :- (Oil coolers) Rising oil temperature causes scale formation and deposits on the insides of tubes. 31ushes are needed to clean or solvents can be circulatedto remove this. o Cil loss inro coolant :- pei-forated tube can cause appreciable leakage - A Iemporxy solution is to seal-off the leaking lubes (with plugs). Hoxvever, afrer a certain number of tubes are blsnked, the efficiency of the Heat exchanger i\ii)l be.drastically affected, hence Wbt: renewal is Pecessary. Cooler designs usually cater for IG%.!oss cf tubes. Replace tubes as soon as possible -This may require the ariliing-out of tube ends and fitting of new olies by suirable expansion tcols.
3
Maintenance To preyent gross wastage due to galvanic a c t i ~ nof the cast iron, o i s:eei, and do-zinciiication of aluminium brass rubes where fit!ed, zinc or ~ n i l dsteel sacrificial anodes are fitted to the tube plates. Altenatively impressed current cathodic protection may be used. r The simplcst method of degreasing the steam side of tubes is as f o l l o ~ s: A vessel containing tri-chloro-ethylene is secured to a bottsin manhole and is wax-med eently. The trichloroethylene vaporises, rises among the rubes, condenses and falls into the vessel, bringing with it the grease and oil from them. This agent is tcxic if inhaled and precautions must be taken. Tube failure is a rare occurrence nowadays: it may occur froin corrosioil/stress cracking or de-zincification of brass tubes, or by corrosion/erosion arising from entrained air in or excessive speed of, circulating water. . When it occurs the defective kib: may be fitted with a wooden plug or a capped ferrule until i t can be renewed conveniently. Tube bores are cleaned by-brushing out, by ase of compressed air. b) There should be a certain minimum velocity of coolant, so that there is no -siltage and the heat exchanger is always runningfull of sea water, ai all times. This is especially important if the location of the heat exchanger is such, that i t is above the water line, when sea water is used as the coolant. Regulation of coolant flow is usually done at the outlet valve and not the inlct valve, thus the fomation of air pockets, which may accelerate corrosion. c ) The tubes are passed through alternate baffles that suppoa the tubes and also dil-ect the fluid flow, so that all the iubc surfaces are swept clean. Any scale, deposits or other insu!ating material forms a film which contributes towards a
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higher reniperature gradient across the tube wall. Thus, the steep te~nperature gradicnr leads to stresses, which contribute towards eventual tube failure.
Q.12 Sketch a longitudinai cross section of a tubular heat exchangerindicating diieriions of flow Briefly describe what rnsterisls you wonid use in the consrruction.of tubular heat exchaagers, giving reasom for your choice. Explain methods used for tube protection in heat exchangers. A m Sheil and tube :ype heat excknnger :
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SHELL AND TUBE TYPE
HEAT EXCHANGER
Wlslerials : Shell - generally of cas! iron or fabricated steel. Tubes - These are of aluminiam-brass (76% Copper, 22% Zinc and 2% A!urniniumj, or cupro-nickel or even stainless steel may be used. The tubes are often ex!~anded in to the tube plate but can be soldered, brazed or welded. The nurnber of tribes always has a fouling allowance. After final assembly, the tube stack is machined to Et in the shell bore (the shel! is also machined) to a.llow easy wididrawal. 7i'iihe-Plntes- The tube piate material should suit the tube material and the method of fixing. Naval brass tube plates are use6 with alnminiuni-brass tubes. Usually assembled, so that the water boxes can be removed, without disturbin: r k rube stack. Wtliei- DOXCS- Cast iron or fabricated steel, always designed to keep riii-bi.tIetice and coated for corrosion protecricn. T u b e P ~ o t e e t i o n: Thtxe is n protective film of iron ions, formed along the rube length, by corl-osion of iron in the system. Unprotected iron in water boxes and in parts of the pipe system, while itself corroding, docs assist in prolonging tube life. In non-ferrous systems, the supply of iron ions is from other sources. Thus, soft ii- or^ sacriticiai anodes have been fitted in water boxes, iron sections have been inszl-tecl ill pipe systems and iron has been introduced into the sea water, in the !ititit of fi:r~oussulphate. The latter treatment consists of dosing the sea water !o a :;ti-ei12th of 1 ppin for an hour, per day for a few weeks and snbsequenr':dosin:: again before enrering and after l e a v i n ~port for a short period.
Electrical contiruity in the sea-water circuiating pipe-work is imponant where sacrificial anodes are installed. Metal connectors are fitted across flanges and Cooler sections (where there are rubber joints and '0' rings, which would otherwise insulale the various parts of the systern). Premature tube failure can be the result of pollution in coastal waters or extreme turbulence due to excessive sea-water flow rates. To avoid the impin~einentattack, czre must be taken with the water velociry thro~ghtrbes. Fcr alu~niniu~n-brass, the upper limit is about 2.5 m/s. Although il is advisable to design to a lower velocity thm this - to allow for poor flow con?roI - it is equally bad practice to save sea-water speeds of less than 1 d s . A more than minimum flow is vital to produce moderate tur5ulei~ce which is essential to the beat exchange process and to rzduce silting and settiemen1 in the tubes. The tube stacks are made up to have one fixed tube plate at one en2 and the tube plate at the ~ t h e end, r which is free to move when the tubes expand or contract. The fixed end :ube plare is sardwiched between the she!! and water box. with jointing material. Synthetic :ubber 'O' rings fgr the s!iding tzbe plate p-rmi~ free expansion The practice of removing the tube stack and replacing it afier rotation radially thro~gh180 degrees, is facilitated by the lype of cooler described. This may prolong cooler life by reversing the flow so that-tube entran~es,which are prone to impingemen! dunage; become ~utlets. Cooler end covers and water boxes are commonly of cast iron or fabricated from mild steel. Unprotected cast iron in contact with sea water, suffers from graphitization. a form of corrosion in which the iror, is removed and only the soft black graphite remains. The shell is in contact with the liquid being cooled which may be oil, distilled <,I. fresh water with corrosion inhibiting chemicals. It may be of cast iron or fabricated from steel. Wl~err Manufacturers recommend 1nat coolers be arranged vertically. horizonral insrailation is necessary, the sea water should enter a1 the bottom aild leave at the top. Air in the cooler system will encourage corrosion and air locks will reduce the cooling area and cause overheating. Vent cocks shonld be fitted for purging air and cocks or a plug are required st the bottom, for draining. Clearance is required at the cooler fixed end for removal-of the mbe stack. Give reasons, whether the following statements relating to Plate type Heat exchangers, are true o r false. Plate corrugation patterns are designed to create turbulence. a) Scantlings of carrying bars and clamping bolts are designed to b) accommodate enlargement of pack. Titanium and stainless steel are used to reduced plate failure. State c) the other materials used. d) T ~ a p e r a t u r c and pressure of fluids handled a r e con~pletely unrestricted.
Advunced Marine En,oineering Kno>:I~dge VoL 111
Ans. a)
b)
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True Corrugations, created by 'troughs' pressed into. the plates, produce sufficiently high turbulence. The plate form can produce turbulent flow with Reynolds number as low as Ten. This type of flow produces a very low fouling rate. True The capacity of a heat exchanger is determined by the number of plates, ar?d this can be increased or decreased within limits, to accommodate different requirements. Thgs, the scantlings should, be able to take care of the maximum number of plates which could be used, with a sufficient nargin of safety.
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True Titaniun and stainless steel are extremely resistant to corrosion, (and also expmsive) and thus reduce the rate of plate failures. hkteriais used in Plate type heat exchangers : Plates Titanium - Stainless steel Frame Coated Mild stecl. loink Nitile nlbber Working pressure : 8 -- 15 5x Temperature : 90 - 110°C
d)
False Plate heat exchanger cannot deal with e&essively high pressures or temperaiures, due to limitations of plate gasket matelia: Also, they cannot deal with large vohme flows associated with law pressure vapours and gases.
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h.. . 3 4 i n a Fresh water generator, the brine concentration should be prevented from falling below a particular value, to minimise one o r a eombination of the foliowing :a) Seale formation on tube nests. b) Loss in czpacity and economy. C) Corrosion in evaporator. Skate the correct answer, supporting with adequatejustification. a)
Ans.
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To reduce scale formation in evaporators and to prevent the breakup of the dissolvedsolids, into harmful constituents, it is essential to operate evaporators at sub-atmospheric pressures. This process makes possible the use of low-grade heat (such as that from the main engine jacket water) in the process. Brine concentration should not be allowed to exceed :-
80
1.5 x 1132 1. I
x 1/52
for Single effecr submerged - coil evaporators for Flash type evaporators.
To maintain these valves, continuous brine extraction is necessary S O T brine temperature, any scale formed wouid be mainly
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CaC0;. ,dbove 80°C brine tempcrawre, m y scale formed would be mainly Mg(OH)2.
Treatment :- Removai by periodic acid clean. By w n t i n u o u injection of chemicals into Brine feed e.g.
2 to 5 p.p.m. Sodium Phosphate 2 lo 4 p.p.m. P3lyelectroiytes
b) True. Fall in Briue wncentration %ou!d affecl h e producrion of disiillatc, since suscien: capacity is only maintained by having a ninimum feel-rate (dictated by the size of the orifice ir, the Brine feed line).
c) False. Corrosion in the Evaporator is prevented by chemical treatment, as well as by a suitable coating on the shell. Thus, change in brine concentration wouid not haye any appreciable effect on the corrosion process.
Compare the advantages and disadvantages of Plate type heat exchangers to Shell and tube type. Ans. Plate type heat exchangers Vs Shell and tube type. The most obvious feature of plate type heat exchangers, is that the heat transfer surfaces are acces;ible and are easily reached for cleaning, during maintenance, unlike tubes, which are difficult to clear. Another major advanage over tube type coolers, is that their higher efficiency is reflected in a smaller size, for the same cooling capacity. They are made up from an assembly of identical metal pressings with horizontal comgations, each with a Nitrile mbber joint. Thus, from the production pointof-view, they are cheaper and easier to manufacture. The plates, which are supported beneath and located at the top by parallel met8.l bars, are held together against an end plate by clamping bolts. This gives a very compact arrangement, with adequate support, unlike tubes, which may not be pro@y supported over their entire length.
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Seals around the ports are so arranged that one fluid flows in alternate passages betwzen plates and the second fluid in the intervening passages, usually in oppositt: directions. This gives a much higher heat transfer rate, since borh fluids are passing in opposite directions, unlike the tube type, where flow through Lubes is at right angles to the flow through the shell. The plat- con%gations promote turbulence in the flow of both f l ~ i d sand so encourage eficienl heat transfer. The turbulence in plate type cannot be so created. Turbulence as dpposed to smooth flow causes more of the liquid passing behveen the plates to come into contact with them. It also bresks up the boundary layer of liquid ~ K c tends h to adhere to the metal and act as a heal ba&r when flow is slow. The corrugations make the plates stiff so permitting the use of thin material. They additionally increase plate aea. Both of these factors also contribute lo hear exchange efficiency. In case of tubes, the thickness cannot be redticed by any appreciable amount, without compromising the strength of the tube. Excess turbulence, which can result in erosion of the pla!e material, is avoided by using moderate flow rates. However, the surfazes o f plates which are r q o d do sea witer are liable to corrosion/erosion and suitable materials must 5s seiecied. Titmiurn s a t e s althotigh expensive, have the best resistance to corrosiorderosion. Stairless stzel has also been used and other materials such as aiumir~ium-brass. The laner may not be ideal fgr vessels wbich operate in apd our of poris with poliuted waters. The Nitrile rubber seais are bonded to the plates with a suitablt adhesive. Fouling on the sea-water side is the most usual cause of deterioration in pzrForniance. The method of cleaning the sea-water side surfaces depends on !he type of deposit and heat -:.-hanger. Soft deposits may be removed by bmshing. Chemical cleanins by imrr.er;ion or in siru. is recommended for st.ihborn deposits. In oil coolers or heaters, progressive fouling may t i i e piz? on Ihr oitiside of the tubes. Manufacturers may recommend achemical flushing to removt: rhis in sity without dismantling the heat exchanger. ::
Q,16 A cornpnny circular draws attention to the fact that bacteria harmful to imcnarts cwn exist in drinking and washing water. a) 1ie:Scr'ibc a system to improve potz3ility of fresh water. $1 S!ir% constrainrs piaced on installation and xse of systems for shipboard production of fresh water. c) Stale maintenance and treatment rccomrnended for fresh water tanks. Anr. Ciiernical treatment f o r potabie w a t e r Manually adding silver salts or solutions, allows for ~recis'edosing of srr~niiatnolints of water produced on board, while larger volume flows are controtled by a metering pump. . . rhs etectrolytic p y s s is the one currently employed by many ships, as it fbrms 1 GO-reatrnent, which is cheap and meets the requirements. 1o t ! ::li:chu:yi1c ~ process, positive siiver ions are released from a silver anode. ?hi* alrlourit of metal released is controlled by the current setting of rhe
electrical supply. This current value is influenced by the conductivity of thc water, which in the case of water produced on b a r d , is low. The operating costs of such a system are relatively low. One electrode has the capacity to treat 4000 m3 of water during its life spa% Such a system is the 'Electro-Katadyn' system. When fiow rates are small, all the water passes through :he .s-tli With larger volumes, only some o f the water passes through the unit, because the concentration of silver sdded is at a higher le-~ei. The required dosage for the full 30w is achieved by mixing the :wo flows. This aIlu;us a wide range of flow rates to be handled, without the significant pressure loss in case of c o ~ p l e t eflow through the system.
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Special propertiss of sil-qer are claimed to offer some advantages over thc chlorine method. Silver is Non-comxive no health hazard (less quantity of silver is take" ir., while drinking water 11 in a day, than when using silver cutiery) p.esidual silver in the water ?revens rz-irxection, and does nor have m y evaporation problem. Thek is no longer m y difierence in regdntions, relating to treatment or purification, between drinking water and wash water. Water for washing is considered the same as drinkicg water. Fresh water &ta:ned From ashore or water produced on board, by evaporatio~'distillationorrevers? osmosis, must be chlorinated or disinfected wi:h Silver ton treatment, before it passes to the Domestic water storage tanks. Ultra-violet sterilizin2 is no longcr considered sufficient as the sole means of sterilization. The free chlorine at outlets must be maintained at 0.2 p.p.n;. In order to maictain this level at outlets, it may well be necessay to keep the chlorine level in the storag- tanks at a much higher value. Sea water suctions to evaporators or reverse osmosis plants must be exclusivt!y for thar purpose and no chemicals (e.g. to Iimit growth of marine organisms) be used at such suctions. These suctions should only be used to produce domestic water when the vessel is more than 20 miles from land or (often wzll in excess o f 20 miles away). remote from estuarial Tins stippty water should be passed through sand filters before being used ir. evaporator or reverse osmosis piant.
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~a'intenance Fresh water sterage t d s to be opened up and manually cleaned at twelve month intervals. Cleaning process should include disinfectian with a 50 p.p.m. chloilne solution. Tanks should also be emptied and hosed out at six months intervals. People inspecting and working domestic water tanks should wear clean clothing and should not be suffering from any communicable disease or skin infection.
AdvuncedMorine Engineering Knowledge YO~. III
The cleaning of the various elements of the domestic water system (such as calorifiers. filters, pumps) must be carried out regularly and a special log of such maintenance be maintained. At every refit or dry-docking period, the complete storage delivery and .. distribution system, from machinery space to the M e s t outlets should be charged with "super-chlorinated" water at 50 p.p.m. free chlorine and left for 12 hours, to disinfect the system. Shower heads and Air Conditioning s p a y nozzles should be cleaned with 50 p.p.m. @orine so!ution, every Pkee months. . .
Q.17 State why fresh water ~ r o d u c e 6by using a low grade Sea$ source can be unfit for haman consumption. Explain bow system should be operated to avoid this problem. Explain how distillate must be treated lo render it Elf for drinking. Ans.
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Fresh water trends to Le acidic, due to its reddy absorption of carbon dioxide. It can tlherrfore be aamagins to the human digestive tract. F3r Chis reasoc, un-treated fresh water is not 6 t for humm consumption. Overall design, of the fresh hot and cold water distribiition systems, should be dcsigxed 19provide maximum circul~ionof the systems and to avoid dead-legs, especially where temperatures could rise lo levels, which mighi provide tke optimum wfiditions fer bacterial growth (viz. 15-C to S O T ) . This psssibility increases as the size of rhe system increases, wlieir sections of the system are no: kepi in continuous use. The freshwater tanks arrangement in every ship should enable tanks to be used in regular rotatim, in order to avoid the associated with stagnation. Calorifiers or pressure tanks should be designed where possible, to avoid. stagnant zones forming and should be fitted with efficient connections at the lowest poin: of the unit, to ensure that ail loose scale or sludge can be completely drained off, after cleaning and maintenance. Calorifiers shouid be provided with adequate access, to enable scale deposits or products of corrosion to be removed and cleaning to be facilitated. Distribution system The various elements of the freshwater production, treatment and deliver:, system, - filters, evaporators, reverse osmosis plant, auto-chlorinator, neutraliser lmineraliser, softeners, pumps, pressure tanks, calorifrer, carbon filter, ultra-violet sterilizer (where fitted) should be inspected, cleancd, flushed out, back washed, re-charged or items replaced where appropriate, i i i accordance with the makers' instructions. During water treatment - all filters, mineralisers, softeners - should have all sea water drawn through suitable sand filters before being introduced to !he water making apparatus and all water produced by such plants in new ships must he disinfected by an auto chlorinating unit, before it is pumped to ihe storage ranks.
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W g h respect to Deck machinery, discuss : Anchor wiiuduass r a k e test, to veriEy efficiency of windlass. b) Steering gepr tests, to demonstrate proper functioning. c) Air Receiver tests.
\.a)'
Ar*chor aild windlass test Ans. a) to This is carried out in the presence of a CIassifis~fio~Soc~ty~Survey~r, of &iig an verify the efficiency of the windlass.\~hewindlass mus: be capable ~-----anchor from a -depth of 82.5 m to a depth of 22.5m12 cable lengths) at a mean _-.. speed of not less than 9 m/min. If the depth of water is inadeqliate, a simulated condition can be con&Although the test does not require both anchors to be lifted simultaneously, on a windlass fitted with 2 cable lifters, this is usually carried cut and the time recorded. A visual check is a h made to enaiye that the anchors stow corre::ly and that chain washing facilities are adequate. ~~~
b) Stee:i.nggyr -/ - S e E i
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test i c t i i e presence of the Classification Society Surveyor to ~. demonstrate the mechanical hncticning of the sear and its ability to satisfy ~-. - ~.~ Rule requirements. !: -~. W h ~ r e=air. steering gear power units z e fitted in duplicare (to avoid fitting auxiliary gear), eachpcweiucit muzt be able to be brouiht spe&iiy into operation and must be ab!e to steer the ship at navigable speed In dddiriun. :he units opcrzring together ..-.. must be able to put the rudder hardover from j j" both -.-.on one side to 3"oil ..- the o'her side, with tile ship at thk maximum service ~~. ~ speed. The_!iie~takn from 3 3 on m e sideto;OO on hr-other side is not . ~ to .~ . exceed -~ 28 seconds. In practice, both units are tesred separately and togeiher. n e automatic steering gear and course recorder may also bc tested at this time.
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c) Air Receiver tests Where the Main engines are manged for direct air starting, the total Air Receiver capacity is to be sufficient, to provide not less t h a n i s consecutive starts, Ahead and Astern, of the Main engine, without replenisbmencif of the reversible type and not less than 6 consecutive starts, if o f the non-reversible type. The above requiren-ent is to be checked in the presence of the Surveyor.
Q.19 Describe how 3 boiler sxfety ~ a l v eis set to the working presscre. How is over-pressure prevented ? How is the correct functioning of this safety valve ensured ? Ans. Boiler safety valves are set to 3 %above the required working pressure, using a standard pressure gtiuge, the gauge having been tested and certified as to its accuracy. A Surveyor is present to see the test and to issue the cei-tificate with the stop stating that it has been duly carried out. The test is ca-ied out-valve and feed check valves shut and the boiler under full firing conditions
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The compression screw is adjusted to let boiler blcw off only at ihc required pressure:The .- .~compression _ _. ring . is then machjned and is f!ttedi!nr;Iace malting certain the compression screw abuts fairly and squarely on the rin!:..f ~. -~ .
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lmporranf : To ensure that the loading of th: safety valves cannot be interfered with, aAcr they have been set, cotters are fined through the spindle and spindle caps %xi then ?adlocked, so that the spindle caps which enc!~se thc compression screws rnak? them coxpierely inaccessible. ?he padlock key is left in charge of the ckiei'enginezr, who thus assumes res?onsi3'ility ior it.
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Q.20 Referring to a spring-loadsd Bciler safety valve, explain what is meant by -the 'Awumulation of pressure' test ? Ail,s.-'
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The pressure in the boiie; is liab!e te rke, even after the safety valves have lifted, due to the increased spring load, caused by the increased c~q:?:;ssion. 'Ilk rise is known as "Accumda$ion of pressur?". it is the one objec;ionabl.z feahre of the spiing-loaded type o f valve, bur ~wiihthe improvement in valve design. its disadvantage has been overcome. It is a well !sown fac! that the more a spring is compressed, ihe greatzr musi be ;hc appiictl load.:The maximum accumulation of pressure allowed i,s ?.O'&&~G.. >woc!tjnu :.pressure. At the time -... of the Safety ... vafGFtest; the test for accurntilation ~~. ~.. . . of pressure is also generzlh 2.. carried out.!
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Q.21. Wit3 rx2'ercnce to marine Boilers : a ) Why is Boiler water treatrned carried out ? b) Discuss the various tests carried out, to ascertain Boilerwater
condition. Ans. -?-da~mier treatme&&c_aitd out to : 1. Keduce !he corrosion of toiler tubes and shell 2. Reduce the scale formation, and hence maintain heat rransfer rates. 3. Extend boiler surveys to the maximum allowed (30 months). Thc following tests may be canied out to asceflain boiler water condition : Chloride : This is s measure of the chlorides which are present, usually a) ac! inrliciiiion of sea water contamination. 89% of the salts present in sea ,water are Sodium chloride and Calcium/ Magnesium chlorides. High chlorides will cause increased scale, acidity, and boiler priming. The isst is a titration of Silver Nitrate (alkali) with a sample of boiler water (acidic), which has Potassium Chromate added as an indicator. The rrlaxirnurn acceptable level of chlorides in the boiler water varies, ilepenclin.u,on the size, the firing rate and its type of construction. Should the chloride level rise, then contamination of boiler water is the most li!dy explanation.
Alkalinity : There are generally two measurements of alkalinity, which are taken, 'P'alkalinity (phenolphthalein), and 'Total' alkalinity. 'P'ajkslinity measures the alkalinity due to hydroxides, and phosphates. This test is a more accurate method of determining alkalinity, than simple pH testing. If alkalinity is too low then corrosion could Qccur, while if it should be too high, then foaming can take piace. The test is carcied out using Phenolphthalein, neutralised by a quantity of Sulphuric acid. Total a l k a l i ~ l ym e a r e s the alkalinity of dl the boiler salts, including bicarbonates. Note that bicarbonates carmot exist under normal boiler conditions, and their presence in t h e boiler water saxple is d j e to exposure of the sample to air, and the transformation o f half the carbonates to bicarbonates, in the phenolphthalein test. Total alkalinity snouid be less than 2 x 'P' alkalinity, and this may occur when Jsrgr qumtities of untreated feed is admitted to the boiler. This test is carrieci out using Methyl-Orange indicator, wher. the boiler sampi-, is neutzalised by Sulphuric acid of a known strength. Phosphates : An adequate reserve of phosphate should bc p;eseni in the boiler water, to neutralize any hardness salts, which may enter. These salts would depmit as scale, on the heating surfaces, if the phosphaie rescrvi. is too !m,while too high a reserve leads re 'foaming' and possible excess production of siudge. If this sludge is not removed; it will settle out and deposit on the heating surfaces. The tests to induce cloudiness or colour change, which indicates the level of phosphate reserve. Totai Dissolved Solids : This is a measure of all thz dissolved soIids in the water including those resulting from the treatment chemicals. It gives an indization o f salt water contruninalion, but is not as accurbte as the chloride tcst. Excess dissolved mlids produce foaming as well as increased deposits. Tests are normally carried out using an electrical conductivity meter, but the sample should be neutralised before testing, as an alkaline sample will affect the TDS reading.
Oxygen Scavengers . These chemicais are added to chemically deoxygenate ths boiler watcr. This treatment is only useful on closed feed systems; as oxygen will enter the feed at an open hot-well. The test is carried out ro establish the level of chemical reserve, o f either one of the two chemicals mainly used, hydradne or sodinm sulphite. If the chemical reserve is low, then corrosion may occur. Too high a level is aiso to be avoided - Excess level of sulphite will raise the density levels, and excess hydrazine will form ammonia, which can attack Popr 1: alloys in the feed system. The test is carried out using a Comparator, which measures the colour change of the boiler water test sample.
Q.22.
Waste heat recovery from exhaust gases is used to improve overall thermal efficiency of marine propulsion engines. Discuss : a) Methods used to recover waste heat energy. b) Factors which determine the amount of heat, which can be recovered. Ans.
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Several nethods ;~r? used to recover v m t e heat energy from exhaust : .~ ~~~~
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: f i s is a boiler, which utiiises the waste he2t from exhaust gases to generate steam. The water is circulated through coils lying in the path of the exhaust, by means of boiler circulating water - pump, takins suction from the main boiler drum, ~with the heated waie+tc~am returning to ~~~. the boiler ste.m@m.. . Exhsusr gii turbine, which is used to drive a centrifugal air compressor (Turbocharger). Here, the energy from the exhaust gases drives the turbine, x t i c h provides the ppwer for the compressor unit. The cow.pressed air is supplied to 'the engine as scavenge air, a k e ~suitable cooling. Exhaust gas turbine directly coupled tc~:he crarks'naft (TCS system). This allows the excess powcr to be fed back to the crankshaft, in a bid to i:nprove the thermal effici6ncy.
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Factors which determine how much heat may be recovered include : b) 1. The saturation temperature of steam at 7 bar is 165 OC. There muyt be a Cemperature difference, between hot gas andsteam, to allow the transfer of energy (say 2 minimum of 15 OC). This limits the outlet temperature to (165 + 15=\180°c. 2. Modem engines utilise fuel more efficiency, thus exhaust temperatures may be slightly lower, which reduces waste heat recovery. 3. Although fow stroke engines may exhaust hotter gases, t!ey discharge only half the volume of gases, (one exhaust stroke every two revolutions), thus total heat output is reduced from a similarly rated engine. 4. It would be possible to remove a greater quantity of heat, if the steam pressure and hence satmition temperature was lowered. But thir would also' reduce the exit temperzture from the waste heat unit; and allow the possibility of cok-end corrosion, from sulphuric acid enack. This factor is also relevant for the feed water entry temperature, if it is allowed ?J impinse on the heating swfaccs of the waste heat boiler. Thus, the feed water temperature should not lower the metal temperature below the dew point of the exhaust gas stream. To avoid 'cold' corrosion, the exhaust outler temperature shouldalways be above the dew point, which is ?round 1 8 0 ' ~ . Thus there is a limit to the amount of heat extraction. i. The space available for the heating surfaces is another limitation. If the heat recovery from an exhaust gas stream were to be dolrhlei:, then the heating surface area needs to be increased by three times as much. As the heating surfaces are arranged vertically, then head space,becomes limited, even if
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ihe funnel space is used. If the 'pinch point' is low, the area must increase, for the same heat transfer. However increasing the size, of the Economiser units, also increases the gas pi-essure drop across them. Thus, in order not to exceed the maximum pressure drop, the gas velocity must be small. However, results have shown that design ga5 velocities below 10 m/s greatly increase the occurrence of soot fires, due to build-up of soot deposits on the tubes, whereas velocities of 20 rnh were, to some extent, self-cleining. When engines are operated for 2xtende6 periods at low power, additional cleaning or frequent higher power runs must be c a n e d out. 6. .Tiie superheat tcmperarure is dependant upon the entry temperature of the gas, to the Waste hes: unit, with an allowmce for a zeta1 temperature difference of around 25 OC. Thus, in order to maximise ihe wasre heat output, without undue corrosions, a; low pressure with high superhea: temperature is desirable. Q.23. Discuss the distin~uishingfeatures of the followinp, Boiler defects : Pitting, Corrosicrn fatigue, Stress Corrosion, Embri:tiement, Overheating. Over pressure.
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Pifting : This is produced by corrosive action, usually in w a y of ths water line, caused by low alkaline conditions @reduces localked deep pits) and oxygen presence @reduces rounded isolxed pits). Corrosion fatigue :This produces fine fatigue cracking which results from alternating stresses caused by poor circulation, or mechanical bending due to pressure effects. The fatigue cracks produced are aggravated by corrosion, which increzses fatigue cracking a d canses coi~osionfatigue. Stress corrosion : This is sin~iiarto the above, but in this mechanisn~~ the alternating stresses expose the bare parent metal, subjecting it to increased corrosive attack. This attack occurs over a wider surface area than corrosion fatigue. Caustic cracking or Ernbrittlernent : This is a form of intercrystalline cracking by water, of high caustic alkalinity, coming into contact with steel, which has been stress relieved. This defect is more common in riveted boilers, and differs from corrosion fatigue in that caustic cracking follows the grain boundaries, whereas fatigue cracks pass across these boundaries. Overheating : If boiler tubes become overheated, either due to direct flame iGpingement, or lack of circulating water, then the high wall temperatures will lower the mechanical properties of the tube. This causes incieased scale thickness build-up and causes the tube to bulge just before rapture. Examination of the edge of the failed tube would show a martensite microstructure, indicating that the metal temperatures
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had gone above 730 OC, and that rapid cooling (under 10 seconds) had taken place. Overpressure : This could be.&assed as the failure .-____ - of the tube by mechanical stresses, rathcr. than thermal stresses. Thus, characteristic . metal failure (plastic and brittle rapture) would be seen, but without the --~-. microstnxture change to h 3 rnarrenslte material or scale build-up. The failure mode colild be more sudden, thus less prominenr tube bulging would OCCUI~~~
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Various valves and fittings are required for the safe and proper working of a boile!.. 'i'i~ose,attached dirccily to the pressure parts of the boiler, are referred to .. as b o i l c ~ o u n t i n e s . i
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Cla.sit'iciition society's minimum requirements
,7 S a k i y ralves. v
i ,Sicam stop va:\,e. iiidepcndmt Feed check valves. -2 indrpcndmt Warer gauge glasses (or equivalent). 1 i'ressaie gauge. 1 Salinoineter cock or valve. I Blow down/scum vsive. c-. 1 independent Low boiler water ievel fuel shut-off device and alarm.
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Safe@ Values :These are to protect the boiler from overpressure. There should be a! least two safety valves on the steam drum and one on the Superheater oat!cr header. The Superheater safety valve must be set to lie before the srealii d c i m sah!;. valve so as to ensure a flow of steam through the superheater under blow-ofi'conditions.
Main Stop Valves : These are momted on the Superheater outlet header and ennbli: rhr: boiler to be isolated from the steam line. If there is more than one boiici-, they must be screlv-down non-return type (to prevent 10% of steam from othm boilers in the event of loss of pressure due to burnt tube) may be fitted w i t h emergency automatic closing device. Auxiliary Stop Valves : As the main stop valves are not connected to auxiliary stearn line, these are needed to control reduced steam flow. Fccd Check Valves : These are Screw down non-return valves. They are nonrctutn, so ihat in case of loss of feed pressure, the boiler water cannot blow back into tile feed line. Main Feed check valves are often fitted to the economiser ir11i:i header. They are C:!ted with extendeb spindles, with positive indication of thc rrpi:i~ arid closed positions.
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Adva?iced !Mnrine Engineering K~o,ulerige Yo/.JJJ
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Water Gauges -~ : There are usually two gauge glasses on the Main stzam drum and a remote level indicator mounted at a convenient position i;l the control --roomi' -. . -
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Pressure Gauges : These are fitted, as required, to the Main steam drum or t'he superheater outlet header.
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Blo:v down vaives : These ar? fit:ed to the water h m s , to enzble the boiler water to be blown-down. Water is blova-dowr, fan :he boiler to reduce its density, or wheu the boiler is shut down to drain it. Usually two valves in series so thai first v.alve fiul1-y open bcfole tile second can be w3rked open. In this way the seating ofthe first valve is protected f r c n damage so reducing the risk af leakage when *e valves are closed. These blow down valves discharge inlo a line leading to a ship side discharge valje.
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Salinometer Valves : I hese are fined to the water drum to enable samples to be drawn off (control 9f fzed treatment). At high pressures, it is necessary to prevent flnsh-off, as the pressure of the sample isreduczd at atmospheric by a ecolirig coil, which reduces the lemperature of the sample to a value below 10o0c.
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Scum Valve : These are fined to b!~wndown the water from the surface. [f there is a possibility of oil contaxtinaticn. this can be taken care nf by blcwing down from the water surface, since oil being lighter floats at the surface. It consists of a shallow pan situated just below the water level, which can remove oil or scum from the surface of the water in the drum. These valves discharge into the same blow-down line.
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The Safety valve lifting pressure (for Main drum and Silperheater) is to be set at a pressure not exceeding 3 % above the approved working pressure of the Boiicr. I t is important that under all rates of evaporation, sufftcient stenm is passing through the superheater, to prevent overheating. On this account, Safity valves are normally fitted to the Superhezfer outlet headcr. Wlicn. 2s is sometimes the case, additional saturated steam safety valves are fitted on the steam drums, i r i s usual for these to he loaded in excess of the Superheater safety valves lifting prsssure, so that the Superheater_~afetyvalve will iift first. ihus ensuring a steam flow through the ~u~ethealefilemeni. The pressure drop, which normally takes place after an over-presswc has been relieved, is known as the 'Blowdown'. This Blow-down is limited (by classification societies) to 5%, though, in practice, a more usual figure would be 3%, as this represents a loss of usable steam. It will be obvious that, for Superheater protection, not only is it important that ths Superheater safety valve lifts before the Main drum safety valve but that the Blow-down factor must be considered, as it is just as important that the Superheater safety valve must also shut last, to ensure protection of the superheater elements.
91
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Describe holy the following conditions can be avoided, in an auxiliary Boiler : a) Furnace distortion. b) Uptake fires. Difficulty in maintaining water le-/el. c) Feed water contamination by oil. d) e) 'Blow-back' in the furnace.
Q35.
Ans. a)
bj
Furnace distortion : Distortion, of the furnace surfaces, cccurs due to overheating, and subsequent reduction in furnace-wali strength. This overheating is caused by the furnace wall being insulated from the water by either scale, mud, or oil. Faulty cm:bustion will increase the problem. Thus the boiler water must be kept in good cocdition, with a good quality of feed water to avoid hardness I scale formation. Also regular blow-downs s h d d be carried out. Uprake fires : This is the burning of the ;oot / ilnburnt fuel which accumulates on the heal transfer surfaces of &.e Econoniiser. This mot is rich in carbon, and is caused by poor cornbustim, usua!ly in the mgine, during low load mming. However these fires often have a zreater intensity than 'normal' fires, and can meit the tubes themselves @which (Meral tires). source of this additional 'fuel? is the i dis$o_ciaterinto hydrogen and oxygen at these ele&ed Temperatures. To ,*>;4.m dissoc~atksteam requires temperature of 2500°c, but at 700'~ the Colio~wingreaction occurs : Heat 3 Fe + 12 H20 -- . 3 FeO, i12 Hz
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Orice the fig is start~d,two types of reaction a e occurring Iron burnicg in the steam, and h) kiydrogen burning in the air, ilnd this type of fire will be self supporting, until the steam supply is cxiiausted.
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A 'metal fire' may occur under the following conditions : I. A metal temperature above 700 OC. 2. Sufficient steam content. 3. The presence of a catalyst, such as carbon ash. 1
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I t is imperatwe to avoid these potentially cat5strophic fires, by keeping
!he heat transfer surfaces in a clean condition, and maintaining a proper s!eam/water circulation through the tubes at all times. If a tube stack is to be bypassed and run dry for any duration. then it must be properly bianked-off, drained and vented. it is also extremely important to clean the surfaces, where soot may collect, if it is to be Iefi dry.
Difficulty i n maintaining water lwei : This may be due to defective control valve, or a defective Boiler feed pump. Foaming, of the boiler water, can also have an influence, as foaming is due to high levels of impurities withir, the boiler water. When the firing rate is increased, the boiler water level will rise, due to foaming 2nd swell, and this may make the control of the water le-?el unstable. The effects of swell and shrinkage is due to boiler design, but their effects are accenmated by the increase of solid contem Fee6 weter cmtamination by oil : If oil enters the boiler, it will coat the heating surfaces, and significantly raise the metal temperatures. The i~dicctic:lof oil in the boiler water, can be seen in the gauge glass, or as an incrcase in the fuel cnnsumption, for the same steam delivery. The msin sotircz of ccntarr.ination could be the fue!lll:be oi: heating coils. To . occurrin" a Weir systeln-----_-~ is used in thz Observation tank of prevrnt this TGK6iwell. Severe contamination~-affects - - the Boiler -operarion.. ~
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Furnace blorv-back : This occurs due to insufficient purging of the Furnace, during bumer firing. The biow-backs occur due to an accumiilation of oil an6 its vapour w i t h the furnace, which is ignited by a re-lit b - ~ r e r i.e. , sudden application of heai. The effect c m also occur when a sudden admission of air occurs into a fuel-rich flame. To avoid this occurrence : There should be en adequate period o f time (of air purge) with 1. the air registers (flaps) fully open, and the forced draught fan 'On'. While lighting up, minimum fuel should be admitted, to maintain 2. a 'minimum flame', ta avoid a?excessive build-up of oil, before ignition occurs. Proper maintenance of burners should be carried ost, to prevent . oil dribbling, while tile bumer is off. Regular inspectior. of the colour of the flame - a deep yellow 4. colour indicates a fuel-rich flame. A correct aidfuel ratio should always be maintained. Give the reasons for failure of Bciler tubes. fi& will you detect tube failure ? Discuss a temporary repair you could carry out, on a leaky Boiler tube, at sea. Ans. Boiler tubes can fail I leak in the following conditions : Excessive corrosion of the tube, which reduces wall thickness to a value I. below that, which can safely withstand working pressures and stresses. This can be caused by oxygen pining or under-deposit pitting. Overheating of the tube due to insufficient water flow, oil depn~itsor 2. heavy scale formation, which insulates the tube, reducing heat transfer.
Advarrced Marine Engineering &owledge
3.
Vol. III
Leakage at the tubdtube plate, use of improperly expanded tubes, increased mechanical stress and movement between tube/ tube plate, or 'forcing' of the boiler, which increases the temperature differential between the tubes, producing increased thermally induced stresses. Note that the causes of tube overheating will also increase the frequency of tube leakage at the tube plate.
Tube h i l u r e and repairs : The method to tist, t 3 rectify a tube leakage, depends upon various factors arid the location. Obvimsly
I . Using a plug or tapered stapper, which is 5:ted at both ends of the failed tube, and tightened into place by a long threaded bar fined inside the tubk. 2. Should a competent welder be available, then stopper plugs could be welded onto the failed tubes, so long as the tube ends have not thinned out or been damaged. l i time permits and suitable materials / spares allwx a permanent repair ro Sc carried out, then the failed tube must be removed a d renewed. One method is to grind the tube Cush with the tube plate, thus remwing all tirc wr:ld/expanded section of rhe tube. n e tube can then be punched-out. Inspection OF the plate should b e carried out to check for thinningkracks. Reillok ail scale from the area of th- weld by lisht grinding. Insert a new tube of proper rating (check material specifications or part nurribc,. stamped onto the tube, to verify), and carry out the tubelplate artrciiinent (either by expmding). Welding is the preferred method (less chance of tube /plate leakage); provided welding is carried out by an approved welder, using acceptable weidirig kct~niqueand consumables, and under the supervision of Class. I'i~efinished weld is normally inspected for visual defects, and by NDT. I-1y:imnlic pressure testing should be carried out on completion, using warm watei., ro tibe working pressure of the boiler.
Kekc to Marine Engineering Practice - for more details of tubes repairs i renew&.
Advonced Mnrine Engineering
Knowledge Vol.i i j
Q.27. Sketch and describe the combustion control system of an auxiliary Boiler. What are the safety devices ic above system.
Ans. In ihe control system shown diagrammatically, the final signal, controlling the fuel sbpplied to the burners, takes account of the Boiler pressure, steam flow rate md also regulates the air - he1 ratio.
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The signal fram the steam pressllre transmitter is f d 10 the Master controller, where it is compared with ihe desired value. The sutput. from the master controller, acts as the set point for the slave controller. The steam flow transmitter signa! (after being linearised by the square root extractor) also goes to the slave controller. Any deviation from its set point changes the signal to the low signal selector. A signal representing the air flow is compared with this signal in the low signa: selector. The air flow signal must be higher, for the selector output to change and alter the fuel oil control valve settin%. The output from the slave controller also acts as the set point for 11ie f;orced dr~ughlFan controller, which operates the danlper. When the stcam lo;~d is increased, the increasing steam flow signal will not allow more oil lo tlow until the low signal selector allows the signal through (when there is an excess of air to ensure combustion). The problem arises due to a fast response of the fuel oil loop to 3 load change, compared to the slow response of the air loop. Note the constant differential pressur- regulator, over the F.O. valve, to nlaintain the fuel oil valve characteristic.
Q.28. Describe an Auxiliary packaged boiler control, with special references to the safeties provided. .4ns. This type of boiler is generally used for auxiliary duties and, as such, i~ . isunlikely that severe changes in demand will be inflicted upon it. Also, being of the smoke tube construction, the critically of feed rate is somewhat dimin.ished, considering the relative mass of water in the boiler and the stetming rate cf the boi1r.r. The water levd control would be less sophisticated - (pcbably 2-iem Proportional ilnregral) - wiih High and Low level cut-outs and alaxns. In the system shown, the b d e r pressure controls :he HighLow flame, by operating the Oil-Fnel spiil tu suction. . . If a h i ~ hfiring rate is required thm no spill would occur. if a low tiring rate were required, then SOIL: of the fuel would be zllowed io spiii back to suction. Ti?e Oil fuel supply is shut-off completely by an Odoff solenoid activated by signals frcm the Water l e d , Steam pressure, Forced-dra~!gh: air acd Fiane failure sensors.
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Tk so!enoid would sl~ur-offthefi\el supply t s the burner under any of the following condirions :
' Low wste; level *
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Kish x+ater level
(a Low-low \vater !eve1 wvcold Iock-out the system. requirfn~a manual reset.) '.
High Boiler pressure Low forced drauyht air pressure Flame failur? Forced draught fan failure These alarms and trips should be tested regular!^, by actually altering the controlled condition manually, under close supervision, to initiate alann nnd trip conditions.
Naval Architecture and Ship Construction Q.f. With reference to the Ship's anchor and cable arrangements, describe how q c b o i t h e following a r e attached to the ship, using simple sketches : a) ,Cable stopper. . . b) Anchor. c) The 'chain locker' end of the anchor cable.
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gf the pipezwin theshain locker is %ell mouthed' and ower fined.&h a solid roundrubbing edge, to prevent !he cable % o n chafing. ~~
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a) The cable stapper is used to lock the chain, to take weight of the anchor off the Windlass. It is in the form of a bar, as shown iri the sketch below. Anchor is pulled tightly into the hawse pipe by means of a bottle screw, called 'Devil's claw', having a book at one end that fits isto the chain link, znd is s a z r d into an eye plate at the other.
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. . b) The connection of ihe'anclior t'o tiis cabie should be such, as to permit the rolarion of the anchoi, witlioi~tallowin: tile cable to get twisted. This is done by means o f two shackles, whic!i art. connected by a Swivel joint. .. :
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i-11- 'c11;tiii iockrp~e i ~ ol f tiizanci13i- cabie is connect?d in s~icha way, tiiat the @ -rhaiil : ca!i he iicliro,cil, an z n ; e ~ ~ ~ e nby c y ,simply rorning the Hand wheel (ken: iii
above.), w h x h rc-si~ir.;i : ~:lie scrr\s being !ifred, aroi~ndwhicli t!ie cable is slid. As r!;e screw iifis. 111,.lice ciitl c i the c&e ivili slip 0111. This is an arrangemenl to pem~iremerswcy elease of the cab!<, wilhont havinx to physically reilch the 'bitter end'.
Q. 2. A) Describe, with a sketch, how an aluminium superstructure is attached
to the s l e d deck. Indicate all materials used. (B) Discuss the use of a l u m i n i u n ~ f o r s h i p construction, explaining i t advantages a n d disadvantages. Ans. Aiuniinii!m is frequently used in ship-bidding to construct d e c k h o x e or even rile entire superstructure (Passenger ships), as the weight saving is considerab!?. this reduction in weight (on top) reduces the need for having permanan ballas1 (in Passenger ships). thus thcre 1s a two-fold beneiit. This benefit, hoxever. may not be significant in other types of vessels, such as bulk carriers or cil tankrrs. which have a comparatively smaller superstructuie. . .
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The direct connection. of a i alun!inium plate io a steel deck. can give rise lo .ga]vanic9 corrosion When two dizsimi!ar metals ( here : stze! and aiuminiun2: a r e connected directly, they form a -galvanic' cell and there is a resuitant potential difference between them, as they differ in their positions in the Galvanic table. The relative pszltion of metals, in the Galvznic series. also depends on whether they are active or passive. Passive means that there is some coating, like a tilm ofoxide, which prevents further corrosion. The common probleni Faced lie:e. in ship construction, is the galvanic cori-ojiol, berwzen the mild &el platin? oi'rhe ship's hull with the bronze or nickel ailo?s of the ~ropeiler.Anoti!e~- comnio!? problem is faced in the attach;neni oj' :rn Aiuininiunl supersti-i~cri~te to ;i stssl deck, as comnioniy fotind in passcngzlvessels and cruise ships. Thc only solution is to ?revent contact between the dissimilar metals. so as 10 prevent the setting-up of galvanic acuun. Various metheds have been tried our. A coating of Barium Chromate betreen the surfaces is one such measwe. '
In other cases, neoplene is used as the insulation in between. The deck-hotrse is bolted on to a steel angle by means of galvanised steel bolts. which are rnci!-cled by Neoprene Fcrmles. The gasket may be of any insulating inaierial (like Plascote). A sealing compound (Aranbee) is used to prevent the ingress of a n y water. which could lead to corrosion.
--ALUMINIUM STRUCTURE COMPOUND
S T E E L RIVET
' 1 S ? E EDECK ~ 99
Q. 3. Where and why are Deep tanks fitted on merchant ships Y Describe with the help of suitable sketches, the important aspects of Deep tank construction, including the scantlings and the method of testing. Ans. Most modem vessels are of the 'Aft accommodation' type. with the Machinery spaccs also aft. Thus, there is a need to have a 'Deep' tank loward, so a s ro achieve the reqxired trim easily. Also, it" only Double bottom tanks were used for ballast, the vessel may become 'too stiff ', i.e. the Centre of Gravity is unduly lowered. For :his reason, a forward Lower Hold is so arranged, as 20 permit the iilling of ballast water, when required; and is'calle8 a s ri Deep tank.' Besides-sea water ballast, the Deep tank may be used for any other liquid, such as fuel oil, i.e. it can b e used ior Bunkers. If the Lower Hold is to be used for caniage of liquids, it obviously needs to be strengthened, in order to resist the maximum possible head due to the liquid carried. A !Vast, plate iz fiired in a longitudinal direction, so as to reduce the Free surface effect. ;IS well as the suige of liquid, when roiling. The Haichway must elso be made water a ~ l doil rig!it, so as to prevent the escape of any liquid. Frames are riomally made at !cast I5 % stron~er. 3ulkheaa stiffeners are spaced nor more than 600 m m apart and have Srackris at tho head and the foot. The deck plating &us[ be at least I mni thicker than that of ihe boundary bulklieads Beams are nonnsi size, provided they are not smafler than the bulkhead stiffenxi. The beams imust be additionally supported by intercosial !;irders on either side o i t h e cenrre lice.
Deep T a n k Hatchway
T a n k Lid
1:) case of Deep ranks being used for oil fuel, the sides and boundary hulkheads are additionally stiffened by means o f deep, horizontal girders. running !ii:ht around the inside of the tank, and spaced not mcre than 3 m apart vertically The :irders are stiffened at their inner edges, and are conriectzd at the tank comers by iliin:ed brackets. They ar~estiffened at every third frame by brackets. A middle line biii!thead is fitted if the tank extends across the fidl breadth of the vessel. which may bi: perforated if required, so as to permit the oil levels to equalise on either side. Light intercostal plates called Stringers are fitted horizontally s o as to meel the sii-dei-5.Heavy oil o f Flash point not less than 60 OC niay be car~ried.Deep ianks are tested by fillins with waier to thk maxi&m head which c'omes in practice, pro\.ided tliat this is nor less tlian 2.44 rn above the tank top.
Q. 4. a) State the reasons Tor freebonl-d requirement.
b) Explain the term 'condifions of assignment' a n d explain h o ~ vthese 31-e maintained for a ship. c) Using a simple diagram, indicate freeboards f o r T y p e A a n d T y p e B giving of the type of ships fal!in:: in either category. one Hns. Freeboard is the distance from the water-line to the e d s e of the ~ ~ p u e r n i o s t continuous deck ai the ship's side, which is usually the nmin deck. A cenain mlnin,i~mfreeboard is assigned to provide adequate reserve buoyancy, so a s to cepe with adverse weather conditions, as well as a possible limited loss of buoyancy in the event of ship runnins aground or colliding, leading to floodiny. T o help i n differentiating the anlouni of freeboard to be assigned lo different type5 of vessels, ships are sub-divided as Class A, which includes all vesgels carrying .bulk liquid cargo ie.y. rankers) and Class B, which includes aii the remaining types. no1 fallins ii: Class .4 (e.2. con!ainer vessels).
Assigned freeboard is the freeboard allowed, obtained by correcting for variation from the standard depth, standard sheer, the extent o f superstructures fitted and for bow h e i ~ h tabove water-line, if deficient. T h e load-line mark indicates the summer load line and the Assigning authority-(e.gi tlo:wj's Resister is shown by !he letters L and R, on either side.) Conditions of Assignment The ship is first assigned a basic minimum freehoard, on standard values o f strength and form. The change in the freeboard allowed would dcpend o n the degree o f water-tightness or weather-tightness, as th: case may he. T h e height and stiffening of hatch coaminjs, the constnlction of hatch-coven and theii- locking arrangements, the machinery space openings including any doors, ventilators n ~ o s thave a means of closino, air pipes extending above the freeboard deck nlrist have a certain minirnunl heiSllt and means of closing, ail ship's side valves, cocks and sea chesrs must be as ,,el- specificntions. overboard valves must have a non-return 31-rangenlent and a means of closini: from above, or liave two valves in the same line, fi~eeingports m w i have ;I minimul1l area, or there may be floatiiiy bulwi~ks,the vessel inmt have adequate and also a minimom range of stability and ri$ting lesel-s.
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& 5 ~ Sketch and describe an arrangement of funnel uptakes, for a motor vessel, giving details of the $&hod of attach'meni of'the Funnel a n d horq suppork is provided, W h a t are the materials used in construction and their scantlings 7 Ans. The funnel is composed of an outer casing of steel p1a:es 6 m n to 8 miit in thickness, which are stiffened internally by angles or flat bers running verticzlly. The funnel is connecred lo the deck by a boundary an&, while the free end on rap is sliffenzd by a moulding, which is half round in section. The support is givsn by means of wire stays, attached by lugs to the funnel and rlie deck. and capable of being tightened by means of rizging screws. Access is by rnezns of a water-righ; door, capable of being opened from both sides. Therz is a p l a t f ~ mabout i m high inside the funnel, throtrgh which the upiakes pass without any connection, in order to take care of the expansion, v!hich is doir~: by tileans c f bellows. The upiakes pass through hales cut in the p1a:fornx. niid have a slidisg riirg sr;angemei!i ro p e n ~ i expansicii. t Tk,e iop of the u?t&es ead a: the lop oftlie fmmel, and are con:iectzd hy ~ncijri-: of :an arigle iron or ring to thc uppet- platfo~m A silciiczr is fitted to e n-~ i n ei:pte:-es,.and is supported on its own seat. Ladders m r ! xiaiiiigs prgvide access for mainrmance and inspection
Section titrough n Funnel
4 . 6 . Descl-ihe with sketches, the co~istrllctio~i of a hatch way, through a deck and show frow the coamirtgs are attached 10 [ h e structure. Explain the steps taken to avoid exctxive stress concentration at the harch corners. Ans. Hatchways are cut aut of the deck, which rewks in reduction i n strength a f the Siruclure. In order to restore the slrtn$, z3"liiional stifiening meassres need l o bc taken, ai shown in the s!:eich below. D /
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The strength of the deck plating is reduced by the o?eningi ciit into it. which i; madp up by Comings, which are vcnical stiffe~ers,welded to the sides of the hatchways. The height of the coaming cl?ould be such, a s to pseveqt the enIIy 3f sea water - on freeboard decks, it is 600 mrn, where they are exposed to the weather. On superstructure decks, aft of 'Aof the length ofthe ship, the height is A 5 0 mm. The loss of resistance due to beams being cut in way o i the hatches is made good by adquate stiffening. Half beams are attached to the deck girder. The deck girder is fitted in line with the side coarnings. Horizontal plates, called Gussets, are fitted under the harch corners to sxengthen the connection &ween the coamingr and the hatch end beams. Stress concentrarions could occw.at [he square edges o f the coa~viiigs.which are reduced by having the deck extending inside the hatchway, as shown. The corners are rounded off, and the radius nlust be at least 1124 of the breadth of the openin:: or al least 300 mm,whichever is more. Full penetration fillel welds are used for seitins ;i good join. If coamings are 600 mm or more in height, they we to be stiffened iurrhcr by bulb bars, at least 180 m m deep, fined horizontally near the upper edge of the coaming. Vertical brackets, called stays, are htted to connect thc above stiffene~sto tlie deck, and these must be not more than 3 11) apart.
Q.7. \\'hat is Pounding ? \\'hat ar-e the effects of pounding, on the s t r u c t u r e of the vessel, a n d whet precnutions ueed to be taken during design, to reduce the effects o f t h i s ? 4.115.
T[
T h e ship experiences severe srress dul-ing pitching, when !he fcre end emerges from the water and slams down with tremendous iorce, which is called as Pounding.
-
..
~
9.np*.e"i,,:
9'&tp
;3$
orc'oukllr bb~t,=m
. 15-3081.
5%L
I
Pounding region
The forward section, fi-om %;h o f the length or 5 %, till 25 - 50 % abaft the siem. is to be strenghened by increasing the rhickness of iiie outer bottoln piatin?~ Also, str-ei~:thcniil: :he coiinections Corn ?he ;ide sheil to rlie inlie: b d ~ o n lor rankside !jrdi:!-. !it !I-ansveisel;. fi~ameddouble borionis. fix pounditis !region hzs solid plate floors at c,,ei y frmi: space, and m i ~ s t ' b econnzcred !o the outer bottom plariny by coniiiiiio:i; ~.velds.In longitudinaily fwmed double bot~onis.plate noors are fitted a i every a!tc7wIe frame sphce.
0 . 8 . a) Sketch a water-tight door and frame. b) Explain how water-tightness of the door is ~ n s u r e d . Ans. Water-tight doors are provided to maintain the watei-tightness o f a bulkhead, while pernitting access. In ships having shafl tunnels, the access to the tunnel iiom the engjne room is through a wa!er-tight door. Similarly, passenger ships reqvire water-tight doors to ailow passage, from one part of tne accomrnodatio& to another water-tight pan. When c u n i ~ gopenings in water-tight bulkheads, care is taken to maintain the stiffness, by framing and reinforcing it, if vertical stiffeners are to he cut in way of tile opening. If the stiffener spacing is t o be increased to accommodate the opening, the scantlings of the stiffeners on either side o f the opening are increased, t o give an eyivalznt strength to that of an un-pierced bulkhzsd. The opening should be as sn;all as practicable, being 1000 t o 1250 mm high .and 700 mm wide, this however k i n g made bixger in passengzr ships. They are normal:y ofthe siidiilg type - either horizontal or vertical. The closing may be by hand (vertical screw thread f r ~ mremote) or by hydraulic rams. In case of hinged water-tight doors a t higher levels, the pins in the hinges ,nust be of 5urr-metal.
The water-right doors must be capable of being closed upto a list of 15' and opening / shutting must be possible both lccally as well as from a remote location above the bulkhead. At this remote location, an indicator is to be provided, showing the status of the door i.e open or closed. These are of mild steel or cast steel, depending on the requirements. Water-tight doors, in cargo vessels (rarely found in modem cargo vessels), are to be tested by a hose test, while those in passerzer ships are tested by submerging under a head ofwater extending to the deck above the water-tizht bulkhead. This is done before the door is fitted in the ship.
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Advanced Marine Engineering Knowledge Vo/. /I/
Metallurgy short notes on : a) Strength, b) Hardness and Elasticity, c) Eior~;niion a n d Ductility, d) Malleability and Toughness, e) Plasticity, 0 Stiffness, g) Brittleness, h) Fatigue Failure 1 f3tigue limit and i) Creep. Ans 1.
YQ.~. Write
7.
-
3.
4.
abrasion. Hardness : The ability to withstmri scra:ching, wcar, indentatian (by a harder body) denoted by the Vickers number (V.P.N.) or Rrilinel number (R.K.N.). EiasiicitJ. : The atility fa mzterial tinder stre!ch to return to its orizinal an impressed force is renmoved. shape 1 dimensicns, ~h.h-r, Elongation : When a sample of a inarerial is pulled (ill a cesiing ti:acIiiite), stretching takes place befcre fracture. The . -. elot~arioi;is [his -. artlount of stretch. jusr bcfore fracr~kre- usually expressed as a psireiiray scia~eiro ductility.
-
5.
Uiiclility: The ability of a material 10. be plasrica!i) d~fornicclwi~hout iiaciure: by being drawn in' me f9rm of a wire.
6.
Malleability: The ability to be physically deformed (beaten into sheers). b y pressing, hammering, rolling. (e.2. Lead).
7.
.l'otigi~ness: The amount of energy a niaterial can absorb hefore ir fractures. when subjectzd to shock loads(e.g. lzod lest).
8.
PI~iticily:The opposite of elasticity. It is similar to malleability. This prlJpcl-ry is necessary for forging, and is generally temperature dependent ( c . 6 Steel is plastic, when 'red' hot).
9.
Stirfness: A nieasure of a component's ability to resist deflection.
10.
l3riltlener;s: Opposite to touzhness. Sudden failwe under load. \ \ - i ~ h iirrie clcibrmarion, e.g. Cast iron is brittle, whiie Nodular spheroidai iron is iess.
1 I.
~FaiigucSailure: T h e B i ~ r eof a consonent, \\ihich has beer3 subjscted to cyclicat applicaiions of load. This may produce a slow, bur progrrssi\? c!i!argc:cn~entof' a small imperfection. until the average stress across the ierrminiu!: ritcia! causes iiactut-e. The loading map he alternative. repetiti~i' ol- iluctnatin~~.
Advanced Marine Engineering Knowledge Voi NI
i?.
Fatigue Limit: This is the greaiest sties8 or range of stresses. whicll can be applied to 3 rnemb-r, for an onlimited number o f cycles. wirhoul caiisini: failure.
Ij
Creep: When a component is loadcd, over a long period o f time, ti:e 111e;aL may exhibit extension and ultimately fail, at a stress well b e l u x the Ultimat: Teqsile Stress (UTS). The effictj: s f creep are serious deformation at high temperatur?.
J
t h e effects of the following elements o n steels : a)Mangane:e. YQ.2Explain . b)Nickel, c)Chrome, d)Molybdenum, e)Vanadium, f)Tungs!er., h)Silicon, i)Sulphur a n d Phosphorous.
g)Cobait,
Ans. V I a n p n e s e : Manganese increases hardenability in steel, but also incl-cases brittieness. It is used with a low carbon steci: to increase tensile stren:th. Nickel: Nickel i ~ c r e a s e sstrength and corrosion resistailce by the formatioi1 01. iinpr %rains, in the material. Upto about 8% Nickel will not affect the ductllii! 'Tliis is used in n1ate;ials subjected to high stiesses - t.5. pump rods. C h r o m i u m : This increases h ~ d n ~ 'and s s resistance to corrosion/erosion. This is used, along with Nickel, for Nickel-Chvome strels. It can be brittle, if irnpropsii> tempered. Molybdenum: This eliminate; '"mper brittleness (in A'ickel-Chrome steels). I t enables increased content of Manganese; without brittleness. !t is used hlincreased strength at high tempeiature, and is thus found in -* superheater tube? and turbine rotors It also increases the 'creep' resistance. _- >__ ..~
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~~~
Vanadium: This i s a De-oxidising agent, i.e. it reduces the iron oxide conten;. i t also increases resistance to fatigue. Used in boiler rube material.
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Tungsten: This refines the grain size, to i:nprove hear resistance and c o r ~ o s i o n resistance. Used in machine tool-bits and cutters in the form ofTungsten Carbidr. .-?
Cobalt: This improves hardness. Silicon: Silicon improves fluidity, and thus improves castability. It is liable to a n p o s e graphite.
Advanced Marine Engineering Knowledge Vol:/I/
Sulphur: This is usually an impurity. It reduces strength and increases the brittleness. - upto 0.6% Sulphur cmtent is a1:owable. I'hosphorous: This is also an impurity. It reduces strength, lowers the meltinspoint, increases fluidity, increases hardness / brittleness.
J Q .W~ J r i t.e s h o r t n o t e s
on :
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-
a ) Case hardening. 5 ) Nitriding. c) Flame H-rdening
c Ans. q0o cL.s&nJ Case hardening This is a process by which the outer surface of a mild steel component can be hardened, ail around or on selective Lieas (you c2n paint the pan of the burface, \vhich is not required to be hardened).This process is done by enrichin: cnrbon content of the surface and applying t h ~ treatment.
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,y$i-;
Thr skin is carbon-enriche: by 'soa iqg the component in some carbonrich material (e.8. charcoal) at a .-temperature 3bove 900 ". The depill of rhe rarjon-enriched skin will depend upon the mate& used for enrichment and on the length oT soak. The depth of the "c2:se" may vary from 0.8 mm. (2 hours) LO h r n (12 hours). Nitrid!ng,
#
(~ 0 - 5 5bc q - l - 3
PLQCm
1
The component is placed in gas-tight container, circulated by Ammoniz gas and Skin depth - 0.125mm (5 hours) to 0.05mm (24 hours). The heated to 5=. change from 'nmreated' to 'hardened' skin is more gradual - this reduct- the risk of exfoliation. Flame - Hardening. Used oli surfaces of Cast steel, Cast Iron and alloy steels. E.g. Gear teeth. The C and quickly quenched by water coinponent is heated locally to about 800 ' - ~. ipray. -....-(e.g Hardening of gear teeth).
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',$I\=
What are the alloys of Copper? Discuss briefly the properties and basic //composftion of various alloys of Copper, such as Brass, Muntz metal, Alum~ntumBrass, Admiralty brass, Manganese Bronze, Admiralty bronze, , Cunrnetal and Monel metal. Ans. C o w : I t is a soft and ductile material but ages / work-hardens very quickly. becoming hard and fairly brittle. Good heat and electrical conductor, --
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~~~
Advanced Marine Engineering Knowledge Vof. Ill
. ~ >-:
~, * . ~ / -
.
3j.
/7
.-R ~ ~ r :s sI t
18 a~l,?!loy o f Copper and Zinc, containing up to 45 % Zilic. h;~:~,~! .-~~~.. brasses are available, proponions depending upon the plirposc anticipaled. Brasses contaiiiing up ro 5 9 % Zinc are known as u Brasses. Brasses c o ~ i i n i r , i ~ ~ l~ between >9 % and 43 % Zinc are known as B-Brasses. They are g e n e i ~ l klio,vl, by their respective percen:ages of Copper and Zinc. bul more c o m n ~ o nalio>s have i p t - c i k nnmes. 70130 Brass is 70 Copper, 50% Zinc - it has model-are strenqri,. 601-10 Brass (knon.n as M u n t z Metal) is stronger, bur is less d11c:ile - it i q a goad ailpurpose Brass. If Aluminium [Al) is added, it intpro\-ts the erosion and corl~osioll iised f c i ~ p ~ o p e r ~ i e sE.2. . (76 Copper, 22% Zinc) Aluminium,Brass is -':* Mi--A\ yd 5- L ~ ! >h9-,.yJ . L * ~;w>w L ~'hi C T . , . ~'.',-. coiidenser tubes and rube-plates. - I-*.-.+: .~..-L i l Adding Tin (51,) fiirther iniiibirs corrosion. g. .4dinir.rity 31-i!;s (70U.; C u and 29 % Zii with 0 l%Sn). A trace of HI-senic (0.01 16 0.05 %) ~-:si~i.; ilczincificatioii. ?'his Brass is aidel!. used for co~idensertube piates. Adding L.C;I(I (l'b). i n h he order of 2%. iiicrea\ts machiiie~biiitya;id also resists ' i ~ n p i n ~ c ~ : : ~ i ; < attack:. Corrosion I-esisrance can be funher improved by addins Nickel (Ni). a p p r ~ s i n a t e i y1 %. . ! 2' L! . i5.2 .i , .. ... . ,. ii?anganese Bronze is a high-tensile Brass (58% CLI. 55% Zc. 7% c ~ i i r i ~ eleriieiirs). I t is hot-v.orking alloy for heavyduly bearings and fur p!~opellers. TIie 7% ,n;y include ZC/b Al, 2% I:bn(Fe), 2% Manganese(Mn) and 1%Sn. ~ d r n i l - a i h t h o n z r is 80% Cu, 19% Al, j?: Fe, 5% Mn. it is stion: and corrosion rssisiont. 2nd is used for pump casings, impellers, tubes and tube-plates. Bronze is an alloy of Copper and Tin (about 10% Sn). E.g. Gunmetni. which is 88% Cu, 10% Sn, 2% Zn. This has good casting qualities fni- pump casings. bearins housings and valves. D r a w n Phosphor Br~onze,94 CII. 5 . 5 % Sn, 0 1 % P5osphorous, is used in the work-hardened condition. and is suitable t b r heavy duty bearings and for steam turbine blades. Moncl Metal, 29% Cu, 68% Ni; 125% Fe. 1.25% Mn. is ducrile. can be hot or cold-~vorkedand forged. I t is highly corrosion resisiant. and is iised ia,imp?lIers and in chemical applications.
,,
e is anodic to copper and lle-rincificati~n: I t is a type of corrosion, w l i ~ ~ziiic a spongy mass of copper. c o r r o d ~ sleavinz , De-~luminilication:Similar to de-zincification, bur conibared by Nickrl (to Bronze).
addilioti 01.
Advanced Marine Engineering Knowledge Vol. NI
5 . What is t h e use and importance of Titanium, a s regards non-Ferrous metals. Ans. Titanium (Ti) is the fikh most abundant metal and has many desirable engineerin2 properties, sucha5:
b) c)
Strength Corrosion Resisrance Titanium has the highest strength-to-weight ratio of acy struc~c:ml metal(about 30% better than either Aluminium or steel). This esceptionrti si~-rngtll to \\-eight ratio is maintained over a wide temperature ranyc (from - 200 "C to 5%)
"Cj.
The prcsence of a thin but tough and tenacio~ts oxide surface tilni. provides escelient corrosien resistznce to borh aimospheric, as weli as rhs sza wat::r environme~t.Being near the cathodic end of the galvanic series. titaniun? peribrms as a 'Ysble' metal. Other properties are:High melting point (compared wit!> steel) ii) (ii) Low thermal canductivity (iii) High electrical resistivity (iv) Low coefficient of expansion. Due to the difficulty of obtaining the metal from its ores, i t is very expmsive, and thus not for general use. Th9 pure metal has a low :ensile strength r i 1 6 i\/l~l/m') and a high ductility (50%) Due to traces impurities in its com~nercialform, it's tensile strength is upto 700 M N I ~and ' the ductility is 20%.' Titanium is one of the few allotropic metals (like steel), and it can exist in two crystallographic forms : At room temperature it has a Hexagonal closepacked s:ructure. At about 900°c, it transforms to a Body Centel-ed Cubic (BCC) structure. Like steel, titanium can be heat treated. Also alloying elements can retain or stabilise a s ecific crystal fxni. Titanium alloys, \vitl; tensile P s r m ~ t h supto 1500 MNlrn-, have been used. The mechanical properties of the mctal are related to the crystai form. In the BCC form, it is niuch stronger. but more brirtle than the HCP form. Fabrication o f titanium is difficult, due to its affinity for Hydrogen. Oxygcr~ c?c Nitrogen - all these impurities can cause embrittlement. Hence. eie\/atcd-ternl)erat~~re plocesses, such as welding, require care and experlisz. Usually casting is carried out under vacuum conditions to avoid oxidation.
Advanced Marine Engineering Knowledge
VoL 111
6. What is Metal-locking ? What repairs a r e czrried out by hletal~-.locking? Ans. k M e t t a l o c k 01 Metal-lnckiug. js.~a 'cold' - repair of brokenhacked ~~.~ .. castings, in lieti of repizcernent of the casting. It has proved popular, due to its cost benefits (as .iompz&f to replacement of the casting). It provides sufficient strength, and enables the casting to be repaired 'in-silu', i.e. without having to disnianrle and remove the componenr to shole workshops. The affected surfaces have to be carefil!ly prepared, by chiseilng or grindine. Broken pieces arc accurately re-alig~edand held in fixtures or clamps. jigs are used co Fosition the 'F3ttern' ~f holes across the crack, at right anglps. Holes are drilled and then 'joined' by accurate cliiseling, to create 'slots- of a specified shape. These will accept pre-made 'Keys'. Q.
~~
~
'Holes'
0 0
o\o
0
'Key' inserted in 'slot'
'Keys' are thin 'peened' into the slots, in lcyers, by pneumatic hammers. Holes are then drilled and tapped, alpn:, the line of fracture. These are careftil!y spaced - alternate h d e s leaving d gap which is less than the hole diamzter. Studs arc thex tightly fined and 'snapped' off, caulked and 'dressed-off. Gaps are then drilled, tapped, plugged and 'dressed' :o complete the seal along the original crack. The studs seal against pressure and exert a tensile stress along t h t 'keys'. The 'keys' restore the rigidity to the casting. Key material is Invar (56% Ni, 0.21%C, 63.79% Fe). I t has a low coefficient of expansion, is soft but work-hardens to zn Ul?imate Tensile Strength (U.T.S.) of 780 MN/ m', afier 'peening'. Advantages : Dampens cornpressior, stresses. No new stiesses or strains.
.
Maintains 'stress-relieved' condition. Can be done "In-Situ" - thus saving in time and costs. Distributes the load to non-critical areas.
.
Disadvantages : Keys are subject to 'creep'
Advanced Marine Engineering Knowledge
Vol. Ill
DifJcrcntini cxmmci~nmn smnctirnrs hc n prohlern
* T l e location of cracks may leave ins~ifficientparent metal to achieve proper ~.. , c7cy:ng' . Q.7.
Explaln the actions of following metallurgical phenomena: Creep a n d stress rupture. b) Brittle fracture.
A x Creep is a time dependent strain. A typical creep curve of strain, on a base of time, shows several distinct stagzs. After the initial sudden extensim AB (occurs in zero time on :he scale used), there is primary creep BC (or trafisient creep), in which the strain rate decrexes with time. This is followed by seccndary creep CD, in which the creep rate is constant with time. Finally, there may b~ le.riary, cr accelerating, creep leading to fractu;~at E. The la!ter is called creep rupture. The total strain at m?tti;e is typic~llyonly a smd1 frxticn of the valm to h c t u r e in a comparative tensile test.
i lOOi
I
i I T the
SUDDEN EXTENSION
I
A'
-
TIME
spccimen is unloaded, at any point on the curve, there is an imrwdizte recovery of strain (according to Hooke's I-dw) but negligible sl.ii)sequent recovery: Creep can occur at lower tenpera:ure but at relatively higher stress levels, as com?ared to high temperature creep. Creep is a function of slrcss and temperature, for a given material. In a room temperature tensile test, creep is readily apparent at stresses approaching the tensile strength. At lower :;tn:sses the creep strain (in a given time) becomes less significant, in relation to thi: instantaneous strain. I t is not possible to define an absolute Limit, below which creep is absent. Iir engineering design, arbitrary criteria have to be selected, ro define when creep will be a factor.
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Advanced Manne Eng~needngKnowledge
Vol. NI
The parameter of importance is the ratio of the operating temperature to the melting point (on an absolute scale). Different pure metals have similar creep characteristics, when tested at the same temperature ratio. For example, lead with a melting point of 3 2 6 ' ~ exhibits similar creep phenomena at room temperature as does nickel (with a rnelting point ot- 155 'c) at 600 OC. Alloying will modify the creep behavior at a ziven remperatwe ratio. Hencr the high meltin? point metals are the basis of creep-resisting alloys. Extrapolalion of creep curves to longer times than the experimental data Is a mbjor fac:oi i i l the cievtlnpmenr ornew alloys and for the provision of design -data' o i estabiisned materials. Tests of even one year's duration are costly (and lenzthy), yet many engineering compotieiits are intended for ten or more years life. 11 is usual lo carry 0111 creep tests over a limited period, at either higher :emperaiures or ili~!lr: ctrcsses, than is expected to be eventually expected ii, service. The main protection against creep, are the a!lcys containing metals with 'high creep resistance, whic!i have high mdting points. Usually the alloys will contzin molybdenum. Considering cornponen!s for high temperatQres, piain carbon and !ow alloy stesls were considered adequate for use upto 550 OC, with ' c r e ~ p 'being domirianr above 400 k.Thesc largely satisfied !he requirements of steam and diessl powzr plants upto the 1940s,. bur subsequent developmenrs forced operating temperatmes above 1his Speci.d stainless steels ( I 8% Cr, go,; Ni, 0.5% Mo) were then used, rollowed by the >!inwnic ( or high nickel) all'ys. For use above 1 OOO'C,refractory metals are being developed. Niobium (melting point 1950 ' c ) , Molybdenum (melting point 2622 U C), Tungsten (melting poim T387 'c), Above 1 5 0 0 ' ~ ceramics, especially graphite, ar? used. Brittle Fracture :
:
i
Some nxtals, norinally considered quite ductile, can fracrure in a brittle manner. E.g. Mild steel, used extet~sivelyin ships can behave in this manner. Normally ductile materials undergo 'slop' within the structure, so that substan;ial plastic deformation takes place, before fracture. However, in some cases, slop can be suppressed, such that the fracture occurs suddenly, with very little prior plastic deformation. Brittle fracture is characterised by very high speed propagation. T i e bright, coarse, crystalline surface often exhibits 'chevrons' ('Vee' markings) which point back to the source of fractuie. The tendency of brittle fracture to occur increases with:-
Advanced Marine Engineering Knowledge Vol. Ill
.
The presence of tri-axial stresses. Hence thicker plate are more prone than thin plates. High swain rates, usually associated with the presence of notches or stress concentrations. Low temperatures. The effect o f low tenqxrature can br: simulated by a notched bar impact test, i: g. Charpy or Izod. I f the energy ro fracture is plorted against tcmperatttrc tl1ei.i: may b: a sudden change from ductile to brittlc bekaviors. This is known as the 'ductile-brittle' transiti~n. It wm!d be impractical to remove every possible notch or stress coriccntration. Also, the working temperature cannot be altered. Thicker plates may be wed, of hisher strength, e.3. high tensile steel. Arresting techniques can be used to prevent the spread of cracks. in o!der ships, rivettd seams would arrest the crack, since the crack could not propzgate GCriiSS cj;%reiil p ! 6 3 rivetzd toge;her. On fully welded s h o p . -notch-ductile' std is i r ~ciitiial posirioils. E.g. shear strake, bilge strake, kezl, comers of open; rigs.
.
With reference to fatigue of engineering centponents, explain t h e i n f u c n r e o C i t r r s s level a n d cyclic frequency c n expcct-d operating life. How i s a 7'c.st piece tested in a inborator) ? Expiain the influence cf rnateriai def:xts, on safe operzting life o f engineering components. Ans. l ' l ~ eload on many str~xtiti-a!components varies repeatedly. This can lead to i'ract~~re, even thougii the maximum load (stress) is very nnxh lower than the tensile sir-rigth, even below it's nominal yield stress. This n~~ ,i,~- t i a lukirnaie 's type of L~ili~re:is known as fatigue failure. 1: occurs in all classes of materials, esccpi glrss artd is one of the most commox causes of failure of engineering cornponenls, in service. fix tiit; simplest rype of laboratory fatigue test, a lest piece is rotztcd coririrutoitsly, whilst supporiing deadwsight loads, and the specimen is rubjecced to aliwnating bending moments. Varioi~sloading arrangements can be used. ( h ~ ,rnaximun~stress at any section occurs at the surface and fluctuates hariiioriically about zero, i.e. between equal maximum tensile and compressive strcssi:s. The specimen is slightly 'waised', ro prevent fracture developing at the loadin:; or sgupporr points - in other words, the region of failure is 'selected' by indilcirig a change iri section. Very gen!le changes of secrion must be used [a avoid stress concentrations, vhict? would seriously lower the observed fatigie strength.
Q.8,
Advanced Marine Engineering Knowiedge Vol. 111
A series of identical specimens are tested to fracture, starting at a high stress level and progressively red~lcingsrress or. succ?sslve specimens. The stress anlp]itudes (S) are plotted against the number of cycles to fracture p),using s~mi-log or log-log scales. These are called as S-N Diagrams. Typical S-N diagrams h r non-ferrous and ferrous metals are shown.
b <
NGN-FERROUS
The S-N diagram indicates that the fatigue strength, or endwance strength, decreases with increasing number of cycles. me S-N curve is sometimes divided into two regions. Below N = lo4 cycles, the effect is known as high stress, or low cycle, fatigue Above N = 10' cycles, it is known as low stress, or high cycie, fatigue. For ferrous metals and a v e v few others, the S-N curve approaches a finite stress aptitude, called the fatigue limit. Below tkis;-'a fraciwe will not develop, however great the number of cycles. The appearance of a fatigue fracrure has several characteristic features(at least i n ductile materials such as Mild steel). In materials with less ductility, such as alumin~mallays, recognition of a fatigue fracture is not so easy. Unlike the tensile fracture, there is no apparent plastic deformarion, adjacent to the fracture.
Advanced Marine Engineering
@
Knowiedge
Vol. Ill
Q. 9. Explain what is meant by fatigue. What is the effect of surface finish on this ? Why does a component, subject to fatigue, have a limited working Life ? Ans. Fatigue. The decrease in usable strength under cyclic leading iz directly attributed to the fact that the material is not an idcal homogenous d i d . In each half cycle, mini~sculeitrains thar are no; completely recoverab!e are pi-oduced. Due to this thcre is a gradoal reduction i l i ducti!iry in the incremenrally strhin hardened areas. This in turn leads ro thz formation of sub-niicroscopic cracks, the effect of these crack is to concentrate stresses, until failure occurs. The minute strains tend to be at grain boundaries and around surface in.egi:arities. Tine surface finish has a tremcndous sfiec: on fatigue strength :Type of finish Surface rougkness Micro inches
Endurance Pounds per sq. inch.
Ground
16-25
9 1 .OOO
Lapped
12-20
100.00
Super-finish
5.6
i 16,000
Designiltg against failure by fztigue, is very much itlore cornp!ex and dific~iltriian designing for static st!-ength.There are a number of reasons for this, which include the high sensitivity :o local stress concentrations and rhe larse dr:)cndence on the corrosive environment. Small specimens of represenrative 111al.wiai give 6ifferent results in laboratory fatigue tests, than do larger specimens or the actual structures. The larger the specimen, the lower the fatigue strength, p;!rticularly when there are stress concentrators. This explains the minute attention that is required to be given to the cross-section, coaxeness of the:surface, .?ciatciies, too-small fillet radii, poor distribution of load between bolts and weld flashes. These are only a few of the frequent causes of fatigue failure. The influence o f cherniczl~actionis complex and difficult to reduce to quarililativc terms. Fatigue cracks usually start from some point of stress concenlralion. such as a key-way, s h a v fillet, micro-structural defect or even a bad tool mark. Any "locked-in" stress from bad welding cool out or thermal stress can rnakr: a signiiicant contribution. Fatigue cracks are not necessarily the result of tiulry niateriai. Sometimes, bad design vill limit the working cross-section of a coinpollcnt subjected to alternating stresses. A knowledge of the behaviour of the
Advanced Marine Engineering Knowledge
Vol. 111
material in fatigue will allow an assessment of the useful life of the component to be made, so that i t can be replaced, after an appropriate working period, such as a bot1om end bolt of a four stroke engine. For plain steels. the fatigue strength in air is nluch higher than in water, even fiesh water Col~osionresistance of a material is more impcrtant then its static tensile strength, in determining the corrosion fatigue strength. For example, plain carbon steels show a marked reduction of fatigue strength in fresh water ; while chromium steels are only slightly affected by water and the corrosion fatigue strength is unaffected. A similar phenomenon is frrtting cormsicn, when two components at-e pressed against each other. Thus slight but repeated relative motion occrrs, as for kxample, in holding-down bolts, when the fretting corrosion destroys the joint faces, when running. The corrosion products formed, like a reddish brown dust Ferric oxide. in the case of steel, can help to detect this condition.
A ~ / Q . I O . Explain how the toughness of steel can be improved, with refereace to hardness and ductiiily. How do the properties of. s t r e ~ g t h ,ductility and fl hardness change wit!> heat treatment ? What are the processes in tile treatment of steel ?
hr
Ans. Steel is xi alloy of Iron, with carbcn as the al!oyinp element. TIlr percentage, of carbon present, determines it's properties. Thete are .wo commsn alloys of iron:
*
-~ ~
a) Sreel m d
b) Cast Iron. Steel has less :han 1.8% carbon, while Cast Iron has between 2% and 4% carbon. Upto 1.8% carbon can exist in steel as a chemical compound called iron Carbide. This enables the steel properties to be modified by heat treatment. The carbon, in 'grey' Cast Iron, exists in the form of flakes of pure graphite. This makes the material weak in tension, but easily machinabl;. There are two factors of importance to the marine engineer :
I.
The properties of steel in its normal state.
2.
The changes ofproperties. when subjected to heat treatment.
Properties of steel in the normal state -- are :_Strength, ductility and Hardness. ~~. As the carbon percentage increases, the following changes lake place:-
Advanced Mar:ne Engineering Knowledge
Vol. NI
Hardness : This increases with increase in carbon content, i.e., steel with 1.2% ' carbon is much harder :hen steel with 0.2 % carbon. Both the low and high carbon sti-el are capable of being machined in the normal state, although the speeds of machining must be low, in case of the high carbon steel, to prevent 'scrface hardening'.
D~tstitily : As the hardness increases with carbon content, the Ductility is reduced, as is the mallcabi!ity. Thus mild steel can be cold worked, bent, and manipulated in presses but medium and high carbon steel has to be 'hot worked'. Strength . She effect of carbon, on the strength is that up to 0.83 %, the strength increiszs, and dfter 0.83%, the streugth reduces, due to brittleness. ,? chari of properties and carbon content is listed below for comparison.
Carbun Content ,
-
MNI~'
Ductility( %)
Hardness No.
!ensile Srreng!
-20%
1
125
Prop::i.tii:s of Cast iron, in its grey form, are:.
htgn-1 250 i
Ductility-
Harness
Negligible
280 Brinnel
I
Compared with steel, the value of the strength of Cast iron is very low. altl~o~.i~;ti the value given is from the tensile test only. The comprcssivr strength of Cast iron is mush higher 690 MN/n12, which indicates that this material is best siiii.i:il to compressive loads. I t is also very brittle, so it should not be subjected ro shwk loails. Cast Ilon has a lower meiring point than steel and is much easier to cast.
!
Advanced Marine Engineering Knowledge
Val. ill
Heat frei~trnentof steel :
There are four processes, by which the properties otsteel can be modified. by heat treatment. These 2re:-
I.
Hardening
2
Tempering
1.
Ameaiing
4.
biormalising
If steel, containing enough carboll, is lieaisd t c certain temperatures, the form, that the cerbon is in, changes. If cooled quickly in water, the ihanges d o not y quenching ii, have time to rever! tack and the steel is hard. !f cooleii slowly= t oil, a partial change occurs and the steel becomes very tough. If a h a d stzel is heated to ibe correct temperature and cooled ~lowly,in drv sand, or in the furnace itself, there be m p l e time fo: changes, thzt tock place due to hrat, tz revert back and stee! %ill be very soft oi- 'annealed'. The cooling-down process is conriolled, by ~radualiycooling down, ;c any streszes are r-lieved. If a steel in an abilorn:ally hard rough oranrl.eaIed state is heaied to lhi. correct temperature and cooled in sti!l air, it wi!! retuiw to its normal state and is said to be 'normzlised'. The temperatwe whele these changes take p i x - depend on carbon content and can be shown in a simple graph.
H U T TREATMENT
C
CHART
1 TEMPERATURE ZONE FOR
AND HARDENING
% CARBON
-- - --Advanced Marine Engineering Knowledge Vol. IIi
,$
Note that the temperature is 900 OC for pure iron, falling to 7 0 0 ' ~ lor 0.83% carbon, after which temperature is constant for increase in carbon content. These temperatures where changes take place are called Critical temperatures, above which change occurs and below which any change cannot occur. To ensure thut the desired changes !lave occurred, the component must be heated to 20 - 50 OC above the critical temperature, before cooling the steel, in th:: rnarmer necessay to give the required properties. It is thus possible in determine the correct temperatures, for treatment of plain carbon s!eels. Q.13. Explain 'Notch toughness' in mzteria!~. 0nt:ine the test carried-out fi, determine iocghness - the Izod Impact Tcst. Discxss how rviil ycu repair a high pressure steam pipe by welding, and ensure that it's properties a r e nut lost? Ans. 'Notch toughness' is a measure of a material's strength, in rhe presence of any stress concentration, such as a notch or a crack. Brittle materials, such as $lass or ceramic, will fracture readily at a notch or surface xratch, but have qu!R iiigir strengths, in the absence of any -stress raiser'. Similarly, low 'notch m!ghr!ess' may occur in metals, the chiet' p1-obIen1being in high strength alioys. Brittle materials, i.e. those that fracture wid? little plastic deformation, in a straigh!. tension test, will have very i o ~ ynotch toughness. However, the opposite is ~mfr,r.t~.ir~ately nor ti~!e.Evec thou$ a material is diictile in a tensile test. with i ~ i $ l eloi~gatiofiand reduction of area, i r may fractrtre in the presencs o f a notch or crack, without appreciable plastic flow and at low average stress. This was found first observed in heat-treated al!oy steels. Various notchedbar. impact tests, s u c , ~as the lzod and Charpy tests, were developed, to measure a marcrial's ability to withstand stress concentrations. It was observed, thzt the. riotch toughness of mild steel, which was a highly ductile and standard construciional material, was questionable, following fractures in many of the early ail-welded ships, due t o i h e absence of any form of heat treatmefit tiler. A crsck can propagate through mild steel, and other similar metals, with little plastic deformation and at low applied stress (75 MN/rn2). The Izod and C h a q y tests are srill [hi. standard tests of notch toughness. lr: the Izod tea, the specimen is a square bar (10 x 10 mnl), in which a 45' ilotch, with u root radius 0.25 mm, iz cut across one face, :o a depth of 12 mni. I: is held verlically in a vice; the top of which is level with the notcli, and the fi-ee end s!si~c!<,on the notched side, by a 27.2 kg pendulum, moving at 3.5 mls. The c n e y y (ir~Joule) absorbed in fracturing the spccinlen or bending i t to a!low fiee pasxi!!:; of' thc pendulum. is obtained fioiii the decrease in amplitude of the l l ~ I . Ctmpy test uses a similar size of specimen, witli a notch cut in
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Advanced Marine Engineering Knowledge Vol. Ill
the middle. 11 is simply supporrd 21 botli ends and st[-itck with a 27.2 kp pendulum in the middle, on the opposite side to the notcli. Tllr energy absorbed 11: a test (at room temperature) was iegarded as a Sllffi~ieiltcriterion of a material's resistance to ii-acture. Nowadays, the energy ?bsorbed is measured as a function of the temperatilie. Over a transition iempera!iire range, the energy value c h a ~ g e sfairly a p i a l y from a high vrtiuc to a low one. i t is considered that a material is satisfactory in service. if used above the transition telmpereture range and unsafe below i t .
Q.i2. With reference to corrosion in See water systems, briefly discuss :
...
Cavi!atic~ 4 t t z s k
b.
'Rate o f Corrosion' Attack.
c.
De-zincification of Brass
d.
Graphitisation
e.
Stress Corrosion
f.
Impingement Attack; Snnd Erosion
g.
Bacterial Attack.
,,
Am. Corrosion Cells If one part of z metal surface is esposed to a higher concentration of oxygen than another part, the higher osyoenated area tends to become positive and r1:e lower area negative (anodic), w i ~ hcorrosion of the latter (anode) taking place. This is refe-red to as Electro-chemical corrosion. Any variation in Ion ccncentration against a mrtaliic suriace will give rise to similar corrosion. Cavitatioa Attack Cavitatian attack is due to t!x hammering effect of streams of vapour buihles, caused by turbulence. wliich b m and collapse, over a sniall aiea of metal surface. This airack not only removes any protective films, but physically removes n~etal,until a hole appears and the component fails. A common example is a partially throtiied sea-waler value. The rate of corroxive attack depends on how much curien~,per unii ar-e;~. acts on the anodic area. (current density). The current is less, if the two nletais are close on the galvanic table. A Iaqe copper sheer nailed down with steel ins
-
- ..
-
Advanced Marine Engineering Knowledge
Vol. Ill
would result in a large cathode area (copper sheet) and a small anode area. Thrse would be an intense attack on the steel pins. liowever. if a large sheet of steel were fastened with copper pins. the attack on the sreel sheet would be negligible, as the anodic area would be very large, as compared with the small cathodic area of the copper pins.
Dc-zincification Of Brass : This is the removal cf the zinc from tlie brass a!iciy, leaving behind a porous weak spongy copper. Certain brasses can be inhibited from this attack by the additior of zrna!l amount of arsenic. (Single 1)hast: brassesj, The more complex brasses can not be ~nhlbi?edby this method. File addition of small amounts of tin (1%) helps to retard this corrosion. Graphitisation In sea w a t q the imn matrix of cast iron can be selectively corroded away. Ie'iving behind a Fragile sile!l, consisting !argely of graphite. This attaCK is aiicn vverlooked, as there is little chaage in outward appearance. Tne galvanic effect of qaphitisarion can be serious on adjacent "no!i-ferrous" compontnts. ?he laypi vi' yiC~phiteremainin2 is more noble than any of t k copper-alloy compoiii-m;. hence their corrosion can follow the graphitisation of cast iron. Stress Corrosion. If a brass component is "cold worked" by being bent and shaped, the mctal Is uoequally stressed. This stress can be enough to set up a galvanic coupit: ibi.Lweeii adjacent areas. The subsequent corrosion is caused by ammonia, whicl: sets u p a concentrated attack at the grain bouxdaries in the areas of unequal stress. lmpinpernent Attack This is the result of the devdopment of high speed turbulenr flow in the waccr; the flow carrying entrained air bi~bbies.This effect can occur even if the tiow velocity is loi.~;however, the faster the flow velocity, the greater the rate of corrosion. The attack on the metal follows the lccal renloval of the (nom~al) protective film. Thesites, where thc impingement takes place, become the anodic amls, b~tiichare surrounded by the large (unaffected) cathodic areas, where the filins al-c undamaged. The continuo~rsimpingement of sea water prevsnts any pissivaiing film from re-forming. Typical factors contributing to impingement attack are badly designed or as:jeinbicd systems, where flanges are out-of-line and there are sharp bends. Partial fouling, by debris and lack of ail- release pipes, can also assist the attack.
Even with sniooth flow, in a well designed system, ~. each material lias an optiil;unl w a k r speed, !o avoid inipingernent attackfAluminium Brass (76% Cu, 22"/oZn, 2%AI) .......... 3 .nl/sec ......r ....... ~3.7rn/sec Cu-Ni (9011 0 with 2%Fe a n d l %Mn)
.:.
Cu-Ni (70130 with I0/3Fe and I%Mn) ......... .r... 4.5 misec Titanitlm cart. sustain.upto- 10-12 inls and is iniperviou$ to any rapid changes in velocity. The natural cxide films can be ieinforced by iron compounds i n the water. A layer or film of hydrate ferric oxide deposits, assist inresisting inrpii?gen~entattack. Hence sacrificial anodes of soft iron not only provide protection, bclr also pro-,ide iron compounds. lion addition can be ferrous sulphate dosing into the sea-yarer stream. Slight overdosing- is not harmful, but excessive amounts can build iip the themal resiztancp to hear transfer. Protection can also be achieved by impressed current. This method controls the rate of coil-osion. by impressing an equal, but opposite current, ihrough the anode. This prrve:lts ciirrosion pf t h t anose material, as well as protects the systenr. Using the impressed current technique has the advaiitaze of avoiding the handling of 1ar;e aruounis of dosing chemicals, which could he a lhazard. ~
-
Sand Erosion.
This occurs in ~ h a l i o w waiers. T h e abras~vzeffect o i sand can ca.:sc a - "enera1 thinning or canassist corrosion by removing protective films.
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Bacteria ~ t i a c k . . . ... This can result. frcm ley-up or prolonged period .in pol1u;ed. watei-s. The . bacteria.(anaeiobic) produce hydroger. sulphide, which can result in considerabi: eorro~iono f the. ferrous cornponenu. . ~
~
. .
Q.13
Describe each of the follorving Non-destructive tests (NDT) a) Radiqgrapky. bj X-rays, Gamma Rays. c) Ultra Sonic. d) Magnetic psrticle test.
Ans. Radioirxplty This reveals rhe presence and nature of discontinuities, in the interioi of welds and castings. Short wave radiation s ~ t c h a sx-rays or gamma rays ai-e passed through the object being examined, and the shadow formed is studied. eithci- on
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Vol. 111
film placed &hind the object. No1 ail a fluorescent screen or on a the radiation penetrates the component, sonic being attenuated (absorbed), the attenuation being a function o f the density and thickness o f the weld. Blowholes, cracks and such defects absorb less radiation than docs the sound metal, and their posiri'ons are marked by regions o f contrast, dark or light, depending on the type o f image obtained. Defects, w small as 2% o f the object thickness. can be detected. and this sensilivity c m be checked by placing or; !he side o f the object, stepped pieces of meaa\l e.g. 0.1,0.2,0.5. 1.0 rnrn to provide contrasting degrees of absorption. X-Rays are generated when a %ream of electrons from a fiiarneni is suddcnly stopped, on strikinx a metal target. The quantity of x-rays emitted. depends on the electron cunent, measured by a millimeter. The quality of the rarliaiinn depends on thz valthge applied to the tube - increasing the Voltage produces harder, rnoie psnerratinz x-rcys. Penetration powcr is given by the thickness of the material required to have the amount of radiation -e.g. for steel. 1000 Tube Voltage (kV) I50 250 Half value :hichess (mm) 4.06 7.00 15.49 car^ is ti, be ~sercisedwith the use of x-rays. a s radiation above a certain levcl cart cause harm to ihc body. G a m m a Rays
This radiation is used for outdoor work and in confined spaces, since neither electrical power nor water supp!y is required, X-rays are produced by d~wornposXonof radioactive substances - but due to decay, the strength of the sour%: decreases with time (Half life). Iriditur, 192 is roughly equivalent to a 500,000 V x-ray set, a s regards per~ct~xting power and has a useful life of 40 days. Cobalt 60 has a half-life of 5.3 yean arid is useful, where a greater pcnefrating power is required. c.g. 60 mm or grre3.m thickness of steel plste. Typical faults revezled by radiography incixdc rim-rnztollicl inclusions (e.g. slag), porosity, cracks and other weld defects, such as i!~adeqitatejoint penetration, incornplere fusicn, casting faults such a s sl~i-in!cagea r ~cavities. i
-
i'his method uses the reflection of sound waves. Being safer for the o!)eriitor, tllan x-rays or other radio-active means, this method is be coin in^ mot-e popl.ilar, tilie to its environmentally friendly nature. However. its limitation is t!ae ext;:iil ul' area covered. Pulses of high frequency sound waves are applied to :he cornpolleiit under test, by a peizo.electrical crystal.
Advaiiced
frlarine Engineering Knowledge Vol. 111
The electrical pulse protluced by the instrument is converted, by a transducer, into n~echanicalvibrations, or sound waves. The sound wave is in the .ultrasonic' frequency range of I - 5 mHz. The sound bean] generated is introduced into the component under test, through a liquid (usually a film of oil), which excludes air and permits the passage of sound. In the intervals between pulses, a crystal detects the echoes e the component, or from any flaws in the pat11 reflected either from the far e d ~ of of tlie beam. The signals received are siiown ori a cathode ray tube (CXT), which has a time-base connected to it, so that the position of the signal, oil the screen, gives an indication of the distance between tlx crystal generator, and !he surface from which it originates~ By inoving a probe siid
Fluorescent : Highly fluorescent liquid with good penetrating qualities is applied ro the surface, and is drawn inrr, small surface openings, like cracks, by capillary action. When penetration is complete, the excess penetrant must be removed, to avoid interference with actual defect observation. A developer, which draws penetrant from a defect and produces fluorescent indications under U~ V. l i ~ h t ,is the:, applied. A dry powder method is commo:ily used, but a colloidal water suspension may be used, applied by dipping or spraying, followed by hot air d~ying.
Dye Penetrant : 'Jses visible dyes. rev:a!ed way to the fluorescent process.
by chalky dtveloper. in a siiniiar
Magnetic Pnrticle Inspection A magnetic iield is established in a colilponent of ferromagnetic material. Disconti~iuiticsin the component cause a break in the path of magnetic flus. so
Advanced Marine Engineering Knowledge Vol Ill
that minute poles are established at these discontinuities. These poles have a sirongel- attraction for fine-n~agne!icparticles, than the surrounding 'sound' parent niatei-ial. Thus. the positions of defecti are ~cvealzdby observation of magnetic jmr?icle distribution or lines of force. which out-line the irregularity. Defects which may be detected are surface cracks of all kinds, sub-surface cracks, weld faults such as incomplete fusion. The magnetic particlr methods has the advantage, that it will reveal defec~sunder thin paint films or plating; it will also I-weal those defects that are not open cracks and therefore not detectable by dye pew:[-ant, e.g. :!lose filled with slag, i t rvill reveal subsurface f l a w . and is h e r and more ecor.omica1 than penetrant inspection, and requires less clean in^. Sowever, this rnelhod can only be used with Fel-ro-magnetic materials - i t cannot be used for non-ferrous alloys or austriitic sreels. Also, any differences in i?!agiictic characterisrics of ma:erials. in dissinilar metal joints, may cleate discontin~iities,whici~elroneously indicate fauits. The outlines of s~b-surface fli~wsmay not be acctirate. since tli; sensitivity drops rapidly, bz>oi:d zbout j mt:~ hclow the surhce Typical processes use DC, AC or rectified cuirent, usual!). hi:h amperage a d low vc!tage, fiequenrly applied thorough two probes. \\;it11 liiizly divided Ferro-magnetic povider particles ( e . ~ .iron i i i i n ~ s )bein2 applied dry or froin a suspension.
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I'.:lciy Ctirrent (Electromagnetic) Testing
Eddy current tests are those, requiiin~the area under test ro be s~~bjecteci to \he influence of an alternating elecrroinagnetic field, and can be used to deteci swhcc cr sub-surface discontinuities, that may occur in the form of cracks. scnirvs, voids and s o on. The effect of an Electro-magnetic fiei.i, on the !est area. may be two-fold : eddy currexts are induced and. if the material is magnetic. ina:pt:tic fields are set up These two effects. in i m g e t i c materials. may not he rcatliiy tlistiiigiiished. but with experience, caxfiil sekction of the mngne~izing ficqucncy, some discrirninatioii may be ctbtained. Muii-mapetic materials 111Eddy ccrrent tesis; the magnitude and direction of the eddy cul-rents are aircierl by ciiscs:itinuities in the metal, and any such change is picked op by a ciekctor coil, that acts upon appropriate electronic circuitry, ro ie:isrrr the tliscut~liruti~y Since eddy citrrcnts may he induced in any conductor. magnetic as wcll as rrowmagiietic marerials can be invesdga~cd.For the laner. an alrernatii~g eli:i;ti.or~~agneric:i: iidd is usually produced Sy an inductor of suitable shape. in close proximity lo thc tezt area.
Advanced Marine Engineering Knowledge Vol. 111
Wlngnetic materi;tls The distribwion of magnetic flus is affected by discontinuities i n the inateria;. changes in the eddy current or the magnetic flux may be interpreted bv means of several different variables - voltage, current, impedance, ph2se or seine combination thereof, a d these variables are analyzed electronicaily, to provide the desired informstion in a usehl form - by comparing faulty and sound material respoilses. The involves sxrounding a coniponent with coil(s) and moving [he two, relative to each other. The frequency of the e.m.f. fieid depends on t l ~ e3,-ptll oipenctr;.tiun, but is usually in the ranze of 500 to 20,000 Hz. Give the analysis of a cast-iron considxed sui:able for cylinder liners. State the mechanical properties of the cast-iron. What impurity n;ust he kept to a minimum ? Ho!v can the tensile srr-ength be incl-exsed incf tile :-esistnnce to wenr bc i n ~ p r o w d ? Q.14.
f\lii.
Analysis sf Cast Iron :
Carton
2.0 YO
Man,c'anese
1.0%
Silicon
0.7 %
Phosphorous
0.4 %
Su!p!lur
C.! "/o
The Silicon content shou!d be kept as low as possible. Silicon pr31110te~ growth in cast-iron, when subjected to continued heating. Cast-ii-on, liaving ;he above analysis, has a tensile strength of appiasimateiy 216bfNIm'. The tcnsile strengtli of cast-iron is iornetimes increased, by the additioii of mild steel scrap to the pig iron chalge, in the fwnace. Alternatively. sinr.Il ainounts of c h r m ~ eand nickel may be used. The wear resistance can be ilnprovrd by small additions of vsnadiun~. Q.15. gescribe the composition of white-metal bearings~
Ans. The tin content for~nsrhe matrix. This ma!& is sufficiently sofi to acconimodate the small changes in alignment betuxen the journal and the beal-inz surfaces. The antin~ony forms cubes or cuboids, which are very hard. 1-hese cuboids take the load from the journal or pin and transmit i r to the s u p p o r r i n ~
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Vol. IN
tilatrix. They also have a high resistance to wear. When the bearing is being cast 3 r d the white-metal is in molten state. the antimony ctbes tend to float and co:igiornerare, a process referred to as 'segregation'. The copper constituent prevents segregation. The copper has a high melting point and solidifies firs, forming long needles which interlace ic a crisscross pattern through the liquid tin. The interlaced copper needles hold the antimony cubes, in an evenly dispersed pattern through the tin matrix.
I I
When the white-metals ingots are melted, prior to casting, es:reine care ! n u s be exercised to p v e n t overheating the metal. Care must be also exercised, when bearings are centrifugally cast, to prevent separation of the consrituer~ts. Cadmium improves the toughness of the bearing metal and helps prevent fatigue.
Q.16. Give the analysis of white-metal, suitable for diesel engine Main and Rotlo~~i-end bearings What are the special requirements of the xvhite-metal ustd in cr?ss:tcad hearin23 ? >\?s.
,Anal-/!;is
Tm
Xi
-
38 ?A
~ntinlony
?-lo%
Copper
4-5
YO
The white metal used for diesel engine bearings, is produced by metal refiners and sold u d e r various brand names. 'flir: loads placed L i the crosshead bearings of modern engines are extt-cmely heavy. The requirements OF the white-metal depend on the design of thc b::ar.ing. For example, very stiff crosshead asseniblies, with a thin whitei!l?tk!l layer, require a different netal, from a more flexible cssembly with a thick layei. Thc requirements, for each bearing, nwst be formulated initially h r ? previous experience, and then modified following service experience, to g i w the desired characteristics.
L i s~ :. W .*,+A + $:
.
38 %
Tin Antimony
8%
Copper
4%
Cadmium
traces
y<
.$&
For modern, slow speed, highly rated crosshead engines, a white-metal is analysci! as follows : 111mIysis
-
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126
.* -.
Advanced Marine Engineering Knowledge Val. / / I
Q.17. Wlint a r c pi;~stics?Where is use made of plastics, in diesel ertgincs and ancillary equipment? Name the p1:rstic used.
. arc Plastics are made u p g f lono, cliains of identicai- moleciiles. i ~ e they polymers. The cotisriruent moiec~rlesw e c a ~ b o nand i ~ y d r o ~ ecompou:xis. n and imay also contain a wide i-anze oisoal or petro:eum oils. Plostics have so far not been sjed for any 31 the rnajor components of dies;\ ensines. Their use is limited to insiru~nen~s and electrical 5ttin:s. The Nylon This iliareid is iised for small bushes and ~earwlieels it1 i~isliulnents.I t is to:!fh: lins n lo\\, coi.fZzli.ni o f li-iclion. and can be machined from rod 0'- piale sections. it is also inilde into fihres. I'oly-mriiryl-nierha-zcry!ete (Pel-;pix). This is a clear vlasric, c o n ~ n > o n ! ~ me6 for instrument glasses, ieve! gauge tubes (not for boiler or high presscrsteam service). arid Gear case si$i glasses.
Poly-tetra-flirro-etliylene (PTFE or- 'I'eilon). This rnatei-iai has a very low coefficient of friction and rezistslieat; i t also has soud chemical rcsisrancr in the presence of oil. I t is used for glanii packing or for coalina coriveniional soft packins. Due to its low fric~ionairesistance. it may be used as a packins hi- pump shafis with !ligh rubbing speeds PTFE is supplied in moulded 0rings, or in shtet or shredded fom:. I t gives se~wicein pump and valve g l a n d s 11 1x1s also been used as an additive in m i l e l~ibeoils. to reduce the fi-iction at start-up Phenolic resins. These resins are ilornially bonded with linen or othm fibrotis materials. They a:-e used hi- bearinss in pumps and on screw si~afts. They swell i i i warei, an acrion to be considei-ed, when calcularin~ bearing i.!easances. One material, which gave ~ o o dservice in old water-iubricared bearings is Tufnol. A wide range of plastic matel-ials are ~iscdlo make marine paints and
surface coatings, t!ierrnal ir~sulation,sound absorbcnls andhohding cements.
\ZZGnd Q.1.
fuels
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I n the case of marine fuel oils, justify the need to have proper ~~erifications. State typical fuel specifications data for a two-stroke main propulsion engine..
Ans.
H e a ~ yfuel oil (marine) Standards have formulated by various o r g ~ ~ i z a t i o e.g. n s IS0 8217, L3S 6843, and CIMAC, which give limi~ingspecifications for each grade of fiiel oil. Engine nanufacturers usually specify the required fnd parameters, for use with their e~gines.Specification for fue!s would indicate: Viscosity Density Flash point Pour point Carbofi resiche-
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handling prehelting and centrifuging usually measured at 15 OC fire risk factor. solidification in tanks and pipelines. J fouling of gas ways and piston rings. Ash abrasion. Water sods!m content of salt water. Sulphur corrosive effect '/anadium undesirable by itself, but when present with sodium, leads to exhaust valve corrosion, cylinder m d turbocharger deposits, which can cause overhcating &nd failure. A mass ratio o f 1:3 o f Na:Va can be iroublesome, especially with high vanadium content. for distillate fnets: a measure of igcition Cetaue surnberquality Cataiytic finesabrasicn A typical fuel specification for a main propulsion marine engine : ncnsiq 991 Kg/m3 (maximum). 7iscosity 700 Cst at 50 OC (maximum). Flash Point 60 OC (minimum). 22% by weight [maximum). ~ o n r a c i s o ncarbon 14% by weight (maximum). Asphalt sulphur . 5 % b y weight (maximum). 1 % by weight (maximum). Water Ash 0.2 % by weight (maximum). 30 m g K g (maximum). A!urnZitium '/madium 600 mgKg (maximum). Sodium . 30% ofthe Vanadium content
Advnrrced ,Marine Engineering Knowledge
YoL 111
Q.2. What are the general parameters to be considered for selection of lubricating oil ?
Ans. Viscosity Oxidation
measure of intermoiecular fiction reaction with oxygen, which forms sludge and acids Flash point the temperaare at wnich an lnflvnmable mixture with air is formed. Neurralisation Value - the ability to neimaiise acids (TBN No.) Foaming Mixture with air causing Cavitation and he3i transfer difficulties. Detergency the ability to prevent deposit fomaticn, by washing them away Dispersancy the abl!ity to absorb particles in suspension. As ? general rule, [rmk engines require a higher TBN No, since thpre is more coutamination of crankcase oil by hel, i~eql~iring mcre =ti-oxidant and detergenUdispersan addi:ives. -
Cylinder oil : Cylinder oii should have gcod. detergent, dispersant propefijes. Also, there niusl be adequate film strength, boundary lubncaiion and oxidation qualities. Iis neutralising qlialitj is given by in TBN Wzmbcr, which depends on the Sulphur coqent oithe fuel, among cther things. Write a brief note on the adverse effect, that a fuel containin%high values of each of the fotlowing, may cause:a) Vauadiurn. b) Sulphur. c) Sodium; d) Ash particles.
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Vanadium Coo A hard, white metallic element, densty - 5500 kgim3. Melting point (of the pure metal) - 1710°C. In marine fuel, it is tied up in the covalent bond structure of the hydro-carbon. which means,that it can not be removed easily. In thkcombusti& process, ii 'can readily combines to form a variety of 'low melting point' compounds, typical of which are : Sodium Metavandate, Sodium Vandate and Vanadium Pentoxide. These compounds, when in the liquid state, can do a considerable amount of damage by liquid metal anack, the main effect being from the Vanadium. This corrosion is rapid with steels but no metals is immune. The process is commonly referred to, as 'high temperature corrosion'. In the solid state, the Vanadium compounds adhere to the metal surfaces;forming needielike deposits. The build-up can be very rapid. p-eh+ sodium l l e d ~ k u U/V COI-S:OQ
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d d v o n c ~ dMurim Engineering Knowledge
Vol. IN
Note: Dust from Vanadium deposits is an imtant to the respirarory systein, it is necessary to provide protection, when cleaning. M0.x SX A".;, Sulphur : Sulphur dioxide, &om the combustion process, is an air pollutant, besides which, it forms acids, especia:ly at temperatures below the Dew point, which is called as 'cold end' corrosion. 2S02 + o2 = 2.S0, V,O~(Catalyst) so, + H,O = E2S0, (acidj. Tnis ~rocessreiies an cozdensation, hence Loth,the ~ a r t i a~res.sure l of the ;vater vapour and the temperahire of the exhaust gas, are important. The presence of free sxygen is required. Rcoocing of the 'excess' air in the combustion process helps in retarding the fennation of the danaging acid, and also helps in woidino, the catalyct action. The use of 'load contro!lcd' coeling has limited the formation of acids, whi-h has significantly connibuted to the reduction o: the 'cold end' conosion problem, however proper fuel treahnzn: is still essentia!. r($o; o[ t h e ;/anadill* cOflM-?
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,/Sodium ': This is a constituent of fuel, and 3ppears in almost all the harm&!, nom.dly part of the moltcular structures of the fuel, so its removal can be achieved.
., low melting point compounds.Sodium is no:
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Ash particles : These consist of earth and metal contarnicwrs, which can be suspended in the oil, like sodium, or may be a part of the chemical stiucture, like Vanadium. In the latter case, they cmnot be sntrifuged. out. They tend to melt or soften in the flame, then solidify on the first cool metal surfade they meet, contributing to slagginer gas side fouling.
4.4.
Discuss the significance of Calorific values in assessing the standard liquid fuel. What is the adverse effect, on Cud quality, of high Asphaltenes and c) Ash. - valve of a) Carbon Residue, b)~.
Ans. Calorific Value : The heat value for carbon is 34 MJkg and for hydrogen, it is 122 MJkg. iIence, for molecules with the same number of Carbon atoms, !he Paraffic molecule, with its greater number of hydmgen atoms, will have a larger heat release, than the equivalent Naphtha o r Aromatic molecule. The Paraffin moleiules are larger than the corresponding Na~hthaor Aromatic molecules, so there are less of them, per unit volume, i.e. the density of corresponding Naphtha and Aromatics is greater than the Paraff~n. Hence there is a relationship between Specific gravity and Caiorific value, the Fuels with the lower Specific gavity having the higher Calorific value. The Calorific value is estabiished with the 'Bomb Calorimeter'.
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Carbon Residue (Conradson Carbon) : This is a measure of the carbonaceous residue remaining, after destructive distillation of a sampk of oil. It gives an indication of the graphite carbon forming tendency and is a 132
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guide lo the Tale of fouling. Piston ring sticking and slagging in boilers is attribstable to high va!ues. ~ t a b i f i t yand Compatibility : A fuel oil can be looked upon as a dispersion of Asphaltenes in an oily medium. In a stable fuel, the Asphaitenes remain in suspensiorr. However, should the equilibrium of the suspension be disturbed, [oossibly by- mixing .. with a second fuel, which (althoueh - bv. itself stable], i--s ncverthe!ess incompatible with the first fuel], precipitation of Asphaltenes will nrclir --.-.n~ .. slndee. .~ Instability can be caused by heating but is unlikely to cccur unlzss the application of heat has been excessive and prolmged, s i x t most icputabie suppliers manufacture their products to provide a reserve o f stability. A refiner cannot always manufacture in such a way, as to guarantee coixpatibility between fuels from different sources. Thus a risk exists, whenever two fuels are mixed. Shouid excessive sludge 3ccur a s a resu!t o f instability Dr incompatibility, it may stan to fall our immerlia:ely in storage tanks, heaters and pipelines, causirg the over!oading of centrifuges aid the clogging of filters. Fuei oil on-board test kits provide a simple way to test for compatibility, using filter p3Ger, however the s a m ~ l etested ashor? in a laborarory will be able to give a more sccurate analysis. ln m y case, it is prudent :o avoid mixing of bunkers - as far ac practicable, store difieienr bunkers separately and change-over anly after finishing-off one type.
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Ash : The residue, free from carbonaceousmatter, which remains afier is burned in air, is referred to as ash. The content and nature of ash depend largely upon the source of the crucie oil and the concentrating effect o f the refinery processes uscd, in producing the residual fuel. Ash-foming constituents include aluminum, calcium, iron, nickel, silicon, sodium and vanadium. These may come from sea water, scale from tanks and pipes, dust, dirt, catalytic iines, in addition to those elements which occur naturally in crude oil. Most of these elements exists as oil-insoluble matter in the &el, but nickel and vmsdium are usually present as oil soluble compounds. The source of Aluminum is usually the catalyst uscd in secondary reining techniques. The presence of aluminum s e 9 y indicates the fear of catalyst fines (from a caralysr cracking process), w_:lich have been known to cause extensive damage to fuel pumps.and -. liners> . Q.5.
With reference to risks of shipboard fires, elaborate on the importance of 'Auto ignition' temperatures of fuel oil/fubricating oils.
Ans. Auto Ignition temperature : Any petroleum vapour, in the flammable range of concentrations can be ignited by a flame or spark, provided that the spark has sufficient energy to initiate ignition.
Advonced Marine Engineering Knowledge
VoL III
For such means of ignition, liquids of intermediate aqd high volatility, are the most hazardous. Petroleum vapour, howzver, will ignite, only if a subs!ar~iial body of it is raised to the auto ignition temperature, which is very I much lower than that of a flame. The minimum temperatures, for 'auto ignition' vary with the hydrocarbon content of the fuel. Typical 'auta-ignition' temperatures are: Fuel oil vapcur . 260' C Lube oil vapour 280 C Low Octane Gasoline vapour 390GC High Octane Gssoline Vapour 470 C Methane . 650 C !t will be seen &om this, that the heavier and thus the less volatiIe the hydmc;lr5oc, the !on~eris the 'auto-ignition' temperature. iri @is context therefore, the vapours of !he more volatile petroleums are ihe less hazardous. Fortunately, vapour requires to be at such temperatures for a long period, before any hazard exists.
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What are the basic purposes of lubrication? Differentiate between d n s r a l lube oils and compound oils. What is Grease and where is it useful ? Explain the foilowiog terms : a) Scuffing. b) Pitting. c) Emulsion. d) Oxidaiion. *): Lacquering. 'What factors can lead to corrosion in White metals bearings ?
Aris. Lui)r.ica?ion: The primary purpose. of lubrication is to reduce friction and wml: i t also helps to keep surfaces clean by carrying. away deposits. In case of pis tor^ iirlgs, the lube oil film provides 2 scnl for compression, and nlzo helps to keq) o l ~dirt. ~ t Besides this, the lubricant carries away heat and thus prevents szizure. ?diueiril lube oils : These represect themajority of lube oil used on board. basi: stocks are obtained from thedistillation of crude oil. Cornpotind lube oils : These consist of 5% to 25% animal or v e e e-.oils ( n o ~ i r i r ~ e roils), a l added to the remaining quantity of mineralox
C r e u e : This is semi-solid lubricant ofhigh viscosity having a filler and soap. i. I t remains for a longer time on relatively slower moving surfaces . I! provides essential lubrication, where there can be no hydm-dynamic 2. lilrn, due to slow speed. It also provides a seal. 3. 4. It is easy to lubricate parts which are inaccessible I difficult to reach..
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Advanced M o r i m Lzrgineering X n ~ w I e d g e Yo/.I11
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Terms used in lubrication : Scuffing : This occurs when there is a break-down o f lubrication or oil flow between surfaces, causing microscopic tack welding. Usually found on the cylinder liner surface, where the lubricant film is diflicult to maintain, due to the adverse conditions in the cylinder during combustion. Pitting : This is seen as minute pits or cracks on bearing surfaces. As a result of a constant high pressure, fatigue prcdiices minute crackirg at contact surfaces.
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Emulsion : When two or more liqrids are non-miscible, then L.ey rend to form an emulsion. Lube oil which is conlaminared, or has deteriorated to jgch a d e g e e that it will not separate &om water, can form an emulsion. Oxidation : Some substances have an affinity for oxygen, and combine 10 form their oxides. Lube oil can get oxidised, due to overheating, producing compomds which may be acidic and cause severe corrosion. Lacquering : These are hard deposits firmed on high temperaturc regions, from this lacqucr layers. On coolers surfaces, sludge df a softer nature is more liable :o be deposited. E.g. ?iston skirts, ring grooves, liners. bearings are tin based. Corrosion 31 White metal bearings : Wh&metai Tin can corrode, if an electrolyte is present. Hard, brittle, tin oxide layers occur in patches. This oxide layer is twice as hard as steel, can damage both the bearing shell, as well as the journzl, if it becomes detached. ~h~ formation of the oxide layer can reduce clearance, cause overheating and seizure. Factors contributing to the formation of tin oxide : 1. Bouadary lubrication, such as at start-up. 2. Surface discontinuities. Eleceolyte (water or other contaminmt). 3. 4. Gil temperature. Stress, in the bewog material. 5. Additives offer somc protection, but care must be taken to avoid oil contamination, which can result in a reduction of additives.
Q.7. What a r e the different types of wear found in marine diesel engines ? Explain the ways in which they can be minimised. W h a t is boundar y lubrication ? Explain the term 'hydro-dynamic lubrication'. Ans. Wear is basically of two types : a) Abrasive wear, and b) Corrosive wear Abrasive wear depends on several factors, such as the quality of lubrication, the size of impurities and the surface finish of rubbing surfaces.
Advonced Marille Engineering ~ t m k e d g e Vol. IrI
When observed under a microscope, even supposedly 'finished' surfaces have an Llneven surface, which consists of 'hills' and 'dales'. When these su-faces ~ b the, points of contact are severely loaded, as the coniact patch is not suff~cientlylarge to dismbute the lqad evenly. This resol!s in wear of the 'high spots', which is part of abrasive w e y Additionaliy, there may be micro particles entrained with the lpbricanr; whlch fuahcr aggravate the problem. Factors such as suddcnly increasing load, a poor surface Snish and insuff~cientmming-in period, resuit in large asperitizs 'work hardening' and breaking-off. Usually these particles are &om the softer material. Ivricrc panicles can ozcasionaily bridge the gap and smooth off the tips of as;jer'.ties, in a sort of 'ginding' process. This improves the surface finish by increasing the contact area, and is what happens when a component has been propedy 'run-in'. Corrosive Wear : This is wear that take2 place as a result qf the col-iosive action of acids, usually formed during the combustion pfocess, by the combination of oxides wirh the cordensed water. This resplts in surface ii::rerioration, by chemical attack, the debris of which is responsible for furthe! wear. Condensing acid vapours 'etch' the -surface, giving loca: area reduction, a d accelerating mechmical wear. Acid can be formed from the r:omhustion products, or from the oxidation of M e oil. Tbe Formation of acidic vapour is assisted by heat &om 'friction welding' of adhesive type wear. Elasto-hydrodynamic lubrication : The formation of a fluid film, which su$~(~orts the shaft and prevents contact behvee.~ metal stlrfaces, is what is reit-rrt-d to a s elasto-hydrodyamic hbrication. Local pockets of lubricant are trapped and the viscosity alterj locally, duc to the high fluid pressure. Thc surface of the metal localli suff&s elastic deibrrr~ationgreater thaz the averzge value, this t e ~ d sto seprirate the points of dosest approach. At start-up, the rotating shaft attempts to 'climb-up' the bearing. Mm-to-metal contact occurs. The only lubrication is due to any retained oil, tt-iippcd in the surface asperities. T h i s absencc of fluid fiim at start-up coiili-ib~ulr;sto the greatest amount of abrasive wear of the bearing. ;'or a bearing of given dimensions, the distance betwqert tne jouma! arid bearirlg, at the point of closest approach, is dependent oh a number of fi~ctol-;,siicit as the lubricant viscosity, the rpm and the shafl lqad. This value deter-rnir~i:sthc type of fiction in a bearing. At small values, thezfriction canses the; bsatirq; to run 'dry', and the bearing is operating under 'boundary lubrication'. It' the value is larger, there is a Ulin film, which may not be :;i~liicienlro takc thc load. When this value is large enough to ensure compleie st:piti.alio
Q.8. Describe how, as Chief Engineer, you would monitor the 'health' of the engine room machinery, using Condition monitoring. W h a t is 'performance trend analysis' and what a r e the means to monitor them ? hs.
The input datz for a computer-based monitoring system is From sensors. Typical sensors measure parameters like Eequency, amplit&e, velocity and so on. These would be continuous measuring devices. The r e s ~ l t s fiom The sensors ran be mdtched against stored data, for most of the engine room mzchiner]. Ferrography : This involves the separation of wear debris of fermus t p e magneticaliy, and arranging in the order of pariic!e size i^ol examination. A sample of oil, for example, is diluted with a solvent, which b r e k s down any gel, which may be around the wear particles. The sample is fed to a transparer,: precipita:~~ tube, cn either side of which are the polss of a ma.met. The magnetic force attracting the paticles is pro?oitional to the size and magnetic susceptibility. Thz larger particks are depozited at the entry rzgion of the tube, and the smaller particles and the oxides of iron gci deposited later on in the tube. The amount of material deposited is measured by the attenuation of light from a !i$ht source placed below the tube. The light passing is tietected by photo-electric transducers. Wear severity is indicated by difference between optical density at two points, distant 5 mm from each other, along the tube length. In normal rubbing wear, the majority of the particles are small. As the severity of the wear increases, so does the number of bigger panicles. In a Ferrc-graphic analyser, the oii containing the wear debris is sorted by size along an inclined glass slide. The magnetic field gradient increases as the particles move downwards. The larger particles get deposired first. The slide is analysed by a n optical microscope. Red light is directed through the objective of the microscope. Opaque objects appear red. Green light is transmitted through trensparent objects, which appear green. Other &jCcts appear in various shades of ye!low- green. This helps in distingcishing metallic objects from oxides. Further, the slide is heated to 330" C, when low carbon and alloy steel particles appear to be bluish, Cast Iron appears Ii&t brown in colour, bronze appears d z k brown, and chromium and aluminium are bright white colours. Thus particle identification is possible. Spectrometric Analysers This technique is called SOAP ( Spectrometric Oil Analyses P r c g a m m e ). Oil samples containing wear debris are taken from the sump and analysed, by any of the following methods : Atomic absorption spectrometer Emission spectrometer.
The Atomic absorption spectrometer is based on the principle that, -very atom absorbs light c f its own specific wave lengfh only. From the combiistion of the oil sample, metailic elements in the oil sqnple are atomised. In a iight beam passing through the flame, certain' wavelengths get extinguished, due to their absorption by the Eree atoms o the metals in the sample. Since each element has its own characteristic wave length, and i t is pxzible to measure the amount of light absorbed, w e are able to get the concentration of various elemznts, in the oil sample, in parts per million (p.p.rn.). The monochromator is tuned tc accept li&t of a certain wave length. A meter indicates the difference behvecn the originai light received directly by the light source, and that received by the monochromator via the s a q l e . The difference is the amount of light absorbed, and indicates the concentration of the particular element present. The procedure has :o be repeated for each of the elements to be detected. In an Emission spectrometer, it is possible to analyse. simultaneously, a number of elements. A. rotating Grqhite disc canies the ojl ran?le, which is subjected t3 high voltage excitation. A spark excites the vanous metallic clem:;~~ts,after which they emit their characreristic wavc lengths, the intensity oC which is measured by various dctxtors. z E!ements which can be analysed b s this method include copper, iron, zluornium, nickel, lead, ~odian,'aluminium,magpsiurn, silicon and silver. IF the cornpositim of various components in the c d c a s e , viz. Bearing material, is h o w n , then it is possible to identify the source of the wear debris in the oil sample.
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i'article counters The si e of vxious particles of the wear debris can be determined by rncans of particle counters, which work on the light blockage principle. The oii sarr~pieis passed thou* a sensor, consisting of a light source and a photo diode. A particle paszing through the window blocks light according to its size, since light travels only in straight lines. Accordingly an electrical signal is sent from the photo diode, accortling to the amount of light re~eived,which is proportionai to the size of tiii: j)a.c?icie.The turbulent flow in the tube results in rotation of the particles, thw the probability of getting the largest particle sizeisvery high.
Corrosion Monitoring Ttit: pzrarneters to be measured are :X!i: m e of coirosion, the total depth of corrosion, or the thickness of good marerial remaining. Direc: observation is the oldest method, based on visual inspection. It im its limitations, since deep corrosion may not be accurately analysed. Chcmical and electro- chemical methods include analytical, potential or galvanic methods. These methods are used to monitor water quality in boiler feed water. The ratio of metallic ions in a sample, may indicate the extcnt of corrosion.
Non-destructive Testing methods (NDT) include radiogaphy, ultrasonics, eddy current, thennography and acoustic emissicn methods. Ultrasonic methods are being increasingly used for flaw detection and thickness measurements in pressure vessek, piping and well a s ship's side plates and other surfaces, wherc it is not possibie to drill holes or cut samples for visual inspection. The disadvantage of this methcd is that only a small surface may be tested at a time, thcs large area coverage is not possible. Radiography is ussful as permanent records can b e kept, howtver it is slow, and hazardous. Eddy current technique is p r i m s l y a surface technique, ~enetrationbelow depths o f 5 to 6 rnm are hard to obtain, hence it is not much ,used for ship's work. Thermogaphy is used to locate increased surface tempera%ies due to carnosion damage in furnaces. Elecrncal resistance technique is based on the principle that, as colrosion decreases ;Es crass- seciion, so its electrical resistance increases. I; it is possible to record the readings on a continuous basis, it is t corrosion rates, thus simplifying the maintenance, by possible to find ~ u the , d i n g preventive actions, before corrosion can develcp to extreme limirs. A?*
P&formance ..:.. Tre@ monitorjng , ~ o n i i o r i n gthe perfoimance of a system involves the measwtment and recording of relevant parameters, an6 detecting any change in pnformmce. Parameters like 6x1 consumption, lube oil consumption, exhaust gas analysis, var;ous :emperatmes 2nd pressures are con:i~;uously monitored. Deterioration in fuel c.onsumption figures could be duuc I:, vaiious parameters changing, which can then be investigated and put right. Similarly, change in the exhaust gas analysis readings could point to faults in the combustion of fuel. Excessive consumption of lube oil could also be detected, by an ificrease in the wear rates of piston rings, if an on-line detecting system like S P W A is fitte5. (Rsfer the text-book 'Marine Engineering Practice' - NC: series, for more details on thi- working of piston ring wear detecting systems). Explain the following terms, with reference to thr analysis of lube oil. Suggest, with reasons, which of the following data is of use, on board ship : Viscosity index Viscosity index improver. Oxidation. Neutralisation value. Detergent I Dispersant oils. Pour point. Cloud point. Foaming point. Cracking point.
Viscosity Index:- (V. I.) This gives the relationship between viscosity and temperature of an oil. An oil, with the least tendency to change of viscosity, wi+ temperature, is preferred., To compare the viscosity/temperame characteristics of different oils, a classification method, based on two reference oils is wed. One reference oil CNapthenic) exhihits a large visosity change, with tempzaiure. This is given a qscosit!, Index, V.I. = 0. The other (Paraffinic), exhibits !ittle change of viscosity with tenlperature. This is given a V.I. = 100. h y commercial lube oil falls within this range. Viscosity Index Improvers : These are high moiecular mass, long chain polymers. These are zd6ed to lubricating oil stocks, to iapede fiow at higher temperatures. (typical examples of these are Isobutylene polymers ind Acrylate co-polymers.) Oxidation : Lsbric&~g oil, being organic, can react cliemi~alljjwi!h oxygen.
The prodccts of this 'decompositicn' are weak organic acids. Oxidation is harmful, as additionai to acid formation, sl~idgeis pioduced. This sludge tends adhere to the meta! surfaces, particulariy in the high temperature region., leading to layers of gum or iamish farming. Certain metals, e.g. Copper and snme lron oxides, are active catalysts in the oxidation pr~cess.However, the main problem is due to e~cessively high temperahire of lube oil. The rate of oxidatior. is doubled, for each 10 'C rise in temperabe. The by-products of oxida:ion are acids and varnishes, which lead to an increase in the viscosity. The basic precaution against exidation is with the right choice of base Sock, from which the oil is manufactuied. This base stock is based on the "Saturated" hydro-carbons, such as paraflins. The oxidation of lubricaticg oil is a chain reaction, involving the initiai f ~ m a t i o nof peroxides. Oils containing carbon have an affinity for these peroxides and combine with them io fonn harmless compounds. In doing sf>, they block fwiher oxidation. Tjvical of these chemicals, which can be used in additive form is Zinc Dialkyld-thiophosphate. Others include Phenols and Aromatic amines. to
Nelrtralisation Value : This is a measure of an oi!'s ability to react with an acid reagent, to achieve complete neutralisation. This is measured as the Total Base Number, TBN, or if measured with a base reagent, it is the Total Acid Number, TAN. The results are expressed as mg of Potassium Hydroxide &OH) per gram of oil, for both TAN and TBN. Detergent oils, in Trunk piston engines, would have an average value of TBN of 25 - 30 ing0(OK)lgrzm. Cross-head type engines use cylinder oils with a T.B.N. of 70 - 80 m~O(OH)/gram.Measuring the acidityialkalinity of the oil is important, when considering its use with fuels of known sulphur content. Additives used are complex compounds, containing a metallic salt, such as Calcium Carbonate (CaCO,).
Detergent - Dispersant Oils : The addition, of a detergent additive, prevents burnt combustion products frvm depositing on piston rings, by washing them awaywith the lubricating oil. To prevent them from coagulating and thus depositing dsewhere, detergent oils usually have a dispersant. additive. This additive enslues that the harmful combustion products and vamishedgums from oil oxidation, are kept in siis~ensionand dispersed evenly throughout the oil. These can thm be removed. These additives are complex compounds, such as mctallic-based su!phonates and phenates. ?cur Point : The pour point of an oil is 30 OC a b o ~ ethe temperature, at which the oil just ceases to flow, under prescribed conditions. Oils require to be about 8 OC above the Pour point, for pumping purposes. Cloud Point : This is the temperature, at which a haze or cloud appears, when the oil is coc!ed, under prescribed sonditiol~s.This 'cloud' is caused by :he iomation c f ax crysta!~.These may prevent the oil flowing. Agitatiol can break cp the wax cryst-I:, allowing flow. The cloud point c f an oil is usually applicable to parafEn-base oils. Foaming : This is csually due to a failure in the design or due to maloperation. Foamlng can causelpumps to ;ct an air-lock, which coujd result in oil starva;ioii and beving failure. Cracking Point : If the oil is heated to a suficiently high temperature, the hydrocarbon molecules break-down thermally or 'crack', into a greater number of smaller molecules.
0.10. Describe, iu brief, the tests carried out on a lubricating oil, to determine its effectiveness, for continued usage, in marine ecgines. Ans. Testing of lubricating oil (a) Crackle Water Test : This is a simple test, to quickly determine i f xater contamination is present, in a given sample of lube oil. Shake the ~ i l s m p l e vigorously, before placing 8-10 drops in an aluminium foil dish. Eold the dish over a naked flame or gas. Ifa 'crackling' sound occurs, this indicates the presence of water. This test can determine the presence of water in lube oil, even at a concentration as low as 0.1 %. @)
Quantitative Water test : This is to determine the actual percentage, by volume, of water in a given oil sample, taken front the engine lube system. Shake the oil sample vigorously, then add 5 ml at the bottom of the reaction flask. Add 15 ml of Xylene solvent to the flask, replace the screwed cap and shake vigorouslv. Add the contents of two sachets of the reagent power, Calcium Hydride, to the plastic 'floater' cup and carefully place this cup on:he surface of the oiVsolvent mixture.
AdvoncedMorine Engineering Knowledge Vo/. III
Secure the screwed cap to the reaction flask and shakc vigorously for one minute. Shake the flask every 5 minutes and read tqf gauge aEer 25 mini~ies.The gauge will give a direct reading of thq percentage, by volume, of water content, up 101.2%. (c) ~ i r o n gAcid Test : This aetermines the presence of strong acids in the eripine lube. oil. as a result of the depletion o f nsrmal alkalinity reserve 1evc;s. Shake the oil sample vigoi-ously and add 6 ml of the s m p l c t9 the test tube. ThzS add 24 mi of 'Bromwresal Green' Indicator and 2 ml OF Kerosene. Replace the stogper and sh&t the test tube wgorously for e l leasi two minutcs. Aliow contents to settle and observe colour of the liquid it! the tloltom of the test !ub% A bice cdour ictiicatts the absewe of strong a i d s in ;he cil and, in terms of alkalinity reserve, the oil is fit for further service. An initial green coJour indicates strong acids in the oil and is rzgarded as a 'border line' case. An initial yellow/g~eencclour indicates the presexe of stroi?g ;ci& in the oil and the oil is uusstisfactory. (dj Comparative Viscosity : This is to determine whether a 'used' !u5e oil i s suiiahle for further service, by viscosity comparison, with a sample o i 'tiesh' oil. Obtaic a representative oil sample from the engine oil systcm, pr-cferably after the main lube oil iilter. Allow the 'used' oil sample to cool to ambient temperature. With the 'Flow-stick' lying fiat, add 3 ml of the fresh oil into the reservoir on the dlarrttel marked with 'OiO' and 3 ml of the 'used' oil into the other i-cservoir. Till rhe 'Flow-stick' on its oblique base, and hoid in this position, until tile 'fresh' oil has nearly reached the 'OJO' reference mark, on its ch:rrmel. Now quickly return the 'Flow-stick' to a horizontar position. The 'fresh' oil should ha1.e stopped directly opposite the 'W0' refsrence mark. Note the point, where the 'used' oil has stopped. If the :msirion of this 'used' oil is within the two marked lines, then the 'used' . . oil is accsptable for further use, on the grounds of viscosity. C x e should be taken to subject the 'used' oil to other checks, before coiklirrtnir~gto use it.
(e) Swoluble content test : This test is to determine the amount of insoluble [iatlicles in the 'used' oil sample, of the engine lubricating oil. Shake the oil :;ample vigorously and then place 3 ml in the test tube. r\dd 3 1111 of 'dilute' oil (or 3 ml of 'fresh' oil of the-same type). ?'hosr)il!ghly mi:i, by shaking the test tubc. Drop a spot onto the test blotter pry::% ?rid allow the spot to develop for at least 2 hours. Compare the spot swiih stewJan1 'spots' of known carbon content in the oil ofbetween 0.4 to 0.3 #x,.
(0 Salt water determination test : This is lo determine of presence o f salt water (e.g. sea water from leaking coolers) in lubricating oil. Add 20 mi of Xylene solvent to the test tube. Shake the oil sample vigorously and then add to the test tube upto 25 ml. Add 10 ml of distilled water, fit the stopper and shake the test tube vigorously, for one minute. Remove stopper and stand the test tube in hot water for 15 to 30 minutes, to enable separation of oil and water. Remove a reasonable sample oiwater from the bottom of the test tube and filter into a smali test lube. Add 3 drops o f Potassium chramate to the sample and shale the test tube. Add Silver nitrate solution, one drop at a t h e (shake sP,er each drop), until the sample o f water just turns a permanent 'reddisn-brown' colour. If only o n t or two drops of Silver nitraie produces a reddish-brown colour, salt water is absent or negligible. If the water remains a yeiiow/greeo cnlour, even after the addition o i Silver nitraie, salt water co~taminationis confirmed.
Q.ll. 2escribe some simple ship-board tests, ro determine t h e q u a l i v and conditiou of : a) Lube oils. b) Fue: oils. h s .
Lube oil tests. The following tests can be conducted on board 1) Water Content : Percentage of water is indicated by the Calcilim carbide test. More than 0.25 % is normall) unacctptab1e.
2) Sea / Frcsh water determination : The sea water conteni is found by the Silver niiratc test described eariier.
3) Alkalinity test : A simple colour indication test, using pH pzpzr, will indicate the presence I absence o f a sufficient reserve of alkalinity. 4) TBN Tpt : The TBN test involves a pressure reaction flask and propel
chart interpolation. Each brand of oil requires its own chart. 5 ) Comparative viscosity : This simple test, using the flow-stick comparator described earlier, is useful as viscosity, a. well as an all-round indicator of oii condition. 6) Flash point : The flash pcint of oil can be checked with the PenskyMartens apparatus. Contamination by fuel oil can significantly affect this. The open flash point is normally taken as the guideline for the suitability for further use.
Advoneed Morirre Engineering Knowledge Yol. 111
following tests can be conducted on board : I)
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Compatibility : Compatibility refers to the ability of two (or mol-e) differmt fuels to mix, without -creating sludge and problems in combustion. As mentioned earlier, it is preferable to avoid mixing bunkers From &fierent sources. However, if that is not practicable, then it is esseniial to perform the compatibility test. A r.ixidre of the hvo fuel is dropped on a filter paper, to create a spot, ivhich is compared with a standard chart, to determine whzther the fuels are csmpatible. Flash point : Most kits have some form of flash point testing apFamus. Flash point is the lowest temperature at which oil gives-off suffiriext vapour to fdrm a11 inflammable mixture with air. The flash point is measured by the Pensky-Martens test described earlier. Minimum acceptable value for machi~lery spaces is 60 OC, as the ambieni tempcraturz rarely exceeds this v21ue. Density : The correct value of deinsi;y is necessary for quantity calculations, and also to %sure proper purificztion procedures. For conventional ceiitrifiiges, this is normally restricted to 991 Kg/rn3, but the A!cap system higher values can be taken by special centrifuges, !kt. @y Alfa I.aval), and similar systems by other nanufacturers (like Weslphaiia). This is normaily measured by hydrometer, and specified nl 15 OC. Viscosity :The resistance to flow, which can be expressed in di~ferent units, such as centistokes (Cst), nomaliy at 40 OC for distillate fuels and at 50 OC or 10QOC, for residuai fuels. It is =ecessay to determine the value, to decide on the amount o f hearing for fuel trvlsfer and injeclion purposes. Viscosi:y, by itself is Lor an indicator of ignition quality. It is measured by various types o f viscomcters (e.g. time t&en by a failing sphere in a standard apparatus). Pour point : This is the lowest temperature that the oil can flow a( before solidificction takes place. Measured by test kits. using some form of refrigeration sprays, to lest at different temperarures.(E.g. 30 OC for a heavy fuel).
Water content : Water contamination can create. problems,. especially with a high range of sodium/vanadium and ash deposits. Catalytic fines : Presence of 'cat fines' is usually a cause For concern, as the abrasive particles present in a fuel could cause fuel pump, injector and vaive damage. Usual limit for residual fuel i c 30 ppm.
Q.12. Microbial degradation, in lube oils, leads to a change in the chemical composition. Discuss the effects leading to corrosion. W h a t a r e the indicators of microbial infection, and what are the guidelioes for situations where the lube cil is found to h e s o infected ? Ans. ,$ &$ Microbial growth in lube oils : /"" Microbes have an+-?bility to degrade organic and scnne inorganic materials. They need a dilt of Carbon, Nitrogen and Phosphorous along with plenty of free water. Various microbes have different ?k$ference for temperature, acid or alkaline cmdition~,light, oxygen availability and so on. They need waier initially, to g c w , but as water is a by-prdlc! o f their )&*I\;;; gowth, they become self sustaininz ir, this respect. Conditions found in the cradcases of proper!! maintained ma;iile engines do not normally support microbe growth, but leakage ofwatei in10 the sump, the use of corrosion inhihitors in :he cooling \v.v;ierwhich could leak in, ma!functio~o f t k pl!ri!iczticn systerr?, rnine~zladdiiiv-s :o oils - al: of these factcrs could increase the suscep!ibility lo microbial attack. Microbe gowth c m cause coirosion by Coming organic acids, hydrcgen sulphide and ?.mmonia. The viscosity and chernicd composition of the oil is altered and any additives zre reduced in effectiveness. Oxygen gradients can be produced, giving rise to anodic pitting, and filters, ~ i ~ e i i n e s a i d n a m w passages becoming i r e q u ~ n tblocked. l~
Indications of microbe infection: %.*., c.5i 3. Unusua! : ! ? Slimin?& m t e 011 3 Corrosion or honey coloured films on jcunals 3 > Brown or black deposits in thz crankcase and sump Heavy siudge accumulation of crankcase and purifier 3 Frequent filter choking or back-flushing. 3
1 4 >Simple
microbiologicaI tests include the use of slides coated with a nutrijive gel, which are dippcd in the 'suspect' oil and inc&fi$ ~~~n$&ti,~ The resultant intensity of red spots are compared to a stanXard c%art,b h ~ c h will indicate the presence of bacteria. Once infection is established, it may be combated by suitable biocides or physical treatment. The most practical physical method is the use of heat in ' for 12-21 hours and conjunction with purification. Renovation at 80 C treating the sump and system with biocides or steam lancing, should be sufficient to kill all infection. The choice of suitable biocides, to be added to a particular brand ofoil must only be done with tile advice of the oil supplier. The following guidelines of operation are suggested:/ The water content of the crankcase oil should never be allowed to : 1. rise above 1 %. 2. The prifier intake must be from near the bottom o f t h e sump. 3. The pu~ifierheat exchanger should be run at 70 + 'C and the oil &ntained at this temperature.
4. The entire sump oil should be purifier: ewxy 8-10 hours.
5. Coolant inhibitor concentrations should be maintained as recommended. 6. Regular testing should be canied out. With respect to lubricating oils, justify the usage cf heavy duty oils and the precautions to be taken in respect to their usage. Briefly compare the r q o i r e m e n t s of Cragkcase oil and Cylinder lube oils in Marine twostroke inain propulsion engines. Ans. Heavy duty oils are generally suitable for use in marine engines, which are working under adverse service conditions. Besides the normal properties, t h a e oils wiii have:. Oxidation stability. Alkaline properties to protect against ccrrocicn. Detergcct - dispcrs;nt, characteristics. :leavy duty operatian is encoun1:red under sustained high speed rur~riir:~ or heavy / fluctuating :oads in adverse weather conditions. These oils arc of napthznic origin, natur?l!y detergent (in compariscn wit;! p i l r d f i n base), i.e. they can 'wash z;:ay' their own oxida:ion products. Ttrcse 'r1D oils have additives which act by increasing the fluid film thickness to take care of heavy loads. Also, deposits between pis tot^ rings m d grooves arc prevented, which reduces the wear rate considerably. However, the high cost has to be taken into con side ratio^^, as compared to their benefits. Prsi:a!~iiorrs recommended with use 0fH.D. oil:i . l'hese oils should not be mixed with normal oils, as t3e accumulated deposits of oxidation products may suddenly cause clogging of oii holes atid damage of bearing%. 7 h c additives used in H.D. may attack antiiiiction materials (containing Cadmium) in bearings. 2. ::ii:zrs or purifiers of adequate capacity / capability must be provided to deal with the Beater quantities of carbonaccous particles which may acc~unulatein forced lubrication system. 3. Che~nicallyactive filters must not be used, othenvise the additives will be rernoved. 4. LLD. lubes are much more expensive than the best quality of straight riiiwxal oils. Care needs to be taken to avoid wastage 1 leakages, which inay pmve expensiv& due to the high costs involved. Caicium phenyi stearate is an example of an effective detergent a J i i e which possesses the ability to break large sludge particles into smaller arrd more manageable ones. Colirparison of Crankcase and Cylinder lubes for main propulsion m ~ i i i c:s Crankcase !ube oil has a high detergent and dispersant quality. Crwiccase system oil, for a typical main propulsion two:stmke marine engine,
has an SAE 30 number. The SAE number of an oil is an indication of its viscosity, based on a classification involving two temperatures. The cylinder lube oil is subjected to more rigorous conditions, i n comparison to crankcase oil, owing to the severe c~nditionsexisting in the combustion chamber, and the difficulty in maintaining a lube oil film of adequate viscosity and film strength, under such adverse conditions. This is a once-thxough use oil, and is thus being regularly changed. Owing to the ccsts involved, it cannot he synthetic oi; or even use many additives, which are comparatively more economical to use in the Crankcase cil, which is infrequently chacged. U'he~.fuels containirg !arge amou3is of impurities are used, the job of the cylinder oil can tend to become very difficult, due to the severe conditions, under which i t does its job. This oil has a comparatively higher detergenc y and needs more al'.a!inity, i.e. Totai Sase nurr-ber is much higher - o f the order of 7 0 or even 80, while Crankcase oils need not have even half as much, depending upon service and type of engine.
Q.14. I n your opinioa, state what zould be the S e a l properties of lubricating oils used in the steerinz gezr ? Expiain what is meant by FLoc Test, and describe, in brief, how it is conducted. Ans. Eesirable properties of Steering gearhbe oil : I. Lube oil must he c!cm and as h;o,h!y pure as possible. The syskm oil must meet 10 micmn standards. It is possible that oil stored in reserve Tanks may include a sizab!e amount of contaminants or corrosion products. T h e r e f o i ~ suitable additives mus! be prwided to prevent deposits. Also, appropriately sized filters, capable of removing 98 % by weight of all particles larger than 10 microns are required. 2. It should be of a sufficient viscosity to assist in sealing, so :hat lr&ages from rams are minimal. It must be ahle t3 work under high pressures (e.3. 63-75 bar), withoct losing its !ubrication qualities. It must not shear in difficuit applications, like hall and socket joints. 3. It mustnot emulsify %withwater. This is because of the possibility of condensation in reservoirs, foiiowing shut-downs or maintenance, or even in low ambient tenperaturzs. 4. It must not react chemically with any metal surface that it comes ir. contacl with. It should not oxidize, and it should not cause a deterioration of any rubber seals or gaskets, with which it comes in contact. 5. It must have a high Viscosity index, i.e. it must not lose viscosity at low ambienl temperatures. Floc test : To determine the tendency of lube oils towards flocculation, a floc test needs to be performed. It is a Low temperature test, performed by giadually chilling a mixture of 10 % oil and 90%Freon 12 until haze and precipitation of wax crystals, as 'floc', are observed. The respective temperatures are noted and recorded a s the 'haze' and 'floc' temperatures.
Shafting, Propellers, Steering G e a r a n d Controls Q . 1 Stare the principle cavses ofvibration and over heating in main shafting and iis hearines. How can such vibration and over heatino he mininrised 7 Explain why the aft position of the Engisie roan1 reduces these problems. Ans. A lone propeller shaft is equivalentto a long beam with intermediate - . . suppor!~, by v.-ay of shaft bearings, t s take up the weight and sag of ti]: shalt Each bearing takes up a certain portion of ihz load, which varies, depending upon :hc luading partern and the stiffne;~of the hull. Any one of th: bearings can get excessively worn, increasing the deflection and a-iEkcting the alignment. This exerts more load on the adjacent bearings, causing them to over-heat. Rough seas often bring the propeller out of the wacer, which causes excessive load to be taken up by the stem tube bearing. Intermediate shafi bearinzs are so placed and aligned, that they help to reduce any tendency to sag. Mc:wevr:r, there still exists a slight whirl i n any ro!a?i?g shaft. Loose foundation boii:; a i loose chacks can increase this effect and over-load adjacent shaCt beasi~igs,causing iheni to over-hezt. Modem shafts are more ilexible, of improved material and reduced scanrlings. Long lengths and fabricated structlures are more prone to hog and sag wi!n %tying seas The shaft, beizg a much stiffer cornpcnent, does no: conform to ihc flexing of the hti!l, resulting in over-heating and vibrations. By siting the Engine room RR, thc length of the shaft is drastically reduced, which reduces the yr-oblrm considerably, even under such drastic conditions. Also, the ship's structure being much stiffer at !he after end. it has a ix+dlucsd ilcx, which produces little or no ill-effect on the shaft. Alignment is based on t i % fair curve method. Ailowances are made, in the a l i g ~ ~ e ntot , ensure-that all irrierrrlediate bearings are proportionally loaded, to a definite pre-calculated load, i ~ ihc i ship's loaded and ballast conditions. 'l'k tail shaft is like a large, overhung mass on the stem wbe hearing. The etid support, i.e. the stern tube, is given a definite slope, to match the permanent sn.: o f rile tail-end shafi, and ensure a larger and continuous support :br the shaft. i'itrvs, rtkr effect of any . .possible flexing - of the shaft is isolated, fi-om the Main engine's crankshaft. ~
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O i x i s s , with reference to shaft coupling designs, 'She advantages of a Muff coupling. "/itti a simple sketch, describe a muff coupling. Llow.is it ensured, that the entire surface will transmit torque ?
A Muff coupling is a shrink fit, and may be used for connection of the [xopcli;:r. shafi, allowing the shaft to be easily withdrawn out- board^
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l i consisis of iwo sleeves. a thit~.inner and a relaiively thicker outer. rhe n?atiiy surface between the t w having a slight taper (l.jO).:The bore. o f t h e innslsleeve, is slig!ltly larzer than theshaft dianietei.. The c o u p l i n ~is made, when thc outer sleeve is driven up the taper of the hydra~iicunit at oce end, which compresses the inner sleeve, so that it grips both shafts. To makc the drive-up easier, the fric.tion, between the mating surfaces of the sleeves, is reduced by injecting oil at high pressure. This oil forms a loadcarrying film, xhich actuallji separates the two components. Whtn the outer sleeve has reached its correct position, the oi! pressme is rzleased and the oil drains cff, allowing normal friction between the sleeves. To check that the correct amomt of 'pull-up' has been achieved. the final ou~sidediameter s f the mrer sleeve is meas-red. An increase, cser the unfitted diameter, of atatit 1.02 rnm is required. This value gives a surface interiace pressure cif at least 1200 bar. For this diameter, rhe nean torque tr;nsmi!ted is about 2 N-m, ar a factor of sefety of between iwo and three. With the flange type, rr push up of 6.5 mm would br required. The twelve bolt holes would be pre-bored to 70 nim and reamed to final dimer~sionsafter mounting.
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To ensure full transmission of torqce and thrust, mating surfaces need to . be machined to a high degree ofaccuracy and finish. ~
Advanced il/rrrilre Engineeri,r,o Knoaleiige
Q.3.
Yol. I l l
F O ~examination of thc shafting, durinz a propeller shaft survey, e x p i a h , with particular attention to key-way and propeller shaft con a) How crack detection methods are x e d . b) Repairs to propeller shaft cone, in case of corrosion damage.
Ans. Crack derecrion may be carried out by [he dye-penetrant method or !he rnasneric particle method described earlier. Masnetic particle inspectioi. prefened, becaiise i t is more sensitive to sub-surface cracks, when properly carried our. in boih cases, the area to be checked inust he ihoroughly cleaned. As far as possihie, qvaiified personnel should be used, who are working to r e c o p i standads. The sensi~ivityof the magnetic panicle inspection is easily checked usirag a fidd strength indicator. If cracks detected are not easiiy removable, lighi po!i:itling, it is advisab!e !o check the dcpth of the crack by ultrasoniq before proceeding.
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:I is iallowed to reduce the diameter by upto 3 % (corresp&dino to a decrease iri torsional strength of 1 0 %) by machining or grinding. Therefore, if the depth of the craz!c is more than 1.5 % of shaft diameter, the shaft should he repleced. I?$pdirs of corroded or cracked shafts, within the limits given above. should Rc :jilluothIy groiind out, to reduce stress concentrations lo a rniniln~ll71. The hollows shotlid be filled with a rneral filler, if in way of sealing r i n g .
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In the case of a pl-opelier-shai7 of normal steel and the propeller h ~ of b srainless steel, ttig holes of !lie propeller flang,e, and the ,flange itself, have to be carefully~.inspected for Eleciro-chemical corrosiog. ,. . . . . .. .. . . .~. Repairs to the propeller shaft cone Corrosion, on the conical pan of the propeller s h a f t m a y be repaired by machining the taper. This wilt result in the propeller moving forward, which must be counter-acted by fitting a spacer, beiween !he shafi couplings. The maximum thickness, -allo;ved f o r this spacer, is 25% of thc intermediate shafis flange thickness. It is therefore th; intermediate sh& flange thickness, which deremines thq maximum amount which can be machined-off the ccne.. -Intermediate shafi coupling Canye thickness 1DO mr.1 then, maximum size of spacer, which may be emp!oyed 35 mm and if, the propei!er shafr taper 1 : 12 25:) 2 the radial amocnt which may be mx:zin~d-off Thus rhe radial imornt is approximately 2 mm. Surface c ~ r t t ~oc itthe propeller bore !o :he shaft cone shoi~ldbe ch*, e.s. by using 'Priissi n blue' TFxre shou!d be a mini!i\uri of 70 % contacr pat~b+$~cii . .--. should be e cnly d~str~buted e.g.
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Bate : Besides rectification of the damage, the cause must also be detem~ilied. Preventative action nceds to be taken, to avoid a recurrence. The usual source of leakage.is from a badly jointed and sealed fairing cone, on the back of the propeller or leakage past the sealing ring andior gasket on the forward face ensure the " 0 ring is the correct size, so that compression and sealin9 is achieved.
Q.3. Draw a cross-section of a keyless propeller with sleeve. What material weuld you suggest for the sieeve, of a tapered, forged, mild steel propeller shaft and why? Explain, in brief, 'the procedure for rernwal of key-less prupellers and the reasons for filting key-less propellers. Ans. A key-less propeller transmits torque by friction and this requires a contact area of around 80 %, between taper and boss. An expensive (afid critical) machining process i n the bore would thus Se nezezsar;. A rough-bored propeller wirh a fitted sleeve elimina~esthis requirement. The keyless propeller, fit:ed by thc hydraulic expansion of the boss, was designed t i kave vcry high boss stresses. in the final 'as fitted' condition. dus to the low coefficient of Friction at the interface. Also temperature differentials can still cause problems. Both these problems can be reduced, by using a cast iron sleeve (which has similar expansion rates to steel ) and the increase in coefficient of friction is considerable. Cast Iron = 0.24 ;Mild Steel = 0.12 : Bronze = 0.08.
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The sleeve is then passed to the propeller manufacturer and is fitted into lhr propeller boss and secured by a high strength epoxy c o m p o u ~ dby a pressure injection process. The propeller is fitted to the shaft by a dry push-on force, from a 'Pilgrim nit'. The material for the sleeve, of a tapered mild steel propeller sh is Pearlitic Cast iron. This has minimal fretting qualities. It is machined, handbedded lo the Tail shaft and pressed-on. Removal of the pro. injection, between sleeve and shaft.
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1Ie;asons f o r titting keyless propeliers :
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When a large amount of power, at lower speeds,' is required to be transmitted, the operstinp torque is very high. The load imposed on a key would be excessively high. . . Use of a key creates a 'nress raiser', which is the site o f stibsequenr d & q e end failure. The larger and heavier propellers require a geater press-on force, which cannot not be achieved, due to friction, during the press-on process. at the tiper, at the nut face and at :he nut threads Often the result is a locked nut or yieldsd threads. There is no method of checking the friction grip, in case of a key. Stresses &t thz proijzller boss are urhiowr.. So=e plastic yield occurs during the f i t t i a ~process. After two or more removals and re-fittings. failure could occur at the kily or key-way. Ahead movement crcates a p s h - u p of the pmpeller on rile key. This n&es it very aiif;cult to remove ( usually a ccnsiderablr axcount of heat needs 10 be applied. which results i n thermal stresses, as well a i stress corrusion cracks. ) -.. pmpeirer and- shah are of different materials,-tke di;ierrnt coef5cienrs ?f expansion can cause 'slip', especially when in warm cot~ditions.
-K
Push up
V
n k 1 1 11 ~ ri 111 1 Mounting at shipyard
0
Q.S.
4.
With respect to the improvements in shaft sealing systems, to ensure no oil pollution occurs, what is the main feature of the 'Pollution-free' ,a sealing system ? What are the materials used for t h e seai ring asse.mbly ?
3
Ans. Comparison of the conventional and the 'Pollution-free' oil seal. In the conventional s ~ a l i n gsystem, all zeal chambers are filled with oi!. The sez water and oil arz directly in contact, through tke sliding sllrface of the seai ring. 7here is thus a possibility of intermixing of sea water ar.d 011 orcurring at l l ~ cseaiing surface. The leakage of oi! and subsequen: I-isk of poll~~tion o f the s r , \taler is not . 9 t h ~ nc;\ s avoidable, to a ce~zaindegrze. The situation becomes still wcrse by shaft vibration. w!iich can damaze the seal. Also, iiseal rings izil, there a x m adequate measures to contain [he oil contamination, and the result of a lea'i?g seal is the loss of the complete stem tube oil into !he sea, doe to the greatex p!:sstiri. c~eaied by ihe head of the sitrn tube oil 9ravitp talks. Even wt1e11the ship's draught changes (ballast or loaded cor2ltion),~iiierr is lin~ited~?lanualadjustment of the oil prexure in the stem tube. t . 5 ~ changing ovci. iron1 low to h i ~ hstern tube gravity tanks), necesswy to xaintain rn adequate 1':.g$ i)u[ no! excessive pressure to ensures a good seal, without having s Ysil - prcssuie :.diiycre~~ce, which increases the'chance of a leakage.
3 !G
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Pollution-free oil sealing system : (Refer Advanced Marine E n g i n k g Voil) This has been developed, by having an air space between kc ail side and 2 the stxi water side. Ar. air cantrol unit has flow controliers, and a n &- rqulator of cotisran! flow rate type. The air pressure, to the aft seal, is controll& by having a cmslan! flow rate. This air blows into the sea water side, throiigh j- lip of the -* seal ring. in case, sea water or stern titbe oii leaks into the air ~ ~ m b e itr is. collccrecl inboard and drains, through piping, to the drain callectian x31.
:
Advantages : 1. Asiti Poilution : The air is ejected f?gm air bamer into Ei-5 ~ o n ~ ~ l e i e ! ~ sepaiates sea water and oil. The leakage oil andfor sea water to after most zharnber is T? discharged autonlatically into the bilge i v i i h z spillin2 0of -2 the oil into the sea. r e free:
No adjustrncnts required, once system is set us. iiutomatic pressure control of oil and air follon%g irl dmli is achieved on every after seal I-ing.
3.
Extension of seal ring sewice life : . . Low and constant pressure is loaded on a h sea! rings. Air film is formed under after most seal ring Forced oil circulation acts on cooling the afi seal ring. The fishing net protector, guards h e aft se'al. -.
Others :
No additiunal air source is necessary for sys'cni Aft seal ring conditicn can be rnonitcred in Engine room. Load on af: seai rings can be adjusted from E n ~ i n eroom One seal ring of aft seal is reserved for emeriency oil leakage. Simple piping s y s t m c3n be a x n g e d , withcst having to fir the usual Stem tube header tanks.
Seal Ring Garrer Spring
Q.5
Rubber Body Provides radial lo2d, madeof hasrr alloy (Nickel allpy) which has the stronges: corrosion resistance.
The survey period for oil hhricated stern tubes, of tapered shapes, is a) 1 year, h) 2 year& c ) 5 year,, d) 10 years. Explain why the fitting of stern tubes, witb a small downward a n d e . may he considered necessaw
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Ans
L.
The Survey period is 5 years. The survey consists o f removing rhe propeller, withdrawing and examining the entireshaft. During each survey. thsforward end of the tapr is examined, by performing a non-destructive test. The exami'ation ir usually by surface crack detectiop methods. which should include the end of key-ways (if fitted). The wear-down of the stem-tube bearing is reccrded and the condition of inboard and outboard seal assemblies assessed and repaired, if required. Where arrangements are such, as to permit effective examination of the forward end of the taper and key-way, to the complete sarisfacrio~l of the surveyor, the tail shaft need not be withdrawn for examination, in its entirety. The downward angle is maintzined to avoid 'Age loading' of shafting, in stem tubes.
3.
Extension of seal ring service life : Low and constant pressure is loadedon aft sea! rings. Air film is formed under after most seal ring Forced oil circulation acts on cooling the afi seal ring The fishing net protector,guards the aft seal.
Others : No addiriunal air source is necessary for sys!cm Aft seal ring conditicn can be monitcred in Engme room. Load on afi s e d rings can be adjusted from Engine room. One seal ring of afi seal is reserved for emergency oil leakage. Simple piping s y s t m c m be a x n g e d , withczt having to f i r the usual Stem rube header raAs. Materials : Seal Ring Garter Spring
Q.5
Rubber Body , haste Provides radial I o ~ dmadeof alloy (Nickel alloy) which has the Strongest corrosion resistance.
The survey period for oil lnhricated stern tubes, of tapered shnpes,
1s
. .
a) 1 year, h) 2 years, c) 5 years, d) 10 years. Explain v h y the fitting of stern tubes, with a small downward angle, may be considered necessary. Ans.
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T h e Survey period is 5 years. The survey consists o f removing rhe propeller, withdrawing and examining the entireshaft. i h r i n g ezch survey. the forward end of the taper is examined, by performing a non-destructive test. The examimtion i.; usually by surface crack der~ctignmethods. which should include the end of key-ways (if fitted). The wear-down of the stern-tube beasng is reccrded and the condition of inboard and outboard seal assemblies assessed and repaired, if required,.Where arrangements are such, as to permit effective examination of the forward end of the taper and key-way, to the complete satisfaction of the surveyor, the tail shaft need not be withdrawn for examination, in irs entirety. The downward angle is maintained to avoid 'Age loading' of shafting, in stem tubes.
0.6, drescrihe the method used for the installation a n d alignm6nt of a coaapkte traismission shaft system, from the propeller shaft to the main propaalrinn unit. Arrs. Good alignment is to ensure that the bearings are conectly loaded and the shaft is not severely stressed. Alignment can be checked by conventional methods bu! the results are uncenain, unless the vessel is in the same condition, with regard to loading, as and when the shaft was installed. 'Jne.en bearing wear, hull d&rrmtion and other factors can affect tile results, giving rise to error. Shaft alignment : The first operation comprises of sighting the centre line of rhe shaft, so that the correctly positioned stem boss is accurately bored o x . This is done before the vessel is taunched, a i d the stern-tube is fitted complete wi?h propeiler and tail end shah. The stem boss should be so positioned, that the engine, on installation, wottid have sufficient chock thickr,ess, when aligned to ihe tail end shaft. The boring of the stern boss is carrieci otit by a boi-ing b2r. Sighting is acciiiateiy ;leterntined by various means, such as an optical telescope. Afier the boring opention is complzie, 5 e bxiiig bar is removed a d :he mt.-ma1 diameter of the stem-t~beis mhchined to accurhtely suit the in iluier diatnelcr of the Sm : boss. The stem-tube is p s h e Z in with the strong-back and fiilallj. secured on stem boss t y tappzd bolt5 having wire lashing. The taii eild s h d i is lowered from inside the Engine room, prior laucching of vessel and insened into the stern-tube (from inside the Engine room). Oil seal rings and propeller are then fitted. Assuming that there is a reduction Gear box, t\e same is Lowered in ::ngine room and .nis is radially and axially aligned, to the outer flange of the tail r:nd shaft. To facilitate the fitting of the chocks, the girder top plate of thr Gear box seating and engine seating is machined slightly, with an outboard facing taper, prior :o lowering gear box /engine. The gear box is inirially installed with jacking bolts, wilich are adjusted to establish its correc: relation with the propel!er shaft. 'She holes in both the flanges (gearbox and propeller shaft) arc findly rmnred arid fit bolts located. The gear box can then be checked. As the holding down bolts and chocks are installed, the jacking bolts on the gearbox can be rcrnawxl. lri a similar manner, the Main engine is lowered and the fly-wheel flange is radially and axially aligl~ed,io the outboard flange of the gear box and the it~stdlationsimilarly completed. I r may be noted, that
Q.7. How is the propeller matched to the main propulsion engine of a vccsel ? What a r e the advantages of using controllable pitch propellers ? 'What is meant by the operating profile of an engine? Ans. The matching of the main propulsion engine and the propeller is essential if maximum efficiency is to he achieved. Power produced by :he engine s h o ~ ~ be ld more than sufficient to take care of the power absorbcd by the propeller. Here the mechanicai efficiency, the transmission losses and propeller efficiency ha\ie to be taken into consideration. Mztchisg involves findin: h e intersection where the torque, power and rpm of the engine are at their mDst desirable values, and then adjusling the engine { opeiatin: poiili j acd the popeller { pitch, diemeter 2nd so on) rill they arc at the desired pari of the curv;. Where special purpose vessels are requiring more flexibility, ~ o n t r o l l a b i ~ pi:ch propellers are being uszd, so that the required alteratior. of pitch can improve the inaneuverahility of the vessel; however ;his is echieved at tile expense of more complicated controls (for CPPi. as well as the canside,-able extra costs involved, which may not be justified for conventional cargo vesseis. Once tht. required engine rating has been es!ahlished, other factors that aflec! the selection of engine? for a particular applicqion musl be considered. Among these are ihe ship's type, the plant weight, the machl~eryspace volun~e,file! ciuaiity and consumption, acquisition cost, reliability, maintenance requirements, and present and future spare pans cost and availability. A requirement for. low weight or minimum machinery volume may be achieved at the expense of high fie) consumption or high maintenance requirements. Selection of proptilsion engines of light weight or low specific fuel con;umption, for example may not result in the lightest or mosi cost-effective power plam The operating profile of zn engine asseses the time spent in various operating modes. All important modes must be considered, and periods of sustained idle or low load operation must be included as well as those at hip11 loads. For propulsion engines, operating modes may include conditions of deep aild ligllt draft, clean and fouled hu!l, cairn and heavy weather, cruising and high ship speed, towing or icebreaking and running free, and operation with and without attached auxiliaries. The plant design and engine Selection will be affected if the profile includes frequent or extended periods of maneuvering or astern running. In selecting propulsion engines, consideration must be given -to whether a single engine of the low-speed, direct coupled type is most suitable, or if requirements are better met by one or more medium or high speed engines driving thc propeller through geakins or electric drive. .~.
Changes in hull resistance, due to fouling of hull, roughness and weather conditions, as well a s limitations on draught (leading to limitations on propeller ciiarnzter) will Further complicate the issue. Consequently, adequate allowance has to be kepr, i t . 'margins' are maintained, so as to reach a suitable matching of propeller and engine.
Q.6. What is Vibration ? W h a t is meant by the terms amplitude, nods, mode, period, frequency, natural frequency and resonance, with respect to vibmtiora ?
Ans. Vibration is the periodic movement of molecules in a zubstance, in vertical, horizontal or twisting (torsional) pla~es. ivla-hii~es, such as diesel engines, frequently suffer from vibration problems, due to out-of-balance forces. Further, these vibrations are transmitted to the ship's structure. 'She magnituj? of osci:lation varies, and the maximum or pzak value is the 'An~plitude'.Since the vibration is of a 'wave' farm, there will be points at which hi: vibration is zero ( viz. the points where positi;.e half cycle crosses io the nsgarive half cycie). These poiiirs are referred r.o as the 'Nodes'. Tile ininher of nudes, logether with the plane of -iibiation is referred to as the 'Mode' of vibrati~n- viz. 2 - node veirtical mode. The time betcveen successive cycles is termed as the 'Period' of vibration. The reciprocal of the Perioi will give the 'Frcqueilcy', i.e. the number of vibrations per unit time. All structures vibrate at a certain frequency, which is termed its Nat81ral freqwncy. The value of this Natural frequency is calculated Sy the expression : 1
4XE3 wherc, I is the second moment of area about its neutral axis, WI is a function of the mass and its distribution, and L is ihe length. The structure is normally vibrating at iis Natural frequency. It can, hoivwer, bc forced to vibrate ai olher frequencies, due to external sources. The final ariiplitude will depend on the external or 'forced' frequencies. 'I'he point at which this amplirude is maximum, is termed as 'Resonance'. At Ilesonance, the frequency of applied forces will match the natural frequtncy, Icatiing to i~nacceptablyhigh amplitude of vibration, which can serioiisly weaken a srructilse and lead to damage. Though the ship may vibrate in a number of modes. h r practical purposes, i t is often considered sufficient to take care of two and ti~rcenode, vertical and horizontal modes. 'i'llc normal amplitude of ship's vibration is limited to 20 mm, whereas deflections tluo to hogging or sagging may be upto 50 rnm per 100 m of length. Tillus, vihalior~is not, by itself, of great danger.
+
However, the cumulative effect of vibration and corrosion, on components already subject to heavy strzss may prove disastrous. Also, high :frequency vibration may cause interference with electronic equipment, . a d -is thus undesirable. . . ... ~. ~.,.~Q.9, Define the following terms : Controlled condition, Monitoring element, 'Measured value, Ceviation, Offset, Control point,.Error signal and Feed.~ . back. Ans. . I. Controlled condition : The variable, that is being controlled, is called the controlled condition. E.g. speed in an engine governor, temperature in the engine cooling system. :
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i
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2.Monitoring element : The element which measures the cnntrolled condirion ( variable ), and 2ioducer a signal conespon6ing to it, which can be used by the control system. Also called zs Sensor, lransdurer. 3. Measured value : Actual value of rhz controlled conditicn, as mea.wred by the sensor. 4. Deviatioc :
the difference benveen the desired value and the measured value. This signai is sent to the Cornpazitor, in orde; to ini:iate some corrective action.
5. Offset : This is sustained deviation, which occurs, when the measured value stabilises (reaches equilibriur,) at some poin: other than the Set vaiue. Tbis could change, with change in load conditions. Offset occurs in simple proportional control: 6. Control point : In a simple proportional control system, the controlled condition will stabilise at some point of equilibrium, other than the Set point, which is termed as the conrrol point. 7. Enor signs1:
The signal produced by the Comparator, after comparing the measured value with the set value. 8. Feedback : The transmission of the measured value to the Cemparator is termed as the feedback.
Q.lO. What is Open and Closed loop control ? Give some of their advantages anddisadvantages, with m a r h e examples. hs. Open loop control : This is the simplest type of control, where the input to the process is independent of the output. Sincc the output is not scnsed, ths input is usually dependant on some other variable. e.g. time. A marine e x m p l e is the oil purifier or centrifuge. Thc puriiler desludg:~ after a fixed time interva!, irrespective of the amount of sludse that may or ;nay no; have built up. The purifier does not check whether or not the siudze has been removcd, or even if there is any sludge at all. Adv-atages of open IGOP : Cheaper (than closed loop). Simpier, thus easier to troubleshoor I repair No hunting Suitable !br systems, where precise control is not essential. ijluadvantages of open loop : Not suitable for complex systems, having considerable laad changes. Excessive deviation from set point. Closed loop control : In a control system, if therc is some means to moijror the output, and generate ;III error signal, which can now correct the input, ( i.e. we 'close' the loop ), then this form of control is called as closed loop control. If a human operator does the job of observing the output and taking the necessary corrective action, then it is a m m ~ a closed l loop. When the operator is replaced by a controller, that uses a semor ( to provide feedback of the controlled condition ), a Comparator to get the ifeviation (frox the measured value and !he set value ), and a correcting signal to the Motor element to take appropriate action m the input, then it is celled as an Automatic closed loop. Advantages of closed loop : Finer control, with less chances of deviation. Suitable for systems having considerab!e load changes. Disadvantages 3f closed loop : More expensive (than open loop). Possibility of hunting (loss of stability). An example of a manual closed loop could be water level control. An operator observes the water level, and accordingly opens or shuts the feed-check valve. to maintain the desired level.
Q.ll With respect to propeller and shafting, discuss the reasons for the stel-n-tube bearing being a t a skope, in the stern frame. What a r e the criteria for shaft alignment ? Describe the procedure of the 'Fair curve' alignment method. Ans. The overhung mass of the propeller has a significant effect on the stern tube bearing. To protect this bearing, the stern frame, in which i t is fitred. is bored at a slope. Excessive flexing of the modem ship of very large izngths, withour a suitable flexibility at the shah, can lead to damage tc gearingkngine or can result in failure of shaft bezrings. In an attempt to reduce the effect of rhe flexing on the shaft. the 'Fair Curve' method of alignment is used, particularly with a h end installations. While checking the shah alignment, the following criteria shouid be me1 :
a j Simi!ar m d known positive ioads, on each braring. b) There should be accepiable le+elsof vibration. C ) There should be maximum distance between bearings (giving f l e ~ i b i l i :; ,~ to allow the shaft to assume the 'fair curve'. d) ~ e n d i ' n gstresser; in each shaft secticn, which cannot be climinzted, 13 te at accepiaS!e levels. e) There shou:d Ge a minimum transfer oFany Seading moment. from the tail shaft to the crankshafi cf ihe Main engins. f) T h x e should b: an adequate sllowance; for rhermal lih of engine, from the 'cold' condition ro actual operating temperatures. e) The above criteria ro be taken care of, in both the ballast, as wei! as the loaded condiiion.
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Bearing Reaction influence numbers for thc arrangement above : (Force per unit of vertical displacement)
This tablc above gives the effect of raising a given bearing unit (here: raising bearhg PJo. 4, which would increase it's load by 165; the effect on the aft shaft bearing would be ;o increase its load by 48. Shaft Alignment
' F a i r corvp' alignment is based on an analysis of the shafting as a conti~iiousbeam, with multiple s u p p ~ r points. : The initial malysis considers the sh:ifiing to be lying on a straight h e datum. The calculations are done b;~ computer:%. Tk.c results give the bearing loads throughout the system and also gi-w: a mics of influence ncmbers. The influence numbers exprcss the change, in load over unit change in vertical movement O.Jeuton/mm), for each bearing in the s y s t a n They also give the effect of raising one bearing, on the other bearings. I)esigners use rhis data to obtain a shaft line, :hat is a smooth (or 'fair') curvz, ?hat allows for a slope at the stem bearing and an acrentable line-up to the engirt;: coupling. The final analysis is carried out on this 'fair curve' shaft line. This gives thz loads on cach bearing, shear forces in the shaft, bending moments in each slinA section and, most important, setting-up data for the shafting. iising rhis data, the stern frame is bored out and, after the launch of the ship, the shaft system is instalied. The shafting installation works from the trail siiaii its a datum. The final part of this process is setting the engine bed-plate at tile wir.ecl height and attitude. Durifig the installation of the shaft and engine, the shaii s y s w n is unsupported (on temporary supports). The couplings are set with gaps antl offsets that are the result of the final shaft analysis. The gaps and offsets are ca1citi;itetl to give the rcquired 'fair curvc', whcn the couplings arc aligncd and bolioil. fi\iicr the engine is finally chocked and boitcd. thc shaft bcarings arc p i o i'he shaft couplings are brought into line and the coupling b o l t fitted. The iirnl lini: of the shafting is the smooth curve required for optimum bearing load, when h: ship is in sewice. The setting up procedure is carried out in Lhc c;cl~:rni: 'Iieht ship' condition, hcnce the data urcd cannot bc uscd to c h d ali~"rw:ot, -..hen Lhc ship is in service.
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The are two accepted methods of carrying out a check on the alignment, when the ship is in service, these are : .~
1. 2.
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Usiag rhe 'Pilgrim' wire, to check...the height of one bearing, relative to another. Measure the actual bearing loads, with a hydraulic jack system.
Pilgrim Wire (alignment) : The pilgrini wire method of obtaining the shaft line is popular and easily understood. A wire is stretched over rhe shafts as a d ~ t u m ,
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pre-tensioned by weights. The height o t t h e shaii to the wire, at each bearing, is then taken, using a seaiiive micrometer arrangemcct. R e micrometer is used to complzre a small electric circuit. The alignment over all the shafiing, inclcding the crankshaft czn be determined, although ii is often racpssery :o swing connecting rods to allow this. This method only gkes the shaf! line, for the exisiing ship conditicn. It does not give the shaft bearing loads. Determination of Shaft alignment : (Checking shaft line with hydraulic jacks) The jacks are insta!!ed and -&e shait is raised ir. increments, with rhe jack load and t h e corresponding lift being recorded m d plotted. This will show two distinct s!?pes. The gradual slope is when the bearing load is being transferred to the jack and is !.he spring constant for the seating, bearkg and housing. The steeper s!ope is a measured bearing influence numbe; (lift per unit forcej. This shou!d show a relationship with the calculated influence number. Extrapolating the steeper slope, back to the zero - lift position, gives the act31albearing h a d (load at zero lift). The jack method is a realistic means of checking the shaft line but care is needed in its application. The jack loads, pmcedure repeated at 90" intervals of shaft rotation, can determine if a shaft is bent faker running agrou~ld).
Q.12. With respect to propeller shafting, discuss : a) Causes of Axialvibration. b) Axial vibration damping. c) Diiferent loading patterns, in shaft system. Ans. a) Axial vibration Causes : These can be Propeller induced. The propeller, working in a non-uniform wake, produces a varying end-thrust. The blade clearances, if too smati, at cenain points of ihe stem frame, can also produce pulsating end forces. This occurs at a frequency, given by the number of blades multiplied by revolutions per second.
They can also be Engine induced (Direct drive diesels). Cmnkshaft deflections (crank 'spread'), over each rcvolution, produces end forces along the crankshaft. These occur at a frequency given by the number of cranks multiplied by revolutions per second. As with all vibration, the natural frequency, established without a forcing or exciting force, is the dominant criterion. If the exciting forces are i r t phase with tine natural frequency, then rescnancr will occu;. This can result in very large vibration amplitudes and ultimately failure. The shaft acts a spring, being subjected to varying end forces, additionally the !h-ts: seating can act a spring. For separate thrust arrangements, the searing design is of great importance. Using an integral tiirust, as with the direct drive diesel, stiffens the thrust seating. This factor, in conjunction with the possibility of t h s t offset, with certain types of thrust, causing an unacceptable bending moment in the crankshaft, are the reasons that engine manufacturers insist on the thrust being part of the engin?. With geared drives, the thrust position is always a compromise, between positioning it as far aft as possible (shortening :he vibmting shaft length), mci positioning it Fa- encugh forward in the wider part of the ship's cross section. The wider section, away from aft has a larger second moment of area and less twisting. 11 alSo is adjacent to the very stiff sectiou near the engine. bj
Axial vibration damping : Damping is usually achieved by viscous fluids. Friction Gpposes motion, which reduces the amplitude of vibration. The major damping, in a ~ i a lvibration, is the propeller, acting like a pistcn in the water. Some damping is achieved by the lubricating oii in the shaft bearings and at the thrust pads. This is small iz value and is usually co~lsidered,along-with the propeller damping. If axial vibration is a major problem (this can be established during Sea trials), viscous dampers are fined to the ends of the crankshaft).
C)
Shaft Loading : The shaft system operates with different loading patterns. The dominant load is the overhung mass of the propeller, causing considerable bending moments, at the aft end of the shafting. This bending moment can be aggravated, by the propeller thrust being offset from the centre line. Whether it is below or above this line, depends on the ship's condition - !oaded or ballast.
Additional to the main bending moments, both misalig,hent &xd changes to the line, resulting from machinery expansion, can contribute to funher bending moment. There are four possible modes of vibration in the shah system and all are encountdied 10 some degree. There is also a variation in the engine torque (direct drive diesels] and the resisting torq!le frcm the propeller will vary, due io i t working in a con-uniform wake pattern. The attitude of the ship, between jight and loadrd condition. has a marked effect on the shaft line. The align men^ is always a cornpromice, 13 meet these ex?reme conditions, and skill main:ain satisfactory bearing loads. hvariably, the shafting aiid machinery are aligned, when the ship is in an extrexe ligfii condition. This has to be allowed for, in the set up.
Q.13. Brief describe ir!vsntages and disadvantages of folierviq types of intermediate shaft bearings : a) Hydro8ynamic white metal bearings. b) Tiiting pad hrarifigs. c) Roller bearings. d) Tllrust b e r i n g s .
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H y drodynamic white metal bearings are usuaHy designed for engine operation with the relat-d need for wear resistance. This is essenrial for turbine-xgine ship:, where slow rotation during warm-up periods and lengthy turning-gear operation, is a normal feature. Tilting pad type bearings are chosen, for particularly heavy duty, such as the extreme aft end, adjacent to the tail shaft. In this location the pads would be in both halves of the bearing. For other bearings there would usually be pads in the lower half. The after bearing usually has a reduced clearance, so that pcak amplitudes are not attained in the rail shafi bearing. Roller bearings offer a very low starting torque and are free from wear, during turniag gear operation. They can be of :he sclid form (requiring a muff coupling) or of the split type to allow easy assembly. Rol!:r bearings are of zero clearance, so they can carry loa6 rzversals. without having to absorb diametric clearance. This is an asset, when transverse vibration may occur and tends to zompensate for t l ~ every low damping qualities of these bearings. They have a specific life (related to load and revolutions per minute) and usually have to be replaced, at the end of their operational life. Thrust Bearings can be directly connected to the engine or gearbox (Integral thrust) or can be separate (old designs).
AdvoncedMorine Engirreeriq Knowledge
Vol. 111
~ i r e c tattachment can result in an unacceptable tilt to the wheel or the after most section of the crankshaft. This pronounced in thrusts of the horse shoe trpe as :The summation of the loads carried by individual pad displaced below the shafi &is. Casing distortion tends to magnify this effect,'unloading the top padsat the zxpense of the lower. Axial vibration effects can be te:rimental, with the integral thrusts, as tiley are transmitted directly to the geer hox (or engine) foundations.
Q.14.
Ans. a)
M'itni reference to propeller shafting, how are fitted 3o:ts insfali What r a c t c n govern the design of silch bolts ? Gorv can failur sucb bolts) take place ? Sketch a typical hydraulicdy tightened Haw are "interference fit' bolts installed and rzmoved ? When fined bolts are used, each hole is ~ x h i n e dand h m e 2 in The hole is slightly tapered, and each bolt is rhen machined to su hc!c. 77:s bolts -re tightened to R specific value of pre-detem~i so a;: tu achieve coaCderable fric?ion at the c,ol;pi;ng faces.
b)
Tli~ nunber of bolts should be such a? ro carry the torque withsut shear '!'$I-:!ii::iion at the coupling'faces doescontribute, but the value of its coniribuiion is debatable. .especially when going astern. Overstressing thc boliz, reducss the degree oiinterference fit.
C)
Bolts ?.re never over-;ighiened, till they reach thcir clastjc limit. thiii:;! rcwrsa1s Lake place - chances of failure (of bolts) arc greater. HYDIWULICT1CI:TENINC
Oil L
d)
Interference fit - aiding attachment. The radial fit bolt consists of a tapered bolt, matching taper bored s1ee.x and two round nuts. The sleeve is assembled loosely on to the bolt and together ,hey are inserted into the coupling hole in the clearance condition. A hydraulic bolt tensioner locates and restrains the sleeve, while pulling the bolt into the taper bore, so expanding the sleeve to develop a radial fit in the bole. Nuts are fitred at each end of the holt, which is then tensioned to induce accuraie tixial loading. The simultaneous high axial and radial loading maintains vital shaft alignment. The radial fii bolt is removed by relzasing the clamping eifect of the nuts, and cil is :hen injected thruugh an oil-way and distribution groove to the tapered interface to bring about separation. The nuts are removed, and the bolt assembly withdrawn from the flange hole.
Q.15. Describe split roller bearings for msin and auxiliary Shafting. How zre they overh-ukd ? Ans~ A Split roller bean'ng (for main and auxiiiary shafting) is designed as a conventiunal roller bearing wi'h the main components halved. Bearing races and rollers are of hardened chrome sreei. Races hav: ang!ed joints to provide continuity of rolling contact. High tensile screws in c!amping rings provide an 'interfereace fit' between inner race and maft. Roller cage me made from liglri alloy or bronze joined by steel clamps. These can be used for shaft d:'ameters upto i250 mm.
Advanced Marine Engineering Knowled~e V d . 111
Overhaul : To dismantle the bearing, lifi the top half squarely, taking care the top half outer race does not faii. Carefully ease out the cagz jcining clip. The prrservative need not be removed, unless contaminated. Outer races need not be removed from the cartridges, but, if doing so, ease back the side screws and remove the radial screws (if provided) for fitting outer race. Maintain the bearing silrfaces clean. Damaged parts should not be interchmged. Complete roller bearings are interchangeable between similar cartridges Ligkly oil threads and mating surfaces. Check shaft diameter fcr roundness and parallelism. For some applications (especially at slow spped and moderate loads) wider :olerznccs, as on stmdard shafting, can be accepted.
1 If the holding-down boits of a thrust bearing should become slack, what effect would it have u p 3 the working of the engine ? Ans. The function of the thrust bearing is to taLs t5e thrust and also all the longitudinal stresses transmitted by the propeiler, so that the crankshaft will :j~ilyhave ro bear the stresses transmitred 5o.m ine pistons. If the i h s t braring bolts hhould become slack, the icngitudinal stress wiil be now be taken by thr: i-tiink shah, which w o d d throw a greater stress on all the working-pans of the erbginz. This may cause the bearin~sto w o r ~hot, as the thrust force traxmitted to the main bearings and bottom ends of the connecting rods woulri cause the components to get out-of-line. If tine thrust bearing should give way suddenly, the whole thrust comes cn the crank shaft, which would be forced forward, untii the web of the crank comes in cmtact with the main bearing brass, which may li.siilt in closing the crank web;, and in some cases cause a total breakdown. Iilc cause of crank web becoming slack on the shaft (or crank pin) in often allributed ro the thrust being partly taken by the main bearings. :().117. Where does corrosion occur on the tail shaft ? How is the shaft protected from this ? What are the methods used to reduce this ? Ans. The tail shaft being in contact with sea ,water, is subject to electrochernicsl corrosion. To reduce the contact with sea water and to provide a good bearing surface, a brass liner is fitted. However, i4 brass and iron are in contact with each 0 t h and salt water gets at the point of contact, the ifon corrodes vapidly, therefore, corrosion can take place on the shaft at the ends of the liner. .i>ns I : 1s due to galvanic action taking place, caused,by the action of the sea water at these points. To minimise the corrosion, the following methods are used : I. Fit a rubber ring between the propeller boss and the end of the liner, so that sea water is kept out. 2.. Cover the unprotected part of the shaft with red lead putty and marline wound tightly round shaft, the whole being then covered with canvas. 3. 'Lb protect the shaft completely from corrosion, the shaft has a continuous liner shrunk on to it.
~ n hollow y pans between liner and shaft are filled up with red lead puny by means of a force pump. Q.18.' If t h e propeller were to start getting loose, with a little 'play' on the shaft, how would this he known ? Ans. :f the propeller were to work loose on the shaft, with side 'play', i t causes z ' h o c k ' to be hzard in 3 c engine-rsorn, whea the shafi is turning, especially when the engines are reversed, from Ahead to Astern. A propeller e the shaft is dangerous, since the ship's propulsion depends on working l o ~ s on this and it is difficult to access this at sea, for m y kind o f repair. Great care needs to be taken in fitting the propeller on the taper, s d emuring that sufficient contact surface exists, to develop adequate friction to hold the propeller tight. It should also be ensured that the tail end nut is adequrtely secured.
Q.19. In large vessels hollow shafts a r e fitfed ir! place of solid shafts, Whz! is the object of this practice 7 How do hollow shafts compare for s t r z ~ g t h , with solid ones ? If a hollow shaft is fitted in place of 2 solidone, whi,-h ;vonld he the heax-ier of the two ? Ans. The object of fining E, hollow shaft, in peference to a s d i d one, is to reduce the weight of rnateiia!. it rmst b e noted that for the same strength, the h$llow shafi is iighler It must also be noted that rhe diameter (extsmal) ~f the solid shafi makes it the stronger. Briefly, the advantage gained (in :olfow shafts) 1s in removing the molecules of metal from the centre core and placing them at the extreme radius, where they will be better able re rerist torslon. The strength of a solid shaft va-ies as it's (diarnete~)~, while the strength of a = (Laree diameterj4- [SrndI diameter)' Hollow shaft varies as Large diameter For stiffness, the shafts may he compared as D~ for solid and (D4-d" for hollow. FOI weight, the shafts may be compared as D' for solid, m d as (D2-d2) for hollow, where D = external diameter, and d = internal diameter. n
Q.20. What are the stresses on the crank shzft, thrust shaft, and propelfer shaft ?Compare tbe diameters of each. Ans. The stresses, in the crank shaft, are - torsion, a te dency to shearing and a bending stress adjacent to the webs. The webs are stressed in bending, and the crank pin is in shear stress. The after end of the crankshaft transmits the torque of the whole- engine, but going towards forward end, the torque becomes gradually less, until at the
foremost journal the torque is nil. For this reason, the diameter of the various con;ponents of the crank shaft need not be the same. They are, however, usun!ly m a d !he same (in diameter) for the purpme of being interchangeable, in the everit OF breakdown and replacement. Ln tke thrust shaft, the stresses are torsion and compression while goiny Ahead; torsion and tension while going Astern; with bending. and shearing at collars~ he propeller shaft is subjected to the most severe stresses of all. In addit.ion to torsion, and compression while running Ahead; or torsion and tension going Asrem; it is subject tc~an alternate bentiing stress, due to ihe overhmging u-eight of the heavy prope:ler and this latter stress is greatly an!$ilied by the movement of the stem of the ship, especially when in a 'heavy' sea. Due to pre-tensioning by the tightening of the tail end nut, the after end of rhc propeller shaft is in 2 contii..ual statt: oftension. The diameters of the various shaft may be compared as follows : Taking a diameter of 600 mrn intermediate shafiing, the thrust shaft could bs 613 mm, the c r o c k shzft 636 mm, and propeller shaft 679 mm respectively. 1 What is meant by rhe 'pitch' cf a screw propel;er ? Zxpfain the di!'frr-enre behvcen a 'right-handed' and a '!eft handed' propeller, and st3te itow carti of them revolves. Ans. The 'pitch' is :he axial movement of the propeller, in one r;wlu
Q 2 2 . i What is the effect. on the main propulsion machinery, if the 'trailing' cdze: of the propeller are worn ? Discuss the term 'Cavitation', with rci'i:iwce to the running of the propeller and how can this. --be avoided ? Ans. The tips of the propeller blade and the 'trailing' edges are particularly siix:;p!ibie lo erosion. They may be completely eroded and tom away by [he ... c k c r s of cavitation. in some instances, the continual pitting causes the entire bIxJi: to be eaten away, leading to cracks and possible failure. lei-osion, due to cavitation, may occur at any part of the propeller blade. \\,heit: l i ! ~s!iclion is high, but is common over three significant regions - at h e
tips (where the rotational speed is highest), a1 0.7 of the radius (where the load is usually at a maximum) and at the 'root' of the blade (where the sections very thick and the pressure distribution is adversely affected by the small gap between the blades). The collapsing cavities give rise to noise effects, accampanied by nigh frequency vibrations, which are undesirable, especislly in the modem 'aft end' accomodation ships. Since cavitation is affected by pressure and temperamre, it is more likely to occur in propellers operating near the surface and will occur more readily in wanner waters, than in cold regions. If this sea water pzsses across the back of the blade znd meets I high suction region, the nett pressure of this water rnky fall be!ow the vapour pressure of the water (at that temperature), catising 3 cavity or 'bubble' to fomj, filled with a mixture of water vapollr and sorrx air, since air is always present in sea water. The creation of these czvities, adjacent to th- p r c p 3 e r biade, is L-gc.m as "cavitation. Cavitation is essentially a boiling phenomena and the action of the propeller blades causes the surrounding water to 'boil' at ordinary sea temperatures. As spceds and power increase, it becomes tiifficuit to avoid cavitation. At extremely high speeds, complete 'back' cavitacim occurs, ilr which the back of the Bkde is completely covered with a sheet of vapour. Means of avoitiing cavita!ion : incrraes rhe total blade area and this reduce :he thrusriunit area of 1) , . blade surface, fcr the sarni: total thrust. This may be accomplished by -3 &ij~. z2>s increasing the Blade area ration (BAR) at constant diameter or increasing the diameter of the propeller, with a resultant reduction in I , 6 ,-.., . r r ~ @ + - iev~iutions. Reduce the blade angles and the angles of incidence, by adopting 7 -~i?' k~ sliehtly "Z--, W ; h %r - larger diameters. over the length of the blade, in order to diminish the load 3) Vary the in critical rcgims. Avoid the occurrence of unduly high sections on the back of the blades, 4) by using section shapes, which gives a more uniform distribution of 8
'.T,-~S/-~
$
.*VL
-
-
7
-
5) 6) 7)
8) 9)
g,"dg&@,, to achieve as uniform a wake field, as possible. Avoid the incidence of locat suction peaks near leading edge, by
the using suitable amounts of camber and a suitable shape of entrance. Reduce the thickness of blades, by using materials which are stronger and more resistant to the effects of cavitation. Provide the maximum immezio; ~mdhle Since the thrust of a propeller varies as the square of the revolurions, rhen reducing the revolutions~wil~ reducecviratm, but will also result in a loss of speed.
Q.23. Describe cach of the following : a) Ducted propeller (Kort nozzle). b) Frce rotating vane wheel (Voith - Schneider type). Sratc their advantages and disadvantages. Ans. Ducted frooeller (Kort nozzle) : a j These are prcpe!!ers ~pcratingin a duct or no7zle. The duct has an aerofoil type cross-section, thus water flowing through the duct gains in veiocity, ai:owing a greater mass of water to be accelerated by the propel!er, resdting in increased thiust. Ttere is also a thrust from the A c t itse!f, due to a low pressure region generated at the duct entrance. Kort nozzles *ere inrroduced on vessels operating at low speed, high t h s t conditions e.g. tugboats and ice breakers, where thrust increasesof upto 40% have been achieved. They have also been fitted to VLCCs, where gains in propulsive efficieccy, of up to 6%. have been achieved, resulting in reduced fuel consumption or an irlcrease i n speed. O i i w advactages inc!ude reduced vibration, dcc to a more unicnnn wake cwiidirion ifi way of the propeller; improved stesring efiiciency, due to ir~cicaii-dwa!er velocity over the mbber; reduced prope!!er diameter: ard psopelli;~proiection, far vessels such as ice breakzrs.
b) Free rota iing .Jane wheel (Voith - S c ~ e i d e type): r yile;
7jhe Voith-Sclmeider propeller unit may be placed forward, aft n~idships,bul does require a flat bottomed vessel.
01
at
Disadvantages : h?ore complicated constrncrion. greater w e i ~ h t and vulnerability, as compared to a screw propeller. Propulsive coefficient is smaller, than with a screw propeller, because gf increesed mechanical losses.
Q.24. With respect to steering gears : a) What arc the materials of the mms, crossheld and other parts? b) What h a p p m s in th? even: of loss of tlxid (f;tilure)? c j Why is the Rudder angle limited to 3 5 O ? Explain. ) State mzterisls ssed forvanes, in the Eotary type steering gear. e) State where clearance for wear is provided, in the vane unit. Ans. a) Cylinder Cast steel, Gunmetal, Bushes R Y ~ S Forged s:ee! (gromd finich), Csst steel, gun metai line<, Crossheai: soiid d r a m steel, forged steel flanges, Pipe work forzed steel. Valve bodies b)
In ihe event of hilure, due to loss offlnid, the level in the operrting taiik falls and an alan:; is set-off: Anotlier, Icwer, float switch activates the controi unit, whicii :I . Energizes rile solenoid, which operates its automatic 1sola:in~ and Bypass v a i x , hence splitting the system into two independent circuits.
2. The system operates on any one pair of rams, the bypass-on the other pair having Seen cpened.
3. If there is no further oil loss, the system continues to operate, as hydraulic integri:y exists. If the leak exists in the presently operating circuit, ,hen the continued loss of oil results in activating the alarm an4 change-over to the orher circui1. c)
Rudder angle is limited to 3j0, because there is an increase in turning circle diameter, as the angle increases, which does not help in steering efficiently.
d) The Vanes are manufactured from spheroidal Cast iron and secured to the (Cast steel) motor and stator, by high tensile Steel bolts and dowels. e) Vertical clearance, of Vane unit, allows for wear down (if a carrier bearing is fitted) and for a ji~mpingclearance. This is 38 mm.
Q.25. Briefly discuss the Steering gear Regulntions with respect to Main ant1 auxiliary steering gear. What are the special requirements of Steering Zear for Tankers ? Wilai iests a n d drills are carried out on the Steering gear, prior to the Vessel's departure from a Port ? Am. Stcering G e a r Regulations : Ships must be provided with an efficient main and auxiliary steering (1) gear, but the auxiliary gear is not required, if the main steering gezr is iirreci with dcpliczte power units and dllplicate connections uplo the Kudder stock. (2)
( 3
-
Means are to be provided, 13 rllow ihe v e s ~ e lto be steered, from a position Aft. All power operated steering gear are to be provided with arrangements for relieviiig shczl;.
(4;
Ccrrifizd hydraclic p i p and electric power cables to bz used for rhe steering gear exclusively, with power cables capablr of withstmdin: i 00% over-load.
(5)
?he main steering gear must be capable of purring the Xuddei over, fioin an angle of 35' on one side, to 354 on the other side, with the ship moving Ahead at maximum sewice speed and with the vessel a: ii's deepest draught. It must also be capable ~i putting the Rudder over from 35' on one side, to 30' on the other side, in not more than 28 seconds, tirider the same conditions (Passenger ship5 with only one of the power units, other vessels with both power units operating).
-j-(6)
The exact position of the Rudder must be kdicated at the main steering position, the method s f indication being independent from the steering control system.
- i,,--17)
An efficient locking or braking arrangement must be titted, to enable the Rudder to be maintained stationary, if necessary.
(8)
A u x i l i y steering gear should be capable of putting the Rudder over from 20 on one side to 20' on the other side. in 60 seconds, with the ship a[ half speed or 8 knots, which ever is the greater
Tankers of 10,000 GRT and above. (Building commenced after Is' Jan 1980) Power supplics :
(a)
In case of power failure, the Power units to start automatically, after powcr is restored.
(b)
An alternative power supply is required, to provide for 30 minutes continuos operation for one Power unit, Rudder angle indicator and Remote steering control system. This is to come into operation within 45 seconds of power failure acd sttould be capable of meeting the requirement (8) above.
(c)
Power nits to tr able to be slarted from the Bridge (manual or automatic) and e!arms fitted on the Bridge to indicate power failure.
Control systems (Tankers 10,000 GRT and above) : [2smMay 1982) Two rer?ote control systems from the bridge, fitted with failure alarms. (2) Csntrol of steering g-ar to be provided in the Steering gear (b) compart-nent. Means of commmication to he provided between Bridge and Steering (c) compartment (Steerizig flat). Angular position of rudder to be indicated on the Bridge 2nd Steering (d) compartment (Steering flat). Steering Tests & Dril!s : Steering system to be tested withir. 12 &ours before leaving Port and entry made in the log book. Tests to include ike operation o f : ;Main / Auxiliap stzering system. yEridge steering position. ,>.Emergency steering. i re rudder angle indicatar, in relation to actual position of Rndder. i Pcwer faiiue alarms. c.;kutornatic isolating equipment. ,'*FullRudder mmemenz. .>Visual inspection of gear and linkages.
h
relevaax
Operating instruction showing changeover procedures to be displayed on Bridge and in the Steering compartment. Q.26. In a control system, explain : a) Double seated control valves and their purpose. b) Materials used for double seated valves'and reasons. c) Various gland packings, with special reference to Asbestos. Ans. a) Double seated (Double ported) valves are arranged so that the fluid forces across the plug are balanced. These usually require a smaller valve movement,
or, ibl- a given movement, have larger flow capaciiies. If metal seats a!-e used iv1111 double poned valves, then complete shut off is not possible ( 1 io 2% leakage is nornial). b) Seat rnzteriais can be metal ( usually stainless s1eel o r m0nel) \vliicIi ~ i v e liigh wear resistance. can be ground in, can cope with most fluids and cal: handle 11i~l1 tem?eratures. Mstal seats are required f o ~emergency shut-off duties. A l ~ e r n a t ~ v e l y z oseats f t can be used, where the material may be nitrile rubbcr~ These c2n a!low complete shut off, wwith dooble poned valves a?d produce a softer action on cloiii~g. Normally the glands on these va:ves are %riy long to give good sealing ) with minimum friction. Tlie type o f giand and p x k i n g will depend oil the contl-ol valve application. Asbestos is necessary for stram and other hig11 teinperatui-e and high pressure niediums, above 15 bar, 260°C. The asbestos s t ~ m i d ssan be spun with brass wire !or strength artd can also be inteivovtn wi:h anti-friction rna:erial. Oil packings are iisually rubber-proofed cGt!on 2nd for loit, pressures, yreasy ipacking of heinp is used. With hazardous'fluids a co~?iplctrlytight, belloivs seal can be used Lubricated packing can be used (P.T.F.E. materials r-equiie no lubiization upto 230 OC). 'Chevrcn' packing, with the 'Vee' expanded rtgai~,st the land. is used. Where the temperature is very hi$, a iubricanr may b2 requit-ed, as thc temperature may quickly destroy any buill-in Iobricanc. Tlis yiand liousing may have cooling fins around it, to dissipaie heat, rlius p r e v e n t i i ~ ~ expansion, an2 imposing increased resistance to valve movement.
4.27. With respect to Reduction pparing : a. State the factors to be considered in selecting steel t o ] - t h e gear. b. Discuzs various surface defects, which cuuld arise in gem ?heels. Ar~s.
a.
Factors to Se cansidered in selecting gear steels will be : (i) (ii) (iii) (iv) (v)
b)
Strergth. Bending. Surface fatigue resistance. Wear resistance. Compatibility with a manufacturing process
Fatigue Tooth Fracture Tooth loadmg creates stresses, which m l y cause fatigue fracture. Stress raisers, i.e. pits in the surface of the teeth, may aggravate the conditioil.
Failure may occur at the root on smaller pitch gears and at the pitch circle diameter (P.C.D.) for larger teeth. Ridging : A form of scratching under heavy load, due to plastic flow, caused by a high spot (usually on the pinion) ploughing through the surface of the mating tooth Rippling : Plastic yielding under heavy sliding action. This is characterised by a fish scale pattern. This is casstd by surface shearing swesses. Q.28. If you were instructed t o carry out an examination of a set of gearing, how would you go about it ? Ans. Prior to making an examination of a set of propulsion reduction gearing, the inspection hole covers 2nd nuts lnust be opened. Clean off so that dir;, paint chips, and Foreign matter will not fall into the geartng, when he covtrs are opened up. The lumiiig gear is put in, the usual precautions having beer. takcn at the engine ccntrol station, prior to tulrizg the propeiler with the turning gear. The first part of the examination wi!l be tr, check gear oil sprayers and oil flow f;-om bearing ends; if they are nn: firted witn drain pockets, either before shuttins the oil pump or by mming the p m p at the ei~do i the examin-tion. If a grid or perforated plate is fined in the run-down connection betwecn the gear casing and the drain tank, it should be inspected for any debris, white metal flakes and so on. The profiles of the pinion teeth should be examined, noting' particularly the wear pattern markings and the cortact surfaces, both ahead and astern sides should be examined. If the contacts surfaces are normal, the alignment wiil be i:, order. Main wheel gear teeth are examined in the same way. If the pinion teeth are hardened, defects in alignment will most likely show up in the gear wheel teeth first, especially if the gearwheel teeth have a softer surface than the pinion teeth. The root fillets in all teeth must be examined for the stw. of any fatigue cracks, even though they are more usual in pinion teeth. If any bearings are fitted within with any wiring (connected to temperature sensors), these are to be examined. The fastenings, clips and connections on lubricating oil pipes to bearings and oil sprayers must also be checked out. Prior to replacing covers, the gear teeth, where cleaned during the examination, should be coated with oil. Make a note of the findings, so that they can be written up in the log-book or work reports, without omissions or inaccuracies.
In
Q.1.
;I,
b, c. Ans. a. .
.
-
/b
,.
b: -
c.
Q.2.
Air conlpressor Air compressor: Whai is 'Bun~ping'clearance ? Flow is it checked ? ~ t a t ~ t reasons he for an increase in clearances. W h a t are the causes 1eadi;lg to reduced v o h m e t r i c efficiency ? What could be the efCect of k a k y valves ?
RO
Bumping clerracce This is t h z t e m given to tile ciearence between the piston and the cylinder cover, at the tap end of the stroke. This is necessary to prevent ,.mc~;iianicai contact between the moving piston and the-valves and g e a r - * I r c m be ye?/ eteiiy checked, by inserting lead gauge wire of known p. ,,lckness, above the -,iston top. Now siowly ?urii the compressor over by hand. The thickness of the !-ad wire is now measured and should nimnaliy be around 1% of the cylinder bore (check the manual For the exaci figire) :f the bumping clearance is rrrorc, then rhe volvmetric er6ciwc; dxreases. This is because rhe volume of tile space above the piston tap dzsicies rhe compressio~ratio and hence the final yressure Having insuificient burnpip: clearance can lead to nechanica; dainayr This is uslialiy adjusted by mean? of shims. The burnp piny' c!earanc~changes due to bcaring wear down or due to piston crown wear, or even 5y insuflicienr thickness o f t h e gasket of the cylinder cover Adjustment is ~lsuallyby shim packs betweenconnecting rod and boriom-end bearing block or even between cylinder cover and block. 'With tandem type pistons, it is necessav to be able to adjust each stage separately (since the piston is con~mon). C ~ u s e of' s reduced volumetric efficiency : Excessive ' ~ u n i ~ i nc!eirance~ g' Di:Fcctive (leaky) valves. .. lle!jtrictior~sin the discharge lints. Resirictinns in Inrercooler i AfttrcooIer or reduced heat transftr. Clio!,ed irttake filter. Vlorri piston rings. . ~. ~~~. :'orsibie effects of leaky valves include : reduction in eFEciency, incretlrc in tire first stage pressure (due to leak in second stase a~cticr?), drop i r i first or second stage pressures ( due to leak in respective suction valvw)~ 111
a) b)
case oSa survey o f the Air receivers : k l o w is the stir-vey inspection carried out ? Whnt are tire Ilegulations pertaining to Air receivers ?
Ans. a)
Dtiririy a sturvey, an inspection of the following will be done : I) Visual icrspection of all concerned parts.
Caiibratlon of liners, pistons, piston rings, crankshaft and bearings Calibration of pressure gauges Pressure testins of Inter / M e r coolers Testing of relief valves. 6 ) Operational test.
2)
3) 4) 5)
b)
Regulations : 1) There should be at least tv..o receivers, of equal capacity. 2) For reversible engines, they must have the capcity t o give 12 or more starts, without any fimher rep!erishing. For non-reversible :ngines, six or more Zans are required. 3) Relief valve should be provided, to prevent an accumulation of C) pressure, to a value no greater than 10% of the working pressur:, witk thz Compressors running and the Air receivers' outlet valves closed^ Fusible plug to be fitted if rhe relief wive czn be isolated. (this is 5) fitted to take care of the presstire rise associated du: to ari abnormal ris2 in the temperature e.2. fire in the Ensine room ) 6 j Outlet valves shculd he of a siow opening type, to prevent a pressure 'surse' in the air pipins
1) 2) 3)
Q.3.
a. b. c. d.
Capacity Calculations:1.5 to 2 times the Engine's total displacement volume, to give the minimum mass of air per sta.1. h ~ l t i p l ythis by 12, to give the total mass of starting air required. This should now equal the total mass of air in both bottles at the maximum stipulated pressure, taking into account the mass of eir which -aovld be remaining in the bottles at the minimum starting air pressure (unusable mass of air). What are the areas to inspect during a survey of Air receivew ? What ore the possible reasons for Starting airiine expiosions ? List the safety devices on S t a r t k g air systems. How will you prevent a reoccurrence of a Starting a i r line explosion ?
Ans. n
Inspection :First confirm that the pressure is properly vented off. 1 . Top inner surface to be checked 2. Bottom inner surface (sludge deposits can be acidic). 3. Circumferential welds. 4. Longitudinal welds. 5. Welds in way of compensation rings. 6. Particular anention to drain connections. 7. Condition of the coating.
b.
Startinp air line explosion : For an explosion to take place, three things -a(required : : 1. Fuel : Lube oil carryoverfrom air compressors. Leaky air starting vdves causes fuel and sparks getting blown back, Excessive ~. lubrication of system components. . 2. Oxygea : This is abundantly available and cannot be controlled. Leaky air s:arting vaives causes sparks to blow back. j Heat: External heat from hot components nsar- by.^ ~
.
. c.
C.
Q.4.
Safety d e v i c s : Flame. traps. Bursting caps. Ftisible plugs. RelieFvalves Non-r~turnvalves. Swrring air !ine esplosiorx may be prevented by : I) Reguiar drairing ofair receivers / lines^ 2) The good condiriw of cylinder starting air valves. 3 ) Preventive maintenance and cleanliness of the air staging sy5te.n
W o w does the stress vary, in the shell material of a compressed a i r receiver, both in the longitudinal 2nd the circumferential directions ?
Ail.;
'!'ha si~cllrliickness, of a compressed air receiver, is i e s (thin), in relation to the tlianreti:r. it can therefore be considered as a thin-walled vessel, where the stress i s uc\iibrnt across the thickness ofthe materia!. Let
. ,
intei-nal d;ameter :rf the air receiver^ shell thickness. 1' working pressurq. . . . ~.~ . . s!ress = load 1 area ii'a scccior~of unit axial length is taken, the circumferential, or 'hoop'stress is . .~ DxP i.-,: = ----21 aild tile lr~ngitudinalstress is Dp DxP -4: ----Dxt 41 l s o m which it cail be seen, that thestress, in the material of the cylindrical siir:Il i n ihe axial or longitudinal direction is only fifty per cent of the hoop or circumferential stress, or the hoop stress is twice the axial . 'stress. . D
[
= . = =.
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Refrigeration, Air-conditioning Q.1.
I
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t
i j
,
.
Describe a self contained refrigeration unit ror a container vessel. How xre the temperRtllres monitored ? What pzoblems could be encountered in the handling of refrigerated cargo ? Ans. Reefer containers ;of about 20 R. length) have their own independent refrigeration system, to keep the inside temperatures uniform. The Reefer container can have i;s refrigeration system powered by an internal combustion engine or by the ship's electrical power. Engines are used at the docKslde and on the l a d leg: of its journey. On it's sea trips, the ship prwidrs the electrical power. The system is asually air coded, but some may require water (for condensers). This requires complicated ship's pipe work. Air c o o l d condenser units tend to oveiheat inside the hold (of a general cargo vessel). Thus, specialized cellular container vessch are used, which can carry sir-cooled units, without danger o f over-heating. Early container ships had insulated holds, for camage c f reefer bo::es but the main insulation was still the container envelope. The hold insulation tried to ensurr an even temperature for the contaicer surroundings. With hiyher tempera!ures occumng naturally under the weather dcck and thr hatches, the uppermost containers iequired the most cooling. Modern cellular container vessels have done away whh the need for dedicated reefer holds. When reefir containers are unloaded, there may be probiems due to chanye of electricity supply. Units h a y need to be chazged to diesel driven compressors or the ship may have to unload a portable diesel genzr3tor, to supply electricily for tke reefer conlai~crs,till shore facilities are available. Temperatures need to be ;cgularly monitored,-in order to prevent the rerfer cargo from getting spoilt A log is normally kept of the temperatures, at r e ~ u l a r intervals. There are various problems, which occur in the handling of reefer cargo. Besides the mechanical problems of therefrigeration system, which could result in inescient or no cooling, there are other problems, due to the iiature o f the reefer c a r p When carrying fniit, various gases are liberated in (he hold. The fruit absorbs oxygen and generates hest, as well a s liberates carbon dioxide. Lowest rate of air circulation (for deciduous fruit and frozen cargo) is approximately 30 to 40 air changes per bour of the empty container, when in a refriyerated ho;d. With bananas, this rate is increased to 70 to 80 air chanses~perhour Dce t~ the larse range of temperatures, at which the fruit may have to be carried, a i~igh rate of air circulation is required. To prevent fruit flies from causing problems, the temperature should be kept in the range 0.6 to 1.S0C.These low temperatures may require the h i t t2 be pre-cooledibefore loading. Micro-organism become inactivated at about -10 OC ar,d chemical deterioralion is lowed to a negligible rate at sub-zero temperatures below -i8 "C.With frozen cargo, ice crystals will form, which may cause mechanical crushing of the meat cells. (Small ice crystals reduce the mechanical damage). Reasons for controlling the carbon dioxide include : 181
A 5 %concentration of carbon dioxide is dangerous (to human life). Some fruits (e.g. Apples) may develop internal browningof the flesh, if . kept in a carbon dioxide content in excess of 2% (Core putrefaction caused by anaerobic bacteria). Some fruits (e.g. Bananas) may give off ethylene, which can cause the (3) remainder to~ripenmore quickly. As carbon dioxide and ethylene are pi-esent toyether, rhe carbon dioxide ccntent is taken as an indicator far t h l ethylene cantsnt S h y l e n e content is difficult to measure). The carbon dioxide content is kept less than 1%. Porr Authorities may require carbon dioxide coGtents to be !ess :han 0.5 (4) %. (1)
(2)
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I n the case of Refrigeration lube oils, discuss : a) Properties, 1;) Flocculation. Explain the principle nf a Psychrometer, used in an ?ir conditionii~g system.
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r i . The purpose of the luSe oil, in a refrigzratior: system, iz to lubricate arid to seal, pacicularly with screw compressors. The lube oil comes into coii,;:c< with the refrigerant and its miscibility is an important factor The oil bei;;i;, a iiydro-carbon, would normally be very miscible wiih the !?eons, in the w::i-k:q range of tsrrperatures. Lube oil is c a ~ i e dover from the Compressor,
bu! most of it is returned periodically by the oii separator. This means that the lev4 in the sump is a balance, between the lube oil in circulation ( m i x d with the %a:;)and the !she oil in the machine. The separator is .lever one hundred percent effective; consequently, some Iiibe oil always goes :hrough the system. ARer the condenser, ihe oil is present i r i a siution with the liquid refrigerant. Preventing the deposition of this oil, on the hean transfer surfaces, is the main problem. A build up of deposited oil will seiiotisly afFect heat transfer. The evaporator coil size is usually desi~nedto ensiul:: a high enough flow velocity of the refriyerant, to entrain the hibe oil. At low iiowis, !he oil will deposit, hence some of this type of contamination u,ill alwti.ys occur. As the lube ~ i reaches l the cold par! of the system, it is essential tiliii irr behaviour at low temperatures does not affect the plant, i.e the oil must not cungeal, hence its Pour point and Viscosity must be correct.
I~'1occulaiioii: FLocculation is defined as the coalescence, of a finely divided precipiiate, into larger particles. It exists in the form of cloud-like tufts (or flocs). Cooling, of reefer lube oil, can cause a wax to form and precipitate, eventually forttiir~gwax crystals. When in the crystal state, the wax is a flocculent, and the retn{?ecatun: at which this occurs is call the Floc poiat. The FIoc point is detti::i~ined by cooling a sample of refrigerant containing 10 % oil - the !i;~i!~;i:rnti.~re at which the wax precipitates, is the Floc point. Wax crystals in a ~ e h ~ ~ , e r a i isystem, orr would have a detrimental effect on expansion valves and riii.it hr: avoided.
I n zeneral, parafin based oils are not used, the naphtha based type being preferred. Refrigeration oils are de-waxed, to achieve a low Pour and Floc point. The 'As new' behaviour of a refrigerat~onoil can be affected, if there is any kind of contamination, that has an effect on the oil. If contamination is suspected, then the lube oil should be changed
Psychrometer : This is a device used to determine the velative humidity of the atmosphere. The basic i~strurilentuses two matched thermometers, one with its bulb suzound by a damp ..vick a ~ :he d other with its bolh dry. In one type cf instmment, the ins:rument is whirled in the zir to give a conside;ablc air movement over the bulbs. In dry air, some o i thc xatzi on the v:et bulb evaporates (absorbs latent heat), which reduces the temperature of this thermomerer. The difference between the two tempcratiires h e . the wet bulb and the dry bulb) is a measure of how much evaporation has occurred (on the wet bulb). This is directly related to the m o i r w e content in the atmosphere (humidity). F& examplr, if it was raining, then no evaporation would take place (because the air is already saturated with moisture), and we say thai there is a high relative humidity. High humidity is undesirable, afid dehdmidifiers are used in Air conditioning systems to reduce this condition so as to keep the air in the 'Comfort zone'. Q.3. Describe a Cargo Kefrige~ationsystem. What are the advantages and
disadvantages of the Brine system. Acs. Cargo hold refrigeration :
This may use primaq- or seccndary refrigerants. In the primary system, one or more compressors may be used, tosupply the refrigerant to various cooling coils, inside the spaces to be refrigerated. The tezperature of a 'cold room' is maintnined, by regu!ating the flow of refrigerant, by means of a thermostatic expansion valve. The expansion valve contmk the cooling effect by varying the amount of liq>d mfiigerant flashing off into vapour - this should be just enough to ade~uaielycoot the space, without allowing excess liquid refrigerant to flow back to the compressor, causing excessi.:e frosting (this-is controlled by maintaining a slight a e p e e of superheat). In the secundary system, the primary refrigerant is used to cool the secondary refrigerant. The secondary refrigerant is circulated through the reefer helds or is led tto 2n air cooie~. Brine (Calcium Chloride) is the most common secondary refrigerant The system is usually fitted with a header tank, to which the Calcium chloride may be added, to maintain the correct density Sodium hydroxide (caustic soaa) may be added for corrosion protection, as this maintains ihe alkalinity.
SECONDARY REFRIGERANT CIRCUIT
Ardv,riii;~r,esof the Brine System : [I) t'siiiiary refrigerant cirsuit is limited to the machinery spaces. This is important i n case of any leakeges, since the primary refn'gerant is sxpensive. (2) Niint: being cheap, is easy to make up, in case of leakages. Different Icnii>eratures can be easily controlled in the reefer holds, by varying the qiiltnrity of brine in circuhtion. This allows greater flexibility in simultaneously canying large reefer cargoes at different temperatures, in difi~:ir:rn reefer holds. Control is achieved by throttling or bypassing brine or by having more than one evaporator in the system and mixing the diiYerent temperature brines. Dkwdvnntngrs of the Brine System : ( I ) Extra pipe work makes a complex arrangement, especially if a number of difkrent temperature brines are used with mixing arrangements. . . (2) Brinc k i n g capable of contamination, corrosion probiems in pipes and heat exchangers can create inefficiency and increased maintenance and costs. (3) I-lavin$ha secondary refrigerant, doubles the number o f heat exchangers in itre system. This adds to the operating costs.
Q
What could be the reasons f o r lube oil t o absorb refrigerant vapour 1 Describe measures you will adopt, to reduce foaming in oil sump^
Ans. Due to the difference in vapour pressure between refrigerant and oil, the lube oil has a considerable ability io entrain or absoib refriserant vapour, especially at low temperature. After a prolonged standjtill, more oil is absorbed by the refrigerant. When starting a reefer compressor, under circunistances where the oil temperature is close to the saturation temperature on the suction side of the piant, the greater p x t of the absorbed refrigerant lvili quickly be frecjaiier the pressure lowers in the crankcase), under a violent f0-m generation. in a refrigerating compiesso; :his involves : Consumption of oil, with the risk oftuo low an oil level in the crankcase. 1) Troubles with the lube cil pump - thz pump cannc~t4rax.v the boi!ing 2) mixture of R22 and lube oil, with the resalt that sufEcient oi! pressur? cannot be genera!& Due to the rnix&in refrigerant, the viscosity of the oil is r?duced, to 3) i ~ c ah degree, that the lubricaiicn properties are greatly reduced, which could cause increased iuear and the possibi!ity of seizure. 'Oil hammer' in the cylinder,, with the subsequent risk of damage to 4) valve piatt-s, .:alve springs and ihe unloadins device. In hermetically sealed compressors, there is an increased risk of 'burn5) out' of the motar Especia!!~ in R??compressors, rhe punctue voltage irom winding to earth is reduced consioerably, with the increasing content ofrefrigerant in the lube oii.
To reduce fnsrning in the oil sump, the following measures can be taken : I)
2)
3) 4)
5)
6)
After starting the compressor, the oil level drops, it is therefore, necessary to allow su%cienr h e for the oil level to re-stabilize. Shon cycling periods, reqcires immediare attention. While starting, take precautions to prevent an oveitlow in plants with forced air circulation over the eva?orators, the fans a) should always be started beiore the compressor. At the slightest sign of liquid 'knock', the compressor should be bj stopped immediately. Reduce the compressor capacity before restarting, until possible knocking sounds have ceased. TIYthrottling the suction side stop. valve. Expansion valves should be correctly adjusted. During standstill periods, isolate the Compressor by closing suction and discharge stop valves. Avoid rapid pressure drops in the crankcase, while the oil sump is cold. If necessary, stan the compressor with reduced capacity. A 'cold' return line from the oil separator indicates the presence of e valve in the return line, before refrigerant in the oil separdroi i ~ stop the crankcase, should be kept closed, until the oil separator rises to operating temperaiur?
7)
Q.S.
If?he compressor has a heating elenent, this should be used for 4 to 8 hours, before starting the compressor. If the oil is heated to about SODC before starting, this reduces the concentration of refrigerant in'oil by about 80%, in relation to the saturation point at 20°C.
3
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In a Refrigeration system, state the effects of following : a) Under charge, b) Over charge, c) 4 i r in the system, d ) Moisture in the system, e) Oil in tht: xystem, and I) Flooding.
Ans.
a) Uatder-charge : Low compressor suction and discharge pressiire.
-,
1 . Xigh superheat at the comprEssor suc:ion. (Possibility of overhearing and riii
breakdown at compressor delivery) 2. !,a~.rgevapour bubbies in liquid sight glass. 1 Compressor ilmriin~Tor extended periods. 4 Compressor cycles intermittently on the pressure switches. 5. R m m temperatures rising 6. krnrnerer reading, for the cGmpresssr motor, lower than normal
OVFR. Charge : The liquid level in the condenser is too high. This reduces the avalab!c; condmsirig sudace, with correspsnc'ing increase in saturation temperature and pressare. There is possibility of txcessive liquid refrigerant gettin3 to the evaporator, giving icing at compressor suciion, and a presswe drop across the expansion valve. The cold room temperatures may rise, if the evaporatoi is flooded. c) Air in the system :!:os.sibility of small air bubbles in liquid sight glass. This may cause the reefer compressor to overheat, with a high discharge pressure (with normal contln~singtemperzrure). If there is excessive air, ir may reduce the cooling . cd p'a kr y or' the system givins long running periods. Air can be removed by ccllrclina the system gas (into the condenser), leaving the condenser cooling water on and venting out the air 6om the top ofthe Condenser. d) I'iioistuw in thesystem : This ~~ormally comes in with the i q r e s s o f air in the system. Moisture may freeze a i tire expansion valve, giving some of the icdications of undercharge. It wi!l contribute to corrosion in the system It may cause lubrication problems a d brcnkdown of the oil. e)
I) i ' l o o r l h l ~: .This is %:an as liquid gettins back to the compressor suction. It may be due to fault*/ oc ii~correctlvadjusted expansion valve. Also due to the Solenoid valve leaking. It may also result from Overcharge. It leads to an iced-up evaporator.
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Emissions With respect to emissions from marine engines to atmosphere, discuss : Need to limit emissions of Nos, SOX,.CO, C a . a) What is 'acid rain', 'smog', 'global warming' a n d bow is it b) d a t e d to the exhaust emissions ? Formation o: SO-, Carbon monoxide, and Sulphuric acid. c) Ans. [ a) Emissions : The mides of Nitrogen (NO,) and Sulphur (SOX) are the "Primary polluiants" of the atmosphere. They -an pollute in two ways : i NOs and SSs dissolve in watm to form NitroudNitric acids. and i 1. i Sulphur~udSulphurica c i A +NOx can combine wkh 01, by isiug ultra-violet radiation (from the 2. sunj, to form 0 3 , i.e. Ozone. .
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The acids are absorbed in the clouds and then become 'Secondary b) poilutanrs', because they cause the formation of 'zcid rain'. This 'cont3minated' rain water has a pH of about 4, and disscives heavily 'toxic' materials, present i n the ground. These toxic materials, being water soluble, will enter the 'water table' This 'contaminated' ground whter is nsed by boih plants and animals, which is increaxingly darnazing their ~ : c w i j iand ia suspected to be one of the reasons for :he increasing incidence of fatal diseases, like cancer. Additionally, Ozone, at low levels, is dangerous, for both animal and plant life, since a combinarion of ozone with hydrocarbons forms a familiar city ~roblem: a photo-chemical 'smog'. This is carcinogenic (cancer forming) and thus t ~ x i cto human and plant life. Hence, thzre is an urzent need to limit the emissions, before the levels get out-of-control and destroy / damage the health r Both NO3 and SGs are emitted from marine engines, and the of ~ u ecosystem. new Emission control regulations have brought about a drastic reduction in the .~ . allowable levels, which are increasingly getting more stringent. Fuel is a hydrocarbon and after complete combustion forms H20 and COz. These are known as "Sekctive absorbers" and they allow ultra-violet radiation to pass through, but absorb infra-red radiation. Hence any infra-red radiation, i.e. heat, that is generated at sea level, is prevented from escaping through the atmosphere, and this can result in 'global -.varming'. Incomplete combustion of the Fuel will result in the formation o f Carbon monoxide (CO). This gas is toxic to human 5fe. Glass is also a 'selective absorber', as are gases such as C02 and N20, which are known as 'greenhouse gases'. C02 is the most damaging, causing an estimsted 55% of the greenhouse effect. Hence there is a need to limit emissions of CO/CO2 from engines.
c)
Formation of NOx : Nitrogen, being a major constituent of air (79%) was previously considered inert in the combustion reactions in the combustion chambers of engines. At the elevated temperatures and pressures of today's highly rated engines, there is a tendency for the Nitrogen to react and form oxides. N2 + O2 + Heat -> 2NO -I his is a reversible process but the cooling of the gases i i l the exhaust system prevents the reverse reaction. Hence, NO is found in the exhaust gas. Also NO + 'A 0 2 -> NO2 and N2 i % Ot -> N20 The various oxides of Nitrogen, scch as NO, NO2 and N2O are represented by the general t e r n N o s . Formation of CO : This is due to the incomplete combustion of fuel, which is a function of: 1 . Tao large a droplet size. i Poor penetratiodturbulence. . Retarded timing of fuel injection. 4 S!ow turbocharger response t3 load change (turbo lag) The result is observed as emission of solid panicu!aks (smoke) and carbon monoxide gas. Tile reaction may be written as : C + 0 2 -ico + % 0; Formation of Sulphuric acid : Sulphur is contained in the asphaltencs present in the fuel When these are burned, the Sulphur is oxidised by o s y ~ e n
Q.2.
With respect to the control of emissions from engines, discuss various design changes to the engine, which haveled to reduction in NOI levels. Ans. Primary methods : These can be summarized under various headings : I. Reduce mass o f scavenge air. (Air contains 79 OA Ni). 2. Reduce the combustion temperaiures. (e-hich reduces NOs formation). C~llssderingpoint no. I :
This seems the most obvious soiution (although not practical). Consider the reactions : 8Nz + 202 -> 4 N 0 + 6N2 4N2 + 0 2 3 2 N 0 i- 3Nz Points $3 note : 1. The practical minimum air requirement is the ctoichiometric figure plus the required excess to ensure complete combustion. The quantity of air available varies as a function of the time available for con~bustioll(two stroke has less time, as compared to four stroke, which is the reason for the increased level of emissions in two strokes).
2.
3.
Scavenge air is used (on both two and four stroke ensines) to reduce the thermal load. Thus any reduction will increase the load on the coolins systems. No sca-densesystem is 100% efilcient. So any reduction of air will affect the combustion efficiency. Also, there will be an increase in the specific fue! combustion.
Considering point no. 2 : It is known that, the higher the cycle temperatures and the longer the residence time ai high temperature, the more the NOS formation. Points to note : lil crder to raise thermsl efficiency and reduce specific fuel combustion, 1. the part-[cad pressures/temperatures were rGsed (V.I.T.). 2. In order to raise propulsive efficiency with direct drive e n ~ i n e s ,the engine sp;ed (rpm) wzs reduced. This increased the time available f ~ r combbstion. which 13 turn lead to higher NOx formation. j In order to rsdlice fuel costs, the fuel quafity was reduced. This lead to an increased bcrn-out $me, .vhich lead to hixher NOz formation. In order to limit after-burning, the bu!k o f the fuel cnarqe is injected 3. during the first ;rhase of cynbustion. This increased the cycle tenpcratures. In order to lower'them, fkber varizrions, such as d w b l e injection are being tried out. In order to reduce corrosion due to condensation of Sulphur gases, 5. highly alkaline cylinder oils were used. With the adoption of loadcontrolled ccoling, the amount of acid produced -has reduced signiticantly. However, this is now seen as an incieased level of Silx in the exhaust Secondary methods : Primary methods try to reduce N o s levels, to existing legislation levels, but when more intensive N o r reductions are demanded, by h t u r e legislation, then secondary methods could be used. Another alternative is to 80 in f ~ r electronic fuel injection with microprocessor contrcl, which can control the combustion proass to reduce the emission levels significantly. Secondary methods involve the use ofthe Selective Catalytic Reduction (SCX)S y s t e n ~ ~ Briefly the SCR system involves : 1. Mixing the exhaust s a s with Ammonia. 2. Passing the resultant mixture through a catalytic reactor at a tempeiature between 300 - 400 OC. ZNH, + Heal + Catalyst -> ?Hz + NZ+ Catalyst. Ifwe mix with exhaust %ascontaining NO + 0 2 2 W r + H + C + ~ N & + ' / ~ O Z - > +~3NH~2 0 + 0 2 + C . Ifwe mix with exhaust gas containing N20 + 0 2 2NH3 + H t C + N1O + 02 -> 2 h + 3HzO + C~
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-Combustion, Indicator cards, Fuel pumps and injectors Wn-it&short notes on following : igrlition quality. .. of changing the Ignition quality. eirs Usage of 'Light spring'. Ignition quality : The ability of a fuel to ignite, when under the no conditions in an internal combustion engine, varies. The Ignition q can be considered as an index of :he ease, with which a h ~ e will l i when injected into the hot ccrrpressed air charge in the cylinder. The time which elapses hetween the first droplets o f fitel en the cylinder and the stG of combus:ior, (the delay period) is a ir, of the ease of igcition. Ignition is the star! of the burning p Curnhusrino is t3e complete burning process. The effect of changing the ignition qua1i:y : As all fuels are differen 1hey diiier in rheir ignition quality. Some fuels have a reduced 'igaitio dslzy' period, and are considered lo have a better igniti~nquality. Reducing ignition qualily increases the delay period. A hi igiitiori quaLily fuel will hzve 13w arotnaticaily and hish p r a cori!enr. The large and rapid'heat release, associated with para iiicls, usually counteracts theeffeci of reduced h e 1 mass at igiirio; to i-educed iynitica delay period. T i e residt is an increased pe-k pr-sii~ and better thennal efficiency. The Fuel quality setring (FQS), on the engine, rakes care of t diffknces in ipition quality, by ad-gancing Gr retarding [he fuel timing. For more details, on the working of the FQS system and a ske showing ihr; varioils components, refer to 'Advanced Marine Esi~ineoringKnowledze - Vol.1'. L i g l ~spring i cards : This is a special spring, used in an !ndicstor, to ger a picture cf the pressure variations during scavenge and exhaust processes The normal (heavy) spring is used to indicate the complete of pressure variations, in the cylinder during the complete c;r-Ie.-r?!nze ~ h i s ,however, is not able to amplify the slight changes in pressure during the process 3f scavenge or exhaiisr. The light spring is used to indicate the pressure variations existing in the cytindtr only during the g?ls exchange process, i.e. scavenging and exhaust processes. Since the spring h;is a light tension, the higher pressures make it compress completely and do not show up on the 'Light spring' card. Light spring cards are used to indicate fouling of the scave ports and the exhaust system. From this diagram, the pressure at exhaust vallfi: opening and BDC can be obtained. By superimposing [he scavengi: mariiii~ltl pressure on the diagram the fouling of the S F can be seen.
190
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Q.2, Ans
Q.3. Ans. 1.
Discuss, in detail, the limitatioxls in the recording and assessment of Indicator cards taken by a conventional indicator. Conventional Indicator cards have the following limitations :The pressure sensing spring of the Indicator relies on pressure I. waves being transmilted down a narrow passage. This brings in an error. Also, the actual site of i ~ i t i o ncan occur randomly in the unit, which affects the magnitude o f transmitted pressure waves. This means that the cylinder pressures (from the same unit) can be registered as slightly different pressures, by the Indicator unit. Thus recorded powel variations may be due to errors in the readings by the Indicatcr unit. Modem Indicators, using piezo-electric transducers, have over-come these limitations !o a large extent. 2. The Indicator card is the basis used to adjust engine parameters. We assume that the same diagram is represtntativc of all cycles of that unit. For an engine running at I00 rpm, in one month, 4.2 million cyclrs would occur. It is extremely unlikely that ail cycles will exhibit the same charactefistics. 3. The calculation of the Indicated powcr [of a cylindel-j relied so the accuracy o f a hand-traced P!animeter. The accuracy of the Planimeter, as well as the assumptions taken in pswer calculation, will be poor. Electlonic indicators have an advantqe, in this respect. These take a voltage inpr;t from cylinder pressure transducers, and also accurately measure exact position of TDC, Ikom a sensor mowred near the flywheel. in addition, microprocessors can calculate !he flex in the crankshaft at different speeds, and use this for re-adjustin8 the timing, sincc TDC positions would vary with the flex. The pressure transducers can measure and store a large number of readings. This rneans that 'rogue' readings wilt not be used, but a corrected 'mean' wiil be used to adjust the engine power se!tings. The hand-held unit will down-load to a PC. Readings can be superimposed, to enable faults !o be easily identified. In addition, softwares can be nsed to help in trouble-shooting, which also display standard power calculations and other relevant information, such as ignition-delay, compression pressure, power and other parameters, which are measured by the data-logger at the time of taking the readings. For a detailed description and sketch of the Indicator, refer to 'Advanced Marine Engineering Kncwledge - V01.1'. In case of a problem arising due to receiving sub-standard quality of bunker fuel, discuss how you, a s Chief engineer of the vessel, mill take appropriate steps. A full a. b. c. d.
record should be kcpl of the following : When the fuel was first burned. When I What were the first signs of fuel related problems. \":?at action was taken to detect and reduce them. What action I repairs could be canied out.
What components were overhauled. When w3s the 'off-spec' fuel last burned and its disposition. Chanse in performance of the engine, once the vessel had ceased to bum the 'suspect' fuel. Previous logs or records should be referred to, to show that the engine (or any specific component) was well maintained and within the normal 'service' life, to confirm that the problem was due to the fuel only. I . Any relevait reports from surveyors, undawriters and engine manufacturers should be kept. 2. .4nf damage to machinery pars should be recorded. Photographs should be takzn of the damaged co!nponenks. 3. Copies of bunker receipts must be preserved. Most imponant of all, the ship's retained drip sample drawn during the bunkering must be kept safe. Also a sample ofe!!? pre.kms bunker. e. f.
x.
The following procedure skc~uldbe followed: Exact recxds should be kept of rhe t a d a in which the suspect fuel was placed on delivery, whether or not thcse tanks were Lnitiaily empty and details of quantirles held in each tank. All trmsfers o f 'suspect' bunkers should be recorded in full. Copies of hunker receipts m s t be p r e s e w d The Ouxer can then put the Charterer andig: Supplier on notice. Fie car1 lodge a complaint with the Charterer or supplier, zlleging that the fuei is ~oCfspec' and has caused damage to his vessel's engines. There are certain limitations placed in supplier's contracts. Many bunker suppliers rely on the 'limitation clausi', which seeks to impose a time consiraint on the Pxchaser. during which the notification of a claim has to be made. For example, one coatract states 'Owners to give notice within 7 (seven) days o; any Quality dispute'. Another more reasonable clause states 'claims on account of quantity, qiiality 3r any other claims shall be communicated in writing to us immediately after discovery'. !n law, such limitations are regarded as enforceabie, i n contractual terns, i f they allow a reasonable length to time to discover any defect in quality, in relation to the specific circumstances govemino the contract. This limitation period ranges From as little as 3-4 days to 3-4 months. Q.4. I) Discuss, in detail, reasons for deterioration in combustion. 2) H O I V will yoti trace the causes of poor combustion 7 Ans. Combustion performance can deteriorate when the following occur : I) Incorrect fuel injection within the cylinder, due to A. a) Fouled injector nozzle, Incorrect assembly of injector nozzle during overhaul b) Excessive wear of injector holes, or seating faces. c)
B.' Incorrect temperature of the fuel. If temperature is too low then, 102
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. i
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a) Viscosity increases, thus fuel droplets become larger, resulting in inadequate surface area for combustion and slower burning. The droplet forms a hard layer around it, which reduces evaporation and hence afiects the burning. b) Fuel spray become more compact, which reduces mixing, and hence increases combustion time and increases exhaust smoke levels.
C. a) b)
Fuel pump internal wear. This will reduce the maximum pressure delivered by the pump, as weil a s produce 'late ir~jection'thus : Fuel econnniy will be affected. Penetration will reduce, as fuel supply is injected later, againsi nigher gas presstires in the cylinder.
Fouling of the turbocharger and thus low charge ai; pressure a) This will reduce the oiitf;ow of exl~ausrfrom [he cylinder and t l l ~ i z reduce cylinder content purity. b) This will also affect the pressure of scaven_eeair, which reduces the compression and thgs ppak pressures. c) Reduce the q n n t i t y of excess air normally supplied, whicn will increase smoke levels and emissions. 6 ) Reduced air 'swirl', as velocity through the scasenge pons .:>ill be reduced. This affects the mixing and combustion process.
D.
E. a7
2)
Foilling of the Exhaust gas boiler. This will increase the hack pressure and hence fernperattire at the end of the exhaust stroke, affectins fuel efficiency.
Tracing the cause of poor combustion requires the following : 1. Draw cards should be taken to analyze the injection process, which includes the checking of injection timing, possihiliry o f afterbuming and checking the power balance between cylinders. 2. Scavenge air pressure and temperattire. The correct air pressure (for the engine load) could be found from test bed ~ s u l t sor previous records. 3. Fuel viscosi:j should be correct. Checks to be made of rhe fuel viscosity using a calibrated visconleter. inspections are made on the following, if individual cylinders are experiencing poor combustion : 4.
5.
Fuel injector condition. The injector should not dribble, injector tip should be clean, and the set pressure should be within 6% of the maker's recommendations. Check the spray pattern. Fuel pump. The plungerhamel wear should be within limits, with no erosion damage present on plunger / control valves.
(2.5.Discuss the common faults which could arise in fuel pumps. 411s. Plungesf Barrel wear This is caused by abrasive particles in the fuel. i t will reduce the anmunr of fuel injected, the maximum fuel pressure achieved and also will inject i! 'late' in the engine cycle. Caviiation damage This is ca~iscdby excessive pressure drop at the spill ports, prodicing erosion damage just above the helix control edge. Usually a design prfiblem, i t i\,;Il inci-ease plitnger wear, rzducing sealing efficiency. Excess leakage of fuel into camshaft system feootami;lation) This is a problem in trunk piston engines, since it can introduce low fiaslr ipoirit (fuel) oil with abrasive pariicles inlo camshaft I sump. This is not so :;iuctt a probleni wirh cross-head type engines. It will increase cam wear and iiicreasc the risk of a crankcase explosion (low flash point). Wc::! at tliscliarg: valve In aiixiliary engine fuel pumps, the discharge valve is pl~ovided io proclk~cicnntrolied reduction in discharse $pe pressure to avoid secoiidary : ~ i j c c ! i u w , and reduce time lac to refill pipe-line at the (lest irijecrioii. If the valve: i s worn, this will r e d ~ c ethe quantity of fuel i t rernoiies from the pipeline w1i;:ri scating (and increases seating velocity), arid hence secondary injections are possible. Iivalve leaks or spring is broken, then a cpater quantity of firel ivili 'cscnpe' and hence less fuel mili be injecte2 (compared is other cylinders) at iit:xt injection, producingcylinder imbalance. Fuel pririlp seizure Se:iztii-e can take place Detween the pltingerharrel due to different ~~~~~~~~~~atrires. or at plunger/seal= due to build-up of cilrbonaceous particles. 111 es!ri:rni. 1;ases. the pump will stop reciprocating verticaily, but just as impor-la:ll is l i i c need to rotate and hencestop fuel uelivery. Q.6. With i-::fer-ence to fuel pumps : a)
b) c)
Describe the cam (profile). giving details or the same. Xxplairi why plungers are driven by cams, rather than eccentrics. 'Xi~enwill you need to inspect the cam and haw is this carried out ? Is the torque on a canishaft constant or varyin.g.3
Ails. The profile of the &el cam is dictated by the following requirements: n) 1. Need ro avoid sudden accelerations, which increases the shock '?adins niid thc inorive force required. 2. Slnoolt~deceleration, to avoid the plunzer i e a v i n ~the cam profilc a i tile c~rdofinjeciion, oi-bouircing. , I. . As bowicing speed is proportional to
I~creasingthe nose radius will reduce k and so increase the pern,issibIe speed, without bouncing. However increasing this radius also reduces the effective stroke of th- cam. The Level ofmax fuel pressure required, dictated by the speed of fuel 4. pulp plunger. This can be at constant velocity, or slightly nsin.~fallingbefore spill ports open. 5. The fuelling rate ~f the engine, depends on pump strokebore size, engine speed, snd effective nozzle ares. There is a rapid decrease in fuel pressure when spill ports open. This 6. needs cuffrcient sized spill ports and slowing plunger speed. The amount of dweli angle provided at the toe of the cam profile may take inlo consideration some of the following : i) checking plunger clearance ii) ensure that on four stroke engines, the fuel injection will rake place near to erhaust valve opening timing. iii) To phase spill and filling sesuences, such that pressure fluct~iations in the inlet chamber are reduced. iv) To comb?.- the fuei and exhaust cam profiles on a single two stroke engine camshaft: for reversible engines. The basic cam profile is extremely complex, :akin2 care of the rate of injection, which varies dliring the injection process. Thus, in case of a cam, any mo6ifica:ions can te'easily cmied out, to produce [he best proftle, which ;iiill be a compromise of the various factors invglved. Considering the above, it is extremely difficuit for a syrnmeriica; harmonic motion unir, such as a crank or an eccentric to achieve this. The cam surface should be :mooth and bright. Any light cracks to be removed. Any flaking indicates over!oad - change cam, and check the fuel oil viscosity. Damage :a cams is usually due to fatiguzd spring and result of extensive shock loading. This is when the cam will- fised replacement. The torque on a camshafi varies considerably through the cycle, due ro the action of the canis on the valves and rhe fuel-pump's operatins mechanism. When a cam is opening an air inlet valve or an exhaust valve, the action of the cam, in opening the valve, compresses the valve springs. When it is time for the valves to close, the reaction a f the compressed spring closes the valve. During the period that the valves are closing, the action af the spring forces the roller on to the cam. Some of the work done in compressing the spring forces the roller on lo the cam and some of the work (done in compression the spring) is returned to the engine through the roller, cam, camshafi m d camshaft drive. The returnstroke o f the fuel pump has a similar effect. The same action occurs with exhaust valves opened by the pressure from a hydraulic pump and closed by the action of a pneumatically opera!-d +ran.
Advanced
Q.7.
Morine Enginecriag Knowledge Yo!. 111
Is the torque on a camshaft constant o r varying ? What effect does camshaft torque variation have on the camshait drive ? Ans. The torque on a camshaft varies considerably through an engine cycle due to the action of the cams on the valves and fuel-pump operatin g mechanism. When a cam is opening an air inlet valve cr an exhaust valve, the action of the cam in opening the valve compresses the valve springs. When it is time the reaction of the compressed spring closes the valve. for the valves to C~OS?, During the period that :he valves is closing the action o f the spring f ~ r c e sthe roller cn io the cam and some of the work done in compressing the spring forces the roller on to the cam 2nd some ofthe work done in compression the spring is returnttd co the engine through the roller, cam, camshaft and camshafi drive. The return stroke of the fuel pump has a similar effect. The same action occurs with exhaust valves opened by the oressure from of 2 pneumatically operated piston. if a hydrau!ic pump and closed by the a c t i o ~ we torque transmitted by the camshaft to a cam is plotted or. 2 polar lwizting mGment diagram, i t will be seen that during the period a va!ve is opening the torque wili be positive and when the valve is closing it wiil be rxgativs. When the sommation of the torq~e're~uirements for each cam is platted, it wiil be seen that the torqoe requirement for driving the catnshaft vaees consi6erably t!xoughou: the cycle. As 'rhc number of cylinders increase, thr vxiation in camshafi torque decreases. As the torque requirement to drive the camshaft varies, the load on the camshaft drive also varizs. This load variation may cause problems in the dliving gears, whick p a y chatter, within the limits of the gear backlash, or cause the roller chains to vibrate and swirg. Gear-wheel chatter, roller-chain vibration or iransverse swing, increase the load on the camshaft drive and may 'cause dificulties in service.
4.8. The roller chains used to drive engine camshafts eventually increase in length, while in service. How is this increase in length accommodated and how is the increased transveise vibration catered for, when the chain stretches ? Ans. At regular intervals, the 'stretdh', or increase in length, of roller chains is taken up, by adjustment of the chain tensioning device. This usually consists of an idler sprocket wheel mounted at one end of a lever. At the other end of lhe lever is a nut fitted in a yoke. A screw passes through the nut. Turning the screw causes the lever to move about a fulcrum, so that the idltr sprocket wheel moves towards o r away from the chain increasing or reducing the tension. in some marine engines, chain adjustment is controlled by measuring the transverse displacement of the free length of the chair.. '
e
Adjusrnlent is controlled by measuring the transverse t l i s p l a c e ~ i i e ~ ~ ~ its mid-position, between two designated sprockeis. In othei- engioes n i>e1icui spring loading device is fitted on the lever nut and screw. As tlie chain stretches, the amount of compression on the spring is reduced; restoring the spring comprzssion te ils proper figure gives the chain its cot-rect telision. The tensio% sprocket is fitted to the slack side of the chain. As th chaiil stretches and is re-tensioned, the camshaft sprocket is giadcaliy retarded. This can affect valve t h i n s end fue!-irijzction timing The camshaft mtist th:n be repositioned reletive to the engine crankssae, so that the retarded tii-,~ingi s corrected. In some engines, the camshaft couplings canfior be moved angularly and relativeiy to the camshaft sprocket wheel, and in s x h cases. the indibidual cams musi be re-timed. Transverse vibration o f the roller chain is caused b y I O ~ ~ U P variations in the camshaft and torsional vibratory movement of tlie crankshafr. The anrplitude is controlled by guides and dampers.
Q.9.
a) With reference to fuei injectors, discuss the criteria a n d the imporlance of the hoie sizes of nozzles. b) Whal is the effect on an ezgine, if operared for long periods on i o , ~ ~ speed a n d power 1eve:s 7 c) What effect will fuei, emulsifli-d with water, have gn engines ? Ans. a) Nozz!e hole size Thc two main parameters are diameter and lengh. Usually the iniio Lld = 3, and the diameter depends lipon the quantity of fuel injectio~:required znt! the nombe1.s oof holes. The quantity of fu-1 s fixed by the power requiremeni of tlir engine. The diameter of the holegjs fixed by the need to penetrate the cylinder to a certain extent (60%) and achieve good atomisation. Tne nombet- o i holes is set by the need to achieve a uniform spray pattern, without iutermixing o f the individual sprays. The shapdsize of the nozzle holes are fixed at:he de9.ign stage, and any wear will increase the nozzle hole shape, an5 burr the entry/cxit profile. Atonlisation is mainiy achieved at the needle seat passage, entry profile to the nozzle hoie, s d change of flow into nozzle hole. Note that many manufacturers smooth the hole entry, which stabilises nozzle characteristics. Thus changes to the diameter and entry profile will reduce the amount of atomisation which occurs. A change to oval from round shape wi:l influence the fuel droplet size spread. Slow stzaming nozzles can be used when regnlar and prolonged engine operation is required between 50 - 80 % power. The nozzle hole diameter is reduced to :
1. 2.
Reduce the penetration, which occu~sinto the less dense cylinder gases. Keep the atomisation level and injection pressure sufficient, as mass now rate is reduced. 197
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b) Low spfzd and power operation If the engine is operated for long periods at low speeds and power levels, with 'normal' size injector nozzles, then - engine noise, mechanical loadins, extiaust miolte, exhausttemperatures and fuel consumption will in&ease. c) Emnlsilied fuels Waier can be introduced into th: cylinder either by a separate injector, or csiiuisiried with the parent fuel. Stable emulsions are more easily formed with FiTO and ihe level o f water in the file1 can rise upto 30% (more thzn this creates prohisms with the fuel pump), but 10% is found to give the best resrlts. When thc iuriihm emulsion is injected into the cylinder, the water droplets cause a fali ii: ihi: ri:suliarit gas temperatures. This leads to a lower level of NOx formztion. 1.
A small reduction in the specific fuel :onsumption, mainly in ~ I d e r i:rigiries, which operate with lower Fuel injec;ion pressurrs.
-.,
A rsdiuced flame temperature, which ieduces the fornationof sdot'and
NC?, Iwels. With a high level ( 30% ) cf water, NOx levels could be rerli~sid by 10 - 15 %, bcr the high consumption of fresh water and the ixi-e&sedspeci5c fuel consumption mike the ecmomics debatable. 3~
An increased ignition delay per;od, witn the increased vaporizarion of hiel, increases both the maximum pressure rise during combustion, ar.d XISO the tendency to 'knock'.
?.
An Emulsion will increase the viscosity of the injected fiie!, thus changes in the fuel HeatindSe~vicesystem is required, i.e. grzatcr heai ir~pui,coupled with an increased discharge pressure, to preven: boiling of i'r,? water content.
Q.lO.
respeel lo combustion in marine diesel engines, discuss effects of: :i. Over Penetration. b. Scored needle, in the fuel injector. c. Weak spring, in the fuel injector. d. Slack needle, in the fuel injector. Ans. a.
O r e r Penetration : This will occur, when the gas density within the cylinder is rzdruczd or with over-size holes. The liquid streamtravels too far into the cylinder, so that a high level of fuel impingement on the liner wall takes piace. Tliis will remove the liner lubrication. This will greatly increase 'e the liner wall temperature (as well as thermal stresses). Gas density .? rzductjon is due to low scavenging pressures and this occurs during low power operations. Hence, when prolonged slow steaming is carried ou[. smaller diameter nozzles must be used to compensate, and avoid Overpenetration. 198
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. . . ..
b. I
s..
Scored needle : Similar to slack needle, which will cause increased friction in the needle operation. The fault differs from 'slack' needle in the observatior: of tlie defect and that fuel leakage will stili be relatively smal!, compared with a 'slack needle' fault. The cause is the same, as that causing a 'slack' nzedle.
c.
s2ring in an injector : This will cause the injecior to open and close at a lowei-pressure. T h x , the fuel droplet sizz will increase during these injection periods. The effect of increased droplet size (at stan of combustion) wi!! increase igr.ition de!ay 2nd an i~creased tendency iwvards diesel knock, wherhereas the effect of &? increased drcplet size (towards the end of combustion) will increase smoke z?d HC levels. Weak spiinss will lead ti, metal fatigue, over a period of time.
d.
Slack needle in an injector : The cause of a slack needle is excessive wear, usualiy due to ?oar fi1:rzticn of fuel. This causes uneven wear between needie a~.dguide. The effecrs are similar :c that o i h a v i l ~a ~ scored needle.
0.1 i . With reference to Cueis, discuss the problems associated with storage of fuel oil and the problems due to inorganic sludge (contaminants). Ans. Storage problems with marine fuel. With the dimillishing reserves of oil, and the consequent rise in prices, i t h;ts Lecome important to extrxt the riaximum out of the crude oil, to make i t economically viab!e. The ability of Kefinenes to extract more and more high giade fuels I distillates from crude oi!, has resulted in a coxentratior. c f contaminants and the deterioration of the quality of the residues. These residues farm the base of marine fuels, which have incre-aed the
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!i
<-
;
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storage problems.
i~
i-
!
!~.
. ~.
Sludge : This separates out from marine fuel oil in storage.-It may consist of carbonaceous material from the fuel, wax from the fuel, water, organic and inorganic substances, such as tank scale. Wax is contained in most fuel oils, the greater amount being in :he residual component. If a fuel oil is kept suitably heated, the wax remains . dissolved. ...
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:
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:.,
::water contamination often occurs during loading, due to . . ~Water .. cdntamination kith sea water (more dangerous). It may occur during .. due to condensation, or during tank heating, due to leaks in coils.
rain, or storage,
Q.1.
Crosshead, Bearings and bolts W h y are there astern faces, on the Cross-heads of unidirectional engines? What are the reasons for limiting the Cross-head guide clearances ? What a r e the reasons, that may necessitate Cross-head replacement ?
Ans. The cross-head guide, which cames the load during the expansion or working stroke, is called the 'ahead' guide. The Ahead cross-head p i d e canies the cross-head side thrust, during the wcrking s!rok=, when the engine is ~ n n i n gin the 'ahead' direction The other guide is termed the 'astern' guide, even if the engine does not run in the astern directim (uni-directionail. The guide is provided to maintain the baiance ofthe working parts. As the Piston passes TDC or BDC, there is a reversal of load on the cross-head ,ouides and slipper. Excessive ciearances would rcsdt in shock loading, when this reversal occurs, with the possibility of mechanical damage to both the top end bearing and the slipper. The oil film would be squeezed from between the surfaces. Misalipnent of the piston rod ill iis $and and of the piston in the !iner could occur. In two-stroke engines, the Cross-head beinng is iini-directionally Isaded, and there is no pericd when the bottom half is 'relieved' of load, un!ike Tour strokes. Thus, the lubrication of this bott3m halfis extremely critical, and special care needs to be taken, to ensure h a t wear is within the ! h i t s stipulated. tiowe-~er,if the 'lcaded' surfaces of bearing or journals are hezvily worn, i.e. more than one third of the contact area is 'scored', or, if roughness has caused a large area of the bearing surface to be 'wiped', then the Cross-head may nerd over-haul. Since it is not possible to rotate journals through 180 " , t o use the unworn surface, the bearing may need renewal. Polishing with hemp rope and mild abrasive may take care of minor scratches, but will not remedy damage due to roughness. Excessive ovality or cracks will also necessitate replacement. Q.2.
State the factors, which govern bearing clearances, in the following components of marine engines : a) Crosshead guide. b) Top end. c) Bpttom end. d) Thrust Bearing.
Ans. Factors governing clearances a) Crosshead guide : Upper limit -
The need to keep good alignment between Crosshead. Piston and Liner, and to avoid impact loads on the Guides, as the Crosshead moves from 'ahead' to 'astern' faces.
L.oir-ct I i ~ i i i t
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1.0allow [he yuides to cxjmnd. and ilvoid s c i z ~ ~:ti- crhc extremes, should alignment nor be conect. If esccsst\-c. pinon rings may contract GI? the liner.
T o p end o r Cross-head bearing : Upper limit To avoid loss ofoil pressure with large convergentidivergent profile.
b)
Loiver limit
To avoid 'edge loading' of the bearing, when the pi11 'flexure' occurs, and the need lo retain clearances, ivllen bearing rsaches the propsr opera8ng temperature.
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Bottonl end o r Crank-pin bearing : I n Need to retain the oil pressure, and prevent rapid jou117aI movememi, which can cause erosion damage ro rllk \\hire m?!al surface.
c) 1
Lower limit
-
Allow some oi: leakage, io prevent nver-heatins and possible reduction of oii vixosiiy, and reduce possible edge-loading.
Thrust hearing d) Uppei limit Prevent Iarse inertia ibrces on bearings, during chansec from anead ! astern. Prwent ensine cranks1i;li mixalignmen:, or running-gear misalignment, wit11 excessive axial movement. ,
Lower
limit
Q.3.
With the Cross head bearing operating under high load a n d a noncontinuous rotation, discuss the importance of : a) A high surface finish. b) Bearing strength. c) Proper lube oil supply. d) Flexing of the bearing.
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Allow rhe thmst pads to tilt and thus generate The required oil pressure.
Ans. a) Surface Finish : The crossheed of a two-srroke marine engine is unidirectionaily ioadcd. This is because the bottom surface is atways under load, whether on rhe compression stroke, or the power stroke. Added to this is the fact that the conversion of vertical movement (of piston) to rotational movement (or crankshaft) mkes place here, so that the resulting oscillatory motion is 1101 sufficient, to generate proper hydrodynamic lubrication. Thns there is no time for an oil film to form, since the lube oil does nor have a chance to enter the bearing easily. The surface finis.': of a hearing is vital. when the oil rthn thickness is so small, that metal-to-metal contact may take
place. In this bearing, the rotational motion ceases twice every revolution, and at this time, boundary lubrication occurs. By improving the surface fini;h, we can delay the onser of boundary lubrication, and reduce its effect occurriny. Also, by increasing the contact area of the bearing, we can reduce tlle specific 1oadir.g. thus spreading the load gradually over a larger area. b) Bearing strength : The strength or fatigue limit o f the bzaring material has a major influence on the operational life of the bearing. If the metal is unable to withs:and the high pressurs imposed on ir, the metal will either yield, or fatigue wiil occur after a 'number' of cycles. To improve the bearing strengh, we can either change the material (to !ln-aluminum), or reduce its ihickness l b y bonding it to a lining material).
c) Proper lube oil supply : The bearings are assumzd to be running 'full' of oil at all times, although it is known that rapid journal movement can starve a hearing. With I!;? crosshead hearing, the stops in the motion and the r e v e ~ a in l movement, makes i t very difficult to maintain oil pressure at the n;min_e or 'bearing' surface. in .order to improve this, some manufactukrs have made supply grooves into !he bearing surfaces. These will act as oil reservoirs;so that the bearing will be quickly lubricated, once motion rc-ockurs. Another improvement is the increase i n the lube oil supply pressure, sincc !he hi+cr pressurc wil: allow tiic bcari81;: to regain the oil film. d) Flcriog of the bearing :
The crosshead pin can be considered as a simple beam, supported a! the ends. If we apply a load in the middle o f the pin length, there is a bendipg moment acting on the crosshead pin. This will causes bending of the pin and c+ae edge loading ofi the crosshead bearing, which the bearing cannst withstand. To alleviate this prob!em different designs are used : 1.
2.
3.
Q.4.
Provide flexible bearing mounts, so that the degree of pin distor!ion is matched by the mounts, and hence edge loading is greatly reduced (Sulzer). Increase the diameter of the Crosshead pin, for the same length. This will increase the stiffness of the pin, so that the degree of bending is reduced (MAN B&W) Mount the piston rod on top of the crosshead pin, so that a full length crosshead bottom bearing can be used, which supponsir~ducesally bending of the pin as well as increases the effective bearing area (B&W MC, Sulzer RTA).
Bearing damages in journal bearing and tilting pad thrust has occurred. Discuss : a) Scoring, due to foreign matter or dirt.
b) c) d) e)
fl
Wiping of bearing. Corrosion. Cavitation erosion. Black scab or wire wool damage. Pitting, d u e to electrical discharge. Damage d u e to faultyassembly.
g) Ans. a) Scoring due to foreign matter or dirt. This can occur due to contamination of the lubricant : Deformation on crankshaft, oil gallerks or cylinder bore, present at the I. time of assembly^ Entertained dirt entering through breathers or air filters, ad panicIes 2. derived from combustion of the fuel. Metallic wear particles resu!!ixg from abrasive wear o f moving pans. 3. 'Din' may cause polisking of the surfaces of whit? metal !in& bearings, burnishing of bronze bearings, abrasive wear of ovei!ays, or 3f other b ~ a r i n g linings, and scoring of both bearing a d mating surfaces, with degrees of severity depending upon the nature a i d size of the dirt particle, or oil fiIn tliichess and type ofbexing material. b) Wiping of bzariog : A 'wiped' bearing ,surface-is one, where surface rubbing, meking and smearing is evidek. This is usuell~due to inadequate iunning clearance, with consequent surface overheating. It may also be due to inadequate oil supply, or to both these causes. A wiped surface may follow disruption of the oil film, due to extreme loading, and shafI vibrations, due to excessive unbalance or journal instability. Wnite metal-tined bearing can be wiped, in both ;op afid bottom halves, due ro inadequate clearance. Overlay-plated copper lead bearing can get wiped due to a barreled journal. If lightly wiped, the bearing can be refitred after cieaning the surface to remove any loose metal, providing sufficient clearance exists. If there is excessive vibration, check the balance, alignment of coupling and so on. In case ofThrust bearings, surface wipiny of white metal lined thrust pads is caused by the imposition of dynamic loads in excess of the fatigue strength of the bearing nlaterial at operating temberature. Fatigue strength, specially o f low melting point materials smh a
Advnneed Marine ~ n & e e r i z i ~Knowledge
Vol. I11
Removal of overlays, by abrasive wear or scoring, by dirt, exposes the underlying lead to attack, in copper-lead or lead-bronze inter-layers. In severe cases, the overlays may be corroded. Inv&tigate the oil condition periodically, . to ascertain the level of corrosion. If severe, it may be necessary to replace the bearing. Eliminate the water in lube oil, b i proper centrih~ing. In case of sulphuric acid corrosion, due to the presence of sulphur, it may be better to avoid phosphor bronze, and use phosphor-6% a!Ioys like lead bronze or sil bronze.
a) Cavitation erosion : A n impac: fatigue attack is caused by the formation and coilz~seof vapour bubbles, in the oil film, under conditions of rapid pressure changes. 'The harder the bearing material, the greater is its resistance to cavitation erosion. in case of cavitation, investieate the practicality of increasing the oil pressure or modifying the groove by tiending the zdges or contours to promote a more stream-!ined flow, or by changing to a h a r k b e ~ . n gmaterial. 9) Black scab or wire wool damage : 'Black scab' or 'wire wool' damage is caused by large die partisles $robably'not less than i m across) czrried into the c!earance space by the lubricating oil, and becoming embedded in !he bearing, which ma.y form a hard scab of material, by contact with thz sieei jcumal or thrust col!ar. This scab will ther. c a d e very severe damage to the mating steel surfacewhich is literally 'machin& away', with the formation of so-called 'wire-wool'. . i The action is self-propagating, once it is started and susceptibility ro scab formation depends upon the nature of the lubricant and the composition of the stee! oft& shaR or collar. Steels c o n e g chromium or manganese, in excess of I%, appear to be particulmly suscep$ble lo scab formation, especially in high speedmachines with bearing rubbing speecis over 20 meters per second.
f) Pitting, due to electrical discharge : Electrical discharge occurs throuph the oil film, between journal and tearing in electrical machinery or on the rotors in fans and turbines. This may occur due to faulty insulation or eanhing, or to the build-up of static electricity. This can occur at very low voltages and may czuse severe pitting on bearing or journai s ~ f a c e sor , both. In exEeme cases damage may occur very rapidly a d the cause is sometimes difficult to diagnose as pitting of the bearing surface is followed ultimately by wiping and failure, .. which may obscure the original pitting. Examine earrhing connections, especially around any insulation, with particular attention to Iittings such as guards, themocouple lcads and water connections, which may be bridging the insulation. ARer replacing the be&ng shells, periodically examine to confirm thereis nb more elect$cal discharge. ~
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g) D&magedueto faulty assembly : Under thisheading are include? 1. Fretting damage dse to inadequate 'interference fit' in flimsy housings. 2. Excessive 'interference fit', causing bearing boredistortion.
Z %.
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Advanced Marine Engineerie: Knowledge
Yol. 111
3. Effect of joint face 'stagger' or absence of joint face relief, in bearillgs causing overheating and damage, in the region of bearing split. 4. Fouling at crankshaft fillets, due to incorrect shaft radii. 5. Misalignment or shaft deflectioa, causing uneven wear of bearing. 6. Entrapment of foreign matter, between bearing and housing, during assembly, causing bearing bore distortion and localised overheating. 7. Effect of ginding the shaft in the wrong direction.
Q.5.
A Main bearing, of a main engine unit, has beec 'opened-up'
for survey. Discuss in detail holy you will access the condition of hearing shells and the actions you propose to rectify the dcfects tintifed.
Ans. The criteria below apply to tri-metal bearing, which consist of a steel shell with lead bronze lming and a galvanic white mctal piating, about 0.03 :c 6.06 mm thick. If a uniform dull grey s-rface is observed, the bearing is iunctionjng perfectly well, m d needs no further attention. Local 1:1ossin:ss: This appearance often occurs on new bearings, afier a short 'running-in' perigd. It is caused by the wear during the 'rnnning. in' of the bearing. The #loss disappears after about a hundred hours of operarion. ~ 'Tozgue-shaped' areas of gloss on one side, the areas being very sharply limited. This is during the ruming-in process, and is caiised by locally uneven lube oil film thickness. These glossy iireas disappear after ~mnins-infor a fairly long time. If large, tongue-shaped and ~ l i g h : ! ~ raised areas of gloss shouid occur after a short running period, they shculd be tightly scraped. Check the bearing clearance. Glossy areas along the hvo edges of the bearing. This indicates that the bearing is overloaded along the two edges. The bearing shells sometimes become concave during operation. If the loss is very in!ense, the bearing should be 'touched up' with fine steel wool or a scraper. Do not use emery cloth. If the crank shaft has been re-ground, check the fillet ~mdiusbetween journal and webs. Heavy pressure on the bearing edge (often one-sided) with revealed bronze aRer a short 'running-in' period. This is not admissibie. It is essential to find and remedy the cause. In most cases;the fillet radius of z re-ground journal is not in order. Heavy pressure near the bearing 'parting tines'. A bearing should be relieved over 10' from the parting line. Remove the marks with scraper or fine steel wool. Heavy pressure below the relief area (on one or both sides). These areas are something characterized by gloss, and sometimes they show cracks and scaling of the plating. Glossy areas should be touched up with tine steel wool, scaling should be treated with a scraper. All loose panicles must be removed.-The bronze may be exposed in the relieving zone.
Advanced Morine Engineering K,rowledge Yol.111
Check the lateral beafing clearance and the bearing cap for misalignment. i Bg~pmarkso~the_oilgraoves. These must be removed, since there is a danger of the oil feed being interrupted. Gaivanic plated overlay worn over a fairly large area (bronze exposed). Aftcr a long period o f operation and with a smooth surface a s well as smooth transitions at the edges, there is no danger. Watch the wear of the shaft, check the bearing fur maximum clearance. If, however, this wzar pattern occurs after a short period of operation, shortage of lube oil may be the cause. Check the surface roughness of the jounial (2/1000mm). Re-polish if necessary For the main beaEngs, check rile crankweb deflections. If necessary, re-align the crankshaft or bed plate. Galvanic plated overlay worn over a large area, bronze exposed, deep score marks both in the lead bronze layer and the plated overlay. Replace the bearing shells, check the surface condition of the journal and smooth it aut, if necessary. Check the bearing clearance. Check the oil filter and the piping between the fi!ter and the engine. If additional hearings are fed ~wiihoil from tius bearing, these beariogs must a!;o be checked. Ensure extreme cleanliness during assembly. Cracks in the plated overlay. These cracks are harmless. The bearing can be fitted again unless a very marked accumulation of crack exists in ?he loaded zone. C&s and scaling in the plate* overlay. Chzck the bond of the overlay. If large pieces break off when isolated patches are pricked wi:h a scraper or another pointed tool, the beariny musr be replaced. Sometimes a dark layer becomes visible. between ihe lead bronze and the plated overlay, when an apparently intact spot is lifted off. This dark layer can appear if the bearings have no intermediate nickel dam. The plated overlay is no longer bonded. iiDepressions caused by erosion or cavitation. These depressions are generally sickie or kidney-shaped. They are recognised by the stepped edges of the damage areas. The borderline itself is very irregular and, generally, smaller are= with depressions exist next to the borderline. These depressions are harmless. However, if they are accompanied by damage in the lead bronze layer, or:--if they occur in large patches (exceeding about 15% of the bearingsurface), the- bearing must be replaced and the engine manufacturer notified. S m h e s in the platplatedoverlaym d in the lead bronze. These scratches - can be caused by dirt, particularly after the initial running-in period. Provided that such scratches are not veIy concenrrated, they are harmless. Deep scratches are best smoothed out with fine steel wool or a scraper. Diagonal glossy areas. Make sure the bearing shells and caps are fined accurately. Measure the :rank web deflections. If the plated overlay contains cracks and scaling, remove the loose panicles immediately.
Wear of the plated overlay over a fairly large area (bronze exposed), but on one side only. Heavy pressure on one edge as a result of inaccurate construction. If the engine has tie-rods, do not align the bed plate by tightening or slackening the tie-rods (frame distortion). If this pattern already occurs after a short running period, it is essential to trace the cause and to replace the bearing fconsult'the engine manufacturer). Pit*. This phenomenon is caused by vibra$ion. Check the bearing for good seating. Check the fix1 injection pump. Check the to~siontil vibration damper. Thc bearing csn remzin. The piated overlay is vely smeary, the iead bronze is partly exposed (mainly along the transversz centre line). This is due to tube oil shortage. Very oitm the outer surface of the bearing is q:ii~eblack with carbon deposits. The she!ls have become convex on the inner surface, as a rcsult of nrnning hot. The bearing shc!!s ;nxst Se rcplacerl and i t is essentia! !o determine the cause. Heavy working traces on the outer surface o i thc bearing awl some areis of fretting corrosion. The bearing must be replaced, paying particui2r attenticln to the assembly instructions, es;lec.ia;;y as regards tightening the b a i n g bolts. If several bearings should exhibit this phenomenon, thz pre-stress of the bearing sheik in the housings must be checked. Pitting on thc joint surfaces (dividing line) of the bearing shells. Check the pre-strrss of thq bearing shells in the housins. Follow the engine manufacturer's instniitioni ?bout tightening the bearing bolt>. ;f pitting occurs on several bearings, notify the engine manufactiirer.
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The plated overiay has much better 'running-ic' and emergency iunninz properties than the lead bronze. Apart from that, journal wear on the crankshafr is much reduced by the plated overlay. If refinishing is necessary, as littie material as possibleshould be removed from tkis over!ay. Since every bearing has to settle in again afler refitting, it is zdvisable not tl open properly running hearings unnecessarily. Q.6.
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Explain why bottom-end bolts a r e prone to failure, eveu under normal running conditions. Identify those features, incorporated into the design of bottxu-end bolts, to inhibit failure. Explain how this tendency is either aggravated o r inhibited-d y i n g main:enance, and what checks are to be carried Oiit. ~.
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The bottom end hearing. bolts o f 4 strokes .are more susceptible to failure, due to the variance of forces imposed on it. In the induction stroke, where the journal pulls the piston down, alternating forces are imposed on the bearing bolts. Hence the fatigue strength of the bearing bolts becomes a factor, as metal fatigue will cause failure below the UTS of a matm-al. Sh, in addition to the pretension load of the bolt due to tightening, an alternative load will be imposed due to the stresses involved. Great care must be
Governors With respects to governors, explain the foilowing terms : Q.l a) Droop. b) Isochronous. e) Governor effort. d) Dead Band. e ) Stability. f ) Sensitivity. Ans p ) Droop' The drop in speed from no h a d to stable full load speed, is called as the Droop
h$ Isochronous : -/V ?so' mms cmstm?; thus an Isochronous governor is one which tries to mkntain a zonzta~i:stable speed, regardless of load. c) Gwernor Effcrt : The resdr~ntfcrce, due to the imbalance between the centrifugal force on the h available to move the fitel contro!. bails and the spring force, w ~ c is d) D e a d Band : The minimun change in speed (increase or jecrzase) required, befare the governor can take any action. This ccuid change, due to the friction in the actuating mechanism. e) S t ~ s i l i t y : The opposite of sensitivity - i.e. the ability to reach equilibrium (stable speed) ,. for different loads, with a minimum of hunting. f) Seusitivity :
This is a measure of the accuracy, with which the Governor vies to maintain a desired speed. Any deviation in speed produces a corrective action. A sensitive or 'fine' governor requires only a small change in speed, to produce a small output movement. This will lead to hunting, which is undesirable. Thus, rhe iiilal output is a compromise between Sensitivity and Stability. Fropulsion engice governcrs are essentially devices, which are required to maintain different speeds. Alternator prime movers, on the other hand, try to maintain a steady speed, in order to reduce any change in gequency, which is undesirable They must be capable of detecting any change in speed and then applying the correcrive action required, with sufficient force to overcome he1 linkage resistance.
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Friction developing between the moving parts of a Governor, linkages and control valves will cause the Governor to : React with insufficient speed droop. a) Fail to react to small speed charges. b) Have excessive sensitivity to small speed charges. c) d) Remain in neutral position. Give the correct answer, and explain what is in built ihto the governor system to overcomesuch friction. Ans. Fail to react to small speed changes, is thp colrect answer. h) If ws take a pizcticai case into consideration, then we must consider the effeci of fi~ictionwithin the Governor and it's linkages. The engin: speed must now rise, by an amount that will generate zn increase in the centrifirgal force, suEcient to overcome the friction in the mechanism, before any corrective governor action can occur. Similarly a small fal! in engine speed will not be sensed, until the decieasc in c ~ m i h g a lfcrce q c a l s the frictional force present, before any movenxnt ~f tne &el linkage car. occur. TNS range of speeds, withotit response fro% the Governor, is termed the 'Dead band' and is inherent in a Governor. Before equilibrium at any speed is reached, there will be a certain amount of speed fluctuation, or 'huntlng', -bout h e desired speed. The period a d magnitude of these fluctuations will depend on the sensitivity a f the Governor, which is turn deperrds on tile masses d the flyxcights, in a mechanical Governor. The greater the mass, the warserrhe reguiarion and this results in a short period but a large temporary deviation. Conversely small r~lassesgive a long period with small temporary deviations. Explain the working of a hydraulic Governor. Ans. With the hydraulic servo governor, the 'ball-head' (fly weight) no longer acts on the file1 linkage directiy, but controls a pilot valve. This, in turn, controls the flow of oil t o the servo or power piston. This arrangement greatly amplifies the governor eKo& while allowing the bail head to be kept small, for 'fine' governing. The flow is directed :o the servo or power piston, connected to the fuel racks. If we consider the engine running at a stable speed, no oil flow occurs. If now the engine ioad is increased, the speed will fall reducing tne centrifugal force. This will cause the pilot valve to move down, under the now greate: spring force, allowing the oil to flow to the servo piston. This, in turn, will cause the fuel rank to move in the 'increase fuel' direction and, after a delay, the engine speed will recover and overshoot. The pilot valve will close off the flow and open the servo space to drain, due to increased centrifugal force. This action will be repeated, until stability at original speed is eventually reached. This arrangement gives a movement (of the pilot vaive) in proportion to the change in the speed. It also gives v W sensitive control, which is inherently unstable, due to the time lag.
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It is quite common to find a conical or mwrnper-shape0 spring, used as thq 'Speeder' spring, as this follows a 'square' law, rather than the linear law of parallel springs. This give a better relationship, wiih the change in centrifugal force, since this increases as the 'square' of the speed. 43.4
The Governor of a typical Main engine (propulsion engine) is hydra~rlic, with feed-back (cmnpexsation). Explain, with a sketch, how the governor is operating and explain what is meant by'compensation'. Ans. HYDRAWTC GOVERNOR
Fuel racks
The arrangement shown here includes a feedback, between the servo or compensating piston and the speeder spring. The operation of the govtmor is basically the same but now, as the servo moves, oil pressure also acts on the feed back or compensating piston, which resets the piston valve. consider an increase in Load, which results in afall ic engine speed. The centrifugal force is reduced; so the spring force is now greater and the pilot valve moves down, allowing oil flow to the servo. As the servo starts to increase the fuel racks, it also acts on the feedback or compensating piston, which resets the piston valve. This action creates stability, since the movement of the compensating piston, reduces the rate of change of ihe fuel rack settiny, allowing an equilibrium condition, at a lower speed. This is termed as Compensation. Compensation brings in stability, at the expense of sensitivity. The compensation can be controlled by ~ i y i n gthe needle valve setting, to achieve a balance, or compromise, between stability and sensitivity.
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How is 'isochronous' performance achieved in auxiliary engine governors ? Explain, with a diagram, the working of the Governor. Show the Droop setting mechanism. Ans. In cases, where 'load shxiny' i; required, as well as stable operation, then a further modification to the hydraulic governor is required. This is achieved by the introduciion of 'speed droop'. This is a form of 'resei' action:
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AdvancedMarine Engineering Knm&dge
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This,.in turn, will depress the transmitting or actuating compensating piston, causing the receiving compensating piston to liff the floating. lever, which resets the pilot valve. This gives a transient equilibrium, at a lower speed. As the oil in the compensating system leaks throuph the needle valve, the floating lever acts to return the pilot valve, to its original setting and equilibrium is achieved, at the original speed but wjth an increased fuel rack position. This therefore corresponds to the increased engine load. The droop setting lever acts to reduce the speeder spring tension proprticnately at higher speeds, thus causing a speed 'droop'. Q.6
What maintenance practices will you f6:low. to keep a governor ia good condition ? Explain the following with reference to Governors : a) Compensation. b j Local speed setting knob. c) Load Limiter h o b . d) Speed Droop h o b . Ans. Hydraulic governors are not nolmally serviced on board, since they contain very complex and delicately balanced parts. They are returned, for overhaul, to the ,makers, in the evem of woin parts or mechanical damage. The only important aspect, of routine maintenance cn board, is the need for extreme cleanliness, especia!!y with respect to the C v e m o r 02 condition. Regular 03 changes, usmlly every 1OGO h-5, is specified as the routine maintenance. Poor oil conditioll, due to w n t d n a t i o n , leads to^ increased Eriction and .. wear. . . This is the usual cause ofgovernor malfunction. .
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Adjustments :C~m~ensationffkedle vaive) : This is only to be adjusted if there is excessive hunting or excessive- sluggishness in normal operation. The adjusrment procedure is normally specified in the manual, but is usually adjusted &em 'full operf, then gradually closed until engine just s t m s to hunt, an2 then opened another '/r turn. There should be no hunting, during normalrunning. Local speed setting knob :
This is in ' b e of remote conrol system failure.The b o b on the dial is turned, td directly change the tension of the speeder spin& to achieve the desired speed. The same thing is zehieved by the remote control, which uses an electric motor (mo,mtid above the governor) to change the spring tension. LoadLimiter knob : T h i s is used to limit the 'stroke' of the power piston Thus, it limits the . .~~ rn&&ximiiGl, and Consequently the maximum load, which can be safely &&Tied.This 'may be required, if engine problems donot g o w load above a certaih'poinf;onthe engine of the prime mover. ., .
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Speed droop knob : This knob controls the Droop setting, for fine adjustments, when load sharing is required to be altered, between different Alternators. This is no: generally adjusted. (1.7.
;I) Explain the functioning of a n Over-speed trip.
b) What a r e the advantages of an 'electronic' Governor? Ans. Over-speed trip : This protects the engine from excessive speed, in the event of a governor failure. This is usually independent of the Governor. When the engine speed exceeds a set zmount (e~g.15% of the normal speed), a weighted bo!t overcomes it's spring force and strikes a lever (knife edge disengages) and fuel is shut off. This requires a nanual reset. For Auxiliary engines : Over-speed is set at 15 % o f the nornial speed. Over-speeci is set at 20 % of the normal speed. For Main engine : Main engine speeds are much lower than the normal rpeed of an auxiliary engine. Aiso, the prcpeller acts as a speed regulator, since i! has a lot of inenia. b) i
Advantages of Electronic Govwnors : Extremely fast respcnse (do not use mechanical weights). S o friztioi;, hence negligible 'Dead band'. (ii) (iii) Simple instaliation and a d j u s t ~ e n t . (iv) Microprocessor controllers, which are fa: superior tc any possible mechanical arrangement. Suitable for load sharing duties (easily adjustable). (v) Easily adapted for extra functions e.b. can use more, than one (vi) sensor, to reduce the possibility of over-speed, or failure of a particular sensor.
Refer to 'Advanced Marine Engizteering Knowledge descr~pt~on of the Electronrc governor
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Q3. With reference to ihe functioning of a Governor, explain : I.
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Why 'Droop' is necessary for parallel operation of Alternators.
The relationship between two engines running in parallel with similar droop characteristics.
Ans. When two diesel enzines are synchronized either electrically or mechanically (geared), then the speeds must be constant, if they are to share the 'load' equally. It is then the governor characteristics, which determine how the load is shared, between the two machines. The characteristic for a governor ~vith droop is a 'drooping' line, as compared to one which is having a straight line or zero droop (isochronous). It can he seen, that for a droop characteristic, there is a proponionate fall in speed, as the lrad is increased. This speed droop is necessary for parallel operation of the machines, if stable operation is to he achieved.
I f we consider the case of two diesel alternators operatins in parallel,, then the required characteristics and the load sharing operation is shown in the following figure :
The top left diagram shows the situation afier synchronizing, with all the ioad taken by machine number i As the speed control of n ~ m b e 2 r is increased, it takes ?art of the load and at the .same time the frequency is increased. The bottom centre diagram shows the situation after the speed control on number one machine is reduced, giving more load to number 2 macnine, and returning frequency back to normal. From this it should be obvious, that load sharing is achieved by alteration of the fuel settiilg of the machines. If the governor characteristics are isochronous, then there will be no crossing points along the characteristics and the load sharing will be unstable. If speed droop is incorporated on a: least one machine, then there will always be a crossing point at any given load, and stable operationwill be achieved. This is shown on the following characteristics, for the case of isochronous governors and also for both having speed droop.
Starting Air Systems With reference to Air starting systems : Q.l a ) Why is the timing of air start affected by the exhaust opening ? b) w h i t is meant by overlap and why is it required ? c) Describe the relevant safety devices a n d interlocks. Exhaust valve I port opening puts a limit to the period of the 'air kick', since any more supply of starting air (after the exhaust opens) would re;ult in the iompressed air passing directly out of the unit, witliout doing any effective work, of turning the engine. This imposes a limit on the effective stroke of air start, and !hus limi!s the period of startinz air supply ro a unit. Overlap is the period, when two or more cylinder air starting valves are simultaneously open. This ensures tt.at another cylinder air sizrtiiig valve is opened, before the earlier unit valve closes. Since the tolque is varying during the starting period, h e to the angularity of the conneciing rod, when one unit >tarts getring air, iis ro:que is insofficknr in magnitude. Due to the overlap, the earlier unit is still getring stariin: air, at a higher torque. This makes it easier to srart the engine. Safety devices and interlocks will compl-ise o f : Pressure relief device - 'reli:f valve or bursting disc. (I) (iij Flsme trap. Son-mum valve in rhe Automa!ic air startins valve. (iii) (iv) Turning gear interlock. (v) Running direction interlock. (vi) Line drains. (Vii) L.O. Low pressure trip. (viii) Jacket cooling water low pressure trip. (ix) piston cooling low pressure. (x) Fuel oil low pressure trip.
Q 2 . Explosions have occurred in the starting air pipelines of marine engines Describe the conditions, which could lead to such an accident, and how an explosion may be caused. What precautions should be adopted to obviate the risk of such a n occurrence ? Ans. Starting air line explosions occur due to a combination of three factors : Fuel - which may be lube oil carry-over from air compressors. or unburnc fuel blowing back i n from leaking cylinder air starring valves. Heat - which could be from the same leaking cylinder air starting valves 01 from anorher heat source near-by. Oxygen - which is abundantly available in the starting (compressed) ail- line.
Precautions : Keep rhe air starring system lines clean and drained. when ilot in im. Regulal-ly over-haul cylinder air starting v:dves. Maintain air compiesso~-r in good condition, to avoid excess carry-over of tube oil into iiir lines.
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Safety devices : All safety devices in the air stariing system should be al\vays in good working condition.
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What normal maintenance of a i r starting valves is required and what are the consequences, if this is neglected ? Ans. Reguiar checking of valve tightness (condition) shou;d be done, by feelins rhe air stanins p i p s by hand, while rile engine is running. Every 6,0C9 to 8000 hrs. dismantle, clean and regrind, where necessary, each cyiinder air stwting valve. Excessive grinding ca< reduce the clearance between the operating piston iuid the valve cover Aiways check that !his clearance is scfficient (this shoilid . normal!y be abou: I mm) The copper joint ring to be renewed at evc:y werhaul.
Consequences of neglect : 1. Leak from a air starting valvc a n gc! worsc a! an iccreasing rare - irrepiirable and costly damage may occur. 2. Hot gases ;unbunt fuel may pass into the sir sta:ting system - fire haznrd. 3. Lcss of compression, during running, will affect h e combustion efficiency. 4
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Describe the actions to be taken, upon receiving a teiegraph order for 'Emergency fcll astern'. What a r e the factors to be considered ? Ans. Inst?uctions :(B&W L-MC) Acknowicdge the Telegraph and slop the ensine (F-~eilever to zero). a) Wait till the engine rpm falls, to the required level. (i.e. the engine speed b) < 40% MCR speed). Ring Astern on the Tekgraph and give the 'air kick', to apply the 'Air c) brake'. The engine is now braked Ci.e. it comes r o a halt, sufficient long enough to reverse the direcrion of rotation). The engine is now to be ran in the Astern direction, at a specified rpm, whichJs-not ro be exceeded. This is because rhe ship may stil! have 'way' on it, and the action of the Astern mnning will put a tremendous load on the engine. Heavy vibrations may occur initially, due to the 'way' on the ship. The d) Engine rpm is to be kepi low, during the first few minutes. Do not waste siaiting air. If the rpm is too high to stop the engine, give a new stop eider and wait for rpm to fall to the specified level. before attempring to restart.
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Explain how the starting and reversing of a large main propulsion engine is accomplished. Describe an arrangement fitted to prevent the engine front being started in the 'wrong' direction. Ans. Starting ar?d Reversing : AIR STARTING SYSTEM
The Air Bottle is linked to the Air starting manifold through l l i t Autornaric air starting valve, which has a non-return valve to pi-event ihc possibility uf a blow-back, from a starting air line explosion. to the Air bottles. The Automatic air starting valve is operated by means of [he pilot v;llw shown, ~b~hict! in t u n is operated by the starting lever through the starting valveTile tturnirig geai- interlock prevents the enginz from being sterted, with [he turning gear engaged. The starting air, from the starting valve. also acts 10 engag: the Distributor. This is a safety feature, which prevents the engine fro111 accidmihly bcilig stasted, in the event of air leakage from the Automatic ailsta-ting valve. The cylinder air starting valves are operated by the Distribt~tol-.ill the cwreci sequence for starting (the Firing order). Rerr:rsil,g : In case of reversal, two things need to be done - fizstiy the fd csnls rieed ro be re-positioned correctly for the astel-n direction, and secondly
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the Distributor cam needs to be repositioned, to give the correct sequence for starting in the astern direction. In the engine shown, the camshaft is repositioned by a reversing Servo-motor, lo do bcth these thing;, since the fLlei cams are mounted on the camshaft, and the Distributor also gets its di-ise the same sout~ce,i.e. the camshaft.
Running Direction interlock : I t the direction o i rotation of the en,'-we is contrary to the command from the Telegraph, we consider i t as the -wrong' direction. In this siruation, the fuel cut our servo must operate to shut-off fuei, in case ihe ecgine is already running. In addition, the stailing air is not allowed to be released, thus preventing the restarting of the engine in the wrong direction. The Rcming direciion interlock in this engine is connected to the camshaft, and wilj be operated by the movement of the camshaft. If [he camshaft does not revct-se, tifen oil pressure doer ::at act on the fuel c w o f f servomotor. as call be seen in rhe sketch below. Thus fuel is cut-off.
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Advanced Marine Engirzeedcg Kmwledge
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. ..... Q.6. .Wit%-refererice. ., ., t ~ a i ~ s ~ r t i n g ; s y s t e m s , oboard n shipigiv@reasons:for. i . .<-.'*~...,&':P.-~Ax.<.+ , -.<,-. .,.~..~,?- .... f a .i. ~~. , u . ~ ~ ~ ~ ~ ~ ~ F l m g 2 ~ ~ ~ Y s t efiiliiie:to'reverse, ~ ~ s ~ ~ s ~ froer aa ::s o n ~ f ~ .. -propulsiok d i & I - & g i n ~ ~.%I : Im- ~3n:ir:;Jr &'o s t r o k e mazn Ans. Failure of a i r starting system : Starting air supply blocked. due to Receiver stop valve being shut, Automatic ail- srarting valve not openin2 or loss of control air to pilot valve operating the Automatic air strrting valve.
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Elocking device operating : Turning gear e n g a g ~ dor the Telegi-nph \vro!iz Jil-eftion inter!ock, Fuel cut-off serT!omotor opei-acing due to operation of 21 ti-ip - viz. Low lube oil presure, iow fuel ?ressuie, low pistoi: coolin: pressure. Air starting valve defect : Cljeck for seized valvr (by turning it). Remote control sj.z!em failure : Firs: check the control air supply. If c o n m i air pressure is satisfactory, siart checking tiiz complete system, from the staning lever to [he autv Air starting valve, to the cylinder air srarring valves - go !hrough the whole siicuir. Cherk that Fuel Limiter has not operated (low Scavenge air pressul-e). Cl:cck for a Gc.;erncr fault. whether the Booster Is operatin:- to move !he iuel racks. whether the boosrer air signal is being supplied. at the Lime o f rtarting Chtck the Fuel supply^ Clicck the linkages in the system. Ciieck the Interlocks, if any haveoperated. Chcck for an Air-lock, due to '2bsing' of the Fuel system Engine fails to Start / Reverse : I 10 Szrvomotor (camshaft
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Check the signal
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Check for Servcmotor faults.
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Check for Cylinder air starting valves faults.
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Distl-ibutor faulty / Governor faulty Booster servomotor faulty.
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Sticking control valve in pneumatic control system.
Regulai Maintenance : I. 2. 3.
Main starting air components. Supply (control) air system. Regulai- testing o f components.
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Engine components Q.1. a) Enumerate the defects which could lead to cylinder cover failures in two and four stroke marine diesel engines. b) Briefly state how repairs to partial cracks could be carried out. Ans. Defects : a) C o ~ m o nfailures arc cracks, which occur in the walls cf rhe coolins space. Cracks propagte from the watsr side. These ofien penetrate par[ 'vay through the wsll. This is associated with scaie or sludge build-up. which redcces the heat transfer rare or corrosion weakening of the materia!, leading [o mechanical saess failwe. Other common defects are: high temperature corrosion of the underside. gas erosion and acidic corrosion. due to leaking exhaust v a h e cage seais. cracks, mechanical stress induced by either over-load condition or tinevrtn tightening-down procedures. Four stroke engine cylinder covex, tn addition to the above. are niio prons to cracks forming at the thin sections, between the e x h a m rnd inlet \,ahQ. seats. This is w a l i y caused by imuificient cooling, alon: with !iigi~ diffei&ntialsin iocai ;ernFzraIures. Partial crackillg can o h he repaired by grinding out and then, afrer suitahic build-up with multi-pass welding. checking with NC1T. This rnetl7od minimiies distortion, as annealing 1s carried our or; i h e previous runs of weld. before tlic x x t stage is~carr~ed our.
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d) a)
With reference ro Cyiinder head vaiws. discuss : Properties required for materials of such valves. Materials suited for valve stems, valve faces, seats a n d valve cages. Valv:: types a n d arrangements. Springs ir? pairs, series o r parallel. Ans. Y roperties required : Mxititain strength at high temperatures. I lave good creep rc:;ista~icc. .1: o : d y machined Good corrosion and erosion rcsistmce. Good computability with guide materials Good wear resistance. Low thermal expansion Typical Nickel based nlfoy
b) Materink : Valse laces ot'ren have a stellite layer welded on, which gives a hard corrosion ~.zc::.istant finish; which is maintained at high temperatures. Valve stems may be sui-lice hardened to irnpro\,e wear (chromed, nitrided). Stellite
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Mo C W(;'ttiigsten) 20?4 IS% 2% 10% V:aive seats are often Kirnonic, with stellite facing. This $ves good corrosion d i d w x i o n resistance, 13 prolong the service life. V ~ l v ecages arc usually Cast iron, with the stem guide being a separale item tisudly pearlitic Casi iron. This is cheap, easy to manufacture and the guide ma;oriai has good computability. 50%
Xigh mnperz:sre corrosion : 1. iish conicnt of fuel. 2. Seat damage increases the temperature ievel, ivhich in turn accelerates the desti-uciign process. 3 . i;ornpoi.ents subject to attern~tingsrress will suffer corrosion dama&!ea1 a iower ten?ptraiure, thin a zompanynt subjeci to a uniform strezs. c] VaIvr tylm 2nd arrangements :, Tiie iuajoiiiy. of vsives in use, are popper typzs and arc fiited in tither sin$e (iarse) or miii (smali) valve arranzements. SiiwJe (!arye) valve : siinpk operating gear. Siriiplevalve construction. Simple cylinder head required. Mi!l!iple (srna!lj valves : Srnailcr valve lift required. Lower inertia, since components usually much lighter. Volunietric efficiency improved. Contbustion chamber design more flexible. Lowcr mdal temperatures. Less distortion of valve lid at operating temperature. d) V:rlve spi.ings : Springs Eire required to support the mass of the valve in the cylinder head. Tiitre are sevei-a1 spring arrangements in common use. some employillg si11+ slxings whilr: others employ multiple springs~ 5in::le spr.iog : This is the simplest solution and it vibrates at a lower natural freqwiicy, lrowever there is [he risk of valve bounce, buckling risk for loll5 sp~ii!.jsarid Iwgc diameter springs have higher stress and bending moments.
Springs in Series : This rcduces bending stresses. Reduces buckling bur is more compler. Springs in Parallel : Parallel springshave reduced inertia, are more complex. this arrangement alters the narural frequency m d so avoids axial vibration. It also gives safety factor in event. .of any one spring breakage. ,
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Air springs : This is the prefened arrangement for modem engines, ai ~ 5 ; s eliminates most of the problems suffered by niechmical springs. There is no inertis, which affects the large mass$s.of the single valve of modern eilgiaes. It also allows thevaive to rotate easily, which significantly reduces the uneven wear. The leakage of air, and the increased rnaint'enance due to the need to supply air constantly, as well as the increased risk to the e n ~ i n ein case ~f air supply failure, are some of the draw-backs.
0.3 Wri:e shori notes on following : a) Exhaust valves defects and indications. b) Importance of cooling and effects of excessive cooling. Pas. a) Valve Defects : Exhaust valves are the most prone to the following, but they apply to all types of valves. 1 . Overheating- Insufiicient cooling, after burning, poor combusiicn. overload, nor rs-seating fully. 2. High temperature corrosion : ?his is essociated with a high sulphur and vanadium content of fuel and high metal swface temperatures. 5. Failure to re-seat : Incorrect lappet clearance, expansion due to too high a temperature creep, jamming in guide, damaged spring. 4. Impactdamage : Thiscould be due to heavy seating, build-up o f debris or? . . .~~ the valve seat. 5. Abrasive damage : This is due ta coctaminants in fuel, air or combustion products of cylinder lube oils. Any of the abovefau1ts:ean cause the valve to fail, leading to leakage, gas cutting into and damaging thevalve and seat. .. .-. indications of leakinz valves : High Exhaust temperatures. . ... . . .~ ~. . . * Smoky exhaust. . . Surging of Turbocharger.' Reduced compression and cylinder power. ~
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Importance of Valve coojing :~ . It causes a reduction in texhper8ture,~especiaily advantageous with HFO. It reduces distortion of material, due to excessive tempeiaturcs. It reduces stem temperatures, allowing smaller guidekern clearances.
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c) Operation of the Mechanical Rotator :
With va!vc iq ;he closcd p o ~ ~ l i o the n , Belleville washer is loaded ayalnst the body o i t h e Rotator. As valve opens, the washer is coxpressed a d ioad is trznsferred ro tht: balls, which are spring loaded. e
Further incr-cse in pressure causes the balk to move along :he ramp, thus causing rotation of the vaive stern. As the valve closes, the washer bears agaixt the rotato; body, thus relic-ving the pressure on the balls. n e ramp springs return ihe bails to their origical positions, and the process continues.
ArI~wncedMarine Engineering Knowledge
Q.5
Vol. 111
Describe common failure of piston crowns, of hvo stroke marine diesel cnginer. Give reasons Tor these failures. Ans.
The above sketch shows the faiiuie areas of the pistan crown. This is of the inrernally webbed type. Cracks occuning on the underside are due to henna1 loading. Cracks on top could be due to stress raisers. The upper right sketch show a method of gauging piston crown burn down using a profile gauge. When grinding to profile, depths cf the c r x k s should be gauged (cheekqd by dye peneriant tezt).
Q.6. I n the case of Cylinder h e r wear, what do you understand by the term 'bore polishing'. Describe types of cylinder liner wear. Ans. There should be a gooti surface finish, to produce an effective of seal between piston rings and the liner wall. This profile is produced initially by using coarse honing tools to remove machining irregularities and to provide good oil retention packets. Then a fine honing process is employed to remove the asperity peaks and provide a bearing surface or plateau. The total roughness is around 8 - 15 pm, with a bearing of around 40-70% contact patch, the roughness should be 2 pm. When thc liner becomes too polished, after a prolonged running, the volume of the oil retained on the liner surface is reduced to near zero. This polishing of the liner is removed, when the pisrodliner is de-glazed using rarborundum stones.
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Arlvmced Mnrine En@nzering Knowledge
Yo/. III
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The rate at which the liner wall becomes polished in service depends upon : a) Oil iced rates : If the liner lubrication is sufficient, the increase in boundary lubrication between lincr and ring will cause the liner wear rate to increase.
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c) Materials of piston ring a n d liner : Some manufacturers laser-harden the liner wall, to reduce wear rates, m d this will also affect liner polishing.
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d> Reduction of combustion prodrkts of the liner / ring interface : The hard p r o h c ~ sof combustion, if allowed to make contact with the liner wail, will increase iiner/ring :vzar, and h e r xvall polishing. To prtvent ;?is, a stepped pistor., having 3 rcdxed diametci at the top cf the piston, has been intxoduced. A sepamte firing ring is provided at the iop of the liner, which the top of the pision enters. This step has proved effective in reducing the amount of coke deposits, which c= mb against the k e r wdl, dwing rhe upward movement and thus preveilt the bre&-down the lube oi! film By redacing the rate of liner polishing, the !iner and pistan ring wcar rates are reduced. When the liner becomes too polished the oil is not retained on the liner wall and passcs through the ring pack a n d thus is burnt. If the liner profile is to^ rough, ringllinzr wear is high, and rhe iubriczting oil consumpiion increaszs due ro evaporation from the rhickcr oil fill.
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Firing or combustion pressure : Increase in firing pressxe with increased nngconract with the liner wall, increasing wear.
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Timing of injedon. Wall temperature of running surface. ej Material of piston ring. Ans. a) Matching of lube oil to the sulphur content of fuel : The TBN value of the cylinder lube oil should be correctly matched to the sulphur content in the fuel to be used. Too high a TBN value wi!l result in excessive alkalinity, which itself is eo&sive. It produces calcium deposits (soft). Too little a TBN value results in failure to neutmiise the acids, resulting in a rapid rise in the conosive wear rate. Crosshead engines use 70 - 80 TBN cylinder oil with a standard value of Sulphur content, which could change with a change in the Sulphur content. c)
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Explain the influence of the iolloviing f3ct*t-s on the cylinder liner / piston ring wear rates, in two stroke main propulsion engines : Matching of lube oil to the sulphur content of fuel. 8) Correct cylinder oil feed rate. b)
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b) Correct cylinder oil feed rate : The feed rates are important, and depend on several factors, such as the viscosity of the oil, the number of points where feed is appiied (single or multi level) and the bore of the engine. Even a slight change in the viscosity (e.g. due to temperature) can cause a fall in the effectiveness of neutralising the acid, since insufficient viscosity would. not allow the oil to spread sufficiently.
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Arivirnced Mnrine Engineering Kno~oledge Yo!. III
would reduce the film thickness. It is difficult to give a firm figiiie - it is easier tojudge this in practice - the rate is correct when the piston r i n q arc slightly damp, the piston rings are free to move and liner wear rates are IOLY ttie quantity of oil feed can be adjusted on eacb cylinder, and each quill position. Cylinder lubricator feed rates can be dependant upon engine speed, or load dependant. Load dependant lubricators have been introduced to ensixre the appropriate lubrication under a11 conditions, as the feed rate should be dependant upon the quantity of fuel injected rather than engine speed. EZ~,....: ~,,lvr:thinning
7 ) 'I'imin~: It is vital to inject the lubr oil at the correct part of the cycle, to ensure
thai t h e lube oil enters the liner when pressures are least, and can spread zrifticieniiy to adequatsly cover the e,~tiresurface to be protected, without loss of Huid iilm thickness.'Ihe'normal ?eri;d of injection w d d bz when the piston Is inovirrg upwards, m d the lubricators are bdwecn the top and seccnd piston i s . Unlike four stroke engines, where copious quantities of lube oil is s(~k!ihi.de n rtle liner wall, the two stroke engine cylinder lubrication is critical. Oil cnrtirol rings, and the clearances of the compression rings wii! dictar tbe awial q~.i?"ti!y of !ube oil, which finally penetrates the top (filing) rino,. d) 0 ~ 1 c i ~ x i iiernper;.ture ri~ / L i n e r wall temperature : For the cylinder lubricant tc opciare satisfactory, th? temperature should not be too high, otherwise : 1. F..;apoiaiion (of the lighter fiac!ions)
will increase, leadiny to loss of
I~.~hi~:ation. 2: 7'lie oxidation level will incresse, which reduces the TBN values.
3. ~I'iierrnalcracking of the cylinder oil occurs (aiound 300°C). Normal wall ' !.?rn@rature depends on the load on the engine, and varies from 180°C to 2jO''C oi more. Engins nimufacnirers attempt ro keep the maximum wail temperature aiound 220''C, in order that ring lubrication caL be effective. Too low a wall t&qxr~turcwould lead to over-cooling and corrosion. Optimum temperature is acili::vwl by i s e .of bore cooling and load controlled cooling system. Ovoicooling of the liner must thus be avoided, hence the liner wall temperature shoiuid be i n the region 2G0 - 220°C.
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Ria$ i'/(;rieriirl : Basic material is Castiron. A steel matrix with free graphite at p i 1 1 bo!~ndaries: This give the piston ring its self lubricating property. The follow in^ alloying materials are commonly used : (I) Silicon, which maintains the carbon in a graphite form. (2) Phospiiorus, vanadium and titanium, which give increased hardness to the ring. Wear resistance of ring is usually attained by chrome plating or plasma c o a i i n ~ .Running-in coatings could be copper, laid on top of chromed or non chrorncd soifaces.
Q.8.
Give values for axial and butt clearances of piston rings, as fitted or, Marine diesel engines. Ans. Importance ofaxizl clearance : The piston ring must be free in it's groovs, from the time tile engine is started, and w e n from 'cold' to 'ii~l!~load'cocdi:io;~s.
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Values : For fcur stroKe engines, the axial ring height should be from 1.6 2.2% of the cylinder diameter. For two stroke engines, the axial ring height should be from 1.9 - 2.6% of cylinder diameter. Butt clearances : Normally 0.5% of cylinder Bore, when fitted in the unworn portion of the cylinder linzr. This value is required for ensuring the correct ring expansioq.
Q.9 Ar. engine bed plztc is s 5 s ~ e c l e dto be cracked. Iiiscuss t h e following : aj How will carry out the inspection ?Suggest remedial repairsfmeastires. 5 ) Reasons for cracking in transverse girders. If cracks arc discovered, what action will you, as the C h i d Engineer, take, to reduce such incidents in future ? What rhccm will you carry c u t ? Ans. a) in order to determine, if cracks are present, inspections should be concc;~i;-atei! on the areas, where craclddefects are most common. The initial search sill probably be carried out visually, with cracks appeari& as paint defects The defecis should be recorded, with reference to position, le;;c+? arrd orientatio~~. If possible, etean the sunounding area and use NDT t G improve impeclioil Dye penetrant is easy t o w e and interpret, but masnetic particle inspection (MPI) will show those cracks, which are beneath thesurface. Note that fatigue cracks cccur with very little plastic deformation, and the absence of any deformation ' makes detecrion more dificult. The foliowing course of action could be taken and the extent o f each defect wili determine the specific action : 1. Many cracks are dormant or will only grow slowly and not pose any ~robiems.Always check tension of the surroun&-bolts. A crack can oniy grow, when the stress levels imposed on it are, higher than the strain energy, which it can dissipate through the parent material. Should the crack be found to be growing, especially if acceleratinp, then 2. preventive action should he taken. If shore facilities are not available. then try to bridge the crack, and place the crack affected area i n cornpreision. This.will include dri!!ing a d lapping, but try to avoid 1101 work, as this will increase stress levefs, instead of reducing then,. Drilling the end of the cpck can be beneficial, however finding the iip ' of ;he crack will be difficult.
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If shore faciiities are extensive, then the crack should be removed and filled by metal similar or superior to the parent metal. This will include gouging, by grinding or arc, then welding with possible pre and post heat treatment. Obtain approval of class surveyor, before and after such work, and closely monitor the area during the work. Try and obtain the Welding procedure approval certificate to be used, so that checks tail be rnede, that the Welder is followi~glaiddown procedures. Poorly carried out work may worsen the defect, rather than improve it, as 'delayed cold cracking' may occur. This type of failure is the most common in higher tensile steel, heavier steel structures, and joints involving castings.
The cnid crack resi!!s from a combination of four factors : t . High residual stress in the joint .J-. Small driect to rrigger?he crack * I. hardening in the heat aKected zone ( t I A Z ) 4. presence of dissolved hydrogen !.?arty two of the above can be elimina:ed t h a the crack is unlikeig to occur b
The major forces, which are transrnieed into tkc trmsverse ~il-deis.are the I-unning gear foicei (tra~ismitiedinto the niain bearins) aui' the combustion reaction forces (transmitted viii the tie ho!ts). The !eve1 of [!we forces are dep-ndant upcn !b.e combtis:ion loads, of adjacen: cylinders, and can be increased by crarksiiaft mlsa1i:nrncnt ioadiny on the main bearing itself Thus a crack could occur from : Overload situation, arising from 1. Cylinder overloaded, by excess power or high cylinder pressures. 2. Crankshaft alignment is incorrect I . Tie bolts are inccrrectiy tensioned (i.e. overlunder) IFa crack is discovered, the following p;ocedures could be followed.
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Check power balance, and totd engine power Check tension of all fastening devices, i.e. tie bolts, holding &ownbolts and chocks in the effected area. 5. Check crankshaft alignment using the deflections method i2eco1.d position and length of the crack, using the best methods available, i.e. MPVDye penetrantlvisual. Try to take a photo or trace the crack, so that it can easily be established whether the crack is increasing i i i size or not. To reduce future inciden~s,regular check should be made of this highly loaded area. These checks will inciude those of 1 to 3 detailed above, as well as inspecting the transverse girder at every crankcase inspcction. The condition of the crankcase paint work can accelerate fatigue Failure, if corrosion is present. Ail steel areas should be protected frulll the lube oil which could become acidic if incorrect!^ maintained.
Ad:~anceu'Mnrine Engineering Knowledge Vo?.111
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Electrical systems L O C A L E"": ~ e s c r i b the e operation of an electrical engine Telegraph system. E C K ,y,,4*)d"dER!'-;cAns. SIATIO,~' The telexraph yrovides a means of tran'sn~ittinr: - ordeis from the bridpe - . . a~ control station to the Steering flat. The electric telegraph employs the synchrostep system. The transmitter consists of fixed face plate contacts and a brush . . .. carrier with 3 contacts revolving coaxially. The receiver ccnsrsts of a 3 phase i:ator winding with a permanent ~ m g f i e rG0r t (wound rotor for A.C. operation) which canies the pointer. By energking each receiver coil separlrrely and progressively, and allowing thefreely rotating magnet 1s find its o.wn equilibrium, i t is possible to achieve subsequent step transmission in three positions. If tv;o or more coils are eiergised, then there is better coritrol of the magnet. The actual instrument incorporates resistors to give partial magne:ization 0. of the coils a d so provide up to 11 definite steps, at 15 intervals. Reply b y the receiving station to the transmitter station requires the system to be dup]icared with a transmitkr handle on ?he bridge, positioning a receiver pointel- in the e n g i ~ room; e and a reply handle iii the engine room pasitioning a reply pointer on the biidge. Alarms : Alarms are : A 'curreiit iailuie' alarm, ro indicare power suppiy failure. 2) A 'wronx-way' alarm ' io indicate propeller djrection iontraiy to b) t e l e p p h o r d ~ r The . wrpng way alnr!n is b;~sically;l rao-~vayslvitcll arrangtment, one switch being operated. Sy rhe receiver- motor. an<: the othefswirch by the :ii:inz/propellcr via ii friction cl!rtci:. ~~~~
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Q.2. Describe She sarious types of Torsion meters used on vesseis, f o r measllritig the shaft torque. Ans. Where the measurement of shafr torque is necessary, a torsion meter is used. Various types of torsion meters can be used. Most types measure the angular distortion of a :haft. The following are the different types : , I . Toothed wheel tvoe : Light beam : A beam is directed through the teeth towards a phoro cell. At zero torque, a tooth on one wheel coincides wi!h a slot cn the othelwheel, so that 'zero' light falls on the photocell. This increases as [he shnft torque increases, due to twisting of the shnft. ~
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Pick-up heads : The pick up heads can be inductive, capacitive or photoelectric type prod&ing two alternating voltages.' When'the shaft twists under torsion. the waveforms become out-of-phase, themag~iitudeof the phase difference is a measure of the torque and also the polarity of the phase difference is an indication of direction. The advantage of the above types is, thatno slip rings and brushes or other sliding contacts on the
ildvnrzced Marirze Engineering Knowledge Vol. 111
shaft are required. A bad contact (of a sliding contact) can produce gleat problems. This is so, especially at the low voltages and low currents involved with measuring equipment.The disadvantage is, that cluiie a long axial length of shaft is required, so that, a measurable angular . .. , distortion of the shaft occurs: ... . ... . .. .~. ~
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2. Strain Gauge Type : Two ?esisrance strain gauges are cem~ntzd:&ifielmt.ia! Transformer type :
'[' riaitsformer k cores are carried on coilars attached ro the shaft. A t zcto torque. the air gaps are equai bill when th2 shaft twists under torsion, one air g p incr~hsesand other dccmases. The twd transformer windings are connected in series, such (hat the primaries carry the same- exciting current and the secondaries are in opposition. 'Niih zero torque both air gaps are equal, both fluxes are equal 3nd the output 'voltage V o is zero. With the shaft torque applied, the shafr twisrs, the fluxes and the secondary emE's ctldnge and an output voltage is produced, being a measure of the to-que and dtiection SIlp rings and brushes are fitted, is robust and ~ndlcatestorque and direction 4 . Torductor type :
Tliis type uses the principle thrt the magneticpemeability'of steel is changed by stress, so !hat the output voltage V,, varies accol-ding to shaft torque. There are no slip r i p ~ por brushes, the unit has a v e r y short axial lengih, is robust and is intended For use with t,eavy shafts.
Q.3.
'Enumerate the safety precautions that you would observe, when working on lrieii vortage equipn~ent,on board vessels. Ans. The foliowing procedure is recommended, before any work is commenced :
.:dvtmced Morine Engineering Ki~owledge Voi. 111
A permit-to-work must be issued by a responsible office;. This rnust de in duplicate and contain five parts. The first pan, signed by a responsible officer, designates the work to be done, the parts to be isolated, the necessary earthing . and where 'Danger' and 'Caution' notices are to be affixed. The second part confirms that isolarion and earthing have been carried out and that 'Danger' notices have been affixed to warn anyone approaching 'live' equipment, and that 'Caution' notices have been affixed to apparatus or its control equipment I O convey a warning against interference with such apparatus or control gear The third part is signed by the s a n e person, when the work is completed or temporarily stopped, and the fifth part is the csncellation GF thpennit-to-work. ~
Precautions : To comply with the firs: part of [he psrmit-to-work, befox work car, caniii~ence on 'Live' equipment, the part upon which work is to be carried on1 inns: be in& 'Ctad' i.z. electrically isolated by the means available. In case of l ~ i g !volt;lze ~ appratus, the conduciors rendered 'Dead' mtlst be efficiently earthed mci sIiol-tcircuired. The points, where these operations are to be cari-ied out, shooitl be specified in rhe permit-to-work. PI-eferlbly, some form of voltage indicating device should be used, to ensure that the csnductors are 'Dead' before earthing is applied. ~ n switching y appa&us, o l the compoiient to be examined, which could make !he circuit 'Liv?? ;nl;s! be lcckcd, acd 'Caution' notices affixed. The keys must berctained by the person-in-charge. Even the 'Caurian' notices must b e iixed and removed by the person-in-charse. When it is necessary to work on 'Live' Low 01-medium-voltage switches, p.ecautions azainsr shocks should be taken, such as using insulating stands, screens, boots, gloves and tools. 'Danser' and 'Caution' notices near 'Live' conductors must be constructed of non-metallic materiai. If the work, to be cni-ried OLI:, entai!s the use of hand-lamps, they must be of im aaproved insulated type. Satisfactory earthing must be maintained thioughout the opel-ational period and any at~tomaticfire prorective system must be temporarily made inoperative. where oil-filled apparatus has to be examined acd possibly I-epienished, smoking or the use of nsked flames must be strict!y p~ohibited. Any auxiliary circuits, which might constitute the means of +I feedback, to the work, must be disconnected. acd if there is any equipment of the bartery-opei-ated type, or the solenoid operated type, the fusz controlling [hi solenoid musi be withdrawn, or, if of the spring type, the spring must be dischar~ed. Although it should be a permanent fixture. and i s statutory, where such work is to be carried out, it should be ensured, that n placard for the treatment of electric shock is affixed in a prominent position.
Advarrced Marine '.~gi:~eeiing Knowlerlgc. Voi. III
Discuss the safety aspects of Battery inspection. Ans. The gases given off from any battery on chat-se, whether lead acid or alkaline type, are e a v e . All battery compartments should be well ventilated lo remove gasses, which are emitted while on charge^ Battery examination should never be carried our with naked li$rs. (31-e shoirld be taken, not to do anythin: likely to cause a s p a 1 new the batteries on charge, since the hydrogen gasgign-offi~a tire hacad. .4iJkikaline and lead acid batte.ries must be charged separarely a&niust not be kept in the s-me room. All terminals should be examined, from time to !ime. to ensure that they are tight. M-t-l jugs must not be used for topping-up Battery water. Loose wires and tools mtist not be placed on top of thc ce!ls. h'elllr?lisil~g agents should always be available, in the event of spillage - Boracic powder for salt or washing soda foi- lead-acid bmeries~ alkaline batteries and household Protective clothing should be used, i.e. G i b e r gloves, !-bier apron and goggles. Avoid getring any acid on your boiler-suit, while topping-up Barrel-ies the acid is highly corrosive. Wash wilh plenty of waten f d * & P,&) Xake a note of !he Fire fighting appliances available nzal-by. A t13tice should be posted prohibiting. naked lights and smoking in battery compeltments. Lead Acid Batteries : Always 2dd acid to water, and not water to acid, which could cause acid to fume. Dilute sulphuric x i d is- not necessarily haniiful io :i healihy skin, provided it is washed off, as soon as poss;ble. A splash in the eye however, requires immediate first aid. It should be swilled with water or a diluted saline soiutisn immediately. (Saline solution :one level teaspoonful of household salt to '/2 a pint of water). Alkaline Batteries : As the steel cr!l containers are 'live', d o not allow metal objects to rest or fall between them. The electrolyte (caustic potash) is corrosive and should be handlzd with care. It should not be allowed to come into cmti-act with the skin or clothing. In the case of bums, immediately cover them with boiacic powd--1- 01-n sntul-ated solution of boracic powder. First aid FOI the eyes - wash out with clew water followed by a solution of boracic powder - one teaspoonful to a pint of water. When mixing electrolyte from solid material, it is advisable to wear protective goggles, mbber gloves and apron. Never put the acids, from lead acid batteries, into an alkaline battery, as this will completely destroy it.. Gas-tight type lighting fittings should be used for illuminating the battery l~oom.
Advanced Marine Engineeril~pXnowlerlge
Q.6.
Vol. 111
In a n electrical circuit, what is the meaning of a short circuit ? Why does it occur? IIow can it beprevented ? Ans. The meaning of 'shdrt circuit' is, that the current has f o ~ n d ~ iparh e r back (from the 'live' wire to the n e u s This means, that current is not passing properiy through the appliance in question. This can lead to excessive ~ I I C of ~ ~ currenr, which causes the fuses to 'blow'. Current always follows the pa;h of leas, -resisance, so always ensure ihat your body does not form that path !
Resistance si the body, when dry, is qui:e high; however, in case cf electrocutio~, !he body's internal rzsist-nce falls rapidly. with f2tal consequences. Wear safety shoes with insulated soles, when dealing with electrical equipmer.t. . . 1nsilla:ed pliers are also useful. Never touch any cond~ctor,unless you are sUre that it is earthed. A 'short' is usuz!ly caused by the condocrcr becoming uncovere5 (ie. n break in the insularix covering). This allows the 'live' :able to come ii~to eltctrical contact with any substance, that is a good conductor. This is most common in ships that are 'single wired', as in the case of circuits ashore, as only one cable requires to make contact withthe hull (earth) of the ship, to cause a short circuit. In the double-wired system (insulated neutral) used on board ship, both 'iive' and neutral wires must be in good contact, to complete 3 short circuit. This is fortunately rare, if care is taken to check for any damiige to insulation and sarth f a u l ~and , these are made good, as soon as practicable. Short circuits can be prevented by good maintenance and safe working practices. Always check the insulation resistance, especially wheri~hereare joints in cables. Electrical wires should always be led elearof sources of water and not be sited in places iiable to water logging.
Q.7
How would you trace 3 short circuit ? How do you trace a n 'earth fault' ? Megger ? Ans. To trace where a short circuit has occurred, each sectional fuse box should be examined, to findthe circuit in which the short has occurred. This will be detected by the fuse of the affected circuit, which would have blown.
. What is a
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Advanced Marine Engineering Knowledge Vol. I11
To locate which part of the circuit has 'shorted!, i.e. a short circuit has taken piace, the whole circuit should be examined. A n appearance of burning accompanies a 'short', as there is a surgeof current during the short circuit, which will cause burning. By checking each section of the 'dead' circuit..by means o f a &Tultimeter, the faulty section can be detected. T o trace an 'earth' fault, each sectional switch should be put 'On', in turn. and the eanh lamps watched. T h e earth will be in the circuit in which a difference in the intensity of the earth lamps rakes place.
TEST FOR OPEN CIRCUT Tr, i w x c ar which particuiar part of circail the car:h has teken place, the swiicii of rhi.; <;iv:uii will be put in and a c h section of the circ-ii wi!i izqni;-e to bz i w c d h y dl- Multimeter, as in the case of a short circuit. !'k Caulty section will be indicated by the deflection of meter needle being less. than ar she sections at which no leakage of current is taking place. A Mezger o r Meg Ohmmeter is just a small generator (called a rnagneio), enclused in a case containing a number of resistances in sei-ies. Thezr can bc piug!:cd in 01- out: in order to Zet the necessary pressure o r voltage. Thr: end of the lead, from the resistance in series, is connected to on,- end o f i i i c coinponcnt io be tested and the other end from the magneto is earthed. If there i v an 'earth fault' of that part being tesced, an indication will be seen on the dial of the Megger.
Refer to 'PAarine Engineering Practice' for details of electrical fault-finding.
Q3. W)aan, is m e a n t by a n e a r t h in the cab& of a n electrical installation, on h p ' d ship ? How is it f o u n d out ? W h y a r e e a r t h l a m p s necessary ? Ans, When a cable becomes 'bare' and makes contact with the hull of the ship, i t is ccrrncd as an 'earth fault'. In the case of a single-wired system, the fuse would blow, as an 'carth', in this case, would also b e -a short c i h i t . I n t h e case of the do~.idli:--wiredsystem on ships, having an insulated neutral, an 'earth fault' on both iile clivc' incl [he neutral wires would be required. to cause a short circuit. An 'earth fault' in the 'live' o r the neutral wires, of a two-wire insulated systein, ~wouidhave n o immediate effect on the operation of the machine. It is an
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t a k p when handling these bolts, as the majcrity o f fatigue failures are initiated a s a mechanical defect. The minor stress concentration at the defect allows a crack to start and grow, which eventually weakens the material, by reducing the area which can regst the applied stress. The design of the boEom end bol% is precise, so that even though 3.fati-we life exists, a 6esignsted service life can be assigned to tham, Main design points: Bolt shanks should be snialler in diameter than the thread root, so tllaf 1. greatest stress occurs there, and not at a stress concentration point. 2.
A portion of the shank has a tight clearance in the hole, so that the nut is tightened 'square' onto spot faced bearing housing This prevents the possibility of yield of threads.
3.
Ni fiiiets are of generous radii, to prevent stress wncmtrarions
4.
Iiuusing are designed to allowbolts to be as long as ?ossihle, hence increasing resilience of tEe bolts.
Whenever bearing ars examined, particular attention must be paid to the bottom end bolts, especially those gn 4 stroke engines. Checks to be made: Check for corrosion by acidic lube oil, discard the bolt if any corrosion 1. is observed. especially on shanks.
2.
Check the f?ee length of bottom end bolts in service, against a new one. If they are longer, it is possible that yield has taken place.
X
Check for mechanical damage, especially on shanks.
3
Check for kactures by NDT or: by 'sins&', i.e. sttiking with a hammer to check if the bolt is 'sound'. A defective bolt sounds diffecent.
5
Check land~ngfaces, to ensure that uneven t~ghtenmghas not taken place
Discard the bolts, when either designed kfe is over, over-speed occurs or piston seizure has occurred
l~rndlex Page Nos.
C
Air Compressors 178 - 180. Capacity calculations 179. Clearances : 178. Explosions i 80. Regulations 178. 179. Surveys Air Receiver, tests 85, 178. Alarms 32 - 3 4 ~ Aux. Power systems 34. EIzi2ical system 34. Main Boiler 34. Motor vessels 34. Oil supply 34. Turbine vessels 34. Anchcr, attachment 37. Autonation 59 - 160. Ccntrolltd condition 159. Contrul point 159. Deviation 15 9 ~ Error sicpal 153. Feed-back 159. Meanred value 159. Monitoring element 159. Offset 159. Open 1 closed loops 160.
%last water 27 - 28. 27. Control measures Evaluation 28. Hazards 28. 28. Loading instrument Safety precautions 27. Ballast syxtem isoiaiion 67. Barium Chromate 99. Bearings 128, 165- 165. Roller . 165, 1 6 7 Thrust, tilt pad 165,168. White metal 128. 34,64,66. Bilge alarm 64. Bilge pumping system Boilers 86 - 96. Aux. Packaged 96. Classreprements 90. Combustion control 95. Defects 89.92
Page Nos. Furnace distdrtion 92. Pressure test 86. Safety valve 93, 91. Settings 86. Tube failure 94. Uptakak-fire 92. Waste r m v e r y 88. Water uearmen: tests z6. Bolt failures 207. 33. Bridge control Buoyanr apparatus 54. Bunkerins 15- 18. Documents 17. Disputes 17. lnsriuctions 16. Letter ofPrs:st 17. Oil spill 15. Operatinns !5. Persistent oils 16. Precautinns 15. Qua;ltitp/quality 17,191. Szmples i 8. T e s kits 17-.IS. Unacceptable perccr~tagcs 17. Volatile oils 16. ~
Cable stopper CaICrifier Camshaft Carbon filter COT. Bulk system Narms Boil-off Relief valve setting Central Cooling .system Advantap Chain Locker Combustion r e t i o n quality Poor combustion Communication Conditions of Assignment Corrosion Bacterial attack Cavitation attack Dczincitication
Evaporates.
Page Nos.
Page Nos.
Fatiwe 89. impingement attack 122. 123. Sand erosim Stress corrosion 89. Cranecircuit, braking . . 74. 200 - 205. CrossheadBearings .200.. Damage 202 - 234. Guides 201. .
HzS, effects of 36. G levels 35. Todc effects . TLV . . E n ~ n rcomponents~ . Beddate. Lra& cylihder cover Euhar?si d v e Liner wear Valve defects Valve springf 221. Piston a w n 225. Rotators 228. Ring clzarances 228. Explosimeter 42 - 44. Calibration procedure 43. Elashb& -xestoi 43. Peak t&g~ 43. E L 42. Measurement 43. Operation 42. ~
Dz
52. Gravlry type 52. Lutiing type 52. Rhdiai type 52. Dnk ~Vachincry 85. Deep t a n k 100. Constru~ion,test$ 100. Detectors 38 -41. Alarms 41. Checking lines $1. Fauk deection 40. Hest sensor 40. M a red sensor 39. Ionisatien sensor 38. Monjtorjng 41. Smoke detectors 39. Testingof deteciors 40. Dry powder 46 - 47. Eulk system for LPGLNG
and Safety 29 - 60. 33. Flooding proteaion Fire wnml plan 29. Fire drills 30. Fie p'h, supervision 31. Fire protection 33. ~ - . Safety measures, W S 30. I;ieeboard,requirements 101. Fresh water Generator 80. Fresh water Storage tanks 83. ~ u e injecton l 197 -199. 130- 144.199. Fuel oil Auto ignition 133, 134. Ash particles 130,133. 132: . ' Calorific value Cxbon residue 132. . Catalytic fines 130. Cetane number 130. C M C 130. ,. IS0 8217 . 130. 132." .~ Sodnrrn Specifications . . , 130. Sto&.&z , . . 199. 132. Sulphur m N ~. ~. 3 1 .
Electrical systems 230 - 237. Battery inspection 233. Cable testing 236. Eanhing 235. 231. Safeties Short circuit 234. Telegraph 230. Torsion meter 230." Emissions 187- 189. Acid rain . . 187. Global warming 187. NOx ievels 188. Smog ;.187: Enclosed spaces 34-36. Hazards 34.
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239
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Iindlew Page Nos. Tess Vanadium Fuel Pumps Cam profile Faults
Page Nos.
144
131 194 - 195. 194. 154.
Gearing 176 -177. Defects 156. Examination 177. Governors 208 - 214. compensation 210. Droop 208, 21 1. Deai band 208. Electronic 213. Wydraulic 209. Isochronous 208. Load Liniter 212. Load seiting 212. Stability 208. Sensit&iry 298. Overspeed trip 213. Parallel mnning 214. &ease 134.
Hatchways, construnion 103. Stresses 103. Hazardous cargoes 41 - 4 2 ~ Fire fighting 42. Medical aid 42. Storage 41. Properties 41. Heat Exchangers 75 - 82. Chemical cieanin: 82. Corrosion 75. Defects 76. Erosion 75. Failure 75. Impingement 75. 77. Maintenance Materials 78. Plate type 78. Shell t y p 81. Tube protection 78. Venting 76.
&WG Code 41, 42. Indicator cards 190- 193. Draw cards 193. Indicator 191. LigFt spring &ds 190. Inec Gas 48- 51. Mars 50. Generator 51. Stahl?ory requirements 49. International Shore couplifig 50.
LNG
47-48.
).dim in case of fire Chaiactzristics LSA
38. 47. 51 - 54.
Cenrrihgal brake 53. Falls 52. Eland brake 53. Lifeboats 51. Life buoys 51. L i e jackets 53 Life rafts 54. Lube oils 130,134 - 143, 145 - 147. Alkalinity, h s t 143. Abrasive wear 135, 136. Cloud pint 141. Corn?mor, Viscosiry 142. Comparison of oils 146. Compound type 134. Condition monitoring 137. 135. Corrosion in bearings Cornsion monitoring 138. 136, 138. Corrosive wear Cracking point 141: Crackle test 141. Cylinder oil 131. Detergent oils 141. Elastohydrodynamic 136. Emulsion 135. Fenography 137. Flashpoint test 143. FIoc test 147. Foaming 141. Insoluble conlent 142. Lacquering 135.
Page Nos. Pumps
61 - 76. Axial Row 70. Characteristics 71. Cargo stripping system75. Centrat priming 63. Cen:rifuga: 61, 69. Clearances 61. D~scharge 72. H 1 Q cun8e 72. Materials 61. 62. Priming pump Deepwell type Lb!G 71 Emergeiicy fire pump 72. Gear 6:. Clearances 64. Materia!~ 64 Tooth dzpti~ 63. NFSH 71. Screw 70. Swash plate 73. Psyckfomeler 182~ ~
Refrigeration 181 - 186. Air in system 186. 183. Cargo refrigeration Container unit 181. Flocculation 182. . Floc point 182. Flooding 186. Foaming 185. Moisture 186. 156. Over charse Propefies 182. Regulations 11 - 13. Annex V 11. Oil discharge (tankers) 18. Oil discharge (chemical)l8. Machinery spaces 12. Monitoring 12. .. Special areas 11. Water treatment 82. Re-ine~ingt a d a 59.
Safe operation, lankms Scale formation
59. 80.
Paze Nos. Sewage pumping 14, 15. Comminutor la^ Chemical type 14. Eductor type 15. Vacuum t j ~ e 15. Tznk capaiiry IS. Shafting 148- 151, 154 - 155, 156, 161 - 171. Alignment 156. Bolts 165, 168. Corrosion 168. Crack deteciion 150~ Fair curve method 161, 162. Overheating 148. P i l e wir: 162. Split rdler karing 167. Stresses 169. Suwey !SO. Snipboard f i e organisziion 30. Sludge incineiation 25 - 26. Disposal 25~ Sofiener 87. Startjng air system 215- 219. Overlap 215. Safety doices 215. !2xplosions 215. Maintenance 216. Emergency 216. StMing / reversing 217. Interlocks 218. Failures, reason; 219. 38, 173 - 176. Steering gear Materials 173. Regdations, tests 38,174 - 175 Shudural Fire protection 54 - 59. Dry cargo ships 56. $MethodI C 57. Method u C 57. 58. Method m c Oil tankers 58. Passenger ships 55. Protectio% openings 56. Protection, stairways 56. Protection, accommodation 56. Protection, ducts 56. Surveyi 5 - 7,205 - 206. Anchor examination 7. Boat equipment 5. 7. Hatch mvers '
Pnge Xos.
Null Examination
7.
Hull Painting IWS operation
7. 5. blain bearing 205 - 206. Surface preparation 7. Tl?ic%nssstxeasuizrnent 7.
: Takin: over
8-11. -4s C E 9-. As C E on new vessel 9. Shipyard programmes 10. Shipyard tzsts 10. Sea :riais 10~
V4a:?r-tisht doors A!taciinient Testing ,%# . v atcr irsaimeqi E1zctiokitadyn Potable Ppm value r_~i*ra violet
Untreated water Windlass brake tea
Vibrations 148, 158, 16: - 1 6 4 ~ Amplitudz . 158. Axial 153. Node 158. Mode 158. Resonancc 158.
