Boiler Tube Failure Analysis

Boiler Tube Failure analysis with Modified Design

FAILURE DETAIL 1.1) Failure Types:

There are six reason for failure in Boiler Tube:1) Water-side corrosion 2) Fire-side corrosion 3) Erosion 4) Fatigue 5) Heating 6) Lack of quality control

1.2) Detail Types Of Failures :1.2.1) Water-Side Corrosion:

There are five types of effect due to water impurities. 1) Caustic corrosion 2) Oxide corrosion 3) Hydrogen Damage 4) Pitting (Localized Corrosion) 5) Stress Corrosion Cracking

1.2.2) Fire-Side Corrosion:

There are two types of corrosion due to firing. 1) Water wall 2) Flue gas

1.2.3) Erosion:

There are two types of failure due to erosion. 1) Fly Ash 2) Coal Particle (clinker)

1

CHAPTER-1


1.2.4) Fatigue:

This is occurs due to following fatigue. 1.Mechanical Fatigue( carry over )

1.2.5) Heating:

There are two types of failures. 1. Short Term Overheat 2. Long Term Overheat

1.2.6) Lack of Quality Control:

There are three types of failures. 1. Maintenance Damage 2. Material Defects 3. Welding Defects

1.3) Feed Water Treatment & Chemical Cleaning:1.3.1) Objectives:1.3.1.1 Pretreatment Of Water 1.3.1.2 Demineralization 1.3.1.3 Chemical Conditioning of Water

1.3.2) De-aerator:1.3.2.1 Function 1.3.2.2 Flow Arrangement 1.3.2.3 Design & Construction 1.3.2.4 Main Parts 1.3.2.5 Accessories

2


DETAIL DESCRIPTION OF FAILURES

Chapter-2

2.1) Water Side Corrosion:2.1.1) Caustic Corrosion:

Problem: Loss on the inside diameter (ID) surface of the tube, stress and strain in the tube wall is increases.



Causes: Caustic corrosion occurs when there is excessive deposition on ID tube surfaces.



If pH value of water is increases, it results in a caustic condition which corrosively attacks and breaks down protective magnetite.

Fig. 2.1.1:- Caustic Corrosion

3


4

2.1.2) Oxide Corrosion:

Problem: Aggressive localized corrosion and loss of tube wall is near economizer feed water inlet on operating boilers.



Causes: Oxygen corrosion occurs with the presence of excessive oxygen in boiler water. It can occur during operation as a result of in-leakage of air at pumps, or failure in operation of pre-boiler water treatment equipment.



This also may occur during out-of-service periods, such as outages and storage, if proper procedures are not followed in lay-up. Non-drainable locations of boiler circuits, such as super heater loops, re-heater tubes and supply lines, are especially susceptible.



Wetted surfaces are subject to oxidation as the water reacts with the iron to form iron oxide.

Fig. 2.1.2:- Oxide Damage


5

2.1.3) Hydrogen Damage: Problem: Due to Internal micro-cracking. Loss of ductility of the tube material leading to brittle rupture in boiler tube.

 Causes: Hydrogen damage is mostly occurs on inner side of tube surfaces, coupled with a boiler water low pH excursion.

 Water chemistry is upset, such as what can occur from condenser leaks, particularly with salt water cooling medium, and leads to acidic (low pH) contaminants that can be concentrated in the deposit.

 Under-deposit corrosion releases atomic hydrogen which migrates into the tube wall metal, reacts with carbon in the steel (decarburization) and causes inter granular separation.

Fig.2.1.3:- Hydrogen damage


6

2.1.4) Pitting (Localized Corrosion) :

Problem: Corrosive attack of the internal tube metal surfaces, resulting in an irregular pitted or in extreme cases appearance of the tube inner diameter.



Causes: Acid attack most commonly is associated with poor control of process during boiler chemical cleanings and/or inadequate post-cleaning passes of residual acid.

Fig.2.1.4:- Localized Corrosion


7

2.1.5) Stress Corrosion Cracking(SCC):

Problem: Failures from SCC is brittle-type crack. May be found at locations of higher external stresses, such as near attachments.



Causes: SCC most commonly is associated with austenitic (stainless steel) super heater materials and can lead to inter granular crack propagation in the tube wall.



It occurs where a combination of high-tensile stresses and a corrosive fluid are present. The damage results from cracks that propagate from the inner diameter.



The source of corrosive fluid may be carryover into the super heater from the steam drum or from contamination during boiler acid cleaning if the super heater is not properly protected.

Fig.2.1.5:- Stress Corrosion Cracking


8

2.2) Fire Side Corrosion :2.2.1) Water Wall:

Problem: External tube metal loss (wastage) leading to thinning and increasing tube strain.



Causes: Corrosion occurs on external surfaces of water wall tubes when the combustion process produces a reducing atmosphere.



For conventional fossil fuel boilers, corrosion in the burner zone usually is associated with coal firing.



Boilers operating with staged air zones to control combustion can be more susceptible to larger local regions possessing a reducing atmosphere, resulting in increased corrosion rates.

Fig.2.2.1:- Water wall side corrosion


9

2.2.2) Flue Gases:

Problem: It most commonly seen as a series of circumferential cracks. Usually found on furnace wall tubes of coal-fired once-through boiler designs, but also has occurred on tubes in drum-type boilers.



Causes: Damage initiation and propagation result from corrosion in combination with thermal fatigue.



Thermal cycling, in addition to subjecting the material to cyclic stress can initiate cracking of the less elastic external tube scales and expose the tube base material to repeated corrosion.

Fig.2.2.2:- Flue gases effect


10

2.3) Erosion :2.3.1) Fly ash:

Problem: External tube wall loss and increasing tube strain.



Causes: It usually is associated with coal firing, but also can occur for certain types of oil firing.



Ash characteristics are considered in the boiler design when establishing the size, geometry and materials used in the boiler. Combustion gas and metal temperatures in the convection passes are important considerations.



Damage occurs when certain coal ash constituents remain in a molten state on the super heater tube surfaces. This molten ash can be highly corrosive.

Fig.2.3.1:- Fly Ash


11

2.3.2) Erosion In Tube:

Problem: Damage will be occurs on inner side of the tube. Ultimate failure results from rupture due to increasing strain as tube material erodes away.



Causes: Erosion of tube surfaces occurs from impingement on the external surfaces. The erosion medium can be any abrasive in the combustion gas flow stream, but most commonly is associated with impingement of fly ash or soot blowing steam.

Fig.2.3.2:- Erosion in tube


12

2.4) Fatigues:2.4.1) Mechanical Fatigue:

Problem: Damage most often results in an outer diameter (OD) initiated crack. Tends to be localized to the area of high stress or constraint.



Causes: Fatigue is the result of cyclical stresses in the component. Distinct from thermal fatigue effects.



Mechanical fatigue damage is associated with externally applied stresses. Stresses may be associated with vibration due to flue gas flow or soot blowers (high-frequency low-amplitude stresses), or they may be associated with boiler cycling (lowfrequency, high-amplitude, stress mechanism).



Fatigue failure most often occurs at areas of constraint, such as tube penetrations, welds, attachments or supports.

Fig.:2.4.1:- Mechanical fatigue


13

2.5) Heating:2.5.1) Short Term Heating:

Problem: Failure results in a ductile rupture of the tube metal and is normally characterized by the classic “fish mouth” opening in the tube where the fracture surface is a thin edge.



Causes: Short-term overheat failures are most common during boiler start up. Failures result when the tube metal temperature is extremely elevated from a lack of cooling steam or water flow.



Tube metal temperatures reach combustion gas temperatures of 1600°F or greater which lead to tube failure.

Fig.2.5.1:- Short Term Heating


14

2.5.2) Long Term Heating:

Problem: Tube metal often has heavy external scale build-up and secondary cracking. Results in tube failure.



Causes: Long-term overheat occurs over a period of months or years. Super heater and reheat super heater tubes commonly fail as a result of creep.



Furnace water wall tubes also can fail from long-term overheat. In the case of water wall tubes, the tube temperature increases abnormally, most commonly from waterside problems such as deposits, scale or restricted flow.

Fig.2.5.2:- Long term Heating


15

2.6) Lack Of Quality Control:2.6.1) Maintenance Damage & Metallurgy Defects:

Problem: Maintenance damage & Material defect are occurs due to methods & materials used in analysis of boiler tube.



Cause: If proper methods & material not used in it , then it cause in boiler. High quality methods & material to be used in boiler tube failure are more expensive.



Insufficient methods & material cause heavy damage in boiler.

2.6.2) Welding Defects:

Problem: Failure is occurs due to dissimilar metals in welding process.



Causes: Failures at dissimilar metal welding locations occur on the ferrite side of the butt weld.



These failures are attributed to several factors: high stresses at the austenitic to ferrites interface due to differences in expansion properties of the two materials, excessive external loading stresses, and creep of the ferrites material.

Fig.2.6.2:- Welding Defects


Feed Water Treatment & Chemical Cleaning

16

CHAPTER 3

3.1) Objectives of Treatment: Treatment and further conditioning of water are necessary for the following objectives: 

To prevent scaling internals of pressure vessels due to dissolved and suspended impurities,



To prevent corrosion of metallic parts of the boiler, with which water / steam come in direct contact,



To establish protective coating over metallic surfaces to prevent corrosion attack.



To avoid salt deposits over turbine blades,



To ensure better utilization of heat energy and to improve on efficiency,



In order to ensure to achieve above objectives, following processes of water treatment are adopted: 1. Pre treatment 2. Demineralization 3. Chemical conditioning

 Following are the impurities in the natural water which are to be cleaned.

1) Dissolved impurities:

Mainly the dissolved solids found in water are mineral salts. These contaminants in water exist as salts of calcium, magnesium and sodium.



To a lesser extent, potassium and iron salts are also present. Nitrates and silicates of such substances are also found to a small degree. Very rarely, Phosphates and a few heavy metals are also found in natural water.



The quantity and composition of the solids present depend upon the soil and strata details and the origin of water.




17

Various gases, mainly Oxygen, and others like Carbon dioxide and Hydrogen Sulfide are normally present in dissolved form and the presence of such dissolved gases alter the composition and concentration of certain salts.

2) Dissolved Oxygen:

Dissolved oxygen plays significant role in boiler feed water. The oxygen accelerates corrosion of water tube material. Mainly oxygen is removed only in De-aerator. Hence performance of De-aerator is very important.



De-aerator performance has to be maintained limiting dissolved oxygen to less than 0.01 ppm level. A figure < 0.007 ppm is considered to be very ideal.

3) Total Solids:

The basic idea is to restrict impurities in feed water as low as possible in order to avoid rise in boiler water concentration and deposition of metal oxides in the boiler.

4) Organic Matter:

It is very difficult to eliminate organic matter totally from water. Presence of organic matter is due to poor pretreatment practice, ineffective D.M. plant performance, resin leaking or condenser leakage.



Quality of raw water at intake point is a deciding factor. Seasonal changes play a vital role.

5) Chlorides:

Presence of chloride in feed water is harmful to the system as whole. Hence it is very advisable to limit chloride concentration in feed water, keeping in mind, limit prescribed for boiler water and restriction as per drum pressure ratings.



Leak proof condenser and efficient demineralization are essential prerequisites to avoid contamination due to chloride.




18

Dosing chemicals fed to the drum can contribute in sizeable proportion to chloride contamination if the chemicals are not of adequate quality. Very low level chloride should be tested by selective ion electrode for accuracy.

6) Suspended and colloidal impurities:

Clay and sand particles constitute a major portion of the suspended matter. Very fine clay remains in colloidal state. Colloidal suspension of dye material and certain organic contents, give color to water, in most cases.

7) General nature of dissolved contents:

Normally the mineral salts dissolved in water are found to exist in the ionized form. Generally there is no uniformity in quantity and proportion of such dissolved salts, since their presence is mainly dependent on the sources of the water.



However fair representations of composition of mineral salts, as widely found in nature are brought out below:

CATION (Basic Radical)

ANION (Acidic Radical)

Ca ++ (Calcium)

HCO3- (Bicarbonate)

Mg++ (Magnesium)

CO3-- (Carbonate)

Na+

(Sodium)

SO4-- (Sulphate)

Fe++

(Iron)

Cl-

(Chloride)

NO3-

(Nitrate)

Al+++ (Aluminium)

PO4--- (Phosphate) SiO2

(Silica)


19

3.1.1) Pre-Treatment of Water :-

Fig 3.1 Feed Water Treatment

3.1.1.1) Clarification:

Pre- treatment to raw water is mainly to make it suitable for further processing of water by Deionization units.



The water entering De-mineralizer plant should be free from suspended, colloidal and organic impurities and the process of pretreatment plays a vital role in ensuring proper feed input is made to Deionization units.




20

Presence of such suspended impurities adversely affects the deionization properties of the resins, which will affect the end quality of Demineralized water.



Suspended & colloidal particles are removed by clarifying the water in a clariflocator aided by suitable coagulating agents. It is further chlorinated to achieve effective oxidation to combat organic contamination.

3.1.1.2) Precipitator Clarifier Section:

A precipitator inlet flow control valve controls the raw water inlet flow into the clarified water basin, through a level controller.



A manual by-pass valve is also provided to the level control valve, so that in case of problem in the level control valve, manual operation can be done to maintain the level.



The raw water enters the inner mixing zone through an open channel from the top and flows downward into the inner conical tank.



The chemicals are let into the open channel to get mixed thoroughly with the raw water flowing along the open channel into the mixing chamber. A water flow indicating mechanism is fitted in the open channel to indicate the raw water flow.



Ferrous sulphate solution is delivered into the “Precipitator” by means of twin head proportional feed 5% solution dosing pump.



The following chemical equations illustrates the chemical reaction of lime with the calcium and magnesium bicarbonates:

Ca ( OH ) 2

+

CO2



CaCO3↓

+

H2O

Ca ( OH )2

+

Ca ( HCO3 )2



2CaCO3↓

+

2H2O


2Ca ( OH )2

+

Mg ( HCO3 )2

Ca ( OH )2

+

MgCO3







MgCO3 + CaCO3↓ +

Mg( OH )2↓

+

21

2H2O

2CaCO3↓

The chemical reaction of Ferrous Sulphate with calcium bicarbonate is as illustrated below:

4FeSo4 + 4Ca (HCo3) 2 + O2

 4Fe ( OH )3 + 4CaSo4 + 8 Co2 + 12 H2O

(Insoluble)



The colloidal produced by Ferric Hydroxide in this reaction is negatively charged, and is an effective coagulant of the positively charged, colloidal precipitates formed in the reaction of Lime with salts causing temporary hardness.



The precipitated chemical in this reaction with the Calcium and Magnesium salts, form an effective sludge in layers, which can be easily removed by blowing down.



Whatever the residual calcium, magnesium and sodium salts and silica still present in the clarified water are removed by the successive ion exchange process with the resin beds in the water de-mineralizing plant.

3.1.1.3) Filtration:

Water filtration is the process of separating suspended and colloidal impurities from water by passage through a porous medium. A bed of granular filter material or media is used in most plant application.



A filter may be defined simply as a device consisting of a tank, suitable filter media and necessary piping, valves and controls.


22



Filters are designed for the following:



Gravity flow: with natural head of water above the filter bed and low point of discharge at the filter bottom, providing the pressure differential needed to move the water through the filter bed.



Pressure units: which, as their name implies, are operated on line, under service pressure, filtering the water as it flows the tank on its way to service or storage

 Backwashing: When differential head between inlet and outlet increases, the filters must be taken out of service and backwashed for removing accumulated dirt and sludge materials.

 For back wash the flow direction is reversed that is the inlet is given at the bottom and water from the top is delivered out to open canal.

 When the water flows in reverse direction it carries all the accumulated dirt and sediments and throws out along with the water. The back washing is to be done until the out flowing water is perfectly clear of all accumulated sediments.

 Chlorine removal:

To avoid algae formation chlorine is dosed into raw water, before the process of clarification in the clariflocator.



Presence of chlorine is harmful to the Ion Exchange Resin and it is essential to ensure that even the traces of chlorine are not present in the clarified and filtered water, before admission to deionizing unit.



Hence before admission to cation vessel, the water is once again filtered treated in a vessel containing a bed of Activated Carbon. Activated Carbon Bed absorbs left over chlorine and the organic matters and feeds clear water to the ion exchange vessels.


23

3.1.2) De-mineralization:

Demineralization is the removal of dissolved ionic impurities that are present in water. Demineralized water is commonly produced by one or a combination of the following processes: 1. Ion exchange 2. Membrane desalination 3. Thermal desalination



The method selected to produce demineralized water depends on the quality of the influent water, the required quality of the effluent water, the availability of resources such as regenerant chemicals and waste water treatment and disposal requirements.



The economics of the processes that produce acceptable effluent quality must be evaluated to determine the most cost-effective method for a specific application.

3.1.2.1) Ion Exchange Process:

Basically the minerals present in water are in ionized condition. Ion exchange demineralization therefore is one of the most important and widely applied processes for the production of high-purity water for power plant services, and it is accomplished using resins that exchange one ion for another.



Cation resins are solid spherical beads with fixed negatively charged sites and exchangeable positively charged sites. Anion resins are solid spherical beads that have fixed negatively charged sites and exchangeable negatively charged sites.



In their regenerated state for demineralization applications, Cation resins are in the hydrogen form and anion resins are in the hydroxide form. The reactions of the resin beads with the dissolved impurities in the water are represented by the following:


Cation resin:

24

R-H+ + C+ ↔ R-C+ + H+ 2R-H+ + C2+ ↔ R2-C2 + 2H+

Anion resin:

R+OH- + A- ↔ R+A- + OH2R+OH- + A2- ↔ R2+A2 + 2OH-

Where, R = resin matrix and fixed charge site; C = cations such as Ca2+, Mg2+, and Na+; and A = anions such as HCO3-, Cl-, and SO4-2 

The hydrogen ions (H+) displaced from the cation resin react with the hydroxide ions (OH-) displaced form the anion resin. The net effect is the dissolved ions are removed from the water and replaced by pure water (H2O).



The ion exchange resins are contained in ion exchange pressure vessels. The ion exchange resin in the vessels is referred to as the resin bed. This process of exchanging dissolved impurities is cyclic.



When a resin bed site is exchanged with a dissolved ion, the site becomes “exhausted” and cannot remove other impurities without releasing an impurity.



Exhausted resins must be regenerated to return the resin beads to the original hydrogen form for cations and hydroxide form for anions before further ion exchange can take place.



Cation resins are commonly regenerated with a strong acid solution of either sulfuric or hydrochloric acid.



Sulfuric acid does not present the fuming problems associated with concentrated hydrochloric acid and is easier to handle (material selection). Consequently, sulfuric acid is frequently the recommended regenerant for cation resins.




25

Anion resins are commonly regenerated with a sodium hydroxide solution. As can be seen from the regeneration reactions listed below, regeneration is the reverse reaction to the impurity exchange reactions.

Cation resin regeneration:

2R-C+ + H2SO4 ↔ 2R-H + C2+SO42R2-C2+ + H2SO4 ↔ 2R-H+ + C2+SO42-

Anion resin regeneration:

R+A- + NaOH ↔ R+OH- + Na+AR2+A2- + 2NaOH ↔ 2R+OH- + Na2+A2-

3.1.3) Chemical conditioning of Boiler water:

Demineralization of water renders raw water fit for use as make up and feed to a boiler. Just feeding a boiler with Demineralize water, which is free of all impurities, alone, will not suffice in reality due to the following reasons.

a) When the steam is continuously being generated and released for process, the water inside the drum gets more and more concentrated, even with the very little and negligible level of impurity in feed water. b) Further the pH of Demineralized water normally remains to be close to neutral and will induce low pH corrosion of tube and pipe materials. c) Demineralized water when under storage absorbs atmospheric air and the level of dissolved Oxygen in water increases. Presence of Oxygen in feed water again renders it corrosive.



Chemical treatment is done at various stages for feed and boiled water, steam and return condensate, in order to ensure adequate protection is established over the entire system, to help prevention of direct attack on parent metal by the corrosive elements present in water / steam cycle.




26

The chemical treatment technique is based on several factors. While the overall conditioning concept remains the same, there would be variations in the methods adopted and the residual levels optimized, it depends mainly on: a) Deferent grades of ferrous and alloy metals which are deployed in the design and manufacturing of various pressure part components, such as, water wall tubes, headers, raiser tubes, economizers, super heaters, reheaters etc. and so many other equipment down the line such as turbines, condensers, various stages of regenerative system heaters and the pumps,

b) Working temperature and pressure at which the steam is generated for supply to end users,

c) The type of fuel used and the firing system adopted,

d) Release of heat flux density, depending on the fuel fired and the design and size of the furnace,

e) Capacity of boiler for steam generation and the level of steam purity expected,

f) Quality and quantity of condensate recirculated within the system. 

The condensate extracted from the Condenser is passed through a De-aerator to get rid of dissolved oxygen, by mechanical stripping action.



The feed water is then chemically conditioned further with dosing of chemicals to ensure that the dissolved oxygen content of the water is reduced to the minimum. Hydrazine hydrate is normally employed for this purpose.


27

3.2) De-Aerator:3.2.1) Function Of De-Aerator:•

To remove the oxygen from feed water to be fed to Boiler.

•

It is a direct contact type heater and is located between LP heater and Boiler feed pump in feed water cycle.

•

The steam from suitable turbine stage is drawn to heat the condensate which is broken in fine particles for effective heating and mass transfer on non-condensable gases.

3.2.1) Flow Arrangement :

The condensate to be de aerated enters the chamber at a top and is sprayed by variable orifice spray valves. The water then falls through perforated tray stack or spill over tray track by gravity.



The condensate is divided in to fine droplets and comes in the contact with the steam resulting in release of non-condensable gases carried by steam moving up wards.



The condensate leaving the de aerator enters and collected to feed storage tank and goes to Boiler feed pump

3.2.2) Design And Construction:

The De-aerator removes the non-condensable gases by heating the condensate which reduces the solubility of non-condensable gases and then allow mass transfer time to reduce non condensable further.



The steam flowing upward provides the atmosphere for mass transfer and condensate droplets while carrying away non condensable gases. The trays divide the condensate in to fine droplets so that heating and mass transfer is achieved quickly.



The steam admittance in storage tank provides steam blanket over stored condensate and thus eliminate sub cooling and re absorption of gases.


28

Fig 3.2 De-aerator

3.2.3) Main Parts Of De Aerating Header:1) Shell:

It is fabricated from steel plate of boiler quality closed with dished end. The shell is mounted over the feed storage tank. It contains steel trays. The distribution chamber is located at the top in which spray valves are provided for spraying condensate.

2) Spray Valves And Trays:•

Spray valves are spring loaded disc. The variable opening is controlled by flow.


•

The tray design is of two type (1) Perforated trays and (2) Spill over trays.

•

Perforated trays have no of small size holes through condensate falls down. The annual and central opening is for flow velocity of steam, non-condensable and pressure equalization.

•

In case of spill over tray a series of channel type trays are arranged such that, condensate spills over from one layer to other layer.

3.2.4) Accessories:1) Safety relief valve 2) Orifice plate 3) Vacuum breaker valve 4) Vent and drain valve 5) Stand pipe 6) Isolation valves for stand pipe and instruments 7) Level gauge 8) Level switch 9) Pressure gauge 10) Temperature gauge

29


30

3.3) Quality of feed water to boiler:

Quality requirement for feed water is very much dependent on drum operating pressure. Hence recommended feed water quality is always expressed for a particular range of boiler operating pressure.

Table No. 3.3.1 :- Quality of feed water Drum operating Pressure Kg/Cm2

60

80

100

120

130

&

above pH at 25 °C

8.5~9.

8.5

0

9.0

Electrical conductivity at 25 °C in < 0.5

~ 8.7 ~ 9.0

8.8 ~ 9.2

8.8~ 9.2

< 0.5

< 0.4

< 0.3

< 0.2

micro mhos/cm Economizer inlet

< 0.01

< 0.01

< 0.005

< 0.005

< 0.005

Total iron ( Fe ppm )

< 0.03

< 0.01

< 0.01

< 0.01

< 0.01

Total copper ( Cu ppm )

<

< 0.005

< 0.003

< 0.003

< 0.003

0.005 Total iron copper & nickel ppm

< 0.03

< 0.02

< 0.02

< 0.02

< 0.01

Total silica ( SiO2 ppm )

< 0.02

< 0.02

< 0.02

< 0.02

< 0.01

Carbonic acid (CO2 ppm )

Nil

Nil

Nil

Nil

Nil

Organic matter mg KmnO4/Litre

Less than 0.1 ppm is considered tolerable

Oil ppm Hydrazine N2H4 ppm )

Nil < 0.1

Nil

Nil

Nil

< 0.05

< 0.05

< 0.02 ~ < 0.02 0.05

Nil


MODIFIED DESIGN

31

Chapter-4

4.1) Types of Modified Design  There are six types of modified design 1) Boiler feed pump and feed control design 2) Arrangement of Orifice Meter 3) Re-heater design 4) Piping Design 5) Design of Cooling Water Sump 6) Air Instrument design

4.2) Detail Description Of Designs:-

4.2.1)

Boiler feed pump and feed control design: Fig 4.2.1(A), (B) and (C) shows the present & modified design of feed control valve arrangement.

 In fig. 4.2.1 (a) drum level controlled by the feed control valve by 60-70 percent open and the differential pressure across the feed control valve is 15 kg per square senti-meter by boiler feed pump scoop.

 Fig.4.2.1 (b) shows the signal flow arrangement of feed water flow, drum level and steam flow.

 Fig.4.2.1 (c) shows the new modified arrangement of the whole control system. So, the new drum level controlled by boiler feed pump scoop and feed control valve 1 and 2 keeps fully open.

 Now in modified design the differential pressure across the control valve is 0 to 0.5 kg per square senti-meter.


Fig 4.2.1(a) Feed Control Valve

Fig 4.2.1(b) Signal Flow

32


33

Fig. 4.2.1(c) Modified Boiler Feed Pump & Feed Control Valve

4.2.2)

Arrangement of Orifice Meter :

Fig. 4.2.2 (a) and (b) shows the piping arrangement of water flow form boiler feed pump to the super heater with control valve arrangement.



In fig.4.2.2 (a), when the inlet pressure is 165 kg per senti-meter square then at outlet the orifice meter gives the pressure reading is 160 kg per senti-meter square. That means there is loss of pressure between inlet and outlet by the control valve.



In fig. 4.2.2 (b) show the rearrangement of orifice meter to solve this pressure drop across the boiler feed pump and super heater.


Fig. 4.2.2(a) Arrangement of Orifice Meter

Fig. 4.2.2 (b) New Arrangement Of Orifice Meter

34


4.2.3)

35

Re-Heater design:-



Fig.4.2.3 shows the water flow from boiler feed pump to the re-heater.



Present pressure in boiler feed pump is 150 kg per senti-meter square. Due to this high pressure the damage occurs in the spray nozzle.



To prevent this damage put the pressure control valve ahead of spray control valve and also make one reservoir tank between the controller and spray control valve.



By this the pressure is reduced up to 50 kg per senti-meter square in reservoir tank. So, in controller the pressure up to 0 to 100 kg per senti-meter square is maintained.

Fig 4.2.3 Re- heater Design

4.2.4) 

Piping Design:Fig. 4.2.4 (a) and (b) shows the piping arrangement of boiler feed pump line.




36

As shown in fig.4.2.4 (a) the pressure difference between boiler feed pump & header pressure is 10 kg per senti-meter square. This pressure difference is occurs due to “T” joint of the pipe line.



This pressure difference can be reduced by changing “ T ” type arrangement into “ C ” or “ Y ” type arrangement as shown in fig. 4.2.4 (b).



Due to “C” or “Y” type arrangement smooth flow of water is made.

Fig. 4.2.4(a) Piping Design


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Fig. 4.2.4(b) New Piping Design

4.2.5)

Design of Cooling Water Sump:-



The simple design is shown in fig. 4.2.5.



In cooling water sump house, make the simple spray type arrangement. By this the natural air passes through the spray and natural cooling is occurs.



Due to this the temperature of the water can be reduced. So, cooling process can be made easily in cooling water sump house.


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Fig. 4.2.6 Cooling Water Sump Design

4.2.6) 

Air Instrument Design:Due to continuous use of air operated instrument (vacuum pressure operated), amount of water and silica particles are increases which cause to stop the working of instruments.



So, to solve this problem make one reservoir tank near the main valve. Due to this silica and water particles fed in bottom of the tank and fresh air can be made and life of the instrument can be increases.



Fig. 6 (a) and (b) shows arrangement of reservoir tank with control valve.


Fig. 4.2.6(a) Air Instrument Design

Fig. 4.2.6 (b) New Air Instrument Design

39


4.2.7)

ADVANTAGES OF MODIFICATION:-



Followings are the advantages of Modified Design.



Ampere loading of Boiler Feed Pumps reduced by 40 A per Boiler Feed Pump.



Reduction in differential pressure up to 15 kg/cm2.



Reduction in feed control valve maintenance.



Reduction in generation cost.



Reduction in load on boiler master controller



Increases plant efficiency.



Reduction in instrument damages.



Easy handling of flow.



Better cooling of water in cooling water sump.



Increases instruments life.



Smooth flow is made.



Smooth control of Boiler Drum Level



Minimum movement of Boiler Feed Pump Scoop.

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CONCLUSION (WASTE DOCUMENTATION)

41

Chapter-5

4.1) Types of Waste:- Solid & Liquid

4.2) Quantity of Waste: - As Per Defect.

4.3) Detailed Description: Waste of material is subjected to defect occurs in the Boiler. If problem caused in it is large than amount of waste material is more.

 But when boiler tube is fail then 1000 kiloliter water is waste. The water used in it is distilling water and it cost is 8 Rs. Per liter. As per market rate.

 So, minimum wastage of money is 80 lakhs per 1 times of failure. If the failure is small or big. This is minimum cost in industry for wastage in distilling water.

 Another loss is Generation loss which occurs due to starting of Boiler. And that cost is Rs.1.5 cr.

4.4) Modified Design:

The modified design is very useful to industry to reduce the generation cost and maintenance of control system.


-: REFERENCES: Books:-

1) Reduced Boiler Tube Failure Analysis, B & W (Babcock & Wilcox) power generation Group. 2) Power plant engineering, by Arora Domkundwar, Dhanpat Rai & co.  Websites: www.swcc.gov.sa  www.PDHcenter.com

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Boiler Tube Failure Analysis

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