12 Turbocharger

TURBOCHARGER Note Book For Marine Engineers INDEX 1.

Principles of Turbocharging

2.

Turbocharger

3.

Charge Air Cooler

4.

Uncooled Turbochargers

5.

Surging of Turbocharger

6.

Corrosion and other defects

7.

Lubricating the Turbochargers

8.

Washing of Turbocharger

9.

Overhauling of Turbochargers

10.

Two-Stage Turbocharging

11.

Turbo Compound System

12.

Trouble Shooting in Turbocharger

13.

Safe Working Practices and Safeties Fitted on Turbocharger

14.

Development in the Field of Turbocharging

BIBLIOGRAPHY Name of the Book

Author

1. Maine Diesel Engine (Questions & Answers)

Lamb

2. The Running & Maintenance of Marine Machinery

Cowley

3. Diesel Motor Ships Engines & Machinery

Christen Knak

4. Marine Diesel Oil Engines

Sothern’s

5.

Shipping World & Shipbuilder (Magazine)

Website 1.

www.abb.co.th

2.

www.wartsila.com

3.

www.manb&w.com

4.

www.dresser-rand.com

5.

www.ask.com/turbocharger

Engineer M. A. Hamid

1

TURBOCHARGER Note Book For Marine Engineers PRINCIPLES OF TURBOCHARGING Turbocharging is the process of charging air into the cylinder at a higher pressure, hence at higher density than atmospheric pressure. Due to this, greater mass of fuel can be burned and a higher engine output can be obtained. Therefore the turbocharged engine can deliver more useful work than a naturally aspirated engine of equal size. In addition the engine performance is further enhanced by the improved scavenging and by the reduction of residual gases at the commencement of each cycle. Of the total heat energy available from the fuel the average Diesel engine discharges some 30 to 35% into the exhaust manifold, presenting a potentially economical source of power. If, therefore, this normally wasted energy be made to drive the compressor, an increase in useful power approximately proportional to the increase in the density of the cylinder charge can be obtained. The turbo-charger achieves this by combining a gas turbine, driven by the engine exhaust gas, with a centrifugal compressor which delivers into the air manifold. Engines pressure-charged with exhaust gas turbo-driven blowers are often referred to as turbo-charged engines. The effect on the cycle is an increase in the intensity or duration of the combustion period and an increase in the work per cycle and, therefore, the mean effective power. It can be seen that an increase in MEP will result in an increase in power output. The power increase will be directly proportional to the increase in MEP if other factors, including cylinder dimensions and RPM, are unchanged. Under normal running conditions all the air required for scavenging and charging the cylinder is supplied by the turbocharger but for manoeuvring and slow running it is sometimes advantageous to have an electrically driven blower or an engine driven positive displacement scavenge pump or rotary blower to supplement the turbocharger output. Methods of Turbocharging Constant pressure turbo-charging: In the constant pressure system the pressure the in the exhaust manifold leading to the turbocharger is virtually steady. That is, the pulses of energy that occur as the exhaust is released from the cylinder are absorbed in the large volume exhaust manifold so that, at the turbocharger, almost steady flow conditions exist. As the exhaust gas blows into the manifold, eddies are set up which help damp out any pressure wave caused by the influx of the exhaust gas. The volume of the manifold must be large enough to accommodate the gas flow from individual cylinders without causing any localized pressure rise in the manifold as exhaust gas leaves the cylinder. The exhaust gas is led from the manifold into the exhaust-gas turbo-charger at a constant pressure. If the exhaust manifold is made too large the response of the turbine to engine load change is slowed up due to the manifold volume taking longer to bring to the higher exhaust pressure. The discharge pressure from the turbine must be higher than the exhaust manifold pressure. When the engine is operated with a low load the air discharged from the compressor may be insufficient; a separate electrically driven blower is then brought into operation. Pulse Turbocharging: The main energy to power the turbine in the system is derived from the pulse or impulse energy in the pressure wave formed during blow down from each cylinder. These waves travel through the manifold to the turbine nozzle where they are converted to kinetic energy at high velocity to rotate the turbine blades. This system gives a rapid buildup of turbine speed when an engine is started or manoeuvring. To maintain the pulses, exhaust connections are of limited diameter, no sharp bends are used and the turbocharger is fitted close to the engine. The peak pressure in pulse waves may be higher than the pressure in other cylinders whose exhaust valves are also open. To prevent a back-flow into these, it is necessary in multi-cylinder engines to subdivide the exhaust between a numbers of manifolds, each connected to a separate nozzle box at the turbine. Up to three cylinders can use each manifold without interference depending on their firing order. A divided exhaust system reduces turbine efficiency and may require more than one turbocharger per engine. The use of pulse converters in modern engines allows individual cylinders to exhaust to a common manifold, giving higher turbine efficiency and a more compact system which allows rapid acceleration when manoeuvring. Advantages / reason for Turbocharging: 1.

The weight of air admitted per stroke is increased.

2.

The fuel consumption, M.I.P., and power developed are increased (about 30% more power).

3.

The cylinder and exhaust gas temperature remain practically unaffected.

4.

Turbulence is improved, and combustion efficiency is increased.

5. The size of the engine is reduced for the same horse power. Engineer M. A. Hamid 2

TURBOCHARGER Note Book For Marine Engineers TURBOCHARGER A modern exhaust gas turbo-blower or turbocharger is essentially a single stage impulse turbine consisting of two sections, the turbine and the centrifugal air compressor. The turbine uses the high velocity and pressure of exhaust gasses to drive the centrifugal compressor which compressing the air and hence is supplied to the cylinder to. Casing: The casings can be considered in two groups, namely the “hot” casings of the turbine and the “cold” casing of the compressor. The turbine inlet and outlet casing if they are air cooled are machined from heat-resisting iron and together form the annular path through which the engine exhaust gases flow. The air-cooled casings are enclosed in a lagged heat shield which is quite safe to touch. The turbine casing being air-cooled as per figure does not suffer from exhaust gas condensation and corrosion, which is sometimes experienced if the cooling water temperature, on a water-cooled unit, is very low. The turbine and blower are housed in a circular casing divided into two separate spaces by a circular division plate, which may be water-cooled, or protected by heat insulation on the exhaust gas side. The volute casing and its rear cover comprise the compressor casings and are cast in aluminium alloy. To allow for the differential expansion between the turbine outlet casing and the volute casing rear cover radial pins clamped by spring plate assemblies are employed. A similar arrangement centers the turbine bearing support casing in relation to the turbine inlet casing. By this means the rotor assembly is maintained in correct alignment in the compressor and turbine casing when the latter expand at the working temperature. A nozzle ring is located in the turbine inlet casing by the nozzle support ring and is free to expand radially. It consists of a set of stainless-steel vanes cast into concentric cast-iron rings, the outer ring being divided segmentally to allow for differential expansion. A diffuser ring is clamped between the volute casing and rear cover, and the latter provides forward location for the mounting foot. An intake air pre-whirl assembly is supported on the front face of the volute casings. A labyrinth ring is bolted to the volute casing rear cover and prevents leakage of air from the impeller tip to atmosphere along the rotor shaft and past the downstream face of the turbine disc. The labyrinth ring is positioned so as to reduce the rotor end thrust to a minimum. An aluminium alloy casting forms the intake bearing mounting which is spigoted to the volute casings, and within which are mounted the oil pumps, drive shaft, oil filter complete with integral by-pass provision, relief valve, compressor bearing, thrust block, and labyrinth bush. The oil tank is cast integrally and its aluminium alloy covers contains a filler and sight glass. An annular oil cooler is mounted on the cast radial support arms which contain passages for oil supply and air supply, also oil drain and leakage air from the turbine bearing housing. Internal pipe work is thereby reduced to a minimum. An air intake filter unit is bolted to the intake bearing mounting and comprises an annular three-ply oil wetted element or a ferrule type may be fitted, housed in aluminium casting. Both types are designed to obtain low intake velocity; the intervals between cleaning are therefore considerably extended. Sound-absorbing material is incorporated in the compressor casings. Rotor: The rotor assembly consist of hollow steel shaft on to which are spigoted the centrifugal impeller and turbine disc. This is supported at the end by bearings. Compressor: It increases the density of the charge air to deliver into the air manifold that is, by pressure-charging, then the weight of the charge in the cylinders is also increased, allowing a greater weight of fuel to be burned. The compressor consists of the following parts:

•

Air filter: They are fitted so as to supply a clean air to the centrifugal impeller.

•

Diffuser: The diffuser forms a stationary passage of gradually increasing cross-section and surrounds the impeller wheel. The air as it passes from the eye to the tip of the wheel gains in velocity is compressed, and therefore the temperature is increased. The inlet edge of the impeller blades is slightly bent in the direction of rotation. This improves the efficiency of the impeller.

•

Centrifugal Impeller: It is made by forging aluminium-silicon alloy to get the properties such as lightness, strength, toughness and smooth surface finish. It consists of channels which begin from the center of the hub and extend radially outwards to the tip. This configuration gives centrifugal force to the air which leaves the impeller


3

TURBOCHARGER Note Book For Marine Engineers tip at high velocity. The displacement of air from the hub area through channel creates the vacuum at hub which induces further air from the atmosphere. Turbine: The turbine blades are made from heat-resisting nimonic-80 which has good fatigue, creep, and corrosion resistance properties. Also austenitic steel can be used for the blades. It should be of the highest grade to ensure long life at the operating temperatures of about 580 to 630oC. The turbine is made in two parts, shot-blasted to remove any loose material and give a smooth interior finish. The roots of the turbine blades, where they are attached to the rotor, are machined to what is referred to as ‘fir-tree’ shape, because it bears some resemblance to the shape of a fir tree. On the rotor are correspondingly shaped slots into which the fir-tree roots of the blades slide. Machining tolerances are extremely tight. In small turbo-chargers used for constant-speed speed diesel engines, such as diesel generators, the blade root can be a tight fit in its slot. In large turbo-charger with long rotor blades, particularly those used for propulsion engines where a wide range of operating speeds is met in service, the fit of blades is such that it can move slightly. When blades are fitted loosely in this manner, a segmented binding wire is fitted through holes near the blade tips which dampen the blade vibration. The blades are held in their slots by upsetting or raising a caulked edge on the rotor at the innermost part of the root slot; this edge prevents the blades sliding out. The turbine disc is machined from a ferritic steel forging. Bearings: The impeller bearing is arranged to take any small axial thrust. Ball races will be fitted at the compressor end to locate the shaft and thereby fix the clearance between the casing and the blades of the compressor impeller. The bearings are enclosed in resilient mountings to protect them from damage by vibration. The turbine roller bearing is arranged to take any expansion that occurs. Each bearing is supplied with oil by a disc-type pump drawing from a selfcontained reservoir. There is a pump and reservoir at each end of the unit. CHARGE AIR COOLER In modern two-stroke turbocharged engines a charge air cooler is necessary. Compression will raise the air temperature and a charge air cooler is fitted to reduce the temperature of the air between the turbocharger and the engine inlet manifold, causing increased air density at lower induction temperature. The engine is maintained at safe working temperatures and the lower compression temperature reduces stress on piston rings, piston and liner. Increased density will raise scavenge efficiency and allow a greater mass of air to be compressed; more fuel may now be burned giving an increase in power. Figure shows a section of a charge air cooler. The air makes a single pass through the cooler and, for efficient cooling; its velocity should be low and cooling area large. This is obtained by making the air inlet connection divergent; the outlet is convergent to restore air velocity after cooling. Condensation of moisture in the compressed air will occur during cooling and a drain is fitted to the outlet side air casing to allow this condensate to be removed. Moisture eliminatory may also be fitted to remove entrained water droplets from the air stream. The drain should be kept open and its discharge noted; this will also indicate if a cooling water leak has occurred. The cooler consists of a tube stack of aluminium brass tubes rolled and solder bonded into two brass plates. Cast iron water boxes are attached to the tube plates and allow salt water circulation within the tubes to make two passes. One tube plate is secured to the causing while the other is free to move axially as thermal expansion occurs. The air seal is maintained by means of a fitted rubber joint ring. An air vent is fitted to the top water box to remove air which may have been released from the salt water system. Corrosion plugs may be fitted within the water space. Thin copper fins are soldered to the outside of the cooler tubes. The air will pass between these plates which greatly increase the area of heat transfer. There are two side plates of mild steel or aluminium alloy. Engineer M. A. Hamid

4

TURBOCHARGER Note Book For Marine Engineers Temperatures and pressures are recorded at each inlet and discharge. Discharge air temperature should not exceed 55oC since temperatures – notably the exhaust temperatures – will increase, with loss in efficiency due to reduction in air density. Air at very low temperatures will also cause thermal shock when in contact with hot liners and pistons. Some medium speed engines’ turbocharger systems may be adapted to improve low speed operation. A by-pass valve may automatically return some charge air to the compressor inlet to improve acceleration when running up to speed. Engines designed to operate with high power at reduced speed may have an excess of charge air at full speed and require a blow-off valve to open at 85% full speed. In recent years the efficiency of turbochargers and their systems has been increased considerably. In addition to increased engine power, greater stability is possible at low speeds. Part of the exhaust gas energy may be used to drive power units which increase the overall thermal efficiency of the plant. When air coolers become fouled, less heat will be transferred from the air to the cooling water. This is shown by changes in the air and cooling water temperatures. Changes will also occur in the pressure drop of the air passing through the cooler. The amount of change will depend on the degree and nature of the fouling. The symptoms of air-side fouling are as follows. 1.

Decrease of air temperature difference across cooler.

2.

Increase of air pressure drop across cooler.

3.

Rising scavenge air temperature.

4.

Rising exhaust temperature from all cylinders.

5.

A smaller rise in cooling – water temperature across the cooler.

Fouling of the cooling-water side is shown by the following symptoms. 1.

Rising scavenge temperature.

2.

Reduction in the difference of the air temperature across the cooler.

3.

Reduction in the temperature rise of the cooling water across the cooler if the fouling is general on all tubes.

4.

Rising exhaust gas temperature from all cylinders.

5.

Increase in the temperature rise of the cooling water if fouling or chocking materially reduces the amount of water flow. The temperature and pressure differences recorded should be compared with those obtained from the engine when on the test bed.

UNCOOLED TURBOCHARGES Uncooled Turbochargers do not have their turbine casing water cooled. In general construction they are similar to those already described, but some changes are necessary to accommodate the higher gas temperatures. The casing is


5

TURBOCHARGER Note Book For Marine Engineers manufactured from cast steel and insulated to conserve heat and meet safety requirements for hot exposed surfaces. Figure shows a cross-section of an uncooled turbocharger as fitted to many large two-stroke engines. The turbine bearing will require cooling: in this case it is protected by insulation and also has water cooled casing which maintains acceptable oil temperatures. Higher working temperatures will eliminate the possibility of condensation and acid corrosion within the gas casing. The final outlet gas temperature may be up to 10oC higher than that from cooled turbines and this attractive for large, slow, two-stroke engines which produce limited exhaust temperatures. This allows more efficient use of a waste heat boiler and therefore adds to the total heat recovery and thermal efficiency of the plant.

SURGING OF TURBOCHARGER Surging: Temporary interruption of mass flow of air from the blower to the manifold which results in reverse flow of air from the manifold to the blower. Surging (variously known as coughing, barking etc.) is a vibration of audible level emanating from the compressor end of the rotation element. The compressor ends of the rotating elements. The compressor, depending upon its speed at any particular time, can only discharge up to a give pressure, if for any reason the pressure in the scavenge space is equal to or higher than this discharge pressure, air will attempt to flow back through the rotation impeller. The back flow of air throws the rotation element in a vibration which produces the so called barking noise. There are many causes of surging. It is usually engine initiated. The turbocharger should be matched to the engine’s air consumption rate and pressure across the whole operating range, this being calculated before the engine is built and tested during the shop trials. An engine traveling from one climate to another will be subjected to great variation in both air density and air temperature. In high temperature, low density climes, usually associated with the tropics, the engine will still have to achieve its rated performance. The turbocharger will be putting out the same volume of air as always but, because the air is less dense, the mass throughput will be reduced. For the turbocharger and engine to be able to provide adequate outputs under these conditions, it is necessary that they are initially provided with a slightly higher rating than is required in more temperate climes. This is referred to as ‘de-rating’. The symptoms of a surge condition are a repeated, irregular violent thud from the air intake at the bower, and rapid surges in the scavenge-air pressure. If you are sanding by the air intake to the blower you would sense that the air was being taken into the blower in a series of ‘gulps’. If surge occurs the engine speed must be reduced immediately; easing the scavenge-trunk relief valves will help to reduce the shocks from air – flow surges. In an engine with a correctly matched blower, surge is caused by a combination of two factors. The first is dirty airintake filters, which restrict the flow of air to the blower and the amount of air discharged. The second is the pressure pulsations, created by the opening and closing of the scavenge ports, and the irregular air flow from this cause. Dirty air filters alter the discharge pressure ration of the blower, and, in effect, change the blower characteristics. The pressure pulsations, when felt at the blower discharge, cause the blower to become unstable and consequently to surge. On engine with two or more blowers discharging into a common casing, the air movement in the scavenge casing may bounce from one blower to the other, causing each blower to go into and out of surge conditions in an alarming manner. To prevent the trouble recurring, the air filters must be immediately cleaned very thoroughly, so that the blower can work within the stable range. CORROSION AND OTHER DEFECTS Engineer M. A. Hamid

6

TURBOCHARGER Note Book For Marine Engineers Some fuels have sulphur content as high as 5% which, when burned, forms sulphur dioxide and sulphur trioxide. These compounds combine with the steam which is always present in exhaust gases to form sulphurous and sulphuric acids. The dew point of sulphuric acid is approximately 160°C and as the wall temperature of water-cooled casings may be lower, highly concentrated sulphuric acid may condense and corrode the casing surface. It is also possible for the casing to suffer erosion from solid particles in areas of high exhaust gas velocity. The problem of corrosion is not as obvious as it was in the past for modern engines, with their higher specific output, are operating at higher exhaust gas temperatures. The cooling water temperatures have also been increased in an attempt to overcome the corrosion problem and some manufacturers of turbochargers have increased the gas casing wall thickness to try and prolong the life of their product. It is recommended that all thicknesses should be checked during every overhaul. Should the gas outlet casing be cracked and no longer water tight, it is possible, in some instances, to run at reduced power for short periods without cooling water but a close watch should be kept on the turbine-side lubricating oil sump where the oil temperature should not exceed 150°C. If a turbocharger vibrates the cause or causes must be investigated. Start by examining the foundations to which it is bolted. See that vibration is not being imposed by gas inlet or outlet pipes or air pipes. Check that expansion pieces are correctly assembled and do not transmit loads from the pipes to the casings. In a properly designed installation one can eliminate the seating and connections to the turbocharger as causes. Vibration alarm instruments are available which can detect eccentricities. They are especially useful in unattended engine-rooms and their use can prevent possible extensive damage to the rotor.

LUBRICATING THE TURBOCHARGES (SEE FIGURE 2). The turbochargers (6) are provided with a separate lubricating oil system, the oil being fed to the turbocharger bearings from a tank (5). The oil level in the tank is about 6m above the turbocharger bearings, which are thus gravity fed. The oil flows to the turbochargers via a pipe from the bottom of the tank, this pipe branching out of the turbocharger bearings. In order to be able to check the oil supply, a sight glass (7) is provided in the outlet branch pipe leading from each bearing. The oil flows from the bearings to a drain tank (1). An electrically driven pump (2) sucks the oil from tank (1), and delivers it through an oil filter (3) and an oil cooler (4) to the gravitation tank (5). In the event of failure of the lubricating oil pump, the standby pump starts automatically. An electrical shutdown mechanism (10) fro the main engines is installed in connection with the gravitation tank, and functions when the oil level in the tank sinks below a certain height. At the moment when the shutdown occurs, there is normally sufficient oil in the gravitation tank for approx 10 min of operation, thus ensuring that the turbocharger bearings are supplied with oil during the run down period of the turbochargers, after the main engine has been shut down. The lubricating oil used must be good turbine oil with a viscosity of 30-40 CST at 50oC.) The inlet pipe to each bearing is provided with a 3-way cock. These cocks can be set to bypass the lubricating oil around the bearings and are used, for instance, after the completion of maintenance work, when the pipelines are flushed through to prevent impurities being carried to the turbocharger bearings. A turbocharger of Elsinore Shipyard manufacture is shown in Fig 3. this differs from the previously described turbochargers in that it has bal bearings instead of journal bearings, whereby the normally required separate lubrication system can be dispensed with, and built-in lubricating oil pumps are used instead. Furthermore, the turbine housing is not cooled, which tends to reduce corrosion on the gas side of the housing.

WASHING OF TURBOCHARGER


7

TURBOCHARGER Note Book For Marine Engineers The NA turbocharger can be kept as clean, hence as efficient, as it is when delivered, by periodically cleaning its turbine side with water. The water washing herein prescribed, however, is no for combating the heavy fouling but, rather, for preventing such a heavy fouling from taking place during a prolonged period of sea service. Only fresh water is to be used in the water-washing to remove dirt mechanically by impingement of fine water particles while running the turbocharger. As the tendency to formation of deposits in the turbocharger depends, among other things, on the combustion properties of the fuel oil used, the intervals between the cleanings should be fixed after an assessment of the degree of fouling of the turbocharger in the individual plants. It is recommended that the cleaning is to be carried out before the turbocharger becomes extremely dirty.

Description: • In the initial period of operation, the cleaning of the turbine side is to be carried out over 200 hours. Afterwards, the cleaning interval is to be adjusted according to the operation conditions. • As a general rule, however, the cleaning is to be carried out when there is change in normal scavenging air pressure or normal exhaust-gas temperature. • Fresh water to be used in the cleaning is to be free of impurities. Never use sea water, kerosene or lubrication oil for cleaning purpose since they will cause corrosion, explosion or other troubles. Do not use fresh water containing additives which are considered to have harmful effects on the turbocharger as well as on the engine. •

After the cleaning, run the turbocharger and engine for the specified period of time to dry them completely.

• If the scavenging air pressure of exhaust-gas temperature persists to remain the same as the pre-cleaning level after the injection of water has been finished, then it is to be suspected that dirt has not been removed adequately. In such an instance, repeat the cleaning. • If vibrations occur in the turbocharger after the cleaning, repeat the cleaning. If the deposits have been so heavy that the vibrations can not be eliminated by the cleaning, the engine load must not be increased, but the overhaul cleaning of turbocharger must be conducted. • If the cleaning should to improve the turbocharger efficiency, then it is to be suspected either that the turbocharger is so heavily fouled as to defy the water-washing or that there are some other causes adversely affecting the turbocharger operation. Please see the instruction manual for probable caused of operational difficulties.

3. Cleaning Procedure 3.1: Method of Cleaning Run the engine as show in fig. 1 while cleaning turbocharger. Set the revolution speed of turbocharger according to table 1. If the engine can be run, the turbocharger may be operated at RMP lower than that specified herein in order to keep turbine inlet gas temperature. Notes: 1)

Allowing more than 15 minutes before the cleaning is necessary to stabilizer the turbine at low temperatures. Start injecting water after making sure that the gas temperature has gone down to 200C or below.


8

TURBOCHARGER Note Book For Marine Engineers 2)

During the cleaning, the turbocharger rpm and also exhaust gas temperatures before and after the turbine will slightly decrease. When the turbocharger is cleaned repeatedly, allow about 5 minutes between injection periods, and make sure that the flow of the drain from the gas outlet casing stops.

3)

All owing more than 10 minutes after the cleaning is necessary to dry the turbocharger and engine.

3.2: Cleaning procedure and operation of cleaning nozzle (1)

Open the drain valve fully at the bottom of the turbine gas outlet casing, and check the passage through the drain valve and pipe. (Illustration III-A and B) caution: Don’t forget to open the both valve.

(2)

Put the cleaning nozzle into the socket pipe on the exhaust pipe immediately before the turbocharger, so that the water jet will be directed towards the turbocharger. In this position the groove in the nozzle and the pin in the socket pipe fit, together, and the nozzle can be fixed by means of the union nut.

(3)

Connect the rubber hoses to the cleaning nozzle and supply pipes.

(4)

Open the cocks on the supply pipes.

(5)

Spray for about 10 minutes, while opening the drain valve at the turbine outlet casing regularly in order to remove the drain water and check the color of drain water.

(6)

Remove the hose immediately after finishing the cleaning, and fit the cap to the nozzle socket pipe.

(7)

Shut the drain valve after all water has been drained away from the turbine outlet casing Turbocharger Turbine can also be cleaned by Injecting Rice Husk or ground walnut powder of 1~1.5 mm diameter. This will clean the blades without damaging it.

OVERHAULING OF TURBOCHARGERS 

Drain cooling water from the turbocharger.



Loosen the inlet and exhaust ducts of both the compressor and turbine ends.



Disconnect and remove cooling water pipelines at the bottom of the bearing casing.



Disconnect cables for vibration monitoring, lube oil monitor and speed measuring device at the terminals and remove cable gland.



Dismantle air filter silencer by removing the wire mesh air filter plates, cone shaped insert. Clean the filter plates of all dirt using compressed air and a brush, to remove greasy dirt the plates are soaked in diesel oil and then blown with compressed air, heavily roped plates and renewed.  Remove bearing cover, on the back of the bearing cover a plate depicting the K value is shown which indicate the relative position of the casing and the end of the rotor shaft. The other picture depicts the relative position and the distance between the ball bearing and housing before dismantling the K value is carefully noted down.  Using the rotor arresting fixture provides by the manufacturer, arrest the rotor and unscrew the nut on the rotor shaft end. The ball bearing assembly jacked out. The full bearing is then dipped in clean diesel fuel and is fully cleaned and on dried with compressed air. Defective ball bearing is replaced.


9

TURBOCHARGER Note Book For Marine Engineers  Remove the volute casing by loosening the bolts pay attention not to damage the compressed wheel while doing so and thoroughly clean the volute casing outlet, inside of the increasing area and the diffuser vanes diesel oil, carbon remove and compressed air are used for effective cleaning.  Withdraw by jacking the rotor shaft. The compressor wheel is then thoroughly cleaned using diesel oil, cloth and compressed air. The labyrinth rings on the back of the impeller should also be cleaned properly. Dismantle turbine end bearing cover and withdraw the bearing clean it with diesel oil and compressed air.  Unscrew the bolts on the exhaust gas inlet casing and withdraw the gas admission casing. Thoroughly clean the nozzle ring of all carbon deposits and dirt accumulation while withdrawing the casing take care not to damage the labyrinth seal and turbine shaft. Then withdraw the turbine and shaft.



Dip the turbine rotor shaft in a pan filled carbon solvent, the rotor is set up on a pair of wooden V notched

trestles that allow the part disc and the blades to soak in the solution. During soaking period the rotor must be regularly rotated so that the deposits are completely removed. After soaking it is cleaned as with a cloth and wiped dry and blown with compressed air.  The whole rotor shaft with the compressed and turbine is then balanced by a experienced operator. The turbocharger is now assembled in reverse order of dismantling. All clearances are to be maintained while assembling according to manufacturers instructions.

TWO-STAGE TURBOCHARGING Two turbo charging may be connected in series on both the air and gas side (see figure) in order to achieve a higher boost pressure than can be obtained by a single turbocharger. This arrangement is termed two-stage turbocharging. If air is compressed in two stages the air can be cooled between each stage of compression and the amount of work done in compressing the air is reduced. Two-stage turbo-charges are generally built up from two separate matched standard size turbo-chargers. The rotor of each stage is on a common polar axis or centre line. The rotors are not mechanically connected and are free to move independently of each other. The exhaust gas stages are usually arranged back to back so that the gas passes direct from the outlet of the first stage into the inlet of the second stage. This reduces loss of heat from the gas in passing between each stage. The second-stage turbine drives the low-pressure compressor. Air is cooled after passing from the low-pressure stage and then enters the high-pressure stage which is driven by the first-stage turbine. The air is again cooled after leaving the high-pressure stage. Two-stage turbo-charging allows higher boost pressure ratios and imparts much higher overall efficiency to the turbocharger unit. Because of the rather recent availability of more efficient turbochargers, interest in two-stage turbocharging for merchant ships has diminished.

TURBO COMPOUND SYSTEM The very efficient turbochargers that are produced today do not need all of the exhaust gas available from the main engine, when this is loaded more than approx 50% of the maximum performance, in order to compress the amount of scavenge air demanded for carrying out the combustion process in the engine’s cylinders. The surplus gas can be utilized by feeding it through a bypass around the turbocharger’s gas turbine for the operation of a gas turbine which generates mechanical energy. This mechanical energy is conveyed back to the crankshaft of the main engine via a Engineer M. A. Hamid

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TURBOCHARGER Note Book For Marine Engineers system of gears. The main engine’s specific consumption of fuel oil can thus be reduced by approx 5 g/kW h. This system is known as TCS (turbo compound system. Naturally, the amount of surplus exhaust gas decreases with reduced loads on the main engine. At approx 50% load, the power from the TCS unit is reduced to approx 25% of the maximum level, and the bypass around the turbocharger’s gas turbine is automatically closed. The exhaust gas is then used exclusively for the compression of scavenge air for the main engine. The result is that the main engine’s combustion pressure when TCS is used is higher at low engine loads than is the case with an engine without TCS. At the same time the specific fuel oil consumption at low loads is reduced by 2-5 g/kW h. With an increased scavenge air pressure at low engine loads, the auxiliary blowers do not need to be coupled until the load on the main engine has been reduced to less than 25% of maximum, causing a saving of electrical energy during maneuvering. TCS, which is standardized for the MC engines, is delivered as a complete unit with the designation TCS/PTI (turbo compound system/power take in). Through bevel gears in connection with a train of gears, the gas turbine is mounted at the foremost end of the crankshaft. The gears are connected to the crankshaft through a flexible coupling (see figure). 1.

Gas turbine

2.

Bevel gear

3.

Reduction gear

4.

Flexible gear

5.

Crankshaft

Since the consumption of electricity is the second largest use of energy abroad a ship, it is also necessary for attempts to be made to reduce fuel oil costs in the production of this energy. One of the possibilities in this connection is to drive an electrical generator through gear transmission from the main engine. Such a system is known as PTO (power take off). However, there are certain difficulties connected with the execution of a PTO system. The first is that the degree of irregularity of the main engine makes it necessary to transfer energy from the crankshaft to the el-generator via a flexible coupling, and the second is that the speed of rotation of the generator rotor must be compatible with the electrical system, even at changing propeller speeds. The latter can be achieved by using an RCF (Renk constant frequency) drive consisting of a planetary gear and a hydraulic clutch (see figure) 1.

El-generator

2.

Hydraulic clutch

3.

Planetary gear

4.

Gear transmission

5.

Flexible coupling

6.

Crankshaft

The TCS/PTI and PTO/RCF units can be built together as shown in (Figure).

1.

Gas turbine

2.

Renk constant

3.

El-generator


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TURBOCHARGER Note Book For Marine Engineers

4.

Crankshaft

5.

Frequency gear

DEVELOPMENTS IN THE FIELD OF TURBOCHARGING ABB TURBO SYSTEMS (Lower Life Cycle Costs) ABB TURBO SYSTEM has announced the launch of its latest turbocharger for large ten-stroke diesel engines ranging from 1,250kW up to the highest outputs available on the market. The new model, the TPL..-B, has been designed as a robust and reliable platform with inboard plain bearings, designed to last for 35,000 operating hours. Using past experience, gained through producing over 30,000 turbochargers, ABB claims to have doubled the lifetime of the turbocharger bearings, as well as simplifying the design and reducing the number of parts, which means lower life cycle costs and easier maintenance. Improvements in design include the compressor, which features a single0piece aluminium alloy wheel with splitter bladed impeller design and backswept blades for higher efficiency and a wide compressor map. ABB claims peak efficiency of over 87 percent can be obtained. In addition, enlarged compressor diameters have increased the volume flow, which allows optimized matching of the turbocharger to the engine application. The main features of this new generation of turbochargers are their pressure ratio and efficiency achieved partly by the characteristics of the turbine, which includes a wide chord blade, without damping wire, for constant pressure applications.

MAN B&W TCA TURBOCHARGERS Reduce Maintenance and Environmental Impact In only a year the MAN B&W TCA turbocharger series has become a bestseller, with more than 120 turbochargers ordered or specified. The TCA series will have a significant impact for MAN B&W Diesel engines. In the case of the updated V48/60B four-stroke engine, the TCA is an important element of the concept as only one unit is used instead of two in the previous design. This results in a reduction of the V48/60B’s overall construction width by more than one metre and a significant increase in engine efficiency. The TCA series offers numerous advantages in environmental impact and economy. Increased efficiency leads to lower fuel consumption and, consequently, reduced emission values, noise emissions below anticipated future requirements, and lower exhaust gas temperature reduced wear. Other improvements have resulted in reduced maintenance requirements. For example, the method of attaching the compressor wheel ensured that the complete rotor does not have to be rebalanced after repair or rework. This has provided another for MAN B&W. All pipes for bearing lubrication and ventilation have been integrated in the casing, continuing MAN B&W’s ‘pipe less engine’ concept. The TCA turbocharger continues two of MAN B&W’s basic principles which have proved successful and have recently been copied in competing designs uncooled hot-gas casings, and internal support of the rotor via plain bearings. Test-bench performance figures indicated turbocharger efficiency of over the 68 percent minimum for the whole engine load range and 71 percent at 75 percent load. Noise level measurements taken at 1 m recorded a level below 105dB Engineer M. A. Hamid 12

TURBOCHARGER Note Book For Marine Engineers (A). All test-bench figures were confirmed during sea trials in Norway and the TCA77 has proved its reliability in over 3,000 operating hours.

TROUBLE SHOOTING IN TURBOCHARGER SYSTEM FAULT Charging air pressure too low for speed and load

Charging air pressure too high together with high turbine inlet temperature.

CAUSES

REMEDIES

Air filter may be dirty.

Clean the air filter.

Obstruction in inlet duct.

Remove obstruction.

Restriction in air manifold.

Clear restriction.

Exhaust gas duct turbine to atmosphere may be obstructed.

Examine duct and clear.

Exhaust gas escaping before reaching turbine

Repair detective part.

Low rotor speed due to mechanical defect.

Examine bearing labyrinths, blade tips, and impeller. Check casings for distortion.

1. Bad combustion.

These are all engine faults and should be taken care of as soon as possible.

2. Incorrect injection timing. 3. Worn or leaky exhaust valves.

Excessive oil consumption

Dirty air filter causing excessive depression at the impeller eye.

Clean air filter.

Choked sealing air passage to labyrinth eye.

Clean air passage.

Bearing labyrinth seals worn or rubbing.

Scrape phosphor bronze bush.

Leak in oil system.

Repair.

Dirty or damaged oil separator

Clear or replace.

TROUBLE SHOOTING IN TURBOCHARGER FAULT

CAUSES

REMEDIES

Rubbing of labyrinths.

Due to distortion of casings caused by local over-heating.

When the unit has cooled and no distortion of the casings, scrape labyrinth bush until free.

Impeller running against casing.

Due to incorrect location of thrust

Examine thrust bearing and re-set


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TURBOCHARGER Note Book For Marine Engineers

Turbine blade tips rubbing against casing.

bearing.

adjusting washer.

Due to distortion, poor combustion causing deposit on turbine casing, or incorrect location o thrust bearing.

If casing is distorted return to makers for re-machining. Clean turbine casing, find cause of poor combustion and cure same. Re-set thrust bearing, adjusting washer.

SAFE WORKING PRACTICES AND SAFETIES FITTED ON TURBOCHARGER. 1.

If the level of Lube oil in the header tank of the turbocharger decrease below a certain limit the alarm is activated and the main engine trips.

2.

If the lube oil pump breaks down the head of the gravity tank is sufficient to produce a continuous supply of oil to the turbocharger bearings till it stops.

3. Before over hauling the turbocharger the manufacturer’s manual should be properly read and understood by the personnel. 4.

Proper tools should be used during over hauling the turbocharger.

5.

The turbocharger should be washed regularly as per the manual.

CONCLUSIONS Most modern engines are turbocharged to increase the power of output and efficiency with only a relatively moderate increase in size, weight and initial cost. The blower compresses air so that it is delivered under pressure to the engine cylinders. Because the air is under pressure, a greater mass can be contained in the cylinder and so more fuel can be burnt per stroke, which increase the power developed. Engines pressure-charged in this way with exhaust-gas turbodriven blowers are often referred to as turbo-charged engines. Turbo-charging tends to reduce fuel consumption, in part because the friction losses of the turbocharged engine do not increase as rapidly as the power output, and in part because the improved charging results in better combustion conditions. A turbocharger is an energy recovery device: a more efficient turbine will recover more energy from the exhaust gas stream; low friction rotor shaft bearings will absorb less of the turbine output; and a more efficient compressor will better utilize the remaining energy to compress more air to a higher pressure. Thus, small but simultaneous improvements in the efficiencies of components, through improved component design and manufacturing, have compound effects on overall turbocharger efficiency. While these improvements tend to result in higher turbocharger cost, the environment of high fuel costs makes the cost increase acceptable because of the resulting improvements in engine fuel consumption and power output. Hence, we can come to a conclusion that by fitting a turbocharged engine a considerable amount of fuel savings can be obtained and also reduction in size and weight of the main engine.


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12 Turbocharger

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