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Turbocharger



Description of Turbocharger


A modern exhaust-gas turbo-blower is essentially a single-stage impulse turbine connected through a common shaft to a centrifugal-type air blower. 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 section of the casing which houses the turbines is fitted with one or more flanged exhaust gas inlets, which lead to
the nozzle-blade ring assembly. The exhaust gases pass through this ring and are directed to the turbine rotor blades. ‘The gases enter the moving blades of the turbine rotor at high velocity. The passage of the gas through the rotor blades causes a change of direction in the gas flow, resulting in a change of momentum, which exerts a force on the turbine blades. This force causes the rotor to revolve at high speed. The exhaust passes from the rotor into a circular space connected to the exhaust-gas outlet branch.
The air-blower casing is fitted with filters at the air inlet to the casing. The entry passages after these filters are usually fitted with splitters to guide the air through the passages and reduce the draught losses caused by a change in the air-flow direction. Sound-absorbent material is used to cover the inside of the air passages and the splitters, to reduce wind and blower noise.
At the end of the inlet passage just before the blower impeller, curved air-guide vanes are fitted so that the air enters the impeller without shock. The impeller takes in the air axially and discharges it radially.
The air is discharged from the blower impeller into the diffuser at high velocity. During its passage through the diffuser, the air gives up much of its velocity and in so doing builds up pressure. From the diffuser, the air passes into the discharge casing, which is volute-shaped and fitted with a flanged outlet connection.
The casings for the turbine and blower are built up from full circular sections held together with circumferential joints. Building the casings in this manner obviates the need to split the casing longitudinally, and reduces design problems and manufacturing costs. The turbine casings are usually water-cooled and connected with the engine cooling system.
The rotor is usually made up of a hollow shaft on which the turbine rotor and air impeller are mounted. The impeller is often made in two sections to make production easier. Three labyrinth gland sections are built into the rotor: one, at the turbine end to seal the shaft and prevent exhaust gas leakage; another, at the blower end to prevent oil from being drawn from the bearing. A third is built up between the turbine rotor and impeller, forming a labyrinth gland in the casing division plate between the turbine and blower spaces. The labyrinth glands are supplied with sealing air from the blower discharge.
Thrust bearings are fitted at one end of the rotor to balance the thrust set up by the difference in gas and air pressures within the casings.
In marine practice sleeve-type bearings or ball and roller bearings may be used, In practice today the use of each type of bearing appears to be equally divided. Some manufacturers prefer to use the ball and roller bearings because the cooling and lubrication of these bearings is simpler and enables the turbocharger to be independent of an external oil supply. Ball-type bearings take up the rotor thrust and roller bearings allow for rotor expansion. In some smaller turbo-charger units ball bearings may be used at each end of the rotor shaft or in the centre of the shaft between the air and exhaust gas sections. This arrangement allows for easy access to the rotor when dismantling the turbo-charger.
‘When ball bearings are used one bearing must be free to slide in its housing in order to accommodate the differences between rotor and casing expansion. The bearings are often fitted into spring-mounted supports.
There are three generally accepted methods of lubricating turbo-blower bearings. One method utilizes the lubricating oil from the engine lubrication system. The oil is supplied under pressure to the bearings, from which it drains back into the engine system. The second method consists of a complete Lubricating-oil system used only for the turbo-blower bearings. The system comprises a pump, which takes lubricating oil from a drain tank and pumps it through an oil cooler up to a gravity supply tank, From the gravity tank the oil passes through filters to the turbo-blower bearings, and then runs down to the drain tank, The third method used with the ball and roller bearings is to make the lower part of each end cover into an oil reservoir and attach a gear oil pump to each end of the rotor shaft. Lubricant and coolant are then pumped by each gear pump from its reservoir into its adjacent bearing. The oil after passing through the bearing drains back to the reservoir. Cooling is affected by air passing into
the compressor over the reservoir at the compressor end and from a cooling water jacket at the exhaust gas end. It must be remembered that ball and roller bearings will operate satisfactorily at much higher temperatures than sleeve bearings.
The rotor revolves at high speed, and it must therefore be dynamically balanced after all the parts of the rotor are assembled. The balancing will normally be carried out on a balancing machine.


Construction and Working


Material
(i) Turbine wheel, blades, Nozzle ring & rotor shaft are made of Nimonic 90 (Ni-75%; CO-18%; Ti-3%; Al-2%; Cr-2%). Impact resistance, strength, thermal stability and creep resistance at high temperature of continuous up to 650℃.
(ii) Turbine casing is made up of cast iron with corrosion preventive plastic coating in the casing of water-cooled turbocharger.
(iii) Compressor impeller, volute casing, diffuser & Inducer: Aluminium alloy for lightweight, strength & smooth surface finish

Refer to the diagram above,
  1. The turbocharger is having an axial flow turbine & a centrifugal compressor. The turbine casing is in two parts and is made of cast iron.
  2. Leakage of exhaust gas to the compressor side is prevented by allowing a controlled air leakage between the impeller and casing, this air passes along the shrouded shaft and also provides shaft cooling.
  3. A grit is provided at the exhaust gas entrance of the turbine side, which prevents the broken pieces of the piston ring from entering into the turbine.
  4. Exhaust side casing(or turbine casing) is water or air-cooled. The un-cooled casing is presently used. This increases the efficiency, also the exhaust gas temperature at the outlet of the turbine is also higher which is utilized in economizer. The turbine casing houses a nozzle ring, which converts the pressure energy of exhaust gas into kinetic energy. Shroud ring is present as a part of exhaust gas casing adjacent to the turbine, which is to prevent corrosion damage of the casing and can be replaced easily when it is corroded. This minimizes the maintenance cost.
  5. The exhaust gas with a high speed acts on the turbine to rotate it, and so the compressor wheel which is mounted on the same shaft rotates. The turbine disc is forged integral with the rotor shaft. The disc is provided with side entry slots to attach the turbine blades. Blade roots are of "fir tree" shape, which provides free room for expansion. Binding wire is used for the purpose of vibration reduction in the blades.
  6. Air from the engine room atmosphere enters the compressor side of the turbocharger through a silencer air filter. Airside casing comprises of inducer, which guides the air without shock, smoothly and efficiently to the impeller.
  7. Impeller discharges the air radially by using centrifugal force through diffuser and volute casing. This high-speed air discharged from the compressor impeller contains kinetic energy, this energy is converted into pressure energy when passing through diffuser and volute casing. The velocity of air is leaving the compressor is the resultant of radial(v2/r) and circumferential(v) velocity. The angle of incident is so designed that to match the inducer inlet angle, which leads to maximum compressor efficiency.
  8. Diffuser and volute casing has a divergent shape causing the speed of airflow to reduce and converting kinetic energy to pressure energy.
  9. Ball bearings or rollers at the ends of the shaft are spring-mounted are used to support the rotating shaft. They are often lubricated by their respective gear-type pumps with oil sump, mounted at the ends of the shaft. Roller bearings are externally or outboard mounted and have a finite service life of 8,000 to 20,000 hrs. They provide greater alignment accuracy but are subjected to brinelling, susceptible to vibration & fatigue.
  10. Sleeve type bearings are generally internally or inboard-mounted and can withstand a higher temperature, quite resilient towards vibration & fatigue. Inboard mounting of bearings makes the machine shorter, provide better air inlet flow, high compressor efficiency but present high friction at load RPM & difficulty in accessing. They are externally lubricated by main engine lube oil passing through a fine filter and require a header tank at a height of 6 m above the bearings to supply for about 15 minutes after the engine is stopped.
  11. Thrust collar or double row angular contact bearings are used to accommodate axial thrust. The thrust bearing at the end of the shaft allows the thermal expansion of the shaft. A radial flow turbine can also be used as a solution for the axial thrust.
  12. Labyrinth glands are provided with sealing air under a slight pressure, which expends in both the direction. They are fitted at each end of the rotor and between the compressor and turbine. They provide torturous paths across a series of fine clearance to prevent any leakage of exhaust gas to the bearing sides and oil to the airside. Oil seals in the form of thrower plates are also fitted at the bearings, to prevent the passage of oil along the shaft.

Scavenging system


In the scavenging system, the scavenge air receiver has an inner and an outer compartment with a set of non-return valves between them.
From the outer compartment, air enters the inner compartment. From the inner compartment, the scavenge air enters the cylinders when the piston uncovers the scavenge ports towards the end of its downward stroke.
An electric motor driven auxiliary blower draws air from the outer compartment and delivers it to the inner compartment via non-return valves. It automatically operates upon the pressure drop and provides sufficient air to supply to run the engine at reduced load.
 


Turbocharger surging


Prevent Turbocharger surging
The following are the ways to prevent turbocharger surging. However, it is to note that some points may vary with the design and construction of the turbocharger.
• Keep the turbocharger intake filter clean.
• Water-wash the turbine and the compressor side of the turbocharger.
• Proper maintenance and checks should be done on turbochargers periodically.
• Soot blow should be done from time to time in case of an economizer or exhaust boiler.
• Indicator cards to be taken to assess cylinder and power distribution of individual units.


Turbocharger Vibration


Turbochargers are high speed rotating machines. In fact, some of them achieve speed higher than any other machinery on-board ship. Hence, they have a natural vibration frequency. Engine bracing is done near the turbocharger to transfer such vibrations to the ship's structure. If the vibration Increases abnormally, stop the engine as it may be due to worn out bearings, abnormal "K" value or lose foundation bolts.
Vibration may be the result of the general deterioration of the turbocharger over a period of time or a sudden change in condition. If allowed to continue unchecked, then serious damage may result. Vibration monitoring may be used to diagnose faults in the turbocharger (harmonic filters). As an alarm/shutdown protection, or as a condition monitoring means in place of inspection.
Gradual increase in vibration normally due to gradual uneven fouling resulting from one or more of the following
  • Ineffective in-service cleaning.
  • Gradual fouling of damping wires/blade roots.
  • Deterioration of resilient mountings for bearings.
  • Slackening of foundation bolts.
  • External excitation.

Vibration can be prevented by maintaining good combustion and carrying out preventive maintenance,  Regular and effective water wash,  Regular bearing replacement, Maintenance of foundation and supports.
A sudden increase in vibration is normally due to a sudden imbalance caused by blade damage resulting from one or more of the following
  • Sudden imbalance due to partial dirt removal on heavily fouled rotors
  • Breakage of damping wires
  • Breakage of resilient mountings
  • Bearing failure.
  • Water ingress due to casing leaks
  • Irregularities in-cylinder combustion
  • Breakage of foundation bolts/supports.
  • Sudden external excitation.

This can be prevented by maintenance of cylinder condition, regular and effective cleaning, lube. oil and bearing maintenance, regular inspection of the casing, foundation bolts, etc.

Matched Turbocharger with engine


In matching the turbo-blower to the engine, a free air quantity in excess of the swept volume is required to allow for the increased density of the charge air and to provide sufficient air for through scavenging of the cylinders after combustion. For example, an engine with a full load Bmep of 10.4 bar would need about 100% of excess free air, about 60% of which is retained in the cylinders, with the remaining 40% being used for scavenging. 
    Modern engines carry Bmeps up to 28 bar in some cases, requiring greater proportions of excess air which is made possible by the latest designs of turbochargers with pressure ratios as high as 5:1. 
    To ensure adequate scavenging and cooling of the cylinders, a valve overlap of approximately 140° is normal within a typical case of a 4-stroke engine, the air inlet valve opening at 80° before TDC and closing at 40° after BDC. The exhaust valve opens at 50° before BDC and closes at 60° after TDC.
A turbocharger is said to be matched to an engine when at any given speed, the exhaust gas energy must cause the turbine to run at a stable speed at which the compressor will supply the correct mass of air to the engine. It must remain matched over the full operating range of engine operation.
In order to do this while designing a turbocharger the following are to be considered:-
(a) Calculate the airflow requirement of the engine at all loads- Quantity and pressure. This approximately fixes the speed range.
(b) Accordingly select the impeller, diffuser and the volute casing of the compressor side.
(c) Based on the above, In the turbine, select the Nozzle ring and the turbine blade size and shape (mixture of impulse and reaction) in such a way that even at some restricted exhaust flow passage, the airflow requirement to the engine does not hamper.

Turbocharger Corrosion


1. Corrosion in waterside of casing:-
This is because of incorrect jacket water treatment, ingress of exhaust gas into water spaces and abnormal stress. The corrosion will result in perforation of casing allowing water into the gas side, mechanical cracking due to weakening of the material, thermal cracking due to reduced heat transfer.
The corrosion in the waterside can be prevented by correct jacket water treatment, maintenance of seals/gaskets, and correct alignment of pipework, supports. etc.
2. Corrosion in Gas side
The Gas side corrosion is caused by acidic combustion products, low operating temperatures and abnormal stress, poor combustion. This will result in perforation of casing allowing water Into the gas side, mechanical cracking due to weakening of the material, thermal cracking due to reduced heat transfer, and reduced turbine efficiency due to nozzle/blade wastage.
The corrosion in the gas side is prevented by maintaining temperatures especially during the low load operation, maintain good combustion, correct in-service cleaning procedure, correct alignment of pipework, supports, etc.
3. Corrosion in Airside
This is caused by airborne corrosive pollutants and abnormal stress. The airside corrosion will result in mechanical cracking due to weakening of the material, thermal cracking due to reduced heat transfer, reduced compressor efficiency due to increased clearances, and impeller/inducer/diffuser wastage.
The airside corrosion can be prevented by air intake taken from the clean area, Correct alignment of pipework, supports, etc., and by correct in-service cleaning procedures.

Operational issues in turbocharger


a. Fouling of Intake filter due to dirty engine room atmosphere resulting in reduced air quantity, high exhaust temperatures, and poor combustion.
b. Faulty intake filter allows airborne contaminants to pass into the compressor and faulty oil sealing arrangements allows oil carryover resulting in reduced air quantity, high exhaust temperatures, and poor combustion. Possibility of imbalance and high vibration levels with consequent bearing damage.
c. Faulty oil sealing arrangement on the Turbine side or improper combustion results in carbon build up in the nozzle and the rotor blades also restrict exhaust passage. Due to this, the turbocharger performance deteriorates, which also can result in imbalance and vibration, bearing damage etc.
d. Maintenance of Intake filters: If the manometer difference is greater than normal, the turbocharger air filter may be choked resulting in reduced Scavenge pressure, Black smoke in the funnel and reduced engine power. It is recommended to put an extra felt filter over the compressor to absorb oily air mixture. The fitted mesh filter must be chemically cleaned bi-monthly or as per the running hours described by the manual.
e. Power Distribution: The turbocharger is driven by the exhaust gases produced by the combustion process inside the engine cylinders. As the engine comprises multiple cylinders, it is important that there is a power balance among all the cylinders. If one cylinder is producing more power due to malfunctioning of fuel valve, it will lead to surging of turbocharger's turbine side. Proper care must be taken to ensure even power distribution in the ship's engine.
f. Turbocharger Over Run: The over-run takes place in constant pressure turbocharged engine. It is caused due to fire and/or detonation of scavenging space. Exhaust trunk fire due to accumulation of leaked or excess lube oil and unburned fuel. Over Run can result in turbocharger bearings and turbocharger casing damage, and engine room fire. Over-run can be prevented by scavenging space regular cleaning, exhaust gas pipe regular cleaning, maintain complete combustion of fuel, liner, piston and rings, fuel vales, cylinder lubrication, maintained in good order and by avoiding operation of M/E under reduced load for long.
g. Accumulation of sludge:- A small cone-shaped accumulation of sludge and oil-ageing residues, mixed with abrasion particles of steel. aluminium and bronze originating from the casing, pump and bearing damping parts, often forms just below the opening of the gear oil pump suction pipe. The residues accumulate at just this spot due to the suction flow current of the working gear oil pump. Most particles just remain there, but some are sucked through the pump and injected into the centrifuge, which also works as a dirt separator, where they are finally collected and can be removed during a standard overhaul. The residues are harmless and have no negative influence on safety or running behaviour. No measures need to be taken to reduce or restrict their formation. Such a sludge/particle mixture size will depend on the level of vibration, newly installed parts, the cleanness of the oil chamber, the purity and quality of the lube oil and the number of running hours. h. Erosion of nozzle and cover rings:- It can be a problem, particularly for installations that run on heavy fuel oil. If left unattended, eventually lead to a drop in turbocharger efficiency and to the premature failure of parts. This kind of erosion is caused by particles being formed during the combustion process and conveyed to the turbocharger by the exhaust gas. The quantity and size of the particles depend on a number of factors, ranging from the properties of the fuel to engine operation.
The best way to avoid erosion is to restrict the formation of particles. Start by ensuring that your engine is top fit. Have a fuel oil analysis performed by a noted laboratory. This will help you to avoid fuels with inferior properties. If you are running more than one generator or auxiliary engine, avoid running them for prolonged periods at low loads. If possible, run fewer generators at higher loads. If erosion cannot be avoided, you may be able to fit erosion-resistant coated nozzle and cover rings.
i. Particle formation:- Factors with a major influence on particle formation are the fuel property CCAI (Calculated Carbon Aromaticity Index) and the asphaltene, vanadium and sulfur content of the fuel oil. Also significant are the fuel oil preheating, compression ratio, injection equipment wear and engine load. The engine part load, in particular, plays a major role in the formation of the larger particles causing erosion.

Operation while a turbocharger is out of service

 Running for longer periods with the Turbocharger out of operation. If it is required to run for a longer period, the rotor of the turbine needs to be locked. If there is any fault in the turbocharger, it may be temporarily cut-off by inserting the blanks. Lock the rotor, with the tool provided.
If the engine has more than one turbocharger, insert an orifice plate in the compressor outlet, and blanks in the turbine inlet and outlet sides.
a. There are restrictions on the load, depending on how many turbochargers are present.
b. If there is only one, and it is locked, reduce to 15% of MCR power or less.
c. If one out of two turbochargers is to be locked, reduce to 50% of MCR power or less.
d. If one out of three turbochargers is to be locked, reduce to 66% of MCR power or less.
e. At this time, if one of the auxiliary blowers is out of action, reduce to 10% of MCR power.
f. Temperature of the exhaust gases should not exceed 350°C.
Running for short periods without blanking the turbine.
If there are heavy vibrations, bearing failure or some other mechanical fault in the turbocharger, a short stoppage will be required, to lock the rotor of the defective turbocharger. If there are two or more turbochargers, one of which is to be cut-off, an orifice plate is inserted in the compressor outlet, so as to supply enough airflow to cool the impeller.
As there will be a loss of exhaust gas through the damaged turbocharger, the minimum power will be limited to 15 % of MCR. Also check the exhaust gas temperatures, which should not exceed the maximum specified. Reduce the lubricating oil pressure for the damaged turbocharger and continue circulating the cooling water, as well as the sealing air for bearings.
To Cut off Damaged Turbochargers for Engine Operation
1. Engines with one turbocharger (Engines with exhaust by-pass)


  • Stop the engine.
  • Lock the turbocharger rotor.
  • Remove the blanking plate from the exhaust by-pass pipe.
  • Remove the compensator between the compressor outlet and the scavenge air duct. This reduces the suction resistance.
  • Run engine with 15% of MCR load and 53% speed.

2. Engines with one turbocharger (Engine without exhaust by-pass)
  • Stop the engine.
  • Remove the rotor and nozzle ring of the turbocharge.
  • Insert blanking plates.
  • Remove the compensator between the compressor outlet and the scavenge air duct. This reduces the suction resistance.
  • Run engine with 15% of MCR load and 53% speed.

3. Engines with two or more turbochargers
  • Stop the engine.
  • Lock the rotor of the defective turbocharger.
  • Insert orifice plates in the compressor outlet and the turbine inlet. (A small air flow is required to cool the impeller, and a small gas flow is desirable to prevent corrosion)
  • Run engine with 20% of MCR load and 58% speed.

Points to consider regarding the continued operation
  • Adequate air supply for the amount of fuel injected. The adequate flow path for exhaust gas produced.
  • Adequate cooling of the idle casing.
  • Isolation of air/gas/lubricating spaces in the event of no sealing air available.
  • Securing of damaged parts to limit further damage.

Methods of operating with defective charger
For situations where one of a number of turbochargers is damaged then isolation of the unit is necessary. This can be achieved in a number of ways depending on the exact nature of the damage. Also, read Turbocharger Temporary or Permanent Repair in the event of a breakdown.
Bypass arrangement:-
  • With this type of arrangement, provision is made to allow fitting of a bypass pipe for the exhaust. The turbine inlet and outlet are isolated by fitting blanks and the bypass pipe is fitted to give a suitable gas path. The compressor side will also require isolation to prevent loss of scavenging air due to back flow from the receiver. Cooling water and lubrication systems may also need isolating if casing or seal damage is present.

Rotor removal:- 
  • This may be necessary so that repair can be carried in the event of rotor damage. It is then necessary to blank the openings of the casing to prevent both internal and external leakage of gas and air. Cooling of the casing should be maintained if possible as the exhaust will still have to pass through.

Locking of rotor:- 
  • This may be necessary if time is limited or no repair facility is available in the near future. The rotor is locked into position by attaching the appropriate fixtures. Rather than blanking off the compressor side completely an orifice plate is fitted to allow a controlled amount of scavenge air to pass over the compressor end of the rotor to prevent overheating due to conduction. Cooling supply should be maintained for the turbine casing since there will still be a gas flow across the locked rotor. Isolation of the lubricating oil supply may be necessary due to lack of sealing air.

Internal water leakage:- 
  • In the event of a holed water jacket it is possible to operate the turbocharger as an air cooled unit by supplying compressed air as the coolant after isolating the water supply. Attention must he paid in operating temperatures especially for the turbine end bearing-

Four-stroke engines with turbochargers out of service.
  • For the case of a four-stroke engine where all turbochargers are out of service and no alternate air supply is available then the engine may be operated as a naturally aspirated unit with suitable attention being paid to operating parameters.
  • For the operation of two-stroke engines, some pressurized air supply must be provided, otherwise, the engine speed is limited to only 15%. and even this figure may be reduced by the factors listed above.

Limitations for operation with a defective turbocharger.
The exact power limits allowable with a defective turbocharger will depend on the actual configuration of the system and on how many turbochargers remain in use. The actual operating temperatures should be taken into consideration and normal values should not be exceeded. The condition of the exhaust should be monitored to ensure that adequate combustion air is being supplied. (Dark smoke) Allowance to be made for the rate of change of air supply during acceleration periods. Remaining turbochargers to be carefully monitored for abnormal operation due to altered gas flow Auxiliary air supply to be used where possible, but this may be affected by remaining turbochargers.

Explosion in the exhaust manifold of a large slow-speed main engine, resulting in turbocharger failure.
1. When either un-burnt fuel or cylinder lubricating oil passes into the manifold during running at low load conditions when the heat in the manifold is not sufficient to ignite the oil. Poor injection equipment and worn fuel pumps will cause un-atomized fuel to enter the cylinder. At low speeds, with worn rings and liner, combustion temperature may not be reached causing the un-burnt fuel to be blown into the exhaust.
2. If the cylinder lubricator requires manual adjustment to alter the quantity injected and this is not reduced when the engine load is reduced below 15%. a lot of the excessive cylinder oil will end up in the exhaust manifold. This can happen during a period of UMS if the officer on the bridge reduces the engine load without notifying the duty engineer.
3. When the load on the engine is increased back to MCR the increased heat in the exhaust manifold with the oxygen in the scavenging air can cause the un-burnt fuel or cylinder oil to vaporize and burn, sometimes violently. The shock wave of the explosion can damage the turbine rotor and gas casing causing fracture of the casing. Possibility of fire from turbocharger lubricating oil contacting gas or exhaust casing. Even if this does not occur, the violent over-speed of the rotor can lead to serious damage to the bearings, resulting in total breakdown.

Actions to be taken for the ship to proceed safely to port following an exhaust manifold explosion and turbocharger failure.
The engine will be stopped and secured and allowed to cool down, (assuming any fire is extinguished safely).
a. Assess damage to turbocharger and exhaust manifold. Remove debris from the exhaust manifold and clean.
b. Carry out scavenge inspection of combustion spaces and exhaust valves. Damage Report sent to DPA.
c. If damage has occurred to the exhaust manifold this will have to be repaired.
d. If the gas casing of the turbocharger is damaged then the turbocharger will have to be bypassed using the supplied by pars pipe if it is a single blower installation. Otherwise, remove the rotor and blank gas casing.

The engine can be run at a low load using the auxiliary blower to supply the air.
a. Performance may be improved by rigging canvas trunking from ER supply fan to blower.
b. In the case of a multi blower installation, then, if the other blowers are undamaged, after blanking the damaged blower, the engine can be run at a reduced load. Do not forget to reduce cylinder lubrication.
c. Check all cylinders are firing correctly, if not then overhaul the fuel injection equipment, or take the affected cylinder out of operation.

Turbocharger cleaning and washing


The exhaust gas from heavy fuel oil during combustion contains particles that attach to every part of the exhaust gas system. In the turbocharger these particles stick to the turbine blades and nozzle ring, forming a layer of dirt which reduces the turbine area and causes a dropped efficiency. To limit this effect, the turbine has to be cleaned during operation.
Getting the cleaning interval right for 4-stroke engines is a bit difficult. If washing is carried out too often the cleaning results will the good, but the thermal cycles increase. This causes material stress and may impact component durability. Especially if the washing temperature is too high. Thermal stress can cause cracking; the more thermal cycles, the faster the cracks develop and propagate.
If the intervals between washing are too long more dirt will build up, causing a drop in turbocharger efficiency, blockage and an increase in the exhaust gas temperature. The layer of dirt can also harden. If this happens it can only be removed by mechanical cleaning of the turbine-side parts.
Too-frequent washing results in a loss of availability due to the necessary load reductions, while worn out parts have to be replaced more often; too-long intervals between washing also lead to a loss of availability due to the unscheduled downtime for mechanical cleaning, and then there's the cost of the work itself.
Watching some key operating parameters - turbocharger speed (rpm), exhaust gas temperature directly before the turbine and air outlet pressure after the compressor - and observing the trend can be helpful in deciding the washing frequency.
With the documented information available for example photos of the nozzle ring and turbine blade, the operator is in a better position to judge whether the cleaning interval should be longer or shorter.
Soot Blowing: The turbocharger performance will get affected if the exhaust passage after the turbocharger is in foul condition (exhaust trunk and economizer). It may lead to surging or even breakage of turbine blades. Therefore it is recommended to soot blow the exhaust gas boiler tubes on daily basis.


Cleaning  the filter silencer

1.1 Washing the filter ring:- To ensure perfect operation during water washing it is advisable to replace the filter ring. Wash or replace the filter ring every 500 service hours. The filter ring is not to be washed more than 5 times.
(a) Rinse filter ring with water (up to 40 deg. cel.) using fine washing powder or, if very dirty, soak it and squeeze out carefully. Rinse in cold water. Avoid high mechanical stress(wringing, strong water jet).
(b) Allow the filter ring to dry completely before assembling.

1.2 Cleaning the silencer:-
During cleaning take care that the felt segment does not get wet.
(a) Loosen the tension bands and withdraw cover grid
(b) Withdraw the cover panels, bend them upwards and remove the felt segments.
(c) Remove dirt with a cloth, a soft brush or compressed air.
(d) Have heavily soiled felt segments replaced.
Damaged tension bands must be replaced with new ones.


Cleaning the compressor during operation.

The proposed cleaning method, carried out periodically, will prevent a thick layer of dirt from forming.
A thick layer of dirt can cause a drop in efficiency and increased unbalance on the compressor side of the turbocharger, which could influence the lifetime of the bearings. The layer of soot on the compressor contains sulfur, which has a corrosive effect on the aluminium alloy and can lead to a considerable reduction in the fatigue resistance of the inducer and compressor wheels.
Compressor pollution depends on the condition of the incoming air. 
The cleaning interval will depend on the environmental condition and on the installed air filter.  
The filters themselves are not capable of removing fine particles of soot or oil vapour, making it very important to seal leaking exhaust pipes and prevent oil losses.
The water injection method is based on the mechanical effect of impinging droplets of water. 
Since the liquid does not act as a solvent there is no need to add chemicals (i.e. solvents) for cleaning during operation. 
The water has to be injected by spraying water into the air inlet casing with the turbocharger running at the highest possible speed. 
Water is injected before the compressor wheel via an injection pipe fitted in the filter silencer or the suction branch in order to clean the compressor stage in operation. 
If solvents were to be used, the speed would have to be lower and the solvent injected for a longer time to have any effect.
Blower side water washing procedure:
1. It can be done when M/E is on full load.
2. Fill up the warm freshwater to the hopper and closed the cover.
3. Open the valve and water will flow into the blower casing and mechanically attack the blower blades and clean the deposits.
4. Close the valve. Open the cover and check the cleaning water must be empty.

2.1 Wet cleaning of the compressor:-Only freshwater to be used without any cooling additives or solvents, which could lead to deposition in the flow ducts. In general, cleaning should be carried out every 24 to 72 operating hours. Cleaning of the compressor stage must be performed with the engine warm from running and as fully loaded as possible (i.e at turbocharger speed).
The success of cleaning can be seen from the scavenging pressure, if required cleaning can be repeated after a stabilisation period of at least 10 minutes. Engine must be run loaded for at least another 15 minutes after compressor cleaning.
The water should be injected during the engine is running warm at the highest possible load (above 75% load). For effective cleaning, inject all the water required within a minute.
The specified water quantity should be observed because entering uncontrolled water into the turbocharger and engine causes trouble. Either charged air pressure or exhaust gas temperature changes after injecting water. If pressure or temperature does not change, it may only be repeated after 10 minutes. If the parameters are not changing upon 3 times injection it can be concluded that the deposit is too hard or the compressor wheel is damaged.
Note:
1. The best results are obtained by injecting water during full-load operation of the engine, i.e. when the turbocharger is running at full speed.
2. The complete contents of the water vessel should be injected within 4 to 10 seconds.
3. Successful cleaning is indicated by a change in the charge air or scavenging pressure, and in most cases by a drop in the exhaust gas temperature.
4. If cleaning has not produced the desired results, it can be repeated after 10 minutes.
5. The interval between compressor cleanings will depend on the condition of the turbocharger suction air. It can vary from 1 to 3 days of operation.
If a very thick layer has built up and it cannot be removed using the method described, it will be necessary to dismantle the turbocharger in order to clean the compressor side.
Principle:
Since the dirt layer is removed by the kinetic energy of the water droplets, the engine has to be run at full load. 
Operation instruction for Compressor cleaning:
1. Cleaning should be performed at high engine load and warm turbocharger
2. Open the filling cap(2) and fill the water container(1) with clean fresh water.
3. Close the filling cap(2) and open the valve(3) for approx. 3 minutes.
4. Close the valve(3) and check, that all water has been sucked out.
5. Repeat steps 2-4.
6. In dirty operating conditions the cleaning procedure (steps 2-4) can be repeated once more, In total, up to 3 cleaning cycles are allowed.
7. After finishing the compressor cleaning procedure the engine should run at least 5 minutes at high load.
8. Compressor cleaning should be performed approx. every 24 operating hours.
Instruction for U-Tube manometer:
1. The U-tube manometer is used for monitoring the filter ring condition.
2. For best readability lightly coloured water should be filled into the U-tube.
3. Under new conditions the pressure loss is less than 120 mmH2O at full load.
4. The filter should be cleaned at the latest when the pressure loss has reached 200mmH2O.
3. Cleaning turbine blades and nozzle ring in operation.


Wet cleaning of Turbine

When heavy Fuel is used the nozzle vanes and turbine blades become dirty due to combustion residue and, though to a far smaller extent, the additives in the lubricating oil. Apart from a very thin coating of additives, turbochargers operating on engines using diesel oil show no signs of dirt deposits. When engines use heavy oil it is necessary to be able to clean the turbines during operation. Depending on the composition of the heavy fuel used and the quality of the combustion, such cleaning of the turbines will have to be carried out more or less frequently. For the turbine, we recommend wet cleaning (water injection) as well as dry cleaning (granulate).
We continue to recommend wet cleaning for installations where the engine output can be reduced. The boost pressure has to be above 0.3 bar to prevent water from entering the turbine end oil chamber and the exhaust gas temperature before the turbine should not exceed 430 °C.
For further details please refer to our technical Information Sheets or to "Cleaning Even when cleaning is carried out regularly during operation, the rotor still has to be removed and cleaned according to a fixed schedule. From the time it should be professionally rebalanced on a proper balancing machine to be sure that it runs smoothly and that bearing loads are minimized. It is recommended that the compressor and turbine be cleaned with the turbocharger running. Periodic cleaning reduces or even prevents contamination, allowing significantly longer intervals between overhauls.

Cleaning the turbine
The combustion of heavy fuel in diesel engines causes fouling of the turbine blades and nozzle ring. The result of this foulıng is reduced turbine efficiency and engine performance as well as an Increase In the exhaust gas temperature. Experience has shown that the contamination on the turbine side can be reduced by regular cleaning in operation and that such cleaning allows longer intervals between the turbocharger overhauls.
Some of the deposits have their origin in soot, molten ash, scale and unburned oil, partially burnt fuel and sodium vanadyl-vanadate. Investigations have shown that most of the residues are caused by the calcium in the lube oil reacting with the sulphur from the fuel to form calcium sulphate during the combustion process. The quantity of the deposits depends on the quality of the combustion, the fuel used, and the lube oil consumption. The frequency with which cleaning has to be carried out depends on the extent of the contamination on the turbine side.
With periodic cleaning during operation, the interval between overhauling can be extended.

Turbocharger cleaning and washing
Procedure for wet cleaning (2- and 4-stroke):
The boost pressure has to be above 0.3 bar to prevent water from entering the turbine end oil chamber.
The exhaust gas temperature before the turbine should not exceed 430℃.
The drain of the gas outlet has to be opened to drain the non evaporated water.
The quantity of injected water will depend on the exhaust gas temperature, water pressure, size of the turbocharger and number of gas inlets.
The interval between turbine cleanings will depend on the combustion, the fuel used and the fuel oil consumption. It can vary from 1 to 20 days of operation.
Principle: The dirt layer on the turbine components is removed by thermal shock rather than the kinetic energy exerted by the water droplets.
Turbocharger Washing: The Turbocharger turbine and compressor sides must be cleaned regularly as per the maker’s recommendations. The cleaning of the turbine and blower sides is carried out to remove carbon, soot, and other exhaust deposits.
Purpose of turbocharger water washing
a. To ensure efficient functioning of the turbocharger.
b. To protect the compressor and turbine from contamination (Deposits).
Disadvantages of turbocharger water washing the turbine side
a. Engine speed has to be lower.
b. Thermal stress and corrosion usually occurred.
c. Longer cleaning time.
d. Very fine hard deposits and residues cannot be removed easily with water washing.

3.1 Wet cleaning:-
To clean the turbine components during operation, the thermal shock principle is applied in combination with a subsequent flushing phase. To achieve the required thermal shock effect for wet cleaning, the temperature before the turbine must lie between 400 & 450 deg cel., to achieve the optimum thermal shock. During the cleaning process, the layer of dirt on the material surface of the turbine components loosens. Only freshwater without any cleaning agent or solvent to be used.
The volume of water and duration of injection must be observed carefully, low volume causes inadequate cleaning and high volume leads to impermissible thermal stresses.
In the case of multiple turbochargers, the cleaning of all turbochargers at the same time causes a great drop in performance. The engine must be running for further 10 minutes after the cleaning is completed.
The frequency of cleaning is approx. 200 service hours.
The absolute static water pressure before the water connection on the turbine casing must be at least 1.5 bar above the absolute turbine inlet pressure.
Turbine side water washing procedure
1. Turbine side water washing can be made with hot freshwater.
2. Inform the bridge
3. Reduce the M/E rpm to recommended speed and hence turbocharger rpm.
4. Check the water washing injection nozzle if fitted. (Directly aim at the exhaust grips before entering the turbocharger)
5. Open the turbocharger drain valve.
6. Open the water supply about 1 bar to the turbine side.
7. Water washing must be made until the clean water comes out.
8. Close the water supply and remove the nozzle.
9. Exhaust side drain can be closed after all water is drained out and dried.
10. Inform the bridge and increase the M/E rpm gradually to sea speed.
11. The turbine side water washing is usually at departure after manoeuvring time.
12. For usual practice cleaning is done every 500 hrs, running hours depending on the cleanlıness of the turbocharger.
Wet cleaning procedure:-

1. Adjust the engine load until scavenging air pressure is in the range of 0.3 to 0.6 bar (gauge pressure). The exhaust-gas temperature before turbocharger has to be below 430 deg cel. and the auxiliary blower must be in operation.
2. Open the drain hole (valve F, 1-3:close, 1-2:open) of the gas outlet casing and check whether exhaust gas emerges.
3. Close the drain valve B.
4. Open valve C.
5. Open valve D slowly until the pressure gauge E indicates 0.2-0.25 bar.
6. Inject water for 5 minutes while keeping the engine load constant.
7. Close valve D.
8. Open drain valve B for a drain for 5 minutes.
9. Close valves B and C
10. Close the drain hole(valve F, 1-3:open, 1-2:close) of the gas outlet casing.
a. the engine should be operated at least for further 10 minutes to prevent corrosion of the turbocharger parts on the turbine end.
b. depending on the load only a little or no water flows out the drain hole F. water drain is not relevant for the cleaning effect. water injection can be confirmed with reduced t/c speed and lower exhaust gas temperature after t/c during cleaning.
c. if more than one t/c is mounted it is recommended to clean one after the other.


Grit cleaning of the turbine

There is no need to reduce load as thermal shocking is not a case in grit washing, generally, it is carried out at high load. Carbon granules or walnut shells are used for this. The frequency of this cleaning is of 24hrs.
Pressurized air is used to push the grits inside the turbine before the nozzle ring.
Grit Washing or Dry Cleaning of Turbocharger
1. Turbine side cleaning is superseded by a walnut shell, with a grain size of 12 to 34 mesh
2. No speed reduction is required and cleaning can be done at full speed, once every day
3. Compressed air of (3 -5 bar) is used to help the grains strike the deposited Turbine Blades and Nozzles, giving effective cleaning of hard particles
4. Air supply pipe is fitted to solid grain container, and grains are injected into exhaust system by air pressure, at the same point (as in water washing) just after exhaust grids
5. Turbine casing drain kept open during cleaning time (about 2 minutes only).

Advantages of solid (crystal) cleaning or grit washing
a. Not required to reduce engine rpm, thus no effect on scheduled voyage
b. No use of water, so no corrosion and thermal stress.
c. Cleaning time is short.
d. Do not wear a turbine blade.
e. Effectively remove combustion residues and hard particle

Procedure for dry cleaning (2-stroke only):
The exhaust gas temperature before the turbine should not exceed.
The boost pressure has to be above 0.5 bar.
Dry cleaning has to be carried out more often than water cleaning as it is only possible to remove thin layers of deposits. A cleaning interval of 1 to 2 days is recommended.
To ensure effective mechanical cleaning, granulated dry cleaning media are best injected into the turbine at a high turbo-charger speed.
The quantity needed will vary from 0.2 l to 3l, depending on the size of the turbocharger.
Experience has shown that the best results are achieved with crushed nutshell or granulate.

Principle:
The layer of deposits on the turbine components is removed by the kinetic energy of the granulate causing it to act as an abrasive.
Devices for both methods are usually supplied by the engine builder and are manufactured in accordance with our recommendations. Experience has shown a combination of the two to be very effective in some cases.
Periodical dry cleaning is the most effective and economical method of cleaning turbocharger turbines on two-stroke engines. Providing the recommended material (e. g. nut shells), and also original spare parts, are always useful.


Checks on turbocharger during operation

Points must be checked while watchkeeping & taking over:-
(a) Speed of the turbocharger
(b) Exhaust gas inlet and out temperatures
(c) Cooling water inlet and outlet temperatures
(d) Turbocharger lube oil pressure and temperature
(e) Differential air pressure in manometer at the compressor side
(f) Sound of Turbo Charger.
(g) Vibration and Noise
(h) Lube oil level.
(i) Leakages if any (oil or gas from any flange or joint)
Check for Exhaust Leakage: Turbocharger handles extremely high-temperature gases. The inlet is from the engine and the outlet connects exhaust pipes to the funnel. These two points are flange-connected with a distance piece in between. It is important to ensure that there is no exhaust leakage from these joints as it may lead to fire or a smoky atmosphere inside the engine room.
Check leakage of Sump Oil: In turbochargers with separate oil sumps, keep constant checks on the oil level and temperature. In some ships, it has been reported that, due to leakage in the turbine side casing, the oil comes in entering in hot spots on the engine body and with the exhaust gas. Such incidents have led to the fire in the engine room.


Monitoring requirements upon turbocharger failure

Watchkeeping has to be rescheduled; at least one engine officer must remain present at all times in the engine room.
The Main engine control has been switched to engine room control.
Following instructions are to be strictly followed:
a. As the Engine is to be run on reduced RPM which is perhaps at dead slow, Monitor the RPM, it should not fluctuate much.
b. Run the auxiliary blower continuously and keep an eye on amperage, vibration and temperature rise.
c. Monitor exhaust temperatures and inform immediately if any increase in temperature is observed.
d. Keep a good eye on exhaust smoke, smoke may be comparatively blackish, but it should not go dark.
e. Every unit should be firing, keep eye on exhaust temperature on each unit.
f. Keep a close eye on the temperature and pressure of cooling water and lube oil.
g. Cylinder Oil has been adjusted to a reduced supply, should maintain it.
h. Maintain proper fuel supply, temperature and viscosity.
i. Ensure a complete set of engine log parameters to be physically checked and monitored in regular intervals.
j. Any deviation on ME parameters to be informed and responded to immediately..


Turbocharger overhauling

As the turbocharger is very sensitive equipment, it has to be handled carefully. It is very important to know the detailed step-by-step procedure for dismantling and also discuss various safety precautions that are to be considered before and while dismantling.
A turbocharger has a turbine on one side and a compressor on another side. Dismantling should always be started from the compressor side in order to measure the critical clearance which must be maintained between the compressor side end cover mounting face and the compressor side end shaft.
This is a very important clearance that is to be maintained and that has to be set back while reassembling.

Safety while Dismantling the Turbocharger
a. Inform the operating personnel accordingly before starting any maintenance work on the turbocharger.
b. As a precaution, place a receptacle for leaking oil under the turbocharger.
c. Before starting work, secure the rotor against turning.
d. Ensure that absorbent material is available to soak up any spillage oil.
e. Ensure that operation and process materials are drained, collected, and disposed of in a safe manner.
f. Ensure that all spares and tools are available for dismantling and assembling.
g. Dismantled safety devices must be reassembled and subjected to a functional test immediately after the conclusion of maintenance and repair.

Tools Required for Dismantling
a. Open and ring spanner
b. Box spanner
c. Claw spanner
d. Tommy spanner
e. Bearing pushing tool
f. Bearing pulling tool
g. Pump disc lock plate
h. pump removing toolset
i. Impeller removing tool set
j. Shaft pushing tool.
k. Clearance measuring instruments.
l. Screwdriver

Before dismantling, exhaust gas from the turbine should be bypassed and a blanking plate should be fitted in the turbine inlet casing.
Drain the lube oil from the built-in sump.
Remove the turbine side cooling water connection and drain all water


Compressor Side Removal:
Dismantling should always be started from the compressor side.
  1. First remove the filter silencer assembly or compressor inlet casing from position.
  2. Remove the compressor end cover and drain plug on the compressor side.
  3. Remove the suction cover and measure the critical clearance .lt is the distance betvwcen the compressor end cover mounting face and shaft end. Mark it as K.
  4. Pull the rotor shaft towards the compressor side until the impeller comes in contact with the insert and determine K2. Impoller clcarance L= K- K2
  5. Thrust the rotor shaft towards the turbine side until the turbinc disc and nozzle ring comes in contact with each other and measure K1. Disc clearance M = K1 - K
  6. The above measured clearance is very important as this will determine the proper functioning of the labyrinth seal between the impeler and exhaust shield and also the alignment of the shaft.
  7. Remove the lube oil pump assembly after remeving the pump locking plate.
  8. Remove the bearing nut and bearing nut wacher.
  9. Fix the bearing pulling tool in position and slowly tighten it. This will pull the ball bearing assembly out. Care should be takan while removing bearing to avoid any damage to the bearing and rotor shaft end threads.
  10. Mark the position of the bearing in position to put it back as it is while assembling.
  11. The ball bearing assembly should not ba disturbed in any casa. If it is damaged, tha whola assembly should ba replacad with the manufacturer's new part.
  12. Now ramove the compressor outlat casing with diffuser.
  13. Remove the impaller nut and impoller washer
  14. Remove the impaller and inducer from position.

Turbine Side Removal
  1. Remove the turbine end cover with sight glass on the turbine side.
  2. Measure the clearance between the turbine end cover mounting face and shaft end.
  3. Check the axial deflection of the pump disc cover. The permissible axial deflection of the pump cover is 0.05 mm.
  4. Check the rotor shaft by turning by hand.
  5. Remove the pump disc lockıng plate.
  6. Loosen the lube oil disc cover and pump washer on the lube oil pump disc by removing the bolt.
  7. Remove the outer shaft end nut and tab washer and then remove the inner shaft end nut.
  8. Remove the lube oil disc from the position.
  9. loosen the bearing nut and bearing nut washer and remove from place.
  10. Fix the bearing pulling tool on a resilient mounting and slowly tighten it, and this will pull the roller bearing on the turbine side slowly out.
  11. Care should be taken while removing the bearing to avoid damage to the shaft outer end threads and bearing.
  12. Do not disturb the bearing assembly as improper bearing position may misalign the rotor shaft.
  13. Before removing, put a punch mark on the bearing in position so that It can be put back as It is.
  14. Remove the turbine Inlet casing from the turbine outlet casing.
  15. Now the whole rotor shaft can be pulled out from the compressor side. While puling out the shaft, care must be taken to avoid damage to the turbine blades and labyrinth sealing arrangements on the shaft.
  16. Remove tab washer and remove seal plate to the turbine outlet casing.
  17. Remove shroud ring and shaft seal from the turbine outlet casing.
  18. Remove nozzle ring assembly from the turbine inlet casing.
  19. Finally remove the air seal adjusting screw, anti-corrosion zinc assembly. sand cover, and other various accessories in position.


General Procedure for Turbocharger Overhaul
  1. Lock-off the engine staring mechanism.
  2. Remove the turbocharger air filter.
  3. Drain off the oil from both drain plugs.
  4. Remove the bearing covers from both sides.
  5. Remove the locking wires.
  6. Unscrew the hexagon screws and remove oil suction pipes.
  7. Tighten again the hexagon screws of the bearing boxes.
  8. Check the deflection of the divergent nozzle by using a pick tester and magnet stand.
  9. Remove the divergent nozzle with a screwdriver.
  10. Measure the K value at the blower side by using depth micrometre or caliper and straight edge.
  11. Lock the rotor with a special tool.
  12. Extract the lubricating disc.
  13. Extract both bearings by bearing extractor.
  14. The various parts should be warped in waxed paper to protect them against dirt and moisture.
Checks on Turbocharger while Overhauling
  1. Check the deflection of the divergent nozzle.
  2. Measure the K value on the blower side.
  3. Change the bearing on both sides with the new one (because bearing service life is same as turbocharger overhauling time).
  4. Clean blower and turbine side with chemical and inspect carefully.
  5. Check the labyrinth seal.
  6. Clear the labyrinth seal air line
  7. Check the casing for crack & wear
  8. Blade condition
  9. After reassembled, check Static Balance
  10. Check Impeller and Casing clearance
Changing turbocharger bearing
  1. As per Running Hour
  2. As per clearance
  3. When damaged
  4. When vibration is heavy

To check the deflection of the divergent nozzle
  1. By using Pick tester &
  2. Magnet stand


Components to check and written down in T/C overhaul
  1. The presence, type & distribution of any fouling is noted & recorded, e.g. soot, scale or corrosion products, slight or heavy, even or uneven, which will give an indication of the efficiency of combustion, water washing, or cooling water treatment
  2. Components are cleaned & surface condition noted & recorded in terms of corrosion, fretting or abrasive damage brought about normal or abnormal operating conditions, e.g. corrosion of casing due to condensation of acidic products of combustion, contact between impeller & casing, or turbine disc with nozzle ring ar shroud ring.
  3. Individual components are then examined and any defects noted & recorded, like Nozzle ring assembly, blades bent or cracked due to mechanical or thermal effects.
  4. Turbine blades, damaged, worn or cracked.
  5. Lacing wire in good condition or otherwise.
  6. Impeller vanes worn or cracked.
  7. The shaft running true.
  8. Bearings are worn evenly or otherwise. (note that diagonally opposite wear on both bearings is indicative of rotor imbalance) Labyrinth glands & thrust bearing are in good condition.
  9. Important dimensions are measured & checked against makers data for unacceptable deviations from allowable tolerances. Dimensions & any replacements are recorded. It should be noted that it is essential to attend to apparently insignificant discrepancies in order to avoid major damage at a later date. Finally, a comprehensive report is prepared for engine records, the shipping company & the classification society.
Turbine blades repair:
  •  These can be damaged by hard particle erosion or foreign object contact. This can reduce turbine surface area and increase tip clearances. The blades can be removed and repaired by welding, which is much cheaper than the cost of new blades.

Shaft repairs:
  •  Occurs mainly in the bearing journals, sealing areas, and shaft ends. High-temperature metal spraying is used to build up affected areas, then machined to size. Lube oil pump drive spigots also suffer wear, and these are replaced complete and machined to size.

Rotor balancing: 
  • Carried out after any work on blades, shaft, etc. The rotor is cleaned, glass bead blasted, then all shaft run-outs and deflections measured. once these measurements are correct the rotor is balanced, with corrective grinding if necessary.

Bearing replacement:
  • As wear on the bearings can not be measured. AB8 recommend replacement after 8000-16000 huuis. Bearing exchange will be a cheaper option, where reconditioned units are provided.

Lubricating oil pump:
  • Those units fitted to the larger turbochargers should be renewed after 16000 hours. Again reconditioned units are a cheaper option. NB These units must undergo testing on dedicated test rigs, which should ensure that the service life is reached without failure.

The procedure for replacing turbocharger bearings:
  1. First, ensure the engine is isolated, i.e. starting system shut down and do not start signs placed. Starting on the compressor end. Remove the drain plug from the bearing housing and drain the oil. Then remove the pump assembly.
  2. Then using the bearing extractor tool, the bearing assembly is removed while the shaft is supported. All dismantled parts should be wrapped in waxed paper and kept clean.
  3. The bearing chamber should be thoroughly cleaned before inserting the new bearing, the bearing assembly is then replaced on the shaft, the pump is now placed. The turbine end bearing is now changed in the exact same way.
  4. Before the bearing housings are reclosed and filled with oil, the "K" value must be measured and checked that it is the same as the "K" value that is stamped on the plate of the turbocharger.
  5. Note: This value should also be checked before dismantling the turbocharger for a bearing change.
  6. True running of the turbocharger shaft should also be checked with a dial indicator for trueness after bearing renewal.
  7. The turbocharger bearing housings end covers are now replaced and refilled with fresh oil.

Check Clearances: 
  • When the turbocharger is opened up for overhauling, all-important clearances like casing and blade tip clearances, the "K" value of the shaft, which determine the correct alignment of the shaft, and proper operation of the labyrinth seal which is fitted between the impeller and exhaust shield must be taken.

    The operational service life is the full period of operation, given in hours, specified for a bearing or pump. After this period, the bearing or pump has to be checked, reconditioned, reset and tested before it can be put back into service for another full period of operation. 
The service lives of bearings and pumps depend on the bearing type and the type of installation.  Gear oil pumps, for example, have a set operational service life of 16,000 h for all types, specifications and sizes.
    In the case of roller contact bearings, the operational service life depends on the type and specification of the bearing, on the temperature and lube oil quality, and also on the type of operation and installation. It usually lies between 8,000 h and a maximum of 16,000 h, after which the bearings have to be reconditioned.
    Reconditioning means that the races or plain bearing body will be entirely renewed and the remaining parts, such as the casings, flanges and bushes, can be thoroughly cleaned and reworked when necessary. 
    All the parts are then carefully measured and checked on the basis of the given specifications, dimensions and procedures. In addition to carrying out a very detailed inspection of the relevant parts, it is essential for a reliable operation that the axial clearance "S" and the axial position "A"  be set precisely.
    Turbocharger condition is dependent on operating conditions rather than just running hours. When rotor clearances are out of tolerance, the rotor will not be able to rotate and there will be a risk of breakdown and serious damage. Exact measurement of the clearances is necessary in order to determine that the rotor is in its working position.
    In order to minimise wear and to ensure optimum lubrication of the bearings, the centrifuge and nipple should be filed in such a way that the given tolerances are not exceeded. For the right tolerances. If tolerances exceed, dismantle, clean all axial contact surfaces, turn centrifuge and /or nipple by 180°, reinstall and check again. 


The K, L &M values in turbochargers

It is a distance between the rotor shaft end and the flange of the bearing cover measured by the blower side.
The purpose of the K value in turbochargers is to ensure that the rotating impeller does not touch the stationary blower casing cover in case of thrust bearing is worn out.
Measurements are taken during auxiliary engine turbocharger overhaul
  • K value, it is a distance between the rotor shaft end and the flange of bearing cover measure at the blower side (axial clearance).
  • Check radial clearance (at plane bearing), by placing clock gauge on the shaft from the top and clamp by screwdriver from the bottom, record the clearance.
  • Rotor and Casing clearance (for new casing or new rotor) (L & M values)

To measure turbocharger axial Clearance: Push the shaft by screw jack and measure by Depth Gauge (0.2 - 0.3 mm) and radial clearance
To measure turbocharger Radial Clearance: Lift the shaft radially and measure by Dial Gauge (0.15 - .02 mm).

Developments in turbocharger

  1. Higher pressure ratio:- ABB turbochargers with aluminium compressor provide a ratio of 4.6; stainless steel or titanium is used for a pressure ratio of 5.2.
  2. Plane bearing with a lube oil supply from the main engine or from a dedicated external system. They are inboard-mounted plain bearings and are used along with axial flow turbines. These bearings work at high temperatures. These turbochargers are un-cooled types and can be dismantled completely from the compressor side due to their modular size.
  3. Un-cooled casing:- lube oil is splashed around the generously sized bearing space to cool the area adjacent to the bearings.
  4. Variable geometry Nozzle ring:- Adjustable blade angle of nozzle ring as per engine load. 
  5. Radial flow turbine mounted on the same, shaft; reduced length and nullified thrust.
  6. High chord blades i.e thick section omits the lashing wire.
  7. The number of blades in the volute is matched to the number of blades in the compressor to reduce noise.
  8. Trust bearing which is subjected to high loading is mounted outside the radial bearing on the compressor and for the ease of maintenance.
  9. Hybrid turbocharger:- An alternator mounted on the shaft within the silencer, can take the sea load. Also, eliminate the need for auxiliary blowers as the alternator can act as a motor for the turbocharger when required.
  10. Multi-stage turbocharging:-In the same turbine volute, a variable geometry turbine (VGT) machine is housed, having high a pressure and a low-pressure turbine. Exhaust gas flow is bypassed over from one stage to another stage through VGT  vanes, thus converting pressure energy into kinetic energy. This high-velocity exhaust gas exerts a mechanical rotational force on the lower pressure turbine wheel.
  11. Bearing system:- Designed in order to reduce friction losses within the bearing. It should withstand high thrust loading, oil contamination, oil supply delay, and hot shutdown. Silicon-Nitride (Si3N4) ball bearing is harder than steel which reduces the contact with the bearing track. The Material properties are low friction, high wear and chemical resistance, high-temperature capacity, high electric resistance & 3 to 10 times longer life. )

Pulse vs constant pressure turbocharger systemscccc

A turbocharging increases power of an engine as more air is available to burn the fuel. It enhances the thermodynamic efficiency thus lower fuel consumption for the same power. For the same power, the engine can be made of a more compact design with savings in engine room space and weight of the engine. the cost of engine per horsepower is less.
Constant pressure turbocharging system:
    In the constant pressure, the turbocharging system shown above each cylinder exhaust gas is piped to a common exhaust gas manifold. From the manifold, a single pipe is led to the turbine. One turbocharger is normally sufficient but for a higher number of cylinders, two turbochargers may be provided.
Advantages
  • Later opening of exhaust, more expansive use of combustion gas
  • Smooth entry of gas, steady load
  • High-efficiency
  • Simpler pipe connections, more flexibility in the location of turbocharger
Disadvantages
  • Poor response on starting
  • Poor acceleration
  • Poor performance at low loads
  • Requires auxiliary blower at low loads

Pulse turbocharging system:
In the pulse turbocharging system shown above each cylinder exhaust gas is piped individually to the turbine. In a 2 stroke 6·cylinder engine, normally 2 turbochargers would be fitted. This is in order to avoid higher pressure exhaust gas from a cylinder blowing back and interfering with the scavenging process of another cylinder. Ideally, there should be a gap of at least 120 deg between the consecutive exhaust openings. For a higher number of cylinders, more turbochargers may be necessary.
Pulse turbocharging was common at earlier times when the combustion pressures and power of the diesel engine were low. In modern high power engines with mean effective pressures of 18 bar or more, the constant pressure system is much more efficient and common.
Advantages:
  • Higher energy of gas at turbine entry
  • Quick response on starting
  • Good acceleration
  • Good performance at part load
  • No need for an auxiliary blower. although often provided
Disadvantages
  • Need more number of turbochargers
  • Complex pipe connections
  • Owing to varying pressure, cannot be optimized for best efficiency
  • At higher mean effective pressures less quantity of air delivered


Troubleshooting of turbochargerxxx

1. Foaming of oil in the bearing chambers: Excessive foaming may be an indication of contaminated oil. Two or three oil changes will usually correct the situation.
Foaming is harmless as long as it does not cause loss of oil and the oil level can still be seen through the gauge glass, but if the foam layer is thicker than about 8 - 10 mm and the oil level can no longer be observed through the gauge glass, the engine has to be stopped as soon as possible and an oil change carried out on the turbocharger.

Exhaust gas temperature too high, engine performance and speed unaltered:
  • Faults in the injection system
  • Lack of air due to a dirty filter
  • Compressor or turbine contaminated
  • Exhaust gas backpressure too high that need to check the boiler.
  • Turbine blade damaged or eroded need to replace the rotor.
  • Cover ring eroded need to replace.
  • Dirt in the cooler need to clean
  • Insufficient cooling water top-up
  • Cooling water inlet temperature too high check / clean cooling system
  • Insufficient ventilation need to improve ventilation

 Charge air pressure too low, engine performance and speed unaltered, air intake normal:
  • Leak in the air receiver repair
  • Leak in the gas duct between engine and turbine repair
  • Injection incorrectly adjusted adjust correctly
  • Manometer indication defective replace manometer
  • Leaks in the line to the manometer repair leak
  • Dirt in the air filter causing, excessive pressure loss clean
  • Dirt in the compressor/turbine
  • Labyrinth seal damaged consult service station for replacement
  • Turbine/compressor blades damaged replace the rotor
  • Nozzle ring damaged replace
  • Cover ring eroded replace
  • Exhaust gas backpressure too high clean boiler or exhaust gas silencer
Charge air pressure too high, engine performance and speed unaltered:
  • Faults in the injection system adjust correctly
  • Engine performance higher than assumed check engine performance
  • Injection system incorrectly adjusted adjust correctly
  • Manometer indication incorrect replace manometer
  • Nozzle ring dirty or partly obstructed clean
Vibration:
  • Rotor unbalance due to heavy contamination of compressor/turbine remove and clean
  • Turbine blades or damping wire damaged replace the rotor.
  • Bearing defect replace bearing, seek the possible cause
Noise on running down, time too short, reluctant starting:
  • Bearing damaged replace the bearing
  • Rotor rubbing call service station
  • Dirt in turbocharger clean
  • Foreign bodies in the turbocharger call service station, replace damaged parts
Leaks in casing:
  • Cracks due to thermal tension replace the casing
  • Insufficient ventilation provide better ventilation
  • Insufficient oo ling water check and top-up
  • Dirt in cooling water space clean

Loss of lubrication oil:
  • Sealing bushes damaged replace sealing bushes
  • Compensation ducts X and Z obstructed clean
  • The gasket of bearing space cover leaking replace gasket 
Constant surging of the turbocharger:
  • Increased flow resistance due to:
  • Dirt in the charge air cooler or silencer clean
  • Heavy deposits of dirt in the compressor / turbine clean
  • Defective check valves in two-stroke engines replace valves
  • Exhaust gas pressure increased after the turbine because the boiler or the exhaust gas silencer are dirty clean
  • Grid dirty clean



























Questions & Answers


Q. How are the turbine blades of an exhaust turbo-blower attached to the rotor disc? Are the blade fastenings a loose or tight fit in the rotor? 
Ans: 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-blowers used for constant-speed diesel engines, such as diesel generators, the blade root can be a tight fit in its slot. In large turbo-blowers with long rotor blades, particularly those used for propulsion engines where a wide range of operating speeds is met in service, the fit of the blade 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.  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 from sliding out. 

Q. Why are turbine blade roots sometimes made to be a sliding fit in the rotor? What is the purpose of the binding wire? 
Ans: The sliding fit of the blade root in large turbo-blowers gives a large amount of damping to the blade to reduce the risk of blade vibration. The binding wire is made in short lengths, each of which extends through four to six blades. All the blades are connected in this way. The binding wire passes through holes near the tips of the blades. It is not a tight fit in the holes. Each segment of binding wire is fastened by welding it to the first blade of the group in which it is fitted. When the rotor is moving at high speed, the flexibility of the binding wire is such that the action of centrifugal force causes it to bear against the outside of the holes through which it passes. If the turbine rotates at a speed at which the blades would vibrate, the friction between the binding wire and the sides of the holes in the blade tips damps out the vibration and prevents ultimate failure, Some turbo-blowers used in engines running at a constant speed, such as diesel generators or alternators, do not have binding wires. 

Q. What is the reason for using a fir-tree form of blade root for turbine blades? 
Ans: The fir-tree root form is superior to other blade fastenings because there is less stress concentration at the junction between the root and the blade. Because there is a more even distribution of stress at the blade root there is less chance of blade failure in this region. 
 
Q. Describe how you would determine whether exhaust turbo-blower air filters were clean or dirty? 
Ans: Small engine filters of the renewable type must be visually examined. In larger blowers with removable filters, a vacuum gauge or U-tube gauges are fitted on the suction side of the blower, so that the air pressure in the inlet space between the filter and blower can be measured. As the air filter becomes dirty, its resistance to airflow increases and the pressure drop across the filter is increased. This is indicated by a lowering of the air pressure downstream of the filter. A comparison of this pressure with the figures recorded at the engine testbed trial will indicate whether the filters are dirty. Sometimes specific pressures at which filters must be cleaned are given in the engine instruction book. 
 
Q. What are the materials used for the filters on the air inlet side of an ‘exhaust turbo-blower? 
Ans: The materials used for air filters on small engines may be special porous paper elements that are renewed when they become dirty. These elements are usually wound in a corrugated form so that a large area of the filter is obtained in a small space. This type is found on smaller engines, as used, for example, for emergency electrical generators. For larger engines, air filters on the blowers have removable elements. The filtering medium within the element may be plastic fibre or a non-rusting metallic ‘wool’ material. Filters made with these ‘materials can be cleaned when they become dirty. 
 
Q. Describe the cleaning of turbo-blower air filters. 
Ans: Most engine builders supply a cleaning pan of the proper size to accommodate the air filters element. The filters are placed in the pan and soaked in alkaline solutions or special solvents. During the soaking period, the dirt adhering to the filtering medium detaches itself and sinks to the bottom of the pan. After the filters are removed from the pan, they should be allowed to drain. Some filtering media may be back-washed with fresh water. Care must be taken to use the cleaning material recommended by the engine builder: Some solvents dissolve the plastic fibre material of which some filters are made. Care must be exercised in the use of many cleaning materials, to prevent skin infections and possible damage to the eyes. This is particularly important when using compressed air for the final removal of cleaning material or rinsing water. 
 
Q. What effects will dirty air inlet filters in turbo-blowers have on the operation of the engine? Ans:  Dirty air filters reduce the amount of air passing into the engine. As the quantity of air is reduced, so the exhaust gas temperature is increased. The degree to which this occurs depends on the restricting effect of the dirt on the passage of air through the filter. 
 
Q. Why are large amounts of lubricant supplied to turbo-blower bearings?
Ans: As the rotor revolves at high speeds large amounts of heat are generated in the bearings due to friction. The friction may be caused by fluid friction as experienced in sleeve bearings or rolling friction as experienced in the ball and roller bearings. The lubricant supplied serves both as a coolant and a lubricant. Generally larger amounts of lubricant are required in sleeve bearings. 

Q. What indications will be given of a faulty lubricating oil supply to the turbo-blower bearings? 
Ans: The lubricant supplied to turbo-charger bearings is supplied under pressure. The pressure of the oil supply is monitored and any fall in pressure gives rise to an alarm condition and call out. Turbo-chargers with sleeve bearings and external oil supply may also be fitted with thermometers on the oil-drain outlets from the bearings together with temperature sensors giving a bearing temperature reading at the control station, Oil flow indicators may also be fitted on the inlet oil line to the bearings. In turbo-chargers fitted with ball and roller bearings and internal oil supply from reservoirs in the end covers, a sight glass is fitted in the reservoir to indicate fluid height. Visual examination and feel of the bearing casings or oil reservoirs also give an indication of satisfactory operation or possible trouble. 
 
Q What do you understand by the term ‘two-stage turbocharging? 
Ans: When air is compressed its temperature rises and the compression may approach adiabatic conditions. 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. Compression then approaches isothermal conditions. Two-stage turbochargers 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 directly from the outlet of the first stage into the inlet of the second stage. This reduces the 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 turbocharging allows higher boost pressure ratios and imparts much higher overall efficiency to the turbo-charger unit. 
 
Q. What causes fouling of the air-side of air coolers? 
Ans: Fouling of the air passages in an air cooler forming part of a turbo-charging system is usually due to oil and oily-water films collecting on the sides of the tubes and tube fins. Lint and similar material adhere to these films of oil or emulsion. The presence of oil may be caused by faulty air filters, or by improperly placed filters that allow the air to pass by the side of the filter element. Sometimes the oil is drawn from the bearing at the blower end of the turbo-blower. The presence of moisture is usually the result of high humidity when the engine is operating in warm air temperatures in conjunction with low sea-water temperatures. 
 
Q. What are the usual causes of oil being found in the air passages between the turbo-blower and the air cooler? 
Ans: If large amounts of oil are found in the air system, the usual cause is faulty oil Dea the compressor labyrinth packing has become choked or shut off, in which case oil or vapour will be drawn through the labyrinth packing. ‘The air breathers on the ends of the turbo-blower end covers must be kept ‘be used to check the air flow into the labyrinth packing, and the leak-off. Ifa Oil vapour will then be drawn into the blower and eventually pass and condense in the blower discharge lines. ‘The remedy is to clear the air supply to the blower labyrinth packing. If the air supply is normal, it is usually indicated by a small amount of oil vapour wafting from the air breather. ‘When turbo-blower casings are painted, care must be taken to prevent the air breather becoming sealed with paint. Q. After a period of service the gos inlet nozzles and blading of the turbine in exhaust turbo-blowers become dirty. What causes the dirt, and how would you know that the nozzles and bloding were dirty? ‘When combustiorris clean, the deposits found within the nozzles and blading of the exhaust turbine are usually sodium compounds, in the form of sulphates, and vanadium compounds, which may be oxides. Some additives used in the cylinder lubricant may also be found in the ash If combustion is dirty, sooty deposits and carbonaceous materials may be found, These deposits may be due to impurities in the fuel, or to poor combustion. Sometimes poor combustion is caused by operation of the engine at low loads for lengthy periods. ‘When nozzle and rotor blades become dirty the engine symptoms are rising cexhaust-gas temperature at the turbine inlet, accompanied by a fall in the speed (rev/min) of the turbine. The exhaust-gas temperature rises because more kinetic energy is given up by the gas before it enters the nozzle blades. This kkinetic energy is converted into heat and consequently increases the temperature. Dirt in the nozzles and blades changes the velocity pattern of the gases passing through them; this prevents the turbine working efficiently, and the speed of rotation falls. When the turbine operates at a lower speed the air delivery is reduced and this in turn causes the cylinder exhaust temperature to delivery is reduc wes the cy Q. Describe how turbine rotor blading should be cleaned. Turbine rotor blading can be cleaned by water-washing while the turbo-blower is in service, or the turbo-blower can be dismantled to clean all parts thoroughly. Water-washing of the blades while the turbo-blower is in service requires special apparatus and connections on the turbo-blower exhaust-gas inlet spaces. This apparatus consists of a probe which passes into the gas space. The probe is in effect a water sprayer which fits into the gas inlet space. The outer end of the probe is fitted with connections for compressed air and water, which are taken through flexible pipes from the supply points at the side of the blower. An automatic cock is fitted into the probe. When opened, it allows ~ water and compressed air to flow to the sprayer. The compressed air atomizes the water as itis blown into the gas space. The usual cleaning time is 10 to 15 minutes, depending on the amount of dirt. The ash found in the nozzle blading is usually water-soluble. The finely atomized water breaks down the ash formations, which then pass up the exhaust pipes with the exhaust gases to the atmosphere. ‘While the blades are being washed the turbine blower speed must be consider- ably reduced, and the water drain cocks from the exhaust spaces must be left open. After washing, the speed of the engine and the turbo-blower must be increased gradually, and the water drain cocks left open until all the water is ‘Some turbo-chargers are fitted with an arrangement to inject small particles of broken wainut shells into the exhaust-gas passages before the nozzle blading. The sharp edges of the broken shells have a good scouring action on the nozzles and blading without damaging the smooth surfaces required for the high- velocity gases to operate the turbine in an efficient manner, with only minimum losses from blade and nozzle friction. When a turbo-blower is dismantled to clean the turbine rotor blades, the rotor is set up ona pair of wooden V-notched trestles that allow part of the rotor disc and blading to soak in a water bath. During the soaking period the rotor must be regularly and frequently turned, so that the deposits are completely removed. Incomplete removal may leave the rotor out of balance. Q. What do you understand by the term ‘surge’, when applied to the centrifugal-type blower used for pressure-charging diesel engines? If the curve of the quantity of air discharged by a turbo-charger plotted against the pressure ratios is examined, it will be seen that a line is drawn from the intersection of the axes. From the right-hand side of this line curves are shown giving the quantity of air discharged against the pressure ratio. No points are plotted to the left of the line. The points plotted on the right-hand side are in the region of stable operation. Instability occurs if the turbo-charger operates on the left-hand side of the line. The line separating the two areas is referred to as the surge line. ‘The surge line passes through the points where the pressure ratio is very near a maximum value; as the amount of air discharged increases the pressure ratio is seen to fall away. Under stable operating conditions, any change in the amount of air taken into the turbo-charger is accommodated by a change in pressure; one change is balanced by another. ‘When the turbo-charger operates under unstable conditions any reduction of air demand on .the discharge side of the compressor causes the pressure to fall. If this occurs the pressure falls rapidly and air flows back from the scavenge air trunk to the diffuser. The increase in pressure then causes an increase in the air flow. As the engine cannot use this air the pressure starts to fall again and the action is repeated. This condition will continue until the air demand is increased and the turbo-charger is allowed to operate under stable conditions again. To prevent these conditions arising the scavenge trunk capacity should be large enough to reduce pressure fluctuations, and the capacity of the turbo- charger must be carefully matched to the engine. It will then operate in the stable range under normal conditions. Q. What are the symptoms when a turbo-blower surges, and how would you rectify the condition? If you had a turbo-blower surge how would you prevent the trouble repeating itself? The symptoms of a surge condition are a repeated, irregular violent thud from. the air intake to the blower, and rapid surges in the scavenge-air pressure. If you were standing 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 combina- tion of two factors, The first is dirty air-intake 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 ratio 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 engines 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. Q. Whot ore the main advantages of the impulse ond the constont- pressure exhaust turbo-charging systems? ‘The main advantage of the impulse system is that at low loads itis more efficient, than the constant-pressure system. It does not require assistance from scavenge pumps, except at very low-power operation. The advantage of the constant-pressure system is that the exhaust-pipe arrangement is more simple. At high engine loads, the constant-pressure system ‘becomes more efficient than the impulse system. At low loads, scavenge pumps, under-piston pumps, or electrically driven blowers are necessary to supply the required air. Q. What provision is made for running turbo-charged propulsion ‘engines at very low power? Whena propulsion engine is running at very low power, the air output from the turbo-blower is insufficient for the requirements of the engine. The air supply can be augmented by the following methods. 1 Some engines are fitted with an electrically driven blower that will give enough air for slow-speed operation, or for emergencies when the turbo- blower is out of service. 2 Other engines retain scavenge pumps, which are connected in series with the turbo-blower, and at slow speeds these scavenge pumps will supply sufficient air: 3 In crosshead, loop-scavenged two-stroke cycle engines the space between the underside of the piston and the diaphragm can be fitted with suction and delivery valves so that it acts as a reciprocating scavenge pump at low engine speeds. At higher engine speeds the pumping action reduces the mechanical efficiency of the engine, but it is possible to open a butterfly valve and, in effect, increase the volume of the clearance space. This reduces the pumping load, increases the mechanical efficiency of the engine and so lowers its fuel consumption. Q. How is the efficiency of a turbo-charger obtained? ‘The true efficiency of a turbo-charger is difficult to obtain. It is not easily adaptable to instrumentation, consisting as it does of a gas turbine and an air compressor combined into a single unit and driven by the power obtained from the exhaust gases of a diesel engine to which it is attached. ‘Turbo-charger manufacturers have their own test stands and testing facilities used both for research and testing for efficiency; due to commercial considera- tions regarding secrecy they do not usually publish details of their work. ‘A basic statement for efficiency is given by Efficiency = Energy in - Energy out)/Energy in but this statement gives only the thermal efficiency; it will have to be multiplied by the mechanical efficiency to obtain the overall efficiency. ‘The energy input must cover both the kinetic energy and the heat energy contained in the exhaust gas. The energy utilized covers the difference between the energy input from the engine exhaust and the energy contained in the ‘compressed air discharged from the compressor. There is no problem in dealing with the heat content in the exhaust gas and the compressed air because these values can be obtained from standard tables and charts, but a problem arises in deciding the datum point for the other forms of energy and whether stagnation enthalpy considerations should be brought in. From this it is seen that an academic approach to finding the true efficiency of a turbo-charger by thermodynamic analysis raises many questions. For simplification the efficiency of the gas turbine and the air compressor are considered separately. ‘The efficiency of the gas turbine is obtained from the values of the tempera- ture and pressure of the exhaust gases entering and leaving the turbine. ‘These values can be set up on a heat-entropy chart for exhaust gases (also known as an enthalpy-entropy chart or a mollier chart). From this chart the adiabatic or isentropic heat drop is obtained; dividing this by the heat input gives the thermal efficiency of the gas turbine. The efficiency of the air ‘compressor can be found in a similar manner by setting up the compressor pressure and temperature values on a heat-entropy chart for ai The efficiency of the turbo-charger can then be found by taking the product of the turbine and compressor efficiencies, then multiplying the value found by the mechanical efficiency. This efficiency is not the true efficiency of the turbo- charger but is accurate enough for comparative purposes. ‘An expression giving the isentropic efficiency of a turbocharger can be developed from basic thermodynamic statements as shown. The turbocharger efficiency is equal to the product of the turbine efficiency, the compressor efficiency, and the mechanical efficiency of the complete unit. Turbocharger _ Turbine Compressor Mechanical efficiency efficiency * _eff Tre = Bev Mee X Maden ncy ” efficiency The turbine efficiency and the ¢ompressor efficiency are now treated separately. Turbine efficiency nx. The efficiency of the turbine may be obtained from a study of the heat given up by the exhaust gases on their passage through the turbine. The gases passing through the nozzles and blading are subjected to friction, this causes a temperature rise in the gases above the temperature rise expected if expansion had been isentropic (adiabatic). This is known as reheating and causes an increase in the entropy of the gases, mass flow x sp. heat x actual fall in temperature of gases ‘70 mass flow x sp. heat x isentropic fall in temperature of gases If the specific heat and mass flow are constant, they may be cancelled. Then: actual temperature drop in exhaust gases ‘+ jsentropic temperature drop in exhaust gases where: T, = temperature of gases entering turbine at pressure p, T, = actual temperature of gases leaving turbine at pressure p, T, = temperature of exhaust gases at pressure p, based on fall of temperature ‘when expansion is isentropic. ‘The following equation may be used to find the value of T; : ‘is the ratio of the specific heats C,/C, for exhaust gas. Compressor efficiency nc, The efficiency of the compressor may be found ina similar manner, then: (Te=Td lon where: re GT) ee T, = Temperature of air entering compressor at pressure p, T, = Actual temperature of air leaving compressor at pressure p, T,_ = Temperature of air at pressure py based on isentropic compression ‘The following equation may be used to find the value of Ty: ( (ps ) x Pu ‘y.is the ratio of the specific heats C,/C, for air. By substituting the expressions for the turbine and compressor efficiencies into the general equation given above an expression for the turbocharger efficiency may be obtained. Then: (T-T) (1-1) nach on TaD) Td (() . =] G=T) 7) Tne All pressures and temperatures must be absolute values. This expression may be further refined by substitution and simplified. The efficiency of the turbo-charger can then be found by taking the product of the turbine and compressor efficiencies, then multiplying the value found by the mechanical efficiency. This efficiency is not the true efficiency of the turbo- charger but is accurate enough for comparative purposes. In studying technical literature covering efficiencies of various turbo-chargers, the method of finding the efficiency should be the same in each case otherwise a fair comparison will not be obtained. Note The thermodynamic analysis of turbo-chargers is beyond the scope of this book. This matter is covered in books on advanced thermodynamics and specialist papers from technical institutions. Q. How does the timing of the air-inlet and exhaust valves differ between naturally aspirated ond pressure-charged four-stroke engines? What is the reason for the differences? In giving valve opening and closing times, it must be remembered that considerable differences occur in different makes of engine. These differences Gepend to some extent on the speed of the engine, and also on the degree of supercharging. The following table gives the valve timings for four-stroke, medium-speed. engines, Naturally aspirated Pressure-charged Inlet valve opens up to 20° bat. up to 80° b.t.d.c. Inlet valve closes up to 25° a.b.d.c. up to 45° a.b.d.c ‘Open period 20° + 180° +25° = 225° 80° + 180° + 45° = 305° Exhaust valve opens up to 45° b.b.d.c. up to $0° b.b.d.c. Exhaust valve closes up to 20° a.td. up to 60° a.t.d.c. Open period 45° + 180° +20" = 245° 50" + 180° +.60° = 290° It will be noticed that the period for which each valve is open is much longer in the pressure-charged engine. ‘When examining the valve timing for each engine, it will be noticed that the inlet valve is opened before the exhaust valve is closed. The period when both valves are open is referred to as the overlap period.”In the naturally aspirated engine, the overlap is (20° b.t.d.c. + 20° a.t.d.c.) = 40°; in the pressure-charged engine, it is (80° b.t.d.c. +60° a.t.d.c.)= 140°. ‘The large overlap period in the pressure-charged engine allows the exhaust {gases to be expelled from the cylinder, except for a small amount of exhaust gas that is mixed with the incoming air. During this period a large amount of air passes through the inlet valve, the cylinder, and the exhaust valve, cooling the parts in the process. This cooling helps to keep the surface temperatures of these parts at a lower value, which in turn reduces thermal stresses. ‘After the exhaust valve closes, less heat is passed into the air that follows into the cylinder, and a greater mass of air is therefore present when compression begins. ‘The overlap period, when both the exhaust valve and air-inlet valve are open together, is sometimes referred to as the scavenge period in pressure-charged four-stroke engines. Q. What pressure-charging system is used on modern slow-speed two- stroke uniflow-scavenged engines, fitted with exhaust valves? Give the crank angles at which the exhaust valves open and close, and the angles ‘at which the scovenge ports open. All modern slow-speed, two-stroke cycle engines operate on the constant- pressure turbo-charging system. The number of turbo-chargers used depends on the size of the engine. Because the efficiency of a turbo-charger increases with an increase in its physical size and output, the number of turbo-chargers is, kept to a minimum. Scavenge ports open at approximately 35 degrees before b.d.c. and close 35 degrees after b.d.c.. The exhaust valve will open ahead of the scavenge ports to give a blow down period and close at some time to leave the correct amount of air in the cylinder for the combustion of fuel. The open period for the exhaust valve will be about 80 to 90 degrees of crank rotation and the valve will open about 45 degrees beforé b.d.c.. It should be noted that considerable variation may be found in valve timing figures for various engines. The figures used in any valve-timing checking work should be taken from the engine instruction book. These figures will take into account lag from hydraulic operated valves and valves with hydraulic lifters. Q. What pressure-charging system is used on modern medium-speed, four-stroke V-engines? Give the timing angles for the air-inlet and exhaust valves. This type of engine is operated with the impulse pressure-charging system. Air inlet valves open 50° b.t.d.c. Air inlet valves close 45° a.b.d.c. Exhaust valves open 40° b.b.d.c. Exhaust valves close 60° a.t.d.c. 5.48 What are the provisions for operating Impulse and constant pressure exhaust turbo-charged engines with a disabled turbo-blower? Impulse pressure-charged engines can have the rotor locked, and the blower outlet blanked off with a blind flange, if the period of operation under emergency conditions is not long. The second blower can be left in operation, and the electric blower brought into use. In certain cases, reducing orifice plates must be fitted into the electric or turbo-blower, to balance the discharge pressures and prevent surge. If the engine is to be operated under emergency conditions for a long period, the emergency exhaust pipes must be fitted, so that the exhaust gases bypass the turbine. This prevents damage to the labyrinth packings and the turbine rotor. It will still be necessary to blank the outlet from the blower, to prevent air backflow from the scavenge receiver. When operating on one turbo-blower with the exhaust gases from the other bypassed, itis still necessary to use the electric blower as mentioned previously. In constant-pressure turbo-blown systems, the gas inlet to the disabled turbine and the air discharge from the blower should be blanked off with blind flanges. If the engine is fitted with scavenge pumps it may not be possible to blank off the air outlet from the turbine, as this would shut off the supply air to the scavenge pumps. In this case the rotor must be locked, or the emergency air- inlet branches to the scavenge pumps opened. The engine will be operated on the remaining turbo-blower. The speed of the engine must be restricted so that the allowable turbine speed is not exceeded. Generally, engines with two turbo-blowers can operate at 60% to 80% of full power when one turbo-blower is disabled. ‘When a blower is isolated from the exhaust by the fitting of blind flanges or ‘emergency exhaust pipes, the lubricating oil and cooling-water services can be shut off. Q. What has been the overall effect of increasing turbo-charger . efficiency? The overall effect of increased turbo-charger efficiency has been and will continue to be shown as a decrease in fuel consumption. : ‘This has fitted in well with the new generation of slow-speed, two-stroke cycle Jong-stroke engines. The increased turbo-charger efficiency has reduced the heat input requirement making it possible to open the exhaust valves later and get a greater amount of work from the fuel due to a greater expansion ratio of the combustion gases. As about fifty per cent of the energy in the fuel passes through the turbine of the turbo-charger there is still plenty of heat available for exhaust gas boilers although in the future the heating surfaces of the boiler will have to be increased. to accommodate the lower-temperature gases, Note Schemes are available to utilize the energy in exhaust gases either to generate electrical power through an exhaust gas-driven turbo-alternator or generator, or to connect an exhaust gas turbine to the propeller shaft through a clutch and reduction gearing. ‘As a point of historic technical interest, similar schemes were used very effec- tively with reciprocating marine steam engines from the early 1900s until well into the post-war years, to utilize some of the waste heat in the exhaust steam before passing into the condenser. ‘One scheme connected an exhaust steam turbine on to acentre-screw shaft in twin reciprocating engine installations, making the ship a triple-screw ship. Another scheme connected the exhaust turbine to the screw shaft through a hydraulic clutch and double reduction gearing. A third scheme used the exhaust, steam to drive a rotary compressor used to compress the steam between expan- sion stages. A fourth scheme used the exhaust steam to drive an electrical generator. The current produced was used in a set of resistance elements to reheat the steam between expansion stages. Q. What precautions must be taken to minimize thermal stresses in modern, highly rated, turbo-charged engines? The modern, highly rated engine requires large amounts of cool air to pass through the cylinders during the oveslap period of the inlet and exhaust valves, in four-stroke engines; and during the period that scavenging is taking place, in two-stroke engines. To maintain this air flow at a maximum it is necessary to keep a careful watch on the air suction pressure to the exhaust turbo-blower. Any reduction in this pressure from normal indicates that the suction filters need to be cleaned. ‘The turbine speed and exhaust temperatures must be carefully watched and any indication of fouled blading should be investigated and rectified. ‘The temperature of the air entering and leaving the cooler must be observed, and any decrease in the difference rectified by cleaning the coolers. The grids fitted in the gas passages to the turbine may also require cleaning if combustion has been poor at any time. Most engine instruction books give both normal values and the values which indicate that corrective action should be taken. Automatic valves fitted in scavenge receivers and under-piston scavenge pumps also require careful atten- tion and regular cleaning, to prevent restriction of air flow. Q. What attention do exhaust gas silencers require? ‘The internal spaces and the baffle plates of exhaust gas silencers become fouled after a period of service. The dirt on the baffle plates increases the back pressure on the engine exhaust system, and must therefore be cleaned off at regular intervals so that the back pressure is kept at a minimum. The silencers are fitted with doors at the sides or bottom of the silencer outer casing to facilitate cleaning, Accumulations of dirt are also dangerous. If the dirt ignites, busaing carbon particles and sparks may be discharged with the exhaust gases from the funnel. 5.52 Why are some turbo-charger manufactures dispensing with water, jackets on the exhaust gas side of turbo-chargers. ‘Water jackets hold the temperature of the castings of the turbine casings at reasonable values, but they have the following disadvantages also. They cool the exhaust gases and reduce the turbo-charger efficiency, They are more expensive to produce than non-jacketed castings due to the complications of coring, cost of cores, the possibility of defective casting due to core shift, and the costs of removal of core material when castings are cleaned. Problems may arise when leakage occurs from the water jackets into the exhaust gas spaces. ‘These factors are prompting some manufacturers to consider dispensing with jackets; others are producing turbo-chargers with no water-jacketed spaces, ‘On the debit side for consideration, better materials will be required for casings without jackets in order to withstand higher temperatures without ‘expansion; the casings will have to be very effectively heat insulated to prevent or reduce the risk of fires and accidents to engine-room personnel. Note Small engine turbo-chargers normally did not have water-jacketed casings. Q. State what can be done to continue operation with a turbo-charger having water leakage into the exhaust gas space. If-the water jacket on a turbine casting leaks, the problem is usually due to ‘corrosion on the cooling-water side of the jacket in the hotter regions of the water space. This is more likely to happen when sea-water is used as the cooling medium and proper attention has not been given to the corrosion protection anodes fitted in various locations in the cooling-water spaces. ‘When leakage occurs it has been the practice in the past to remove the plugs on the cover plates, or the cover plates covering the core holes in the cooling spaces. The aim is to set up a thermo-siphon action and cause air to flow. through the cooling space to hold the casting at a safe operating temperature ‘until a new casting is obtained and fitted. . In some engine rooms where ventilation around the gas side of the turbo- ‘charger is inadequate, the cooling may have to be augmented with an air hose ‘connected to an ait supply, or by leading air through a temporary duct from a ventilator outlet. ‘This form of temporary operation has been carried out many times, but should not be attempted without the cognizance of the Company's technical department and the turbo-charger’s manufacturer. ‘The operation is more easily carried out where owners operate their engines at some value below the maximum continuous rating. (Fig. 5.5) \ Cin cat ertble ergreroon ‘rout Exhausts ile! fenge lg. 6.6 Emergency air cooling arrangement for turbocharger turbine water jacket when leakage occurs across water/gas space boundary. Improvements in Turbocharger oils / reduced maintenance requirements. Turbocharger development has contributed to a progressive rise in the efficiency of marine engines, which can produce more power but leading to higher stress levels. Modern turbocharger designs (ABB) have two bush bearings in the centre, with lube oil supply from the main engine lube system. This has lead to the use of special low friction synthetic oils, which increase the life of the oil, leading to 5000 hours between oil changes. Oxidation resistance and thermal stability properties improve the performance at high operating temperatures. This oil also resists hydrolysis and acidic corrosion. Un-cooled Turbochargers are now in use, where the casing is un-cooled, while the cooling is only for the bearings, to prevent seizure. The reason that water jackets were used was to bring down the casing temperature. However, this had the disadvantage that the exhaust gases were cooled, which reduced the turbocharger efficiency and also the thermal efficiency. The presence of water also meant an increase in the possibility of corrosion, as well as water ingress into exhaust spaces, leading to turbocharger damage. The use of un-cooled casings means that much better materials are required, which can safely withstand the higher temperatures without distortion.
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