Scavenging and turbocharging

Following a recent turbocharger overhaul it is observed that the scavenge air pressure is lower than it was previously and the engine power output is also reduced. a. State, with reasons, the possible causes of the problem. b. State, with reasons, what engine operational information should be gathered in order to enable the possible cause of the problem to be detected. c. Explain how the defect may be rectified.

prevent similar incidents.

Answer:

a. A turbocharger overhaul consists of removing the rotor and nozzle ring for inspection and cleaning, renewing the bearings where necessary, inspection of the labyrinth glands, cooling water spaces and gas inlet protection grid. Causes of low scavenge pressure are: – Dirty air filter. Overlooked cleaning the air filter before refitting. – Scavenge dump valves left open. – Damage to turbine blades. This could have occurred when removing refitting rotor. The efficiency of the turbine will be reduced if the blade profile is damaged. May also cause vibration. – Damaged impeller; again while removing or refitting. See above. – Defective or incorrectly fitted bearings allowing rotor to rub against casing labyrinth glands may be damaged as a result. – Foreign bodies in protection grid, nozzle ring, turbine blades, compressor or diffuser. Compressor axial clearances incorrect due to badly fitted bearing; allowing air to leak back from discharge to suction. May also cause back of compressor wheel to rub against casing. – Waste gate or a charge air by pass valve, may be jammed open or not operating correctly (if fitted)

b. Operational information to be gathered: – Turbocharger speed: The faster the turbocharger revolutions, the greater mass flow of air produced, and the higher the scavenge pressure. Slow turbocharger rpm could be an indication of all of the above faults. – Vibration (and noise): Vibration can indicate damaged unbalanced rotor; noise indicates rotor in contact with casing. – Exhaust gas inlet temperature to turbo-charger: A higher than normal temperature (along with higher exhaust valve temperatures)are an indication of a high exhaust gas back pressure could be caused by choked nozzle ring. – Exhaust gas temperature difference across turbine: A smaller than normal differential will indicate that the exchange in energy is not taking place across the turbine. – Pressure drop across air inlet filter. -Temperature differential across air cooler water inlet and outlet. – Surging of the turbocharger

c. If the T/C is vibrating or unusually noisy: – Engine must be stopped and the cause investigated. This will entail removal of the rotor. Check compressor wheel, shaft, and labyrinth, turbine wheel for signs of damage or rubbing. Check casing and nozzle ring for damage. If damage has occurred then it must be rectified before the rotor is rebalanced and refitted with the correct bearings. If there is no vibration or noise: – Check dump valves, bypass and waste gate if fitted. – Check that the air filter is clean. – clean the air cooler on the air side. If none of these are successful, then it may be necessary to open up the turbo-charger and check that it hasbeen assembled correctly with correct axial clearances and that there are no blockages in the nozzle ring or protection grid.

d. The overhaul of the turbocharger should be overseen senior member of the engineering staff as appointed by the Chief Engineer.

– Only chemicals recommended by the manufacturer are to be used at the recommended strengths.

– After cleaning the turbine rotor and compressor wheels are to be examined closely for any sign of damage before re-assembly.

– New bearings to be fitted if the running hours are within 20% of the recommended maximum hours for the bearing.

– Only manufacturers spares, sealed in tins are to be used.

– Axial clearances must be checked and recorded after comparison with the original clearances

– Ensure that no gloves, rags, tools etc are left in the casing or exhaust pipes before boxing up, and that the protection grids are clean and in position.

– The air filters and air cooler is to be cleaned.

– Bypass valve and waste gates are to be checked for correct operation on completion of overhaul.

Turbocharger vibration:

Sudden vibration:

– Sudden imbalance due to blade damage

– Sudden imbalance due to partial dirt removal.

– Breakage of damping wires

– Breakage of resilient mounting.

– Bearing failure

– Poor cylinder combustion.

– Damaged support – External excitation

Remedial action:

– Regular and effective in service cleaning.

– Effective LO management.

– Regular inspection of bolts and casing.

– Maintain optimum cylinder combustion condition.

– Protection grid condition

Gradual vibration:

– Ineffective in service cleaning.

– Gradual fouling

– Slackening of bolts

– Deterioration of resilient mounts for bearings.

– Ineffective PMS.

Remedial action:

– Maintain PMS

– Effective Water/solid wash.

– Regular bearing exchange – Regular inspection and maintenance of the supports.

Q: State the Methods of operating with defective turbocharger. Methods of running the engine with defective turbocharger depend upon design consideration and the repair facility.

Answer:

Bypass arrangement:

Some design provides a different path for the exhaust gas to pass to the uptake. Engines with exhaust by-pass

1. Stop the engine.

2. Lock the turbocharger rotor.

3. Remove the blanking plate from the exhaust by-pass pipe.

4. Remove the compensator between the compressor outlet and the scavenge air duct. This reduces the suction resistance.

5. Run engine with reduced load.

Engines without exhaust by-pass:

This system allows the damaged rotor to be taken out of the engine. This may also allow repair work of the rotor is possible.

1. Stop the engine.

2. Remove the rotor and nozzle ring of the turbocharger.

3. Insert blanking plates.

4. Remove the compensator between the compressor outlet and the scavenge air duct. This reduces the suction resistance.

5. Cooling should be maintained for the turbine casing if possible.

6. Run engine with reduced load.

Engine with locking the rotor:

1. Stop the engine.

2. Lock the rotor of the defective turbocharger.

3. Rather than completely blanking off the compressor an orifice plate is fitted to allow controlled amount of cooling air to pass to prevent overheating of the rotor due to conduction

4. Cooling should be maintained for the turbine casing if possible.

5. LO supply to stop.

6. Run engine with reduced load.

Q. Describe how the following conditions are caused and dealt with

A)Gradually decreasing in scavenges air pressure.

B) Suddenly increasing in scavenges pressure.

C) Scavenge air temperature at consistently above normal.

Answer:-

(a)Scavenging air pressure drops in spite of the normal speed of the turbocharger means that resistance has increased in the passage between air inlet and the scavenge air valves.

• Problem with the turbocharger • Negative pressure set up inside scavenge trunk.

• Air suction filters fouling.

• Faulty scavenge air non-return valve.

• Charge air cooler fouling.

• Turbocharger deceleration.

• Scavenge fire.

• One of the cylinder units misfiring.

• Fouling exhaust system.

• Scavenge manifold leakage and scavenge drain valve open.

• Engine room pressure is negative due to bad weather.

(b) The scavenge air pressure may be increased suddenly because of the following facts:

• Adverse weather and strong wind.

• Overloading of engine.

• Faulty exhaust booster system. • Auxiliary blower cut in.

• Just after water washing.

(c) The scavenging air temperature may be rising due to the following conditions:

• Fouling of charge air cooler.

• High engine room ambient temperature.

• Faulty cooling water control bypass system.

• Engine overload.

• Scavenge fire.

• Piston blow-past.

Remedies:-

So as to avoid these conditions already mentioned above, to carry out the following measures:-

• Charged air cooler is cleaned when the pressure differential exceeds the max limit, by reading the differential pressure at manometer fitted at charged air cooler for airside cleaning.

• Turbocharger blower side air filter is cleaned by checking the pressure different shown at “U” tube manometer attached at Blower side air filter housing.

• Charge air cooler’s seawater is cleaned by checking the cooling S.W inlet and outlet temperature difference.

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