Turbocharger and overhaul

Q) Following a recent turbocharger overhaul it is observed the scavenge air pressure a lower was previously and the engine power output is also reduced.

State, with reasons, the possible causes of the problem.

State, with reasons, what engine operational information should be gathered in order to enable the possible cause of the problem to be detected.

Explain how the defect may be rectified.

State the instructions, which should be issued with respect to future turbocharger .. order to prevent similar incidents.

Ans)

(a) A turbocharger overhaul consists of removing the rotor and nozzle ring for inspection and cleaning, renewing the bearings where necessary, inspecting of the labyrinth glands, cooling water spaces and gas inlet protection grid,

Causes of low scavenge pressure on reassembly could be due to:

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.

Compressor axial clearances incorrect due to badly fitted bearing: Allowing air to leak back from discharge to suction, or excess air to leak down back of wheel to exhaust. May also cause compressor wheel to rub against casing. If the turbocharger system is fitted with a waste gate or a charge air by pass, then these could be jammed open or not operating correctly,

(b) Turbo-charger 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): some turbo-chargers have vibration meters fitted. If not, monitor visually and by placing the end of a screwdriver or special “listening rod” on the casing, and listening at the other end. 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 damaged balding not allowing smooth passage of gas.

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: If the turbocharger is operating at the normal speed, but the scavenger pressure is low. Then the compressor is not operating efficiently. This could be because the axial clearances are incorrect or because of damage or choked compressor wheel. The pressure drop across the air filter will be lower than normal. Could also be an indication of dirty air filter or choked air cooler(pressure drop across air cooler will be high) is the turbocharger prone to surging, esp at load changes? If so then it indicates disruption in either the gas or air flow through the charger.

(c) If the charger is vibrating or unusually noisy then it must be stopped and the cause investigated. This will entail removal of the rotor. Before removal check run out of bearings with a clock gauge. Check compressor wheel, shaft, 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 simplest possible faults first. Check dump valves, bypass and waste gate if fitted. Check that the air filter is clean. It may be necessary to 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 has been 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 by the 2^nd engineer or another senior member of the engineering staff as appointed by the chief engineer. When the rotor is removed for overhaul/cleaning, only chemicals recommended by the manufacturer are to be used at the recommended strengths. After cleaning, the turbine and compressor wheels are to be examined closely for any sign of damage before re-assembly. New ball and roller borings to be fitted if the running hours are within 20% of the recommended maximum hours for the bearings. Normally this will be the case. Only manufacturers spares, sealed in tins are to be used. When fitting run out must be recorded and should be within 0.01 mm .Axial clearances must be checked and recorded after comparison with the original clearances (stamped on makers plate) As soon as the casings are oil tight the compressor and turbine end are to be filled to the correct level with the correct grade of oil. The overseeing engineer is to 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 at the same time that the turbocharger is overhauled. Bypass valve and wastes gates are to be checked for correct operation on completion of overhaul.