TURBOCHARGER OVERHAUL RESULTING IN LOW SCAVENGE AIR PRESSURE AND LOSS OF POWER

1. Faulty turbine/ compressor overhaul - e.g. incorrect assembly, wrong clearances, misalignment of the rotor. Reason: increased internal friction/leakage reduces efficiency.
2. Damaged or incorrectly fitted blade profiles, or damaged nozzle ring. Reason: poor gas flow to the turbine reduces the power produced by the turbine.
3. Worn or incorrectly set bearings - journal/thrust clearance too big or too small causing the rotor to rub or to vibrate. Reason: vibration and rubbing destroy performance.
4. Rotor/ shaft out of balance or bent. Reason: increased vibration, seal damage and reduced speed.
5. Plugged or incorrectly refitted turbine gas inlet/outlet (exhaust duct or nozzle ring), or a closed/partly closed exhaust gas by-pass valve. Reason: reduced gas flow.
6. Leakage at the gas inlet/outlet joints or the blower casing - air leaks back to atmosphere or from the discharge. Reason: lost scavenge air.
7. Dirty/ fouled compressor or turbine blading (after overhaul unit not cleaned). Reason: reduced flow efficiency.
8. Damaged or incorrectly fitted labyrinth/ piston ring seals allowing air or gas leakage between compartments.
9. Incorrect rotor end clearances set at overhaul, causing rubbing of wheel to casing.
10. The air filter or cooler was disturbed/ blocked during the overhaul and not replaced; restricted suction raises pressure drop and lowers delivery.
11. Compressor surge or surging outlet pressure due to engine/charging system condition (e.g. a dirty scavenge cooler, exhaust back pressure) - this is a charging-system cause rather than the T/C itself.

1. Exhaust temperatures before and after the turbine (per cylinder and average). Reason: high turbine inlet temperature with low boost shows poor turbine gas flow/efficiency; uneven temps indicate cylinder problems.
2. Turbocharger speed (rpm) and boost pressure (scavenge/charge air pressure) at set loads. Reason: a speed that is low with normal exhaust back pressure shows a turbine/compressor fault; ratio of boost to rpm indicates compressor efficiency.
3. Compressor delivery temperature relative to speed. Reason: indicates compressor efficiency/fouling or leakage.
4. Charge-air cooler temperature drop (air before/after cooler) and sea water delta-T. Reason: an inefficient/warm cooler lowers charge air density and boost; shows whether the cooler was disturbed at overhaul.
5. Cylinder compression and firing pressures (indicator diagrams) and max combustion pressure at the load. Reason: confirms whether the loss of power is due to low scavenge pressure or to a fuel/cylinder fault.
6. Exhaust gas back pressure downstream of the turbine (after-turbine pressure). Reason: indicates system restrictions (exhaust boiler) that load the turbine.
7. Electronic engine management/ process data (if fitted): scavenge receiver pressure valve, T/C speed, temps logged against engine load and fuel index.
8. Lubricating oil pressure/temp to T/C bearings and condition (debris). Reason: indicates bearing fault or misalignment vibration.
9. Air filter differential pressure. Reason: a choked filter reduces compressor suction.
10. Fuel index/rack position versus rpm at set load. Reason: a higher fuel index to hold the same power shows lost charging efficiency.

1. Use only the maker's manual, correct clearances and locking/torque values; follow the OEM procedure step by step.
2. Record all as-found and as-fitted clearances (bearings, seals, end/axial float) and component serial numbers on the overhaul record sheet.
3. Balance the rotor as a complete unit (or per maker) and record the balance report; never swap or individual-balance wheels without a rig.
4. Clean both air and gas sides thoroughly with approved methods; avoid wire brushing that damages blade surfaces.
5. Renew all gaskets, O-rings, joints and locking devices on re-assembly; use new components where the manual requires.
6. Check nozzle ring, diffuser, blade profile and rotor fretting; renew damaged parts rather than refitting.
7. Reset and verify correct axial/radial clearances and rotor end-fits before final torquing.
8. Ensure correct lubrication - clean oil, correct pressure and flow, and a primed oil system before run-up.
9. Pre-commission: check free rotation by hand, check for rubbing, confirm rotation direction, and perform a slow run-up checking vibration, speed and boost against recorded values.
10. Perform a post-overhaul performance comparison (speed, boost, temps) against the baseline and initialise a fresh trend record.
11. Involve a qualified second engineer/officer to check the work, and follow a completed job-observation/release-to-operation procedure.
12. Keep proper records of the overhaul and of any abnormal findings so future work is carried out in a consistent manner.