Scavenge Air Fire on a Single-Turbocharger Main Engine
At 22:10 the 6S60MC main engine is running at 78% MCR on one turbocharger; the second unit was isolated two days earlier after a bearing temperature alarm. The watch notices No. 4 unit exhaust gas temperature at 415 °C against 355–365 °C on the other units, scavenge air temperature after the cooler at 58 °C, and repeated turbocharger surge at 70–75% load. The scavenge space drain shows black oily sludge, and there is a hot, oily smell in the engine room. Load was reduced to 60% 20 minutes ago. The bridge has asked for 14 knots to meet the pilot station ETA; current speed is 12.5 knots. The duty engineer must decide whether to stop the engine, continue at reduced load, or isolate the unit, and what firefighting preparation is needed. The chief engineer is asleep and the second engineer is on watch.
1. State the immediate actions you would take, in priority order, and explain the reasoning behind each. Include what you would tell the bridge and why.
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Priority is to keep the engine under control, prevent escalation and preserve the option to fight a fire without causing thermal shock.
- Inform the bridge and call the chief engineer. Report a suspected scavenge fire, state that speed will be reduced and that an engine stop may become necessary. Do not allow the ETA request to dictate the technical decision.
- Reduce load to about 50–60% and cut fuel to No. 4 unit. Keeping the engine turning maintains scavenge air flow and reduces the risk of local overheating and distortion; cutting fuel removes the heat source.
- Increase cylinder lubricator feed for No. 4 and keep the scavenge drains open and monitored. Do not open the scavenge space.
- Monitor exhaust temperatures, scavenge air temperature, turbocharger speed and surge, oil mist detector and bearing temperatures. Prepare the fixed CO2 system, portable extinguishers, boundary cooling and the fire team.
- If temperatures continue to rise, smoke increases or the turbocharger overspeeds, stop the engine, keep the turning gear engaged if possible, and apply CO2 to the scavenge space. Do not open it until cooled.
Stopping immediately is not automatically correct: a sudden stop can cause loss of cooling and seizure, and may not extinguish a hot-spot fire. Controlled load reduction is preferred unless the fire is clearly uncontrolled.
2. How would you distinguish a scavenge air fire from a leaking exhaust valve or a crankcase oil mist condition using the indications given?
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Use the combination of indicators, not one reading.
A scavenge fire typically shows high scavenge air temperature, smoke or oily sludge from scavenge drains, blistering paint, turbocharger surging and a hot oily smell. The exhaust temperature may be high, but the scavenge air temperature rise is the stronger clue in this case.
A leaking exhaust valve gives sustained high exhaust temperature on one unit, often black smoke and a popping exhaust note, while scavenge air temperature remains near normal. A leaking valve can ignite scavenge oil, so both conditions may coexist. After cooldown, the valve should be inspected.
A crankcase oil mist condition gives an oil mist detector alarm, high crankcase pressure and no primary scavenge temperature rise. The crankcase must not be opened because of explosion risk; the engine should be stopped and the oil mist system treated as the immediate hazard.
Here, high exhaust temperature plus high scavenge air temperature, surge and drain sludge point first to a scavenge fire, but the exhaust valve remains a possible root cause to check later.
3. After the situation is stabilised, what inspections and records are needed before the unit is returned to normal load?
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Allow controlled shutdown and cooldown, then isolate and open the scavenge space under a permit to work. Inspect cylinder liners, piston crowns, rings, scavenge ports, air cooler and turbocharger nozzle ring and turbine for overheating, distortion and oil carryover. Clean the scavenge space and drains, renew damaged gaskets and check the oil mist detector and crankcase for contamination.
Investigate the root cause: fouled air cooler, faulty injector, exhaust valve leak, turbocharger defect or incorrect cylinder lubrication. Correct the cause before load increase.
Run at low load and monitor exhaust temperatures, scavenge air temperature, turbocharger behaviour and drain flow. Increase load in steps only when stable.
Records should include the engine log, defect report, maintenance history, spare parts used and any class or company notification if damage affects class or safety. Update the planned maintenance system so the air cooler and turbocharger checks are not deferred again.