Causes, Control, and Prevention of Scavenge Fires

Scavenge fires are caused by lube-oil, diesel or heavy fuel oil gathering around the area of the scavenge ports. This is ignited by sparks form the combustion area passing the piston rings and coming in contact with the flammable oil. It is part of the watch keeping engineers remit to be familiar with the causes, control and prevention of scavenge fires in the main engine. It is also imperative that the scavenge spaces should be cleaned out at every opportunity in port and regular inspection of the scavenge drains to the sludge tank carried out by the watchkeeper.

The following sections examine scavenge fires and the actions required of the senior watch keeping engineer,

The senior watchkeeping engineer on a main engine diesel engine should familiarise himself with the fuel system. This entails knowledge of how the fuel is purified by centrifuge and stored in the ready use tanks as well as the fuel pump and injection system. He should also be aware of the Chief Engineers standing orders regarding fuel oil injector nozzle cooling temperatures, fuel oil temperatures, fuel oil filter change intervals and maintain these as part of his watchkeeping duties.

This is normally carried out by the third engineer; I used to do it whilst on watch during quite sea passages. I sailed with one Chief who insisted that the Second engineer carried out the maintenance. In any case the Chief or Second should at least witness the testing of the injectors.

The ship should carry plenty of spare fuel injectors; enough to change a couple at every port and leave a couple of spares to change after scavenge fires.

The spare injectors should be stored in racks with the nozzles well clear of the deck to prevent accidental knocks which can block the nozzle atomising holes.

The injector should be gripped in a vice which has jaw protectors and the nozzle removed. The parts should all be washed in diesel and any clinker carefully removed from the face of the nozzle with a copper scraper.

The nozzle should then be gripped in the vice and the needle ground into the seat using fine grinding paste. Once again wash the nozzle and needle carefully making sure all the grinding paste has been removed. Dry all parts and reassemble the injector, connecting the inlet to the injector testing pump outlet. Pump up the pressure to the recommended level and adjust the spring accordingly until the injector needle lifts and fuel is injected into a suitable receptacle.

At the recommended pressure the atomisation of the fuel should be visible and a squeak should be simultaneously heard. No dripping from the nozzle after atomisation is permitted.

Record the injector serial number, pressure that it lifted at and date of test, in the injector maintenance log and re-rack, ready for witnessing by the Chief or Second if required. Be sure to mark on the injector that it has been tested.

Finally, a word about safety - marine diesel engines fuel supply is at high pressure and if using heavy fuel oil will be very hot; I still have the scar on my wrist from an oil spill when I was changing an injector. That was 1967 when I was an engineer on a Sulzer engined Denholm Company Sulphur Tanker. Yuc! What a reek, glad I wasn’t a Deck Officer.

Also when testing a fuel valve on the test rig do not put you hands anywhere near the atomising fuel, and wear goggles!

A sketch of an injector and a scavenge space is shown below;

Regular maintenance of the piston, rings and liners should be carried out, as per the Chief or Second engineers maintenance programme which will involve the periodic pulling of a piston (more so if a particular cylinder has a record of scavenge fires) and the examination of the rings and liners. A survey of these should be carried out to ensure that all components are within the makers recommended specs.

Rings should be checked for defects and clearances, liners being checked for scratching, wear and ovality.

Regular purifying of the fuel oil by centrifuge, be it diesel or heavy fuel must be carried out to ensure an adequate clean supply of fuel for the main engine. I had trouble with scavenge fires on one ships engine and was getting a lot of stick as I maintained the fuel injectors. However we had a new Fourth who, against his Junior Engineers advice failed to clean the oil filters or centrifuges regularly. The dirty fuel blocked the injector nozzles causing them to leak after atomizing, resulting in scavenge fires. The junior finally came to me for advice; I promoted him and the Fourth got the boot at the next port and sent home – no more scavenge fires, so I cannot emphasis enough the importance of fuel oil filtering and centrifuging.

Page 2 continues the causes of scavenge fires; and the method of extinguishing them.