Scavenge Fire on Marine Engines, causes, actions to be taken and prevention measures

Accumulation of sludge in the under piston area

If flakes of burning or glowing carbon deposits drop into the oil sludge at the bottom of the scavenge air box, this sludge can be ignited and, if very combustible material is found here, serious damage can be done to the piston rod and the scavenge air box walls, the latter possibly causing a reduction in the tension of the stay bolts. For a fire ti take place, there should be a combustible material, oxygen to support the fire and a source of heat high enough to start the combustion. The combustible material can be the oil that get accumulated in the under piston area that has been drained from the cylinder spaces. It may also contain fuel oil as a result of defective injectors, injectors with incorrect pressure setting, fuel particle striking on the cylinder. Incorrect fuel pump timing and similar reasons. The air required for the combustion is readily available in the under piston area coming from the scavenge manifold. The source of ignition comes from piston blowpast as a result of broken piston rings or liner wear or incorrect lubricaton, slow ignition, after burning or excessive exhaust gas back pressure. To keep the exhaust resistance low, heavy deposits must not be allowed to collect on protective gratings. Nozzle rings and turbine blades.

 Measure to be taken 

 Owing to the possible risk of a crankcase explosion, do not stand near the relief valves - flames can suddenly be violently emitted. 1) Reduce speed to SLOW, if not already carried out automatically, and ask bridge for permission to stop. 2) When the engine STOP order is received, stop the engine and switch-off the auxiliary blowers. 3) Stop the fuel oil supply. 4) Stop the lub. oil supply. 5) Put the scavenge air box fire extinguishing equipment into function. Do not open the' scavenge air box or crankcase before the site of the fire has cooled down to under 100 deg Celsius. When opening, keep clear of possible fresh spurts of flame. 6) Remove dry deposits and sludge from all the scavenge air boxes. 7) Clean the respective piston rods and cylinder liners. and inspect their surface condition, alignment, and whether distorted. If in order, coat with oil. Repeat the checking and concentrate on piston crown and skirt, while the engine is being turned. Inspect the stuffing box and bottom of scavenge box for possible cracks.

Scavenge Fire

 The Extinguishing medium used are CO2 or Dry chemical powder or smothering steam. Boundary cooling is also carried out to avoid the risk and effect of scavenge fire.

 Prevention

 Regular cleaning of the under piston area and the scavenge manifold is the proper way to get rid of the scavenge fire. The under piston drain pockets should be cleared out regularly to avoid the accumulation of sludge in those areas. During Piston overhauling, the stuffing box rings should be cleaned and put back and the clearance should be with the limit. The piston rings should be regularly inspected for their proper functioning. The fuel injectors should be kept in good condition, timed correctly and the load on each unit should be properly adjusted.