- It’s also called as Scavenge Air Fire or Scavenge Space Fire.
- It’s the most deadly fire on ship.
Note on Fire
- Fire has three essential components i.e. Fuel, Heat and Oxygen.
- Source of fuel may be unburnt fuel, leaking lubricating oil or carbon deposits.
- Source of heat may be blowing by gases through liner and piston rings or frictional heat.
- Source of Oxygen is scavenge air.
Cause of Scavenge Fire
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 serious damage can be done to the piston rod and scavenge air box walls, possibly reducing the tension of the stay-bolts.
Ignition of carbon deposits in the scavenge air box can be caused by:
- Prolonged blow-by
- Slow combustion in the cylinder, owing to incorrect atomization, incorrect type of fuel valve nozzle, or misaligned fuel jets.
- Blow-back through the scavenge air ports, due to an incorrectly adjusted exhaust cam disc or large resistance in the exhaust system (back pressure).
To keep the exhaust resistance low, heavy deposits must not be allowed to collect on protective gratings, nozzle rings and turbine blades.
If the auxiliary blowers do not start during low-load running, on account of a fault, or the switch for the blowers not being in Auto-position, unburned fuel oil may accumulate on top of the pistons.
This will involve the risk of a scavenge air box fire.
In order to avoid such fires:
- Obtain permission to stop the engine.
- Stop the engine.
- Remove any unburned fuel oil from the top of the pistons.
- Re-instate the supply of scavenge air.
- Start the engine.
The switch for the auxiliary blowers should be in AUTO-position during all modes of engine control.
- An increase in the exhaust temperature of the affected cylinder,
- The turbocharger may surge,
- Smoke from the turbocharger air inlet filters when the surging occurs,
- The scavenge air box being noticeably hotter.
If the fire is violent, smoky exhaust and decreasing engine revolutions will occur.
Violent blow-by will cause smoke, sparks and even flames to be blown out when the corresponding scavenge box drain cock is opened, therefore keep clear the line of ejection.
Steps to be followed
Owing to the possible risk of crankcase explosion, do not stand near valves – violent flames can suddenly be emitted.
- Reduce speed/pitch to Slow and ask permission to stop.
- When the engine Stop order is received, stop the engine and switch-off the auxiliary blowers.
- Stop the fuel oil supply.
- Stop the lubricating oil supply.
- 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 degree C. When opening, keep clear of possible fresh spurts of flame.”
- Remove dry deposits and sludge from all the scavenge air boxes.
- Clean the respective piston rods and cylinder liners, and inspect their surface condition, alignment and whether distorted. If in order, coat with oil. Also inspect piston crown and skirt, stuffing box and bottom of scavenge box for possible cracks.
- All engine parts must be returned to normal operating temperature.
To ensure proper draining of oil sludge from the scavenge air boxes, thereby reducing the risk of fire in the scavenge air boxes, we should:
- Daily check scavenge air drain pipes with the engine running.
- Cleaning of drain pipes at regular intervals.