Scavenge fires occur when oil and carbon particles from the engine cylinder accumulates in the scavenge space.
For a fire to begin there must be combustible material, air to support combustion and source of heat at a temperature high enough to start combustion.
A) In case of scavenge fires, combustible material is oil. This oil can be:
· Cylinder oil which has drained down from the cylinder spaces.
· Crank case oil carried upwards on the piston rod because of faulty stuffing box.
· Fuel oil which may be contained in the cylinder oil residues due to defective injectors or injectors with incorrect pressure setting.
B) Oxygen necessary for combustion comes from the scavenge air which is in abundant supply for the operation of the engines.
C) The source of heat for ignition comes from:
· Prolonged piston blow by.
· Slow combustion in the cylinder due to incorrect atomization or misaligned fuel jets.
· Blow back through the scavenge air ports due to incorrectly adjusted exhaust cam disc or large resistance in the exhaust which causes back pressure in the exhaust system
CAUSES OF SCAVENGE FIRE
1. Blow past of combustion products caused by leaky, sticky or broken piston rings, worn out liner, faulty cylinder lubrication, or insufficient axial clearance of the piston rings.
2. Overheated piston dissipates heat to the under piston area caused by faulty atomization and injection pressure, faulty fuel pump timing, loss of compression, engine overload, failure of coolant circulation or insufficient cooling due to formation of scale.
3. Blow back of exhaust gases caused by exhaust back pressure or deposits on exhaust ports, fouling of grid before turbine inlet, fouling of turbine blades, choking of Exhaust Gas Boiler or economizer gas outlet.
4. Presence of fuel oil in the scavenge spaces due to defective fuel injectors, incorrect pressure setting of injectors or fuel particles landing on the cylinder liner due to excessive penetration.
5. Excessive cylinder lubrication which is drained down to scavenge spaces.
6. Fouled scavenge manifold.
7. Defects in cylinder liner.
INDICATION OF SCAVENGE FIRE
Scavenge fire can lead to grave consequences hence it is better to read the signs earlier. It is difficult to tell about a scavenge fire from outside unless it has reached serious proportions. Following are some signs of a possible scavenge fire. These symptoms can be noticed if the watch keeper is alert all the time.
· A sudden loss of power and high exhaust temperatures.
· High scavenge air temperatures.
· Irregularity in the running of the engine or excessive gases in the exhaust.
· Scavenge fires also leads to heavy surging of turbochargers and sparks from the scavenge drains.
If the above signs are noticed, appropriate action must be taken and chief engineer is to be informed immediately. Bridge watch keepers are also likely to observe such signs. They should also be briefed that in case they notice unusual smoke from the exhaust or sparks from funnel they should inform the engine room.
ACTIONS TO BE TAKEN
If any of the above is noticed or there is a suspicion that a scavenge fire might have started, the first thing is to inform Master / Chief Engineer and not to panic about the situation. While taking corrective actions two objectives must be kept in mind:
1) To contain the fire within the scavenge space.
2) To prevent or minimize damage to the main engine.
A minor fire may burn out shortly without damage and conditions will return to normal. The affected units must be run at reduced power until inspection of the scavenge trunk and overhaul of the piston and cylinder can be carried out at the earliest opportunity. Any oil residues found under cylinders during scavenge space inspection must be cleared.
In case of a major fire following are the steps to be taken:
1) Inform bridge and reduce the speed of the engine but the engine is NOT to be stopped.
2) Cut off fuel supply to the affected unit and shut off valves.
3) Cylinder lubrication must be increased.
4) Coolant flow through jacket and piston to be maintained.
5) The scavenge drains should be tightly shut to prevent flow of spark in engine room.
6) Once engine is stopped open indicator cocks, engage turning gear and turn engine to prevent engine seizure.
7) Scavenge air duct flap valve before engine to be shut. The turbocharger air suction can be covered to prevent air ingress into the engine.
8) Monitor temperature of the scavenge spaces.
9) Boundary cooling of scavenge spaces may be carried out.
10) In order to extinguish the fire carbon dioxide, dry powder, steam or CO2 smothering etc. (as applicable to the vessel) may be supplied to the trunking.
11) It should be kept in mind that no attempts are made to open the scavenge trunk as it may lead to a severe explosion
12) Keep clear of scavenge space relief valve (if fitted) to prevent injury to personnel.
INSPECTION AFTER SCAVENGE FIRE
Following inspections to be done after the scavenge fire has been confirmed extinguished and scavenge spaces cleaned. While cleaning check how many units have dry residues as special attention should be paid to these units during the inspection:
1) Check by turning the engine and watch movement of piston in the liner, check for any occurrence of binding at part of stroke (Binding indicates misalignment of piston).
2) Check spring on scavenge space relief device (if fitted), if the device was near the seat of fire.
3) Piston rod packing spring also should be checked, as it may have become weakened by overheating.
4) Check piston rings and liner for any distortion or reddish burning mark.
5) Check diaphragm and frame near affected part for distortion or cracks.
6) Check guides and guide shoes.
7) Check tension of tie bolts.
PREVENTION OF SCAVENGE FIRE
Since prevention is always better than cure, a few simple steps would help to ensure that a scavenge fire never occurs on the ship or at least minimizes the chances of its development. Here are these simple tips on avoiding scavenge fires.
1) Scavenge trunking and scavenge ports should be kept clean.
2) Piston rings, cylinder liner, piston skirt and the water seals should be inspected for any defect.
3) Vibration of the engine should be prevented by tightening the tie bolts properly.
4) Excess cylinder lubrication must be avoided.
5) In case of timed lubrication, the time has to be checked as per PMS.
6) Piston rings must be properly maintained and lubricated adequately.
7) Piston rod stuffing box must be maintained to prevent oil ingress in the scavenge space.
8) Prolonged engine or any cylinder over loading to be avoided.
9) Cylinder liner wear must be within admissible limits.
10) Routine inspection and maintenance of the trunk should be carried out.
11) The unburned oil and carbon particles should be removed from the trunk.
12) Scavenge drains should be drained and cleaned periodically to prevent choke up.
13) High temperature alarm system should be fitted to provide an indication of rise in temperature above the normal limit.
14) Pressure relief valves (if fitted) should also be tested on regular basis.
15) To keep the exhaust resistance low, heavy deposit must not be allowed to collect on the protective grating, nozzle rings, and turbine blades. Back pressure after turbo charger must not exceed 3.5 kPa.
16) If auxiliary blower does not start during low load running, un-burnt fuel may accumulate on top of pistons, which will involve risk of scavenge air box fire.
