Scavenge Fire

In a scavenge air space there is a normal accumulation of carbonized cylinder oil, unburnt fuel and carbon from the residual products of combustion. They will ignite under certain faulty operating conditions (such as flame blow by) indicating a fire in the enclosed scavenge air box is scavenge fire.

Causes

1. Blow past of combustion products.
2. Overheated piston throw away heat to the under piston area.
3. Blow back of exhaust gases.
4. Presence of fuel oil in the scavenge spaces.
5. Excessive cylinder lubrication. 

Indications 

1. Scavenge temperature will start increasing.
2. Turbochargers will start surging.
3. High exhaust temperature.
4.  engine lose its power and reduction in engine rpm.
5. smoke or sparks coming out from scavenge drains.
6.  will be forming of Paint blisters on the scavenge doors.

In case of Large fire - 

• Peeling or Blistering of paint.
• large reduction in engine rpm. 
• surging of turbocharger.

Actions to be taken

For small fires

1. Start reducing the engine rpm and reduce it to slow or dead slow.
2. Increase the cylinder lubrication of the affected unit. additional attention to be given for this as this does not feed the fire. In case of increase of fire should not increase the lubrication.
3. The fire may be due to leaky fuel valves, so lift up the pump of the affected unit.
4. Keep scavenge drain closed.
5. Keep monitoring the scavenge and exhaust temperatures and let the fire starve and wait for it burn itself out.
6. After confirming that the fire is out, start increasing the rpm slowly.
7. Keep monitoring the scavenge temperature for any sign of re-ignition.

For large fires

1. Immediately inform Chief Engineer and Officer on watch Bridge. Then start standby Generator. Slowly start reducing the Main Engine rpm to slow ahead, if same is not reduced automatically.
2. usually fire should extinguish on its own within 10-15 minutes.
3. If the fire does not extinguish, stop the engine and switch off auxiliary blowers. Engage turning gear and keep rotating with turning gear.
4. Stop F.O & L.O supply pumps after ensuring main engine relevant temperatures has already reduced acceptable level.
5. Open main steam inlet valve located near air cooler of main engine. open smothering steam valve and close drain valve in order to supply steam in the scavenge air receiver.
6. should not open the scavenge air box or crankcase door before the site of fire is cooled down.
7. Before re-starting main engine, should clean the scavenge space, investigate and rectify the cause.

Inspection after scavenge fire

1. Piston alignment should be checked.
2. Check piston rings and liner for any distortion. 
3. Check spring on scavenge space relief device.
4. Piston rod packing spring also should be checked.
5. Check diaphragm and frame near affected part.
6. Check guides and guide shoes.
7. Check tension of tie-bolts.

Prevention of scavenge fire

1. Clean scavenge space and drain at regular intervals.
2. Keep scavenge space drain open.
3. Excess lubrication must be avoided.
4. Piston rings must be properly maintained and lubricated adequately.
5. must be maintained Piston rod stuffing box  to prevent oil ingress in the scavenge space.
6. Cylinder liner wear must be within the admissible limits.

Scavenge space protection devices

1. Electrical temperature sensing device which will automatically sound an alarm in the event of excessive rise in local temperature.
2. Pressure relief valves consisting of self closing spring loaded valves are fitted. 
3. Fixed fire extinguishing system. (CO2 , Dry powder or Steam)