“Fire onboard” sound itself gives a frightening feeling to us all but the real scenario is quite more chilling than it seems. On board, we have limited resources, manpower, and conditions. Hence, it’s un-imaginable to fight the fire under situational stress. In the recent past (2018), Maritime Industry had witnessed a lot more than a few incidents, caused mainly by fire on board which even led to the sinking of Vessel. In some cases, an even dead body was not found.
In few cases, scavenge was the main cause of the fire on the ship that made a huge fire. As the saying goes, “Prevention is better than cure”, we will be going to discuss each and every important aspect regarding scavenge fire.
What is the Scavenge fire?
If there is any fire in the scavenge space of the main engine is called scavenge fire or scavenge space fire. To catch fire in the scavenge space the basic element of fire should be present. Fire is a continuous chemical reaction of fuel and oxygen in the presence of heat. For this chemical reaction that means to catch fire, there should be a proper ratio of the element (heat, fuel, oxygen).
- Sufficient oxygen is present with scavenge air.
- Lube oil and unburned carbon deposits act as fuel in this case.
- Only heat is missing here. If sufficient heat will enter in this space, a fire will occur.
Note – for continuous fire 16% 0f oxygen in needed.
Causes of scavenge fire –
- Blow past- leakage of combustion products from combustion chamber caused by damage (sticky or broken) piston rings, worn out liner, improper cylinder lubrication, or insufficient axial clearance (clearance between the inner part of ring and groove) of the piston rings.
- Overheating of the piston– Failure of flow of piston cooling oil will cause overheating of the piston, it will dissipate the heat to the under piston area. Other reasons may include bad combustion because of faulty atomization, injection pressure, wrong fuel pump timing, loss of compression, overload on the engine.
- Presence of the fuel oil –If the unburnt fuel oil is present 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.
- Blowback of the exhaust gas –It will give heat to scavenge and it is caused by the high back pressure of exhaust gas, deposits on the exhaust ports, fouling of grid before turbine inlet, fouling of the turbine blades, choking of exhaust gas boiler or economizer gas outlet.
Other deposits under scavenge space include:
- Inadequate draining of the scavenge spaces.
- Excessive cylinder lubrication will cause deposits of lube in scavenge spaces.
- Leakage of the stuffing box.
- Fouled scavenge manifold and space.
The indication of Scavenge Fire –
Being an engine room watchkeeper, we must be aware of the indication of scavenge fire. The indication will help us to identify easily then we can go further to control the fire. Below mentioned points are the indications of scavenge fire.
- Increase in scavenge temperature of one unit as compared to the others.
- The affected cylinder will not receive the fresh air leads to an increase in the exhaust temperature.
- Engine revolution drops as the power generation in the affected cylinder is reduced.
- Black smoky exhaust due to incomplete combustion in that unit.
- Spark, flame or smokes at scavenge drain from scavenge air boxes.
- Rise of pressure and temperature of exhaust and scavenge system.
- Visible evidence of fire if the transparent window is fitted.
- Increase in temperature of cooling water outlet temperature of the affected unit.
- Rough operation of the engine with a slight rpm drop and turbocharger may cause surging.
Action to be taken in scavenge fire –
Once confirmed that scavenge fire is there then the first aim should be too confined the fire in that area and extinguished to avoid any secondary fire and to the minimum the damage of the engine. For this, we can proceed with these steps –
- Inform bridge and reduce the RPM of the main engine.
- Cut off the fuel supply of the affected unit.
- Maintained the Cooling of the jacket and piston.
- Increased the cylinder lubrication.
- Shut off the drain to prevent the flow of flame or spark in the engine room.
- Personnel should keep clear of scavenge relief door to avoid the injury.
Fire should be extinguished after some time. If the fire spread in the other scavenge spaces along with the scavenge manifold, then –
- Inform bridge and stop the main engine
- Stop the fuel oil supply.
- Open indicator cocks, engage turning gear and slowly turn the engine to prevent seizure of the engine.
- Maintained the engine jacket cooling and cylinder lubrication.
- Cover the turbocharger fresh air inlet( or shut off the scavenge air duct flap valve before the engine).
- Release the smothering gases (Steam or CO2) in scavenges space to extinguish the fire.
- Maintain the boundary cooling of scavenge space.
- Wait to complete extinguishing of the fire.
- Before opening the scavenge door ventilate the space thoroughly if CO2 is released to extinguish the fire.
Inspection after Scavenge Fire –
After complete extinguishing of the scavenge fire, we need to cool down and ventilate the space thoroughly. Further, we need to inspect the area and component to avoid failure and lead to any breakdown of components.
Note: component affected by scavenge fire includes piston rod, cylinder liner, stuffing box, piston and rod alignment, scoring or cracks in the liner and tie rod. Thoroughly inspect the affected space for any damage and cause of scavange space.
- Check the piston alignment which may upset due to intense fire.
- Check the spring of relief device.
- Turn the engine and check the movement of the piston in the liner, check for any occurrence of binding at liner which will indicate the misalignment.
- Check the piston rod packing spring, which may have become weakened by overheating.
- Check the piston crown for any crack or any leakage from the injector.
- Check piston rings and liner for any distortion or reddish burning mark on the surface.
- Check diaphragm and frame near the affected area.
- Check guides and guide shoes of the affected unit.
- Check tension and tightness of the tie bolts.
Preventions of Scavenge Fire –
It will be really better if we can avoid such fire. Being a good watchkeeper it’s our duty to avoid such accident and save the life and for this, we can proceed with these below-mentioned steps –
- Check the scavenge space at regular intervals. It should always be open in running condition.
- Clean scavenge space as per PMS or manual.
- Avoid the excess of cylinder lubrication which cause accumulation and act as fuel in scavenge fire.
- The time has to be checked as per PMS, in case of timed lubrication.
- Piston rod stuffing box must be maintained to prevent oil ingress in the scavenge space.
- Piston rings must be properly maintained and lubricated adequately because of leakage of it cause blow past.
- Prolong engine or any cylinder over loading to be avoided
- Cylinder liner wear must be within admissible limits, to be checked as per manual.
Scavenge Space Protection Devices –
- Electrical temperature sensing device fitted within the trunking, which will automatically sound an alarm in the event of the excessive rise in local temperature.
- Fixed fire extinguishing system may be CO2, Dry Powder or Steam
- Pressure relief valves consisting of self-closing spring-loaded valves are fitted and should be examined and tested periodically.
Hope this blog has given you a basic information about Scavenge space fire. Please do read about types of scavenging. In my next blog, we will discuss exhaust gas fire and crankcase explosion. Suggestions and discussions are always welcomed.