One of the most common reasons for a fire in a ship’s engine room, is that scavenge fires are the deadliest of all fires. They have caused several major accidents on ships in the past and it is for this reason that they are considered to be the most dangerous natural hazard that sailors must guard against.
In order to understand the scavenge fire, it is important to know the basics. In this article, we have compiled everything you wanted to know about scavenge fires – from causes to effects, actions, and preventions. With this info you’ll be aware and in a better position to tackle them.
- What is Scavenge Fire
- What are the Causes of Scavenge Fire
- What are the indications of Scavenge Fire
- Actions to be taken in event of Scavenge fire
- How to prevent scavenge fire in engine
What is Scavenge Fire
Carbonized lube oil, unburnt fuel oil, and carbon from the residual products of the combustion spaces are accumulated in the scavenge spaces with the running of the engine. Under certain faulty running conditions of the engine, these may ignite causing a fire in the enclosed scavenge space, known as scavenge fire.
The above is the standard textbook definition that can be used to score marks in your exam. But lets further break down what is scavenge fire.
Basics of Fire.
For a Fire to occur, the Fire Triangle needs to be completed. The components of the Fire Triangle are as follows.
- Oxygen O2
- Source Of Ignition
Well, all the above mention components are present in the Scavenge Space of the main engine. Oxygen is available in abundance in the Scavenge space, Fuel is injected into the combustion chamber unburnt residues of which find their way into the scavenge spaces due to piston movement.
The source of ignition is provided by the compression of the gases in the compression cycle and by the heat generated during the expansion stroke. These hot gases may pass through the piston rings into the scavenge spaces raising the temperature. Also, friction between liner and piston may result in the generation of heat source.
Note: The above fire triangle further needs another component where the air mixture should be neither too lean nor too rich in order to catch fire
After the basics of the Fire triangle now let’s revisit the definition of Scavenge Fire and break it down for ease of understanding.
Carbonized lube oil, unburnt fuel oil, and carbon from the residual products of the combustion spaces are accumulated in the scavenge spaces
When the Main Engine is running Fuel is injected in the combustion spaces. This fuel can find its way to the scavenge spaces due to piston movement. The Cylinder Lubrication if in excess may also be scrapped by the piston rings and accumulate in the scavenge space.
As the combustion occurs its never an ideal case where all the fuel is burnt completely to generate energy. Some of the fuel may remain in unburnt form and that may accumulate in the scavenge fire as the temperature there is relatively low compared to the combustion space.
The accumulation of such fuel and lub oil and residual carbon products may build up over a period of time which if overlooked may result in promoting scavenge fire.
Under certain faulty running conditions of the engine
Faulty running conditions of the engine. What does that exactly mean. Some examples of the faulty running conditions of engines can be as follows
- Faulty fuel injector
- Improper cylinder lubrication.
- Worn out engine components.
- Lack of maintenance
Further down will cover more reasons for scavenge fire in depth that will cover the engine condition. These conditions will increase the pile-up of the above-mentioned carbon residues fuels and lubes in the scavenge space.
With the rich mixture and introduction of a source of ignition, the above may lead to a fire in the enclosed scavenge space of the engine.
Now let’s understand the Causes of Scavenge fire in engines.
What are the Causes of Scavenge Fire
Reasons leading to Scavenge fires
The cumulative effect of the following faulty engine running condition and other unavoidable conditions may lead to scavenge fire.
- Dirty or fouled scavenge space due to the following.
- Lack of maintenance
- Ship calling ports frequently.
- No spares onboard, hence attitude to prolong the maintenance
- Lack of maintenance
- Poor combustion due to leaking fuel valves or improper timing.
- Excessive cylinder lubrication which is drained down to scavenge spaces.
- Poor Quality of Fuel may further lead to following
- Damage to fuel injectors
- Damage to combustion space like liner damage piston rings damage etc.
- Off-specs fuel received.
- Blow past of combustion products caused by following conditions in the engine.
- Excessive wear of the liner.
- The piston rings might be worn out or have loose ring grooves.
- Broken piston rings or rings seized in the grooves.
- insufficient axial clearance of the piston rings.
- Non-genuine spare parts.
- Overheated piston and improper heat disspation to the under piston area caused by
- Faulty atomization and injection pressure,
- Faulty fuel pump timing.
- Loss of compression.
- Engine overloaded.
- Insufficient circulation of coolant.
- Insufficient cooling due to the formation of scale.
- Vapour lock in the cooling spaces.
- The blowback of exhaust gases caused by the following
- Exhaust back pressure or deposits on exhaust ports.
- Fouling of grid before turbine inlet.
- Fouling of turbine blades.
- Clogged EGB or economizer gas outlet.
- The presence of fuel oil in the scavenge spaces due to
- Defective fuel injectors.
- Incorrect pressure setting of injectors.
- Improper fuel automisation due to faulty injector nozzles.
- fuel particles land on the cylinder liner due to excessive penetration.
By now we have a decent understanding of what is scavenge fire and what may be the causes of occurrence of scavenge fire. It’s time to know how to identify the scenario so that it can be adverted.
What are the indications of Scavenge Fire
Signs of scavenge fire and its effects on engine.
- Engine revolution drops as the power generation in the affected cylinder is reduced.
- Unstable operation of Turbocharger may cause surging.
- The exhaust temperature of the affected cylinder increases, due to a lack of fresh air.
- Black smoke with exhaust due to incomplete combustion in that unit.
- Discharging of spark, flame, or smokes through drain cocks from scavenge airboxes.
- Rise of pressure and temperature of the air in the air box below the piston.
- Visible evidence of fire if a transparent window is fitted.
- The cooling water outlet temperature of the affected unit is increased.
- In ships where the engine room is designed as UMS, temperature sensors are fitted at critical points within the scavenge spaces. So, activation would cause an automatic slow down of the engine.
- Paint getting peeled off or blister formation near the affected unit.
- The parameters in the control room may show huge fluctuations in temperature for the affected units.
Actions to be taken in event of Scavenge fire
How to Fight Scavenge fire
Action to be taken in case of a scavenge fire depends on the type of the fire, whether small or large. In the case of a large fire, the following signs will be easily visible while in the case of a small scavenge fire it may even go unnoticed if parameters are not properly monitored.
In case of scavenge fire All Engine crew to be informed to stay alert near the scavenge space to avoid any casualty.
For small Scavenge fires
- Start reducing the engine rpm and reduce it to slow or dead slow.
- Increase the cylinder lubrication of the affected unit. Special attention is to be given to this as this does not feed the fire. In case of an increase in fire do not increase the lubrication.
- The fire can be due to leaky fuel valves, so lift up the pump of the affected unit.
- Keep scavenge drain closed.
- Keep monitoring the scavenge and exhaust temperatures and let the fire starve and wait for it to burn itself out.
- After confirming that the fire is out start increasing the rpm slowly.
- Keep monitoring the scavenge temperature for any signs of re-ignition.
For large Scavenge fires
- Inform the bridge immediately, and stop the engine immediately if possible.
- Stop fuel oil booster pump.
- Open indicators cocks, engage turning gear and turn the engine to prevent engine seizure.
- Normal engine cooling and lubrication are maintained.
- Scavenge air duct flap valve before the engine is shut.
- Extinguishing the fire with a fixed fire fighting system for scavenge fire. This may be a CO2 system or a steam connection for smothering the fire.
- On a very old ship, if there is no fixed system then external cooling is provided to prevent distortion due to heat.
- Before opening scavenge door, ventilate the space if CO2 is released.
Now once the scavenge fire is under control a few steps are required to be followed in order to troubleshoot the cause of the fire.
Checks and Inspection after Fighting Scavenge Fires
- Intense fire can cause distortion and may upset piston alignment. It needs to be inpected to identify the cause.
- Check by turning the engine and watch the movement of the piston in the liner, check for any occurrence of binding at part of the stroke (Binding indicates misalignment of the piston)
- Check spring on scavenge space relief device, if the device was near the set of fire
- The piston rod packing spring also should be checked, which may have become weakened by overheating
- Check piston rings and liner for any distortion or reddish burning mark
- Check diaphragm and frame near affected part
- Check guides and guide shoes
- Check tension of tie bolts.
- Clean the Scavenge space to remove excess deposits to prevent further occurrence.
What can be done in order to prevent the occurrence of scavenge fire in the engine? Lets discuss them down below.
How to prevent scavenge fire in engine
- Clean scavenge space and drain at regular intervals.
- Keep scavenge space drain open at regular intervals.
- Excess cylinder lubrication must be avoided.
- In the case of timed lubrication, the time has to be checked as per PMS.
- Piston rings must be properly maintained and lubricated adequately.
- The piston rod stuffing box must be maintained to prevent oil ingress in the scavenge space.
- Prolong engine or any cylinder overloading to be avoided.
- Cylinder liner wear must be within admissible limits.
I guess I have covered almost everything about scavenge fire over here. In the next article will discuss more on the safeties installed in the engine to identify, fight and avoid casualities in the vicinity of the engine.
Have you ever faced a scavenge fire before? Till now I have noticed only a minor scavenge fire which was taken care of immediately. Do share your experience and doubts or any comments regarding the same in the comments section down below.