Scavenge Fires

The fire of flammable mixture (cylinder oil, unburned fuel and carbon) which can collect in the scavenge space of an engine. Fire in the scavenge air space can be extinguished by steam, water mist or CO₂.

One of the most common reasons for a fire in a ship’s engine room, scavenge fire is the deadliest of all fires. Scavenge fire has been the reason for several major accidents on ships in the past and it is for this reason that it is termed as the most dangerous cause for accidents on a ship.

Causes Of Scavenge Fire

1.Blow past :- From Combustion Chamber ( cylinder liner ), leakages of combustion products, residual fuel which is caused by damage

• Cylinder liner excessive wearing
• The piston rings of the engine might be worn out or have loose ring grooves
• Improper cylinder liner lubrication
• Insufficient axial clearance of piston rings.

2.Overheating of piston:-

• Due to failure of piston cooling oil flow .It causes piston overheating and dissipate heat to under piston area.
• Bad combustion , due to not proper atomisation,improper injection pressure,wrong fuel pump timing,loss of compression & overloading of the engine.

3.Presence of fuel oil :-Presence of un burnt fuel oil in the scavenge spaces because of

• Defective or Faulty fuel injectors
• Injectors pressure setting incorrect
• Fuel particles landing on cylinder liner due to excessive penetration.

4.Blowback of the exhaust gas :-It occurred due to high back pressure of the exhaust gas or deposits on exhaust port,fouling of grid before turbine inlet ,fouling of the blades of turbine,exhaust gas boiler chocked or economizer gas outlet.

Indications Of Scavenge Fire

You must aware of the below indication of the scavenge fire so that to identify and control the fire.

These are :-

1.Increase in scavenge temperature of one unit relative to the other.

2.The affected cylinder does not obtain fresh air leading to an rise in exhaust temperature.

3.The engine speed decrease due to power generation in the affected cylinder is reduced.

4.The exhaust smoke of engine is blacky due to incomplete combustion in that unit.

5.There is spark,flame or smokes at scavenge drain from scavenge air boxes.

6.The exhaust and scavenge system pressure and temperature increase.

7.Check visible evidence of fire through transparent windows fitted .

8.The outlet temperature of cooling water of affected cylinder is increased.

9.The operation of engine rough with slight drop in rpm and may cause surging of turbocharger.

10.loss of power , irregular running of the engine and reduction in rpm of the engine.

Action on Scavenge Fire

In the event of a scavenge fire occurring:

1. Contact the Bridge and request to slow down immediately and stop engine as soon as possible
2. Contact Chief Engineer
3. Stop engine, and shut off the fuel supply to the engine
4. Stop the auxiliary blowers
5. Operate the steam or CO2 extinguishing medium on the affected cylinders
6. Engage and operate turning gear with pre lubrication.

In the event of the scavenge fire occurring and that the main engine cannot be stopped, then the following course of action would be taken:

1. Contact the Bridge and request to slow down to the lowest power possible
2. Increase the cylinder lube oil feed rate to the affected cylinder
3. Lift the fuel pump on the affected cylinder, using the manual activation of the air cylinder (option fitted for fuel pipe leakage system).
4. Prepare the fire fighting equipment to tackle any fire that may be emitted from the scavenge receiver relief valve.
5. Move all personnel away from the engine, should the scavenge fire burn long enough to trigger a crankcase explosion.
6. The scavenge fire should burn out, once all the oil is consumed.
7. Stop the engine as soon as possible, to allow the fire (if still burning to be extinguished) and the affected areas within the engine to be inspected.

Inspections After Scavenge Fire

After the scavenge fire is completely extinguished, we need to cool down and ventilate the room thoroughly. In addition, we need to check the area and component in order to prevent failure and to cause any breakdown of the components.

Components affected by scavenge fire involves piston rod, cylinder liner, stuffing box, piston and rod alignment, marking or cracking of the liner and tie rod. Inspect the affected area thoroughly for any damage and cause of scavange area.

• Piston misalignment :- Check for the binding occurrence at the part of stroke ,it indicates the piston misalignment.
• Check spring on scavenge space relief valve.
• Check piston rings and liner for any distortion or redfish burning mark on the surface.
• Check the crown of piston for any crack or any leakage from the fuel injector.
• Check diaphragm and frame near affected area.
• Tensions and tightness of tie bolts.
• Guide and guide shoe of affected area should be checked.
• Check the movement of piston in cylinder liner by turning the engine.Check binding at liner which shows indication of misalignment.

Preventions Of Scavenge Fire

• Scavenge Spaces checked at regular intervals of time.It should be open always in running conditions.
• Clean scavenge space as per manuals or PMS.
• Excess cylinder lubrication will be avoided which cause accumulation and it act as fuel in scavenge fire.
• Engine piston rings must be maintained properly & lubricated adequately .If not then leakage of it cause blow past.
• The piston rod stuffing box must be properly maintained for preventing oil ingress on the scavenge spaces.
• Fuel injectors should be kept in good condition,timed correctly and load on each unit properly adjusted.