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.
In order to understand scavenge fire it is important to learn the basics. In this article we have brought to you everything you ever wanted to know about scavenge fires – from causes to actions. Understand and fight scavenge fires the way it should be.
Getting the Basics Right: What is Scavenge fire?
For any fire to occur we need three elements which make the fire triangle. The three important elements for any type of fire are:-
1. Oxygen –this is available plenty in the scavenge space.
2. Heat source- this could happen because of blowing by of gases between piston rings and liner or as a result of any rubbing between two surfaces.
3. Fuel- this can be from un-burnt fuel, carbon or cylinder lubricating oil leaked into the space
When all these elements are present in a proportion ratio and lie within the flammable limit inside the scavenge space the later become a hot spot for eruption of fire. The fire which thus results is known as the scavenge fire.
Causes of scavenge fire
There are many reasons for scavenge fire. However, the main ones are as below:
1. Excessive wear of the liner.
2. The piston rings might be worn out or have loose ring grooves.
3. Broken piston rings or rings seized in the grooves.
4. Dirty scavenge space.
5. Poor combustion due to leaking fuel valves or improper timing.
6. Insufficient or excess cylinder lubrication.
Indications of scavenge fire
There are a few signs which indicates a scavenge fire. One should be extremely cautious in case any of the below mentioned conditions are observed.
1. Scavenge temperature will start increasing.
2. The turbochargers will start surging.
3. High exhaust temperature.
4. Loss of engine power and reduction in rpm. This happens because a back pressure is created under the piston space due to fire.
5. Smoke coming out of the scavenge drains.
6. The paint blisters will be formed on the scavenge doors due to high temperature but this will occur only in large fires and extreme cases.
Actions to be taken
Action taken in case of a scavenge fire depends on the type of the fire, whether small or large. In case of large fire the following signs will be easily visible – the peeling or blistering of paint, large reduction in engine rpm and surging of turbocharger.
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. Special attention to be given for this as this does not feed the fire. In case of increase of fire do not increase the lubrication.
3. The fire can 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 to burn itself out.
6. After confirming that the fire is out start increasing the rpm slowly.
7. Keep monitoring the scavenge temperature for any signs of re-ignition.
For large fires
1. Stop the engine immediately and engage turning gear, and keep engine rotating with turning gear.
2. Extinguish the fire with fixed fighting system for scavenge fire. This may be co2 system or a steam connection for smothering the fire.
3. In case fixed system is not available on very old ships an external cooling is provided to prevent distortion due to heat.
4. Once after confirming that the fire is extinguished. The scavenge space is allowed to cool down and later opened for inspection and cleaning of the scavenge space.
Reference: The Operation and Maintainence of Machinery in Motorships By N.E Chell