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Of all the fires that are related to marine engines, scavenge fires are considered the most hazardous. They have been a reason for numerous disastrous accidents on ship and they still impose a major threat to the safe operation of marine engines. So what are these scavenge fires and why do they occur? Also what are the preventive measures that needs to be taken to prevent such fires? Let's find out.
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Causes of scavenge fires
Scavenge fires occurs when the oil and carbon particles from the engine cylinder accumulates in the scavenge space. This partially burned oil and carbon particles are generated as a result of numerous factors. The chief among these factors are leaky or defective injector, improper setting of fuel timing, usage of low grade of fuel, insufficient supply of scavenge air, chocked exhaust, overloading of the engine, piston rings leakage, defect in cylinder liner and improper lubrication.
These carbon particles along with the half burned oil gets concentrated in the scavenge spaces where they get heated due to high temperatures. This mixture in presence of a limited supply of air and an ignition source can lead to massive fires. This ignition can be high temperature particles or a spark from the leaky piston rings. In this way the initiation of scavenge fire occurs.
Take a look at the sketch below and you will clearly understand the scavenge system setup on ships. The diagram is clearly labelled and self descriptive
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Indication of scavenge fire
It is better to read the signs earlier rather than facing grave consequences later. Since you cannot tell about a scavenge fire from outside unless it has reached serious proportions here are a few symptoms which need to be observed for signs of a possible scavenge fire. Needless to say, these symptoms can only be noticed if the marine engineer is alert all the time.
- A sudden loss in power and high exhaust temperatures
- Irregularity in the running of the engine or excessive gases in the exhaust
- Scavenge fires also leads to heavy surging of turbochargers and sparks from the scavenge drains.
If you notice the above signs be sure to take appropriate action and inform the chief engineer about the situation. If you are the CE yourself then you would know what should be done, otherwise you can continue reading the article to know what needs to be done in such a situation. First take a look at the image below which shows the second symptom mentioned above and should be a cause for concern. Of course since the people on the bridge are more likely to observe such signs they should also be briefed that in case they notice unusual exhaust they should call up the engine room.
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Actions to be taken
In case you notice any of the above symptoms or otherwise have an inkling that a scavenge fire might have started, the first thing is to inform everybody and not to panic about the situation. Then try to following the steps listed below
- Reduce the speed of the engine
- Shut the fuel and air supply of the whole engine/particular cylinder
- The scavenge drains should be tightly shut
- If the things don't come to normal after sometime, a thorough inspection of the engine's trunking and cylinder needs to be done.
- Fire extinguishing elements such as carbon dioxide, dry powder etc should be supplied to the trunking to prevent fire as a result of generation of hot spots.
- It should be kept in mind that no attempts are made to open the scavenge trunk or it might lead to a severe explosion
- It is good not to panic but it is foolishness to portray yourself as a hero so just ensure that a safe distance is maintained from the scavenge trunk and relief valves by everyone in the vicinity
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Since prevention is always better than cure, a few simple steps would help to ensure that a scavenge fire never occurs on ship or at least it minimizes the chances of its development. Here are these simple tips on avoiding scavenge fires.
- Scavenge trunking and scavenge ports should be kept clean
- Piston rings, cylinder liner, piston skirt and the water seals should be inspected for any defect
- Vibration of the engine should be prevented by tightening the tie bolts properly.
- Routine inspection and maintenance of the trunk should be carried out
- The unburned oil and carbon particles should be removed from the trunk
- Scavenge drains should be drained and cleaned periodically to prevent choke up
- High temperature alarm system should be fitted to provide an indication of rise in temperature above the normal limit
- Pressure relief valves should also be tested on regular basis
- Fire alarm should also be tested on regular basis.
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Diesel Engines by A.J Wharton
Scavenge System of Ships - Marine Engineering Knowledge Website by Brian