Technical notes of interest to Marine Engineers

Crankcase Explosions, Scavenging & Economizer Fires

Authored by: Martin Leduc - 10.1998

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Explosions can occur when three ingredients are present; air, fuel, and heat. In a Marine Diesel engine and its crankcase the danger of explosion can exist. When air mixes with an abundance of fine particles of oil, the mixture, also know as the stoichiometric mixture, is able to maintain combustion. Combustion occurs with the introduction of heat. Such a source of heat could be the friction of two surface of metal, hot spots, such as a failed bearing would provide. This hot spots can be the source of ignition, as well, serve to increase the danger as the oil coming in contact is vaporize, furthering the fuel available for combustion.

The gravity of explosions is proportional to the physical size of a given crankcase and it’s gas volume. But nonetheless explosions are destructive on any scale. Damages can be limited by explosion relief doors. Designed to vent the explosion but stop flames and the continuation of feeding oxygen to maintain the combustion.

Crankcase explosion can be prevented by monitoring the problem of oil mist. This is performed by monitoring the space’s oil mist density. The monitor's photoelectric sensor achieves this by measuring the mist density from the space and comparing it with a sample of air from outside the space. Once a given value as been exceeded, an alarm notifies the engineer. Fans can also be used to evacuate oil mist from the space and to create a sub atmospheric environment. This would help keep the atmosphere below flammability range.

Once a situation as been detected, immediate action is needed; primarily to prevent an escalation. To prevent it, a slow down of the engine is needed, subsequent stopping and investigation of the source of oil mist carried out. These sources would be hot surfaces, puncture oil lines and such. Precautionary measures such as readying fire equipment, shutting access to engine room and evacuating personnel should also be carried out.

Scavenging Fires

Scavenging air fires, on larger two stroke, slow speed engines, can occur where an excess of combustible, such a coke, builds up in the scavenging air manifold. This can be aggravated by oil collection at the bottom of the manifold. Coke can be a result of blow back, this could be as a result of poor exhaust valve operation, injector performance, or poor fuel quality which delays burn until scavenge ports are exposed.

Indications of a scavenge fire can be an increase of manifold temperature. Other indication are, violent blow backs, smoke and/or flames from the manifolds’ drains cocks.

Once detected, the engine should be stopped and introduction of fire extinguisher should be carried out. The manifold should be allowed to cooled, and then source of blow back investigate and remedied.

 
In early 2005, kind visitors submitted these suggestions on the Galley Wireless.

"With scavenging air fires you must stop the fuel by hanging the pump to the cylinder and run on reduced RPM until it goes out eventually (With help of steam), if you stop when there is a fire going on the risk is imminent for a crankcase explosion."

"Normal practice depends on how severe and who you talk to. (slow steaming RND 90 anyone) some say slow right down, put on your aux blowers, and put in extra cylinder lubrication. Others, stop the engine, keep the engine turning with the turning gear, aux blowers off, no cylinder oil. The reason for continuing to turn the engine is to prevent it seizing. This is also the reason some people advocate extra cylinder lube oil; on this point I'm not sure.

All ships I sailed on had a steam line connected to the scavenge air receiver, So you could smother the fire, using steam, you were less likely to thermally stress anything, blasting water or CO2 in would lead to a lot of cracks. Although all the steam lines I've come across had no drains, so the first blast would have been nice cool water."

"Regarding fires in the economizer you should reduce RPM or stop and try to extinguish them by the soot blowers (Steam). Do NOT stop the circulating pumps or you might melt the whole economizer, if some tubes burst this will also stop the fire, if you stop circulation you will probably have a lot of explanation to do for the company..."

"Again there are many schools of thought, to stop the engine or not, the debate of which allows more Air to the fire. All agree to sootblow though, and kept the circulating water flowing. Personally, I have only had smaller fires in the economizer, (crap fuel) and or these I would burn through, with a lot of sootblowing."

"All of the above you have the option of boundary cooling of course."

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