Under certain conditions the deposits may dry out and ignite. Piston blow past due to sticking or broken piston rings, or even excessive liner wear, is a major cause of the problem. Faulty combustion due to late injection or incorrect atomisation may also be responsible as may blow back through the scavenge ports caused by a restriction in the exhaust.
In all cases the increased temperature leads to a drying out of the oily deposits in the air box and subsequently ignition takes place. A scavenge fire can cause serious damage to the piston rod diaphragm gland as well as leading to possible distortion of the air box and cracking of the liner. Tie rod tension will almost certainly be affected.
The worst case scenario for a scavenge fire is it leading to a crankcase explosion.( B&W on some designs fit a cooling jacket between the air box and crankcase to help prevent this.)
The fire may also spread outside the scavenge box due to relief doors leaking or oil deposits on the hot casing igniting. For these reasons a scavenge fire should be dealt with as quickly as possible.
B&W and other engine builders recommend that in the event of a fire the engine should be slowed as soon as possible and preferably stopped. The turning gear should be put in and the engine continuously turned to prevent seizure. Air supply should be cut off by enclosing the turbocharger inlets, for mechanically operated exhaust valves the gas side should also be operated, (hydraulically operated exhaust valves will self close after a few minutes). The individual isolating valves on the scavenge drains line are close to prevent the fire transferring to other boxes. Boundary cooling may be employed.
Fixed extinguishing mechanism should be used at the earliest possible time. When the fire is extinguished the air box is allowed to cool, then the air boxes should be well cleaned, the tie-rod tension checked. The causes of the fire should be ascertained and remedied.
Dry powder- will cover the burning carbon and oil but is messy. As the fire may still smoulder below the powder care must be taken when the scavenge doors are removed as the powder layer may be blown away.
Steam-plentiful and effective
Water spray- perhaps the ideal solution giving quick effective cooling effect to the fire.
Indications that a fire is imminent are a reduction in flow through the drains and a temperature rise in the scavenge. This temperature rise can be detected by temperature wires or mats.
When a fire is in progress there is a slow down of the engine with the blowers surging as the fire consumes the oxygen. Sparks are seen issuing from the drains and there is a temperature rise in the scavenge space.Fire fighting media
Carbon dioxide- will put out a fire but supply is limited. Susceptible to loss if dampers do not effective prevent air flow
