Scavenge fires

Oily carbon deposits build up in the scavenge trunking during normal operation of the engine. Drains are provided in order to remove such deposits and so help keep the scavenge trunking reasonably clean. 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.

• 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.

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

• 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 turbocharger surging as the fire consumes the oxygen. Sparks are seen issuing from the drains and there is a temperature rise in the scavenge space.

Scavenge belt relief door

• Fitted to both ends of the scavenge belt and set to lift slightly above the maximum normal working scavenge air pressure