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Medium speed engines. Scavenge fires

For a scavenge fire to occur there must be the three sides of the fire triangle: air, fuel and a source of ignition. The removal of any one of these would not only extinguish fire but prevent it occurring in the first place. It is impossible to prevent air flow through the scavenge spaces as scavenging implies air flow. However, fuel should never be present in the scavenge spaces so a clean scavenge space can never ignite. Ignition itself could occur were there to be any blow past the piston or were the piston to begin to seize in the liner. It may even be possible for the piston rod gland to overheat to the point where it could cause ignition.

The easiest way to avoid scavenge fires is to ensure that the scavenge spaces are maintained clean and free from oily deposits. The ease of this depends, to some extent, upon the engine design, with respect to its breathing, and in particular depends on the pressure of exhaust gases still in the cylinder at the opening of the scavenge ports. However, the engineer can limit fouling of the scavenge spaces by ensuring that combustion is being carried out as cleanly and crisply as possible; there is good fuel timing, atomisation, penetration, air fuel ratio and so on. Similarly, the lubrication of the ring pack needs to be controlled to prevent a build up of lubricating oil in the scavenge spaces. There is a possibility that oil may pass over with the scavenge air from the turbocharger, particularly if the air filters are fouling up. Oust brought in with the air may also be a source of fuel within the scavenge spaces. The liner/ring interface should be well maintained. Use good quality rings and renew them and the liner in good time.
A scavenge fire will manifest itself as a drop in power. There will also be a rise in the exhaust and jacket temperatures local to the fire area, the turbochargers may begin to surge and a smell of smoke/hot paint will be apparent. Automatic alarm systems are available, many of which are wires, the resistance of which alter with changes in temperature, the corresponding change in current flow activating an alarm.
Scavenge fires are capable of generating conditions favourable to a crankcase explosion because they put heat into the top plate of the crankcase. That is one reason why the fire should be extinguished as soon as is reasonable possible. 
When a fire occurs, the watchkeeper should, apart from raising the alarm, reduce speed (checking with the bridge first), shut the fuel off the affected unit, and slightly increase the cylinder lubricating oil to the affected unit to prevent, if possible, seizure and wear. If the fire does not burn itself out quickly then stop the engine (bridge), put in the turning gear and commence turning the engine. Without turning it is possible that the localised overheating of a piston or piston rod may lead to distortion and subsequent problems. The tie bolts are generally shielded by tubes from the extreme temperatures. Otherwise they may ‘stretch’ and relax their grip on the structure. In any case it is prudent to check the tension of these bolts after a large scavenge fire. 
As with crankcase explosions, the doors should not be removed until the fire has subsided and temperatures have dropped. The early ingress of air may allow an explosion to occur. Air flow though the engine will occur naturally, even when it is stopped. This is due to the convection currents generated by the heat in the uptakes. Wrapping canvas around the turbocharger filters can limit this. The injection of carbon dioxide will rapidly extinguish the fire, but time must be allowed to pass before opening the doors, for a hot spot could cause re-ignition. Carbon dioxide could cause thermal cracking of the hot components within the engine. The use of dry powder would add to the cleaning up required once the emergency is over. Steam is ideal in this situation, provided the line is adequately drained first and the valves have not seized with corrosion. However good it is as a fire fighting agent, steam is not recommended because of the problems associated with corrosion, water slugs preceding the steam and the need to generate it in the first place.

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