As a practical safeguard against explosions which occur in a crankcase, explosion relief valves or doors are fitted.
These valves serve to relieve excessive crankcase pressures and stop flames being emitted from the crankcase.
They must also be self closing to stop the return of atmospheric air to the crankcase.
Various designs and arrangements of these valves exist where, on large slow-speed diesels, two door type valves may be fitted to each crankcase or, on a medium-speed diesel, one valve may be used.
A light spring holds the valve closed against its seat and a seal ring completes the joint.
A deflector is fitted on the outside of the engine to safeguard personnel from the outflowing gases, and inside the engine, over the valve opening, an oil wetted gauze acts as a flame trap to stop any flames leaving the crankcase.
After operation the valve will close automatically under the action of the spring.
