Crankcase explosion relief valve

A crankcase explosion relief valve is a spring-loaded, flame-arrested vent that opens to relieve an oil-mist explosion in the crankcase and reseats to stop air re-entry that would feed a secondary blast. IACS UR M9 makes it mandatory on engines with a cylinder bore of 200 mm and above or a crankcase volume of 0.6 m^3 and above, with at least one valve per engine end and added valves for longer engines. It works alongside oil-mist detection (UR M10).