Crankcase overpressure relief valves have three functions:
(1) Rapidly relieve excess pressure inside the crankcase
(2) Prevent flame inside the crankcase from escaping and causing further damage
(3) Rapidly close after the crankcase pressure is relieved to prevent the air from entering into the crankcase.
The figures show that the relief valves have a light spring that holds the valve tightly against its seat. The pressure inside the crankcase is relieved, and the spring closes the valve automatically. Figures show an image of a properly operating crankcase relief valve. The valve is still closed on the left of the picture while an internal explosion is about to open. On the right, the internal pressure has forced the valve open, compressing the spring while the hot pressurized gas, but not the flame, is vented to the atmosphere. Once the pressure is relieved, the compressed spring closes the valve.
API 618 requires a relief area to crankcase volume ratio of 3.0 in²/ft³ (683 cm² /m³), which is higher than any of the engine standards. With properly sized and installed overpressure relief valves, experience has shown that the risk of damage and injury from crankcase explosions can be eliminated. Most compressor and engine manufacturers offer them as standard equipment on large machines and options for smaller frame sizes.
The crankcase explosion relief valve (ERV) rapidly relieves excess pressure, prevents flame, and rapidly closes after the crankcase pressure is relieved.