A valve is a device to permit relatively unrestricted flow of gas in one direction but to block all flow in the opposite direction. Each operating end of a cylinder must have two valves  or sets of valves, one to admit gas before compression (Suction Valve), and the other to discharge gas after compression (Delivery/Discharge valve).

All valves in use today are automatic in operation – they open and close solely through differential gas pressure between the inside of the cylinder and the cylinder gas passage, with the assistance of a spring in the closing.

Every time a valve opens there is an impact against the stop plate and every time it closes there is impact against the seat. Although the majority of valve failures are due to impact (the discharge impact being the more severe) there are other causes often not so apparent.