Check valves are self-activating safety valves that permit gases and liquids to flow in only one direction. The purpose of a check valve is to prevent process flow from reversing in the system which could damage equipment or upset the process. They are classified as one-way directional valves. Check valves are commonly found protecting pumps in liquid applications or compressors in gas systems where backflow could cause the pump or compressor to shut down. They are also applied in process systems that have varying pressures, which must be kept separate. They do not need an outside power supply to operate since they use the pressure drop created by the media flow.