Check valves, also called one-way directional valves, are safety valves that allow gases and liquids to flow in a single direction. The main purpose of check valves is to prevent the flow from going in reverse direction which would otherwise cause the system or equipment to damage, partially or completely, causing a major setback.
Check valves are applicable in systems such as compressors or pumps where flow of gas or liquid in reverse direction would cause the system to shut down completely. They are also applicable in conditions where the system possesses varied pressures and they all need to be kept separately. They operate on the pressure drop created by the medium in which they reside, hence they do not require any external power supply.