The check valves are non return valves that activate themselves and then allow the flow of liquid in a single direction. This kind of valve ensures there is no reversal in the process flow in a system which can later spoil equipment. They also flow in a single direction and are usually found in the compressors or liquid applications where the compressor can be shut down as a result of backflow. The check valve can also be used in process system which has different pressure and must be kept separate. You do not need an external power source for the smooth operation because they use pressure drop that is created as a result of media flow.
The check valves are used for the prevention as well as maintenance of pressure. They are mostly used as a backup whenever there is a pump failure so as to help compressors and reciprocate the functions whenever the system pressure is more than the needed capacity. Check valves can also be seen in industries such as chemical, petrochemical, steam, water refining etc.