Check valves are important elements in any compression system. Singleand two-stage reciprocating compressors use a number of check valves in their systems. There is normally an input and output check valve in each pump head. These units will also incorporate an in-tank check valve, which is necessary during unload periods. Atypical in-tank check valve is usually in the form of a National Pipe Thread (NPT) extension fitting. They have a ceramic poppet that is loaded against the seat with a spring. The spring and poppet are held in place with a spring deck and snap ring. Most in-tank check valves are equipped with a 1/8-in. (F) NPT unloader port. These valves are available with either NPT input threads or compression fittings for use with copper tube.

above Figure shows an in-line check valve. The internals of these valves are similar to the in-tank valves and can generally be used in the same applications. These valves also carry a 1/8-in. (F) NPT port. Be cautious that the in-line check valve you select has a temperature rating that is suitable for compressor service. Do not select a flap type valve for compressed air service. These valves have the same outward appearance as the in-line check valve but are designed for liquid service and may not have the pressure and/or temperature rating required.