Check valve
Also known as: non-return valve, one-way valve
Short definitionA valve that lets flow pass one way and closes by itself when the flow tries to turn back. When the system stops, it keeps gas and liquid from escaping to where they should not go.
Inside there is a weak spring and a disc or ball that it pushes onto the seat. Flow from the right direction lifts the disc; when flow stops or reverses, the spring seats it again. It needs no cable, setting or coil. The arrow on the body shows which way to fit it. Depending on type, a difference of 0.05–0.3 bar is needed to lift the disc, and that is a permanent pressure loss in the line.
In a project it appears mostly on the discharge outlets of parallel compressors, between condenser and receiver, and on hot gas defrost lines. Its size is set by flow rate, not by pipe diameter. In an oversized valve the disc stays half open, chatters and wears quickly; an undersized one costs too much pressure. No liquid should be left between two closed components; as it warms it expands and can burst the pipe. Since it is not fully tight, it cannot stand in for a solenoid valve in pump-down.