Q.What is check valve?
A check valve, clack valve, non-return valve (NRV) or one-way valve is a valve that normally allows fluid (liquid or gas) to flow through it in only one direction. Check valves work automatically and most are not controlled by a person or any external control; accordingly, most do not have any valve handle or stem.
There are various types of check valves used in a wide variety of applications. Basically, check valve is doing the function of avoiding reverse flow no matter what the type is