NON RETURN VALVE

Non-Return Valve or One-Way Valve, Check Valve, clack valve is a valve that normally allows fluid (liquid or gas) to flow through it in only one direction.
The bodies (external shells) of most check valves are made of plastic or metal. Check valves have two openings in the body, one for fluid to enter and the other for fluid to leave thus they are two-port valves. There are various types of check valves used in a wide variety of applications. Check valves are often part of common household items. Although they are available in a wide range of sizes and costs, check valves generally are very small, simple, or inexpensive. 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.
Typically the check valve is designed for and can therefore be specified for a specific cracking pressure. An important concept in check valves is the cracking pressure which is the minimum upstream pressure at which the valve will operate.