Plastic Check Valve

Check valves are two-port valves, meaning they have two openings in the body, one for fluid to enter and the other for fluid to leave. 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. The bodies (external shells) of most check valves are made of plastic or metal.
An important concept in check valves is the cracking pressure which is the minimum upstream pressure at which the valve will operate. Typically the check valve is designed for and can therefore be specified for a specific cracking pressure.
There are several different types of check valves. A ball check valve consists of a ball that sits over the valve's opening. When liquid flows past the ball, it pushes the ball away from the mouth of the valve, which gives it room to flow through the pipe. When the liquid begins to flow the other way, however, the ball fits tightly over the valve opening, essentially plugging it.