Check Valves are used to control liquid flow in one direction. A check valve, clack valve, non-return valve or one-way valve is a valve that normally allows fluid (liquid or gas) to flow through it in only one direction. 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. Spring actuated Check valves are valves that a kept closed by a spring in one direction, This check valve can mounted and will operate in any orientation. However the springs used are usually made of stainless steel or a type of plastic. Gravity actuated check valves use a weighted ball or flap that must be mounted in a specific orientation to work properly. These valves are used quite often, when corrosive liquids that could attack components in other types of check valves.