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Pilot-to-close check valves
There is also a pilot-to-close check valve, but it is seldom used. It is rarely necessary to have a valve that always stops flow in one direction and also is capable of stopping it the opposite direction.
Notice in the cutaway view in Figure 10-7 that the spring-loaded poppet does not have communicating holes through it to the spring chamber. Flow passes freely from inlet to outlet until a pilot signal is fed to the pilot port. Because the pilot port side of the main-flow poppet has more area than the inlet side, this valve can be closed against free flow.