Check valve type SCheck valve type S is used to allow fluid flowing in one direction and leakage-free closure In opposite direction .When fluid flow via port P1, fluid push poppet (2) to move right way overcoming the spring(3), then connects the chamber P1 and P2. When fluid flows P2 to P1, poppet (2) moves left way and then the check valve closes. As for the check valve without spring, the fixing position should ensure that the poppet can locate itself closed under its deadweight.S.pdf