Discuss pilot operated check valve with neat sketch............

When pilot signal of pressurized oil is used to control movement of poppet in the check valve, it is called as pilot operated check valve. It is used when no flow characteristics of the valve is desired only for a portion of the system cycle. Figure shows the pilot operated check valve. A pilot piston is introduced below moving poppet. This pilot piston can move up by introducing pilot signal. Working: In normal position there is no flow from (A) to (B) because the movable valve poppet has blocked the flow. Now pilot signal is given through port (C). This oil will push up the pilot piston upwards, thereby compressing springs (S1). The piston rod of pilot piston will push the movable poppet in upward direction thereby compressing the spring (S2). Now the flow from (A) to (B) will start. As and when we cut-off the pilot signal the flow from (A) and (B) will continue. When pilot signal will be cut-off, spring S1 and S2 will expand and moving poppet will again block the flow from (A) to (B).