*They are employed to action pilot pressure and drainage, thus, for achieving the sequence control, pressure releasing control, counter-balancing control.
*The check valves are generally used at bi-pressure circuiting to prevent pressure auto dropping.
*A sequence valve is a pressure-operated, normally closed, poppet or spool valve that opens at an adjustable set pressure. Some designs use a spring acting directly on the spool or poppet, others are pilot-operated.
*A sequence valve always has an external drain port to keep from trapping leakage oil. Trapped fluid modifies set pressure at best or keeps the valve from opening at worst.
*Fluid at the inlet port of the valve cannot pass to the secondary circuit or outlet port, until reaching set pressure.
*Upon reaching set pressure, the valve opens enough to let excess pump flow pass on to the second operation.
*The check valve built into both valves is there to provide free flow in the reverse direction. In effect, it takes the valve out of the circuit when flow is reversed.
*The sequence valve is used to ensure that a certain pressure level is achieved in one branch of the circuit before a second branch is activated.