• A sequence valve symbol is similar to a relief valve symbol. The main difference is that a sequence valve always has an external drain line — and often has a bypass check valve for reverse flow. • A sequence valve is a pressure-operated, normally closed, poppet or spool valve that opens at an adjustable set pressure. • 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. • For reverse flow capabilities, use the integral bypass check valve shown in the symbol.
Main idea • 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
• in some circuits the pilot signal to open the valve is from a source other than the line feeding it. • An external pilot-operated sequence valve opens and allows flow when a remote operation reaches a certain pressure.