example, it would be undesirable for the landing gear to retract before the gear compartment doors are
completely open. A sequence valve actuated by the fully open door would allow pressure to enter the
landing gear retract cylinder.
The sequence valve consists of a valve body with two ports, a ball and seal spring-loaded to the closed
position, and a spring-loaded plunger. Compressing the plunger spring off-seats the ball and allows the
passage of fluid to the desired actuator. The typical sequence valve is mechanically operated, or it can
be solenoid-operated by means of microswitches. In either case, the valve is operated at the completion
of one phase of a multiphase hydraulic cycle.
Figure 2-14. Mechanically Actuated Sequence Valve.
A priority valve is installed in some hydraulic systems to provide adequate fluid flow to essential units.
The valve is installed in the line between a nonessential actuating unit and its source of pressure. It
permits free, unrestrained flow of fluid to nonessential units as long as system pressure is