A compound pressure relief valve is one which operates in two stages. They are designed to accommodate higher pressures than direct acting relief valves at the same flow rate capacity. To have a broad understanding of how a compound pressure relief is internally designed, a cutaway view of an actual valve manufactured by Vickers INC., Detroit is shown in Figure 6.18.
The first stage of the pilot relief valve includes the main spool which is normally closed and kept in position by a non-adjustable spring. The pilot stage is located in the upper valve body and contains a pressure-limiting poppet, which is held against a seat by an adjustable spring. The lower body contains the port connections. The balanced piston in the lower part of the body accomplishes diversion of the full pump flow.
In normal operation, the balanced piston is in a condition of hydraulic balance. Pressure at the inlet port acts on both sides of the piston, through an orifice, that is drilled through the large land. For pressures less than the valve setting, the piston is held on its seat by a light spring. As soon as the pressure reaches the setting of the adjustable spring, the
poppet is forced off its seat. This limits the pressure in the upper chamber. The restricted flow through the orifice into the upper chamber results in an increase in pressure in the lower chamber. This causes an imbalance in the hydraulic forces, which tends to raise the piston off its seat. When the pressure difference between the upper and the lower chamber reaches approximately 1.5kg/cm3 (approx. 21 psi) the large piston lifts off its seat to permit flow directly to the tank.
If there is a flow increase through the valve, the piston lifts further off its seat. However, this compresses only the light spring and hence very little override occurs. Compound relief valves can also be operated remotely by using the outlet port from the chamber above the piston. This chamber in turn can be vented to the tank through a solenoid-operated direction control valve.