Compound Relief Valve (pilot operated relief valve)

The "pressure override" which is caused by the direct-acting relief valve may find applications in some areas. However, in others it may result in a considerable input-power loss owing to the fluid lost through the valve during the wide "pressure override" band. Owing to the inertia of the valve poppet and the spring, there is sudden increase in pressure on the valve inlet side which gives rise to pressure peaks beyond the adjusted "full flow pressure" and may well exceed the maximum permissible pressure rating of other components within the circuit. The compound-relief valve lowers the "pressure override" to the minimum i.e. approximately 100-150 kPa. It also eliminates pressure peaks almost completely, thus the compound relief valve provides a safe and economical solution, and is the most commonly used type in industrial systems.

The adjustment screw 9 is used to adjust the relief valve setting pressure. According to the Pascals Law; as long as the pressure in the hydraulic system is below the setting of the relief valve, the static pressure in chambers 2, 4, and 6 remains equal . When the system pressure is enough to force pilot poppet 7 off its seat ("cracking pressure"), the fluid starts to flow to the tank through internal drain 12 at a very low pressure. The resulting pressure imbalance, due to the pressure drop (p) across orifice 3, forces valve piston 11 upwards which results in compression of the piston spring 10 and opening of the tank port T, which prevents a further rise in pressure. Increased flow through the valve causes the piston to lift further off its seat, but as this only compresses a very light spring, very little "pressure override" is encountered.