What Is a Hydraulic Pressure Relief Valve?

Hydraulic pressure relief valves protect operators and equipment.

Hydraulic pressure relief valves are valves used to prevent excessive pressure in hydraulic systems. As the normal pressures in hydraulic systems range from 1,000 pounds per square inch to more than 5,000 pounds per square inch, it is easy to see that a rupture in a hydraulic system can be hazardous to personnel and damaging to equipment.

Types

There are three types of hydraulic pressure relief valves. All three act to relieve the pressure in a hydraulic system by opening when pressure rises to near-dangerous levels. The differences are in the means of sensing the pressure and opening the valve. In a direct-acting valve, the system pressure pushes directly against a piston or spool that is held in place by a spring. When the pressure becomes greater than the spring force, the valve opens. In a direct-operated pilot-type valve, pressure in the system at the valve opens a pilot valve that in turns opens the relief valve. A remote-actuated pilot-type valve operates in the same way except that the pressure to the pilot valve comes from another location in the system.

Construction

Engineering a hydraulic pressure relief valve is complicated business. Hydraulic pressure relief valves must be strong enough to withstand pressures higher than their relief pressure. They must be designed to connect to the hydraulic system (either inline or off-line) itself, and they must provide a port for discharging the relieved fluid to a sump or a lower-pressure place in the system. The valve must open smoothly when the relief pressure is reached and close properly when the pressure falls, without any chatter.

Regulation

A hydraulic pressure relief valve should have a pressure-sensing tap, to which a pressure gauge and a means of adjusting the relief pressure can be installed. The adjusting mechanism is often an adjusting knob with a locking nut that prevents drift in the setting.

References

• "Audel Pumps and Hydraulics"; Rex Miller, Mark Richard Miller and Harry Stewart; 2004
• Hydraulic Supply: Eaton Vickers Relief Valves