Pressure-Relief Valves

Pressure-relief valves limit the maximum pressure in a hydraulic circuit by providing an alternate path for fluid flow when the pressure reaches a preset level. All fixed-volume pump circuits require a relief valve to protect the system from excess pressure. Fixed-volume pumps must move fluid when they turn. When a pump unloads through an open-center circuit or actuators are in motion, fluid movement is not a problem. A relief valve is essential when the actuators stall with the directional valve still in shifted position.

A relief valve is similar to a fuse in an electrical system. When circuit amperage stays below the fuse amperage, all is well. When circuit amperage tries to exceed fuse amperage, the fuse blows and disables the circuit. Both devices protect the system from excess pressure/current by keeping it below a preset level. The difference is that when an electrical fuse blows, it must be reset or replaced by maintenance personnel before the machine cycles again. This requirement alerts electrician’sabouta possible problem before restarting the machine. Without the protection of a fuse, the electrical circuit would finally overheat and start a fire.

Similarly, in a hydraulic circuit, a relief valve opens and bypasses fluid when pressure exceeds its setting. The valve then closes again when pressure falls. This means that a relief valve can bypass fluid anytime, or all the time, without intervention by maintenance. Many fixed-volume pump circuits depend on this bypassing capability during the cycle, and some even bypass fluid during idle time. A well-designed circuit never bypasses fluid unless there is a malfunction, such as a limit switch not closing or an operator over-riding the controls. This eliminates most overheating problems and saves energy.

There are two different designs of relief valves in use: direct-acting and pilot-operated. Both types have advantages and work better in certain applications.

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