In brief
A pressure relief valve opens as soon as the system pressure reaches a set value, thereby protecting pumps, lines and consumers against overload.
Contents
What is Pressure relief valve?
The pressure relief valve is one of the pressure control valves and is present in almost every hydraulic circuit, usually directly downstream of the pump. When the set pressure is reached, a valve element lifts against a spring and diverts the excess flow to the tank. This keeps the pressure limited, even when a cylinder is at its end stop or a line is blocked.
Direct-acting valves are simple in design and respond quickly, but are better suited to smaller flow rates. Pilot-operated valves use a small pilot valve that opens the main spool; they operate with a stable characteristic even at high flow rates and can be remotely controlled or electrically unloaded. Key parameters are nominal size, pressure stage or adjustment range, maximum flow rate and design as a cartridge, subplate or in-line valve. It should be distinguished from the safety valve for pressure vessels, which has its own inspection and approval requirements.
Wear on the seat, broken springs or dirt lead to leakage or an incorrect opening pressure. Frequent operation generates heat. As a central protective element, the pressure relief valve should be checked for leakage and cracking pressure during inspections. In the worst case, a sticking valve can overload the pump, hoses and cylinders.
In practice: maintenance and procurement
For a replacement, the nominal size, design, adjustment range and set pressure must be known; the set pressure should be documented before removal. Adjustments to the valve must only be made by qualified personnel and with a pressure gauge check.
Synonyms and abbreviations: relief valve · PRV · overpressure valve · pressure limiting valve
Frequently asked questions
What is the difference between a pressure relief valve and a pressure reducing valve?
The pressure relief valve limits the pressure at its inlet by opening when the set value is reached and diverting oil to the tank; in normal operation it is closed. The pressure reducing valve, by contrast, reduces a higher inlet pressure to a lower, constant outlet pressure for a sub-circuit; it is open in normal operation.
What is the difference between a direct-acting and a pilot-operated pressure relief valve?
In a direct-acting valve, the system pressure acts directly against the spring of the valve element. This is simple and fast, but the characteristic rises noticeably as the flow rate increases. In a pilot-operated valve, a small pilot valve opens first and relieves the main spool. As a result, the pressure remains stable even at high flow rates.
What information is needed to replace a pressure relief valve?
Important are the design as a cartridge, subplate or in-line valve, nominal size, mounting pattern or thread, pressure adjustment range, set pressure, maximum flow rate and type of adjustment. For pilot-operated valves, remote control or electrical unloading must also be specified. The nameplate or the marking stamped on the valve body provides the exact designation.
Is a pressure relief valve the same as a safety valve?
Not quite. Pressure relief valves in hydraulic systems are functional components that limit the working pressure. Safety valves on pressure vessels and steam systems are safety accessories and are subject to their own standards as well as inspection and marking obligations. Which requirements apply depends on the plant and the relevant regulations.