What Is a Pressure Relief Valve?

A pressure relief valve (PRV) is a safety device that opens automatically when pressure in a hydraulic or pneumatic system rises above its set value, releasing fluid or gas so that pressure cannot climb further. When pressure falls back to a safe level the valve closes again. Relief valves protect pumps, pipework, vessels and the people working near them from overpressure.

How a pressure relief valve works

In the basic spring-loaded design, a valve element is held on its seat by a spring. The spring is adjusted so that its force matches the force of system pressure acting on the element at the desired set pressure. While pressure stays below that value the valve stays closed. Once pressure overcomes the spring, the valve lifts and the excess fluid is routed away, in a hydraulic system typically back to the reservoir, until pressure drops and the spring closes the valve.

In practice valves do not open and close at exactly the same pressure. The valve begins to open at its cracking pressure, the flow through it grows as pressure rises, and it reseats at a somewhat lower pressure than it opened. These characteristics are what a relief valve test measures.

Types of pressure relief valves

Direct-acting relief valves: the system pressure acts directly against the spring. They are simple and fast, and suit smaller flows.
Pilot-operated relief valves: a small pilot stage controls a larger main stage, which allows large flows with a more stable set pressure.
Safety valves and relief valves: the terms are often used for valves in gas, steam and liquid service, and the precise definitions depend on the applicable code. Safety valves in gas or steam service usually open fully at once ("pop"), whereas relief valves in liquid service usually open in proportion to the overpressure.

A relief valve is one kind of pressure control valve; see control valve types for the other valve families.

Why relief valves matter in pressure testing

A test bench takes components to high pressure on purpose, so a pump, a faulty regulator or a closed valve can push pressure past the intended limit very quickly. A correctly set relief valve limits the pressure the equipment and test article can see. In high-pressure systems more than one protection device may be fitted, and the relief valve setting is chosen to sit above the test pressure yet below the maximum allowable working pressure of the weakest component.

How relief valves are tested

A relief valve is tested to confirm it opens and closes where it should. The usual checks are:

Set (cracking) pressure: the pressure at which the valve begins to open.
Relieving behaviour: how the valve passes flow as pressure rises above set pressure.
Reseat pressure: the pressure at which the valve closes again.
Seat tightness: leakage through the closed valve below set pressure.

The Neometrix safety and relief valve test bench page describes engineered-to-order benches that check set pressure, relieving behaviour, reseat and seat tightness, using dry air or nitrogen for gas service and water for the liquid pattern. For hydraulic pressure valves, the hydraulic control valve test bench and the valve test rig cover related valve testing. See also the hydraulic valve testing glossary, which has entries for pilot-operated relief valves and cracking pressure.

Frequently asked questions

What is a pressure relief valve?

It is a safety device that opens automatically when system pressure exceeds its set value, releases fluid or gas to limit the pressure, and closes again when pressure returns to a safe level.

What is the difference between a relief valve and a safety valve?

Both protect against overpressure. In common usage, safety valves are associated with gas and steam service and open fully at once, while relief valves are associated with liquid service and open in proportion to the overpressure. The exact definitions depend on the applicable code.

What is cracking pressure?

Cracking pressure is the pressure at which a relief valve first begins to open and pass a small flow. It is normally lower than the pressure at which the valve passes its full rated flow.

Where does the fluid go when a relief valve opens?

In a hydraulic system it is normally routed back to the reservoir. In gas or steam systems it is discharged to a safe location through a vent or discharge line.

Why are relief valves tested regularly?

Springs relax, seats wear and debris can hold a valve open or stuck shut. Periodic testing confirms the set pressure and seat tightness are still within tolerance.

To discuss relief valve testing, contact the Neometrix team.