Difference Between Pressure Regulating Valves and Pressure Relief Valves

The terms “pressure regulating valve” and “pressure relief valve” are often used interchangeably even though they are not identical in function; one is a continuous control device and the other is an emergency device.

Pressure Regulating Valves

Purpose

A pressure regulating valve is a mechanical way to ensure downstream pressure doesn't exceed a set amount by continuously bypassing excess flow. The pump  is sized with that bypass in mind.

Mechanics

The body has three ports: inlet, outlet and bypass. Inlet and outlet are 180° apart. The bypass is 90° off that line.

An orifice sits over the bypass port. Above it is a spring-loaded plunger. Spring load is set with a cap screw on top of the valve. That load determines the back-pressure required to lift the plunger off the seat to enable bypass of flow.

Setting the Pressure

With the pump running, tighten the cap screw slowly until the discharge gauge reads the maximum pressure for your system.

Operation

Once the discharge pressure reaches the set point, the plunger will lift off the seat.  If the pump is dead-headed, the valve bypasses 100% of pump flow and keeps the pump and piping at the set pressure. When the process needs less pressure, less fluid is bypassed.

The set point can sit above normal operating pressure, in which case nothing bypasses. Its recommended the pump is sized to cause some flow to bypass at the operating pressure because it reduces pressure spikes associate with the initial lifting of the plunger form its seat.

When spray nozzles or a filter are downstream, bypass flow changes with the process. Worn nozzles take require high flow to maintain pressure, so less is bypassed and nozzle pressure holds even as the orifices wear. A clogged filter will not result in over-pressurization because the pressure regulating valve will bypass the excess flow.

Pressure Relief Valves

Purpose

A relief valve is a safety device. It protects the pump and system from overpressure.

A regulating valve also limits pressure, but a relief valve is normally closed. It opens only when system pressure exceeds the set point, passes just enough flow to bring pressure down, and reseats. Flow through it is not proportional. It is open or closed.

That is why a relief valve is for emergency protection, not for continuous regulation of a positive-displacement pump.

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