Difference Between Pressure Regulating Valves and Pressure Relief Valves

We represent Wanner^® Engineering for sealless positive-displacement pumps and pressure regulating valves.

The names “pressure regulating valve” and “pressure relief valve” get used as if they were the same part. They are not. One is a continuous control device. The other is an emergency device. The comparison below is how they differ in design and where each belongs.

Pressure Regulating Valves

Purpose

A pressure regulating valve is a mechanical way to hold downstream flow and pressure by continuously bypassing the excess. The pump is sized with that bypass in mind.

Mechanics

The body has three ports: inlet, outlet and bypass. Inlet and outlet sit 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 is the pressure that lifts the plunger off the seat.

Setting the Pressure

With the pump running, tighten the cap screw slowly until the discharge gauge reads the maximum pressure you want.

Operation

Discharge pressure cannot climb above that set point. Extra flow compresses the spring farther and leaves through the bypass. If the discharge 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. We still size the pump so some flow bypasses in normal service. Regulation is smoother when the valve is already cracked open than when pressure has to spike to unseat it first.

On spray nozzles or a filter, bypass flow changes with the process. Worn nozzles take more flow, so less is bypassed and nozzle pressure holds. A clogged filter will not push system pressure past the set point; differential across the element rises as more flow goes to bypass.

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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