Pressure Relief Valves - How They Work

Pressure relief valves are designed to balance system pressure and prevent damage by maintaining pressures below a set threshold. These valves can reduce downstream pressure to a constant level when it exceeds a certain limit, maintain sustained pressures either downstream or upstream, and reduce pressure peaks or pulses to protect downstream equipment. They are crucial components in various industries, including oil and gas, chemical processing, and manufacturing, where precise pressure control is essential for safety and efficiency. This article provides a comprehensive overview of pressure relief valves, detailing their design, operating principles, applications, and installation procedures.

Table of contents

• Design and operation of direct-acting pressure relief valves
• Pressure relief valve applications
• Differences between pressure relief valves and safety valves
• Selection criteria
• Pressure relief valve installation
• FAQs

Design and operation of direct-acting pressure relief valves

A direct-acting pressure relief valve (Figure 2) has an inlet, outlet, and a flow control mechanism (e.g., poppet) supported by an adjustable spring. These valves have two designs for adjusting the spring: external and internal.

• External: An external design allows adjustment of the spring with a knob or handle that bolts onto the valve's outside.
• Internal: An internal design has a nut or screw within the valve's housing that turns in either direction to loosen or stiffen the spring. The internal design requires partial valve disassembly to adjust the spring's stiffness.

A direct-acting pressure relief valve limits pressure in a system rising above a set limit. The valve's spring (Figure 2 labeled A), which is either adjustable or not, determines the limit. If the system's media pressure is not high enough, it will not open the valve’s opening mechanism (Figure 2 labeled B).

The opening mechanism begins to open when the media pressure reaches the valve’s cracking pressure and the valve discharges media into the atmosphere. This slows down pressure build up in the system.

Finally, when the system's pressure reaches the valve’s set pressure limit, the valve is fully open, and all media discharges through the valve. This stops the operation of any downstream components until pressure reduces enough for the valve to begin closing. Known as the pressure override, the difference between the valve's cracking pressure and true pressure relief valve setting is typically at least 13.8 bar (200 psi).

Pressure relief valve applications

Avoiding cavitation

Cavitation occurs when the pressure in a liquid drops rapidly below the vapor pressure. If a centrifugal pump pumps against a closed system, overpressure must discharge within the pump housing. This causes areas of low pressure, potentially causing cavitation.

By opening proportionally with pressure increases, pressure relief valves bypass the housing. This slowly discharges the excess pressure. By avoiding cavitation, it increases the lifespan of the pump. Learn more by reading our guides on cavitation and flashing and cavitation in pumps, valves, and pipes.

Cooling/heating circuits

Flows may vary significantly with intermittent loads or during startup or shutdown. Reactions from connected boilers or reactors may cause pressure to rise or fall disproportionately to the input that user-controlled equipment (e.g., pumps and heat exchangers) generates. Pressure relief valves help avoid unexpected pressure changes in heating circuits.

Systems with sensitive equipment

Similarly, excessive pressure may damage equipment in pneumatic systems with multiple components. A pressure relief valve can avoid premature equipment failure and be part of a preventative maintenance plan.

Differences between pressure relief valves and safety valves

Pressure relief valves open gradually and keep fluid systems balanced during normal operations. They are part of the system's operations. Safety valves, however, open fully when needed to prevent system failure by releasing excess pressure. They continue to release pressure into the atmosphere until it is safe, even if it drops below the threshold.

The way these valves handle pressure is different. Pressure relief valves close quickly after opening. They close near the set pressure. Safety valves close further from the set pressure and have a slow build-up before they open fully.

Read our article on the differences between pressure safety valves and pressure relief valves to learn more.

Selection criteria

When selecting a pressure relief valve, consider the following criteria. Read our selecting pressure relief valves and safety valves guide for more information.

• Set pressure: Ensure the valve's set pressure, whether adjustable or not, matches the pressure necessary to work in the system.
• Body & seal materials: Ensure the valve components are compatible with the media.
• Blow off: Ensure the valve is the proper size to sufficiently discharge during an extreme scenario.
• Adjustable/non-adjustable: An adjustable valve is advantageous if the desired set pressure is unknown or possibly changing during the valve’s lifetime.

Pressure relief valve installation

Pressure relief valves work best downstream of high-pressure zones in a system. A common application is installing a valve near a pump’s discharge. Read our guide on installing pressure relief valves and safety valves for more information.

FAQs

Does a pressure relief valve reduce flow?

Yes, but this only occurs when pressure exceeds the set pressure. During normal operation, pressures should not exceed the set level, and the valve remains closed.

What do you need to look for when selecting a pressure relief valve?

Maximum flow, maximum pressure, and the nature of the medium. Corrosive media might require different seals, diaphragms, or bodies. Ensure that the valve has the needed approvals for your application.

Can pressure relief valves prevent water hammer?

Yes, certain models can. To fully avoid water hammer, use a specific model that can accommodate both the high flow and pressure differentials common in a water hammer scenario.

Are there disadvantages to using pressure relief/overflow valves in a bypass?

The biggest disadvantage is the loss of energy. Everything that is being pumped through the valve is by definition not being used downstream. For short bursts, this is generally not an issue.