Relief Valves and Pressure Safety: What Every Engineer Needs to Know

In any system involving pressurised fluids or gases, pressure relief valves are non-negotiable safety devices. They protect pipelines, vessels, and equipment from the catastrophic consequences of overpressure — including rupture, explosion, and injury. Despite their critical importance, relief valves are often misunderstood, poorly specified, or inadequately maintained. At Valves UK, we believe every engineer working with pressurised systems should have a solid understanding of these devices.

What Is a Relief Valve?

A pressure relief valve (PRV) is a type of safety valve that automatically opens to release excess pressure from a system when it exceeds a preset limit, and closes again once the pressure returns to a safe level. Unlike a shut-off valve that is manually operated, a relief valve operates automatically in response to pressure — it requires no operator intervention or external power supply to function. Safety valves (typically used on steam and gas systems) open fully and rapidly (pop action), while relief valves (typically used on liquid systems) open gradually in proportion to the overpressure.

How Does a Relief Valve Work?

The simplest and most common design is the spring-loaded relief valve. A spring holds a disc or ball against a seat. The spring force is set to correspond to the desired set pressure. When system pressure exceeds this set point, the fluid force overcomes the spring force and lifts the disc, allowing fluid to escape through a discharge port to a safe location (typically a drain, vent, or holding vessel). Once pressure drops back below the set point, the spring re-seats the disc and the valve closes. The difference between the pressure at which the valve opens (set pressure) and the pressure at which it closes (reseat pressure) is known as the blowdown, typically expressed as a percentage of set pressure.

Types of Relief Valves

Spring-Loaded Relief Valves: The most common type. Simple, reliable, and self-contained. Available for liquids, gases, and steam. The set pressure is adjusted by changing the spring compression.

Pilot-Operated Relief Valves: Use a small pilot valve to control a larger main valve. These offer tighter pressure control (lower blowdown) and are better suited to high-pressure applications. They can also be adjusted while in service.

Balanced Bellows Relief Valves: Designed to handle back-pressure on the discharge side of the valve. The bellows isolates the spring from the process fluid and compensates for variable back-pressure. Used in applications where the discharge line may not be at atmospheric pressure.

Rupture Discs: A thin membrane that bursts at a preset pressure. Once ruptured, it does not reseat, providing a one-time, irreversible pressure release. Often used as a secondary protection layer below a relief valve.

Sizing a Relief Valve

Correct sizing is critical. An undersized relief valve will not relieve pressure quickly enough to prevent overpressure, while an oversized one may chatter (rapidly open and close) and wear out prematurely. Sizing requires knowing the set pressure (maximum allowable working pressure), the relieving capacity required (expressed as flow rate), the properties of the fluid (phase, viscosity, specific gravity), and the back pressure conditions. Relief valve sizing calculations follow international standards such as EN ISO 4126, API 520/521, or ASME Section VIII.

Materials and Maintenance

Relief valves must be compatible with the media they protect. Brass is suitable for water, air, and neutral gas applications at lower pressures. Stainless steel (316) is appropriate for corrosive chemicals, steam, and hygienic applications. Carbon steel is used for high-pressure steam and process gas. Relief valves must be tested and inspected regularly to ensure they will operate correctly when required. Most industrial installations test relief valves annually or during scheduled shutdowns, verifying the set pressure, checking for leakage at the seat, and inspecting the condition of the spring and internals.

Regulatory Compliance

In the UK, pressurised systems are governed by the Pressure Systems Safety Regulations 2000 (PSSR 2000), which require a written scheme of examination and regular inspection by a competent person. Relief valves are typically included in the scope of these inspections. Failure to comply can result in improvement notices, prohibition orders, or prosecution.

Conclusion

Pressure relief valves are critical life-safety devices that must be correctly specified, installed, and maintained. At Valves UK, our technical team can advise on the correct selection of relief valves for your application and operating conditions. Browse our range online or contact us today for expert guidance on pressure relief and safety valve solutions.