Pressure relief valves are among the most safety-critical components in any pressurised system. Where a pressure reducing valve manages pressure continuously as part of normal operation, a pressure relief valve sits in reserve, doing nothing until it is needed, and then acting to prevent a potentially dangerous overpressure event. Getting the specification right matters considerably.
This guide covers what pressure relief valves are, how they work, the main types available, installation requirements, maintenance, and how to select the right valve for a given duty.
What Is a Pressure Relief Valve?
A pressure relief valve (PRV) is a safety device that protects pressurised systems, vessels, and pipework against overpressure. When system pressure rises above a pre-set level, the valve opens automatically to discharge fluid, reducing pressure until it falls back within safe limits. Once pressure has normalised, the valve closes and resets.
Pressure relief valves are used across pneumatic, hydraulic, steam, and process systems wherever there is a risk of pressure exceeding the safe working limits of the equipment or pipework downstream.
It is worth clarifying terminology here, as abbreviations can overlap. PRV is used for both pressure relief valves and pressure reducing valves. In safety contexts, pressure relief valves are also referred to as safety relief valves (SRVs) or pressure safety valves (PSVs). These terms are often used interchangeably, though there are technical distinctions between them covered below.
What Does a Pressure Relief Valve Do?
The function of a pressure relief valve is to act as the last line of defence against overpressure. Overpressure occurs when system pressure rises above the maximum allowable working pressure (MAWP) of the system. This can happen due to process upsets, blocked outlets, thermal expansion, equipment failure, or operator error.
The consequences of unchecked overpressure can include vessel or pipe failure, release of hazardous materials, fire, and injury. A correctly specified and installed pressure relief valve limits these risks by providing a controlled, automatic pressure release path before system pressure reaches a level that could cause damage or danger.
Pressure relief valves are a requirement, not a recommendation, in the majority of pressurised systems. In the UK and Europe, their use is governed by the Pressure Equipment Directive (PED) and the Pressure Systems Safety Regulations 2000 (PSSR).
How Does a Pressure Relief Valve Work?
The operating principle is straightforward. A spring-loaded disc or piston is held against a valve seat. The spring is set to a specific tension, which determines the pressure at which the valve will open, known as the set pressure.
When system pressure acting on the inlet side of the valve exceeds the spring force, the disc lifts off the seat and fluid is discharged through the outlet. As pressure falls, the spring returns the disc to the seat and the valve closes. The pressure at which the valve fully closes is called the reseating pressure, and is typically slightly below the set pressure. The difference between set pressure and reseating pressure is known as blowdown.
In spring-loaded direct-acting relief valves, the set pressure is adjusted by increasing or decreasing the tension on the spring, typically via a cap screw or adjustment mechanism at the top of the valve. Once set, the valve operates entirely passively, with no external power or control signal required.
Pressure Relief Valve vs. Pressure Reducing Valve: What Is the Difference?
The two are often confused due to shared abbreviations and overlapping terminology, but they perform fundamentally different functions.
A pressure reducing valve operates continuously during normal system operation. Its role is to lower and maintain downstream pressure at a set value, modulating flow as demand changes. It is an active control device.
A pressure relief valve is a safety device. It remains closed under normal operating conditions and only opens when system pressure exceeds a threshold. It does not regulate pressure during normal operation; it provides emergency pressure relief when something has gone wrong, or when thermal expansion or other transient effects push pressure above safe limits.
Both may appear in the same system. The pressure reducing valve manages operating pressure; the relief valve protects against the consequences of it going too high.
Types of Pressure Relief Valve
Spring-loaded direct-acting relief valves
The most common type. A spring holds a disc against a seat. The valve opens when inlet pressure overcomes spring force, and closes when pressure falls. Simple, reliable, and available across a very wide pressure and size range. Suitable for the majority of gas, liquid, and steam duties.
Pilot-operated relief valves
A small pilot valve controls the opening of a larger main valve. At normal pressures, the pilot valve directs system pressure to hold the main disc closed. When pressure reaches the set point, the pilot vents, allowing the main disc to lift. Pilot-operated designs offer tighter set pressure tolerance, lower blowdown, and better performance at high flow rates. They are preferred where precise pressure control is needed or where large bore relief is required.
Safety valves
Safety valves are designed for compressible media, particularly steam and gas. They open rapidly and fully at set pressure, providing maximum discharge capacity. The disc lifts clear of the seat quickly rather than opening gradually, which is important for compressible fluids where a small lift would be insufficient to relieve pressure fast enough.
Safety relief valves
Designed for both liquid and gas service, safety relief valves combine characteristics of safety valves and liquid relief valves. They are widely used in process plant where the phase of the fluid at relief conditions cannot be guaranteed.
Pressure and temperature relief valves
Also known as T&P valves or combined pressure/temperature relief valves, these open in response to either overpressure or overtemperature. They are standard equipment on hot water storage vessels and commercial boilers, where thermal runaway is a specific risk.
Pressure Relief Valves on Boilers and Hot Water Systems
Boilers and pressurised hot water systems present a particular risk because pressure and temperature rise together. A blocked flue, failed thermostat, or control failure can cause both temperature and pressure to increase rapidly.
Pressure and temperature relief valves on boilers are typically set to open at a specific pressure and at a specific temperature, whichever threshold is reached first. The discharge pipe from the valve must be routed safely to a drain or tundish, not sealed or terminated where discharge could cause harm.
Regular inspection of boiler relief valves is part of any responsible maintenance schedule. Signs that a boiler relief valve may be failing include persistent dripping from the discharge pipe, which can indicate the valve is not reseating correctly, or conversely a valve that has not operated in a very long time and may have seized. Replacement of boiler relief valves on gas-fired systems in the UK must be carried out by a Gas Safe registered engineer.
How to Install a Pressure Relief Valve
Correct installation is as important as correct specification. A poorly installed relief valve may fail to operate when needed, or may operate unnecessarily.
Orientation. Most spring-loaded relief valves must be installed in a vertical upright position with the inlet at the bottom. Installation in other orientations can affect the set pressure and valve performance. Always check the manufacturer's installation instructions.
Inlet pipework. The inlet pipe must not be smaller in diameter than the valve inlet connection. Undersized inlet pipework restricts flow and prevents the valve from achieving its rated discharge capacity.
Outlet pipework. The outlet pipe must be at least as large as the valve outlet connection, and ideally larger. Back pressure in the outlet pipework reduces the effective set pressure of conventional spring-loaded valves. Where significant back pressure is unavoidable, a balanced bellows or pilot-operated valve should be specified.
Discharge routing. The discharge must be routed to a safe location. For steam and hot water systems this means a drain or tundish. For hazardous or toxic media, discharge must be contained and handled appropriately. Outlet pipework must be supported to prevent mechanical loading on the valve body.
No isolation valves. A pressure relief valve must never be isolated from the system it protects. Any isolation valve upstream of a relief valve must be locked open and managed under a written scheme of examination.
Selection Criteria
Selecting a pressure relief valve requires the following parameters to be defined:
Set pressure. The pressure at which the valve must open. Typically set at or below the system's maximum allowable working pressure.
Required discharge capacity. The valve must be capable of relieving the maximum credible flow at set pressure without system pressure rising excessively. Discharge capacity is calculated based on the relief scenario, fluid properties, and allowable overpressure.
Fluid. Liquid, gas, and steam require different valve configurations. The fluid phase at relief conditions must be established, as this affects both sizing and valve type selection.
Operating pressure. The normal operating pressure should be no more than 90% of the set pressure in most applications. Too small a margin causes the valve to simmer or weep at operating pressure, which damages the seat over time.
Back pressure. If the outlet discharges into a pressurised system, or where outlet pipe losses are significant, back pressure must be accounted for in sizing and may influence the choice of valve type.
Temperature. Affects fluid properties, material selection, and in some applications determines whether a combined pressure/temperature relief valve is appropriate.
Materials. Body and trim materials must be compatible with the process fluid. Standard options include bronze, carbon steel, and stainless steel. Soft seat materials must be checked for compatibility with the fluid and operating temperature.
Maintenance and Testing
Pressure relief valves are life-safety devices and require a documented inspection and maintenance regime. In the UK, pressurised systems covered by PSSR 2000 must operate under a written scheme of examination, which will specify inspection intervals for relief valves.
Functional testing should confirm that the valve opens at or close to its set pressure and reseats correctly. Many designs allow for manual lift testing via a test lever, though this should only be carried out when system pressure is sufficient to verify operation. Bench testing against a calibrated pressure source gives a more reliable result and should be used where precise set pressure verification is required.
Valves that have operated during a relief event should be inspected for seat damage and function-tested before being returned to service. Repeated operation, particularly with liquids, can damage seating surfaces and prevent the valve from reseating cleanly, resulting in continuous leakage.
Pressure Relief Valves and Pressure Reducing Valves Together
In many systems, pressure reducing valves and pressure relief valves are used together. The pressure reducing valve maintains operating pressure at the correct level under normal conditions. The pressure relief valve provides protection against the consequences of the reducing valve failing open, thermal expansion on a closed system, or any other scenario that could cause downstream pressure to rise above the safe working limit. For more on selecting pressure reducing valves, see our guide on pressure reducing valves.
Summary
Pressure relief valves are not a commodity item. Set pressure, discharge capacity, fluid compatibility, installation configuration, and regulatory compliance all need to be worked through carefully. A valve that is incorrectly sized, incorrectly set, or incorrectly installed may fail to protect the system it is intended to safeguard.
If you have a pressure relief application to specify, our team can assist with selection across a wide range of duties and operating conditions. Contact us to discuss your requirements.