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Understanding The Structure of Pressure Relief Valves

Dec. 21, 2022


A pressure relief valve is a safety device that is designed to protect a pressurized system or component from overpressure by releasing excess pressure. It is used in a wide range of applications, including steam boilers, gas storage tanks, and other pressurized systems.

The pressure relief valve consists of a valve body, a spring, and a pressure-sensitive element (such as a diaphragm or a bellows). The pressure-sensitive element is designed to respond to changes in pressure within the system and open the valve when the pressure exceeds a predetermined set point. The spring is used to apply a counterforce to the pressure-sensitive element, which helps to hold the valve closed until the pressure reaches a certain level.

API Pressure Relief Valve / PRV

A pressure relief valve is comprised of three functional elements:

1) Valve element, typically a spring loaded poppet valve.

2) Sensing element, typically a diaphragm or piston.

3) Reference force element.  Most commonly a spring.


In operation, the pressure relief valve remains normally closed until pressures upstream reaches the desired set pressure. The valve will crack open when the set pressure is reached, and continue to open further, allowing more flow as over pressure increases. When upstream pressure falls a few psi below the set pressure, the valve will close again.

(1) VALVE ELEMENT (poppet valve)

Most commonly, pressure relief valves employ a spring loaded “poppet” valve as a valve element. The poppet includes an elastomeric seal or, in some high pressure designs a thermoplastic seal, which is configured to make a seal on a valve seat. In operation, the spring and upstream pressure apply opposing forces on the valve. When the force of the upstream pressure exerts a greater force than the spring force, then the poppet moves away from the valve seat which allows fluid to pass through the outlet port. As the upstream pressure drops below the set point the valve then closes.

High performance Pressure Relief Valve for steam service

(2) SENSING ELEMENT (piston or diaphragm)

Piston style designs are often used when higher relief pressures are required, when ruggedness is a concern or when the relief pressure does not have to be held to a tight tolerance. Piston designs tend to be more sluggish, compared to diaphragm designs due to friction from the piston seal. In low pressure applications, or when high accuracy is required, the diaphragm style is preferred. Diaphragm relief valves employ a thin disc shaped element which is used to sense pressure changes. They are usually made of an elastomer, however, thin convoluted metal is used in special applications. Diaphragms essentially eliminate the friction inherent with piston style designs. Additionally, for a particular relief valve size, it is often possible to provide a greater sensing area with a diaphragm design than would be feasible with a piston style design.


The reference force element is usually a mechanical spring. This spring exerts a force on the sensing element and acts to close the valve. Many pressure relief valves are designed with an adjustment which allows the user to adjust the relief pressure set-point by changing the force exerted by the reference spring.

In conclusion

Pressure relief valves are important safety devices that are used to prevent accidents and damage to equipment by releasing excess pressure. They are typically installed in systems where the pressure could become dangerous if not properly controlled, such as in steam boilers or gas storage tanks. They are also used in many other applications where the pressure must be carefully regulated to ensure the safe operation of the system. We are a pressure relief valve supplier. If you are interested in our products, please contact us now!

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