What is a Pressure Relief Valve?
What is a Pressure Relief Valve?
What is a Pressure Relief Valve?
A pressure relief valve (PRV) is a type of safety-related component used in the pipeline or with vessels in pneumatics. Relief valves are used to limit the amount of compressed air pressure in a system so that it doesn't build up above a pre-set level and cause damage to machinery or people. These types of valves are therefore used to stop over-pressurization, where air pressure exceeds the maximum allowed for that system.
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Also known as...
A pressure relief valve is also referred to as a PRV, a relief valve, or a pressure release valve.
How do pressure relief valves work?
Relief valves are designed to protect compressed air systems from over-pressurization by releasing excess pressure.
Pressure acts against the valve seat and the force generated opens the valve against a spring tension. Turning the control knob can increase or decrease the tension in the spring, therefore adjusting the pressure required to open the valve.
Should pressure rise above a pre-determined setting, pressure will be relieved through a spring-loaded mechanism which opens due to the increased pressure upstream of the valve, compared to the set point. Once air has been exhausted out of the system, and the pressure has fallen down to its pre-set level, the pressure relief valve will close.
To stop a tank or vessel from becoming over-pressurized, pressure relief valves can be used to exhaust air into the atmosphere quickly and are an important component in terms of both safety and the reliable running of an air compression system.
How do I select pressure relief valves?
There are a wide variety of pressure relief valves available and choosing the appropriate valve will depend on the type of application you will be using it for. Norgren's comprehensive range includes valves designed to fit with the Excelon and Olympian Plus ranges, as well as valves for pressure vessels and those with the ability to be utilized in system pipework.
Before selecting your pressure relief valve, you should consider the following:
Operating pressure range / maximum pressure level
Flow requirement for system delivery
Failure mode
What temperatures the valve is required to act at
Where it is utilized in the compressed air system
The port size required
How it will be mounted
Material type required: e.g. brass, steel
Environment to be used in: e.g. corrosive
Types of pressure relief valves
Norgren's range of pressure relief valves include:
Excelon modular system pressure relief valves
Olympian Plus relief valves
Miniature ported pressure relief valves
Pop Type relief valves
Typical applications of pressure relief valves
Pressure relief valves are commonly seen in any application which uses compressed air, such as pneumatics systems in industrial automation.
They're also an important component in high pressure applications such as oil and gas and defence and nuclear, where industrial and process gases are stored and transported at high pressure and need to be regulated down to a safe working pressure before use.
Pressure relief valves should always be used where there is a risk to any individual or process from over-pressurization. It is good practice to automatically include them in any system design.
Do I need anything else to make pressure relief valves work?
Accessories required for use with valves depend on the type selected, and can include wall mountings and gauges. Tubing and fittings will be required to connect the relief valve into a system. Tamper resistant kits are also available on some pressure relief valves.
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INTRODUCTION TO PRESSURE RELIEF DEVICES
A pressure relief device is a safety device designed to protect a pressurized vessel or system during an overpressure event. An overpressure event refers to any condition which would cause pressure in a vessel or system to increase beyond the specified design pressure or maximum allowable working pressure (MAWP). When the pressure inside equipment such as a boiler or pressure vessel increases beyond this specific value, the excess pressure may result in a catastrophic failure. To avoid that calamity, pressure relief devices are used at predetermined set pressures to protect the equipment by opening and ejecting mass from the process. The ejected mass contains energy; the removal of this energy reduces the process pressure.
Pressure relief device is the 'last line' of defense for safety of the pressurized equipment. If all other safety devices or operating controls fail, the pressure relief device must be capable of venting excess pressure. Accidents are often caused when the pressure relief devices themselves fail to perform the function for which they are designed.
CODES AND STANDARDS
All pressure relief devices for set pressure of more than 15 psi are designed, constructed, inspected, stamped and certified in accordance with the ASME Boiler and Pressure Vessel Code (ASME Code). Each device is stamped with a certification mark, and one of the eight certification designators. See Figure below.
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The eight certification designators under the ASME Code are:
V Safety valve for steam boilers
NV Safety valve for nuclear components
HV Safety valve for heating boilers
UV Safety valve for pressure vessels
UV3 Safety valve for pressure vessels with more than 10,000 psi maximum allowable working pressure
UD Rupture disk for pressure vessels
TV Safety valve for transport tanks
TD Rupture disk for transport tanks
The performance of pressure relief devices is determined by ASME Pressure Test Code (PTC) 25. The American Petroleum Institute (API) has also published codes and standards for sizing, selection, installation and inspection of pressure relief devices. API 520 contains guidelines for sizing, selection and installation of pressure relieving devices; API 521 is applicable to pressure-relieving and vapor depressuring systems; and API RP 576 is a recommended practice that describes inspection and repair practices for automatic pressure relieving devices.
TYPES OF DEVICES AND VALVES
Many types of pressure relief devices are available in the market today. They can be generally classified as reclosing and non-reclosing pressure relief devices.
Reclosing Pressure Relief Devices
A reclosing type pressure relief device is designed to close after operation. They are pressure relief valves whose primary purpose is to open to relieve excess pressure, reclose and prevent further flow of fluid after normal conditions have been restored. A secondary purpose is to minimize damage to other system components. A pressure relief valve designed under ASME Code is stamped with the certification mark, and one of the certification designators: V, NV, HV, UV, UV3 or TV. The many types of pressure relief valves that exist are based on different designs and construction. Generally, they are classified as safety valves, relief valves and safety relief valves.
Safety Valves
The principal device used to prevent overpressure in steam plants is the safety valve. It is actuated by inlet static pressure and characterized by rapid opening or pop action.
Relief Valves
A relief valve is a pressure relief valve actuated by inlet static pressure having a gradual lift generally proportional to the increase in pressure over opening pressure. It may be provided with an enclosed spring housing suitable for closed discharge system application and is primarily used for liquid service, especially for lower capacities and thermal expansion applications.
Safety Relief Valves
A safety relief valve is a pressure relief valve characterized by rapid opening or pop action, or by opening in proportion to the increase in pressure over the opening pressure, depending on the application and may be used either for liquid or compressible fluid application.
Non-Reclosing Pressure Relief Devices
A non-reclosing type pressure relief valve is designed to remain open after operation. Generally, a manual means of resetting is provided. If it's not, the device is replaceable. Non-reclosing pressure relief devices are of two types: rupture disks and pin devices.
Rupture Disks
A rupture disk is designed to rupture at a predetermined pressure and temperature. If designed under ASME Code, the rupture disks are stamped with the certification mark and one of the certification designators UD or TD. Rupture disks are used where instantaneous and full opening of a pressure relief device is required to protect vessels, piping and other pressurized systems from excessive pressure and/ or vacuum. The rupture disk is oriented with the process fluid against the concave side of the disk. As the pressure of the process fluid increases beyond the allowable operating pressure, the rupture disk starts to grow. This growth continues as the pressure increases until the tensile strength of the material is reached and rupture occurs.
Pin Devices
Pin device is actuated by static differential pressure or static inlet pressure. They are designed to function by the activation of a load bearing section of a pin that supports a pressure containing member. If designed under the ASME Code, the pin devices are stamped with the certification mark, and one of the certification designators UD or TD. Pin devices are often used in applications where rupture disks must be replaced because of frequent failures.
Rupture pin technology was developed in to solve valve failure problems for Exxon and Shell in Holland. In the US, rupture pin technology was introduced in . ASME approved the use of the buckling pin under ASME Code Case # in May of . Since then, ASME has permitted the use of pin devices designed under the ASME Code.
PROBLEMS THAT CAN PREVENT NORMAL OPERATION OF PRESSURE RELIEF DEVICES
Pressure relief devices must operate as designed in order to perform their required function. Care must be taken during design to prevent problems during normal operation:
- The inlet piping connected to the device must not be smaller than the inlet opening of the device.
- The discharge piping connected to the device must be no smaller than the discharge opening of the device.
- Multiple devices discharging into a discharge manifold or header is a common practice. The discharge manifold or header must be sized so that the cross sectional area is equal to or greater than the sum of the discharge cross sectional areas of all the devices connected to the discharge manifold or the header.
- Constant leakage of the device can cause a build-up of scale or other solids around the discharge opening. This build-up can prevent the device from operating as designed.
- Discharge piping connected to the device must be supported so as not to impart any loadings on the body of the device.
- Some devices, especially on larger boilers, may have a discharge pipe arrangement which incorporates provisions for expansion as the boiler heats up or cools down. These expansion provisions must allow the full range of movement required to prevent loads being applied to the device body.
- Drain holes in the device body and discharge piping, when applicable, must be open to allow drainage of liquids from over the device disk on spring-loaded valves. Any liquid allowed to remain on top of the device disk can adversely affect the operating characteristics of the device.
- Most jurisdictional requirements state that device must be 'piped to a point of safe discharge'. This must be accomplished while keeping the run of the discharge piping as short as possible.
The above is a very brief introduction to pressure relief devices. The material for this article has been taken from the following sources:
- Back to Basics: Introduction to Pressure Relief Devices by Mohammad Malek
- Introduction to Valves ' Pressure Relief Valves from Explore the World of Piping
- Pressure Relief Devices from The National Board of Boiler and Pressure Vessel Inspectors
Ramesh Tiwari provides consulting and training services for pressure vessels, heat exchangers and storage tanks. He may be contacted at .
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