What is Pressure Relief Valve and How does it Work?

What is Pressure Relief Valve?

A pressure relief valve (PRV) or relief valve is a type of safety valve used to control or limit the pressure in a system; pressure might otherwise build up and create a process upset, instrument or equipment failure, or fire.

The pressure is relieved by allowing the pressurized fluid to flow from an auxiliary passage out of the system. The pressure relief valve is designed or set to open at a predetermined set pressure to protect pressure vessels and other equipment from being subjected to pressures that exceed their design limits.

When the set pressure is exceeded, the pressure relief valve becomes the “path of least resistance” as the valve is forced open and a portion of the fluid is diverted through the auxiliary route.

In systems containing flammable fluids, the diverted fluid is usually routed through a piping system known as a flare header or relief header to a central, elevated gas flare where it is usually burned and the resulting combustion gases are released to the atmosphere.  

In non-hazardous systems, the fluid is often discharged to the atmosphere by suitable discharge pipework designed to prevent rainwater ingress which can affect the set lift pressure, and positioned not to cause a hazard to personnel.

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 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 the 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.

IMI 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

The three basic types of pressure-relief valves are conventional spring-loaded, balanced spring-loaded, and pilot-operated.

• Conventional spring loaded. In the conventional spring-loaded valve, the bonnet, spring, and guide are exposed to the released fluids. If the bonnet is vented to the atmosphere, relief-system backpressure decreases the set pressure. If the bonnet is vented internally to the outlet, relief-system backpressure increases the set pressure. The conventional spring-loaded valve is used in noncorrosive services and where backpressure is less than 10% of the set point.
• Balanced spring-loaded. The balanced spring-loaded valve incorporates a means to protect the bonnet, spring, and guide from the released fluids and minimizes the effects of backpressure. The disk area vented to the atmosphere is exactly equal to the disk area exposed to backpressure. These valves can be used in corrosive or dirty service and with variable backpressure.
• Pilot operated. The pilot-operated valve is combined with and controlled by an auxiliary pressure pilot. The resistance force on the piston in the main valve is assisted by the process pressure through an orifice. The net seating force on the piston actually increases as the process pressure nears the set point.

Types of safety Valves

There is a wide range of safety valves available to meet the many different applications and performance criteria demanded by different industries. Furthermore, national standards define many varying types of safety valves.

The ASME standard I and ASME standard VIII for boiler and pressure vessel applications and the ASME/ANSI PTC 25.3 standard for safety valves and relief valves provide the following definition.

These standards set performance characteristics as well as define the different types of safety valves that are used:

• ASME I valve – A safety relief valve conforming to the requirements of Section I of the ASME pressure vessel code for boiler applications which will open within 3% overpressure and close within 4%. It will usually feature two blowdown rings and is identified by a National Board ‘V’ stamp.
• ASME VIII valve – A safety relief valve conforming to the requirements of Section VIII of the ASME pressure vessel code for pressure vessel applications which will open within 10% overpressure and close within 7%. Identified by a National Board ‘UV’ stamp.
• Low lift safety valve – The actual position of the disc determines the discharge area of the valve.
• Full lift safety valve – The discharge area is not determined by the position of the disc.
• Full bore safety valve – A safety valve having no protrusions in the bore, and wherein the valve lifts to an extent sufficient for the minimum area at any section, at or below the seat, to become the controlling orifice.
• Conventional safety relief valve – The spring housing is vented to the discharge side; hence operational characteristics are directly affected by changes in the backpressure to the valve.
• Balanced safety relief valve – A balanced valve incorporates a means of minimizing the effect of backpressure on the operational characteristics of the valve.
• Pilot operated pressure relief valve – The major relieving device is combined with, and is controlled by, a self-actuated auxiliary pressure relief device.
• Power-actuated safety relief valve – A pressure relief valve in which the major pressure-relieving device is combined with, and controlled by, a device requiring an external source of energy.

The following types of safety valves are defined in the DIN 3320 standard, which relates to safety valves sold in Germany and other parts of Europe:

• Standard safety valve – A valve that, following the opening, reaches the degree of lift necessary for the mass flowrate to be discharged within a pressure rise of not more than 10%. (The valve is characterized by a pop-type action and is sometimes known as high lift).
• Full lift (Vollhub) safety valve – A safety valve which, after commencement of lift, opens rapidly within a 5% pressure rise up to the full lift as limited by the design. The amount of lift up to the rapid opening (proportional range) shall not be more than 20%.
• Directly loaded safety valve – A safety valve in which the opening force underneath the valve disc is opposed by a closing force such as a spring or a weight.
• Proportional safety valve – A safety valve that opens more or less steadily in relation to the increase in pressure. Sudden opening within a 10% lift range will not occur without a pressure increase. Following opening within a pressure of not more than 10%, these safety valves achieve the lift necessary for the mass flow to be discharged.
• Diaphragm safety valve – A directly loaded safety valve wherein linear moving and rotating elements and springs are protected against the effects of the fluid by a diaphragm
• Bellows safety valve – A directly loaded safety valve wherein sliding and (partially or fully) rotating elements and springs are protected against the effects of the fluids by a bellows. The bellows may be of such a design that it compensates for influences of backpressure.
• Controlled safety valve – Consists of the main valve and a control device. It also includes direct acting safety valves with supplementary loading in which, until the set pressure is reached, an additional force increases the closing force.

EN ISO 4126 lists the following definitions of types of safety valves:

• Safety valve – A safety valve which automatically, without the assistance of any energy other than that of the fluid concerned, discharges a quantity of the fluid so as to prevent a predetermined safe pressure from being exceeded, and which is designed to re-close and prevent further flow of fluid after normal pressure conditions of service have been restored.
• Directly loaded safety valve – A safety valve in which the loading due to the fluid pressure underneath the valve disc is opposed only by a direct mechanical loading device such as weight, lever, and weight, or a spring.
• Assisted safety valve – A safety valve which by means of a powered assistance mechanism, may additionally be lifted at a pressure lower than the set pressure and will, even in the event of a failure of the assistance mechanism, comply with all the requirements for safety valves given in the standard.
• Supplementary loaded safety valve – A safety valve that has, until the pressure at the inlet to the safety valve reaches the set pressure, an additional force, which increases the sealing force.

Applications of Pressure Relief Valves

Sanitary pressure relief valves are widely used in high-rise buildings, areas with overly high-water pressure in urban water supply networks, mines, and other conveyance systems to ensure that various points in the water supply obtain the appropriate service water pressure and flow.

In light of the fact that the water leakage and degree of waste are almost proportional to the water pressure of the water supply system, pressure relief valves have the role of improving the running working conditions of the system and can potentially save water.

According to recent statistics, some pressure relief valves have water-saving effects of about 30%.

6 Precautions to Take When Using Pressure Relief Valves

• Before the pressure relief valve leaves the factory, its opening pressure should be adjusted one by one to the set value required by the user. If the user proposes the working pressure level of the spring, it should generally be adjusted according to the lower limit of the pressure level.
• Before installing the pressure relief valve on the protected equipment or before installation, the user must re-adjust at the installation site to ensure that the set pressure value of the pressure relief valve meets the requirements.
• You can adjust the opening pressure by changing the spring’s compression level. Simply rotate the adjusting screw within the spring’s working pressure level range, usually indicated on the nameplate.
• Before rotating the adjustment screw, the valve inlet pressure should be reduced to below 90% of the opening pressure. This prevents the valve disc from rotating when the adjustment screw is rotated, which may damage the sealing surface.
• In order to ensure the accuracy of the opening pressure value, the media conditions during adjustment, such as the type of media and temperature, should be as close as possible to the actual operating conditions.
• The conventional pressure relief valve is used to mitigate excess back pressure. When the opening pressure is adjusted after inspection (the back pressure is atmospheric pressure at this time), its setting value should be the required opening pressure minus the additional back pressure.

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