A pressure relief valve is a safety device designed to secure a pressurized system during an overpressure occurrence during operation. The system is widely available today as an electronic, pneumatic, and hydraulic system. The various types serve the same purpose in different applications and are called pressure relief valves, relief valves, or safety valves.
Depending on the type of system, the power source could be electricity or compressed air for operation. Its purpose is to control the pressure so that life and property can be safe.
A pressure relief valve must be capable of serving for a long period of time and capable of operating at all times. And a professional operator with experience must be in charge to ensure proper working, as there is no room for error. Well, in this reading, we’ll explore what a pressure relief valve is, its applications, parts and diagrams, types, and how it works
Let’s get started!
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What is a Pressure Relief Valve?
A pressure relief valve or relief valve is a special type of safety valve system used to control or limit the pressure in a system. It can be manually or automatically controlled from a pressurized vessel or piping system. The pressurized fluid or gas is discharged to a reservoir or atmosphere to relieve pressure over the maximum allowable working pressure (MAWP).
The primary purpose of a pressure relief valve is to protect pressure vessels or systems from catastrophic failure. Catastrophic failure could be disastrous during an overpressure event and could be either liquid or gaseous.
This device is widely used in the petrochemical, petroleum refining, and chemical manufacturing industries. Industries where natural gas processing and power generation occur, as well as water supply industries, also make good use of pressure relief valves.
Though it’s generally known as a relief valve, depending on its field of application, it can be called a pressure relief valve (PRV), pressure safety valve (PSV), or safety valve. You should note the design and operation of these valves are slightly different.
A relief valve is often installed in a hydraulic system after the pump because it provides a direct, effective, and rapid response when the device opens to bleed excess fluid into a reservoir.
The fluid could be liquid, gas, or liquid gas. This reduces the pressure equal to its spring setting. the device opens as the pressure caused by backpressure or downstream is high enough to force the poppet or spool to open against its spring.
Applications
Below are the applications of pressure relief valves:
- Widely used in high-rise buildings, urban water supply networks, mines, and conveyance systems.
- Used for appropriate service water pressure and flow.
- Used to improve system running conditions and potentially save water.
- Used by firefighters and water supply industries.
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Parts and Diagram of a Pressure Relief Valve
How Does A Pressure Relief Valve Work?
The working of a relief valve is quite complex but can be easily understood. It consists of a ball, poppet, or spool opposed by a spring, which is placed into a cavity or ported body. A hydraulic system is often used to limit fluid pressure in the part of the circuit they are installed in.
The poppet is in the form of a disc- or cone-shaped object that is mounted within an opposite machine seat. If the part is forced closed by spring pressure, very low leakage will be provided.
The spool is a cylindrical, machined steel rod with metering grooves or notches that’s also stopped by spring pressure. It leaks more than a poppet valve but offers superior metering effects.
In the working of a relief valve, excessive pressurized fluid is provided from an open path to a tank with the purpose of reducing work port pressure. As soon as the fluid pressure begins to rise, the force is applied to the bottom of the spool or poppet. This allows the valve to open modestly at first, bleeding a little fluid as required to maintain the downstream pressure.
But if the downstream pressure continues to rise, the force acting upon the poppet or spool will be pushing it further towards the spring until the point spring force is balanced by the hydraulic force.
The pressure rise is a result of load pressure combination, backpressure, and energy required to flow through the valve itself. The initial fluid force overcomes the seated force of the spring with the help of the cracking pressure.
As the valve flows more fluid to the tank, the rate of pressure rise is safe because the forces of the pressurized fluid counteract the compression rate of the spring. If the valve is nearly fully open, pressure rise increases again as the valve bottom opens due to the flow forces.
As the operation or backpressure decreases, the valve begins to close. This is done at differing rates than the opening. The difference between the opening and closing curve is called its hysteresis; it’s indicative and instructive of the quality of its construction.
This is because higher-quality valves with advanced construction allow lower pressure rise with better hysteresis.
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Types of Pressure Relief Valve
There are three basic types of pressure-relief valves, such as
Conventional Spring Loaded
the conventional spring-loaded types of relief valve that contained the bonnet, spring, and guide in the released fluids. Its relief pressure backpressure decreases the set pressure if the bonnet is vented to the atmosphere. But if the bonnet is vented internally to the outlet, the relief system backpressure increases the set pressure.
These types of pressure relief valves are used in noncorrosive services where backpressure is not up to 10% of the set point.
Balanced Spring Loaded
The design of a balanced spring-loaded valve is to protect the bonnet spring, guide it from the released fluids and reduce the effects of backpressure. Its disk area is vented to the atmosphere and another disk is exposed to backpressure. The vented disk is equal to the exposed disk.
Balanced spring-loaded is used in corrosive and dirty service and with variable backpressure.
Pilot Operated
The pilot-operated valve is controlled by an auxiliary pressure pilot. The resistance force on the piston in the main valve is achieved by the pressure during the operation through an orifice. The net seating force on the piston actually rises as the process pressure nears the set point.
Safety Valve
a safety valve is a pressure relief valve that works by inlet static pressure and is designed to rapidly open with a pop action. It is widely used for air and stream services. A safety valve is available in two forms:
- Low-lift safety valve: in a low-lift safety valve, the disc lifts automatically in a way that the disc position determines the actual discharge area.
- Full-lift safety valve: in this safety valve the disc is also automatically lifted but the actual charge area is not determined by the position of the disc.
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Relief Valve
A relief valve is a pressure relief device designed with an inlet static pressure for its working. It has a gradual lift generally proportional to the increase in pressure over opening pressure. It’s suitable for a close discharge system as it is enclosed in spring housing.
Safety Relief Valve:
A safety relief valve is a pressure relief valve characterized by its rapid opening or pop action. Also known for its opening in proportion to the increase in pressure over the opening pressure based on its application. Safety relief valves are used either for liquid or compressible fluid. The common type is the conventional and balanced safety relief valve, which was earlier explained.
Power-Actuated Pressure Relief Valve
A power-actuated pressure relief valve contained a relieving device that is joined with and controlled by a device requiring an external source of energy.
Temperature-Actuated Pressure Relief Valve
A temperature-actuated pressure relief valve may be actuated by external or internal temperature or by pressure on the inlet side.
Vacuum Relief Valve
A vacuum relief valve is a pressure relief device that admits fluid to prevent an excessive internal vacuum. It’s designed to reclose so as to prevent further flow of fluid after normal conditions have been restored.
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Things to Consider Before Selecting a Pressure Relief Valve
Below are things an engineer or professional in this field considers while selecting a relief valve:
- Design Considerations include the size, material, product, complexity, etc.
- The individual relief load should be determined.
- Causes of overpressure.
- Various types of relief devices and their characteristics should be considered to see the best option.
- Selecting appropriate relief devices to handle the imposed loads. Several components must be taken into consideration. Parts like set pressure, backpressure, dual relief valves, and multiple relief valves must suit the capacity of the relieving device.
- Installation considerations
- Inlet piping and discharge piping
- Reactive forces
- Tailpipe considerations
- Rapid Cycling
- Resonant chatter occurs when the inlet piping produces excessive loss at the valve inlet.
- Liquid discharge considerations.
Pressure Relief Valve Safety Precautions
Consider the following to ensure safety while using a pressure relief valve:
• Adjust opening pressure to the user’s set value before leaving the factory.
• Re-adjust at the installation site to ensure set pressure meets requirements.
• Adjust opening pressure by changing the spring’s compression level.
• Reduce valve inlet pressure to below 90% of opening pressure before rotating the adjustment screw.
• Ensure media conditions during adjustment are close to actual operating conditions.
• Conventional pressure relief valve used to mitigate excess back pressure.
• Setting value should be the required opening pressure minus additional back pressure after inspection.
Conclusion
A pressure relief valve (PRV) is an essential safety device used to protect equipment and systems from overpressure. By automatically venting excess pressure when limits are exceeded, it helps maintain safe operating conditions in various applications—ranging from plumbing and heating systems to industrial and hydraulic machinery.
Regular inspection and proper installation of pressure relief valves are critical for ensuring system safety, efficiency, and compliance with safety standards.
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FAQs on Pressure Relief Valve
What is a pressure relief valve?
A pressure relief valve is a safety device designed to open automatically and release excess pressure from a system to prevent damage or explosion.
Where are pressure relief valves commonly used?
They are used in:
- Boilers and heating systems
- Pipelines and compressors
- Hydraulic systems
- Chemical and process industries
- Air and gas tanks
How does a pressure relief valve work?
When system pressure exceeds a set limit, the valve opens to allow the excess pressure to escape. Once normal pressure is restored, the valve closes automatically.
What’s the difference between a safety valve and a relief valve?
- Safety valves are typically used for gas and steam and open fully and instantly.
- Relief valves are more common in liquid systems and open more gradually.
How often should a PRV be tested?
It depends on the system, but typically once a year or per manufacturer or regulatory guidelines.
What causes a pressure relief valve to fail?
Corrosion or rust
- Blockages or debris
- Incorrect pressure settings
- Wear and tear from frequent activation
Can a pressure relief valve be reused after it opens?
Yes, most PRVs reset automatically and can be reused, but repeated activation or visible damage may require replacement.