Pressure relief valves are generally classified into three types:
- Relief Valve
- Safety Valve
- Low Lift
- High Lift
- Full Lift
- Safety Relief Valve
- The conventional spring-loaded safety relief valve
- Pilot operated relief valve
- Balanced-bellows type relief valve
- Power actuated
- Temperature and Pressure actuated relief valve
Relief valves are spring-loaded and have a smooth opening and closing movement. They are ideal for incompressible fluids and are operated by upstream pressure. Adjustable relief valves allow pressure settings to be modified via the outlet port. Electronic relief valves have no leakage and use electronic controls to monitor and manage system pressure.
Safety valves, on the other hand, are utilized for compressible fluids (gas and vapors) and are distinguished by their rapid opening and closing functions. Safety valves are commonly used in steam plants to protect against boiler overpressure. They are divided into three groups based on the amount of travel or lift they experience during the pop-up. Low-lift safety valves have a limited capacity and lift just 1/24th of the bore diameter. High-lift safety valves cover 1/12th of the bore diameter. Full-lift safety valves, on the other hand, move at least 1/4th of the bore diameter and are ideally suited for steam services.
Depending on the application, the safety relief valve can be utilized for gas or liquid service. They can open quickly or gradually.
Conventional Pressure Relief Valve
Spring-loaded conventional pressure relief valves are best suited for applications with no back-pressure. Back-pressure has a direct impact on the operational features of these PRVs.
There are three basic components of a conventional pressure relief valve
- An inlet nozzle to be attached to the system has to be protected.
- A moveable disk for controlling fluid flow; and
- A spring that regulates disk position
Seat leakage should be checked while designing a normal pressure relief valve because leakage indicates continual loss of system fluid and can damage the valve seating surface. Conventional pressure relief valves are classified into Soft-seated valve and Metal-seated valve types based on the seating material
Pros & Cons of Conventional Pressure Relief Valves
- If correctly sized and operated, this is the most dependable type.
- This versatile type can be used in a variety of services.
- Pressure relief caused by back pressure
- If the back pressure is too high, it is subject to chatter.
Balanced Bellows Type Pressure Relief valve
Spring-loaded balanced bellows pressure relief valves are being designed to lessen the effects of back-pressure. The PRV design has a bellow to minimize the effect of back-pressure. The bellow keeps the spring, bonnet, and guiding surfaces away from the process fluid.
Pros & Cons of Balanced Bellows type Pressure Relief Valve
- Protects spring from corrosion
- Possess good temperature and chemical properties
- Can handle higher built-up backpressure
- Relieving pressure not affected by back pressure
- Bellows are susceptible to fatigue/rupture
- May release flammables/toxics to the atmosphere
- Requires a separate venting system
Balanced bellows and balanced bellows with auxiliary balancing piston are two types of balanced bellows safety relief valves
Pilot-operated Pressure Relief Valves
A pilot-operated safety relief valve is a pressure relief valve with a self-actuated auxiliary pressure relief that regulates the pressure relief. The pressure of the flowing medium controls the opening and closing of the relief valve. A pilot is used to measure process pressure and pressurize or vent the dome pressure chamber, which regulates valve opening and closing. The three main components that comprise a pilot-operated pressure relief valve are the primary valve, an unbalanced floating piston assembly, and an external pilot.
The pilot regulates the pressure on the top side of the main valve’s imbalanced moving chamber. A robust seat is usually connected to the bottom end.
Advantages of Pilot Operated Pressure Relief Valve
The following are the primary benefits of pilot-operated safety relief valves:
- The valve backpressure does not affect the set pressure.
- Because the operating pressure of the system determines the opening of the relief valve, the system can be run at maximum working pressure.
- They are less expensive than other types.
- Less susceptible to chatter
Pilot-operated Pressure relief valves can be classified based on various parameters as mentioned shown below:
Depending on the type of moving members
- A piston-type.
- A diaphragm-type.
Based on the type of pilots
- A pop-action pilot
- A modulating-action pilot
Based on the flow of pilots
- A flowing-type pilot.
- A non-flowing-type pilot
Power Actuated Pressure Relief Valve
Power-actuated pressure relief valves are operated by an externally powered device. The opening and closing of the pressure relief valve are controlled by energy sources such as water, electricity, or steam. They are typically utilized in nuclear power plants for forced-flow steam generators with no fixed steam or waterline.
Temperature Pressure actuated Pressure Relief Valves
A temperature and pressure-actuated pressure relief valve (also known as a T & P safety relief valve) is actuated by the temperature or pressure on the relief valve’s inlet side. The valve is controlled by two major elements: a spring and a thermal probe. They perform functions of preventing the temperature from rising above a certain level and over-pressure from rising above a certain limit. They are typically used to convey hot fluids in vessels, tanks, and heaters.
Vacuum Relief Valve
A Vacuum Relief Valve is designed to avoid an excessive internal vacuum by admitting fluid. They reclose and stop further fluid flow once the normal condition is restored.
When to Use a Spring-Operated Pressure Relief Valve
- Losing entire contents is unacceptable
- Fluids above the normal boiling point
- Toxic fluids
- Need to avoid failing low
- Return to normal operations quickly
- Withstand process pressure changes, including vacuum
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