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Pressure Release Valve [3] |
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What are Relief Valves?
A relief valve is a type of safety valve designed to open at a preset pressure, and discharge fluid until pressure drops to acceptable levels [1]. The valve's release of elevated liquid, gas, or steam pressures acts to prevent damage to the system.
Components of a Relief Valve
Relief valves are comprised of three main elements:
- Valve element - typically a spring loaded poppet valve [1]
- Sensing element - typically a diaphragm or piston [1]
- Reference force element -
commonly a spring [1]
Generic Pressure Release Valve Diagram [2] |
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Types of Relief Valves
Conventional Spring Loaded
With a conventional spring loaded relief valv, 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. [5]
Conventional Spring Loaded [4] |
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Advantages | Disadvantages |
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- Most reliable type if properly sized and operated
- Versatile — can be used in many services [4]
| - Relieving pressure affected by back pressure
- Susceptible to chatter if built-up back pressure is too high
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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. [5]
Balanced Spring Loaded [4] |
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Advantages | Disadvantages |
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- Relieving pressure not affected by back pressure
- Can handle higher built-up back pressure
- Protects spring from corrosion [4]
| - Bellows susceptible to fatigue/rupture
- May release flammables/toxics to atmosphere
- Requires separate venting system
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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. [5]
Advantages | Disadvantages |
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- Relieving pressure not affected by backpressure
- Can operate at up to 98% of set pressure
- Less susceptible to chatter (some models)
| - Pilot is susceptible to plugging
- Limited chemical and high temperature use by “O-ring” seals
- Vapor condensation and liquid accumulation above the piston may cause problems
- Potential for back flow
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Operation of a Relief Valve
Contributors:
Faculty Advisor: Mary Robinson