Pressure Relief Valve

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A pressure relief valve, abbreviated as PRV, is used to protect pressure vessels and storage tanks from overpressure.  It is designed to protect critical system components such as vessels, tanks and flange ratings.  This valve removes excessive pressure from a system by limiting its pressure level to a safe level. The valve opens gradually during normal operation to maintain an optimal pressure level inside the vessel.


Pressure Relief Valve Design Classification

  • Pressure Valve  -  Both the relief and safety valves work to release excess pressure, but each go about it is a little different.
    • Pressure Relief Valve  -   Relief valves removes excessive pressure from a system by limiting its pressure level to a safe level.  The valve opens gradually during normal operation to maintain an optimal pressure level inside the vessel.
      • Relief Valve  -  Relief valves are spring-loaded and characterized by gradual opening and closing.  They are actuated by the upstream pressure and are suitable for incompressible fluids.  Adjustable relief valves allow the pressure setting adjustment through the outlet port.  Electronic relief valves offer zero leakage with electric controls to monitor and regulate the system pressure.
      • Safety Valve  -  Safety valves are a type of automatic valve designed to automatically open and discharge fluids when the pressure reaches the designated value, then automatically close again when the pressure falls below the designated value.  The safety valve can be used for gas or liquid service depending on the application.  They are used as a safety device to protect equipment, instrumentation, and piping from excessive pressure.
      • Safety Relief Valve  -  Safety valves provide relief from pressure.  This valve opens quickly to avoid overpressurization when a set pressure is reached, preventing a potential safety incident.
        • Balanced Bellow/Piston Pressure Relief Valve  -  This valve is a spring-loaded valves that contain a bellows arrangement to keep back-pressure from affecting the set point.  The bonnet is vented to atmosphere and a bellows is installed so that the back-pressure acts both downward and upward on the same area of the disc.  The forces created by the back-pressure always cancel and do not affect the set point.
        • Conventional (Spring Loaded) Pressure Relief Valve  -  A pressure relief valve which has its spring housing vented to the discharge side of the valve.  The operational characteristics, opening pressure, closing pressure, and relieving capacity, are directly affected by changes of the back pressure on the Valve.
        • Pilot Operated Pressure Relief Valve  -  This valve uses pressure to keep the valve closed.  It automatically controls the actuator pressure to either open or close the valve depending on the pressure setting of the pilot vs the actual system pressures.  They are used on liquid storage tanks and other vessels to prevent structural damage due to excess internal pressure or vacuum.  System pressure is fed from the inlet into and through the pilot control and ultimately into the main valve's dome.  The system pressure will prevent the main valve from opening.
        • Power Actuated Pressure Relief Valve  -  This is a valve in which the relieving device is combined with and controlled by a device requiring an external source of energy.
        • Temperature Pressure Actuated Relief Valve  -  The purpose of the valve is to release water, and in turn lower the pressure inside the tank, in the event that the temperature or pressure gets too high in the system.  Without this valve, the system would run the risk of exploding if the temperature or pressure got higher than the tank was designed to handle.


PSV Orifice Designation

The effective discharge area is the area that the fluid passes through when the valve is relieving.  It is dependent on various properties of the fluid & process including temperature, relieving pressure, flow rate and compressibility.


 Size (in2)Size (mm2)
D 0.110 70.97
E 0.196 126.5
F 0.307 198.1
G 0.503 324.5
H 0.785 506.5
J 1.1287 830.3
K 1.838 1,186
L 2.853 1,841
M 3.60 2,323
N 4.34 2,800
P 6.38 4,116
Q 11.05 7,129
R 16.0 10,323
T 26.0 16,774


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Tags: Valves Safety