Conventional Relief Valves Working Principle

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The figure above shows a cross section of a conventional relief valve. Conventional relief valves can be used if the header back pressure is low. The conventional relief valves are commonly used on onshore facility where relief valves are fitted with individual tail pipes. On offshore facility, the valves are used mainly as small threaded valves for fire and thermal relief and for liquid relief on pumps.

conventional safety relief valve diagram
Conventional safety relief valve diagram

Conventional relief valve consist of a spring which holds a disk closed against the vessel pressure. A bonnet covers the spring and is vented to the valve outlet. The outlet pressure P2 acts on both sides of the disk, balancing the pressure across the disk except for the portion of the disk open to the vessel pressure P1. The net opening force is equal to P1 times the area over which P1 acts. The closing force is the spring force Fs plus P2 times the same area where P1 acts. When the open area times the difference in pressures, P1 minus P2, equals the spring force, the valve begins to open. Increasing the pressure on the back of the disk, P2 or the backpressure, will hold the valve closed. “Back-pressure” is the pressure that builds up in the relief piping and at the outlet of the relief valve. It consists of constant back-pressure in the system, back-pressure due to other relief valves relieving, and self-imposed back-pressure due to the valve itself relieving. If P2 increases because the valve is installed in a header system with other valves, then the amount of pressure in the vessel (the set point) required to overcome the spring force increases.

The set point of conventional relief valve increases directly with back-pressure, so the back pressure must be maintained below 10%. The valves should only be used where the discharge is routed independently to atmosphere.

 

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