Figure 13-2 shows a cross section of a conventional relief valve and Figure 13-3 is a schematic that shows the valve’s operation. Convention al relief valves can be used anywhere that back-pressure in the relief header is low. They are common onshore where relief valves are fitted with individual tail pipes. On offshore platforms, they are used mainly as small threaded valves for fire and thermal relief and for liquid relief around pumps. In a conventional relief valve, a spring 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.
Conventional relief valves should only be used where the discharge is routed independently to atmosphere, or if installed in a header system, the back-pressure build-up when the device is relieving must be kept below 10% of the set pressure so the set point is not significantly affected. The set point increases directly with back-pressure.
Conventional relief valves may be equipped with lifting levers or screwed caps. The lifting lever permits mechanical operations of the valve for testing or clean-out of foreign material from under the seat. Screwed caps prevent leakage outside of the valve, but also prevent overriding the spring if foreign material or ice become lodged under the disc.