ARNOLD, K. (1999). Design of Gas-Handling Systems and Facilities (2nd ed.) Episode 2 Part 6 docx

Pressure Relief 361

Figure 13*2. Conventional

safety-relief valve. (Courtesy

ofAPI.)

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, bal￾ancing the pressure across the disk except for the portion of the disk open

to the vessel pressure Pj. The net opening force is equal to P] times the

area over which Pj acts. The closing force is the spring force F§ plus ?2

362 Design of GAS-HANDUNG Systems and Facilities

Figure 13-3. Operation of conventional

safety-relief valve. (Reprinted with

permission from API RP 520.1

times the same area where P} acts. When the open area times the differ￾ence in pressures, PI minus P2, equals the spring force, the valve begins

to open. Increasing the pressure on the back of the disk, P2 or the back￾pressure, 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 con￾sists 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 affect￾ed. 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 over￾riding the spring if foreign material or ice become lodged under the disc.