Choosing relief and control valves for GCC plants.

Relief valves: start from the worst case

A pressure relief valve is sized for the worst credible overpressure event, such as a blocked outlet, a fire around the vessel or a failed control valve. Sizing follows API 520, and flanged steel valves follow the standard sizes and ratings in API 526. The choice of type depends mainly on the back pressure from the flare or vent system.

• Conventional spring-loaded: simple and robust, for low and constant back pressure.
• Balanced bellows: for back pressure that varies, and to keep corrosive fluid away from the spring.
• Pilot-operated: for operating pressures close to set pressure, large sizes and high back pressure.

Send us the set pressure, the relieving case and flow, the fluid and its temperature, the back pressure and the inlet and outlet ratings. In the Gulf, also tell us if the valve sits outdoors in direct sun, since that affects spring and seal choices.

Control valves: size for the real operating range

A control valve should work between roughly 20% and 80% open across normal operation. Oversized valves are the most common cause of poor control and early trim wear. Sizing follows IEC 60534, using the minimum, normal and maximum flows with their pressure drops.

• Globe valves give the most accurate throttling and suit high pressure drops.
• Ball and butterfly valves suit large lines, high flows and lower pressure drops.
• Trim and materials must match flashing, cavitation, sand or sour service.
• Actuator and positioner must fail to the safe position and suit your control system signals.

Fewer trips, longer life

Most unplanned trips caused by valves come from sticking, leaking seats or worn trim. A simple plan helps: record every valve with its datasheet, test relief valves at set intervals, and check control valve signatures during shutdowns.

Our partner Young & Cunningham has published practical guides on specifying relief valves, choosing gate valves and valve automation. Ask us and we will send you the ones that fit your project.

Relief valves →   Globe and control valves →