Gate, globe, ball, butterfly or check: choosing valves for oil and gas
Each valve type is good at one job. The right choice comes down to whether you need isolation, control or backflow protection, plus size, pressure and the fluid.
Start with the job
Valves do three main jobs: isolate (fully open or fully closed), regulate (throttle flow) and prevent backflow. Most valve problems come from asking a valve to do a job it was not designed for, such as throttling with a gate valve.
| Valve | Best for | Avoid using it for | Common standards |
|---|---|---|---|
| Gate | Isolation with minimal pressure drop | Throttling, which erodes the seats | API 600, API 602, API 6D |
| Globe | Throttling and regulating flow | Services where pressure drop matters | ASME B16.34, BS 1873 |
| Ball | Fast, tight quarter-turn shut-off | Precise throttling with standard designs | API 6D, API 608, ISO 17292 |
| Butterfly | Large-diameter isolation in limited space | Tight high-pressure shut-off with concentric designs | API 609 |
| Check | Preventing reverse flow | Oversized installations, which chatter | API 594, API 6D |
| Plug | Isolation in dirty or abrasive services | Frequent throttling | API 599, API 6D |
Ball valves: floating or trunnion?
In a floating ball valve, line pressure pushes the ball against the downstream seat to seal. That works well in smaller sizes and lower pressure classes. As size and pressure rise, the force on the ball makes it too hard to turn, so trunnion-mounted designs hold the ball on fixed shafts and seal with spring-loaded seats instead.
Butterfly valves: concentric to triple offset
Concentric butterfly valves have a rubber or polymer lining and suit water and utility lines. Double-offset, or high-performance, designs lift the disc away from the seat as it opens, which reduces wear. Triple-offset designs add a conical seat for metal-to-metal sealing in high-temperature and critical hydrocarbon services.
Check valves
Swing checks suit horizontal lines with steady flow. Lift checks suit small-bore, high-pressure lines. Dual-plate wafer checks are compact and light and close quickly, which helps limit water hammer. Size a check valve to the actual flow, not just the line size, so it opens fully and does not chatter.
Fire-safe and sour service
Soft-seated valves in hydrocarbon service are usually specified fire-safe and certified to API 607 or API 6FA, so they still seal after a fire has damaged the soft seats. For sour service, materials and hardness must meet NACE MR0175 / ISO 15156.
Pressure testing
Every new valve is pressure tested before it leaves the factory. API 598 covers inspection and testing for most process valves, with a shell test of the body and seat tests in the closed position. Pipeline valves to API 6D are tested to the requirements in that standard. If your project calls for extra tests, such as gas seat tests or tests witnessed by an inspector, include them in the enquiry.
End connections
Flanged ends, raised face or RTJ, are the most common in process piping. Butt-weld ends suit high-pressure lines and pipelines, where fewer joints mean fewer leak paths. Socket weld and threaded ends are used on small-bore lines, usually 2 inches and below.
What to put in a valve enquiry
- Valve type and design standard
- Size and pressure class
- End connections and flange facing
- Body, trim and seat materials
- Operator: lever, gear or actuator
- Service: fluid, temperature, sour or fire-safe requirements
- Test and certificate requirements
Frequently asked questions
Can I use a gate valve to throttle flow?
It is not recommended. A partly open gate valve vibrates, and the flow erodes the seats and wedge. Use a globe valve or a control valve for throttling.
What is the difference between API 6D and API 608 ball valves?
API 6D covers pipeline valves, typically larger and with features such as double block and bleed. API 608 covers metal ball valves for general refinery and process piping.
What does fire-safe mean?
A fire-safe valve has been type-tested to API 607 or API 6FA to show that it keeps sealing to an acceptable level after exposure to fire.
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