Isolation and Regulating Valves

A valve does one of three jobs: isolate, regulate, or prevent reverse flow. Choosing a valve designed for one job and using it for another is the most common cause of valve trouble in a plant.

Isolation valves

• Gate valve — a wedge or slab moves across the bore. Full bore when open, low pressure drop, tight shut-off. Slow to operate, and not for throttling: a partly open gate vibrates and erodes the seat, after which it will never seal again.
• Ball valve — a bored ball rotated a quarter turn. Fast, full bore, excellent tight shut-off, the workhorse of hydrocarbon service. Soft-seated designs have a temperature limit; metal-seated designs handle high temperature and abrasive service. Trapped pressure in the body cavity of a double-seated ball valve is a real hazard and is handled by a cavity relief feature or a self-relieving seat design.
• Plug valve — a tapered or cylindrical plug, quarter turn. Compact, good in slurry and dirty service, widely used on wellheads and manifolds. Lubricated types need their sealant maintained.
• Butterfly valve — a disc rotating in the flow. Light, cheap and compact in large sizes, used for water, air and low-pressure service, and capable of coarse regulation. The disc is always in the flow path, so it is not full bore and cannot be pigged through.

Regulating valves

A globe valve forces the flow through a tortuous path past a plug and seat. Its characteristic suits throttling, it is repeatable, and it is the basis of most control valves. The penalty is a high permanent pressure drop, so a globe valve is the wrong choice for pure isolation on a long pipeline where that loss is paid for continuously.

A needle valve is a fine-regulation globe derivative used for instrument and sample lines.

Double block and bleed

Positive isolation for maintenance normally requires two valves in series with a means of venting the space between them, so that leakage past the first valve is detected and released rather than accumulating. Understand what your site's isolation standard requires before opening any hydrocarbon system.