Gate Valve

Bonnet choice follows pressure and temperature, not preference

A bolted bonnet is the default across most of the Class 150–900 range and is what API 600 and API 602 describe for general cast and forged service. Its limitation is the gasketed joint: as line temperature rises the bolting relaxes and the gasket stress falls, so on main steam and high-temperature hydrocarbon lines the bonnet joint, not the body, becomes the first thing to leak. A pressure-seal bonnet inverts that relationship. Line pressure acts on a segmented sealing ring so the joint tightens as pressure rises, which is why pressure-seal construction is normal above roughly Class 900 and is what carries the upper end of the Class 2500 range shown above. The trade is maintainability: a bolted bonnet can be opened in the field with hand tools, while a pressure-seal bonnet is a workshop job. Decide this from the service conditions and the maintenance regime together, not from price alone.

Wedge type decides whether the valve binds

A solid wedge is the simplest disc and tolerates turbulent flow well, but it is a single rigid piece, so when a hot valve is closed and then cools, the body contracts onto a disc that cannot yield. The result is thermal binding — a valve that will not open without excessive stem force. A flexible wedge, machined with a relief groove around the hub, lets the two seating faces deflect slightly under that load and is the standard choice on API 600 valves in steam and any service with wide temperature swings. One point applies to both wedge types: a gate valve is an isolation device. Holding one partly open to throttle exposes the unsupported disc edge to high-velocity flow, which erodes the seat faces and sets the disc vibrating against the guides. Where throttling is required, a globe valve or a control valve belongs in that position instead.

Seats, stem sealing and where service life is actually lost

Seat facing is the usual life-limiting item. 13Cr hard facing is adequate for clean hydrocarbon and water service; a Stellite overlay is the normal step up for higher temperatures, erosive duty and any service where the valve is stroked frequently. Both are worth specifying explicitly, because a soft-faced seat in an abrasive line will not survive a maintenance cycle. Stem sealing is the second consideration and is increasingly the one that drives selection: a conventional packed gland can be arranged to meet most emissions targets, but where fugitive emissions must be eliminated rather than limited, a bellows-sealed stem provides a static metal barrier between the medium and the packing, which is the reason the bellow gate valve exists as a separate type in the range above. Finally, the OS&Y rising-stem arrangement earns its place — it keeps the stem threads out of the process fluid and gives an unambiguous visual indication of valve position, which matters on any line where an operator has to confirm isolation before work begins.

What to include on the enquiry

• Design standard and class — API 600 cast or API 602 forged, and whether BS 1414, DIN 3352 or GOST equivalence is required
• Bonnet type: bolted or pressure-seal, and the expected maintenance access
• Wedge type: solid or flexible, and the seat facing — 13Cr or Stellite overlay
• Body and trim material against the actual design temperature, not the nominal one
• End connection: RF, RTJ, BW, SW or NPT
• Stem sealing and any fugitive-emission target — packed gland or bellows seal
• Operator: handwheel, bevel gear, electric or pneumatic actuator
• Sour service (NACE MR0175 / ISO 15156) if the line contains wet H2S, and EN 10204 3.1 certification if required

Design guidance and standard references on this page are general engineering guidance for valve selection. Confirm the final specification against your line list and the applicable design code for the project.