Forged compact vs cast bolted-bonnet — where the line falls
A forged A105 gate valve and an API 600 cast gate valve solve different problems. Below about NPS 2 the forged route wins: an API 602 body can be machined from solid with an integral or bolted bonnet, socket-weld or threaded ends, and a wall section that carries Class 800 to 2500 without the pattern and riser work a casting needs at that size. Above NPS 2 the economics invert and API 600 cast construction takes over. If your line list mixes both, expect the changeover somewhere around NPS 2 rather than a single standard covering the whole range.
Wedge choice and thermal binding
A one-piece solid wedge is the most robust design and the most likely to stick. Close a solid-wedge valve hot, let the line cool, and differential contraction between body and wedge can drive the wedge into the seats hard enough that normal handwheel effort will not free it. A flexible wedge — a solid disc with a machined relief groove around the hub — lets the two faces move independently and absorbs that contraction, which is why it is the default for steam and for any line with a wide thermal swing. Parallel slide gates go one step further and take seat load from line pressure rather than wedging action.
Pressure locking, and why gate valves must not throttle
A double-seated gate valve traps fluid in the bonnet cavity when it is closed. If that cavity then heats up, the trapped liquid expands with nowhere to go and can hold the wedge shut against the operator or, in the worst case, overpressure the bonnet. The standard remedy is a relief hole drilled through the upstream side of the wedge, or an external bonnet relief line — specify which one you want, because it is not fitted by default on every design.
Gate valves are isolation devices. Held part-open, flow accelerates through the narrow gap between wedge and seat and erodes the seating faces, and the disc can chatter against the guides. A valve used for throttling will not pass a seat test a year later. Use a globe valve where you need to regulate.
What to include on the enquiry
- Bonnet construction: bolted, welded, or pressure-seal for higher classes
- Wedge type — solid, flexible, or parallel slide — and whether a relief hole is required
- Trim: 13Cr as standard, hard-faced or F316 where the service is erosive or corrosive
- Backseat requirement, and stem-seal emissions class if ISO 15848 applies
- Sour service (NACE MR0175 / ISO 15156), stated at enquiry stage rather than after award
- A bypass arrangement if the valve must open against a large differential pressure
Material behaviour and standard references on this page are general engineering guidance for grade selection. Confirm the final specification against your line list and the applicable design code for the project.
