Views: 0 Author: J-VALVES Publish Time: 2026-10-27 Origin: Site
API 609 splits butterfly valves into two categories, and neither is about the body. Wafer, lug and double flanged describe how the valve mounts. Category A and Category B describe how it seals. A Category A valve has a resilient seat, a polymer insert that the disc edge presses into. A Category B valve has a metal seat and holds pressure by contact stress rather than by conforming. Leakage expectation, temperature ceiling, torque and dead-end suitability all follow from that sentence. Most project arguments about butterfly valves are really arguments about which category the service needs.
In a resilient seated valve the seat is a soft ring set into the body, and the disc closes against it with a designed interference. The elastomer or PTFE deforms into the disc edge, which is why the design reaches tight shutoff at very low differential pressure, in both directions, on its own seating torque. Line pressure is not needed to complete the seal, and in dead-end service that matters. The price is paid at the seat, which sees the medium on every cycle. Temperature and chemistry are limited by the insert rather than the shell: a PTFE seat is normally capped around 180 °C, PPL around 280 °C and PEEK around 260 °C, whatever the carbon steel body could take.
A metal seat cannot flow into the smallest imperfection on a disc edge. It seals because the two surfaces are pressed together hard enough, and because the geometry brings them together in the final degrees of travel instead of dragging them across each other. That is what the eccentric designs exist for. In a double eccentric valve the shaft is offset from the seat face and from the pipe centreline, so the disc swings clear of the seat for most of the stroke and lands on it only at the end. A triple eccentric valve adds a conical seat, turning that last contact into a wedge action. The result survives high temperature and fire without a polymer to hold it, and never seals independently of torque or pressure.
| Point | Category A, resilient seat | Category B, metal seat |
|---|---|---|
| Sealing source | Polymer conforms to the disc edge | Contact stress plus pressure assist |
| Seat insert | PTFE, RTFE, PPL, PEEK or elastomer | Stainless or hard faced metal ring |
| Temperature ceiling | Set by the insert, not the body | Set by the body and the metal pair |
| Tightness at low differential | Yes, by design | Only with enough contact stress |
| Dead-end service | Yes, if the body style allows | Yes, if rated bidirectional |
| Fire exposure | Needs an added metal backup | Already metal on metal |
Four situations decide the choice, and each favours a different answer. Dead-end or low differential duty favours A, because a metal seat with no pressure assist and no thermal growth can weep at a fraction of a bar. Thermal cycling favours B, because a metal seat expands with the body instead of relaxing away from the disc. Slurry and solids favour B, because a soft seat pocket packs with grit that the disc then drags across the sealing face. Flammable service also favours B, since a metal seat is already the secondary seal that a fire-safe soft-seat design has to add deliberately.
Two phrases turn up together in tender documents far more often than they should: metal seated, and zero leakage. They contradict each other at low pressure, and a supplier who accepts both is either quoting a soft seat behind a metal-looking ring or planning to argue at the witness test. The honest version names the leakage class and the pressure at which it applies. The second trap is asking for a fire-safe certificate on a Category A valve and expecting the seat to stay as it was. Fire-safe construction on a soft-seated valve means adding a metal backup that takes over after the insert burns, which raises torque and changes the seat design. It does not turn a Category A valve into a Category B valve. A related point sits behind both: the seat test at 1.1 times rated pressure proves the seat at its design point, not at the 0.3 bar that a drain line will actually see, so where low-pressure tightness matters it has to be written into the order as a separate test.
Five lines settle the category. Name the category rather than describing the seat in prose. State the seat material with its temperature ceiling, and confirm that ceiling sits at or above the design temperature. State whether the valve is bidirectional or unidirectional, because that decides whether a dead-end block is permitted. State the leakage class and the pressure at which it will be measured. And if there is a fire requirement, state it by standard and edition rather than by adjective. One more line belongs there for dead-end duty: a wafer body is clamped between two flanges and cannot hold a dead end on its own, while a lug body can. That point is separate from the category, and it is missed often enough to be worth writing down.
J-VALVES builds both categories: resilient seated butterfly valves with PTFE, PPL or PEEK seats, and metal seated double eccentric and triple eccentric butterfly valves. The range runs from 2 to 112 inch (DN50 to DN2800) in ASME Class 150 to 900, in wafer, lug and double flanged bodies, in WCB, CF8, CF8M and bronze. Graphite body gaskets and metal-backed secondary seats are quoted for fire-rated duty. Shell tests run at 1.5 times and seat tests at 1.1 times rated pressure, with EN 10204 3.1 certificates on request.
Is a double eccentric butterfly valve Category A or Category B? Category B, because the category follows the seat construction and a double eccentric valve seals on a metal ring. The eccentric geometry is what makes that metal seat workable: the disc swings clear of the seat before it rotates, so the two metal faces meet at the end of the stroke instead of grinding across each other.
Can a Category A valve be used for dead-end service? Yes, because a resilient seat completes its seal without help from line pressure. Two conditions apply. The body must be lug or double flanged rather than wafer, since a wafer valve relies on the downstream flange to stay in place. And the valve must be rated for the differential in both directions.
Why does a metal seated butterfly valve leak on a low pressure air test? Because a metal seat develops its seal from contact stress and pressure assist, and at a few tenths of a bar there is almost no pressure assist. The disc and body also sit at shop temperature, where the pair is not bedded in the way it will be in hot service. If low-pressure tightness is a process requirement, specify that test pressure and its acceptance limit separately.
