Choosing a bellows seal valve is a material-and-duty decision before it is a size decision: process fluid, temperature, pressure class, and required throttling accuracy all drive the spec before the nominal DN is even written on the RFQ [S3].
The dominant configurations in current industrial use are the bellows seal gate valve (sizes 2″-32″, PN16-PN250, working temperature -29 °C to +350 °C) and the bellows seal globe valve, with the globe variant preferred wherever throttling or frequent cycling is required [S1][S2]. The two share a welded metal bellows as the primary stem seal, but they solve different flow-control problems and should never be substituted for one another without re-checking the duty.
Define the Duty Before the Brand
Zero-emission service is the core reason a spec moves from a packed industrial valve to a bellows seal design: a welded stainless steel bellows isolates the stem from the process media, replacing the graphite packing that gradually wears during cycling [S1][S2]. The bellows is typically welded to both the stem and the bonnet, so the fluid-containing area is hermetically separated from atmosphere even after thousands of stem travels [S3]. For most hydrocarbon, steam, and chemical-plant services, multi-ply SS316L is the baseline bellows material because it combines fatigue life with broad media compatibility, and a flexible graphite packing is retained only as a secondary backup seal [S2]. If the duty is on-off isolation only with no throttling, the gate geometry is the lower-cost answer; if the duty includes flow regulation, repeated modulation, or tight shut-off against a pressure differential, the globe geometry is the correct one [S1][S2][S3].
Body and Trim Material Mapping
Body material selection is driven by corrosion, temperature, and code compliance rather than by the bellows itself. Carbon steel ASTM A216 WCB is the workhorse for Class 300 steam and refinery service, with a published reference build at 3 Inch, ASME Class 300, RF flanged end [S2]. For sour, cryogenic, or chloride-bearing services, the cast equivalents shift to LCB/LCC for low temperature, CF8/CF8M for stainless, and CN7M, Alloy 20, Hastelloy C, or 904L for aggressive chemicals, all of which appear as standard body options in current bellows seal gate-valve catalogues [S1]. Trim pairing matters as much as body: a WCB body is routinely delivered with 13Cr or Stellite overlay on the disc and an integral or Stellite hard-faced seat to extend shut-off life under thermal cycling [S2]. Stem material follows the trim, with ASTM A182 F6a or SS410 specified for WCB bodies to keep galling and corrosion in balance [S2].
Pressure Class, Temperature, and End Connections
For gate valves, current OEM data sheets show PN16-PN250 as the standard pressure envelope and -29 °C to +350 °C as the standard working-temperature envelope, with end connections including RF flanges, RTJ flanges, butt weld, hub end, and groove [S1]. For globe valves built to BS 1873 with ASME B16.34 pressure design and ASME B16.10 face-to-face dimensions, Class 300 (300LB) is the most commonly stocked rating, with the bellows element itself rated for the full body pressure class because it is a pressure-containing part, not just a seal [S2]. Whenever the process exceeds 350 °C or drops below -29 °C, the body and bonnet material must be re-selected (for example to LCB for cryogenic, or to a high-temperature alloy) before the pressure class is even discussed, because the pressure envelope of standard carbon steel collapses outside that band [S1].
Bellows Geometry, Ply Count, and Fatigue Life
The bellows is the failure-limiting component of any bellows seal valve, and it is the part a buyer must interrogate on the data sheet. Multi-ply SS316L is the current default because adding plies increases the number of convolutions that share the axial stroke, which raises fatigue life at the cost of higher stiffness and a slightly taller bonnet [S2]. Single-ply designs remain in use for lower-cycle duties and for services where minimum bonnet envelope is required, but the published reference build for hazardous-service globe valves uses multi-ply SS316L specifically to extend the maintenance interval [S2]. The bellows is welded to both the stem and the bonnet, so the welded joint, not just the convolution, is a fatigue hot spot; a vendor that publishes cycle counts to leak rather than just burst pressure is the safer pick [S3].
Standards and Compliance Stack
A spec-grade bellows seal globe valve is normally built to BS 1873 for globe-valve design, ASME B16.34 for pressure-temperature ratings, ASME B16.10 for face-to-face, ASME B16.5 for flange dimensions, and API 598 for inspection and testing [S2]. Gate-valve equivalents reference ASME B16.34 for pressure design and gate-valve body standards such as API 600 or API 602 depending on size, with end connections per ASME B16.5 or butt-weld per ASME B16.25 [S1]. For fugitive emissions, the relevant benchmark is ISO 15848, which rates stem-seal leakage classes; buyers should request the class on the data sheet rather than accept a generic "zero leakage" claim. Where the service is in a hazardous area, the mechanical seal reference architecture of the bellows assembly is conceptually similar to a pump seal, and the same ATEX/IECEx certification thinking applies to the actuator and stem area.
Gate vs Globe: A Side-by-Side Decision
Use the gate variant when the duty is full-bore on-off isolation with minimal pressure drop, the cycle count is low, and the line is large (2″-32″ in the current GMK catalogue) [S1]. Use the globe variant when the duty requires throttling, frequent cycling, or tight shut-off against differential pressure, with the published reference at 3 Inch, Class 300, RF end as a typical build [S2]. On cost, gate bodies are cheaper per size class because the bonnet is shorter and the trim is simpler; on lifetime, a globe with Stellite-trimmed seat and disc will outlast a gate in any modulating service, which is why most steam and thermal-oil plants standardise on the globe pattern. Do not use a gate valve for throttling: the partially open gate is exposed to erosion and vibration, and the bellows will fail prematurely even if the body survives.
Who Should NOT Pick the Bellows Seal Option
If the fluid is non-hazardous, the plant tolerates minor packing leakage, and maintenance crews are comfortable repacking valves, a conventional packed industrial valve is the lower-cost choice and the bellows premium is not justified. If the duty is high-cycle modulating service on a slurry or a solid-bearing stream, even a multi-ply SS316L bellows will be destroyed by erosion long before its fatigue limit; a different valve geometry or a different sealing technology altogether is the right answer. If the line size exceeds the catalogue envelope (above 32″ gate or above the published globe size range), the buyer must move to engineered specials with longer lead times, and the standardisation benefit of the bellows design is partly lost. Similar to the way oil seal selection works on rotating equipment, the right answer is the simplest design that meets the duty, not the most exotic one. [S2]
RFQ Shortlist Logic
A shortlist-ready spec should lock in four items before vendor selection: (1) the geometry (gate or globe) justified by throttling requirement; (2) the body and trim material pair (for example WCB + 13Cr/Stellite for steam, CF8M + Stellite for corrosive chemicals, LCB for cryogenic) [S1][S2]; (3) the pressure class and temperature envelope, with confirmation that the bellows itself is rated to the full body class [S1][S2]; (4) the standards stack, typically BS 1873 + ASME B16.34 + ASME B16.10 + ASME B16.5 + API 598 for globe, and the equivalent gate-valve body standard plus ASME B16.34 for gate [S1][S2]. Vendors that publish bellows cycle count to leak, ISO 15848 emission class, and full material traceability should rank above those that publish only burst pressure and a generic "zero leakage" line. Buyers who need a checklist for adjacent equipment, such as the protocol layer on a pressure transmitter tied into the same hazardous-area loop, can follow the same spec-first approach used for manometer protocol compatibility.
Track the next two signals: revised ISO 15848 class definitions for bellows-only designs, and any OEM data sheet update that publishes bellows cycle count to first leak rather than burst pressure, which would let buyers compare fatigue life across vendors on a single number.