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Already have the specification? The enquiry checklist is in the last section; send the specification to enquiry@flowsynergy.ae for valve selection and technical support.
How does a bellows seal valve work?
Dixon describes the bellows as a cylindrical metal tube that expands and contracts like an accordion, welded to the stem and bonnet. It is compressed when the valve opens and expands when the valve closes, so it moves with the stem and leaves the gas or liquid no leak path out along the stem.
Dixon's primer on bellows seal valves says that because the bellows is welded to the stem and bonnet, there is no leak path for the gas or liquid to escape. Bonney Forge lists seal welded bellows designed to isolate the valve packing from potentially caustic gases.
Bellows are made in two ways. Dixon describes welded bellows, built from thin washer-like plates welded at their inner and outer edges, and hydraulically formed bellows, pressed from a single tube. For the same stroke, formed bellows are two to three times longer than welded ones, and Dixon says they are more economical but need more vertical space. Dixon's business development manager Dan Fisher puts the choice this way:
"Form factor is really the major reason to choose one over the other."
Dan Fisher, Dixon, Bellows Seal Valves: A Primer (2021)
When to use a bellows seal valve: what does a specification name?
A specification calls for a bellows seal valve where any leakage through the stem is not acceptable, such as toxic or regulated fluids, heat transfer oil and steam. Conval says its bellows seal valves are used when packed valves may not reliably contain light gases or hazardous system fluids.
The Dixon G8 brochure names the services:
"For use in applications where leakage into or out of the valve is unacceptable such as heat transfer oil, toxic fluids, steam, and regulated media."
Dixon Eagle, G8 Series bellows seal valves (2019)
Lines that run hot and cold are a second reason. Fisher says traditional packed valves tend to loosen up with the temperature changes, and that a bellows seal valve does not because it is hermetically sealed, which is why Dixon calls bellows seal valves the valve of choice in heating and steam services. Dixon's brochure adds that its bellows keep corrosive or harmful atmospheric conditions from entering the process. The other valves on a steam line are covered in steam and pressure protection, and how the service decides the rest of the valve is in valve selection by service condition.
When is a bellows seal valve not the right choice?
A bellows seal valve is a poor fit where the valve will be operated far more often than its bellows is rated for, or where the size is beyond what the maker offers. The bellows flexes on every stroke.
Dixon's bellows flyer says its cycle life tests are done with the bellows under pressure, at ambient and elevated temperatures, and gives minimum requirements of 2,000 cycles for gate valves and 5,000 for globe valves in sizes 1/2" to 4" up to Class 800. A valve that will be stroked many times a day should be checked against the bellows' rated cycle life before it is specified. Dixon offers a five year bellows fatigue warranty on its standard products.
Twisting is the second limit. Conval says its rising, non-rotating stem prevents destructive torsion of the bellows. Size is the third: Bonney Forge lists its bellows sealed valves from 3/8 NPS to 2 NPS, Dixon's G8 globe valves run from 1/2" to 2", and Conval's from 1/2" to 4". Those are the ranges of the three makers cited here, not of every maker; for a larger line, ask what is available before writing a bellows seal into the specification.
How is a bellows seal different from a low emission packed valve?
A packed valve seals at two surfaces, while a bellows seal valve closes off the stem with a welded metal barrier. Conval says packed valves need a 360 degree seal on the stem surface and another on the stuffing box wall, and that its bellows seal valve removes these two leak paths.
Bonney Forge says the packing on its bellows sealed valves is high quality packing to API 622 for reliable tightness and low emission performance, in full compliance with API 624, and lists API 624, ISO 15848 and TA Luft under fugitive emission. Dixon says its bellows have been emission tested per API 622.
What a fugitive emission result on a packed valve tells you is in our guide to ISO 15848-1 fugitive emission classes. Seat leakage, through the closed valve rather than out past the stem, is a separate question, covered in valve seat leakage classes.
| Point | Bonney Forge bellows sealed valves | Dixon G8 bellows seal globe valves | Conval bellows seal valves |
|---|---|---|---|
| Bellows | Seal welded bellows | Metallic bellows (Inconel) | 6-ply Inconel welded bellows |
| Packing | Packing to API 622, in compliance with API 624 | Graphite packing | Graphitic stem seal that backs up the bellows |
| Backseat | Not stated on the page | Backseat in the open position | Backseat in the full open position |
| Documents named | Design API 602, ASME B16.34, MSS SP-117; testing API 598, MSS SP-117 | Tested to ASME B16.34 / API 598; meets MSS SP-117 | Meets the requirements of MSS SP-117 |
| Sizes and classes | 3/8 NPS to 2 NPS, Class 150 to 2500 | 1/2" to 2", ANSI Class 150 to 800 | 1/2" to 4", through ASME Class 2500 |
Which documents does a bellows seal valve line name?
The document the bellows seal makers cited here share is MSS SP-117, which Dixon names as Bellows Seals for Globe and Gate Valves. Dixon also cites API 602 Annex C, Type Testing of Bellows Stem Seals, and the valves around the bellows are named to ASME B16.34 and API 598.
Bonney Forge lists MSS SP-117 under design, under inspection and testing and under marking, alongside API 602 and ASME B16.34. Dixon says its metal bellows are designed and tested to meet or exceed MSS SP-117 and API 602 Annex C, and Conval says its bellows seal valves meet the requirements of MSS SP-117. This guide does not reproduce any requirement from either document; confirm what they ask for against the current edition. What an API 598 shell and seat test covers, against its European counterpart, is in EN 12266-1 valve pressure testing, and gate valve standards are compared in our guide to API 600 and API 602.
Bolted or welded bonnet, and which bellows material?
The bellows seals the stem, but a bolted bonnet still has a gasketed joint, so makers offer a welded bonnet as well. Bonney Forge offers a bolted bonnet with a spiral wound gasket or a welded bonnet with a threaded and seal welded joint, and the bellows material is a separate line on the enquiry.
Bonney Forge's bolted bonnet uses a male-female joint and a spiral wound gasket in F316L and graphite, with ring joint gaskets on request; a full penetration strength weld is available on request for its welded bonnet. On materials, Dixon offers Inconel 625 bellows in all its valves, a 321 stainless steel bellows as standard on its welded bonnet design, and Hastelloy C-276, which Dixon says is commonly used for chlorine applications. Dixon's G8 series also has a hardfaced Stellite 6 seating surface.
What should a bellows seal valve enquiry state?
A bellows seal valve enquiry should state the fluid and why stem leakage matters, how often the valve will be operated, the documents the specification names, and the size, class and bonnet, because those decide whether a bellows valve fits and which one. These six points let a supplier quote the valve the specification means:
- The fluid, its temperature and pressure, and why stem leakage matters for it.
- How often the valve will be operated, so the bellows' cycle life can be checked.
- Gate or globe, size (NPS or DN) and class.
- The documents named: MSS SP-117, ASME B16.34, API 602, API 598, and any fugitive emission standard for the packing.
- The bonnet, bolted or welded, and the end connection.
- The materials of body, trim and bellows, if the specification sets them.
Send the list and the specification to enquiry@flowsynergy.ae or call +971 4 313 2786, Monday to Friday, 08:30 to 17:00 Dubai time. What our team checks is on technical support and testing; valve choice by standard is in the valve selection guide.
Frequently asked questions
Does a bellows seal valve still have packing?
Yes, on the designs of the three makers cited here. Dixon's G8 globe valves carry graphite packing as the second of three stem seals, Bonney Forge fits packing to API 622 on its bellows sealed valves, and Conval says its graphitic stem seal backs up the bellows.
Can I get a bellows seal valve in large sizes?
The three makers cited here list small sizes: Bonney Forge from 3/8 NPS to 2 NPS, Dixon's G8 globe valves from 1/2" to 2" and Conval from 1/2" to 4". For a larger line, ask the supplier what is available before writing it into the specification.
Is a bellows seal valve needed on every steam line?
Not by default. Dixon names steam among the services where stem leakage is unacceptable, and says packed valves tend to loosen on lines that run hot and cold, but the specification decides which steam lines need a bellows seal. Name it on each line where it is required.
Is a bellows seal the same as a fugitive emission rating?
No. A bellows seal is a design, while API 624, ISO 15848 and TA Luft are the documents Bonney Forge lists under fugitive emission. Bonney Forge says the packing on its bellows sealed valves is to API 622, in full compliance with API 624, so one valve can have both.
Sources quoted on this page
- Bonney Forge, Bellows sealed valves: bonnet designs, sizes, classes, applicable standards and product features, read 28 October 2026.
- Dixon Eagle, G8 Series bellows seal valves, brochure (2019), and Benefits of Bellows, flyer (2019), read 28 October 2026.
- Dixon, Bellows Seal Valves: A Primer, 17 March 2021, read 28 October 2026.
- Conval, Bellows Seal Valves, brochure, read 28 October 2026.
How this guide was compiled: from the three makers' own published pages listed above, each read on 28 October 2026; product details are the makers' own descriptions. No clause, table or test value from MSS SP-117, ASME B16.34, API 598, API 602, API 622, API 624 or ISO 15848-1 is reproduced. Reference data only, not a claim that Flowsynergy stocks or supplies any of these valves: confirm every line against the current edition and the maker's datasheet.