Bellows Seal Globe Valve
General / GeneralDefinition
A bellows seal globe valve replaces the conventional packing gland with a metal bellows welded to the stem and the bonnet, creating a hermetic barrier between the process fluid and the atmosphere. The bellows flexes as the stem moves up and down, maintaining a continuous metal seal with zero path for leakage. This design achieves fugitive emission rates far below any packed-stem valve, making it mandatory for services involving toxic, carcinogenic, flammable, or environmentally sensitive fluids where even trace leakage is unacceptable.
Working Principle
The bellows is a thin-walled, multi-convolution metal tube welded at one end to the stem and at the other to the bonnet body. When the handwheel or actuator moves the stem, the bellows compresses or extends like an accordion, accommodating the stem travel while maintaining a continuous metal barrier. Process pressure acts on the outside of the bellows (or inside, depending on design), but the bellows wall contains it. A backup packing gland sits above the bellows as a secondary seal — if the bellows ruptures, the packing contains the leak until the valve can be taken offline for repair.
Key Components
| Bellows | Multi-convolution metal tube (316SS, Inconel, or Hastelloy) |
| Inner stem | Connects the bellows bottom to the disc; moves with the bellows |
| Outer stem | Connects the handwheel to the bellows top; pulls the bellows |
| Bonnet | Houses the bellows and provides the upper weld point |
| Backup packing | Graphite or PTFE packing above the bellows as secondary seal |
| Disc / Plug | Globe valve closure element that seats against the seat ring |
| Seat ring | Machined or replaceable ring in the body |
Types & Variants
| Bellows Type | Construction | Stroke | Pressure Limit | Common Service |
|---|---|---|---|---|
| Formed bellows | Hydroformed thin-wall tube | Short to medium | Lower (to Class 300) | General chemical, heating steam |
| Welded bellows | Individual diaphragms welded edge-to-edge | Longer | Higher (to Class 600+) | Toxic, high-pressure, cycling service |
Materials
Bellows: 316L stainless is standard for general chemical service. Inconel 625 for chloride-containing and high-temperature environments. Hastelloy C-276 for aggressive acids. Monel for hydrofluoric acid service. Body materials match the service: carbon steel (A216 WCB) for steam and general chemicals, stainless (CF8M) for corrosive media, chrome-moly (WC6/WC9) for high-temperature steam. Disc and seat facings: Stellite 6 or 13Cr for erosion resistance.
Applicable Standards
ISO 15848-1 (fugitive emissions) — bellows seal valves easily achieve Class BH (bellows, highest tightness). TA-Luft (German Clean Air Act) compliance is standard for bellows designs. API 602 covers forged steel globe valves, including bellows variants. ASME B16.34 for pressure-temperature ratings. API 622 and ISO 15848-2 address packing emissions testing for the backup packing.
Applications
Chemical plants handling benzene, vinyl chloride, phosgene, and other carcinogens where OSHA and EPA regulations require zero detectable emissions. Refinery hydrogen and hydrocarbon services where fugitive VOC emissions must meet LDAR (Leak Detection and Repair) program thresholds. Pharmaceutical processes with sterile or toxic intermediates. Chlor-alkali plants handling chlorine gas. Nuclear power plants on primary and secondary coolant loops. Any service where a packing leak would create safety, environmental, or product contamination consequences.
Advantages
- Zero fugitive emissions through the stem — hermetic metal seal
- Meets the most stringent environmental regulations (TA-Luft, ISO 15848-1 BH)
- No packing adjustment or maintenance — the bellows does the sealing
- Backup packing provides secondary containment if bellows fails
- Eliminates
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