Diaphragm Valve vs Bellows Valve

Published 2026-11-05 · Updated 2026-11-05 · Valonix Engineering

Both valve types remove the sliding stem seal from the gas path, but they solve different problems. A diaphragm valve uses a flexible metal diaphragm as the closing element; a bellows valve keeps a rigid plug and seals the stem with a welded metal bellows.

Sealing principle

In a diaphragm valve the diaphragm is both the closure and the containment boundary, so there is no cavity behind the seat. In a bellows valve the bellows only seals the stem; the closure is a plug or disc pressing onto a seat.

Purity and dead volume

Diaphragm valves win on internal volume and particle generation, which is why they dominate UHP distribution lines. Bellows valves have a larger body cavity but tolerate higher temperatures and more aggressive chemistry.

Temperature and aggressive media

Where a line is heated, or where halides and hydrides attack a thin diaphragm, a bellows sealed valve with compatible wetted alloys is the safer specification.

Leak testing and maintenance

Both are helium leak tested. Diaphragms are a wear item eventually replaced on a schedule; bellows assemblies are tested across the full stroke and are generally not field-repaired.

Which one to specify

Use diaphragm valves for the cleanest UHP distribution, automated gas panels and point-of-use isolation. Use bellows sealed valves for toxic, corrosive or high temperature services where a hermetic stem seal is mandated by the safety case.

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