A bellows-sealed valve replaces the sliding stem-packing interface with a welded metal bellows — a flexible, hermetic barrier that stretches and compresses with stem travel. It reduces stem leakage to essentially zero, which is why bellows seals are specified for toxic media, heat-transfer oils and emission-regulated services.
Conventional packing seals by squeezing a compliant material around a moving stem — a design that must leak a little to work at all. A bellows seal removes the sliding interface: a thin-walled metal accordion (typically multi-ply 316L or Inconel, hydraulically formed or edge-welded) is welded to the stem at one end and to the bonnet at the other. The process fluid is now enclosed entirely by welded metal; the stem moves by flexing the bellows, not sliding through a seal.
The result is measured hermeticity — helium leak rates in the 10⁻⁶ mbar·l/s range — plus a safety net: a conventional packing set above the bellows serves as backup, often with a monitoring port between the two that reveals bellows failure before anything escapes. The design's finite life is its one caveat: bellows are rated in mechanical cycles (commonly 5,000–10,000 full strokes) and suit rising-stem gate and globe valves far better than quarter-turn designs.
Typical applications: thermal oil to 400°C, phosgene and chlorine, hydrogen, vacuum lines and any EPA/IED-monitored VOC service. VTXFLOW bellow-sealed gate and globe valves use multi-ply 316L bellows with backup packing and inspection port.