Bubble-tight shutoff means a closed valve shows zero visible bubbles during a low-pressure air or nitrogen seat test with the downstream side monitored under water — effectively zero leakage. It corresponds to FCI 70-2 Class VI, ISO 5208 Rate A, and the resilient-seat acceptance of API 598.
The term comes straight from the test bench: the seat is pressurised with 4–7 bar air, the other side is flooded or piped to an inverted water column, and the inspector counts bubbles for the standard duration. Zero bubbles equals bubble-tight. Because air at low pressure finds defects that high-pressure water masks, this humble test is the most discriminating seat check in routine valve inspection.
Formally, different standards express the same idea in different vocabulary: API 598 allows no leakage for resilient-seated valves, ISO 5208 calls it Rate A ("no visually detectable leakage"), and control-valve specification FCI 70-2 labels it Class VI with a tiny numeric bubble allowance that scales with size. Soft-seated ball valves achieve it as a matter of course; metal-seated valves need premium lapping to approach it and are normally sold to a small measurable allowance instead.
Specify bubble-tight where trapped leakage matters — gas service, tank isolation, anything with a flare or metering downstream — and accept a defined leakage class on metal seats where temperature or abrasion rules soft seats out. Every VTXFLOW soft-seated valve ships bubble-tight, verified by low-pressure air seat test on 100% of production.