The phrase comes from seat testing, where air pressure on one side of a closed valve shows any leakage as bubbles out of a tube under water. Fitters and buyers use it loosely to mean a valve that holds absolutely, the expected result for soft-seated ball and butterfly valves. Metal-seated valves usually have a small allowable leak rate and are not bubble-tight.
We need bubble-tight isolation for that hot work, a metal-seated gate isn't going to cut it, blind it.
Also heard as
- zero leakage
- drop-tight
- drip-tight
Related terms
Shutoff class
A rating of how much seat leakage a closed control valve is allowed, from Class I to Class VI.
Seat
The surface inside a valve that the closure member presses against to stop flow.
Passing
Leaking through the seat while closed, so fluid still gets past a valve that is supposed to be shut.
Ball valve
A quarter-turn valve that opens and closes by rotating a ball with a bore through it.