A passing valve is shut but lets flow by, usually from cut, eroded or debris-held seats. It is the term in oil and gas and power plants for internal leakage, as opposed to a packing or body leak to atmosphere. Passing valves defeat isolation, waste steam and product, and are found with temperature guns, acoustic leak detectors or by bleeding the downstream side.
Block valve is passing, the bleed won't go dead, we'll need to double block it off the next one upstream.
North Sea crews say a valve is letting by; Gulf Coast crews say it's leaking by. Both mean the same thing.
Also heard as
- letting by
- leaking by
- letting through
Related terms
Seat
The surface inside a valve that the closure member presses against to stop flow.
Bubble-tight
Shut off with no visible seat leakage when tested with air or gas under water.
Double block and bleed
Isolation using two closed seats in series with a vent or drain between them to prove the isolation holds.
Wire drawing
Erosion of thin grooves across a valve seat by a high-velocity jet leaking through a tiny gap.