Three offsets do the work: the shaft sits behind the sealing plane, the shaft is off the pipe centerline, and the seat is a cone whose axis is tilted from the pipe axis. Together they make the disc cam into the seat without sliding contact, so a laminated or solid metal seal can give near-zero leakage at high temperature. These valves are torque-seated and compete with gate valves in steam, hydrocarbon and isolation service.
We're putting a triple-offset on the HP steam header instead of another 20 inch gate, it weighs a third as much.
Most are designed with a preferred pressure direction; install one backwards and shutoff gets worse when pressure rises.
Also heard as
- TOV
- triple offset
- TOBV
Related terms
Butterfly valve
A quarter-turn valve with a disc on a shaft that rotates in the bore to open, close or throttle flow.
Seat
The surface inside a valve that the closure member presses against to stop flow.
Fire-safe
Describes a valve built to limit leakage through and around it during and after an external fire.
Gate valve
A multi-turn isolation valve that lifts a flat or wedge-shaped gate clear of the flow path to open.