The stem drives the ball through a slot, so the ball is free to shift slightly along the pipe axis. Upstream pressure pushes it against the downstream seat, and that seat does the sealing. The design is simple and cheap, which suits smaller bores and lower pressure classes; in large sizes or high pressures the seat load and operating torque climb too high, and designers switch to trunnion mounting.
Up to 4 inch we buy floating ball valves, anything bigger in Class 600 goes trunnion.
Also heard as
- floating ball
Do not mix it up with
- Trunnion-mounted ball valve: A trunnion ball is fixed on its axis and the seats move to it; a floating ball moves into the downstream seat.
Related terms
Ball valve
A quarter-turn valve that opens and closes by rotating a ball with a bore through it.
Trunnion-mounted ball valve
A ball valve whose ball is anchored top and bottom on trunnions while spring-loaded seats press against it.
Seat
The surface inside a valve that the closure member presses against to stop flow.