Because the ball cannot float, line pressure loads the seats instead of the ball, so operating torque stays reasonable in large bores and high pressure classes. Each seat is pushed against the ball by springs and by pressure behind it, which lets the valve seal on the upstream seat and gives double block and bleed capability. It is the standard pipeline valve, with a body cavity that must relieve trapped pressure.
That 20 inch trunnion ball valve needs the gearbox and a full stroke check before we put the pig through.
Also heard as
- trunnion ball valve
- trunnion
Related terms
Floating ball valve
A ball valve whose ball is held only by the seats and is pushed downstream by line pressure to make the seal.
Double block and bleed
Isolation using two closed seats in series with a vent or drain between them to prove the isolation holds.
Ball valve
A quarter-turn valve that opens and closes by rotating a ball with a bore through it.
Pressure locking
A closed gate valve that cannot be opened because trapped pressure in its bonnet cavity jams the wedge into both seats.