The trunnion ball valve is a form of quarter-turn valve which uses a hollow, perforated and fixed/supported ball to control flow through it. A trunnion mounted valve means that the ball is constrained by bearings and is only allowed to rotate, the majority of the hydraulic load is supported by the System constraints, resulting in low bearing pressure and no shaft fatigue. The line pressure drives the upstream seat against the stationary ball so that the line pressure forces the upstream seat onto the ball causing it to seal.