Trunnion Mounted Ball valves are straight-through flow valves which provide positive shut-off with minimal pressure drop and flow turbulence. The barrier to flow is a ball which is rotated 90 degrees to the direction of flow.
In Trunnion Mounted valves, the ball is held in a fixed axial position by upper and lower stems (trunnions) and can move only in a rotational mode. The resilient seats are contained in metal carriers which are spring loaded against the ball. Hoe ever metal seats normally tungsten carbide coated can also be used. Line pressure applied to a close valve increases the spring load on the upstream seat to effect a tight seal. The primary seating is therefore on the upstream seat permitting trunnion mounted ball valves to be used for “block and bleed” service.
A variation of this is the Double Piston valve in which the upstream seat works as a standard trunnion mounted seat, but the downstream seat can also be pulled on with pressure by pressure in the body cavity, this is often used in top entry welded in line ball valves were a test can be applied into the body after an over haul without pressure testing the entire pipeline.
Because the stem motion from full open to full closed is a 90 degree rotation, ball valves are ideally suited for automatic operation.