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. The mechanical anchoring of the ball absorbs the thrust from the line pressure, preventing excess friction between the ball and seats, so even at full rated working pressure operating torque remains low. This is particularly advantageous when the ball valve is actuated because it reduces the size of the actuator and hence the overall costs of the valve actuation package. Advantages of trunnion ball design is the lower operating torque, ease of operation, minimized seat wear (Stem/ball isolation prevents side loading and wear of downstream seats improving performance and service life), superior sealing performance at both high and low pressure (a separate spring mechanism and upstream line pressure is used as the sealing against the stationary ball for low pressure and high pressure applications). The trunnion is available for all sizes and for all pressure classes but they are not suitable for throttling purposes. An article published in valve magazine discusses the unique operating dynamics of a trunnion ball valve.
For a very long time, the idea has existed that a trunnion-mounted ball valve can be opened under full differential pressure if the piping system can withstand the rapid increase in pressure and the forces induced by it. As valve sizes and operating pressures increase, however, several factors need to be considered if that valve is opening with little or no pressure downstream. Continuing reading.