Self Relieving Seats (single piston effect)
The single piston effect is the the standard design for trunnion mounted ball valves. Pressure from both upstream and downstream sides pushes the seat rings against the ball. If the force created by the body cavity pressure is greater than the preloaded spring force plus the force created by the pressure from upstream or downstream side, the seats are pushed away from the ball. Thus, any overpressurein the body cavity is released automatically in the valves' fully open or fully closed position.