When the valve element is stopped abruptly at the guard or seat, it may not land evenly. Typically, one edge contacts first, which induces a bending moment. This flexing will eventually cause fatigue failure of the valve element. Several steps can be taken to reduce the flexing. Reducing valve lift is one remedy, but frictional power loss may increase. Providing balanced spring arrangements that allow the valve element to return to the seat as evenly as possible and/or providing a gas cushion can reduce fatigue failures of the valve element.
The gas cushion is produced by a close clearance between the valve element and a groove in the valve seat, which also locates the valve element. As the gas trapped in the space between the underside of the valve element and the walls of the groove escapes along the edges of the element, the element is cushioned as it returns to the seat, as shown in Figure 300-51.
Another type of damping, mass damping, is used in the “damped plate” design. A disk ported the same as the valve element is attached to the valve element, usually at its center. This damping plate adds inertia to the valve element and therefore retards the impact velocities of the valve element against the guard and seat. This type of damping is useful for high speed compressors where valve velocities are high. It is also more desirable than gas damping, where the gas stream is dirty or heavily lubricated. A damped plate valve is shown in Figure 300-52.