The internal elements of a valve are collectively called as valve’s trim. The trim typically includes a disk, seat, stem, and sleeves needed to guide the stem. A valve’s performance is determined by the disk and seat interface and the relation of the disk position to the seat. Contact of disk with seat matters most when evaluating the performance of valves.Disk to seat contact determines the presence of leakage from a valve.
Because of trim, basic motions and flow control are possible. In rotational motion trim designs, the disk slides closely past the seat to produce a change in flow opening. In linear motion trim design, the disk lifts perpendicularly away from the seat so that an annular orifice appears.