For a valve having a bonnet, the valve disk is the third primary principal pressure boundary. The valve disk provides the capability for permitting and prohibiting fluid flow. With the disk closed, full system pressure is applied across the disk if the outlet side is depressurized. For this reason, the disk is a pressure-retaining part.
Disks are typically forged and, in some designs, hard-surfaced to provide good wear characteristics. A fine surface finish of the seating area of a disk is necessary for good sealing when the valve is closed. Most valves are named, in part, according to the design.
The seat or seal rings provide the setting surface for the disk. In some designs the body is machined to serve as the seating surface and seal rings are not used. In other designs, forged seal rings are threaded or welded to the body to provide the seating surface.
To improve the wear-resistance of the seal rings, the surface is often hard-faced by welding and then machining the contact surface of the seal rings. A fine surface finish of the seating area is necessary for good sealing when the valve is closed.