VALVE FUNCTIONS AND BASIC PARTS
The cover for the opening in the valve body is the bonnet. In some designs, the body itself is
split into two sections that bolt together. Like valve bodies, bonnets vary in design. Some
bonnets function simply as valve covers, while others support valve internals and accessories
such as the stem, disk, and actuator.
The bonnet is the second principal pressure boundary of a valve. It is cast or forged of the same
material as the body and is connected to the body by a threaded, bolted, or welded joint. In all
cases, the attachment of the bonnet to the body is considered a pressure boundary. This means
that the weld joint or bolts that connect the bonnet to the body are pressure-retaining parts.
Valve bonnets, although a necessity for most valves, represent a cause for concern. Bonnets can
complicate the manufacture of valves, increase valve size, represent a significant cost portion
of valve cost, and are a source for potential leakage.
The internal elements of a valve are collectively referred to as a 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.
Because of the 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 designs, the disk lifts perpendicularly away from the seat so that an annular orifice
Disk and Seat
For a valve having a bonnet, the disk is the third primary principal pressure boundary.
The 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 of their disks.
The seat or seal rings provide the seating 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 ring. A fine surface finish of the
seating area is necessary for good sealing when the valve is closed. Seal rings are not
usually considered pressure boundary parts because the body has sufficient wall thickness
to withstand design pressure without relying upon the thickness of the seal rings.