Introduction to Valve Parts or Components

valve types
Various types of industrial valves
(courtesy of Flowserve)
Industrial valves come in many shapes and sizes and they are classified according to different functions they have. Regardless of type, all valves have the following basic parts or components, the body, bonnet, trim, seat, stem, actuator, and packing.

Valve Body

The valve body is the primary boundary, housing or container. It serves as the main element of a valve assembly. It is the framework that holds all the parts together.

The body contains the fluid pressure loads from the inlet and outlet piping. The body connects to the piping via through threaded, flanged, or welded joints.

Valve Bonnet

The cover for the opening in the body is the bonnet, and it is second most important boundary of a pressurized valve. Like valve bodies, many designs and models are available for bonnets. A bonnet acts as a cover on the valve body, is cast or forged of the same material as the body. It is commonly connected to the body by a threaded, bolted, or welded joint.

Valve Trim

Valve trim is a collective name for the replaceable parts, in a valve. A typically trim design includes a disk, seat, stem, and sleeves needed to guide the stem.

Valve Disk

The disc is the part which allows, throttles, or stops the flow, depending on its position. In the case of a plug or a ball valve, the disc is called plug or a ball. Disks are generally forged, and in some designs they are hard surfaced to provide good wear properties.


The seat or seal rings provide the seating surface for the disk. A valve may have one or more seats. In the case of a globe there is usually one seat, which forms a seal with the disc to stop the flow.

Valve Stem

The valve stem provides the necessary movement to the disc or plug for opening or closing the valve. It is responsible for the proper positioning of the disk. It is connected to the valve hand wheel, actuator, or the lever at one end and on the other side to the valve disc.

Valve Yoke

A Yoke connects the valve body or bonnet with the actuating mechanism. The top of the Yoke holding a yoke nut, stem nut, or yoke bushing and the valve stem passes through it.

These are the important and most basic parts of a valve and they play a very important role in functioning of a valve regardless of its type or model.