The Major Components of Valves

| Power Mechanical

In order to most effectively work with valves, you need to know the most important components of valves. Valves are assembled products that consist of 7 major parts so today, you will learn about the different parts and what each component does.

Valve Body

The valve body not only houses other valve parts including the disk and seat but it also is the main pressure-retaining part of the valve. It is also where fluid passes through the valve as both ends of the valve body are connected with piping. The ends can either be threaded or flanged or finished with butt-welding types that depend on the size and pressure of the piping system. Valve bodies come cast or forged in a variety of materials including cast steel, stainless steel, and alloy steel depending on the requirements of the valve as a whole.


The bonnet is the second pressure-retaining component of a valve and is also referred to as the cover. It is connected to the valve body to create the valve enclosure. In the case of globe, diaphragm, gate, or stop check valves, it has an opening where the valve stem passes through and provides access to the internal parts of the valve when maintenance is needed.


The valve stem’s purpose is to transfer the required motion to either the disc, plug, or the ball in order to open and close the valve. It is connected to the valve actuator, lever, or handwheel at one end and the valve disc at the other end.

In a gate or globe valve, the stem uses the linear motion of the disc to open or close the valve while in plug, ball, and butterfly valves, the disc is rotated to open or close it.


The disc is the part of the valve that either allows or stops the flow, depending on where it’s positioned. A valve disk can be forged, fabricated, or cast.


The seat is one of the main parts of the valve that directly affects the rate at which the valve leaks and is an integral part of the valve body. Valves can have one or more seats depending on the type of valve. For example, a gate valve has two seats, one that sits on the upstream side and the other on the downstream side.


Valve trim is the collective term for the removable or replaceable internal parts that come into contact with the flow medium. These parts include the valve seat, disc, glands, spacers, guides, bushings, and internal springs.


The actuator is the mechanism that operates the valve and is connected to the stem and disk assembly. It can be manually operated by a handwheel, lever, gear, chain, or through the motor, solenoid, pneumatic, or hydraulic mechanism.

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