Parts of a Gate Valve

by Carlos Mano; Updated September 26, 2017
Gate valves

We have all seen gate valves, although we might not know the proper name for them or exactly what their purpose was. The most visible part of a gate valve is the round knob sticking out of a pipe. They can be seen on water pipes, steam pipes or pipes that carry air. They are most often seen in the hallways or basements of buildings. Their purpose is to completely stop (or allow) the flow of something at one particular point. They don't control the flow--they just turn it on or off, and they do this with just a few simple parts.

Handle and Stem

The handle is the part you turn and the stem is the axis of the handle which rotates when the handle is turned. There are two types of handle and stem arrangements: the rising stem and the non-rising stem. The rising stem is a simple rod and handle. As you turn the handle, the handle and stem assembly protrudes further and further from the gate valve. It is easy to see if the valve is opened or closed. The non-rising stem is a more complex assembly that does not protrude from the valve in either the open or closed position. Gate valves with non-rising stems are found in places where there is very little room.

Disk and Seat

The disk is the end of the stem which fits snugly into the fixed seat when the valve is closed. Turning the handle causes the disk to be removed from the seat, which means that fluid can flow. Despite the name, these two parts come in a wide variety of shapes. Often the disk is a wedge that fits into a V-shaped seat, or the disk and seat are two parallel faces of the same shape. It is important that they fit together snugly when the valve is closed, and that they do not impede fluid flow when the valve is open. The valve should never be partially open as this causes the valve to vibrate.

Body and Bonnet

The body and the bonnet form the outer parts of the gate valve. The body is the part that connects to the pipe carrying the fluid the gate valve is controlling. The bonnet is the bulbous part in the center that the stem goes through. The bonnet contains the disk and seat. The body and bonnet are usually metal, but may be special alloys for special fluids. For low-pressure and low-corrosive fluids, the body and bonnet may be plastic.

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