A gate valve is a type of stopper placed on a pipe system to block flow. These valves simply prevent the passage of liquid using a wedge that slides in and out of the pipe. In most cases, the gate valve is designed to be completely opened or completely closed. As a result, these valves are rarely used as a means of flow control; they generally just stop flow completely or are unused. Partially-closed gate valves may increase pressure in a system in unpredictable ways or cause vibration in the liquid.
The construction of a gate valve is quite simple. They generally have two threaded connectors that hook into the pipe system. It is possible for a gate connector to have more than two connections, but this is very rare. A post, called a bonnet, rises above the connectors; this area holds the stopping wedge and the mechanism for raising and lowering it. Lastly, a handle, which controls the rising and lowering of the stopper, sits on top of the bonnet.
When the handle is turned, it moves the stopper inside the valve. The stopper slides down into the pipe and blocks liquid from passing. The valve will typically indicate the position of the stopper on the outside of the valve, or the handle will screw in and out to show the relative height of the stopper. The valve casing is slightly wider than the connected pipes, allowing the stopper to totally cover the opening and make it completely liquid-tight.
The method used to connect the bonnet to the valve has an impact on the use of the gate valve. If the bonnet is simply screwed on, the valve will work well for light-duty applications. Bolted bonnets work well for heavy-duty jobs, but cannot be taken apart for repair or cleaning. Union-bond bonnets may be taken apart easily, but they also have the highest rate of leaking. The last style is a pressure-sealed bonnet—it works well in many different environments, but it is also the most expensive style.
Since a gate valve is supposed to be either all the way open or all the way closed, it shouldn’t have any impact on overall pressure. A properly-designed valve will be the exact same size on the inside as the pipe, since any narrowing or widening of the opening will change the speed of the flow. When the gate is closed, the pressure should be constant in the system, as there is no movement at all.