A stop valve is any sort of valve system that totally stops the flow of liquid through a pipe. There are many styles of stop valves, but two of the most common are gate valves and globe valves. Gate valves don’t restrict flow when not in use, but they take longer to move into position and are more likely to leak. A globe valve will interrupt flow at all times, but will move into position very quickly. Both of these valve types are common on piping systems of all shapes and sizes.
Most of the time, a stop valve is necessary as a safety feature. These valves completely stop flow of any kind through the system, often to avert a crisis before it can get out of hand. As a result, these valves are generally close to important machinery or a human workstation. A stop valve may have a manual crank or may be incorporated into an automated system. Manual stop valves typically have a brightly-colored screw handle, the kind common in many pipe systems.
Since any valve design that completely blocks flow is a stop valve, there are many different designs and styles. Even so, the gate valve and the globe valve are two of the more common types. These valves look a lot alike from the outside, so it is often difficult to tell them apart. The both have threaded openings to connect to pipes, a bulbous middle area that contains the valve and a long steam that holds the screw mechanism. On the inside, they are built quite differently.
A gate valve typically has a solid piece that slides up and down in the pipe that completely blocks flow. When the gate is up, there is no restriction at all and the fluid moves freely through the valve. When the gate is down, the blocked area is slightly larger than the opening. This is the reason this valve type has a bulbous body; the gate is slightly larger than the pipe to guarantee an adequate seal.
A globe valve has two chambers, one in the bottom of the valve and one in the top. Liquid moves into the bottom chamber and through an opening in the top. This opening leads to the upper chamber, where it leaves the valve. This valve style creates an interruption in the speed and pressure of the moving liquid even when not in use. As a result, this stop valve isn't used in places with tight tolerances. When closed, a stopper comes down from the top of the upper chamber and plugs the hole.
By M. McGee