An isolation valve is a valve in a fluid handling system that stops the flow of process media to a given location, usually for maintenance or safety purposes.
They can also be used to provide flow logic (selecting one flow path versus another), and to connect external equipment to a system.
A valve is classified as an isolation valve because of its intended function in a system, not because of the architecture of the valve itself. Therefore, many different types of valves can be classified as isolation valves.
To easily understand the concept of an isolation valve, one can think of the valves under a kitchen or bathroom sink in a typical household. These valves are normally left open so that the user can control the flow of water with the spigot above the sink, and does not need to reach under the counter to start or stop the water flow.
However, if the spigot needs to be replaced (i.e. maintenance needs to take place on the system), the isolation valves are shut to stop the flow of water when the spigot is removed. In this system, the isolation valves and the spigot may even be the same type of valve. However, due to their function they are classified as the isolation valves and, in the case of the spigot, the control valves.
As the isolation valve is intended to be operated infrequently and only in the fully-on or fully-off positions, they are often inferior quality globe valves. These less expensive styles lack a bonnet and stem seal in favor of threading the stem directly into the body. The stem is covered with a rubber washer and metal cap similar in appearance to a gland nut. Because they lack a stem seal they will leak unless fully closed and installed in the correct direction or fully open, causing the disk to compress the top washer against the stem.