A ball valve is a valve that uses a ball shaped disk to start and stop flow. Ball valves are one of the simplest valve types, and are made in 1-piece, 2-piece and 3-piece configurations. They tend to be the cheapest valve type and one of the easiest to maintain.
One of the main advantages of ball valves is that they offer a very quick shut off, usually by turning the stem 90º. This is as opposed to gate and globe valves, for example, that require multiple turns to shut-off. For the very same reason, ball valves are not suited for flow-control or throttling, and user would prefer to opt for globe valves for these purposes. Ball valves are often sub-divided into categories that distinguish between the seat material, such as PTFE seated ball valves, which is the most common version or metal-seated ball valves for high-temperature applications.
Photo courtesy: The Alloy Valve Stockist