A ball valve (often called a quarter turn valve) is a valve that opens by turning a handle attached to a ball inside the valve. The ball has a hole, or port, through the middle so that when the port is in line with both ends of the valve, flow will occur. When the valve is closed, the hole is perpendicular to the ends of the valve, and flow is blocked. The handle position lets you "see" the valve's position.

The body of ball valves may be made of metal, ceramic, and/or plastic. The ball may be chrome plated to make it more durable.

There are three general types of ball valves: full port, standard port, and reduced port.

• A full port ball valve has an oversized ball so that the hole in the ball is the same size as the pipeline resulting in lower friction loss. Flow is unrestricted.

• A standard port ball valve is usually less expensive, but has a smaller ball and a correspondingly smaller port. Flow through this valve is one pipe size smaller than the valve's pipe size resulting in slightly restricted flow.

• In reduced port ball valves, flow through the valve is two pipe sizes smaller than the valve's pipe size resulting in restricted flow.

Manually operated ball valves can often be closed quickly and thus there is a danger of water hammer. Some ball valves are equipped with an actuator that may be pneumatically or motor operated. These valves can be used either for on/off or flow control. A pneumatic flow control valve is also equipped with a positioner which transforms the control signal into actuator position and valve opening accordingly.

There are also three-way ball valves, with a T-shaped hole through the middle. With such a valve the flow can be directed to either one or the other or both sides or be closed off completely.