A ball valve, one type of quarter turn valve, is quite literally a ball placed in a passageway through which fluid flows. The ball has a hole through it, by which the valve opens and closes. When the ball is positioned so that the hole runs the same direction as the passageway, the fluid simply flows through the hole, and the valve is open. However, when the ball is positioned so that the hole is perpendicular to the passageway, the fluid cannot pass through, and the valve is closed. The ball is controlled from outside the valve, often with a handle that is turned 90 degrees, or a quarter turn, back and forth to open and close the valve.

The basic ball valve, described above, is a two-way valve. This ball valve has a single, straight passageway bored through the ball, making two openings: one on each side, an inlet and an outlet. A ball valve can also be a three-way valve if a third hole is bored partially through the ball, until it meets the main hole, forming a T. A three-way ball valve can shut off one or all of the three passageways it connects.

Because of the nature of the ball valve, it does not work well in situations in which fine control of the valve is needed, such as the valve that controls the throttle in a car. However, a ball valve works very well for situations in which a flow needs to be completely shut off, such as the shut-off valve on the main water line in your house. Ball valves also do not tend to develop problems if they are not used for long periods of time; they will still work perfectly when needed again.

There are three different types of ball valve. A full port ball valve offers no flow restriction, which means that when the ball valve is open, the liquid can flow freely through it. This is achieved by making the ball larger than the passage size, so that the hole bored through it can be the same size as the passage. A standard port ball valve does not have an oversized ball, and as a result the hole is one size smaller than the passageway. This presents a small amount of flow restriction as the fluid passes through the ball valve. A reduced port ball valve, on the other hand, has an even smaller ball and an even smaller hole, which creates significant flow restriction as the fluid passes through the valve.