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 it, and the valve is open. The ball can also be positioned so that the hole is perpendicular to the passageway so that the fluid cannot pass through, and the valve is closed. It is controlled from the outside, often with a handle that is turned back and forth to open and close the valve.
The basic version described above is a two-way valve. This type 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 valve can shut off one or all of the three passageways it connects.
Because of the nature of this valve, it does not work well in situations where fine control of the valve is needed, such as the valve that controls the throttle in a car. It does work 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 a 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 valve offers no flow restriction, which means that when it 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 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 creates a small amount of flow restriction as the fluid passes through. A reduced port valve, on the other hand, has an smaller ball and an even smaller hole, which creates significant flow restriction as the fluid passes through it.