In the Ball Valve design, a spherical ball with a passageway cut through the center rotates to allow fluid more or less access to the passageway. When the passageway is parallel to the direction of fluid motion, the valve is wide open; when the passageway is aligned perpendicular to the direction of fluid motion, the valve is fully shut (closed).
The following set of photographs shows a hand-operated ball valve in three different positions, from nearly full closed to nearly full open (left to right):
Simple ball valves with full-sized bores in the rotating ball are generally better suited for on/off service than for throttling (partially-open) service.
A better design of ball valve for throttling service is the characterized or segmented ball valve, shown in various stages of opening in the following set of photographs:
The V-shaped notch cut into the opening lip of the ball provides a narrower area for fluid flow at low opening angles, providing more precise flow control than a plain-bore ball valve.
Also See : Ball Valve Animation