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Ball Valves
This illustration shows the internal structure of a ball valve. The control element for this valve consists of a sphere which has an opening cut through it. To open the valve, the opening in the ball is aligned to the fluid flowpath. On the other hand if you turn the ball a quarter of a turn, the solid surface of the sphere faces the fluid, thereby shutting off the fluid flow.
Notice the operating mechanism and the handwheel which is used to operate the valve.
In this figure the ball is shown in the open position by "C." The operating mechanism is shown by "A" and "B." Notice the handwheel shown by "A." This handwheel is rotated to move the ball by quarter of turn to take it from full open position to full close position. In this particular design, the handwheel is required to be rotated through many turns to cause the operating mechanism to rotate the ball through 90 degrees (quarter turn.)