Valves are used for adjusting a flow rate and include ball valves.
A conventional ball valve includes a ball housing communicating with a pipe, a ball rotatably provided in the ball housing and having a linear flow path passing through a center of the thereof, and an actuator for rotation of the ball. Thus, when the ball rotates by the actuator so that the linear flow path of the ball is disposed in a direction of the pipe, an inner fluid of the pipe flows from a high-pressure side to a low-pressure side of the pipe.
However, the conventional ball valve has a problem that the actuator considerably increases in size because large driving torque is required due to large frictional force during the valve operation because of original characteristics of the ball valve even though a flow coefficient is high because the linear flow path is opened to almost the same size as an inner diameter of the pipe. In addition, the conventional ball valve has a problem that it is impossible to control the flow rate because an inherent flow coefficient curve of the ball valve has flow characteristics of a quick-open valve.