This is another type of flow control valve shown in Figure 6.31.
It is made up of a ball with a through hole which is rotated inside a machined seat. The manner in which flow control is exercised can be understood better with the help of Figures 6.32(a) and (b).
From Figure 6.32(a), it can be seen how flow assists opening and opposes closing of the valve. Conversely, from Figure 6.32(b), the flow is seen to assist closing and oppose opening of the valve.
Figure 6.33 shows the balanced version of a ball valve. This valve uses two plugs and two seats with opposite flows resulting in very little dynamic reaction onto the actuator shaft, although at the expense of higher leakage.