Consists of a short cyclinder generally rubber lined both for corrosion protection and to act as a seal for the disc. The disc forms the closing device and is locked to a spindle. The spindle gland generally takes the form of an o-ring or v-seals.
The torque acting on this disc is caused by the dynamics of fluid flow across it. When the valve is partially open the flow across the top half is much smoother than the bottom. This results in a torque acting on the driving spindle
The torque rises from zero to a peak when the valve is in mid position and falls again to zero . This non-linearity makes for difficult accurate positioning. To overcome this it is normal to fit oversized actuators. The flow through the valve is also very non linear rising quickly after intiall openig and near full flow being achieved before the valve is 50% open