Butterfly valves, Bianca, Desponia

How to consider break away torque?

Afshin Khosroshahi

Jun. 27, 2022 | 5 min read

A centric-disc butterfly valve has a break away torque as long as the disc is in contact with the liner. The torque then falls to around 20% of the initial value, once the disc is moved.

The above figure shows the torque behaviour of butterfly valve, Desponia^®, and pneumatic actuator IA motion. When the valve is closing, the disc comes again into contact with the liner, and the torque is increasing to the max. value at 0°.

How to consider break away torque?

Break away torque has a direct influence on actuator selection, as they should conquer this torque at the very beginning and closing of the valve. To consider break away torque, a safety factor is applied to actuator selection as:

In normal conditions:

On the rubber-lined butterfly valves, Desponia^® and Desponia^® plus, the safety factor for the actuator selection is included.
On the Teflon-lined butterfly valves, Bianca, since PTFE has a much better friction coefficient than rubber, a safety factor of 1,1 is applied.

In severe conditions:

For media between 20 °C and 80 °C, the torque values for “normal conditions” and rubber-lined butterfly valves can be used.
For all other media, the torque values for “severe conditions” must be used. This value is 1,35 higher than the normal torque values.
This does not apply to the Teflon-lined butterfly valves, since PTFE is not being influenced by liquid, temperature, etc.