Butterfly Valve Stem Diameter Calculation

1. It has to survive, with an appropriate safety factor, the maximum torque that the actuating device(s) can apply to it under all process conditions that it is rated for.
2. The diameter has to be standard so as to fit a shaft sealing device so that it can turn without letting the valve contents out.
3. It has to be easily manufactured from standard metal stock, allowing for the process materials that one might find inside the valve and all components' resistance to corrosion by those contents.

How big is the valve?

When I designed valves in the region of 3 or 4 m dia the main factors were torque from actuator or the flow medium (this could produce a large torque, even on a balance flow) and bending moment due to pressure and valve "blade " weight.

Failures in shafts that I have had to investigate were normally caused by fatigue effects from flow pulsations - standing waves, diesel exhausts or other turbulent flow. You must remember if it is a high temp application that the shaft allowable stress is reduced. If there are known pulsation factors, such as in large diesel exhausts (dia 1m to 2m) then you need to think more about the disc (blade) and sealing design as this can be used to reduce the stresses on the shafts. Do not forget to take account of the blade to bearing and bearing to actuator dimensions.