The only way one could calculate this would require knowing what elastomers were used, dimensions and tolerances, and other unavailable information. The manufacturer or representative should be contacted directly for torque data. Or just let them furnish the actuator along with the valve, after you tell them the pressure conditions.
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The torque values are only of use if one is selecting an actuator. Farm the job out. Get the local friendly Valve Distributor to select the actuator to go with the required valve, and provide quotations for the required combinations. That way, if it's wrong, it's the Valve Distributor's problem. And it's not likely to be wrong, as a Valve Distributor that gets it wrong doesn't stay in business very long.
Use the telephone, and not the internet, for goodness' sake!
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Fisher Butterfly Torque Calculation
Re: Fisher Butterfly Torque Calculation
