The actuator sizing is based on the torque requirement for the valve to be actuated. From the valve data sheet obtain the working torque Nm (torque required to turn the valve at the required pressure, temperature and media viscosity) and add a minimum 30% safety factor. Always check breakaway- torque for the valve. Especially on butterfly valves this can be higher than the working torque because of the unseating of the valve disc off the seats. Using this find the corresponding actuator with this figure (or greater) and select. Also check the valve mounting pad configuration against the valve hole pattern. It may be necessary to step up an actuator size to accommodate ease of mounting.