A valve has an installed authority of 35%. The design flow rate is 350 gpm. For good low-flow
control, we wish a minimum flow rate of not more than 2 gpm. We can calculate the required
R = (350 / 2) x (0.35)½ = 123
This means the valve must have a manufacturer’s rangeability rating of 123:1 or greater. A
typical commercial valve generally has a rangeability of about 50:1.
Industrial valves can have a rangeability as high as 200:1. This is part of the reason why industrial valves are so much more costly than a commercial valve.