Sizing & Selection
Rangeability
Rangeability is the ratio of the maximum to minimum controllable flow a valve can throttle within its specified accuracy at constant pressure drop — a trim property, e.g. R = 50 for a typical globe cage. It is the theoretical span of the characteristic.
Turndown is the related but installed number: the usable flow span in the actual application. Turndown is always less than rangeability because the installed authority, shutoff leakage, actuator resolution and control signal resolution all eat into the usable bottom end. A valve with R = 50 may deliver an installed turndown of 10–20 in a low-drop system.
Sizing consequence: if the process needs a 30:1 operating span, don't check rangeability alone — estimate installed turndown, and if it falls short, either select higher-rangeability trim (characterized cages, some rotary elements reach R = 100–300) or split the duty between two valves in parallel (large + small). Trying to run one valve at 1–2 % of travel to reach the low end is where instability begins.