A globe valve with 50 to 1 rangeability can control from its maximum Cv down to one fiftieth of it before the characteristic breaks down near the seat. Rotary valves such as segmented ball valves often claim 100 to 1 or more. Installed rangeability is lower, because system pressure drop changes with flow and because stiction and positioner resolution limit how small a step the valve can make.
Startup flow is 5 percent of design, the valve doesn't have the rangeability, so we're putting in a small one in parallel.
Also heard as
- turndown
Do not mix it up with
- Cv: Cv is the capacity at a given opening; rangeability is the ratio between the highest and lowest usable capacities.
Related terms
Cv
Flow coefficient: US gallons per minute of 60 degree F water a valve passes with a 1 psi pressure drop.
Flow characteristic
The relationship between how far a valve is open and how much flow it passes.
Control valve
A power-operated valve that modulates flow on a controller signal to hold pressure, level, flow or temperature at setpoint.
Stiction
Static friction that makes a control valve stick, then jump, instead of moving smoothly with its signal.