What Is Valve Rangeability?

Quick Answer

Valve rangeability (also called turndown ratio) is the ratio of the maximum controllable flow to the minimum controllable flow through a control valve. A valve with a rangeability of 50:1 can control flow from 100% down to 2% of its maximum Cv. Higher rangeability means the valve can handle a wider range of process conditions without losing control accuracy. Typical rangeability for globe control valves is 50:1; for rotary control valves (ball, butterfly), 100:1 to 300:1 is achievable.

Definition

Rangeability = Maximum controllable Cv / Minimum controllable Cv

The “minimum controllable Cv” is the point below which the valve can no longer maintain stable, predictable control. Below this point, the flow becomes erratic due to friction, dead band, and the valve operating too close to the seat.

Inherent vs Installed Rangeability

Type	Definition	Typical Value
Inherent rangeability	Rangeability measured with constant pressure drop across the valve (laboratory conditions)	50:1 to 100:1 (globe); up to 300:1 (ball/butterfly)
Installed rangeability	Rangeability in the actual piping system where pressure drop varies with flow	10:1 to 30:1 (typically much lower than inherent)

Installed rangeability is always lower than inherent rangeability because real piping systems have friction losses, pump curves, and pressure variations that change the pressure drop across the valve as flow changes. At low flows, more pressure drop is available across the valve; at high flows, piping losses consume more of the system pressure, leaving less for the valve.

Rangeability by Valve Type

Valve Type	Inherent Rangeability	Installed Rangeability (Typical)
Globe control valve (equal %)	50:1	15:1 to 30:1
Globe control valve (linear)	30:1	10:1 to 20:1
V-port ball valve	100:1 to 300:1	30:1 to 100:1
Segmented ball valve	100:1 to 200:1	30:1 to 80:1
Butterfly control valve	50:1 to 100:1	20:1 to 50:1
Eccentric plug valve	100:1 to 200:1	30:1 to 80:1
Needle valve	10:1 to 20:1	5:1 to 10:1

Why Rangeability Matters

Process conditions change. A control valve sized for a normal flow of 100 m3/h may need to control at 10 m3/h during turndown or startup. If the valve has insufficient rangeability, it cannot maintain stable control at the reduced flow. The valve will either oscillate (hunt), close completely, or operate in the seat contact zone where wear accelerates.

Scenario	Required Rangeability	Typical Solution
Normal process variation	10:1 to 20:1	Standard globe or rotary control valve
Large turndown (batch process)	30:1 to 50:1	Equal percentage globe or V-port ball
Extreme turndown (startup to full load)	50:1 to 100:1+	Split-range with two valves, or high-rangeability rotary valve

How to Improve Installed Rangeability

• Use equal percentage characteristic: provides better control at low flows compared to linear
• Avoid oversizing: an oversized valve spends most of its time at low opening where rangeability is poor
• Split-range control: use a small valve for low flows and a large valve for high flows, both controlled by the same signal
• Digital positioner: reduces dead band and improves controllability at the extremes of travel

Rangeability vs Flow Characteristic

Characteristic	Low-End Control	High-End Control	Rangeability
Equal percentage	Good (small Cv change per % travel)	Good	Highest
Linear	Moderate	Good	Moderate
Quick opening	Poor (too sensitive)	Poor (too flat)	Lowest

Equal percentage is the default for most process control loops because it provides the widest usable rangeability. The small Cv change per degree of travel at low openings gives the controller fine resolution where it needs it most.

Pro Tip

When the process requires a turndown ratio exceeding 50:1 with a single valve, specify a characterized V-port ball or segmented ball rotary control valve. These designs achieve 100:1 or better inherent rangeability, translating to 30:1+ installed; often enough to avoid the cost and complexity of a split-range arrangement.

Read the full guide to valve types