Valve Rangeability and Opening Calculator
Check that a selected control valve can control both the minimum and maximum flow. Enter the rated Cv and the Cv needed at each condition to see the valve travel for linear and equal percentage characteristics.
- Equal %: travel = 100 × (1 + ln(Cv ÷ Cvrated) ÷ ln R)
- Maximum flow: 86.9 %, minimum flow: 28.1 %
Inherent characteristic of this valve
| Travel | Cv | Percent of rated Cv |
|---|---|---|
| 10 % | 2.958 | 3.0 % |
| 20 % | 4.373 | 4.4 % |
| 30 % | 6.467 | 6.5 % |
| 40 % | 9.564 | 9.6 % |
| 50 % | 14.14 | 14.1 % |
| 60 % | 20.91 | 20.9 % |
| 70 % | 30.92 | 30.9 % |
| 80 % | 45.73 | 45.7 % |
| 90 % | 67.62 | 67.6 % |
| 100 % | 100 | 100.0 % |
Valve Characteristic Equations
Equal percentage: Cv = Cvrated × R(travel/100 − 1)
R is the inherent rangeability, commonly 50 for globe valves with equal percentage trim. Each equal increment of travel changes the flow by the same percentage of the current flow, which suits most process loops.
Worked example
A Cv 100 equal percentage valve (R = 50) needs Cv 60 at maximum flow and Cv 6 at minimum. Travel = 87 % at maximum and 28 % at minimum. The maximum is above 80 %, so a slightly larger valve or trim would give more control margin.
Frequently Asked Questions
What is control valve rangeability?
It is the ratio of the maximum to minimum controllable flow coefficient. Globe valves typically offer 30:1 to 50:1; rotary valves can reach 100:1 or more.
What valve opening is best for control?
A common guideline is 20 to 80 % travel for equal percentage trims and 10 to 90 % for linear trims, with normal flow near 60 to 70 %.
When should I use a linear characteristic?
Linear trims suit loops where most of the system pressure drop is across the valve, such as level control or bypass service. Equal percentage suits most flow and pressure loops.
Learn more on the blog
Basic parts of a control valve
A practical, field focused guide on instrumentationblog.in that explains the theory behind this calculator and how engineers apply it on real plants.
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