Installed Valve Characteristic Calculator
A valve’s inherent characteristic changes once it is installed in a system where pressure drop varies with flow. See the installed flow at each travel for linear and equal percentage trims.
- q = 1 ÷ √(N ÷ φ² + 1 − N), where φ is the inherent relative flow
Flow versus travel
| Travel | Inherent flow | Installed flow |
|---|---|---|
| 0 % | 0.0 % | 0.0 % |
| 10 % | 3.0 % | 5.4 % |
| 20 % | 4.4 % | 8.0 % |
| 30 % | 6.5 % | 11.8 % |
| 40 % | 9.6 % | 17.3 % |
| 50 % | 14.1 % | 25.2 % |
| 60 % | 20.9 % | 36.4 % |
| 70 % | 30.9 % | 51.1 % |
| 80 % | 45.7 % | 68.4 % |
| 90 % | 67.6 % | 85.9 % |
| 100 % | 100.0 % | 100.0 % |
Installed Characteristic
q is installed flow as a fraction of maximum, φ the inherent relative flow coefficient at that travel (linear: x; equal percentage: Rx−1) and N the valve authority. Pressure drop in the rest of the system is assumed to vary with flow squared.
Worked example
An equal percentage valve (R = 50) at 50 % travel passes 14.1 % of maximum flow inherently, but with N = 0.3 it passes about 25 % installed.
Frequently Asked Questions
What is an installed characteristic?
The actual relationship between valve travel and flow in a real system, where pressure drop across the valve changes with flow.
Why does authority change the characteristic?
At low flow the system loses little pressure, so the valve sees more ΔP and passes more flow than its inherent curve suggests.
Which characteristic should I choose?
Equal percentage for most flow and pressure loops with moderate authority; linear when the valve takes most of the system drop.
Learn more on the blog
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