Control Valve · Glossary

What is Valve Authority?

Symbol: N0 to 1Control Valve

Valve authority is the fraction of the total system pressure drop taken by the control valve, which sets how well it controls.

Valve Authority Explained

If the valve takes a large share of the drop, its characteristic is preserved. If it takes only a small share, opening the valve changes flow mainly at the start of travel and control is poor.

A common guideline is an authority of at least about 0.25 to 0.5, balanced against the pumping energy that a large valve drop costs.

Valve Authority Formula

N = ΔP_valve ÷ (ΔP_valve + ΔP_system)

ΔP_valve	valve drop at design flow
ΔP_system	rest of the circuit drop at design flow

Example: Valve 0.5 bar, system 1.0 bar: N = 0.5 ÷ 1.5 ≈ 0.33.

Calculators for Valve Authority

CalculatorValve Authority CalculatorFree online calculator with formula and worked example.CalculatorInstalled Valve CharacteristicFree online calculator with formula and worked example.CalculatorValve Pressure Drop CalculatorFree online calculator with formula and worked example.

Free Downloads

Free downloadControl Valve Sizing Excel SheetFree download in Excel and PDF.

Frequently Asked Questions

What is Valve Authority?

Valve authority is the fraction of the total system pressure drop taken by the control valve, which sets how well it controls.

Is high authority always better?

For control, yes, but each bar across the valve costs pump power. Designers balance the two.

Is authority used outside HVAC?

Yes, the same idea appears in process sizing as the ratio of valve drop to total dynamic loss.

Learn more on the blog

Control valve flow coefficient (Cv and Kv)

A practical guide on instrumentationblog.in with more detail on valve authority and how it is applied on real plants.

Read Full Article

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