Design-point valve authority

Compare a fully open valve drop with the total pressure drop of its controlled variable-flow path.

How this calculation works

Valve authority compares the fully open valve's pressure drop with the pressure drop of the entire controlled path at the same design flow. Enter the drop across the valve and the combined drop across the rest of that path. Adding them gives the denominator; dividing the valve drop by that total gives authority, displayed as a percentage. Both values must represent the same flow condition and a clearly defined differential-pressure basis. The result is a simple design-point ratio rather than a simulation of the valve's installed characteristic across its operating range. It does not identify an acceptable target or certify control performance. Networks with pump controls, bypass paths or pressure-independent valves may require a different analysis rather than this two-part pressure-drop balance.

Inputs and units

Fully open valve drop at design flow (kPa)
Other controlled-path drop at design flow (kPa)

Method and formula

Authority = Δpvalve/(Δpvalve+Δprest), using pressure drops at the same design flow in the controlled path.

Worked example

Example inputs

Fully open valve drop at design flow: 20 kPa
Other controlled-path drop at design flow: 30 kPa

Calculation steps

Controlled-path total drop = 20 + 30 = 50 kPa.
Valve authority = 20 / 50 × 100 = 40%.

Example results

Valve authority: 40 %
Controlled-path total drop: 50 kPa

Assumptions

A simple variable-flow path with a specified differential-pressure basis.

Limitations

Not a full network or installed-characteristic simulation.
Pump controls, bypasses and pressure-independent valves can require a different authority analysis.
Preliminary educational check; verify inputs and equipment data before design or operation.

Sources

Belimo: Globe valve project planning: Printed pages 10, 12 and 20: Kv relation and valve-authority definition