# Valve Authority

## The valve authority expresses the ratio between pressure drop across the control valve compared to total pressure drop across the whole system

Control valve authority can be expressed as

N = dpv / (dpv + dpc)                          (1)

where

N = valve authority

dpv = pressure drop across a full open control valve (psi, Pa, kPa, bar)

dpc = pressure drop across the circuit (psi, Pa, kPa, bar)

To obtain control it is recommended that the control valve size is selected so that its authority is never less than 0.5. In general valve authority between

• 0 - 0.25 -> unstable to fair control
• 0.25 - 0.5 -> fair to good control
• 0.5 - 1.0 -> good control

### Example - Control Valve Authority in a Heating System

The total pressure drop in a heating system circuit at full load is 15 kPa. The volume flow is 1.4 l/s.

For decent control the valve authority is set to N = 0.8. The pressure drop across the control valve dpv can be calculated by transforming (1) to

dpv = N dpc / (1 - N)

- or with values

dpv = 0.8 (15 kPa)  / (1 - 0.8)

= 60 kPa

The flow factor for volume flow 1.4 l/s can be estimated to approximately

Kv = 7

by using this flow factor diagram.

## Related Topics

• Control Valves - Control Valve terminology, bodies, trim, flow characteristics, Cv and Kv sizing, noise, actuators and positioners

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