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# Valve Authority

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)                     (1b)

- or with values

dpv = 0.8 (15 kPa)  / (1 - 0.8)

= 60 kPa

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

Kv = 7

by using this flow factor diagram. For a Straight Through Diaphragm Valve of "Saunders" type this corresponds to a 15 mm valve.

## Related Topics

### • Control Valve Sizing

Control Valve terminology, bodies, trim, flow characteristics, Cv and Kv sizing, noise, actuators, positioners and more.

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### Control Valves - Cavitation

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Calculate flow factor Kv for liquids, steam and gases - metric units.

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### Straight Through Diaphragm Valves - Flow Coefficients

Flow coefficients - Cv - and flow factors - Kv - for straight through diaphragm valves.

### Valve Selection Guide

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### Valves - Specific Services

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### Valves - Typical Operating Temperatures

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### Valves Types - Operating Size Ranges

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### Water Control Valves - Flow Factor Kv Diagram

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### Water Control Valves - Kv Calculation

Designing water process control valves and their Kv values.

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