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)
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.