Valve Authority

Control valve authority can be expressed as

N = dp_v / (dp_v + dp_c)                          (1)

where

N = valve authority

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

dp_c = 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 dp_v can be calculated by transforming (1) to

dp_v = N dp_c / (1 - N)                     (1b)

- or with values

dp_v = 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

K_v = 7

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