General information Useful calculation data
7
Valve authority The ratio of pressure drop across the valve when fully open to that across the complete circuit is termed the 'Valve authority' (N) and is expressed as:
P 1 P 1 + P 2
N =
Where:
N = Valve authority P 1 = Pressure drop across the fully open valve P 2 = Pressure drop across the remainder of the circuit
The diagrams below illustrate P 1 and P 2 more fully. Valve authority is a means of selecting a valve size on a water system with due regard to economic viability and good control. When selecting a valve size, the valve authority should be between 0.2 and 0.5 (and preferably 0.5). This will ensure that each small valve movement will influence some authority over the flow whilst not excessively increasing pumping power costs. Valve authority will always relate to the circuit which has a varying flowrate.
P 1
P 2
Valve authority - Two-port valve
P 1
7.5 16
P 2
Valve authority - Three-port mixing valve
P 1
11.5.13
P 2
Valve authority - Three-port diverting valve
TI-GCM-09 CM Issue 2
Page 2 of 3
Water Valve Sizing Chart
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