3 WAY MODULATING VALVE SELECTION PROCEDURE
BASIC FORMULA FOR VALVE SIZING FOR WATER:
Kv = Flow in m3/hr .
√ Pressure drop in bar
Valve Authority VA
Available/Total pressure drop
Ideal VA = 0.5
Acceptable VA = 0.4 to 0.65
Taking the example of a AHU
Flow rate = 63.4 gpm = 14.39 m3/hr
Coil pressure drop = 10.57 ft = 0.317 bar
Therefore Kv = 14.39 = 25.56
√0.317
Since the calculated Kv works out to be 25.56 we have to select a valve which has a Kv near to 25 and gives valve authority between 0.4 and 0.65.
From the page 2 of 8 of the valve literature we observe that the 40 mm valve has a Kv
of 25 which is near to the desired calculated Kv of 25.56.
The valve model selected is VXP459.40-25 which has Kv = 25,
For the selected valve model the actual pressure drop across the valve is calculated as under
25 = 14.39 ‘
√Pressure drop in valve in bar
Therefore Pressure drop across the valve = 0.331 bar = 11.06 ft (against the actual coil pressure drop of 10.57 ft)
Valve authority = 11.06 / (10.57 + 11.06) = 0.51
For the same ideal Kv of 25.56 if we select the valve of one size higher with Kv 31
(50 mm valve size) the actual pressure drop across the valve works out to be
7.2 ft (against the actual coil pressure drop of 10.57 ft) and valve authority to be 0.41.
Similarly for the same ideal Kv of 25.56 if we select the valve of one size lower with
Kv 16 (32 mm valve size) the actual pressure drop across the valve works out to be 27.1 ft and valve authority to be 0.72
From the above selection it can be conferred that the valve with Kv = 25 will give the
best results.
