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G-18
Butterfly Valves
Example
The line size is 6" and the required ow is 600 GPM with
a maximum pressure drop of 5 psi. The square root of 5 is
equal to 2.236. When divided into 600, the required Cv for
this application is: 268.336.
2. Select your valve size: Using the Flow Coefcients
(Cvs), select the appropriate valve size. If your required
Cv is in between valve sizes, choose the larger size
valve. When selecting a 3–way assembly, the Cv of the
run should be selected.
Example
The line size is 6" and the calculated required Cv is
268.336. The valve selected is a 4" with a rated Cv of 647.
Buttery valves are high capacity valves and require
very little pressure drop to control ow, which allows for
reduction from the line size when sizing valves. This pipe
reduction affects the ow characteristics and will reduce
the effective Cv of the valve. This phenomenon is known
as the piping geometry factor (Fp), which brings us to the
nal step in valves sizing.
3-Way Flow Coefcients (Cvs)
Size Angle of Disc Opening
Run 0° 10° 20° 30° 40° 50° 60° 70° 80° 90°
Branch 90° 80° 70° 60° 50° 40° 30° 20° 10° 0°
2" 130 102 79 66 63 66 67 84 110 139
2-1/2" 213 175 142 107 79 97 113 155 207 252
3" 331 277 226 153 79 136 161 252 327 403
4" 500 445 373 270 131 257 297 449 574 664
5" 774 688 559 416 200 406 466 689 909 1,049
6" 1,095 967 766 566 335 541 627 929 1,255 1,454
8" 1,935 1,874 1,512 1,155 721 1,064 1,242 1,763 2,376 2,584
10" 3,153 3,058 2,490 1,886 1,155 1,704 2,003 2,849 3,833 4,221
12" 4,760 4,674 3,822 2,889 1,792 2,543 3,041 4,306 5,782 6,334
14" 5,384 6,055 4,683 3,164 2,377 3,433 3,973 5,577 7,959 7,559
16" 7,235 7,037 6,200 4,885 3,267 4,574 5,245 7,381 9,329 10,109
18" 8,664 8,431 8,018 6,302 4,486 5,847 6,740 9,492 11,302 12,227
20" 11,402 11,041 9,971 7,840 5,541 7,237 8,383 11,813 14,746 16,013
Table Notes:
Three-way valve assemblies Cvs are corrected from published two-way Cvs to account for line losses generated by the tee, and are
calculated values only. The pipe friction losses are a function of uid velocity through the pipe and the three-way Cvs listed are apparent
for full ow through the pipe. Operation at less than full capacity (lower velocity) will increase the actual Cvs
Sizing Example
With this information and assuming the media is water or
a similar media (glycol/water mix), a control valve can be
properly sized for the application by following these steps:
1. Calculate the required Cv: Using the following formula
and the information required above, you could calculate
the ow coefcient (Cv) of the control valve.
Whereas: GPM = The maximum ow requirement
P = The max. pressure drop (5 psi)
Cv=
GPM
∆P
√