Brochure

103F-27855-10
Water Flow Coefficient (Cv)
Sizing a valve requires selecting a flow coefficient (Cv), which is defined as the flow rate in gallons per minute (gpm) of 60°F water that will pass
through the fully open valve with a 1 psi pressure drop (∆P). It is calculated according to the formulas shown in Cv Equation for Water and Cv
Equation for Steam.
Since the flow rate through the heat exchanger is usually specified, the only variable normally available in sizing a valve is the pressure drop.
The following information can be used to determine what pressure drop to use in calculating a valve Cv. Using the calculated Cv, consult the
Water Capacity table on this page or the Steam Capacity to select the valve body with the nearest available Cv.
Caution: Be sure that the anticipated pressure drop across the valve will not exceed the close-off pressure rating and the maximum pressure
differential rating listed in the Vx-8xxx Selection Guide, F-27199.
Two-position
Two-position control valves are normally selected “line Size" to keep pressure drop at a minimum. If it is desirable to reduce the valve below line
size, then 10% of “available pressure” (that is, the pump pressure differential available between supply and return mains with design flow at the
valve location) is normally used to select the valve.
Proportional
Proportional control valves are usually selected to take a pressure drop equal to at least 50% of the “available pressure.” As “available pres-
sure” is often difficult to calculate, the normal procedure is to select the valve using a pressure drop at least equal to the drop in the coil or other
load being controlled (except where small booster pumps are used) with a minimum recommended pressure drop of 5 psi (34 kPa). When the
design temperature drop is less than 60°F (33°C) for conventional heating systems, higher pressure drops across the valve are needed for good
results (see the table Conventional Heating System below).
Conventional Heating System Pressure Drops
Design Temperature
Load Drop °F (°C)
Recommended Pressure Drop
(% of Available Pressure)
Multiplier on
Load Drop
60 (33) or More 50% 1 x Load Drop
40 (22) 66% 2 x Load Drop
20 (11) 75% 3 x Load Drop
Secondary Circuits with Small Booster Pumps: 50% of available pressure difference (equal to the drop through load, or 50% of booster pump
head).
Water Table
Water Capacity in Gallons Per Minute for VB-82x3 Series
Valve Body
Part Number
Cv
Rating
Differential Pressure (DP in psi)
1 2 3 4 5 6 7 8 9 10 15 20 25 30 35
VB-82x3-0-5-12
56 56 79 97 112 125 137 148 158 168 177 217 250 280 307 331
VB-82x3-0-5-13
85 85 120 147 170 190 208 225 240 255 269 329 380 425 466 503
VB-82x3-0-5-14
145 145 205 251 290 324 355 384 410 435 459 562 648 725 794 858
VB-82x3-0-5-15
240 240 339 416 480 537 588 635 679 720 759 930 1073 1200 1315 1420
VB-82x3-0-5-16
370 370 523 641 740 827 906 979 1047 1110 1170 1433 1655 1850 2027 2189
C
v
Equation for Water
Where:
C
v
= Coefficient of flow.
gpm = Flow rate of water that will pass through fully open valve, measured in U.S. gallons per minute (60°F (15.6°C) water).
DP = Differential pressure (pressure drop), measured in psi.
C
v
=
GPM
∆P =
GPM
C
v
( )
GPM = C
v
∆P
2
∆P
VB-82x3 Water Flow
Coefficient and Capacity
5. VB-8/9000 Globe Valves
Sizing and Selection
5. VB-8/9000 Globe Valves
Sizing and Selection