User Guide

F-27197-4 © Copyright 2008 TAC All Rights Reserved. 7
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).
ΔP = Differential pressure (pressure drop), measured in psi.
Cavitation Limitations on Valve Pressure Drop
A valve selected with too high a pressure drop can experience erosion of discs and/or wire
drawing of the seat. In addition, cavitation can cause noise, damage to the valve trim (and
possibly the body), and choke the flow through the valve.
Do not exceed the maximum differential pressure (pressure drop) for the valve selected.
The following formula can be used on higher-temperature water systems, where cavitation
could be a problem, to estimate the maximum allowable pressure drop across the valve:
P
m
= 0.5 (P
1
– P
v
)
Where:
P
m
= Maximum allowable pressure drop
P
1
= Absolute inlet pressure (psia)
P
v
= Absolute vapor pressure (psia) (refer to Table-5)
Note: Add 14.7 psi to the gauge supply pressure to obtain the absolute pressure value.
For example, if a valve is controlling 200°F water at an inlet pressure of 18 psig, the
maximum pressure drop allowable would be:
P
m
= 0.5 [(18 + 14.7) – 11.53] = 10.6 psi
(Vapor pressure of 200°F water is 11.53 psi.)
Therefore, if the pressure drop for this valve is less than 10.6 psi, cavitation should not be a
problem.
Systems where cavitation is shown to be a problem can sometimes be redesigned to provide
lower inlet velocities. Valves having harder seat materials should be furnished if inlet
velocities cannot be lowered.
Table-4 Water Capacity in Gallons Per Minute for VB-8303 Series
a
Valve Body
Part Number
C
v
Rating
Differential Pressure (
ΔP in psi)
123456789101520253035
VB-8303-0-5-12
80 80 113 139 160 179 196 212 226 240 253 310 358 400 438 473
95 95 134 165 190 212 233 251 269 285 300 368 425 475 520 562
115 115 163 199 230 257 282 304 325 345 364 445 514 575 630 680
VB-8303-0-5-13
110 110 156 191 220 246 269 291 311 330 348 426 492 550 602 651
120 120 170 208 240 268 294 317 339 360 379 465 537 600 657 710
VB-8303-0-5-14 190 190 269 329 380 425 465 503 537 570 601 736 850 950 1041 1124
VB-8303-0-5-15 290 290 410 502 580 648 710 767 820 870 917 1123 1297 1450 1588 1716
VB-8303-0-5-16 500 500 707 866 1000 1118 1225 1323 1414 1500 1581 1936 2236 2500 2739 2958
a
Refer to Table-1 for details on multiple Cv ratings.
C
v
gpm
P Δ
-----------=
Δ P
gpm
C
v
------------
⎝⎠
⎛⎞
2
= gpm C
v
P Δ=