User Manual

TI597SI Page 3
VALVE SIZING AND SELECTION (WATER)
The sizing of a valve is very important if it is to render good service. If it
is "undersized", it will not have sufficient capacity. If it is “oversized”, the
controlled variable may cycle, the trim can be exposed to excessive wear or
wire drawing, and you could expect reduced valve life. To help select the
right valve, it is important to understand your application and its variables
(controlled fluid, temperatures, pressures, min/max load, etc.). When
your system variables are known and you have calculated actual demand, it
is possible to select the right Powers valve for your application. The
following technical data should help you in selecting a valve for your water
control applications. For fluid applications other than water, contact
Powers’ application engineering.
On/Off Control:
These types of applications are normally line sized to reduce pressure drop
and pump size. In these applications it is important to verify valve seat
leakage will not result in system overheat or damage. If this is a concern, it
is necessary to take precautions to alleviate this potential problem.
Proportional Control:
In applications where the close-off pressure at the valve is below 20psig,
use a pressure drop of 5psi.
In applications where the close-off pressure at the valve is above 20psig, it
is generally recommended to take 25-50% of the system pressure drop at
the control valve to maintain good valve/system performance. Certain
applications can successfully utilize lower pressure drops across the valve
(5-25%) if system fluctuations are kept to a minimum. If not, the valve is
considered oversized it will not effectively throttle until it is nearly closed
thereby resulting in poor control.
Refer to the following table for flow….
Table - 1 Water Capacity in Gallons Per Minute
Valve Cv
Size Rating Differential Pressure ( Δ P in psi)
5 10 20 30 40 50 60 70 80 90 100 125
2.5 56 125 177 250 307 354 396 434 469 501 531 560 626
385190 269 380 466 538 601 658 711 760 806 850 950
4 145 324 459 648 794 917 1025 1123 1213 1297 1376 1450 1621
5 240 537 759 1073 1315 1518 1697 1859 2008 2147 2277 2400 2683
6 370 827 1170 1655 2027 2340 2616 2866 3096 3309 3510 3700 4137
CAVITATION LIMITATIONS ON VALVE PRESSURE DROP
A concern in high temperature water systems is the potential for cavitation/flashing, which is caused by the downstream pressure being lower than that of the
vapor pressure of the fluid. This basically causes the water to "boil" and can result in reduced flow/capacity, excessive noise, vibration, wear and should be
avoided if possible. Use the following equation to estimate the maximum allowable pressure drop across the valve:
Pmax = 0.5 (P1 – Pv)
Where:
Pmax = Maximum allowable pressure drop
P1 = Absolute inlet pressure (psia)
Pv = Absolute vapor pressure (refer to psia - Table 2)
Absolute pressure = gauge pressure + 14.7
Table-2 Vapor Pressure of Water Table
Water Vapor Water Vapor
Temp. Pressure Temp. Pressure
( ° F) (psia) ( ° F) (psia)
40 0.12 140 2.89
50 0.18 150 3.72
60 0.26 160 4.74
70 0.36 170 5.99
80 0.51 180 7.51
90 0.70 190 9.34
100 0.95 200 11.53
110 1.28 210 14.12
120 1.69 220 17.19
130 2.22 230 20.78