Brochure
Valve Sizing Information for Water
F-27414-1 © Copyright 2006 TAC. All Rights Reserved. 173
For example, if a valve is controlling 200°F water at an inlet
pressure of 18 psig, the maximum pressure drop allowable
would be:
Pm = 0.5 [(18 + 14.7) - 11.53] = 10.6 psi
(Vapor pressure of 200°F water is 11.53 psi)
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.
C
v
(FLOW COEFFICIENT) DETERMINATION
The Water Valve Sizing Table or Slide Rule (refer to the
following page) is based on the following formula:
or
Where: C
v
= Coefficient of flow
C
v
is defined as the flow in GPM with ýP = 1 psi
GPM = U.S. gallons per minute (60°F, 15.6°C)
= Differential pressure in psi (pressure drop)
Other forms of this formula are:
and
These formulas can be used to calculate one of the three
quantities if the other two are known.
Flow coefficients (C
v
’s) for valve bodies are given on valve
specification pages of this catalog.
Metric (SI) Units
Kvs is defined as the flow in m
3
/h with ΔP = 100 kPa (1.0 Bar)
with the valve completely open.
Flow is calculated using the following formula:
Where:
ΔP = Differential pressure (pressure drop) in Bar
(1 Bar = 100 kPa)
m
3
/h = Cubic metres/hour (15.6 °C)
Pressure drop is calculated using the following form of the
above formula:
These formulas can be used to calculate one of the three
quantities if the other two are known.
TABLE 1. Vapor Pressure of Water Table.
Cv
GPM
PΔ
-------------=
Cv GPM
Specific Gravity
PΔ
---------------------------------------=
PΔ
PΔ
GPM
Cv
-------------
⎝⎠
⎛⎞
2
=
GPM Cv PΔ=
m
3
h⁄ kvs PΔ=
PΔ
m
3
h⁄
kvs
--------------
⎝⎠
⎛⎞
2
=
Water
Temperature
°F
Vapor
Pressure
psig
Water
Temperature
°F
Vapor
Pressure
psig
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