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Chapter 5. Sizing equations for control valves
Correction factor Fm
1.0
0.8
0.6
0.4
0.2
0.0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Volume ration Xv
0.7
0.8
0.9
1.0
Figure 5.7.-1: Correction factor Fm at two phase flow mixtures dependent on the volume share
As it becomes obvious from Figure 5.7.-1 the surcharge or correction factor (1+Fm), by
which the sum of the two Cv-values for gas and liquid is multiplied, lies between 1.0 and
2.0. This confirms the habit of the practitioners (who use mostly method b) to simply
double the calculated flow coefficient.
5.8 Multi-phase flow: liquid with dissolved gas
Valve data-sheets from Refineries as well as from upstream gas- and oil fields sometimes
specifying hydrocarbons named gas oil in combination with other liquid hydrocarbons or
pure fluids.
Surprising is often, that at moderate low temperatures << 100 °C the vapor pressure of
gas oil pv is identically with higher upstream pressures 10 to 60 bar. Does that mean, that
gas oil is a boiling liquid?
The answer is that gas oil is a type of oil with dissolved gas under the max. pressure p1.
As far the pressure is released specific fractions of dissolved gas versus pressure and
temperature recovers to a liquid – gas mixture. Even small mass fractions of gas will
dominate in the valve throttling area “vena contracta” and therefore the flow sizing result
to an acceptable larger Cv-value if setting pv = p1 for the total mass flow.
5.9 Supercritical fluids above the critical point
p1 > pc ; T1 > Tc
Predictable troubles with control valve sizing in case of supercritical flow
conditions.
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