Liquid Control Valve Sizing with Large Quartiles of Dissolved Gases

[chemdesign]

Can anyone provide a reference for sizing flashing liquid control valves when the base liquid contains a significant amount of a dissolved gases, but the liquid itself (exclusive of the dissolved gases) is well below its bubble point?

The traditional method outlined in the Fisher Catalog requires uses the liquid's vapor pressure and critical pressure to size the valve. The problem is, what to use for the liquid vapor pressure and critical pressure when they are both strongly influenced by the presence of dissolved gases.

 

[buzzie]

For this problem in the past I have calculated two Cv values. One for the percent of flashed vapor expected downstream of the valve and a second one for the liquid fraction that does not flash. Then the two Cv values are added together as the valve Cv used for final valve selection. This method has always served pretty well.

Not clear from your description if the dissolved gases increase or decrease tendency of your liquid to flash but suggests that you do have an idea of the magnitude of the effect. This may be all you need to get a reasonable value for valve selection. Remember that a typical control valve operating Cv is 50 to 60 % of full capacity so you have significant room for error in this estimate as long as other liquid and hydraulic parameters of you system are understood.

If the flashing is significant, careful consideration must be given to multiphase flow transition downstream of the valve and attention to valve material to mitigate erosive effects of cavitation and probably noise.

Regards