2 replies to this topic

[Subject_Name_Here]

Hi all,

 

I have slight delima. I am trying to size a control valve for water but am uncertain on the pressure drop I should use. The water is at 70 F, 40 PSI inlet, 10-40 gal/hour through 1/2" tubing. About 20 feet after the valve air will be injected at 100 PSI and about 10 inches later the water/air will be released to atmosphere by means of a misting spray bar. Please see the attached PFD.

 

If I assume a 40 PSI drop, my Cv is about 0.026, which is very small so I don't think this is correct.

If a typical 10 PSI drop (1/4 the inlet), I get a Cv of 0.053, which seems a bit more realistic. My formula is Cv=Flow (GPM) * Sqrt(S.G./dP (PSI))

 

1. Which dP should I use for the sizing?

2. Does the air injection complicate this at all or should I ignore it?

 

Thanks in advance!

Attached Files

•  Water Valve Cv.pdf   63.06KB   18 downloads

[Steve Hall]

You need to calculate the pressure at the downstream side of the control valve. If there was no air, this would be a simple matter of calculating the pressure drop through the 20 feet of tubing plus the pressure drop through the spray bar. The pressure drop = pressure at downstream side of the control valve.

 

It's the same answer with air injection, but now you have the additional pressure drop from the air injection point through the spray bar. The air must also be controlled through a valve, or it will simply flow at a rate that either equates to 100 psi pressure drop or at sonic velocity, whichever is lower (it will be sonic velocity). If you know the amount of air you could use a homogenious two-phase flow model. But this is all more complicated than what you need since your water flow has a wide range. I would calculate the downstream pressure at 40 gal/h and see what that gives you. It will set the wide open Cv (or 80% open if you need tight control) with no air flowing. Then I'd look at available valves and pick the next largest size. Then I'd "rate" that valve with different assumptions for pressure drop due to the air addition. If it seems fine, I'd look at the 10 gph flow with no air and see if the valve can handle it while remaining at least 10% open. If that all works out then I'd call it a day.

[Subject_Name_Here]

Thank you, Stephen.