Geez, this is a toughie.  There are two critical flow equations for steam that I know of and there are likely others, too.  The one I am most familiar with is the James equation.  There is also a modified James equation.  These are used in commercial applications for two-phase steam flowrate measurement across an orifice plate.  As I recall you have to assume a quality.  We used that eqn with great success for flowrate determination across orifice plates in critical flow for oilfield steamflooding.  I have a copy of the James paper and I will dig it up for you.  I don't remember the name of the other equation but I will find out.  You might be able to do a Google search and find James' paper on the web.  I have it in my files, somewhere...

The two eqns are easily handled by Excel, but pretty cumbersome to do by hand.

Thanks!
Pete
pjchandl@prou.com

Ah, this is more information than in your post in the other forum.  Unless you have a very small pipe or a convoluted piping path, most of the pressure drop should occur at the nozzles, which should be located at the blowdown vessel.  Therefore, you shouldn't have problems with two-phase flow in the piping.  I think what you are asking is simply how to calculate the design flow rate for blowdown.  Since your inlet and outlet conditions (pressures, temperatures) are specified and fixed, the blowdown flow will solely be dependent on the size of the nozzle.  In the design phase, you should be considering what the desired blowdown flow is, and then calculate the size of the orfice (nozzle) you need.  

Thanks for the responses!

To be specific, I am trying to size the steam and liquid outlet lines on the blowdown tank.  To get this flow, we need to know the inlet flow from blowdown inlets.  The inlet flows are to be based on choked flow from a 2" nozzle (850# blowdown), and a 2½" (400# blowdown), both saturated liquid at the boiler outlet.  

Unfortunately, this is in the "design / build" phase (quick ship) and (as luck would have it) our customer (the design engineer for the job) is unwilling to tell us how to calculate this flow (they don't know the flow or haven't calculated it, but they specify the blowdown tank size... go figure!).

If anyone knows of any sources for information, I would greatly appreciate the help!

jproj

P.S.  Pete: I tried searching on James or James equation, but I didn't find anything.  If you could point me to where I could find it, I would be much appreciative!

Thanks!!!

I can sympathize with your frustration.  However, you can use some rules of thumb to help.  Boiler blowdown flows generally range from 0.5% to 3% of designed feedwater flow.  Pick a number within that range to estimate the blowdown rate, and then it's a simple flash calculation for each stream to figure steam and water rates out of the blowdown vessel.  Better yet, since the customer can't or won't tell you what the blowdown rate is, then calculate three conditions (maybe 0.5%, 1.5%, and 3.0% of boiler feedwater flow rate) within that range and let him/her pick which one they think is right.  

Actually, after giving it some more thought, I think you may be able to determine the blowdwon flow rates given the nozzle sizes.  The Crane manual (Flow of Fluids Through Valves, Fittings, and Pipes) covers calculation of flow of liquids through nozzles and orifices.  You may need some more data like the ID of the blowdown lines, but it looks pretty straightforward.