What would be the proper methodology for sizing a control valve with liquids flashing downstream?

Fisher recommends allowable pressure drop method that results in a larger valve than using actual pressure drop. The question is, how can I get the required pressure drop if the much lower allowable pressure drop is used for sizing?

Fisher has extensive valve sizing help on their website including provisions for flashing service.  If memory serves, it is their Catalog 12.  If not, it will be in the same group of technical data where Cat 12 is to be found.  It may be the next one or the third one listed.  I think however, the third one is all about noise.

Sorry, while I refer to them often, I downloaded them long ago (onto my work computer), so I don't have to go to where I am sending you so I don't know the path right off hand (and I will leave looking it up to you).

>>how can I get the required pressure drop if the much lower allowable pressure drop is used for sizing?<<

When flashing occurs in a valve, the flow is choked.  "Choked" is an unfortunate term but when choking happens the actual downstream pressure is independent of the flowrate.  The effective DP is P1-the pressure at which the flow becomes choked.  

Download the sizing program from a major control valve manufacturer.  Flowserve (Valtek) , Emerson (Fisher), Dresser (Masoneilan) all offer free software.  

Water expands around 1000 times its liquid volum when it flashes completely, although typical flashing usually vaporizes around 15% of the mass of the liquid. That's enough to make the downstream flow very fast. Rotary valves serve well for flashing because the outlet is straight out and the high-speed droplets don't scour the valve internals.  It's a good idea to calculate the velocity downstream of the valve and to transition to a larger pipe diameter to keep the velocity within reasonable limits so the downstream system erosion is managed.  Also, Chromium-bearing alloys resist flashing erosion a lot better than carbon steel, so select piping accordingly.  

Calculate the percent flash by setting up an isenthalpic balance:

Hf1=XHg2+(1-x)HF2 Where H is Enthalpy, F is Liquid, G is vapor, 1 is upstream and 2 is downstream. X is the percent of vapor phase.   

Once you have the percent flash you can calculate the specific volume, then the velocity.

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