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?

Any idea or commends would be appreciated.

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.

MORE NEWS