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.
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.   
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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