I am designing the piping system for my treater desanding system. the treater is operationg at about 80 psig and and i have 2" sch. 160 line coming out of treater going into an atmospheric tank. the piping is on the water side of the treater. now i have to install a butterfly valve on this 2" line to take some pressure drop across the valve. water is at 130 C in the treater (i know this is very high temperature but i am dealing with very heavy oil thats why temperature is too high). now as soon as i come out of the treater and accross the valve the pressure will drop and there will be some flashing of water due to high water temperature and pressure drop across the valve. i cant increase the size of my valve because i need certain pressure drop across the valve. my questions is what else can be done to avoid flashing if i cant change the valve size and i need lot of pressure drop? what damage flashing can cause in the piping and valve? is there a better way to avoid this situation?
I am not trying to control the flow here all i need is some pressure drop across the valve. the valve is snap acting not modulating and i cant modulate the valve because of the sand in water which will eat away the valve seat very quickly if i try to modulate it.
any sugesstions?

Flashing WILL happen regardless of the style of valve if the discharge pressure is less than the vapor pressure.  

If you have a full line of hot fluid and snap open a butterfly valve, the flashing might happen throughout the system.  This could lead to slug flow and steam hammer.

Usually flashing damage is erosion from the high-velocity droplets downstream of the throttling orifice.  You did not specify what type of butterfly valve but a rubber-seated valve is unlikely at these temperatures.  I assume then they you have a high-performane double-offset valve.  
 
If you'll mount the butterfly valve ON the flash tank, preferably with the shaft upstream, most of the flashing will happen as the flow drops into the flash tank and not inside the valve or piping.  You can modulate with a butterfly valve, just don't pick an operating point below about 30 degrees open or the flashing and its attendant high velocity may occur across the seats and that may cause erosion as you feared.  

I ran the numbers for water.  You'll get 5.5% flash, with an expansion rate of 8.7.  You didn't specify the flowrate, but if you come into the valve at 10ft/sec, the  average velocity of the mixture out would be 87 ft/sec.  That's faster than I would want sandy water droplets flowing through MY pipe.  

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