Flashing Accross the Control Valve

[soma0324]

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?

 

[CJKruger]

soma0324, maybe you should post a sketch, but here are my thoughts:

- I think you need a modulating valve, else the water flow rate and/or treater pressure will be uncontrolled.

- Flashing across a valve is fairly common. It is a severe environment though, so state the inlet/outlet conditions correctly so the vendor gives you a valve that's suitable.

- Maybe you are thinking of cavitation, which we try to avoid.

 

[hacksaw]

not a good application for a high recovery valve, try a globe valve instead



flashing by it self is not harmful, cavitation is probably what you are worried about

 

[JimCasey]

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.

 

[soma0324]

CJKruger: I am trying to control the flow rate by eating away all the pressure in the line from my treater to atmospheric tank.
my flow rate is approximately 250 USgpm and i am trying to maintain this flow rate. thats why I have to take such a high pressure drop across the CV. at this point i am not sure what kind of butterfly valve will that be but of course not the rubber seated. operations is planning to change the butterfly valve often.
the slug flow and steam hammer issue makes sense so i should inform my client about these issue.
here is the rough sketch of the system.

http://files.engineering.com/getfil...4d20-8289-428107b8ab5f&file=Doc090429-001.pdf

 

[CJKruger]

soma0324, I don't think the system will work. The pressure drop through both the 2"and 3" pipes are excessive. I actually get a negative pressure drop for the control valve.

 

[soma0324]

CJKruger,
what are u using to calculate the pressure drop. actually i should have mentioned earlier that I am changing the flow rate to get the positive pressure. so my flow rate is less than 250 USgpm which i mentioned in my sketch. it is 220 USgpm. i tried to do it in HYSYS but it didnt work. i am doing it in my spreadsheet.
Thanks

 

[786392]

I don't know or understand if you have seriously considered(water,Steam Hammer Dammage possibilities these can be horrible practically during operation,may affect adversely the useful life.

Additionally the sand particulates;while released along with flashing water stream could prove very serious erosional damage source.
with greater severity at the ell's and
could even foil the system if for any reason flow stops and settling is initiated.
Kindly do consider these issues if not already addressed!
Hope suggestions prove useful.

Best Regards
Qalander(Chem)

 

[SeanB]

If you are going to do this you should install the valve as close to the tank as possible. At least you will minimize the amount of flashing taking place in the upstream piping.

 

[CJKruger]

soma0324, I used Korf Hydraulics (

http://www.korf.co.uk)

for the calc. Disclaimer: I am involved with its development.

Hysys should give you a reasonable answer. If Hysys failed, it just confirms my suspicion that the pressure drops (partly due to flashing) are excessive.

For the control valve suction pipe you should target 0.4 psi/100 ft. For the discharge maybe 1 psi/100ft.