Flashing control valve sizing

[fdomin]

I have some doubts regarding flashing control valve CV sizing. Available software indicate flashing service but they don´t take into acccount downstream vapour fraction.

I´ve seen manual procedures for sizing flashing liquid valves where separate CV´s are calculated for both flashed liquid and produced vapour, then Cv´s are added.

It´s my opinion that this could lead to an oversized valve as a larger orifice is required for both liquid and vapour flows, but actually only liquid is flowing through the valve orifice.

Can anyone put some more ligth on this subject?

 

[pleckner]

I'm confused. Are you asking about liquid flow through a control valve that flashes downstream of valve or liquid flashing in the valve?

Anyways, a relatively recent article on this subject was published in Hydrocarbon Magazine, March 2005.

 

[fdomin]

I took a look at that article, but it mainly presents a new method for handling two-phase flow calculation, while my question refers about liquid flashing in the valve orifice.

 

[roker]

hello fdomin,

1. several orifices can be installed in same valve size.
2. it is recommended to give the control valve supplier to size the orifce and valve, ask for the calculations and compare.
3. to my opinion there is a pressure gradient in the orifice so flashing may start in the orifice although it is thin.

regards,
roker

 

[JimCasey]

I have used the control valve sizing software from most of the major control valve companies and they have all addressed sizing with flashing through the valve. It is NOT correct to calculate the Cv for the liquid fraction and the vapor fraction and to add them together, even for 2-phase flow of a liquid and a nonsoluble gas.

Flowserve's "Performance!" sizing software will report percent flash sometimes, but other times it just reports flashing, and I'm not sure what causes it to differentiate.

The significant application consideration with flashing is the downstream velocity. With water flashing to steam, the expansion can be several hundred times the liquid volume. There are always entrained droplets, and these can be erosive if you don't control the downstream velocity. Frequently the downstream piping needs to be several sizes larger to keep the velocity down. Directly discharging the valve into a pressure vessel is also a good practice.

It is pretty easy to calculate the downstream quality. If you know the state point of the fluid coming in to the valve, determine its enthalpy. the enthalpy remains constant for a throttling process. So follow the enthalpy line on the mollier chart down to the outlet pressure, and that gives you the quality of the downstream product.

If you only have a table and not a mollier chart:
Hf1=(1-x)Hf2+xHv2. In English: Enthalpy of the fluid at the inlet is equal to the enthapy of the fluid fraction out(1-x) plus the enthapy of the vapor fraction out(x). You DO calculate the density by adding the fractions together, and use that in your velocity calculations.