Inlet piping pressure drop for two phase flow

[rendarsmith]

I've sized a PSV for two-phase flow (water/steam) and I've found the various isentropic mass fluxes. What I need to do now is find a maximum length for the inlet pipe with a pressure drop of 3% of the set pressure. It's a heater that uses hot water to supply heat and the scenario is abnormal heat input (due to cooling water failure upstream for the gas), the gas can come in really hot and vaporize the water, but since the water side is liquid full we will most likely see two phase flow through the valve. I found a method for finding pressure drop with two phase flow, the problem is I'm not sure which vapor/liquid ratio to use since this will vary. There is a TV on the water that will close if it reads a high temperature in the gas, so I anticipate the water will not be moving during this scenario. My first thought is to use the same V/L ratio that gives the maximum mass flux that I used in sizing the valve. Anyone have any experience with this? If so, any advice on what V/L ratio I should use?

 

[LittleInch]

I would just do a series of calcs going from 0%/100% to 100%/0% in 10% stages to see which is the worst case. If you can't readily work out what is going to happen with any certainty you need to be conservative - it's just that the most conservative case is not apparent, so just look at a range.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[gocougs1]

For an equivalent mass flux, 100% or maximum vapor will require the greatest PSV orifice area and have the greatest line losses. However, if you are starting with 100% hot liquid water, then you should size for two phase flow through the piping using the highest expected inlet water temperature and maximum vapor rate in.

Also if you are concerned about having too much inlet piping, consider using a larger diameter inlet pipe?