Two phase downflow pipe sizing

[mbt22]

I'm trying to confirm the sizing of a line from a partial condenser to a flash drum. The line runs vertically down from the condenser to the drum. I've got a two phase mixture, roughly 50:50 mass flow liquid to vapour. I'm attempting to confirm that fluid hammer will not be a problem (or as an alternative, the flow regime).

My first though was Fr<0.31 for self venting flow, but I can't convince myself that this is valid for co-current vapour and liquid down flow. Any suggestions are gratefully received.

 

[davefitz]

There are flow regime maps, such as the Baker plot. You can also install spiral ribbon turbulators to force an annular flow regime onto the downcomer.

 

[mbt22]

Thanks very much for the prompt reply. Does something similar to a Baker plot exist for vertical flow? I've only ever seen them for horizontal. It's only 3/8" pipe, so internals may be a problem.

 

[insult2injury]

For such a small pipe, it's hard to imagine water hammer being a problem.

I2I

 

[Latexman]

If the liquid and vapor are in equilibrium with each other as they should be coming out of a partial condenser, I don't see the potential for much fluid hammer. The worst hammer occurs with condensable vapor and subcooled liquid.

Good luck,
Latexman

 

[BigInch]

.... usually at startup or shutdown.

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

 

[katmar]

It depends on what your goal is with this installation. It seems very small and is probably a lab or pilot scale unit. If your aim is to get information that you can eventually use for commercial design then it is important that you get the right answer now. I am a great believer in the Fr<0.31 rule for gravity drains, but I also doubt whether it is the right procedure here. However, if you just want to make it work, why not just put in a 1" line and be done with it?

An alternative would be to install 2 lines - I have seen it done this way for condensers. Put a gooseneck (or water seal or P trap - whatever you like to call it) in the liquid line to ensure only liquid travels down that leg. And then put in a separate line from the condenser, connected where no liquid will enter it, to allow dry vapor to travel to the flash drum.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[mbt22]

Thank you all for your helpful replies.

As was rightly stated, it's a pilot plant, so I'm more interested in smooth operation (and therefore data). A liquid leg and vapour balance line was one of the possible process options I considered for this.

I'm going with the easy option though, pushing the line size up a bit.

 

[CJKruger]

mbt22, in general, there is no reason to design a 2-phase outlet pipe from a condenser to a drum for self venting flow.

I would target about 0.3 psi/100 ft and be done with it. Also, a larger pipe cannot hurt you, as it is downflow. Downflow is almost always annular flow (thus you cannot use typical flow regime maps).

Also, there typically is no pressure recovery (due to elevation change) in 2-phase down flow.