CO2 Pipeline Blowdown

[ibby003]

Hello all,
My first post so please be kind! I am looking at how to blowdown a CO2 pipeline with the following conditions;
16" diameter
20 mile section to be depressured
operating pressure: 2200 psia
temperature: 50F (buried pipeline)
Phase: I beleive that since I'm below the 88F critical temperature that this is a dense liquid phase (not supercritical phase)?
composition: 98.75% CO2, 1.1% CH4, trace N2, H2S, COS, C2
4" pipe connection to above grade that will vent the pipeline segment contents to atmosphere.

I am trying to understand how to calculate the venting rate, and the time to depressure the pipeline. If this were a high pressure gas (such as a NG pipeline) I think this would be pretty simple to calculate using a choked sonic velocity equation for vent rate and then the ideal gas isenthalpic temperature change to determine how cold the remaining gas in the pipeline was getting. And perhaps that approach will work once I am down below the pressure where the liquid will convert to gas. But for the initial high pressure venting where the pipeline contents are a liquid, I'm a little puzzled as to how to determine how my pipeline contents pressure and temperature are changing as CO2 is flashing out of the vent pipe.

Can anyone refer me to a good resource for this scenario?

Thanks in advance.

 

[zdas04]

If you'll look at the attached T-P diagram you'll see that your initial conditions (152 bar and 10C) you have liquid in the line. Blowdown of a liquid is mostly isothermal. That will get you down to about 45 bara (652 psig), and at the saturation line the world changes. You'll follow the saturation line down to the triple point boiling off liquid. At about 5 bara (72.6 psia) and -58C (-73F) you'll start getting freezing and sublimation. Things get really cold before you get all the way blown down. Almost certainly cold enough to be in the brittle fracture region and even if you don't break anything, re-pressurizing will be seriously scary.

I've never done this, thank goodness, but the people who tell of major leaks in CO2 systems on this site tell some pretty scary stories. If it was me, I'd blow it down to about -20F and 220 psia and then add enough warm nitrogen or air to get to nearly 100% vapor (maybe warm the line up to zero while blowing down the other end to maintain a constant pressure, you want to be decisively super-heated) and then continue blowing the line down.

Not a trivial undertaking.

David

 

[ibby003]

Thanks David,
I see that the boiling of the liquid is the main concern here in that it will reduce the temperature of the pipe.

I wonder if people who have done this sort of thing have done it in stages and used "waiting periods" in between to allow the soil around the buried pipeline to warm the pipeline back up. If that would work (just allowing earth's heat to warm the pipe), it would be much cheaper than compressing a lot of air or nitrogen into the pipeline. As a worst case for N2 filling/purging, a 20 mile section of 16in pipe at 200 psig would contain about 80 tons of N2 (if I've done my math right). But hopefully you would only need a fraction of that amount in order to add enough heat back into the pipe to prevent pipe concerns.

I don't THINK this is an insulated pipeline. Again, pipeline engineering is new to me. I'm not sure if the period of time for heat to conduct from the soil back into the pipe metal would be ridiculously long or not.

I would be very interested in any experience forum members have with this.

 

[zdas04]

You are almost certainly right. The pipe is sitting in a huge heat sink. It makes a lot more sense to define soak periods (i.e., maybe blow down isothermally to 650 psia, then blow the line down to 400 psi, soak it till the pressure gets back above 600 psia, repeat as necessary). When pressure doesn't build back up in a reasonable time, call it super-heated and blow it down to zero. I don't know what a "reasonable time" is, but you can assess it in your first couple of blow and soak cycles.

That sounds a LOT better than my lame idea.

David

 

[BigInch]

Soil has poor thermal conductivity, so it makes an excellent insulator. Wet soil has high thermal capacity, so takes a lot of heat to get warm again. At a one meter depth, once cold, it will stay cold for awhile. You could try accurately modeling the pipeline's thermal performance during blowdown with Stoner Pipeline Similator, or Olga2000, or at least get a handle on time to reheat an already cold section using a simple Excel sheet. I quickly found some example soil values here. Didn't look at it yet, but will check later on. If its not good, I've got some more data somewhere.

http://apollo.lsc.vsc.edu/classes/met455/notes/section6/2.html

What would you be doing, if you knew that you could not fail?

 

[MortenA]

One of the real worries would also be that if you follow the isothermal depressuring curve (you can find an excellent copy here:

http://www.union.dk/media/CO2-GB.pdf)

then you find that you get solid CO2.

Use the PH diagram in my reference to get a worst case low temperature. Or use Biginch suggest to model it in more details. However, be warned that many commerical simulators e.g. HYSYS cant handle solid CO2 and the phase transition at all.

Best regards

Morten