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LPG line Pressure Drop Calculation

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robjul

Chemical
Nov 6, 2007
24
Happy New Year to my colleagues!
I was given an assignment to calculate the pressure drop on a 10"ND pipeline where 600MT/hr of refrigerated Propane (-42deg. C) would be discharge from an LPG cargo vessel (via 2,000 KM of submarine pipeline) at the Jetty then, goes 45deg upwards diagonally then horizontally onshore for 500m long up to several "bullet type" storage tanks. I assumed the storage tank temperature would finally be in equilibrium with an ambient temperature of 25deg. C. Discharge static elevation is 25m.
I don't know what condition to use (to based on -42deg. C or 25deg. C) for my pressure drop calculation since I am aware that, with the lenght of the pipeline, Propane will pick-up heat and therefore, cannot assume an isothermal condition. I am thinking of a weighted averaging calculation where an isothermal condition can be assumed for every kilometer or every 100m of pipe lenght then, add the individual pressure drops to get the total. This is of course a tedious calculation but is this a right approach?
Please share if you have any sample calculation or spreadsheet. Actually, my boss prefers a manual calculation because he was used to it during his 1970's schooling and then, I have to countercheck it with an online spreadsheet. I actually had done some manual calculation using Crane's Technical Paper 410 but my counterchecked with an online spreadsheet yielded with an answer 10 times greater than the manual calculation! Hence, obviously as per my engineering "feel", the manual calculation is wrong and can't find the error even with further reviews and checks of the input data and calculations.
My online spreadsheet calculation yielded an acceptable pressure drop but failed on velocity. The criteria I used for a 10"ND line and above is: Allowable velocity = 3.0 - 4.6m/s whilst Allowable max. delta P = 0.45 bar/100m. Are these criteria correct which was just given to me by a colleague.
I had done so many pressure drop calculations in the past but it looks like calculations involving LPG mix, propane & butane is a bit complicated as its density varies along the lenght of a long pipeline.
Indeed, many thanks for the help/assistance.
 
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Are you sure the inlet fluid is total gaseous?- otherwise you'll have two phase flow for some of the distance which is even more complicated.
 
robjul,

Did you really mean "2,000 KM of submarine pipeline? Wow, that jetty is WAY off shore. I could believe 2 km.

I think you should do some heat transfer calculations too. Do enough all along the pipeline (beginning to end) so that you understand what is going to happen to the state (gas/liquid/two phase) of the propane (or is it really LPG). This knowledge would have also helped me understand your system better too, because I have no experience in your application.

The criteria from your colleague seems reasonable to me.

You did not mention how the propane is transferred. Is it pushed with pressure or pumped? What pressure is the propane at near the tanker and in the bullet tanks? These details can help others envision your system. Of course the best option is to attach a detailed. well labelled sketch.

Finally, you may consider, after correcting any errors and adding more detail, if needed, to repost this on the "Pipelines, Piping and Fluid Mechanics engineering" forum. I think you will get more responses there. If you do this, please Red Flag this post and ask Site Admin to delete it. Cross posts are not allowed.

Good luck,
Latexman
 
Hi Latexman/Cloa - indeed, many tnx for the prompt reply. I just had a meeting with our client and there have been several developments/options in the project. I'll get back to you after I had consolidated all the data and the sketch. And yes,sorry,it's 2,000 meters (now 2,900 approx.). Propane is transferred from the cargo ship's pump via a submarine pipeline for the first 2,300m then goes upward onshore (on the balance of the pipe lenght)up to the LPG storage tanks. Afterwhich, Butane is the next one to be pumped and mix with the propane in the tank equals LPG.
An option was proposed to install 2 booster pumps onshore that will run in parallel and a Reheater at the discharge side. It gets complicated but I'll try to draw the sketch with the data/info in it.
What I understand is that Propane and Butane exist as saturated liquid under slightly above atmospheric pressure??
 
Pure propane at 231 K is a liquid at pressures above 1.0117 bara and pure butane at 273 K is a liquid at pressures above 1.0264 bara (from webbook nist).
 
Hi ione, - many thanks for the valuable info.
 

For two-phase flow the following link , chapter 13, may be of some help:

www.wlv.com/products/databook/db3/DataBookIII.pdf​
 
I wouldn't think that you'd get that much heating along a 10" pipeline unless your flowrate was very low.

Higher temperatures will require yet greater pressures to keep a liquid phase.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso
 
Hi Biginch, - belated Happy New Year! I would like to agree with you but there's no way to escape something embedded and left undone at the back of my mind. Pablo is great but he's also human,.....is to err!
 
Did the construction of this submarine pipeline add significant insulating value to the entire length of the pipe? Like some kind of coating system to prevent corrosion? Or, is it buried? Or, is it not so "insulated"? I am not familiar with submarine pipelines, but robjul, this will affect the heat transfer to the pipe. You can think of this as a single, long tube in a water bath and estimate a heat transfer coefficient for the classic Q = UA[Δ]T. At 2.3 km long, the A is very large. The [Δ]T is medium sized IMO; probably over 100 F on average year round. So, it depends on the U. If the pipe has little insulating value, I would think the heat transfer would be significant. If the pipe has a lot of insulating value, I would think the heat transfer would not be significant.

Good luck,
Latexman
 
Many tnx for the prompt reply, Latexman. Actually, what will be discharge first from the cargo ship to the Jetty manifold is refrigerated Propane which has a boiling pt. of -42deg.C then, followed by Butane. From the Jetty it will drop down to seabed where it will be heated by the seawater temp. The submarine pipeline has a proper coating and I would presume it is properly insulated.
As per instruction to me, I can initially assume that the Propane was heated by seawater to 10deg.C (for purpose of having a ballpark figure on delta P)and operate isothermally along the lenght of the pipeline with lots of insulation. When it reached the storage tank, its temp. inside the tank will rise to a point where it will be in equilibrium with the ambient temp. I considered a worst case scenario where the temp. will rise up to an average 25deg.C during summer.
1. What is IMO?
2. Had you seen the LPG loading system profile (with some data/info) I attached to my post above on last 13 Jan.,9:59?
3. Do you have any sample calculation on this particular LPG system which I can use as template? Thanks!
Best regards,
Roberto
 
IMO is In My Opinion (Internet shorthand).
 
2 - Yes. It seems that over 80% of the pipe is at an average -35 C, right?
3 - Sorry, no.

Can you estimate/deduce the line pressure right before the bullet tanks? Why don't you share your pressure drop calculations you have done so far? And, tell us about the input data to the online spreadsheet and it's results. That may trigger some helpful discussion.

Good luck,
Latexman
 
Hi Latexman,
Pls find attached my calculations. The schematic diagram had been attached in my earlier post as shown above. Hopefully, some comments would shed light if my calculations was done on the right path.

Many thanks
Roberto
 
 http://files.engineering.com/getfile.aspx?folder=9793dded-05bb-4d6e-ad0e-0c7e14d70f80&file=12inch_Pipe_x_500MT_per_hr_C3@_10deg.C.doc
My spreadsheet said [Δ]P = 67 psi. Very close!

Good luck,
Latexman
 
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