Thermal losses calculation for a burried pipe 1

[sheiko]

Dear all,

I am working in an Engineering Company.

The context is the following:
- The system is: inlet pipe + control valve + outlet pipe.
- The fluid is natural gas
- The outlet pipeline is burried.
- No outlet pipe insulation.
- The minimum allowable temperature in the outlet pipe is -20°C.
- The minimum temperature at the control valve outlet flange is about -15°C (worst scenario)

The problem is that i need to calculate the length of outlet pipe so that the fluid temperature increase to 0°C.

My data are:
- Outlet pipe material: carbon steel (L360)
- Outlet pipe internal diameter: 570 mm
- Outlet pipe thickness: 20 mm
- Outlet pipe is burried 1 m deep.
- Average air temperature: 11°C
- Wind velocity: 10 m/s

My questions are:
1/ Do you know where i can find thermal conductivity data for ground? or do you have data? I know it strongly depends on the ground composition but i don't have anything...
2/ Could you please share any Excel spreadsheet to perform that calculations?

Thank you


"We don't believe things because they are true, things are true because we believe them."

 

[davefitz]

This is not exactly my area of expertise, but this is how I would solve it:

a) the nat gas pipeline industry has dealt with this issue for at least 100 yrs, so they must have published a standard solution by now- a review of the tech papers issued by that industry or its design standards may result in a standard solution.

b) the ground conductivity is one variable, but more important is the ground temperature vs depth and vs time of year( there is an annual variation of ground temp in parallel with seasonal climate variations). ASHREA design fundamnetals volume ( circa 1998) has a section on calculating heat loss vs ground depth and vs seasson and location - one may use those methods.

c) solution curves of conductivity loss from buried pipes can be found in heat trasnfer texts by Kreith, originally plotted in the 1930's- both ground temp as well as soil concutivity are requried.

d) modern methods use finite element simulation , knowing ground temp vs depth , ambient air temp vs time, soil conductivity- maybe the soltuion shuld be solved voer the 3 coldest months of the year.

 

[vesselguy]

Sheiko,

This is also not my area of expertise, but I have done much research on this similar topic and will share this with you. Other may know more.

Soil conductivity:
Best is to get actual soil report that covers the actual soil thermoconductivity. But with such info, you can gleam some insight by reading a chapter that is available on this web link

http://books.google.ca/books?id=qBB...-onpBQ&sa=X&oi=book_result&ct=result&resnum=1

It presents a graphical method to determine outlet fluid temperature of air, plus it gives you some recommended values on soil thermoconductivity to use.

Another source I found on the web, I can't remember where, its been a long time quotes the following:

ASHRAE handbook of HVAC systems and equipment deals with this in details. The following approximate values for Ks can be considered for the soil if you don't have your soil analysis.

sand silt clay
moisture<4% 0.17 0.08 0.08
moisture4 to 20% 1.08 0.75 0.58
moisture>20% 1.25 1.25 1.25

Soil resistance(Rs) = ln[(d/ro)+((d/ro)2-1)1/2]/2?Ks for d/ro>1

and Rs = ln(2d/ro)/2?Ks

Where d is burial depth of pipe to the centerline in ft
ro is outer radius of pipe in ft
Ks is conductivity of soil in btu/hr.ft.F

Heat loss Q = (Tf-Ts)/R

Where Tf is fluid temperature and Ts is undisturbed soil temperature. For yearly average heat losses, soil temperature can be approximated to the average dry bulb temperature.

As for method of calculation, you can use the nice worksheet at

http://www.cheresources.com/inssoftwarezz.shtml

The worksheet for Process Temperature by Chris Haslego is perfect for what you want to do. I have checked its result against the graphical method in my above reference book for air and it matches up.

Hope this help you. It would be good if you come back here and share your final methods used and resources on this problem. I would like to know.

 

[sheiko]

Thanks both!

A star for you vesselguy,

For your info. attached are some soil conductivity data found in HYSYS.

I have additional questions.

1/ What is the condition for "Rs = ln(2d/ro)/2?Ks"? is it d/ro<1?
Please feel free to attach the reference document if in Word or Excel.

2/ Using the spreadsheet, it seems that i will have to consider the following in my case (using your terms):
- Insulation thickness = d
- Transfert coefficient outside pipe = 1/(2*ro*Rs)
- Thermal conductivity of insulation = Ks
- Ambient temperature = soil SURFACE temperature = ambient air temperature

Do you agree?

3/ Do you have an unlocked version of the Cheresources spreadsheet so that i can check the maths?



"We don't believe things because they are true, things are true because we believe them."

 

[sheiko]

Plus,

4/ In the spreadsheet, it is not possible to enter the tube thickness. By default, the thickness is 1/4 inch. Is it always suitable?

"We don't believe things because they are true, things are true because we believe them."

 

[vesselguy]

Sheiko,

First, it is not my spread sheet so I can't comment on unlocking it.

I have done my own calculations using MathCAD to check the spread sheet's calculations and it is spot on. I have no doubt about its numerical accuracy. AS for the theory, you can read Chris technical basis on his Cheresources web site. Read it and if you agree with it then use it.

Yes, I don't know why he fixed the pipe thickness but frankly, it does not matter that much unless you have thickness greater than 3/4" Steel don't hold back much heat.

As for the "insulation" thickness d to simulate buried pipe, I played with 1m, 1.5m etc as "insulation" and find it's about right. Since there is no wind to help remove the heat, there is no outside convective heat transfer coeff. So, what I do there is just put in a very small value just to keep the spread sheet from coming back with either NULL value.

For soil temp, depending on the season that you want to simulate. You have to use your engineering sense to come up with a number. Of course, it is dependent on how deep the pipe is buried. Since you now have the soil k, you can just do a simple conduction HT cal to come up with a soil temp next to the pipe.