Minimum burial depth of insulated pipe to avoid freezing (moving fluid)

[JoshuaMan]

Hello everyone

I have been looking in to the required minimum burial depth of insulated pipes and I have become stuck.
I have read through the attached pdf however I am getting weird values (0.00034mm depth).

I have searched on-line for a while but no joy, there where a few pieces of information but none that fitted my requirements

Does anyone know of a formulae to calculate the minimum burial depth of an insulated pipe to avoid freezing?

Thank you for your time
Josh Man
Beng Mechanical engineering

 

[EdStainless]

It depends where you are.
You are working in Chapter 12, right?
Someone has supplied you with the soil conditions?
They should also have told you the depth of freezing.


= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[JoshuaMan]

wow what was a quick reply !!! thanks.

yes i am currently on page 12-14
using X = m(Ig)½ that expands in to using the correction coefficient, my main issue is how to calculate Ig?

The pipe will be buried in London and have to withstand the winter months.

when you say depth of freezing is that frost permeability?

Thanks
Josh

 

[ione]

It’s a transient analysis problem. You can simplify the problem assuming the soil remains at constant temperature and proceed as follows:

α is the soil thermal diffusivity in [m2/s],
Tsurf is the soil surface temperature [°C],
Ti is the initial soil temperature [°C]
x is the depth of the water pipe [m]
t is the time the soil is exposed at Tsurf
T(x,t) is the temperature at depth x and time t

[T(x,t) –Ti]/[Tsurf – Ti] = erf [x/(2sqrt(αt))]


Where erf is the error function (

http://www.miniwebtool.com/error-function-calculator/)

You put T(x,t) = 0 °C and solve for x

 

[ione]

For more details you can go for:

"Heat transfer a practical approach" by Yunus A. Cengel (2nd edition - McGraw - Hill):
4–3 Transient heat conduction in semi-infinite solids

 

[JoshuaMan]

ione, thanks for the reply, this formula doesn't take the pipe insulation or is this the soil thermal diffusivity?

 

[ione]

Joshua,

The value you get with the approach above is conservative as it doesn’t take into account pipe’s insulation.

 

[JoshuaMan]

ione, again thanks for the response.

http://www.cdeep.iitb.ac.in/nptel/M...le 4/interactivepages/prob4.5co.html#comments

the link above is a similar calculation to yours.

how would I go about incorporating the pipes insulation values in this calculation?

Thanks
Josh

 

[fegenbush]

JoshuaMan,

There may be a much easier solution. I know that AWWA D100 has a figure which shows the minimum cover over buried pipe to avoid freezing. There may be a similar BS that covers the same material. Alternatively, you could look at weather conditions and select an similar US city and use that value as a starting point.

 

[CarlB]

City of London standard drawing W-CS-68 shows that standard minimum depth of cover over water lines is 1.7 m (5.5 feet). Water line bury depth is usually a good indicator of maximum frost depth.
That detail shows the depth can be reduced to as little as 0.6 m (2 feet), when insulated with 3" thick by 4-ft wide insulation.
I'd think you could select annular insulation to allow as little as 2' bury also.

 

[ione]

If you want to take into account the pipe insulation you could consider the whole scenario as a frost penetration problem in a multilayer medium. Assume just two layers: uniform soil and pipe insulation. It is still a transient problem, much more difficult than the one which considers just uniform soil with step change in temperature and I not aware about the existence of an analytical solution for this problem. Probably a finite element simulation is the way to go, anyway I have taken the liberty of attaching a technical report of the US Army Corps of engineers dated back 1957, where you can find some guidance about the way to approach the frost penetration in multilayer soils.

File in next post...

 

[ione]

Here is the file...

 

[JoshuaMan]

it worked this time, thank you.

i will have a read through, it looks very retro.

 

[CarlB]

That method of estimating 1-D frost penetration in multi-layer soils is essentially how London came up with those depth of bury/insulation guidelines. Tempered with experience. They account for 2-d effects by varying the insulation width.

There's been a lot of advances since that old Corps paper. Alaska DOT offers a free dos-based program to solve this, called "BERG2". Even that program is 20 or so years old. It is most useful for determining insulation of roadways, doesn't apply well to narrow elements like pipes.

 

[ione]

Thanks to the Eng Tips staff who have fixed my previous post

 

[JoshuaMan]

hi guys

thanks for all the responses, I think I have worked it out.....

I have calculated the depth of pipe with no insulation, I have then calculated the temperature of the OD of the pipe and worked out the heat loss with the insulation.

I have then done the same calculation using the thermal diffusion for the insulation and taken the insulation depth from the non insulation depth.

hopefully this is done correctly?

 

[MortenA]

If i were you i would go with CarlBs methodology. A city like London has been there for more than a 1000 years, so their standard will most likely be pretty optimised taking everything into account. But then again i cant see that you mentioned whats in the pipes.

 

[JoshuaMan]

hi guys

I have used the formulae from the link below, I have then calculated the outer casing temperature and then used this value to calculate the time taken to fully penetrate the PU foam and used the values to calculate the minimum depth after a specified time period - the time taken to penetrate the insulation of the pipe, sorry if I haven't explained this too well.

http://www.cdeep.iitb.ac.in/nptel/M...le 4/interactivepages/prob4.5co.html#comments

attached is my attempt :/ if some one would like to have a quick scan to see if I have made any major mistakes it would be appreciated guys.

Cheers
Josh