Depth of buried gas flowline

[Weko]

Hi all,

I am currently designing a buried 1.2 km gas flowline (ASME B31.8). The area is besides the province road.

Although we will put saftey signs. Probably local citizen would build a "personal" road in the future, crossing on top of the flowline.

My question is, how depth it is needed for the flowline to be buried?

A hint/reference to a standard/code would be very useful. Thanks.

 

[BigInch]

The minimum cover you would want to have is 3 ft, or commonly "metrisized" to 1.00 m clear top of pipe to surface.

Beside the road, but still within the road's "Right of Way", the province or local governing body will have jurisdiction and they may want you to bury it deeper, 4ft is common, but could be more, if they will let you put it in the road's right of way. You will normally need the local government's road and transportation department's permission before locating your pipeline there.

If you are paralleling the road, but outside the right of way, you can usually go back to 3 ft clear cover.

3 ft clear cover is normally adequate for light traffic, but not for highway loads, so if you expect roads to be built on top in the future, you should watch for that type of construction activity taking place and stop and prohibit all of it and any construction within your pipeline's right of way.

You will find all the minimum burial depths listed in B31.8

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[zdas04]

In addition, I try to stay below the frost line (which can require more cover than the normal 3-4 ft) because even in a gas line, you need to try to avoid condensation freezing.

In many places (like the south of Spain), the "frost line" hasn't existed for millennia, but in others it can be pretty deep. It is generally a good idea to know approximately where it is most likely to be and go deeper than that.

David

 

[BigInch]

I've put in 6" to 36" gas transmission lines from Gulf of Mexico to Canada none of which were below the frost line. The only ones that plugged up were the ones in the gathering system between Laredo and Zapata, and that was for the hydrates that formed up in winter when the temperature was still well above freezing. We just lit up the line heaters and the crystals melted away.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[zdas04]

Interesting. I've seen candle-wax freezes in buried lines in New Mexico where the pipe wall is below freezing, a drop condenses and touches the pipe wall and freezes, then another, etc, until the ice has built up like the wax on a chianti bottle in a cheep Italian restaurant. When I started having that problem, I started being more careful of the frost line.

David

 

[BigInch]

If it looked like wax it was hydrates. It's been a while since I've looked at it, but as I recall they can start forming up at around 40F if the pressure is around 200 psia. See table 2 here,

http://www.ogbus.ru/eng/authors/Fatahh/Fatahh_1.pdf

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[BigInch]

Transmission quality gas isn't going to suffer from hydrates. Maybe some small quantities of water will collect from time to time and freeze somewhere, but it usually isn't enough to stop up the works and it has to be pleanty cold, below -10F or more ... usually, but, from time to time it has been known to happen. The closer to the gathering systems you are, the wetter the gas, so the more problematic it can be. I think Denver had a gas supply problem in the winter of 2004/5 or so. Actually I think I remember now that it was a nearby gathering system that froze and they had to shut it in, so it wasn't the transmission lines at all.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[ione]

A couple of documents which could be interesting.

http://www.bakerprojects.com/phmsa/pdf/Pipeline Consensus Standards - FINAL REPORT.pdf

http://pubs.aina.ucalgary.ca/cpc/CPC5-389.pdf