Minimum_Design_Temperature_Toughness

[numerical1]

Hello,
For gas pipe (buried at around 5 to 6 ft), I am trying to determine the Minimum Design Temprature for toughness purpose. I am assuming that minimum designTemperature of the pipe can be the same as the minimum ground temperature at that depth (5 to 6 ft).

The question is how to obtain that temperature. I do have the minimum air temperature for the subject location but I was wondering if I need to do ground heat transfer analysis to obtain the ground temperature at that depth.

Thanks for help.

 

[rconner]

[you may be interested in (probably many) past discussions, and with perhaps other considerations as well e.g.

http://www.eng-tips.com/viewthread.cfm?qid=260205

]

 

[Krausen]

For buried steel pipe on crude cross-country lines in the northern climes I've used 20 F as the MDMT. This assumes only a minimum of 3' cover. There are heat transfer effects, but there are many factors to consider to really dial in your ultimate pipe wall temp. On typical crude lines over long distances the process (and outer pipe wall) temp stay pretty consistent in each season in steady state operation. Not so sure on gas lines though. FYI, If this line is not designed to stress the pipe over 20% SMYS then you may not need to worry about any proven toughness testing, which can add expense & extend schedules. The liquids transportation code (B31.4) has this allowance.

 

[SNORGY]

Find the frost line depth. Below that, you are at about 0 C or 32 F. At grade, you are at ambient. Draw a straight line through the points and you have a very crude estimate of the ground temperature at whatever depth. Maybe it's not a straight line, so you might try a 1/3 or 2/3 rule for the temperature profile as a function of depth. How accurate does your estimate need to be?

 

[saplanti]

You had better go through the following discussions:

http://www.eng-tips.com/viewthread.cfm?qid=406156

 

[numerical1]

Thanks all .
SNORGY: it is a good approach to linearly interpolate between using the minimum air temperature and the frost depth.
I guess the interpolation should be between the minimum ground surface temperature (not the air temperature and the frost depth). I think one should have accuracy within 5 degree , at least.
The issue is that it is not easy to calculate the minimum ground temperature (from the air temperature) because it depends on surface condition, snow cover, etc. I am wondering how this issue/point is not covered well in literature. When installing pipe in cold region, knowing the ground temperature at burial depth is very important. No guidelines in the pipeline literature are provided to determine this temperature.