Heat Transfer in Soil 4

[Pipeliner2008]

Need help finding a simple equation to calculate the cover for a pipeline to not being impacted by a forest fire aboveground.
I am assuming homogeneous soil and only 1 direction for the heat wave to travel (down).
So far I've found this one based on Fourier's law, but not sure if it can be used.
h/kA = T1-T2/Q
where:
h: depth
k: thermal coef. of soil
A: area of impact (assuming 1 square unit)
T1: Temp. above ground
T2: Temp. at the pipeline (unchanged)
Q: Thermal energy of fire

 

[SREisme]

I don't see why the Fourier formula wouldn't be valid. I would model it as heat flow in one direction across a uniform resistance, the soil. I would probably use a cross sectional area equal to the diameter of the pipe times the length of the affected area to be conservative, given the required depth is probably much greater then the radius of the pipe (effectively makes the pipe a square instead of round cross section) The value for q may be a bit tricky. I believe the Q in the formula is the amount of heat conducted through the resistance, not necessarily the thermal energy of the fire. You could start with a flame side temp of about 3000F, a ground temp of 55F, calculate a resistance per foot of earth and see what kind of heat flow you get. That heat will be absorbed by the product in the pipe, and may or may not be removed from the system.

 

[Pipeliner2008]

SREisme, Thanks for responding. Somehow the units on the equation won't work.
Any other equation you may know of?

 

[BigInch]

This IMO is kind of a red herring. No way you will have any impact on the pipe at normal burial depths of 2 feet in rock or 3 feet in soil. Consider the thermal heat transfer across the interface of atmosphere and soil and you will see how little heat is actually transferred to the soil, heat capacity of the soil and warm up time, and length of time required to heat up just 1 foot of soil and you'll see that there is little effect on the soil for anything near a normal fire duration time (24 hours in the same spot or so might be a very long time. I wouldn't think more than 4 hours would be sufficient at such temperatures). Then, if you still need to, add to that the cooling effect of moving product inside the line on the pipe wall itself and you can see where this will take you. You very rarely see roots charred to any significant depth.

But if you want to do it, look at the "overall heat transfer coefficient" equation to get the units correct.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[dcasto]

here is a practical dtat point. There was a huge pipeline fire in Friendswood Texas in the early 1990's. A series of pipelines were crossing a canal above ground and a mower cut into one, a propane line. The line ruptured and propane ignited from the diesel engine. Within a few minutes the line a few feet away ruptured as did a couple of others. Our buried line was safe.

Back at the site after the fuel was exhausted we went back to look at the dfamage. A metal barbwire post used as a pole for an API pipeline marker that was within a few feet of the rupture was laying down and was just a melted line of metal in the hardened clay. The 11" diameter aluminum sign was just an eery silver shawdow in the clay.

The clay was solid "rock" not the stuff you normally expect. For a 100 foot radius around the fire, the ground looked like the moon. There were little mountains that stuck up about 4 inches and were about a foot in diameter. they were scattered every 8 to 10 feet. I kick one of those moun ds and a 1 to 1 1/2 " of fired clay broke off. Then the ground started to crawl as the fire ants came running out to defend their turf.

 

[BigInch]

dcasto, LOL

THAT'S WHY THEY CALL THEM FIRE ANTS !!!

Since the hottest part of the fire is not at the bottom of the flame and the gasous interface between the fire atmosphere and soil prevents effective conduction into the ground, little heat is transferred to the soil. Even if you throw gasoline on it. The only real means of heat transfer available is radiation, which is not nearly as effective as conduction. Most of the fireball and the heat it has within it goes up. Relatively little heat goes down. As decasto says, radiation can be effective and will warm the upper 1" of soil, but not really that much. Its even common for pine cones, either still in the tree or lying on the ground, not to be burned by a typically fast travelling forest fire. The pine cones open at first rain and release seeds to get the forest restarted straight away.

There are a lot of ranchers that would gladly burn their grasses using gasoline, if only the fire would get rid of those ants. The liquid gasoline itself would probably only kill a handfull of them by chemical contact.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[sailoday28]

Pipeliner2008 and SREisme
A shape factor for a long isothermal pipe buried beneath an isothermal surface is given in:


See table 1 of

http://poplab.ece.illinois.edu/files/ece598/Sunderland-ShapeFactorsTABLE-ashrae64.pdf

 

[SREisme]

I agree that most of the heat from a fire will go to the atmosphere, especially a fast brush fire or tree top burn. However, I have seen the aftermath of forest fires where the ground is scorched to bare soil where the needle mat has been entirely consumed. I have heard other reports of major forest fires changing the soil quality, presumably by scorching all the organic matter out of the soil.

Pipeliner- as mentioned, Q is heat transfer rate, not thermal energy, its units are Btu/hr. Use h in ft, k in Btu/(hr*ft*F) A in ft^2 T in (F). In these units the equation balances.

I am assuming you need to convince or prove to someone that your cover depth is adequate to withstand a forest fire. I would do that by estimating what the heat transfer rate would be at your standard depth, for an estimated duration of a fire event and making some assumptions on where that heat will go once it reaches the pipe.

 

[IRstuff]

Dry Earth 1.5 W/m-K, assume 2ft depth and 1200°C delta to pipe in straight 1D analysis

(1.5W/m-K)*1200°C/2ft = 2952 W/m^2

diatomaceous earth is listed as 0.06055 W/m-K


TTFN

FAQ731-376

 

[BigInch]

Just out of curiosity, who in the heck is requesting that you do this? I'm sure that the BLM or USFS wouldn't.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[Pipeliner2008]

Myself. There will be a series of controlled burns for the vegetation to regrow in W. TX where we have several pipelines and everybody @ work, including me, think the fires will not represent a safety issue to our facilities, but I just wanted to mathematically prove it with an equation.

 

[BigInch]

Doubt it. It's not a problem in south Texas burning off the prickly pears and the mesquite.

OK. Have fun. Watch them rat'lers.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[BigInch]

and of course the ...

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/