High thermal conductivity backfill

Out of curiosity, what is the fluid being conveyed? Where are these pipes located; urban ROW, countryside, are there roadways over the pipes?

It is planned to be either liquid ethane (about minus 100C) or LNG (minus 160C). The pipes will be highly insulated, but there will still be some heat input required as the contents stay circulated and at those temperatures 24/7. The pipes are planned to be buried approx. 1.5m cover in a congested industrial area. Most of the piping run is above ground but we have a shortish (1.5km) section which they want to bury. Backfill doesn't have to be anything special in terms of compressive strength, road crossings will be done differently.

The insulation vendor has done some calcs and because we have two in close proximity (don't ask) they are predicting close to zero C after some time if we don't have soil of relatively high thermal conductivity - approx. 2W/m/K, which is the inverse of electrical cable conductivity from what I can gather.

Therefore although the heat flow is in the other direction these seemed to be similar to special high thermal conductivity backfill used for cables and I'm just searching to find some experience of it and any contacts / companies / papers / websites which can help me.

If this isn't the right forum then please advise, but it came up a few times when I searched for this topic here.

Thanks

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

I suggest reducing the amount of insulation for the buried section of piping and use a poorly graded, granular backfill (3/4" rock) wrapped in filter fabric. Reducing the insulation will enhance the heat absorption required for fluid circulation. The poorly graded, granular backfill will have plenty of void space for air to pull heat from the surface. Carry the backfill to the surface to allow light vehicle passage. Wrap the fabric around the corridor with room for a 6" layer of granular backfill on top to match grade. Slope the ground parallel to the pipe corridor down away from the rocked surface to prevent rainwater from surrounding areas washing into the pipe envelope.

Livewire,

thanks for input, but reduction of insulation is not an option and heat input would cause a problem, not solve my current one.

The insulation is very good, but not perfect and therefore there is some small heat transfer from the soil onto the pipe. My issue is that the calculations show that without a relatively high thermal conductivity, over time this could get to or close to freezing.

Therefore my current query is about knowledge and experience of using high thermal conductivity back fill.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

Understood. If you look at a table listing the thermal conductivity of different materials, and target 2 W/m-K you mentioned initially, you're left with only a few options: asbestos-cement, concrete, ice, saturated sand. If you consider the last two you're best option may be an envelope of clean, free-draining sand. Best of luck with your project!

From my understanding, what you want to do is prevent the soil around the pipes from freezing.

The first question that arose is: Is that really a problem? Honestly I don't know.

If it is :
You could circulate a fluid through the soil to constantly ''feed'' some heat to the system.

Reversely, you could make sure no water will be present around pipes, so that no swell will occur upon freezing.

If the project would be in a remote area i would say just freeze it. But since there are buildings nerby, I don't know.

Charles



Ingenieur Minier. QuTbec, Canada.

Hi LittleInch, your solution is to install pipes and recirculate fluid (brand name temper, You need plastic pipes (search horizontal geothermal heat exchanger) and a dry cooler which will works as a heater.

Thanks and appreciate that one solution to prevent freezing is to add heat to the soil, but if I can get a thermal conductivity just a little bit higher than normal "soil" then I don't have to install anything. I have seen this before in cable trenches where conductivity is also important and hoped someone on here had come across this before and could provide some idea of what is possible or who supplied this special backfill material.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

Hi LittleInch, the increase in thermal conductivity will change the temperature profile of soil but also this depends from mean surface temperature (air temp fluctuation) and the pipes' heat flow (there is a vast ΔΤ). Consequently for your application the installation of extra pipes perhaps is the only solution. If you run a 2D transient thermal model you will find the time to reach the steady state and then the necessity to install extra pipes.