Insulation Enhancements for Soils

[BigInch]

I have 1000km of hot pipeline (180F) that must be buried in a trench. $400MM is a lot to pay for spraying polyurethane foam on the pipe and wrapping with an outer HDPE cover.

Does anyone know of the "magic" treatment I need that is relatively cheap and can be mixed directly with soil to improve the soil's insulation properties? It's a moderate climate, not subject to freezing.

Learn from the mistakes of others. You don't have time to make them all yourself.

 

[miningman]

Granulated polystyrene, and / or "pop corn" chips that are used for packing fragile items during shipping. Anything that introduces air pockets into the soil???

 

[dhengr]

BigInch:
I assume the HDPE cover is to protect the sprayed-on polyurethane. Do you have to insulate the whole pipe, or would it suffice to just inhibit the heat transfer up to the frozen surface? Fill the trench to the top of the pipe and compact as usual. Then lay a 4'x8' sheet of rigid foam insulation over the top of the pipe, you pick the thickness, and finish filling. This will at least improve the heat loss situation. The rigid insul. can also be carried down the sides of the pipe at about a 45̊ angles for some depth, maybe only 2' wide sheets, with considerable more placement work. The idea is to reduce the heat loss in the worst direction and the rest of the soil will start to come to some equilibrium temp. Try Google on “insulated shallow foundations, &/or slabs on grade.”

 

[BigInch]

Thanks for picking up the phone guys. Both good ideas. I have already been pulling my hair out for some months now [&bald*spot;] and I seem to have learned that any insulating material, such as the popcorn chips, that might still allow the soil in the trench to become saturated will by virtue of that water content increase the conductivity of the moist soil mass surrounding the popcorn to the point where it effectively negates the insulating value of the popcorn as well. If I'm not mistaken, popcorn can adsorb water too. In fact anything wet seems to fall off my charts.

dhengr, Yes the HDPE is to protect the PUF. The climate is moderate to hot, to very hot equatorial zone, so no freezing ever occurs in air, or underground. Additionally with the minimum allowed pipe temperature of 150F (65C) it appears that enough heat will be lost to lateral and lower soils to reach 140F (60C) rather quickly, initiating wax precipitation and pipeline plugging. This is one of the highest wax content oils in the world. We are planning to electrically trace the whole length just in case we have to shut down long enough for this stuff to cool off and it solidifies within the pipeline. We want to be able to at least remelt it over a week or so with a 40+/- W/m heat input. I kid around that we are actually making a long shoe polish factory. We are considering lowest soil ambient is around 70F (21C).

Voids of air space, while also having excellent insulation properties when dry, should they become wet, I believe would turn the soil mass into a saturated state and all soils when saturated move to the worse ends of their respective ranges of thermal conductivities. Dry sand has relatively excellent insulation value, 0.2 W/m-K is low in the range of soils, but when saturated it rises 2 to 3X, and quickly as soon as only a little water is added. I can tolerate up to 0.25 W/m-K, so the secret seems to be that it would be essential to maintain absolutely dry conditions around the pipe. Digging the trench, laying a heavy polyethylene sheet and padding with sand, laying the pipe, filling most of the remaining trench, closing and overlapping the upper PE flaps of the U from side to side of the trench, maybe with a downturn on each side, sealing?, then topping off the upper 12" of trench burying the PE completely with the last of the backfill?

To be more difficult still, compacted soils also tend to rise to their highest thermal conductivity ranges too. Unfilled voids not good; Compacted not good. Dry sand not too bad, but almost all other soils are starting to become too much of a heat sink. Moist clays and saturated gravels are the thermodynamic equivalent of the kiss of death.

I had brain flashes of adding some kind of hydrophobic, environmentally benign, chemicals to the soil, blending it in well, effectively repelling water at the boundary??? But I have no idea if such stuff exists.

So, unless we can come up with that magic potion, what do you think about the sheet wrapping technique?



Learn from the mistakes of others. You don't have time to make them all yourself.

 

[dik]

If subject to moisture, you likely need an extruded polystyrene, polyurethane or something of that ilk... popcorn type stuff is often not waterproof...

Can you create a utilidor type of containment in concrete with an extruded sheet insulation around the perimeter?

Dik

 

[dhengr]

BigInch:
What’s the water table along the trench, if the trench filled with rain water and how long would it take to drain off in the native soil? Typical soil and soil conditions in the area of the trenching: will trench sides stand vert. or need to be braced in some way, and change installation methods; is soil full of sharp rock which might puncture the PE sheet; should there be a sand cushion (vert. protection layer/wall) btwn. the native soil and the PE sheet as well as around the pipe? What are the dimensions of the trench and the pipe, so we get some idea of the size of things. Mound the top of the fill over the pipe before you flap over the PE sheet, almost like a gable roof, so it will drain off to the sides. So that you can work on the PE sheet joint only on one side of the trench: on the right side, make the vert. sheet length (height?) the trench depth plus a couple feet, and fold this up onto the mound, and pin it down; on the left side, make the vert. sheet length the trench depth, plus the trench width, plus a couple feet, and fold this over the mound and tuck it down along the right side vert. sheet.

There are blocks of foam now being used to lighten deep road fills and the like. The inner half area (inner volume) of a cross section is filled with these foam blocks to lighten the total weight of the fill. This foam has pretty good compressive strengths, don’t know about its high temp. resistance, would have to do some digging to find some of the manuf’ers. names, and mat’l. properties, and available sizes. I would have the manuf’er. cut two half circles out of two mating foam blocks, so that one block fit below the pipe and the other fits over the pipe. Then I would fill the trench around them. In road work, I don’t think they do anything particularly special to protect these foam blocks as they backfill over and around them. Obviously, they don’t drive a Cat directly on it, but pushing 8-12" of fill over them will then allow cautious Cat operation. Filling a trench with normal equipment would seem to be pretty straight forward.

The discussion about friction factors and pipe support systems (the Piping Forum) was an interesting thread wasn’t it? Young engineers seem to get so tangled up with one number or a formula that they fail to look at the bigger picture from all directions. They want a design example or guide into which they can just plug their numbers for the definitive answer; no real thinking or real understanding of the whole problem is required.

 

[oldestguy]

I'd do this post in a forum for chemical engineers who deal in plastics and look for a two part mix that results in a closed cell foam that will not take on water. It may be possible to set the pipe in the trench and fill with that mix and be done with it.

Have you explored placing the pipe above ground and encasing it in multiple layers of spaced aluminum sheets, in the manner used to insulate freezers? Pre-assembling in an enclosure may be better than field assembly.

 

[BigInch]

The water table within a 1000 km of pipe is likely to be anywhere. Where there is a possibility that a water table might rise greater than bottom of trench depth, we will have to bite the bullet and use the HDPE/PUF system. All saturated soils are just too highly thermally conductive to allow any potential contact. At least that's what I'm assuming today. Equatorial rainfall won't help.

For a 24" pipe the minimum trench excavation must is 4.5'w x 7.5'h with the HDPE/PUF insulation system. 3 to 4' top of pipe to normal surface elevation; min of 1' sand bedding below (and all around) the pipe. Trench will be topped off with remaining excavated materials. I may need to enclose a minimum dry region around the pipe of a 4 ft Radius cylinder; squaring off by lining all sides of a 8 X 8 square box works geometrically. Actually that may be too costly to excavate. The foam blocks sound like they could be used to improve trench insulation properties and reducing excavation, if that is wrapped with PE sheeting to keep that water out.

Thanks, but don't "do any digging" to find those mfgrs. All we need to do here is to come up with some ideas to give to our engineering contractors to research, which typically means that they tell me why nothing else but the most expensive solution works. It would be nice to turn that around.

Oldestguy, I didn't want to ask the chemical engineers. I'm pretty sure that most of the hitech insulation products they might propose have already been screened out during previous studies. I'm really looking for the old forgotten methods that the kids today simply don't know, or those "overlooked" methods that have become unfavorable simply because these days that more often than not means "there's just no money to be made on that stuff".

Learn from the mistakes of others. You don't have time to make them all yourself.

 

[Denial]

I worked on a pipe like this multiple decades ago, albeit a much shorter one at about 100kms.[ ] You have probably been through these thought processes already, but two of the most effective (although far from cheapest) things you can do are:
» Increase the pipe diameter, to lessen the pipe's surface-area to content-volume ratio ratio.
» Pump faster, to lessen the time the oil spends in the pipe.
Obviously these two items are closely interrelated.

The standard joke on our team was that it we got it wrong we would end up with a giant candle.

 

[dhengr]

BigInch:
A generic name for the foam blocks is “EPS Geofoam” (expanded polystyrene). It weights 1 or 2lbs./cu.ft, and comes in large sizes, like 6-8' long and 2, 3 or 4' on a side. I Googled “Foam blocks/highway fills,” and found some info. Of course, the entire system could become buoyant if the water level got too deep.

 

[BigInch]

Denial, Yes we've had a look at hydraulic constraints. One mountain range has pressures increased to the limit already, no matter what diameter we choose and, as this is a rather long, remote pipeline, minimum pump stations and viscosity & head loss are controlling diameter, rather than velocity. With the 100 km distance, I presume you didn't need any intermediate pipeline heating stations.

dhengr, Thanks for the Geofoam target. I'll take a look. We won't use that in flood plains or near river crossings. As Denial noted, the conductivity of wet soil will make one hellofabig candlestick and this diameter is large enough that it is buoyant when flooded, so we'll go with the PUF in potentially liquified soils and at river crossings.

Learn from the mistakes of others. You don't have time to make them all yourself.

 

[BigInch]

Ran into this landfill geotextile with embedded clay layer that sounds interesting, but it appears to be more suitable for for planar applications.

Learn from the mistakes of others. You don't have time to make them all yourself.

 

[dhengr]

BigInch:
That’s probably a bentonite clay product. Bentonite clay is very fine and it expands when it gets wet, and acts as a water sealing layer. I have no doubt that it could be used in some geotextile material which acted as a water retaining layer in a pond or landfill. And, it certainly might come in some blanket form (rolled sheet) which could be formed (folded) to fit some gradual contours. The way I’ve seen it used is as a foundation waterproofing system. It looks kinda like a thick piece of corrugated cardboard box material, with all of the corrugation space filled with the bentonite clay product. These sheets are applied to the found. wall, and then backfilled. When it expands it forms an impregnable layer, almost like a PE or rubber membrane. But, it must be constrained btwn. two fairly resistant (compressively resistant) masses, so as it expands it fills voids and compresses on itself to form that tight layer. It is also somewhat self healing for small enough punctures. I didn’t find this in your link, but it’s probably what they are talking about, I only glanced at it.

If you used the rigid foam blocks stacked around the pipe, would it really make any difference if a small amount of water did get into some of the joints? The water really wouldn’t flow to dissipate heat to any extent, and I don’t think the foam blocks take on water, they are closed celled foam blocks. There might actually be some system for taping the joints, from the foam manuf’er.

 

[Denial]

BigInch.[ ] You are correct.[ ] At a length of 100km no intermediate heating/pumping stations were required.[ ] Good luck with the project.

 

[BigInch]

Thanks all. If anyone turns up that magic bullet, let me know.
At least I have a couple red herrings in the fish basket now.

Learn from the mistakes of others. You don't have time to make them all yourself.

 

[cvg]

instead of sand bedding, you might consider a lean mixed, light weight concrete mixture. would keep water away from the pipe, and depending on the aggregate, you might get some decent insulating qualities also.

http://www.nrmca.org/aboutconcrete/cips/36p.pdf