Underground Storage

[CNY CAT MAN]

Hi guys new here am looking at installing econoburn 200 maybe 300 with 2000 gal storage / Has anyone put there tanks underground ? I am considering putting in 2 - 1000 gal propane tanks insulated with foam 3' deep backfilled with sand :coolsmile:

 

[dogwood]

Would you be coating them with something to prevent corrosion?

Mike

 

[101x81]

well, if you have a leak you will be in trouble

 

[CNY CAT MAN]

Mike ; I am looking at getting 2 used 1000 gal underground propane tanks I will recoat them and install a new anode then spray with closed cell foam probably 4" thick + - R24, I am hoping the ground will also act as an insulator, Looking any and all input

 

[stee6043]

Me thinks R24 will be SEVERELY inadquate for an in-ground installation. The ground will not act like an insulator, it will act like a heat sink (quite the opposite if insulating). I think R60 would be a bare minimum and even then I think it's light. I have R40 on my indoor tanks and wish I had more.

Everything I have read seems to suggest in-ground is likely one of the least desirable locations for high temperature thermal storage....

 

[ewdudley]

Mike ; I am looking at getting 2 used 1000 gal underground propane tanks I will recoat them and install a new anode then spray with closed cell foam probably 4" thick + - R24, I am hoping the ground will also act as an insulator, Looking any and all input

If you put them in your basement they would be underground, but they won't rust unless there is moisture.

As long as there is sufficient drainage to keep the water table below the tanks at all times you should have no problem, especially if you keep the tanks at least good and warm year-round so as to keep them well above the dew point.

Just follow normal practice for burying propane tanks as propane tanks, and unless there is some reason a hot tank of water should be more vulnerable that a cold tank of propane, what could go wrong?

--ewd

 

[Nofossil]

You'll want to be sure that your insulation does not become waterlogged over time. I'd go with no less that a foot of insulation, and 2 feet over the top. Unless you want to build a greenhouse over them. That would keep out the water.....

 

[ewdudley]

Me thinks R24 will be SEVERELY inadquate for an in-ground installation. The ground will not act like an insulator, it will act like a heat sink (quite the opposite if insulating). I think R60 would be a bare minimum and even then I think it's light. I have R40 on my indoor tanks and wish I had more.

Everything I have read seems to suggest in-ground is likely one of the least desirable locations for high temperature thermal storage....

Right, this would be the big problem. I did a calculation for a 4' X 10'6" 1000 gallon tank at R30 and came up with 1300 btu / hour heat loss to a 50F space. For two tanks at R24 you could be looking at upwards of 3000 btu / hour. Maybe scale back to one tank and add another 4-6 inches of styrofoam board?

I'm sure your darling would love a glassed-in sun room with a whirlpool, now you know where to build it!

--ewd

 

[CNY CAT MAN]

ewd ; Thanks for getting back with the calcs. I could add more spray foam the foam I would be using is closed cell appiled directly to the tank it should act as mositure barrier as well. I just used it for my run of 1" pex 100' from garage to house , please dont tell me I made a mistake by doing that. PS I'll make sure not to let my sweet thing see your post that sunroom on a cold winter day would sound better to her than a boiler :coolsmile:

 

[ewdudley]

ewd ; Thanks for getting back with the calcs. I could add more spray foam the foam I would be using is closed cell appiled directly to the tank it should act as mositure barrier as well. I just used it for my run of 1" pex 100' from garage to house , please dont tell me I made a mistake by doing that. PS I'll make sure not to let my sweet thing see your post that sunroom on a cold winter day would sound better to her than a boiler :coolsmile:

I wasn't suggesting styrofoam board as a superior alternative to sprayed-on closed cell, the spray-on is in all ways superior from what I gather. I was just thinking once you had the tank sealed off vapor-tight with the spray-on you could switch to board on an outer layer to save some cost since the vapor-tight advantage of spray-on wouldn't be important for the outer layer.

For underground pipe I believe closed cell sray-on is how it's done if it's done right, but even then you see some of the installations form-up the trench with 2" styrofoam and cap it off with syrofoam to save cost.

--ewd

 

[ewdudley]

And speaking of styrofoam board under ground, in building my boiler shed I had to excavate around some water lines that -- since they were unavoidably near the surface -- I had insulated with a couple inches of styrofoam board under a layer of plastic and six inches of gravel to keep them from freezing. I believe the plastic kept water from seeping down, but there was condensation from below, so everything was moist for a few years.

At any rate, for some reason I had a mix of the blue stuff and the pink stuff, and there was no comparison: The blue stuff was crisp and dry whereas the pink stuff was spongy and wet.

--ewd

 

[powerspec]

Glad you asked. I posted the same question here some months ago and found no one had done it before. So I put a 1000 gallon used propane tank underground this past summer. I used 4 inches of high density (2 lbs per cubic foot) urethane foam to insulate it. The tank is buried right beside the house with about 18 inches of dirt cover over the top of the spray faom. The tank is resting on two sleeper beams (6" x 12") laid in the earth under the tank feet to prevent the tank from settling and straining the piping. I filled the tank with warm water from my old boiler and waited to see how long it would take to cool off. I lost 1 degree overnight from 100F to 99F. That was last fall when the weather gave us frost every night.
If I had to do it again I might go with 5 inches of foam but not more. My 1800 sq ft house can go all day at 30 below F temp without a boiler fire. On such a day the tank will be at 170F in the morning and down to 120F by the time I light an after supper fire.
Make sure you use the high density foam not the light stuff that is used for sealing windows and doors. Also make sure your foam contractor has his head on straight. Mine didn't and tried to charge me 70% more than his estimate because he goofed on the amount of product the job required. One needs calculus to really figure out the volume of product needed because with every pass the tank surface gets bigger. My guy didn't do calculus that's for sure.
I am looking for a source for DIY urethane foam either spray or in a bulk mix. Dow chemical used to sell a product in two parts that you mixed together and in a few moments the foaming started. But this is no longer available as a standard industrial product that I can find. I used some left over chemical like this to insulate the tank to house piping (all sch 40 steel 1.5 inch and covered with a tunnel of foam 3 inches thick).
With the tank underground my house is 1000 gallons bigger than it would be if I put the tank inside as originally planned. I am not a believer in cathodic protection for an installation like mine or yours. The urethane is a very good electrical insulator.
I have two PT100 RTD's inside the tank in two thermowells for temperature measurement, one lower than the other by just one foot or so to give me some idea of the stratification.
The return piping and supply piping are rather close together in the tank I used, about 14 inches apart. The return pipe has a dip tube to within a few inches of the tank bottom so returning water is directed to the bottom coldest water in the tank. I have no separate pumping loop for the tank. When the tank is the only heat source (cold boiler) the hot water will rise by gravity to supply the house load header pumps. When the boiler is firing the hot outlet water passes the header connections to get into the tank. Simple and it works. Nofosil's advice to install the tees so the flow favours the tank was good advice. The boiler (EKO 40) feet and tank feet are at the same vertical elevation.
In case someone asks my tank is .375 inch thick, very heavy and weighs 1400 lbs. This is not the standard wall thickness I am sure. Heavier is better for underground service. My backhoe had flat tires lifting it off the truck.
A greenhouse would do well on top of this tank but mine is against a north wall where there isn't much sun. If I could I would have buried a bigger tank but not two of them.
Go for it. Good luck.

 

[Chris Hoskin]

thanks for sharing powerspec - very interesting stuff!

 

[ewdudley]

Here are some better numbers than my earlier mental calculation. Should be fairly pessimistic since the tank won't be 190F on all sides at all times, and ground temperature would be higher than 40F maybe.

Single 41" D 192" L tank as a simple cylinder (end domes would be slightly larger than simple circles):

You have: (1/24.0 Rvalue) * ((2*((41/2)^2*pi)) + (192*41*pi))in^2 * ((190-40)degF)
You want: btu/hour
        1187.9829

You have: (1/30.0 Rvalue) * ((2*((41/2)^2*pi)) + (192*41*pi))in^2 * ((190-40)degF)
You want: btu/hour
        950.38632

You have: (1/36.0 Rvalue) * ((2*((41/2)^2*pi)) + (192*41*pi))in^2 * ((190-40)degF)
You want: btu/hour
        791.9886

Two tanks side-by-side taken to be two half cylinders with a 41" X 41" X 192" box in the middle, two ends and two sides of which are exposed:

You have: (1/24.0 Rvalue) * ((2*((41/2)^2*pi)) + (192*41*pi) + (2*(41^2)) + (2*192*41))in^2 * ((190-40)degF)
You want: btu/hour
        2017.2364

You have: (1/30.0 Rvalue) * ((2*((41/2)^2*pi)) + (192*41*pi) + (2*(41^2)) + (2*192*41))in^2 * ((190-40)degF)
You want: btu/hour
        1613.7891

You have: (1/36.0 Rvalue) * ((2*((41/2)^2*pi)) + (192*41*pi) + (2*(41^2)) + (2*192*41))in^2 * ((190-40)degF)
You want: btu/hour
        1344.8242

 

[CNY CAT MAN]

ewd & Powerspec ; Thanks for the world of info the calcs are a little over my head is the line next to the bottom my actual btu's per hour needed to maintain the 190 degree temp ? / Powerspec I believe I have a source for DIY urethane , I have seen it used on jobs filling in small areas , I have seen it at a contractors supply that we use , it comes in 2 disposable tanks that are like 20# propane tanks ,I think that you have to purchase a reusable nozzle , I will call and get more info. I also am glad to here that someone else has experimented with underground storage, you mentioned you would go to a 1 larger tank rather than multiples , one reason for my multi tank thoughts is availability and I may need more than 2000 gallons of storage I am heating 10,000 sf home and 2,000 sf garage new construction ,radiant heat ,high ceiling hgt. so I am really unsure of total storage needed and want to make sure I can add onto it if needed THANKS GUYS FOR THE HELP !

 

[RobC]

Two quick comments. You guys are ALL better at the math than I, but I don't believe you can compare the insulating qualities of any spray foam to fiberglass, blown in or air trapping types of insulation. The way it insulates is different. I'm not sure about R values but I don't believe that comparing R values is apples to apples. Foam at 3" is like 97% efficient, 2"is like 87%. ( looked quick but couldn't find the chart I was looking for ) But, this is why foam contractors say spray 3". Anything after 3" you will never get your money back. If burying I would put some board around it, it case, over time, the back fill weight started to compress the foam.
Foam contractor sprayed more foam than expected. Make sure contractor has sprayed on steel before to avoid this. One way to do it is to shoot a quick sealer coat to break the contact with the steel surface. Then you go back and re-coat.
Dow's product is Froth Pac it's still out there. I got OK results.
Rob

 

[powerspec]

Rob C did you bury the tanks you insulated with Froth paks? Catman you must make sure to use high density foam and I think the DIY kits make it easy to get a lower or varying density in each pass, something you ust avoid. A reputable contractor will test the spray gun flow at and during each spray job to make sure the density is up to spec. The raw foam will stick like mad to any moderately clean oil free metal surface and the cured foam only needs protection against UV rays. So direct burial is no problem. You may want to provide a bottom drain but I didn't since this is just another place that needs to be protected from freezing. So if I ever drain this tank I will pump out from the top.
Cat Man if your home was in our country 1000 gallons wouldn't be enough. Perhaps it will be for your new construction if everything is really up to the latest insulation specs. See your propane dealer for used anhydrous ammonia tanks or 2000+ size propane tank. One tank is way better than two if you can handle it. The surface area saving alone makes it worth it for very much less heat loss.
When insulating pipes and round conductors there is point where the added insulation makes more surface area to radiate heat away, called 'critical thickness'. So don't add more than 5 inches of evenly applied high quality foam, that's my take on things.
Press on!

 

[in hot water]

Insect protection is another concern. They love warm blueboard. I have some pictures of a basement that we exacavated to do an addition. Termites, or maybe ants had turned the 2" board into swiss cheese! Bug experts tell me the need a source of moisture nearby to bore the foam. In this case there was some roof drainage issues near the basement wall.

About 20 years ago they took the termicide out of foamboard due to an EPA ruling. Probably a good idea. They have yet to come up with a suitable replacement. Borax works and is in some foamboard products, but it doesn't last and needs re-treatment.

In some countries they now require a stainless steel mesh around any foam with earth contact. I met an ICF builder in Australia, at the homebuilders show. He told me about a product they use that is mesh and can be coated with a waterproofing also, for double duty. Sounds expensive.

Underground thermal storage would be my least favorite choice, look for other options.

A neighbor in using old shipping containers to put his wood boiler and old dairy tanks for storage. The plan is to "earth shelter" much of the container, should be interesting.

Here is some reading on foam and pests

(broken link removed to http://www.perfectionpest.com/PDFs/BuildersIRBCode.pdfhr)

hr

 

[tom in maine]

Although I have never buried a tank, I have messed around with a lot of foam--both spray and sheet.

Any foam will water-log over time, if the soils do not drain well or even if they only occasionally get wet.
Critters love to live in foam (a lesson there)--wet or dry.

I would insulate as much as possible, given the water issue. None of that heat loss is usable, except to
make the tulips bloom early!

Froth pak or any self-applied foam is not as good as that installed by someone who knows what they are doing.
I have done a lot of DIY foam. Professional installers offer much better results--higher density and more consistent results.

If I was going to the hassle of digging a big hole to bury a tank, I would dig a basement and have it in the basement.

Out of sight is not necessarily a good thing.

 

[powerspec]

Insect protection! Never would have thought about it. Maybe a concrete missle silo would do. Seriously though direct burial of high density foam is well approved up here and underground bugs are the least of concerns. Check your local codes?

 

[ewdudley]

Insect protection! Never would have thought about it. Maybe a concrete missle silo would do. Seriously though direct burial of high density foam is well approved up here and underground bugs are the least of concerns. Check your local codes?

Bad news it was 0F/-20C for a week. Good news no termites, kudzu, roaches, poisonous snakes, centipedes, fire ants, scorpions, Pill & Sow bugs, pythons, killer bees...

--ewd

 

[Nofossil]

When insulating pipes and round conductors there is point where the added insulation makes more surface area to radiate heat away, called 'critical thickness'. So don't add more than 5 inches of evenly applied high quality foam, that's my take on things.
Press on!

As I understand it, 'critical thickness' is the amount of insulation that MAXIMIZES heat loss by providing additional surface area. Beyond that, heat loss is reduced. There is the issue of diminishing returns - each time you double the insulation (beyond the critical thickness) you roughly cut heat loss in half. If the first six inches take you from 10,000 BTU loss down to 5,000 BTU, then the next six inches take you from 5,000 to 2500. The next 12 inches of insulation will take you down to 1250, and so on.

It's important to consider that temperature loss is proportional to the temperature drop across the insulation. When we insulate our attics with 15" of fiberglass, we're looking at an average winter delta T of about 45 degrees in Vermont (average winter temp is 25, average indoor ceiling temp is 70). If you have a storage tank at an average of 160 and soil at an average of 30, that's a 130 degree delta T - you'll get a LOT more heat loss. It's even worse than that, because both water and soil are far better at transferring heat into and out of the system than air is.

I'm not saying that buried tanks are a bad idea, but it would be really difficult to add more insulation if you find out that you don't have enough. Here's a chart that shows the critical thickness issue. The units are for different insulating materials, but hopefully this helps explain what's going on. The actual math is kind of hairy. This isn't really much of an issue with large diameter items such as tanks.

 

[powerspec]

Nofo is correct, critical thicknes refers to the magic value of insulation thickness that best defeats the purpose of the insulation, that of reducing heat loss. The critical thickness for spray foam on these tanks would be very much greater than anyone would afford to pay for. But the same is not true for small diameter pipes. The 5 inch max limit is a practical one as I see it to save money and get the least heat loss for the money spent. I feel that all other forms of insulation are poor compared to the performnce of properly applied high density spray urethane foam. Flat sheets of blue styro are a very poor substitute for underground service on a round hot tank.
Powerspec

 

[Nofossil]

Nofo is correct, critical thicknes refers to the magic value of insulation thickness that best defeats the purpose of the insulation, that of reducing heat loss. The critical thickness for spray foam on these tanks would be very much greater than anyone would afford to pay for. But the same is not true for small diameter pipes. The 5 inch max limit is a practical one as I see it to save money and get the least heat loss for the money spent. I feel that all other forms of insulation are poor compared to the performnce of properly applied high density spray urethane foam. Flat sheets of blue styro are a very poor substitute for underground service on a round hot tank.
Powerspec

This probably isn't worth getting too obsessive about, but I just can't help myself. Critical thickness and the point of diminishing returns are two different things.

Critical thickness is the amount of insulation that you need to have before adding more reduces heat loss. It only applies to small objects, and usually makes a difference only if the insulating material isn't a very good insulator. On very small pipes, it's measured in millimeters. The larger the diameter of the object being insulated, the thinner the critical thickness is. The critical thickness for a flat sheet is zero - adding any insulation at all will reduce heat loss.

Once you're beyond the critical thickness, more insulation reduces heat loss. Doubling insulation cuts heat loss in half. The critical thickness phenomenon means that you need more insulation on round objects, not less.

The point of diminishing returns is where another dollar of insulation does not save a dollar in heat loss (over whatever time period you care to consider). For most residences with a winter delta T of around 45 degrees, the point of diminishing returns is around R20 for walls, and around R50 for the ceiling. If you extend that logic to a tank with a 130 degree delta T, you'd want R60 on the sides and R145 on the top. That's what I did for my own storage.

The best foam I've seen comes in at R8 per inch, or around 18" to get R145. Again, I'd suggest more because soil and water will transfer heat to and from the insulation better than air will. Of course, the point of diminishing returns is a personal decision and may be limited by other factors.

The soil can provide a bit of insulation, but I wouldn't count on it to add much.

 

[CNY CAT MAN]

ewd & Powerspec I called the supply house today about diy urethane spray foam they carry Fomo it is a closed cell two part it is pricey the most common kit is the 2 - 20# tanks with a nozzle covers 605 bf it costs a heafty $649. per kit. They told me you can also get much larger tanks by ordering it. I just had a 24 x 40 garage insulated with closed cell at a price of .95 bf or 3" @ $2.85 sf / much cheaper than the kits and the mess is someone elses , also from what I have read it seems the diy kits are not as good a foam as what a foam contractor would give