Underground Hot Water Storage Tank Question

[Eric Johnson]

This came up in another thread, but in case they don't see it, I was wondering if some of our engineering types would care to answer a question I have about creating a pressurized, underground storage tank.

Would it be possible to pressurize an underground storage tank, say a 2,000 gallon steel or fiberglass fuel tank? Would the support of the ground, in other words, allow you to pressurize such a tank to more than 30 psi?

I'm thinking that it would be pretty easy to dig a pit, line it with plastic or some other waterproof liner, place the tank in the pit, get it all plumbed up, and then spray a thick layer of foam around it. Could you engineer it so that the foam would provide sufficient support for the pressure? That sure would simplify things and allow you to get a large amount of pressurized, virtually invisible storage for pretty cheap.

 

[wdc1160]

Eric I am constantly surprised what they can do with some of the fiberglasses they put in the ground. Indiana has several companies now that put fiberglass pools directly into the ground in northern indiana. Ground heaving and temperature variations seem to have little effect and the pool comes with a "20 year no cracks" policy.

I am not sure what the ideal setup would be, but I think the materials could withstand the pounding.

 

[kuribo]

Stresses will be high around fittings. That would most likely be where you would see failure....no real way to strengthen those areas....

 

[wdc1160]

In indiana we believe that duct tape with strengthen anything....
Even a car........

 

[jebatty]

Almost anything, including an egg, will take pressure with an equal opposing force. The issue will be ground movement as the tank expands. Also, not all fiberglass can take the temps needed for heat storage. Be sure to look at fiberglass rated for the heat of the liquid you intend to store.

 

[Eric Johnson]

Are you talking about a plastic/fiberglass tank or a steel tank, or both? What would a failure in a steel tank look like? I was under the impression that the biggest problem with pressurizing big tanks comes across long spans of unsupported steel--like the tank bulges until it splits open or the welds fail. Would there really be enough force at the fitting at 15 or 20 psi to blow out the weld? I'm just trying to understand what we're up against here.

On the question of plastic or fiberglass taking the heat, I'm wondering if you could push the envelope a bit considering that the thing isn't going to collapse or blow out, since it can't physically do either. I'd hate to find out the hard way that I was wrong, but it's another point to consider.

And I assume that in most jurisdictions, none of this would be up to code.

 

[Reggie Dunlap]

Sounds like a big propane tank might be a good option. I don't know how well foam would hold up in contact with wet soil and ground water. I insulate a lot of houses with the closed cell foam and it works great for that application. It might be simpler to build a shed around the tank and not put it in the ground. Another thing is that foam is expensive enough that you want to do it right the first time.

 

[kuribo]

I assumed you were talking about a fiberglass, plastic, or other non-rated tank used as a pressure vessel....

Overheating will shorten the life of the tank, which could be drastic at high temps....

 

[Eric Johnson]

I was, but I was also thinking about steel.

This is just idle curiosity on my part. I'm just wondering how practical it would be.

I guess what I'm really talking about now is something like an underground fuel storage tank or a stainless steel milk tank--a big old cylindrical steel tank that wouldn't hold pressure above ground, but ought to (it seems to me) if it were buried. You need the foam to insulate it, I think, but perhaps it would also create a more consistent material for the tank to push against, compared to sand or gravel or some other backfill. Maybe vermiculite or perlite could be used instead, but that stuff is probably a lot more expensive than foam.

Does that sound like a reasonable idea?

 

[wdc1160]

kuribo what do you mean non-rated?

 

[kuribo]

Tanks are rated for pressure or not. A non-rated tank is one that has not been certified/tested for use as a pressure vessel and is not meant to be used as a pressure vessel.

If a tank, plastic, fiberglass, steel, or otherwise, is not meant for pressurized usage, you are only asking for trouble if you pressurize it. As I mentioned, the areas around the fittings are stress risers and will be prone to failure. I would simply use a tank meant to be pressurized, i.e., a propane tank, or go non-pressurized....

 

[wdc1160]

Do you think a pressure rated firberglass tank exists? Or, that fiberglass has a properties to even have this type of purpose?

 

[kuribo]

Yes, fiberglass pressure tanks are made. Wel-X-trol makes well pressure tanks out of fiberglass....I think a 120 gallon unit retails around $800.....

 

[atlarge54]

I seriously doubt spun glass tanks could withstand the combination of high temp. with the pressure. I don't have any data to back this up but I'd be willing wager on the side of failure.

 

[kuribo]

You are most likely correct....If you add high temp service to the pressure requirement, it may be too much to ask for...In any case, the cost would be much greater than the old propane tank.....

 

[Bartman]

Depends on what resin was used for the construction of the tank. Polyester resins are the cheapest and most common, epoxy resins are the strongest and most expensive. It's not so much the glass that's the problem, the resin that binds the glass has to give and take the heat. Spun fiberglass tanks are very strong because the glass is in different directions like biaxial glass mat which is considered the strongest of the fabrics.

When it comes to local codes for underground tanks, I would guess that there are not many codes for water storage. Fuel, and septic, there would be, but water isn't really considered hazardous.

 

[hkobus]

Eric,

Why would you want to pressurize the tank. I have still not made up my mind on the storage I need, but with the cost of copper and plate HX I am thinking more and more about circulating the same water from the boiler to the tank and trough the heat load. This would increase the water amount circulating in the system from 50 to 100 gal, to potentially 1000ths of gallons!
I do believe I can feed the tank at the top and return to the boiler from the bottom and use water from the top and return to the bottom or the middle. I would build a diffuser to reduce the speed of the charge and load return water, much like in a DHW tank.
When using this amount of water in the system, it would make for a long time to eradicate oxygen from the system, even when presurized. The OWB dealers sell conditioners to buffer their open systems and protect them from corrosion.
Now my thoughts are going more this direction and making an open system with the whole storage volume able to circulate through the boiler. Just for the DHW supply I would use HX, side arm or other.
If you can tell me what you use to make your great drawings, I will draw my idea and see what everyone thinks. But I think you will grasp the concept, and maybe there is a good reason this is not done much.
With all the water storage talk you can almost split the forum...

Henk.

 

[Eric Johnson]

So what you want to do is convert your whole system to an open one, is that right, Henk?

You might have a problem if you have to pump water up to a second story, but I'm not sure about that. I think my dad does that with his OWB, and it works OK.

I've always run pressurized system, which is why I asked about pressurizing the tank.

I use Adobe InDesign, which is a desktop publishing program to do my drawings, then I convert it to a pdf, open it in Photoshop and convert that into a jpeg. It's not the most efficient or sophisticated way to make a drawing, but I use those programs all the time at work, so it's second nature. A more direct approach would be to use a drawing program. I have Adobe Illustrator for that, but I hate it, mainly because I've never taken the time to learn how to use it. My favorite drawing program is Corel Draw 2.0, but I cooked my operating system trying to install it on XP.

You might be able to make decent drawing in Word.

 

[hkobus]

So what you want to do is convert your whole system to an open one, is that right, Henk?

You might have a problem if you have to pump water up to a second story, but I'm not sure about that. I think my dad does that with his OWB, and it works OK.

Yes, I have been thinking about that, and the point you brought up. The second floor is heat deprived at this time, but plans are on the table for in floor now. I was thinking radiators, but with an eye on efficiency, I think in floor would be better and make better use of stored heat. I also plan to run higher temp downstairs and lower upstairs. With downstairs return in the middle and upstairs in the bottom of the storage.

I'm sure that when the return is submerged in the tank and the same with the draw end, it should not drain, as no air can enter the system. Different story when air enters from a leak in the system.. :bug: Some head space in the tank may prevent spills in the basement for an emergency.

I also plan to draw the supply for the floor heat through a Nat gas fired DHW tank as backup heater, that way when the tank is set for the min. required temp for the in floor, it will kick in on it own. This way the storage will frost protect the boiler and supply lines when fitted with a timer and frost sensor in the boiler room.

Henk.

 

[antknee2]

I was checking out some solar hot water equipment and stumbled on this fiberglass tank , wonder if can handle pressure .Anthony

http://kingsolar.com/catalog/mfg/other/desertsun750g.html

 

[EForest]

the underground storage idea has been on my mind for weeks.
What if you pour a concrete tank, skim the walls for a smooth finish, then line it with .060 epdm for ground water barrier, then 2-3" of foam insulation on walls and floor, another layer of epdm for storage, then seal the deal with an epdm lid covered with @ 5-6" of foam, finally pour concrete (or frame lid) over the tank and run it as an open system with 4 way valve ( for flow reversal) and flat plate HX with bronxe pump. Share your thoughts good or bad....
thanks'
Ed

 

[trehugr]

the underground storage idea has been on my mind for weeks.
What if you pour a concrete tank, skim the walls for a smooth finish, then line it with .060 epdm for ground water barrier, then 2-3" of foam insulation on walls and floor, another layer of epdm for storage, then seal the deal with an epdm lid covered with @ 5-6" of foam, finally pour concrete (or frame lid) over the tank and run it as an open system with 4 way valve ( for flow reversal) and flat plate HX with bronxe pump. Share your thoughts good or bad....
thanks'
Ed

Im haunted by the same questions gasifirewanabee. My latest obsession is using the existing 2 walls of the foundation, adding 2 more walls, and possibly building a tank inside the new foundation walls so I can insulate between the walls and the tank.

My biggest problem is how to:

1. make the lid
2. access the lid and HX
3. cover up the whole works so it looks like it did before I started.

 

[kuribo]

excavate that area, drop in a concrete septic tank....no cold joints.....

 

[trehugr]

excavate that area, drop in a concrete septic tank....no cold joints.....

What is a cold joint ?

 

[kuribo]

the interface between new and old concrete......not a lot of strength......