Shallow Depth Water Lines 4

[JustAPM]

I live in an area that is served almost extensively by wells. At this point the area is more suburban than rural and there has been contamination of the wells in many areas. There has been a push to put in water lines but the local water company always throws out $200 per foot to install water lines so it is an expensive proposition, as the frost depth in the area is generally considered to be 42 inches and we have quite a bit of ledge in the area.

Thinking outside the box I was wondering if a double walled pipe with the outer pipe being tied to a geothermal closed system ground loop might do the trick. If the potable water was encased with water at 50 degrees could water lines be installed at 1' deep? I understand there would be the associated cost of running pumps to circulate the water and there would be some special connections required to tap into the water main but my understanding is that the pumps in vertical loop closed system geothermal wells can be fairly small as you are simply moving water in two columns having a balanced load. Maybe the pumps could even be powered by photovoltaic panels.

Drilling vertical loop closed systems run about $14.00 per lineal feet, including all piping. I doubt if a heat pump would even be necessary, as the goal is just to keep the water above the freezing point and the water that would be circulated would be at 50 to 55 degrees. Geothermal well drillers and Mechanical engineers in the area assume about 1 ton or 12,000 BTU/HR per 150' of depth, so 100,000 BTU/HR can be purchased for about $21,000. 2000 feet of water line at 42" deep cost $400,000 or more, depending on how much ledge is encountered. So that is the heart of the equation. What length of water main could be kept above freezing per 100,000 BTU/HR assuming a design heating load of 0 degrees and pipes being buried 1' below a paved surface?

Anyone ever hear of this use for geothermal technology? Does anyone think it makes sense. Creative thinkers only please?

 

[bimr]

What you have proposed is not normally done because of the maintenance issues. Most utilities would not want to be responsible for keeping such a system working.

Heat tracing and insulating the pipe is probably feasible for short distances.

However, insulating with another fluid is not practical. Not sure that you would ever verify that there was no leakage into the water main.

You do not state whether fire flows are a consideration. It would probably not be practical to try to heat a water main sized for fire flows.

Moving water does not freeze. It may be more practical to just keep the water moving with a looped system

 

[cvg]

majority of waterlines around here are buried from 4 - 6 feet deep, so 42" frost depth would not be an issue. Unless you are installing in solid bedrock, $200 per foot for waterline seems high for normal sized pipe. I would expect about half that. Have you asked any contractors for an estimate?

 

[Hoagie]

The idea is no doubt radical, but I think it a worthwhile pursuit.

I would recommend that you pilot study the idea to determine actual heating gain--which seems to be your biggest challenge.

My initial thought is that you will need to determine the best trench section...perhaps bundling the product lines together within a shallow-narrow insulated trench may worthwhile? I would explore different types of backfill, for insulating potential.

 

[JustAPM]

Bimr,

I am listening to what you are saying and actually I think I am incorrect in talking about a double wall pipe. Double wall pipes are used all the time in solar hot water heating systems to run potable water through a heated storage tank fed by solar collectors. The air space between the double wall pipe is open somewhere and if water is observed you know you have a leak. tht id tequired by code. To accomplish the same thing in this case it would have to be a triple wall pipe with air in between the geothermal loop and the potable water with some sort of leak detection available in the sapce between the out pipe and the potable water pipe. The outer pipe would proably require insulation so the pipe would probably be expensive and may offset the cost of going deeper, even with ledge.

As far as larger mains for fire flows are concerned it may be desired but realize the potable water source would orignate from a water main located 4' to 6' down. The system would not be raising the temperature of the potable water. It would just be maintaining a 55 degree envelope around the pipe so the water doesn't cool down. The potable water would be at 55 degrees and the geothermal water in the outer pipe would be at 50 to 55 degrees. Teh air inbetween the two pipes should then also be at 55 degrees. It would be a matter of house fast the water in the outer pipe would dispate heat,

Still trying to understnd how to calculate the heat loss from the outer loop filled with a geothermal medium. I guess it would come down to BTU/HR/lineal foot but how can that number be calcualted - for arguements sake?

There would be maintenance but it seems it would be minimal - a few solar collectors and a few small pumps to circulate the geoterhmal medium. Most geothermal well driller give 50 year gaurantees on their systems.

Keeping the water moving to prevent freezing is an interesting approach. As water usage is probably varied by hosuehold there shuld be some movement anticipated most of the time, except at night when people are asleep However, would it require large pumps. Would the water still be balanced. it still requires maintenance for the pumps and powere supply.

CVG - maybe that makes the most sense - get some bids from some independent contractors - generally quasi-judicial entities are not very price competitive. It might be half the cost the water district is tossing around. They already told me they have to have an inspector there eight hours a day. I told them tht is ludidcrous. They could snd and inpector for 1/2 hour once a day. Sounded like job security to me.

Hoagie - Thanks for the encoragement. Insulation would probably defintely be required and I hear what you are saying the naorrower the trench the less direct heat loss. Compressing the inulation would also be a problenm that wouldhave to be overcome.

Maye it isn't practical?

 

[eea]

Good idea. You might check on the long term performance with some of the municipalities featured in the US DOE link regarding geothermal heat for snow removal on streets and walks. Might be closer to your situation than the residential heating systems.

http://www1.eere.energy.gov/geothermal/geothermal_basics.html

I have small town in Central Illinois with a 100' run of sanitary force main that was installed shallow due to a box culvert crossing. Long story and thankfully one that I can blame someone else for. We've looked at electric heat tracing it, but will probably relocate the check valve upstream of the shallow burial since the downstream liftstation is nearby.

Keep us posted.

 

[bimr]

Here is a link that details some of the options available to you for freeze protection:

http://books.google.com/books?id=3j...=municipal water mains freezing&f=false

A number of freeze proofing techniques were referenced in the article including bleeding, looped systems with circulating pumps, warm water supplies, heat tracing, etc.

I agree with CVG's analysis of the costs. However, it is not really clear what the $200 per linear foot covers. Does it cover the house services or just the water main? Each house service will be several thousand dollars. If you are only talking about street installation of small diameter piping and excluding the larger mains required for fire service, the installatin cost will be considerably less. A 2" plastic pipe can be installed for around $25-40 per linear foot in decent soil.

It is not clear where you are located but the results from the latest demonstrations of rock trenching equipment are quite impressive. I recommend that you seriously evaluate the modern trenching equipment:

http://www.tesmec.com/get/517/32/0706_RMC.pdf

http://www.trenchtech.com/

The double wall piping that you reference would actually have to be triple wall pipe in order to have three different fluids. I am not familiar with a source for a triple wall pipe.

My understanding is that solar systems use a heat exchange tank to transfer the heat with the piping contained within the tank. Such a tank will be labeled as a double-walled heat exchanger.

"The fluids that are circulated into the collectors are separated from the heated water that will be used in the home by a double-walled heat exchanger.

A heat exchanger is used to transfer the heat from the fluids circulating through the collectors to the water used in the home.

The fluids that are used in the collectors can be water, oil, an antifreeze solution, or refrigerant.

The heat exchangers should be double-walled to prevent contamination of the household water."

http://solarheatcool.sustainablesources.com/

Considering all the issues that you are facing, I expect that you will find that trenching will probably be the most feasible. Good luck.

 

[JustAPM]

Bimr

The trenching equipment you sent links to looks pretty impressive, especially since the machines create their own backfill material!! One issue I see though is those big steel tracks. All the pictures show the equipment being used ahead of paving and it looks like ahead of road base being laid down. We need to be able to trench on exiting asphalt paved streets in such a manner that we don't have to repave the entire street. Local municipal engineering departments generally require the pavement to be removed and replaced 18" on either side of the trench an dit is more cost effective to minimize the paving work. I'm going to make some inquiries into the trenching equipment on Tuesday.

Do you know if this equipment has ever been used in the application I am describing?

Thanks!!

 

[bimr]

One of those references is in Colorado mountains. Can't be much different that what you are describing.

Installing in the roadway adds to the expense because you will have to use trench backfill (aggregate),

 

[rconner]

It has not yet been mentioned on this thread that very shallow cover on piping can result in perhaps non-obvious extreme trench total loads, due to traffic and impact etc. Some pipe materials in rather common use at other depths may not be able to handle the higher loading, and also the fatigue due to repetitive applications of same @ very shallow cover.

As another has mentioned, when a system as described is correctly designed and "working" this would appear to be theoretically possible from a heat transfer standpoint; however, all pumping systems or power supplies for same etc. have an uncanny habit of eventually breaking down at the most inopportune time (e.g. over winter holidays, when noone but "mad dogs" are working or on the roads, and it is extremely cold?)

 

[civilman72]

What is the actual minimum covere requirements of other water districts in your area? Just because the frost level is 42", does not mean the pipe only needs to be buried 4' deep. As an example, our frost levels vary from 3'-5', but minimum burial depth is 8' for water and sewer.

I've had success with a pre-insulated pipe with tracer wire for projects with less than 5' of cover. The product I use is called Urecon. Go to their website, and they have some handy charts on heat loss by pipe size and ambient temperature.

Due to traffic loads, you will probably have to bury your water line at least 2' deep, and if you are only required to bury the pipe 4' deep, then the additional expense of the pre-insulated pipe is probably not worth saving the additional 2' of excavation. However, if you minimal burial is 7'-8', then investigating the pre-insulated pipe may be worth it.

 

[cvg]

if frost depth is 42 inches and the pipe is installed 6 feet deep - it seems that the risk of freezing the pipe is minimal, insulation and geothermal systems are then completely optional

see attached for rock saw that could be used on your project

 

[civilman72]

Frost depth obviously varies from year to year, and also greatly varies depending on location (frost depth under plowed roads can be a few feet deeper than areas adjacent to the road that are covered in a few feet of snow) . If the 42" frost depth is an average, a 6' bury may not be enough under a plowed road and during a colder than average winter. Just depends.

 

[rconner]

We should probably also mention the Federal Highway Administration at

http://www.fhwa.dot.gov/pavement/ltpp/pubs/08057/07.cfm

and the NGS et al at

http://www.ngs.noaa.gov/PUBS_LIB/GeodeticBMs/#figure13

have looked at and/or have actually monitored in some fashion “extreme frost penetration” (while the former at least apparently considers paved areas, that one would think are mostly plowed, who knows if influence of conveyed fluids in pipelines or pipe trenches or crossings etc. makes a difference from whatever sites they have monitored/are monitoring?)

As far as arguably more applicable standard I believe AWWA D100 standard for tanks has also traditionally included a similar appearance and headed table for extreme frost penetration, as well as a Figure 4 “recommended depth of cover” table for water pipes along with the guidance, under Sect. 12.9.2 Pipe cover, “Pipe cover shall be provided in accordance with Figure 4, unless local conditions dictate that greater or lesser cover should be used.”(and I do believe some utilities take them up on the latter!)] I don’t know if there is necessarily full agreement with all these references, but I guess there might be.

In any case, from what I hear from frosty relatives up North I think it is probably penetrating pretty good right now – everyone have a good weekend!

 

[BigInch]

Get your own contractor, install the water lines, sell them back to the water company for $ 200/ft, smile all the way to the bank.

 

[ClarkBart]

Hi:

In reviewing this discussion, I was reminded of a study performed by the ACOE Cold Regions Group. The study looked at wheter insulation could be used to "delay" freezing for a long enough period to allow shallow cover of water mains. You can look at the study at this link;

http://www.crrel.usace.army.mil/library/crrelreports/CR98_04.pdf

I have used some of the techniques to protect shallow water lines and force mains in limited locations without the finite element analysis with good success. If you were going to use this type of installation to take advantage of the earth's heat on a larger scale, further calculation to insure that the insulation package selected would be appropriate.

 

[JustAPM]

Just wanted to tell you you guys are great and you have been keeping me pretty busy with the suggestions and articles.

I have contacted a local excavator and both of the trenching companies in the articles to get some quotes. One of them has done a major project in our area installing underground cable lines so they may have some data regarding ledge available. They are looking into it. That info may reduce cost of test bores.

To fill in some blanks regarding design parameters and costs:

1) The local water department will require installation with ductile iron pipe. Plastic is not an option.
2) The local water department requires a 5' bottom of pipe depth with 1' of bedding so that equates to a 6' deep trench.
3) The main is required to be 8" ahead of a fire hydrant and 6" behind fire hydrantws.
4) The $200 per lineal foot ballpark number includes the water main and stubs out to curb stops and installation of curb stops. The average distance between homes is 140' and there is a house on each side of the street in most cases. It also includes repaving "as necessary" - to be defined but probabaly the entire street as we have stubs to both sides. It also includes fire hydrants located by the local FD. My understaning is that they'll basically take what they can get but the installation of hydrants should be within ISO standards to provide the benefit of lowering insurance rates.

Running the line from the curb stop to the house would be an additional expense.

I'm trying to hone in on numbers for a more conventional system (as suggested) - via a private contractor first.

I haven't given up the original thought about geothermal and I have been reviewing the shallow depth and insulation articles that have been provided.

I still think geothermal may make sense as it could accomplish raising the frost depth to the level of the pipe. I am also beginning to think that a double or triple wall pipe would not be necessary if the sytem could use ground water as the heating medium as there is ground water around the pipes anyway. The issue then becomes keeping the geothermal system pumping enough to prevent it from freezing so an anitfreeze solution is not necessary. It all comes down to economics, acceptance by the local water department and maintenance issues as pointed out.

Thanks!!

 

[BigInch]

Sounds like you'd want to have 3x50% geothermal pumps for reliability reasons. You wouldn't want a flow failure and a week to fix it in freezing conditions.

I would suggest that in this, as in all projects, you design to meet your number one objective's criteria FIRST; water supply. Then and ONLY THEN, you might go back and see what modifications you would need, what you can realistically do and how much extra money you would require to do it, how much your payback would be improved, IF you started adding the bells, whistles and gold plating. Don't even think of allowing the tail to wag the dog at this stage of a project.

 

[davefitz]

At $200 /ft, you might be better off going into the trenching business .

While the trench is still open, you could distribute the charges to other utilities by including fibre optic and LP sewer , and kill 3 birds with one stone.

It would seem that a lower cost alternative would be providing activated carbon filters at individual residences on their primary drinking water connection. It might be a model followed by all the towns that may be affected by the hydro-fracking for nat gas , and the subsequent effect on aquifers.