Underground Temperatures - Very Technical Question

[CurlyDave]

Does anyone know how to estimate temperatures at shallow depths, say at the bottom of a 2' or 3' backfilled trench in high ambient temperatures (summer)?

The real problem is this. My contractor has already installed 700' of 2" schedule 40 PVC pipe from my well to my house, without ever asking me whether this was what I wanted.

Because of the elevation change (150 feet) from the house to the wellhead, the pressure at the wellhead will be 70 psi static head, plus the pressure setting of my pressure tank, say 55 psi maximum, or 125 psi at the wellhead. (Pressure tank will be at the house.)

2" PVC usually rates for 280 psi at 73 degrees. So far so good, plenty of safety factor. Well, actually there isn't. PVC has to be de-rated for temperature according to the table found here: PVC de-rating schedule

Now I can easily envision a Summer temperature of 110 degrees and inside a wellhouse, with the sun beating down, it might get to 130 degrees, so we have to de-rate the pipe to ~87 psi, which is less than the actual pressure.

Now I can deal with this for the aboveground pipe by using galvanized, but is the pipe in the trench going to be OK or not?

This reference: Groundwater Temperatures tells me that groundwater, which should be at the same temperature as the earth it comes from, is 2 to 3 degrses higher than the mean annual temperature at depths as shallow as 30 feet, I know shallow caves are cool year round. Even basements are usually cool. But, basements and caves are usually deeper than 2 to 3 feet.

The real question is whether the pipe in a 2 to 3 foot trench is going to stay under 100 degrees (which would allow the use of the existing pipe) even on the hottest days in Southern Oregon?

My gut feeling is that it will easily stay under 100 degrees, but it would be really nice if someone knew for sure before they backfill the trench.

Any thoughts guys?

 

[_RaT_]

Dave, at a family members place in Palo Cedro CA., a small town just east of Redding, we have a 2" sch 40 water line that has 2 booster pumps that kick in as needed in order to get the domestic water up the hill to the home. The elevation change is over 200'. The pipe is in a trench 2' deep and about 2000' feet in length. We have never had a pipe break due to pressure but have had 90's etched out at the crotch. I know it is because the velocity of the water is much higher then 2" can handle. The chances of your pipe bursting at those pressures seems remote since I am pretty sure your temps ( in ground or air) will not be any greater then Redding, CA. The important thing is in my opinion to use plenty of check valves going up the hill. We have 3 and need another 3. When the pumps shut off, the water surge bumps the pressure gauge to about 175+ psi diminishing with each hammer coming to a precharge pressure of about 145 psi. That changed after adding 2 check valves. I did change out all of the above ground 2" galvanized to 2" type L copper. This was due primarily for the physical demands placed on the pipe inside the pump house. All of the PVC is protected from the sun.


If we had it to do over, I would have:

1. 3" sch 40 or greater. I am not sure where class 200 exceeds sch 40, but I think 3" or 4" is getting close. Class 200 is rated at 200 PSI where schedule changes with size.

2. more check valves.

3. We have entertained the idea of going to a tank system. A 3500 gallon tank at the top with a pump inside to pump water to the desired pressure (about 60 PSI) while the pump at the bottom simply fills the tank dropping the pressure by 60 PSI. The tank would be concrete and buried.

Be sure to add enough conduit for control wiring purposes.

Your system sounds good, 2" coming off a well pump should be more then adequate. We are using booster pumps in 1.5 and 5 hp sizes. Both have about 60 PSI being supplied to them. The 1.5 hp is 60/45 cut out/cut in psi and the 5 HP is 55/40. The 1.5 is always the first to start and the last to run. They are also protected with Pump Techs which determine the correct operating current to keep the motors protected in under current situations (no water) and over current (bad pressure switch stuck on). Best wishes in your adventure. Mark

 

[Egon]

Check around and see if any others in your area have the same pipe and if they encountered problems.

Also remember the engineered safety factor for the pipe's rating.

You could also place a thermometer with remote reading at the desired depth to check your temperatures.

 

[JerryG]

Dave,
You won't have any problem with the run the way that he did it. At that depth it is all cool. I run many times the amount of water that most people do and I have lines that run around 200 psi that are above ground. The only problem that I have ever had is with joints that weren't glues correctly. I know that others will have different ideas, but this is what has worked for me for seven years.

 

[stumpfield]

It would be difficult to estimate the temperature 2' or 3' below ground. It depends a lot on your specific area and type of soil. If it's expose to full sun and you got dark, dry, volcanic rock mixed with sand type of soil, it can trap heat and the temperature can be hotter than the air temperature .
If water is moving every once a while from normal usage, I think you should be fine. The water come out of your well should be cold and it will help cool the pipe.

 

[turnkey4099]

From your description of the pressures involved, it sounds like you have your pressure tank and controls at the well head. A better system to keep pressures lower would be a check valve at the house, tank at the house with a booster pump there. That only requires about 75 psi at the well head. Of course it would take a second tank and controller at the well head to operate that pump.

I do not see any problem with the pipe/temperature.

Harry K

 

[_RaT_]

He mentioned that the pressure tank is at the house.

 

[BSVLY]

CurlyDave,

According to data viewd at CEMP (good data I believe) the official ground temps are running as much as 12 degrees lower than air temps and vary by the hour. Look at this example as follows.

Desert Research Institute CEMP Weather Data Page


Do this:

1) Click on any "active station" on map,

2) choose "advanced" on left side bar

3) choose "Monthly Summary (w/Et data) "

You will now be able to view ground and air temps for any reporting ststion-for any month!

I am not certain how deep these soil temp measurements are taken. I do know that fancy, upscale amature weather stations have ground probes, so there must be a standard depth for the ground temp probes to be placed for accurate data gathering.

Interesting question you raise!

Rich

 

[CurlyDave]

Unfortunately check valves are not possible. I need to have a water source at the wellhead, and the easy way to do that is just to tap the main pipe right there and add a spigot.

From your description of the pressures involved, it sounds like you have your pressure tank and controls at the well head.

Actually, the pressure tank will be at the house, but the motor controller for the pump will be at the wellhead. I am intending to have the usual pressure switch at the pressure tank, and it will actuate a relay back at the wellhead. This avoids a 700' run of heavy gauge wire. I have a 2" conduit for the signal wire already in the trench, and a spare 2" conduit.

The water come out of your well should be cold and it will help cool the pipe.

Of course this is true under most conditions, but the system has to be designed for the most stressful situation. Think about what happens if we go on vacation for two weeks in the middle of summer. There is no water flow and no cooling for that time.

My situation is not really comparable to 99.9% of the other installations in the area because of the elevation change. Neither the contractor, nor the pipe installers understand the concept of static head. We have temporarily located the pressure tank at the wellhead so I can get water for my RV, which is at the same elevation, about 60' from the well. The pressure tank is the usual, on at 40 psi, off at 55. The contractor and the pipe installers keep thinking that when they hook up the pipe to to the temporary pressure tank they will get water flow up at the house location 150' higher. It is beyond my capability to explain that the water will only go about 120 feet of elevation change up the pipe and then stop.

Anyway, you guys have collectively convinced me that the temperature will be OK in the trench, and I have just told the contractor to use galvanized in the parts of the pipe which are above ground at the wellhead. Up at the house, PVC is acceptable everywhere.

RaT's real life experience is very helpful.

 

[_RaT_]

Actually, the pressure tank will be at the house, but the motor controller for the pump will be at the wellhead. I am intending to have the usual pressure switch at the pressure tank, and it will actuate a relay back at the wellhead. This avoids a 700' run of heavy gauge wire. I have a 2" conduit for the signal wire already in the trench, and a spare 2" conduit.



That is in my opinion the best way to do it. We have 4 #12 control wires running to the pressure switches located with the 3 large pressure tanks. 2 wires for each pump. The relays are at the pump house. I put in oversized relays for the pumps to provide longevity. They are 120V coils. 2" conduit is large, thats beneficial. Its good to have the spare as it would be needed if you use low voltage for any reason since high voltage and low cannot be in the same conduit. I know I did not address your temperature concerns, but I know nothing about that area other then I have yet to ever see PVC fail from high water temperatures short of male adapters failing at above ground pumps when the pump is dead headed.

 

[dfkrug]

I have many hundreds of feet of sched 40 PVC pressure lines, 1", 2", and
4". Most are buried only one foot as backfilling was all manual labor on a
50% slope. I have no doubts that the lines stay cool enough. I have
avoided any exposure of PVC to the air, due to UV deterioration and also
the rare freeze I might get in the winter. My 4" wharf fire hydrant is
reqd to have steel 4" riser, by fire dept requirement. My water system is
all gravity pressure (no pressure tank) and the water storage is 5000
gal concrete with plumbing underneath. I did find out that there are no
22.5-degree elbows made for 4" pipe (only 4" dwv PVC) and gluing up
20' 4" lines is a 2-person job....very tough! All pipes de-burred, cleaned,
primed before gluing.

Finally, if you are subject to inspection, you probably have to do a pressure
test. I had to convince the bldg dept to let me use air, since I could
not fill the pipes with water at the time. It took hours to fill the
pipes with air, but it is important to find/fix leaks BEFORE you backfill.

 

[Westonium]

Me? I went with 2" 200psi poly for my 600ft for a lot of reasons.

One concern you haven't mentioned is thermal shrinkage. I believe the formula is 100' of PVC will shrink (or lengthen) 1 inch for 50 degrees of temperature change. You have 700' of PVC. If the PVC has changed in temperature (cooled) 12.5 degrees, that 700' of PVC wants to be 698' .25". It should be a moot point because there is probably slack that was taken in when you filled the pipes with cold water before you backfilled (right?)

Honestly, I think you'd be hard pressed to make the pipes in your pump house 130 degrees or even 90 degrees. If you are concerned about the pipe temps in the well house, insulate them. I forgot to note your location, but you want to do that anywhere it freezes anyway (and maybe have heat wrap under the insulation). The rolls of fiberglass pipe insulation are dirt cheap. I put 2 good layers on and put both included vapor barrier layers on the outside.