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tankerator (Mechanical) (OP)
18 Apr 11 17:22
We're building a hot tank (380F design/300F op) in the place where a similar tank once was.  The service is asphalt.  Multiple hot tanks in our complex, from sulfur to asphalt, have pipe vents radially through the tank ringwall foundation.  None of the Civil guys can tell me much about why they are needed.  If anything, they think the vents would aid in further dessicating the clay/silt soil and cause additional settling.

I was wondering if anyone had experience with hot service foundations for storage tanks and if you used vents and why.

Thanks in advance for any replies.
IFRs (Petroleum)
19 Apr 11 0:00
Have some experience, never used foundation vents.  Interesting thought, though...if you find out why, please post!
simplemath2 (Aeronautics)
19 Apr 11 13:12
Radially buried vent pipe with riser to remove heat by natural convection.  
tankerator (Mechanical) (OP)
19 Apr 11 17:02
I guess I should've said that I understand how the vents work and the reasoning behind installing them, but what I was questioning is whether they are needed at all.

We have a similar tank in sulfur service that's 150'D with 16 vents radially around the foundation.  Each 4" pipe vent extends almost to the center of the tank.  It is unknown if the inside ends have caps on the pipe or not. The pipe is perforated along its length.

Understanding that natural convection will pull heat out from under the tank, why would you want to do that? The tank exterior is insulated so you don't want to lose appreciable amounts of heat from the bottom since a temperature above 300F must be maintained.  Also, logically, the surface area of the perforations in the vent pipes is so small that I don't foresee them pulling much heat out compared to the heat load generated by the tank.  The sand, aggregate, etc. under the tank is acceptable for the design temperatures.  Assuming the foundation is built correctly, there should be no water that seeps under the tank so there will be no need to drive it off.

All that being said, why would you bother venting the area under the tank?
 
Chance17 (Chemical)
24 Apr 11 9:39
I do not know the right answer but it seems venting of heated ground water would be a possible answer.
Most hot tanks I have seen have insulated bottoms.
Are these tank insulated on bottom?
Helpful Member!  Duwe6 (Industrial)
27 Apr 11 9:10
If you set your tank on a ringwall 2-3 feet tall, and fill the ringwall with sand, venting is not needed, and the dry sand will act as a fair insulator.  Always a good idea to conserve the heat you paid for.  Never put Hot Product tanks at grade, or you get groundwater accelerated underside corrosion, and serious heat loss every time it rains.

For construction details:

Settle/compact the sand by wetting it.

If the underlaying soil is not very permiable: Put 3/4 or 1-inch pipe penetrations in the ringwall as 'footer drains', about every 10-15 feet apart around the wall.  Allows the excess compaction water to vent off.

On initial filling of tank with hot product, warm it up slowly to allow the small ammount of remaining moisture to steam off.
MJCronin (Mechanical)
27 Apr 11 15:03
Could there be some confusion here between "vents" and leak detection piping ??

   

tankerator (Mechanical) (OP)
28 Apr 11 9:49
MJCronin, I know what a leak detection system is and looks like.  Thank you though.

Duwe6, thanks for the info.  I'm a mechanical and usually don't get involve with CivE issues but I'm trying to help out my coworkers on this one.  I thought that the vents might be just for a start up/reaching steady state condition.  Our current plan is similar to your suggestions.  We are excavating  4ft below grade, backfilling from the bottom up with compacted cement stabilized sand, light weight aggregate, compacted sand and crushed limestone. In the end, the ringwall will extend 2ft above grade.  

One general suggestion we've heard for a lot of our new foundations is to compact and hold the sand in place by spraying mineral oil.  There a problem with high winds out here (being right on the bay) and sometimes all the sand leveling and compaction can be undone in a few hours.  Have you heard of using mineral oil?  

Thanks!
 
Duwe6 (Industrial)
28 Apr 11 16:56
Nope, but it sounds like a very good idea.  In addition to stabilizing the sand, it is also a rust inhibitor.  Most tanks are "killed" by underside corrosion.  However, yours will be too hot to have much corrosion -- water will seldom reach the floor.  You should probably expect some smoking during initial heat-up.

Thanx, I'll add that one to my 'bag 'o tricks'.
MJCronin (Mechanical)
29 Apr 11 13:46
It looks like these people advocate multiple small-diameter tanks for asphalt service.

http://www.heatec.com/literature/images/HeatecAsphaltTanks.pdf

The Heatec tanks appear to be on concrete pads with no vent pipes. FOAMGLAS is used for the insulation on the bottom.

Assuming that the tank is shop-fabbed, that puts the diameter of these asphalt storage tanks at around 12 ft....

What is your tank diameter and proposed insulation material ?

   

Duwe6 (Industrial)
29 Apr 11 16:07
Small tanks for hot [or cold] service don't make good engineering sense.  A sphere had the least surface area per unit volume, and for a cylinder, the best ratio is for the diameter to ewual the height.  Thus a 40-ft diameter tank will loose the least heat [surface area-to-volume ratio] if it is 40-ft tall.

12 & 13-foot diameter tanks are cheaper, being shop-fabbed and transported w/o the need for permits.  But the heat losses go on forever, and these tanks are only cheap once.
tankerator (Mechanical) (OP)
2 May 11 17:25
This is actually a 92'D x 40'H tank.  The exterior is insulated but the area under the tank is not insulated with anything but the backfill materials.  This tank is a basically a wide spot in the line with a constant in and out flow that regulates the temperature.  There is an internal fired heater for hurricane type situations where the flow in and out is shut off.  It's only used 1-2 times every 10 years.
Duwe6 (Industrial)
3 May 11 11:58
That insulation sounds reasonable.  Dry backfill is a pretty fair insulator.  It's the water that pulls the heat away, mostly.
simplemath2 (Aeronautics)
3 May 11 13:17
Pipe is needed for prevention of soil dryness resulting from heating and water loss.Geotech engineer shall be able to tell max temperature bottom of foundation. Then you can tell if sand insulation is adequate.

The pipes per your description feels like not for passive cooling.  

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