TM 5-852-5/AFR 88-19, Volume 5
and this heat loss reduction can be further enhanced
to allow installation on a curved tank surface. Two
by installing insulation. Moisture-resistant insulation
layers of insulation are preferred so that the joints
materials must be installed in contact with the tank
between boards can be staggered (see figure 5-2).
at inaccessible locations since moisture from leaks,
Sheet metal cladding is then applied for weather and
condensation, rain or ground water can drastically
vandal protection. Insulation boards composed of
reduce the insulating effect.
high density foams (compressive strength about
c. Foam insulations. Near-hydrophobic plastic
1500 psi) are sometimes placed under a tank (figure
foam insulations are readily available and commonly
5-3) for protection of the frozen subgrade.
used. Polyurethane can be obtained as either
5-3. Tank design.
prefabricated boards or foamed in place by spraying
directly onto the tank. The latter has been the more
Water storage tanks must be designed to prevent the
common approach in Alaska. To ensure a good
formation of ice in the tank under all foreseeable
bond all loose scale or paint flakes are removed, the
circumstances and tanks must be completely drain-
surface solvent-cleaned if it is oily, and a compatible
able. Floating ice in the tank can destroy interior
primer applied. Foaming-in-place requires a dry
appurtenances, and ice formed on the walls can
surface, winds less than 3 miles per hour, air
collapse and cause structural failure or punctures in
temperatures above 36 degrees F and tank surface
the tank bottom. Surface icing can be avoided by
temperatures above 50 degrees F. The hardened
maintaining the water temperature above 39 degrees
foam must then be protected from vandalism,
F and there will be a continuous circulation. In some
weather and ultraviolet light. This is commonly done
cases the return line of a circulating water
with low temperature elastomers or similar coatings
distribution system is discharged to the storage tank
compatible with polyurethane that are sprayed onto
to promote circulation and maintain temperatures.
the insulation. Two or three coats are recommended
In other cases a small amount of water is
with the first application within one day of foam
withdrawn, heated with a boiler or heat exchanger
application. Polystyrene or polyurethane boards can
and pumped back into the tank.
a. Appurtenances. Breather vents will be located
be glued and strapped (with 1-1/2 inch wide
stainless steel banding, 18 inches on center) to the
on the inside of the tank and vented into an attached
exterior of the tank. Large tanks will require clips.
pumphouse or building rather than directly to the
The insulation boards will be less than 3 inches thick
outside. Ice will form in an exposed vent due to
5-3
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