TM 5-852-5/AFR 88-19, Volume 5
CHAPTER 12
THERMAL CONSIDERATIONS
12-1. General considerations.
temperatures, which range from -40 to -75 degrees
F. These extreme surface temperatures are attenu-
The thermal aspects of utility system designs are
ated by burial depending on the thermal properties
among the most critical elements for cold regions
of the soil. For example, the air temperature at the
systems. The potential problems are failure of pipes
surface might have an annual range of 150 degrees
due to freezing of water, thaw settlement or heaving
F. At a depth of six feet the temperature may vary
of foundation soil, thermal strains and the associated
slightly with the season, and at 30 feet seasonal
stresses, the cost-effective selection of materials and
temperature fluctuations are negligible. Frost
insulation thicknesses, and economical operation.
penetration will be greatest in rock or bare, dry
This chapter presents criteria and design examples
soils. A snow cover will typically reduce the depth
for the most critical thermal calculations that might
of frost penetration by an amount equal to the snow
be required for design of pipes, utility structures and
depth. Locating utility lines away from snow-
appurtenances.
plowed surfaces will take advantage of this poten-
tial. There is a time lag involved with frost penetra-
12-2. Freezing of pipes and tanks.
tion so that maximum depth of frost penetration will
Damage or failure occurs due to the expansion of
occur long after the extreme winter temperatures.
water changing to ice. The hydrostatic pressure on
At a depth of six feet the lag time may be one to five
the still-unfrozen liquid can reach several hundred
months after the onset of freezing conditions at the
atmospheres and it is this pressure, not the contact
surface. The specific time depends on soil properties
of the ice, that typically causes pipe failure. Preven-
and moisture conditions.
tion of freezing is accomplished via the most cost-
a. Direct burial. Water and sewer mains are typi-
effective combination of insulation, heat trace, cir-
cally buried below the maximum depth of seasonal
culation, etc., using the methods presented in this
frost. In cold regions, the frost penetration is often
section. Insulation alone will not necessarily prevent
greater than the common pipe burial depths of 6 to
freezing. It reduces the rate of heat loss and extends
10 feet, and may be 20 feet or more in exposed dry
the freeze-up time. Small diameter service
soil or rock. Deep frost penetration, high ground-
connections may have a freeze-up time measured in
water, hilly terrain, rock or other factors will make
minutes or a few hours. These are the most vulnera-
it more practical and economical to install all or
ble portion of the system and will usually freeze
portions of the utility system within the frost zone.
first. Thawing capability is mandatory for these
In these cases, the degree of freeze protection
small diameter pipes.
necessary will depend upon the ground temperatures
at the pipe depth. Where pipes are only
12-3. Thawing of frozen pipes.
intermittently or periodically within frost,
Remote electrical thawing methods that can be
conventional bare pipes will be adequate, provided
incorporated in the original design include skin
a minimum flow can be maintained by circulation,
effect, impedance, and various resistance wire and
or
consumption.
Frost-proof
commercial heating cable systems. Frozen wells
have been thawed by applying a low voltage from a
also be necessary. Heat loss and freeze danger are
transformer to a copper wire located inside the riser.
significantly reduced by insulating the pipes.
Once a small annulus is melted, the flow can be
Insulated pipes can be installed in shallower trenches
restarted and it will thaw the remaining ice. Chapter
or within berms at ground surface. In these cases,
6 contains details on thawing of frozen pipes.
the minimum depth of cover would be 1.5 to 3.0
feet for exposed ground surfaces. Greater depths
12-4. Heat loss from pipes.
will be necessary if heavy surface traffic is expected.
The total heat loss and the freeze-up time are
b. Insulation barrier far buried pipes. Buried
dependent on the ambient and internal temperatures
pipes within seasonal frost can also be protected by
in the pipe system. Above-ground piping systems
placing a layer of insulation board, usually polysty-
must be designed for the lowest expected air
rene, above the pipe (fig 12-1). This method, using
12-1
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