TM 5-852-5/AFR 88-19, Volume 5
energy. To accurately measure the fluid
require coatings to prevent water absorption, since
temperature, they should be put in a pipe well or
freeze-thaw cycles of the moisture can lead to
attached to the pipe surface with heat transfer
deterioration of the insulation.
b. Extruded polystyrene, particularly the high
cement, particularly for plastic pipes. They should
be located where the lowest pipe temperatures
density products (3 pounds per cubic foot), suffer
within the section being controlled are expected,
the least from moisture absorption and freeze-thaw
such as at exposed windswept areas or shallow bur-
but the outer 0.25 inch of unprotected buried insula-
ied sections.
tion should be disregarded in thermal analyses.
Molded polystyrene will absorb some moisture and
for smooth pipes with fluid velocities less than 6 ft/s
should not be used in moist conditions. Polystyrene
(feet per second), which is about the desirable upper
is available in board stock or beads. The former has
limit for flow in pipes. At high velocities frictional
been extensively used to reduce frost penetration.
heat is significant, but deliberately increasing the
Beads are useful for filling voids in utilidors while
velocity for this purpose is an inefficient method of
retaining easy access to pipes. Although the thermal
heating since the energy is supplied by pumping.
conductivity of polystyrene is higher than that of
The equations for frictional heat input are presented
urethanes, the volumetric cost is usually lower.
c. Glass fiber batt insulation is the most common
in figure 12-4.
building insulation, primarily because it is fire-
12-7. Insulation materials.
resistant and relatively inexpensive. Its insulating
value is significantly reduced when wet, and is
Common insulating materials are plastics, minerals
reduced by half if 8% by volume is water. For this
and natural fibers, or composite materials. For
reason, glass fiber should not be used underground
design purposes, the structural and thermal prop-
but may be considered wherever dry conditions can
erties for the worst conditions must be used. These
be ensured. Cellular glass is very water-resistant but
conditions occur after aging, compaction, saturation
is seldom used because it is brittle and difficult to
and freeze-thaw cycles. Other selection consid-
work with. Lightweight insulating concrete made
erations are ease of installation, vapor transmission,
with polystyrene beads, pumice or expanded shale
burning characteristics, and susceptibility to damage
can be formulated with relatively high strength and
by vandals, animals, chemicals and the environment.
thermal resistance. It can be poured in place around
The insulating value of a material depends more or
piping but must be protected from moisture to
less directly on the volume of entrapped gas in the
prevent freeze-thaw deterioration.
material. If the material becomes wet and the voids
filled with water, the insulating properties are lost
12-8. Thermal calculations.
since the thermal resistance of air is about 25 times
that of water and 100 times that of ice. In the past,
The analytical thermal equations presented below
the lack of a near- hydrophobic insulation made the
use a number of simplifying approximations. The
design of piping in moist environments very difficult
user must determine their applicability for particular
and is a major reason for the development of above-
problems and consider the various models and a
ground utilidors.
range of values for physical and temperature condi-
a. Polyurethane foam is used extensively in cold
tions. This chapter includes time-independent
regions to insulate pipes and storage tanks, and is
steady-state heat flow procedures as well as calcula-
also used in some buildings and foundations. Ure-
tions to determine ground temperature and the
thane will bond to most materials. Piping or other
depth of freezing and thawing. The symbols used
components can be pre-insulated or polyurethane
are defined in table 12-1 and the thermal conductiv-
can be applied on-site from the raw chemicals,
ity of common materials in table 12-2. Solutions to
which are about 1/30th the final volume. Field
typical utility system problems are given to illustrate
applications are restricted by climatic conditions,
the procedures involved.
a. Steady-state pipeline heat loss. These include
and the density and thermal conductivity will often
be higher than values attainable under factory con-
typical cases for bare and insulated pipes, and single
ditions. The foam must be protected from ultraviolet
and multiple pipes in above- and below-ground
radiation. A metal skin has proven effective to
configurations. These methods are presented in
prevent the loss of entrapped heavy gas which can
figures 12-5,12-6 and 12-7.
increase the thermal conductivity by about 30%
(1) Figure 12-5 deals with heat flow from a
above the theoretical minimum value. Densities over
bare pipe, an insulated pipe, a single pipe in an
6 pounds per cubic foot are essentially impermeable,
insulated box, and a utilidor carrying multiple pipes.
but lighter foams, which are better insulators,
In each case, some of the major approxima-
12-5
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