TM 5-852-5/AFR 88-19, Volume 5
materials. It is not recommended for plastic pipes
will increase the heat generated by a factor of four.
which could melt or be damaged if the procedure
Generally, as much current (heat) as practical, with
used is improper.
safety limits, must be provided so that the thawing
c. Service line thawing. Small-diameter service
time is reduced. The approximate times required to
lines of any material may be quickly thawed by
thaw different sizes of steel pipe using different
pushing a flexible 0.5 in. or smaller, plastic tube into
currents are given in figure 6-13. Copper pipe has
the frozen pipe while pumping warm water into the
about one-ninth the resistance of steel and a smaller
tube. Water pressure can be obtained from a nearby
cross-sectional area. Therefore, when thawing
building, either directly or by connecting to the
copper pipes, these current values must be increased
building plumbing. A conventional hand pump filled
by about 10 percent for 1/2-inch pipe, 25 percent
with warm water can also be used (fig. 6-11). A
for 1-inch pipe, and higher values for larger copper
commercial unit produces a pulsating stream of
pipes. However, when copper pipe with soldered
water to pump warm water through a tube attached
joints is to be thawed, it must not be heated to the
to the frozen pipe by a special fitting to ease the
point where the solder melts (silver solder can be
installation and reduce spillage.
used to alleviate this). Steel lines with continuous
joints can be expeditiously thawed with welders.
metal pipes using electricity is fairly common. Either
The following precautions must be taken when
portable gasoline or diesel generators, welders, or
thawing pipes electrically:
heavy service electrical transformers (110 or 220
(1) Use on underground or protected pipe only
volt) have been used (fig 6-12). AC or DC current
(not indoor plumbing).
at high amperage and very low voltage (seldom
(2) Don't use a high voltage. Twenty volts with
more than 15 volts) can be used. The amount of
50 to 60 amps is sufficient. (Do not use a constant
heat generated when current is passed through a
voltage power source because there is usually no
pipe is
control for limiting the current.)
(3) Make good, tight connections to the
pipeline.
(4) When conventional arc welders are used for
thawing, do not operate them at their maximum
rated amperage for more than five minutes. Only use
The rate of thawing of a frozen pipe is directly
about 75% of rated amperage if longer times are
proportional to the square of the current applied, the
needed.
mass of the pipe (cross-sectional are times length),
(5) Disconnect electrical wires grounded to the
and the material's effective resistance to the passage
water pipes in the buildings, or disconnect the
of electricity. For example, doubling the current (I)
6-8
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