TM 5-852-5/AFR 88-19, Volume 5
6-4. Service lines.
loss of about 0.2 feet per service connection is
suggested as a conservative design value. The head
The design of service lines must prevent freezing of
losses in the plumbing in the pumphouses used for
the contained water but must also consider the
circulating water in the mains must also be
effects of permafrost thawing, frost heaving and
considered as they are substantial on most systems.
differential settlements between the pipes and the
Backup freeze protection is usually provided by a
building. These structural concerns are covered in
thermostatically controlled electrical heat tracer. If
TM 5-852-4/AFM 88-19, Chap. 4. Circulation in
copper pipe is used for the service lines, an electrical
these service connections is the recommended
connection between the two service pipes at the
method to prevent freezing. Heat tapes are also
main will allow electrical resistance thawing from
commonly installed as a backup. The two most
within the building as a second backup system.
common methods for circulation are either a small
Figure 6-4 illustrates both of these approaches.
pump inside the building or the use of pit orifices. A
Typical details of above-ground service lines and
typical pit orifice installation is shown in figure 6-4.
connections are shown in figures 6-5 and 6-6.
There will be a small velocity-induced pressure dif-
Figure 6-7 illustrates the details of an underground
ferential between the upstream and downstream
valve and box on water mains. The valve box
orifices, which in turn induces flow in the service
assembly should be packed with a low temperature
loop. To operate properly, the flow velocity in the
non-harden mg grease to prevent infiltration and
main must be at least 2 feet per second and this can
freezing of water around the mechanism in the
be technically difficult and energy intensive for large
winter. A pair of adjacent buildings can employ a
diameter water mains. If the service lines are longer
common service loop as shown in figure 6-8. This
than 80 feet, the pressure differential in the main is
will be less expensive than individual service
not sufficient and the pit orifice will not function. A
connections and will be more reliable because of the
small circulation pump must be added in the building
two circulation pumps on the single loop.
for longer distances. The presence of these pit
orifices will result in head losses in the main. A head
6-3
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