TM 5-852-5/AFR 88-19, Volume 5
insulation from moisture. Insulation will also be
cycles. This can be accomplished by an electrically
placed beneath the station when permafrost is pres-
operated ball valve in the line to allow drainage back
ent at a shallow depth to prevent settling due to
into the wet well between pump cycles. If this is not
ground thaw. A plastic film or some other bond
possible the line must be placed in a heated utilidor
breaker will be required to reduce frost jacking in
or heat traced. Another option would be to time the
the active layer (see further discussion, para 11-3).
pumping cycle so that wastes stay in the line for a
If thawing and settling under the station are antici-
calculated period, and to size the wet well at the
pated, pile foundations extending well into the
pump station to hold at least the volume of the force
permafrost will be required. All stations must be
main.
attached to concrete base slabs to provide sufficient
7.5 Pressure sewerage.
weight to overcome the buoyancy of the station.
Pressure couplings or flexible connections are
The main advantage of pressure sewers is that spe-
required for the inlet and outlet pipes to prevent
cific grades need not be maintained throughout the
station differential movement from breaking the
system. Typically grinder pump units are used so
lines.
that smaller diameter pipes can be installed without
the risk of clogging. Grinder pumps may be installed
pumping station must not be installed without
in each building or in a holding tank serving several
immediately placing the heater and dehumidifier into
buildings. An alternative to grinder pumps is to
operation. Condensation caused by the surrounding
install a two-compartment septic tank with a
cold earth could corrode the controls and electrical
conventional submersible pump in the second
equipment before the system is actually put into
compartment. The hydraulic design of these
service.
pressure systems is not unique in the cold regions
c. Alarms. Alarms must be provided in accor-
and criteria found in TM 5-814-2/AFM 88-11, Vol.
dance with the requirements of TM 5-814-2/AFM
2, will be used. The pressure piping must be
88-11, Vol. 2. All critical components, such as
designed to drain by gravity to a low point or sump
pumps and compressors, must be duplicated in each
in case the system has to be shut down in the winter.
station. In addition, alarms must be set to warn of
The grinder pump units, holding tanks and septic
freezing temperatures in the station and to warn of
tanks must have a firm foundation and must be
sump pump malfunctions. Alarms must be annun-
protected from frost heaving as discussed previously
ciated as described in TM 5-814-2/AFM 88-11,
for manholes (para. 7-3a). Water conservation
Vol.2.
measures in each of the buildings served is required
d. Standby power. Due to the dangers of freezing
to reduce the costs of equipment and energy for
associated with extremely low temperatures, stand-
pumping.
by power facilities must be provided for each major
pump station.
7-6. Vacuum sewerage.
e. Maintenance. During freezing conditions,
Vacuum sewers do not depend on a specific grade
each pump station must be checked daily by the
for successful operation. They operate at a vacuum
operator. All entrance manholes must extend
of 8-10 psi, and so are limited to an elevation differ-
sufficiently above the ground surface to be above
ence of 15 to 20 feet within the system. The hydrau-
any flooding or snow drifts. All pump stations will
lic design is not unique to the cold regions. The
be supplied with devices for measuring flow rates.
concept depends on providing traps in the system to
Corrosion protection must be provided in
maintain a vacuum. Since these traps are full of
accordance with TM 5-814-2/AFM 88-11, Vol.2.
water for extended periods, they must be insulated
Sacrificial anode type systems do not work well
and/or heated for extreme low temperature condi-
when the ground surrounding the anode or pump
tions. The traps should also be drainable under
station is frozen.
emergency situations. A 50 house vacuum system in
f. Force mains. Force mains will be designed to
Noorvik, Alaska, was installed in 1977 and has
have scour velocities during pumping (2.5 to 3.5
operated successfully, since that time.
feet per second) and to drain between pumping
7-5
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