TM 5-818-5/AFM 88-5, Chap 6/NAVFAC P-418
CHAPTER 6
OPERATION AND PERFORMANCE CONTROL
6-1. General. The success of a dewatering opera-
the system; the pump operator should do this daily.
(2) A wellpoint leaking air will frequently cause
tion finally hinges on the proper operation, mainte-
an audible throbbing or bumping in the swing-joint
nance, and control of the system. If the system is not
connection, which may be felt by placing the hand on
operated and maintained properly, its effectiveness
the swing joint. The throbbing or bumping is caused by
may soon be lost. After a dewatering or pressure relief
intermittent charges of water hitting the elbow at the
system has been installed, a full-scale pumping test
top of the riser pipe, In warm weather, wellpoints that
should be made and its performance evaluated for ade-
are functioning properly feel cool and will sweat due to
quacy or need for any modification of the system. This
condensation in a humid atmosphere. A wellpoint that
test and analysis should include measurement of the
is not sweating or that feels warm may be drawing air
initial water table, pump discharge, water table in ex-
through the ground, or it may be clogged and not func-
cavation, water table in wells or vacuum in header sys-
tioning. Likewise, in very cold weather, properly func-
tem, and a comparison of the data with the original de-
tioning wellpoints will feel warm to the touch of the
sign.
hand compared with the temperature of the atmos-
6-2. Operation.
phere, Vacuum wellpoints disconnected from the
header pipe can admit air to the aquifer and may af-
a. Wellpoint systems.
fect adjacent wellpoints. Disconnected vacuum well-
(1) The proper performance of a wellpoint system
points with riser pipes shorter than 25 feet should be
requires continuous maintenance of a steady, high vac-
uum. After the system is installed, the header line and
capped.
(3) Wellpoint headers, swing connections, and
all joints should be tested for leaks by closing all
riser pipes should be protected from damage by con-
swing-joint and pump suction valves, filling the header
struction equipment. Access roads should cross header
with water under a pressure of 10 to 15 pounds per
lines with bridges over the header to prevent damage
square inch, and checking the line for leaks. The next
to the headers or riser connections and to provide ac-
step is to start the wellpoint pump with the pump suc-
cess for tuning and operating the system.
tion valve closed. The vacuum should rise to a steady
25 to 27 inches of mercury. If the vacuum on the pump
b. Deep wells. Optimum performance of a deep-well
is less than this height, there must be air leaks or worn
system requires continuous uninterrupted operation of
parts in the pump itself. If the vacuum at the pump is
all wells. If the pumps produce excessive drawdowns
satisfactory, the gate valve on the suction side of the
in the wells, it is preferable to regulate the flow from
pump may be opened and the vacuum applied to the
all of the wells to match the flow to the system, rather
header, with the wellpoint swing-joint valves still
than reduce the number of unite operating and thus
closed. If the pump creates a steady vacuum of 25
create an uneven drawdown in the dewatered area.
inches or more in the line, the header line may be con-
The discharge of the wells may be regulated by vary-
sidered tight. The swing-joint valves are then opened
ing the pump speed (if other than electric power is
and the vacuum is applied to the wellpoints. If a low,
used) or by varying the discharge pressure head by
unsteady vacuum develops, leaks may be present in
means of a gate valve installed in the discharge lines.
the wellpoint riser pipes, or the water table has been
Uncontrolled discharge of the wells may also produce
lowered to the screen in some wellpoints so that air is
excessive drawdowns within the well causing undesira-
entering the system through one or more wellpoint
ble surging and uneven performance of the pumps.
screens. One method of eliminating air entering the
system through the wellpoints is to use a riser pipe 25
c. Pumps. Pumps, motors, and engines should al-
ways be operated and maintained in accordance with
feet or more in length. If the soil formation requires
the manufacturer's directions. All equipment should
the use of a shorter riser pipe, entry of air into the sys-
be maintained in first-class operating condition at all
tem can be prevented by partially closing the main
times. Standby pumps and power units in operating
valve between the pump and the header or by adjust-
conditions should be provided for the system, as dis-
ing the valves in the swing connections until air enter-
ing the system is stopped. This method is commonly
cussed in chapter 4. Standby equipment may be re-
quired to operate during breakdown of a pumping unit
used for controlling air entry and is known as tuning
6-1
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