TM 5-814-2/AFM 88-11, Vol. 2
Figure 5-3. Pump suction connections to wet well.
speeds near the HWL, and at minimum speeds near the
LWL. However, pumps are started and stopped by level
switches. Constant level control is seldom used, but
a. Water hammer. Sudden changes in flow and
may be required where a very narrow band of operation
velocity in force mains can occur as a result of pump
is necessary. In a constant level system, one level is
startup, pump shutdown, power failure, or rapid closing
set as the control point, and pump speed is adjusted in a
of a valve. These velocity changes can produce large
stepless fashion as the liquid level rises above, or falls
pressure increases or surge phenomena known as water
below this point.
hammer. The most severe water hammer conditions
c. Alarms. Alarms will be provided to signal high
are usually caused by a pump shutdown or power
and low liquid levels in the wet well, pump failure, or a
failure. An analysis of water hammer will include
malfunctioning speed control system. The high level
calculating the critical time, determining the maximum
alarm will be set above the start point of the last pump
pressure increase, and selecting a method of control.
in the operational sequence, but below the start point of
b. Critical time. When flow is suddenly changed in
the standby pump, if used. The low level alarm will be
a force main, a pressure wave is generated which
set below the shutoff point of the lead pump. An
rapidly travels the entire length of conduit, and back
emergency, low level pump cutoff will be set below the
low level alarm.