1 March 1997
7-3. PUMP CONTROLS.
a. General. Instrumentation at a pumping station includes automatic and manual controls
used to sequence the operation of pumps, and alarms for indicating malfunctions in the pumping
system. Automatic control of pumps will usually be based on the liquid level in the wet well.
Paragraph 6-4 contains a discussion of the various modes of pump operation, pump control
systems, and a description of level detection devices. Manual control of pumps is always
required in order to operate the pumps during emergencies, for maintenance purposes, or when
automatic systems fail. Manual override will be set to bypass the low level cut-off, but not the
low level alarm.
b. Selection of control points. A control range of at least 1 meter (3.0 feet) is required
between maximum and minimum liquid levels in the wet well. A minimum of 150 millimeters (6
inches) will be required between pump control points used to start and stop successive pumps,
or to change pump speeds. For small stations, the control range may be less, however control
points will not be set closer than 75 millimeters (3 inches).
(1) Constant or adjustable speed pumps require simple on-off switches to start or stop
pumps, or to change from one speed step to the next.
(2) Variable speed pumps require a more complex control arrangement. The two basic
types of level control for variable speed operation are (a) variable level, and (b) constant level.
For variable level control, a narrow band of control points is established in the wet well. Pump
speed is then adjusted in steps by the level detection system (usually a bubbler tube) as the level
varies. Pumps operate at maximum 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 may be required where a very narrow band of operation is necessary. In a
constant level system, one level is set as the control point, and pump speed is adjusted in a step
less fashion as the liquid level rises above, or falls below this point.
c. Alarms. Alarms will be provided to signal high and low liquid levels in the well, pump
failure, or a malfunctioning speed control system. The high level alarm will be set above the start
point of the last pump in the operational sequence, but below the start point of the standby pump,
if used. The low level alarm will be set below the shutoff point of the lead pump. An emergency,
low level pump cutoff will be set below the low level alarm.
7-4. PUMP SCHEDULING AND OPERATIONS. Reliability of a pump station is a critical part of
design and operation. The number of pumps depends on present and future needs. An
economic analysis should be performed to determine the number of pumps to be installed. In
smaller stations a single pump may be most economical to meet the peak demand. However
whenever a single pump is sufficient, two equal size pumps, each able to handle the peak
demand, must be provided and set-up to alternate. Whenever two or more pumps are cost-
effective to meet the peak demand, additional pump capacity or pumps must be installed so that
peak demand can be met with the largest pump out of service. All pumps should alternate.
7-5. WET WELL DESIGN.
a. General. Wet wells will be constructed at pumping stations for the purpose of storing
wastewater flows prior to pump operation. The storage volume required depends upon the
method of pump operation, i.e., whether pumps are constant, adjustable or variable speed. In
addition to providing adequate storage volume, wet wells will be designed to (1) allow for proper
pump and level controls, (2) maintain sufficient submergence of the pump suction inlet, (3)