TM 5-814-2/AFM 88-11, Vol. 2
(b) Bubbler tube. One of the most
to variable speed control in wastewater pumping when
commonly used systems employs a bubbler tube which
speed adjustment is not required too often. Where
is suspended in the wet well and is fed by compressed
automatic operation is needed pulleys or sheaves can
air. The backpressure on the open end of the tube is
be positioned through the use of pneumatic, hydraulic or
sensed by pressure switches, and then transduced to a
voltage or current signal. These signals are transmitted
(3) Variable speed. Variable speed operation
to a controller which operates the pumps. This system
will usually be required at large pumping stations where
has no moving parts in contact with the wastewater, and
the entire wastewater flow, or major portions thereof,
requires very little maintenance. The constant flow of
must be pumped to the treatment facility, and where it is
compressed air keeps the tube free of solids
desired to match the incoming flowrates in order to
maintain a smooth, continuous flow into the plant.
(c) Electrodes. A series of electrodes
Pumping stations will normally require more pumps
are mounted at different elevations so that when the
under a constant speed system than one utilizing
liquid level rises and contacts an electrode, an electric
variable speeds. Also, the size of the wet well can be
circuit is energized. Electrodes are used primarily in
reduced greatly when pumps operate on a continuous
pneumatic ejectors where the compressed air serves to
basis. Variable speed operation is less efficient than
keep the electrodes clean. They will not normally be
constant speed when pumping at reduced rates,
used in wet wells due to frequent fouling by grease and
however friction losses and thus power costs are
generally less for the smaller flows.
(d) Sonic meters.
A sonic meter
b Speed control systems. The selection and
measures the distance from the liquid level to the meter.
design of the speed control system will be coordinated
They are difficult to install free of obstructions, and must
closely with the selection of the pump and drive units.
be isolated from stray electrical or acoustic signals.
The simplest system which allows pumps to accomplish
the required hydraulic effects will be chosen for design.
diaphram sensors. These types of controls will not
Factors to be considered in selecting a system include
normally be used due to fouling by the wastes.
cost, efficiency, reliability, structural requirements, ease
(2) Variable speed. A bubbler system will in
of operation and degree of maintenance necessary.
most cases be employed to control the operation of
The last two items are critical at military installations
automatic variable speed pumps. In these systems, the
where adequate personnel cannot always be provided.
backpressure from the bubbler tube is transduced to a
Pumping stations will normally be designed for
pneumatic or electronic signal for use in on/off and
automatic on/off operation of the pumping units, with
variable speed control of the pumps. On/ off controls
manual override by pushbutton or selector switch.
are usually provided by pressure or electronic switches.
(1) Constant and adjustable speed. Most
Variable speed control devices consist of (1) magnetic
automatic constant speed and adjustable speed systems
will operate from level signals. Pumps are turned on as
voltage controls, (4) variable frequency controls, and (5)
the liquid level in the wet well rises, and are turned off
wound-rotor motor controls.
Magnetic and liquid
when it falls. Pumping systems utilized in treatment
clutches have been available for many years as
plant processes are sometimes controlled by flow or
controllers for variable speed pumps. These older
pressure sensors. Level detection systems in standard
methods are inefficient in that the slip losses which
use include the following:
developed are lost as heat. The recent development of
(a) Float switches. The simplest type
solid state electronics has led to the introduction of
of switch consists of a float attached to a rod or tape,
newer methods of variable speed control suitable for
and suspended in the wet well. The float rod opens or
both squirrel-cage and wound-rotor induction motors.
closes a switch, depending on the rise or fall of the float
The variable voltage and variable frequency controls are
riding on the liquid level. The float may also be
suitable for use with squirrel-cage motors. Variable
suspended in a tube or cage. These units usually
frequency drives are possible in efficiencies up to 95
require frequent maintenance as grease, scum and
percent, and are available in sizes up to 250 hp.
debris in the wastewater build up on the equipment.
However, variable voltage units are inefficient and are
Another type of float control incorporates a mercury
not recommended. Wound-rotor motor controls come in
switch encapsulated in a corrosion resistant ball, and
five categories, (1) fixed step resistors, (2) liquid
suspended by cable in the wet well. This unit is not
rheostats, (3) reactance/resistance controllers, (4)
dependent upon the smooth, vertical movement of a
electronic rheostats, and (5) regenerative secondary
rod, and thus is not subject to the maintenance
controls. Of these, the regenerative secondary
problems described above.