1 March 1997
can be reduced greatly when pumps operate on a continuous basis. Variable speed operation is
less efficient than constant speed when pumping at reduced rates, however friction losses and
thus power costs are generally less for the smaller flows.
b. Speed control systems. The selection and design of the speed control system will be
coordinated closely with the selection of the pump and drive units. The simplest system which
allows pumps to accomplish the required hydraulic effects will be chosen for design. Factors to
be considered in selecting a system include cost, efficiency, reliability, structural requirements,
ease of operation and degree of maintenance necessary. The last two items are critical at
military installations where adequate personnel cannot always be provided. Pumping stations will
normally be designed for automatic on/off operation of the pumping units, with manual override
by push-button or selector switch.
(1) Constant and adjustable speed. Most automatic constant speed and adjustable speed
systems will operate from level signals. Pumps are turned on as the liquid level in the wet well
rises, and are turned off when it falls. Pumping systems utilized in treatment plant processes are
sometimes controlled by flow or pressure sensors. Level detection systems in standard use
include the following:
(a) Float switches. The simplest type of switch consists of a float attached to a rod or
tape, and suspended in the wet well. The float rod opens or closes a switch, depending on the
rise or fall of the float riding on the liquid level. The float may also be suspended in a tube or
cage. These units usually require frequent maintenance as grease, scum and debris in the
wastewater build up on the equipment. Another type of float control incorporates a mercury
switch encapsulated in a corrosion resistant ball, and suspended by cable in the wet well. This
unit is not dependent upon the smooth, vertical movement of a rod, and thus is not subject to the
maintenance problems described above.
(b) Bubbler tube. One of the most commonly used systems employs a bubbler tube
which is suspended in the wet well and is fed by compressed air. The back pressure on the
open end of the tube is sensed by pressure switches, and then transduced to a voltage or
current signal. These signals are transmitted to a controller which operates the pumps. This
system has no moving parts in contact with the wastewater, and requires very little maintenance.
The constant flow of compressed air keeps the tube free of solids accumulations.
(c) Electrodes. A series of electrodes are mounted at different elevations so that when
the liquid level rises and contacts an electrode, an electric circuit is energized. Electrodes are
used primarily in pneumatic ejectors where the compressed air serves to keep the electrodes
clean. They will not normally be used in wet wells due to frequent fouling by grease and waste
(d) Sonic meters. A sonic meter measures the distance from the liquid level to the
meter. They are difficult to install free of obstructions, and must be isolated from stray electrical
(e) Capacitances tubes and pressure diaphragm sensors. These types of controls will
not normally be used due to fouling by the wastes.
(2) Variable speed. A bubbler system will in most cases be employed to control the
operation of automatic variable speed pumps. In these systems, the back pressure from the
bubbler tube is transduced to a pneumatic speed equipment to cover service conditions, or