Pump Ed 101
Joe Evans, Ph.D.
Almost Utopia:
Lift Station Level Control
Back in the old days, level control had little or nothing
to do with saving energy. In fact, it was often a necessary evil. Today, that is no longer true – the VFD
offers the potential for power savings in lift station applications that range from a few hundred gallons per minute to
those that have to move thousands of gallons each minute.
A Little History
In an ideal world, the flow of wastewater into a wet well
would be constant. We could then size a pump (or pumps)
that could run continuously and remove that entire inflow at
the same rate as its entry. The result: smaller pumps that run
successive pump down cycle. Another method is to stage
multiple smaller pumps and attempt balance outflow with
inflow (level control). Although both of these methods work
well in many installations, there are times when the necessary
wet well volume becomes unrealistic or the number of staged
pumps required cannot be accommodated.
An alternative approach would be to vary the pumping
rate by changing pump speed. This would allow outflow to
be closely matched to inflow, and thus significantly reduce
the number of pump starts.
In the 1960s, well before the arrival of the VFD, a variable speed technique became available that employed a motor
originally developed for constant
torque applications. Figure 1
shows the rotor of the “wound
rotor” induction motor.
Rather than using the imbedded bars found in a standard rotor,
this rotor is wound with coils of
insulated copper wire that terminate at a set of slip rings. Brushes,
similar to those found in DC
motors, allow a resistance to be
connected to the coils during
motor operation. By varying the resistance of the rotor coils,
the slip speed of the rotor can be altered and that range varies
VFDs offer potential power savings in lift
station applications ranging from a few
hundred gpm to those that must move
thousands of gpm.
at BEP 24/7. We learned early on that this approach would
be impractical unless we could gain total control over the
habits of the populace. Although this has been attempted in
other parts of the world, history has taught us that it seldom
works. These revelations led to a less desirable but more practical alternative known as “pump down.”
Pump down is extremely simple – when the water in the
wet well rises to some maximum level, a pump starts and
pumps the well down to some predetermined lower level.
The pump then shuts down and waits for the water to rise
again. Usually a wet well is sized for some minimum pump
run time in order to keep the number of pump starts within
the guidelines of the manufacturer. Some may be oversized
and employ multiple smaller pumps in an attempt to emulate that Utopian system we envisioned originally. Overall,
pump down can be a very effective and relatively efficient
process.
Historically, one of the challenges of lift station design,
especially high flow ones, has been keeping pump starts to
an acceptable level. In some cases, this can be attained by
installing multiple pumps and alternating them with each
