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Flow Control Network > PastIssues >jun2002 > Pump
Station Control
Pump Station Control
by Walt Boyes and David W. Spitzer, P.E.
A common application of control equipment in water and
wastewater treatment is the management of pumpingstations. Pump stations are found in such varied
applications as the clearwell of gravity filtration systemsin wastewater treatment plants, at the base of water
storage tanks, as booster stations for feed to distribution
grids and even on golf courses for irrigation. Inwastewater treatment, pump stations are found before
the influent of the treatment plant (called lift stations),and at the outfall of the treatment plant to move the
treated wastewater to the actual discharge point.
Pump stations are available in two forms. Sometimes, anengineering firm is retained to design and draw plans for
a custom pumping plant. Other times, the water or
wastewater utility purchases a pre-designed packagepump station complete from a vendor, and hires a
contractor to install it and connect it to the system. Thereare advantages or disadvantages to both forms.
Before the design is started, or before the requisition tobuy a package pump station is written, there are issues
on which the end user must decide, including the overallcontrol strategy. Selecting an inexpensive control
strategy may result in the installation of equipment that
has a lower purchase cost, but a much higher operatingcost that could offset the savings in a short period of
time.
On/Off Pump Control Strategy
A common control strategy is on/off pump control. Thisstrategy is often selected when operating a single pump
in a pump station with one pump fed from a sump or
tank. The pump is turned on when the level in the tankor sump is high, and turned off when the level is low.
This can be implemented using two limit switches --- oneto detect high level and another to detect low level. The
outputs of the level switches are used to operate thepump motor starter, where appropriate provisions are
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Current Issue
Next IssuePast Issues
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Media Kit
Contact Us
Flow Control Network > PastIssues >jun2002 > Pump
Station Control
Pump Station Control
by Walt Boyes and David W. Spitzer, P.E.
A common application of control equipment in water and
wastewater treatment is the management of pumpingstations. Pump stations are found in such varied
applications as the clearwell of gravity filtration systemsin wastewater treatment plants, at the base of water
storage tanks, as booster stations for feed to distribution
grids and even on golf courses for irrigation. Inwastewater treatment, pump stations are found before
the influent of the treatment plant (called lift stations),and at the outfall of the treatment plant to move the
treated wastewater to the actual discharge point.
Pump stations are available in two forms. Sometimes, anengineering firm is retained to design and draw plans for
a custom pumping plant. Other times, the water or
wastewater utility purchases a pre-designed packagepump station complete from a vendor, and hires a
contractor to install it and connect it to the system. Thereare advantages or disadvantages to both forms.
Before the design is started, or before the requisition tobuy a package pump station is written, there are issues
on which the end user must decide, including the overallcontrol strategy. Selecting an inexpensive control
strategy may result in the installation of equipment that
has a lower purchase cost, but a much higher operatingcost that could offset the savings in a short period of
time.
On/Off Pump Control Strategy
A common control strategy is on/off pump control. Thisstrategy is often selected when operating a single pump
in a pump station with one pump fed from a sump or
tank. The pump is turned on when the level in the tankor sump is high, and turned off when the level is low.
This can be implemented using two limit switches --- oneto detect high level and another to detect low level. The
outputs of the level switches are used to operate thepump motor starter, where appropriate provisions are
SEARCH THE SITE
search criteria GO
Current Issue
.
Page 1 of 6Pump Station Control
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8/14/2019 Pump Station Control by Walt Boyes
3/12
8/14/2019 Pump Station Control by Walt Boyes
4/12
made for the installation of a hand-off-auto (HOA) switchthat allows for manual or automatic pump operation.
Note that on/off pump operation results in zero flow
when the pump is off, and full flow when the pump is on.This operation might have significant effects on the
process. For example, using this control strategy in thewet well of a wastewater treatment plant can cause the
flow through the entire plant to stop. If the controlstrategies in the plant do not take this into account,
chemical additives like chlorine could continue to flow
while the pump is off. This can cause concentrations ofadditives that not only cause instability, but also waste
chemicals. The additional cost of the chemicals could besignificant, especially as the amount of time at zero flow
increases. If chlorine is being added while there is noflow, a significant hazard can be created, as well.
This controls strategy can also be used when a spare
pump is installed. In this case, a sequencer may be usedto alternate the pumps to distribute wear between the
two pumps. Should one pump fail to start, the otherpump can automatically be started to handle the load. In
addition, a high-high level switch could be added to
operate both pumps under high flow conditions. It isuseful to take another contact from a current sensor to
make sure that the pump started and is running at load.
The level switch signals can also be generated from a
signal from a level transmitter by using signal tripmodules or software in the sequencer. The significant
advantage of this approach is that it provides levelmeasurement information instead of the presence or lack
of level. In many cases, the level measurement canprovide information that allows the diagnosis of apotential problem prior to the problem affecting the
process. In addition, changing the level at which thepump turns on or off can be performed electronically
instead of mechanically.
The calibration of the level transmitter should be checkedoccasionally to ensure proper operation. On the other
hand, a level switch typically requires little maintenanceuntil it fails. Note that failure of either a level transmitter
or level switch can cause major problems. However, it is
often possible to use the level measurement information
to detect a problem prior to it affecting the process.
In some applications, one operating pump cannot handle
the design load. In this case, additional level switches (or
trips) can be used to sequence the operation additionalpumps and alternate the pumps. For example, a three-
pump station may be sequenced 1-2-3 or 1-3-2, and mayoperate based upon:
Level Operating Pumps
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90% alarm
80% 3 pumps
70% 2 pumps
60% 1 pump
20% off
As the level rises, more pumps are turned on to handlethe load, creating four flowing rates --- zero, one pump,
two pumps and three pumps. Due to pumping and piping
hydraulics, when at least one pump continues to operate,turning another pump on or off will not affect the flow
rate as much as the single pump operation describedabove. Therefore, designing to keep at least one pump
on continuously tends to reduce these abrupt flow ratechanges.
Modulating Pump Control StrategyThe pump station can also be controlled such that flow ismodulated. When operating a single pump in a pump
station with one pump fed from a sump or tank, themodulation could be based upon a signal from a liquid
level transmitter. This modulation can be achieved usinga control valve on the discharge of the pump, but issues
associated with operating high on the pump curve (dead-
heading) should be considered. In part for this reason, itis more common to vary the speed of the motor driving
the pump. In addition, varying pump speed is moreenergy efficient than using a control valve and can
reduce pump wear and reduce pump maintenance.
Modulating control allows flow to be varied continuously
from near zero to maximum flow. When the incomingflow into the sump or tank is relatively steady, the
pumped flow can be manipulated to match the incoming
flow. When this occurs, the level can be controlled toremain relatively stable. If the level is relatively steady,
the incoming flow is approximately equal to the pumpedflow, and therefore the pumped flow will be relatively
steady.
In contrast, the on/off control might cause the pumped
flow to jump from zero to one pump flow several timeseach hour, as the level is allowed to rise and fall
significantly (from 20 to 60 percent in the example).These flow changes can have significant adverse effects
on the process, as outlined above, when appropriatecompensation is not included in the control strategy.
Using a modulating strategy to stabilize the pumped flowcan make the process operate better with less complex
control strategies.
As previously stated, modulating flow using a variable
speed drive to vary pump speed is more energy efficient
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90% alarm
80% 3 pumps
70% 2 pumps
60% 1 pump
20% off
As the level rises, more pumps are turned on to handlethe load, creating four flowing rates --- zero, one pump,
two pumps and three pumps. Due to pumping and piping
hydraulics, when at least one pump continues to operate,turning another pump on or off will not affect the flow
rate as much as the single pump operation describedabove. Therefore, designing to keep at least one pump
on continuously tends to reduce these abrupt flow ratechanges.
Modulating Pump Control StrategyThe pump station can also be controlled such that flow ismodulated. When operating a single pump in a pump
station with one pump fed from a sump or tank, themodulation could be based upon a signal from a liquid
level transmitter. This modulation can be achieved usinga control valve on the discharge of the pump, but issues
associated with operating high on the pump curve (dead-
heading) should be considered. In part for this reason, itis more common to vary the speed of the motor driving
the pump. In addition, varying pump speed is moreenergy efficient than using a control valve and can
reduce pump wear and reduce pump maintenance.
Modulating control allows flow to be varied continuously
from near zero to maximum flow. When the incomingflow into the sump or tank is relatively steady, the
pumped flow can be manipulated to match the incoming
flow. When this occurs, the level can be controlled toremain relatively stable. If the level is relatively steady,
the incoming flow is approximately equal to the pumpedflow, and therefore the pumped flow will be relatively
steady.
In contrast, the on/off control might cause the pumped
flow to jump from zero to one pump flow several timeseach hour, as the level is allowed to rise and fall
significantly (from 20 to 60 percent in the example).These flow changes can have significant adverse effects
on the process, as outlined above, when appropriatecompensation is not included in the control strategy.
Using a modulating strategy to stabilize the pumped flowcan make the process operate better with less complex
control strategies.
As previously stated, modulating flow using a variable
speed drive to vary pump speed is more energy efficient
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than installing a control valve on the pump discharge. Ina control valve installation, the pump generates hydraulic
pressure, some of which is dissipated across a restriction(the control valve) to manipulate flow. In a variable
speed drive installation, pump speed is manipulated togenerate only the hydraulic energy needed to pump the
desired flow without any restriction.
In some applications, the difference in electrical energyconsumption between the two installations can be
sufficiently large to justify replacing a control valve
installation with a variable speed drive installation.Potential energy savings are especially large when the
pump is relatively large as compared to the operatingload. For example, this can occur when a pumping
system is sized for the maximum possible flow (such asduring start-up), but normal operating flow is much
smaller.
In some applications, one operating pump cannot handlethe design load. In this case, variable speed drives
should be installed on all of the operating pumps to varytheir speeds in unison. When the pumps are identical,
operating the pumps at the same speed will distribute the
flow equally between the pumps. This strategy shouldavoid potential backflow situations that can occur when
the pressure produced by one pump operating at highspeed overcomes the pressure produced by another
pump operating at a lower speed. As in on/offapplications, the pumps can be sequenced to distribute
wear and change the number of operating pumps as the
load changes. Control strategies used to determine thenumber of operating pumps include the use of pump
speed and/or pumping flow rate information.
Modulating control as described above can be
implemented using a single control loop consisting of alevel transmitter, level controller, and variable speed
drive(s). In most applications, superior control can beachieved by using a cascade control strategy that
includes information from a flow transmitter that
measures the pumped flow. In some applications, furtherimprovement can be obtained using feedforward control
techniques that use other process measurements.
Package and Custom Installations
Pumping stations can be purchased as a package systemor designed as a custom installation. Some packagedsystems can be provided with either on/off or variable
speed drive control strategies. Custom pump stations canbe designed using any of the control strategies discussed.
The choice of package versus custom pump station
controls is often a matter of their economic impact on theprocess and the ability of the designer to recognize and
mitigate these effects.
Package pumping station systems generally do not offer
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than installing a control valve on the pump discharge. Ina control valve installation, the pump generates hydraulic
pressure, some of which is dissipated across a restriction(the control valve) to manipulate flow. In a variable
speed drive installation, pump speed is manipulated togenerate only the hydraulic energy needed to pump the
desired flow without any restriction.
In some applications, the difference in electrical energyconsumption between the two installations can be
sufficiently large to justify replacing a control valve
installation with a variable speed drive installation.Potential energy savings are especially large when the
pump is relatively large as compared to the operatingload. For example, this can occur when a pumping
system is sized for the maximum possible flow (such asduring start-up), but normal operating flow is much
smaller.
In some applications, one operating pump cannot handlethe design load. In this case, variable speed drives
should be installed on all of the operating pumps to varytheir speeds in unison. When the pumps are identical,
operating the pumps at the same speed will distribute the
flow equally between the pumps. This strategy shouldavoid potential backflow situations that can occur when
the pressure produced by one pump operating at highspeed overcomes the pressure produced by another
pump operating at a lower speed. As in on/offapplications, the pumps can be sequenced to distribute
wear and change the number of operating pumps as the
load changes. Control strategies used to determine thenumber of operating pumps include the use of pump
speed and/or pumping flow rate information.
Modulating control as described above can be
implemented using a single control loop consisting of alevel transmitter, level controller, and variable speed
drive(s). In most applications, superior control can beachieved by using a cascade control strategy that
includes information from a flow transmitter that
measures the pumped flow. In some applications, furtherimprovement can be obtained using feedforward control
techniques that use other process measurements.
Package and Custom Installations
Pumping stations can be purchased as a package systemor designed as a custom installation. Some packagedsystems can be provided with either on/off or variable
speed drive control strategies. Custom pump stations canbe designed using any of the control strategies discussed.
The choice of package versus custom pump station
controls is often a matter of their economic impact on theprocess and the ability of the designer to recognize and
mitigate these effects.
Package pumping station systems generally do not offer
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the flexibility of custom designed stations. In addition,many package pumping stations have limited areas for
maintenance and limited headroom, making maintenancemore problematic. Custom pumping stations are
generally built to suit, and the designer often allows theoperator more room to work in doing the required
maintenance on the station after it has been installed and
operational.
Key design criteria for pumping stations include the
ability to handle a wide variation in diurnal flows from
minimum through average to peak flow without causingplant upset or flooding the wet well. In addition,
designers need to provide a means for equalizing theoperating hours, and therefore, the wear on multiple
pumps. Designers need to be mindful of both operationalissues and maintenance requirements when designing
the physical layout of a pump station.
SummaryDesigning pump station controls correctly can result in
years of excellent operation with minimal maintenance.Designing pump controls incorrectly, or purchasing a
package pump station that does not meet your criteria
can swell the cost of operation, increase downtime andmaintenance, and result in premature replacement of
equipment.
References
1. Flow Measurement (2ndEdition), David W. Spitzer,
ISA, 2001 (838 pages).2. Industrial Flow Measurement, David W. Spitzer, ISA,
1990 (441 pages).
3. Variable Speed Drives: Principles and Applications forEnergy Cost Savings, (2nd Edition, Revised), David W.Spitzer, ISA, 1990 (201 pages).
About the AuthorsDavid W. Spitzer, P.E., and Walt Boyes have more than
50 years combined experience in instrumentation and
process control, and are the principals of Spitzer andBoyes, offering product development, marketing,
specifying, installing, start-up and troubleshootingservices for manufacturing and automation companies.
Both Mr. Spitzer and Mr. Boyes have written numerousbooks and articles, have taught seminars, and are
members of standardization committees. Mr. Spitzer can
be reached at 845 623-1830; [email protected] can be reached at 425 432-8262;
Subscribe | Site Map | Advertise
Page 5 of 6Pump Station Control
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the flexibility of custom designed stations. In addition,many package pumping stations have limited areas for
maintenance and limited headroom, making maintenancemore problematic. Custom pumping stations are
generally built to suit, and the designer often allows theoperator more room to work in doing the required
maintenance on the station after it has been installed and
operational.
Key design criteria for pumping stations include the
ability to handle a wide variation in diurnal flows from
minimum through average to peak flow without causingplant upset or flooding the wet well. In addition,
designers need to provide a means for equalizing theoperating hours, and therefore, the wear on multiple
pumps. Designers need to be mindful of both operationalissues and maintenance requirements when designing
the physical layout of a pump station.
SummaryDesigning pump station controls correctly can result in
years of excellent operation with minimal maintenance.Designing pump controls incorrectly, or purchasing a
package pump station that does not meet your criteria
can swell the cost of operation, increase downtime andmaintenance, and result in premature replacement of
equipment.
References
1. Flow Measurement (2ndEdition), David W. Spitzer,
ISA, 2001 (838 pages).2. Industrial Flow Measurement, David W. Spitzer, ISA,
1990 (441 pages).
3. Variable Speed Drives: Principles and Applications forEnergy Cost Savings, (2nd Edition, Revised), David W.Spitzer, ISA, 1990 (201 pages).
About the AuthorsDavid W. Spitzer, P.E., and Walt Boyes have more than
50 years combined experience in instrumentation and
process control, and are the principals of Spitzer andBoyes, offering product development, marketing,
specifying, installing, start-up and troubleshootingservices for manufacturing and automation companies.
Both Mr. Spitzer and Mr. Boyes have written numerousbooks and articles, have taught seminars, and are
members of standardization committees. Mr. Spitzer can
be reached at 845 623-1830; [email protected] can be reached at 425 432-8262;
Subscribe | Site Map | Advertise
Page 5 of 6Pump Station Control
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8/14/2019 Pump Station Control by Walt Boyes
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2003 Witter Publishing, Inc. All rights reserved. Unauthorized use prohibited
Privacy Policy Terms of [email protected]
Page 6 of 6Pump Station Control
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2003 Witter Publishing, Inc. All rights reserved. Unauthorized use prohibited
Privacy Policy Terms of [email protected]
Page 6 of 6Pump Station Control