pump station control by walt boyes

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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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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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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




7/12

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




8/12

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




9/12

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;

[email protected].

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10/12

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;

[email protected].

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pump station control by walt boyes

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