Variable Frequency Drives for
Water/Wastewater
Discussion on Pump Optimization Principles
and “Need to Know” Drive Technology
Pumps in Water Wastewater
2
WWW Challenges: Drivers/Trends
● Demand for WWW
● Age of infrastructure
● Legislative compliance
● Reduced financial resources
● Energy efficiency awareness
● Energy use
1.4%
0.5% 0.3% 0.1%
Typical wastewater process
energy usage breakdown
Aeration
3.2%
3.9%
Collection
Anaerobic Digestion
8.1%
Lighting and Buildings
14.2%
54.1%
14.3%
Belt Press
Clarifiers
Grit
RAS
Chlorination
Process = 91.9%
Gravity Thickening
3
Water
energy use 71% of consumed electricity is used to
turn motors
65% of this energy is used for fluid
applications
Wastewater
energy use Finnish Technical Research Center Report:
“Expert Systems for Diagnosis of the Condition and
Performance of Centrifugal Pumps”
Evaluation of 1690 pumps at 20 process plants:
● Average pumping efficiency is below 40%
● Over 10% of pumps run below 10% efficiency
● Major factors affecting pump efficiency
● Throttled valves
● Pump over-sizing
● Seal leakage causes highest downtime and cost
5
System Curve Uncertainty
Results in Uncertain Pump Operation
- and higher costs
6
Pump Curves
7
Pumps System Overview and
Fundamentals
8
Overview
●  The pumping system:
Components
–
–
–
–
–
–
–
–
Pumps
Motors, engines
Piping
Valves and fittings
Controls and
instruments
Heat exchangers
Tanks
Others
End-use
–
–
–
–
–
Water treatment
Wastewater treatment
Water distribution
Power generation
Irrigation
9
Overview, continued
Electric utility
feeder
Transformer
Motor breaker/
starter
System Approach
●  Component optimization involves
segregating components and analyzing in
isolation
●  System optimization involves studying how
the group functions as one as well as how
changing one component can help the
efficiency of another
Adjustable
speed drive
(electrical)
Motor
Coupling
Pump
Fluid System
Served
Process(es)
10
Pump Fundamentals
There are two basic types of pumps:
1.  Centrifugal
2.  Positive
Displacement (PD)
●  Use a rotating impeller to increase
velocity of a liquid and its
stationary components direct
discharge flow to convert velocity
to increased pressure
●  Types include axial, mixed flow,
and radial
●  Move a set volume of liquid and
pressure is obtained as the liquid is
forced through the pump discharge
into the system
●  Types include piston, screw, sliding
vane, and rotary lobe
11
Pump Fundamentals, continued
Centrifical Pumps
●  Impart energy to the
liquid by increasing
its speed in the
impeller and then
converting the speed
to pressure through
diffusion in the volute.
12
Pump Fundamentals, continued
PD Pumps
●  Impart energy by applying
mechanical force directly
to the liquid through a
collapsing volume
13
Energy Efficiency in Pumps
•  Load Characteristics
Water Wastewater Load Characteristics
Variable
Torque
Constant
Torque
Constant
Power
Typical
Applications
Centrifugal Pumps
and Blowers
Positive
Displacement
Pumps, Blowers,
Mixers, and
Chemical Feed
Pumps
No applications
Energy
Savings
Potential
Substantial
Potential – Largest
of all VFD
applications
Lowest Potential
No Potential
The Main
Target ( first
priority)
The Next
Step ( second
priority)
14
Pump Fundamentals, continued
●  Centrifical pumps are constant head devices, where
head loss causes a pressure drop.
●  Frictional head loss increases with the square of velocity
change of the liquid in the pipe.
●  Static head is the energy needed to overcome an
elevation or pressure difference between the suction and
discharge vessels.
●  In most cases:
15
Pump Fundamentals, continued
System Head Curve Produced by US DOE PSAT Software
Friction Head
Static Head
16
Pump Fundamentals, continued
Friction
●  May occur in pump systems due to irrecoverable hydraulic
losses in:
●
●
●
●
Piping
Valving
Fittings (e.g., elbows, tees)
Equipment (e.g., heat exchangers)
●  Used to control flow or pressure by:
●  Automated flow and pressure control valves
●  Orifices
●  Manual throttling valves
17
VFD Benefits with Pumps
18
Energy Efficiency in Pumps
•  Motor Costs
19
Energy Efficiency in Pumps
•  Energy Wastes
How your money is wasted!
Car example :
…try to regulate the speed of your
car
•  keeping one foot on the accelerator
•  the other on the brake.
Pump example :
… try to adjust the pump output
•  running the motor at full speed
•  control the flow with a throttle valve
Still one of the most common control methods in industry …..
with a considerable waste of energy
20
VFD Benefits with Pumps
•  Physical Laws for Centrifugal Loads
Its Pure Physics:
Due to the laws that govern centrifugal pumps,
the flow of water decreases directly with pump speed
Affinity laws of centrifugal loads :
Flow = f (motor speed)
Pressure = f (motor speed)2
Power = f (motor speed)3
21
VFD Benefits with Pumps
•  Physical Laws for Centrifugal Loads
A motor running at 80% of full speed requires 51% of
the electricity of a motor running at full speed.
22
VFD Benefits with Pumps
•  Physical Laws for Centrifugal Loads
A motor running at 50% of full speed requires 12.5% of
the electricity of a motor running at full speed.
23
VFD Benefits with Pumps
•  Physical Laws for Centrifugal Loads
•  A small reduction in speed produces a significant reduction in power
•  Relevant applications : Pumps
•  The resisting torque of centrifugal pumps varies with the square of the
speed : T = kN²
•  Power is a cubed function P = kN³
EX 50HP 10Hrs/day, 250 days @$.08
With 15% average speed reduction
ATL = $7,460
VFD = $4,188
Savings = $3,272
Today, less than 10% of these motors are controlled with Variable Speed Drives
24
Efficiency of Pumping Systems
25
Efficiency of Pumping Systems
•  Equations for efficiency
Pump Energy Usage
kW =
(Q * H )
(5308 *η Motor *η Pump *η Drive )
Q = Flow( gpm)
H = Head ( ft )
η = efficiency
For Water Wastewater
PSI conversion to ft = PSI * 2.31
Pump Efficiency Measurement
kW
16,667 * kW
=
MGD
Q
(Q * H )
(5308 * kW )
(Q * H )
=
(5308 * kW )
η Motor *η Pump *η Drive =
η Wire to water
26
Sample Pump Calculations
Actual Pump Data
•
Single Pump Running Measurement
Measured VFD Speed
Measured Power
Measured flow
Measured head
54.1 Hertz
32.3 kW
716 GPM
74.5 PSI
Two Pumps Running Measurement
Measured VFD Speed
Measured Power
Measured flow
Measured head
48 Hertz
82.5 kW
1452 GPM
75 PSI
Three Pumps Running Measurement
Measured VFD Speed
Measured Power
Measured flow
Measured head
?? Hertz
241.5 kW
5125 GPM
79.4 PSI
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•
•
27
Sample Pump Calculations
FREQ
?
48
54.1
KW
PSIG
TDH
GPM
GPM X TDH KW X 5308 EFFICIENCY
241.5
82.5
32.3
79.4
75
74.5
183.414
173.25
172.095
5125
1452
716
939996.75
251559
123220.02
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FREQ
KW
GPM
EFFICIENCY
KW X 16667 kW/MGD
?
48
54.1
241.5
82.5
32.3
5125
1452
716
4025080.5
1375027.5
538344.1
785.38
946.99
751.88
1281882
437910
171448.4
0.73
0.57
0.72
PUMP QTY
three pumps
two pumps
one pump
PUMP QTY
three pumps
two pumps
one pump
28
VFD Benefits with Pumps
Other Benefits
In addition to Energy Saving, using a VFD has many other
advantages:
•  Less mechanical stress on motor and system
•  Less mechanical devices - Less Maintenance
•  Process regulation with PID regulators, load management
functions
•  Reduce noise, resonance avoidance
•  Performance and flexibility, range settings, above base
operations
•  Easier installation and settings, drive mechanics
•  Can be controlled with Automation, Communication networks
29
Steps to Obtain Pump
Optimization
30
Pump Optimization
Complete a detailed Pump Assessment
Pumps are usually consuming more energy than necessary:
●  The pump is oversized and has to be throttled to deliver the right amount of
flow. Energy is lost in the valve.
●  Pumps that are not running close to their best efficiency points (BEP) operate
at lower efficiency. Throttled pumps usually fall into this category.
●  Pumps are running with by-pass, or recirculation, lines open.
●  Pumps are running although they could be turned off.
●  The pump is worn and the efficiency has deteriorated.
●  The pump/system was installed or designed incorrectly (piping, base plate
etc.)
31
Pump Optimization
Complete a detailed Pump Assessment
To determine whether these reasons apply, some basic information
is needed:
●
●
●
●
●
Actual system demand (flow and pressure)
Operational flow rate as a function of time (the duration curve)
Flow controls
The pump curve
Where the pump operates on the curve
32
Process Energy Optimization
Automation is the Key
•  Develop consistent and appropriate milestone and deliverable expecta4ons •  Standardize program schedule tracking requirements •  Establish key energy management performance metrics •  Produce meaningful reports that allow for clear and concise decision-­‐making •  Install addi4onal monitoring equipment as needed 33
Considerations for
Variable Frequency Drives for
Water and Wastewater
34
VFD Topics
● Type(s)
● Enclosure / Environment / Packaging
● Harmonics/ Harmonic Mitigation IEEE 519
● Accessability
● Sustainability
35
VFD Considerations
● The industry has standardized on PWM 6 pulse drives.
● Where 6 Pulse refers to the Front end of the
Drive and a bridge of 6 diodes converting
incoming AC to DC power.
● A DC Bus (capacitor)
● Insulated Gate Bipolar Transistors (IGBT) as
the output components.
● The output of which generates a simulated
RMS waveform with a constant V/Hz ratio.
36
One of These…
37
Packaging…
● Open
● Enclosed
● MCC
38
Harmonics Reduction
● This continues to be a big topic for us in Water and Wastewater
● The Motor loads on VFD’s are a large
percentage of the total load.
● Many Consultants have standardized on designs by HP requiring Line
reactors or Multipulse Drives (typically 18 pulse).
●  There are multiple sloutions
● One size does not fit all.
●  Schneider Electric offers as standard…18 pulse VFD, Matrix Filter
VFD and Active Harmonic Mitigation.
39
18 Pulse Drive Using the same 6 Pulse Inverter…
● STD 6 Pulse Inverter
● Line Reactor
● 18 pulse Diode
Bridge
● Phase Shifting
XFMR
40
Matrix Filter Drive using the same 6 pulse Inverter…
STD 6 Pulse Drive
Matrix Filter
41
Matrix Filter Drive
● Matrix Filter Drive Harmonic Mitigation as good or better than 18
pulse.
● Better Mitigation given Voltage Imbalance
● Footprint of Drive is Typically smaller than 18 Pulse.
● Efficiency of Drive is better than 18 Pulse
●  Losses of 18 Pulse bridge + Transformer +
Line Reactor > Matrix Filter.
● Cost is Typically Lower than 18 Pulse
● Output to the Motor is Identical.
What’s Not To Like?
42
● Data on side by side
comparisons of 18
Pulse and Matrix Filter.
43
Accusine used with one or many 6 pulse drives…
44
The Variable Frequency Drive for W/
WW
●  The Altivar ® 61 is our Standard 6 pulse inverter for variable
speed applications used in centrifugal pump and fan / blower
applications offering the highest level of features, functions,
and flexibility.
This same inverter is the heart of our configured enclosed
applications, 18 Pulse Drives, Motor Control Centers and our
new Matrix Filter Drive..
● All the Inverter parts, programming,
troubleshooting, wiring, interfacing, etc is
common.
● 45
Other Drive/System Application Considerations
● Enclosed Drive or Packaged Drive Short Circuit Current Rating.
● SE = 100k amps as standard
● Power Loss Ride Through – especially for pump stations.
● SE meets Semi F47 standards
● Communication Capabilities
● SE offers Modbus Serial and 11 additional
Protocols as options.
● Built in Web Server and diagnostic web displays
with Ethernet.
● 46
Appendix
47
● Standard
Six Pulse
Inverter in…
● 6
pulse
Drive
● Drive in
● MCC
● 18 Pulse
● Matrix
Filter
Drive
● 48