Variable Speed Waste Water Pumping
Challenges & Opportunities
Kristel Zaman
Marketing & Business Development Manager – Flygt Large Systems
Xylem North America
Background
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VFDs have become common during the last 10-20 years
Pump industry is looking for ways to increase operational flexibility
Budget constraints drive the need for energy savings and efficiency
VFD prices have steadily gone down
Reliability has gone up
Intelligent pump controls are being
introduced and are gaining ground
What’s Special with Waste Water Pumping?
Non-clog pumps are designed to function best at a certain speed
Modern Trash - Tough stringy solids tend to cling to impeller leading edges
High starting torque and operational torque needed
Reduction in speed (flow) equals decrease in power, this is reduction of
power that keeps a pump clog-free
 Pipe velocities must be above certain minimum value to avoid
sedimentation of heavy solids
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VFD + Wastewater Pump Aspects:
Pump sump and system analysis
Pump controls and variable speed drive
Specific energy
Pump mechanical & hydraulic
Process control
Pump sump level control
Pump and Pump System Aspects
- Pump Sumps
• Floating debris
• Sedimentation
• Odor problems
• Clogging
Start level
Stop level
Snoring level at
cleaning
Pump and Pump System Aspects
- High Energy Consumption or Risk of Sedimentation?
When flushing the discharge line on regular basis, it is possible to reduce the
minimum fluid velocity below 2.5 feet/s without having sedimentation problems.
Pump and Pump System Aspects
• Check Valves
• Swing Check Valves have lower
friction losses at lower velocities
than ball type check valves.
• Water Hammer
• VFD’s can be used to soft start
and soft stop pumps.
• Won’t soft stop in power fail
• Minimum Speed Limitations
• Cooling Systems
• Shaft Resonance
• Recirculation
• Other factors
Pump Controls and VFD’s
• External Control
• Programmable Control
• Programmed for pump alternation, level control, speed references
• Internal Control
• VFD controls each pump individually based on program inputs.
• Inputs are connected directly to the drive and the drive has
program variables for control based on the inputs.
• No external controller
EMC and Motor Cabling Requirements
Ensure a power supply free from excessive harmonics and follow good EMC
(Electromagnetic compatibility) engineering practices:
Built in Harmonic Filters
Addition of Output Filters for long cable lengths
Protect against voltage spikes due to switching frequency
Use the shortest possible power cables
Use shielded sensor cables and position the cable shield as close as possible
to the connection terminals
• Separate signal and power cables by more than 1.5ft in parallel runs
• Use twisted sensor leads along the entire cable route
• Ground the cable screens at both ends
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Specific Energy
The most useful measure to compare energy efficiency in pump systems
10
Inflow Duration Diagram for Waste Water Pump Station
Process control aspects
-Variable speed pump systems
How can we minimize energy consumption?
Maximizing
pumping efficiency
Reducing the total
pumping head
Hstat
12
Process Control Aspects
-Minimizing energy usage
Specific Energy =
13
Energy H
∝
Volume η
Energy optimization function – SmartRun VFD
Specific Energy
value
0.7
0.6
0.5
0.4
Pumping cycle 1
0.3
Pumping cycle 2
Pumping cycle 3
0.2
0.1
0
10
20
30
40
50
60
Every pump cycle a new speed is being calculated based on the
energy efficiency of the last cycle
Pump Hydraulic Aspects
- Clogging is Common
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Long operational times tends to pack solids hard in impeller
Lack of pump cycling eliminates beneficial back flushing
Reduced speed reduces available energy at impeller to pass solids
Soft stopping prevents clogs from dislodging naturally
Pump mechanical aspects
- Reverse rotation
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Maximizing Pump Efficiency in VFD Systems
Select pumps that:
• Deliver sustained efficiency (self-cleaning pumps)
• Operates to the right of the best efficiency point at full
speed in order to achieve maximum efficiency when the
speed is reduced.
• Determining the optimal performance of a typical
variable speed pump system requires the analysis of
pump curve
Pump Sump Level Control at Variable Speed
- Two Level Control Principles
Traditional constant level control
Optimized constant level control
Opportunities and Challenges
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Better process control
Energy savings
Smoother operation
Reduced maintenance
costs
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No energy savings
Increased energy use
Increased clogging
Increased
maintenance costs
Conclusion:
All pump system and pump station aspects need to be
considered in order to achieve a well operating pump
station.
Flood & Wastewater Pumping
Complete Solution
PDF of the white paper
Questions?
Thank you!
Kristel Zaman
Marketing & Business Development Manager – Flygt Large Systems
Xylem North America
Kristel.zaman@xyleminc.com