Variable Speed Waste Water PumpingChallenges & Opportunities

Kristel ZamanMarketing & Business Development Manager – Flygt Large SystemsXylem North America

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

Background

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

What’s Special with Waste Water Pumping?

Pump sump and system analysisPump controls and variable speed drive Specific energyPump mechanical & hydraulicProcess control Pump sump level control

VFD + Wastewater Pump Aspects:

Pump and Pump System Aspects- Pump Sumps

• Floating debris

• Sedimentation

• Odor problems

• Clogging

Snoring level at cleaning

Stop level

Start level

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- High Energy Consumption or Risk of Sedimentation?

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

Specific Energy

10

The most useful measure to compare energy efficiency in pump systems

Inflow Duration Diagram for Waste Water Pump Station

Process control aspects-Variable speed pump systems

12

Reducing the total pumping head

Maximizing pumping efficiency

Hstat

How can we minimize energy consumption?

Process Control Aspects-Minimizing energy usage

13

ηH

VolumeEnergyEnergySpecific ∝=

Energy optimization function – SmartRun VFD

Specific Energy value

Pumping cycle 1

Pumping cycle 2Pumping cycle 3

Every pump cycle a new speed is being calculated based on the energy efficiency of the last cycle

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

10 20 30 40 50 60

Pump Hydraulic Aspects- Clogging is Common• 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

16

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

+ Better process control + Energy savings + Smoother operation + Reduced maintenance

costs

- No energy savings - Increased energy use - Increased clogging- Increased

maintenance costs

Opportunities and Challenges

Conclusion:All pump system and pump station aspects need to be considered in order to achieve a well operating pump station.

Flood & Wastewater PumpingComplete Solution

PDF of the white paper

Questions?

Thank you!Kristel ZamanMarketing & Business Development Manager – Flygt Large SystemsXylem North America

Kristel.zaman@xyleminc.com