Applying Variable Frequency Drives in Water Treatment
Introduction
Over the past five years, the application of variable frequency drives (VFD) in the water treatment processes has seen significant growth. Several factors are driving this change:
- Changes in the water and waste-water treatment processes from batch to continuous
- Advances in the reliability of VFDs
- Escalation of energy costs
This article examines each application of the VFD in the water treatment process, industry trends, and provides insight into the selection of medium voltage drives over low voltage drives for 500-1000 hp applications.
VFDs in Water Treatment
Table 1 details the function, horsepower range, and trends of each VFD application in the water treatment process.
|
Step in Process |
VFD Usage |
VFD Function |
Range of hp |
Notes |
 |
70% |
Raw Water Pumps
Regulate water flow through the plant, matching customer demand |
100 - 1500 |
In the past, water treatment processes ran in a batch mode, operating at full throttle throughout the day filling up the storage tanks and shutting down at night.
A trend has emerged in recent years of running the process in a continuous mode, regulating the flow to match demand. This has resulted in better regulation of the chemical processes and less wear on the mechanical equipment. |
 |
70%
|
Rapid Mix Motors
Regulate the rate of mixture between chemicals and raw water |
< 20 |
The feed rates of both raw water and chemicals vary in the rapid mixing chamber, prompting the need for a variable rate of mixture. |
 |
50% |
Flocculation Mixer Motors
Regulate mixing speed for formation of floc particles |
1-5 |
In this step of the process, floc particles are formed. Dirt particles accumulate to form floc particles, which are large enough to settle by gravity in the sedimentation basin. Mixing at the proper rate is essential for the formation of floc particles. |
 |
30% |
Filter Backwash Pumps
Regulate backwash flow into the filters |
25-75 |
In the past, backwash flow has been regulated with throttling valves. Using a VFD to control the flow saves energy and mechanical wear on the piping system. |
 |
90% |
Coagulent Chemical Feed Pumps
Regulate chemical feed based on flow rate of raw water into the system |
1-10 |
In the past, the coagulent chemical feed rate has been regulated with throttling valves. Using a VFD to control the flow saves energy and mechanical wear on the piping system. |
 |
35% |
High Service Pumps
Regulate pressure in the water distribution system |
100 - 1500 |
Running the water treatment process continually, regulating flow to meet demand, requires pumps to have a VFD versus on/off control. |
Table 1
VFDs in the Wastewater Treatment Process
Table 2 details the purpose, horsepower range, and trends of each VFD application in the wastewater treatment process.
|
Step in Process |
VFD Usage |
VFD Function |
Range of hp |
Notes |
 |
50% |
Conveyance Pump Station
Regulates the flow of sewage from the collection areas to the treatment plant |
2000-4000 |
Based on the time of day and weather, there are large fluctuations in the rate of incoming sewage for treatment. |
 |
100% |
Main Lift Pumps
Regulate flow to plant for optimal treatment, regardless of incoming sewage from the collection system |
10 - 2000 |
A wastewater treatment plant is designed for a particular capacity that dictates the rate at which it can treat sewage. VFDs on main lift pumps regulate the flow to meet the plant's rate of treatment. |
 |
90% |
Chemcial Feed Pumps
Regulate the chemical feed rate to match the flow of sewage being processed |
1-20 |
In the past, chemical feed rate has been regulated with throttling valves. Using a VFD to control the flow saves energy and mechanical wear on the piping system. |
 |
20% |
Aeration Blowers
Throttle back the power when process allows, saving electrical energy |
50-1000 |
Energy savings realized by running a motor 25% of the time at half power, versus full power, is substantial over the course of equipment life. |
 |
75% |
RAS Pumps
Regulate the flow of return activated sludge (RAS) back to the beginning of the process |
50-1000 |
In the past, RAS feed rate has been regulated with throttling valves. Using a VFD to control feed rate saves energy and mechanical wear on the piping system. |
 |
75% |
WAS Pumps
Regulate the flow of waste activated sludge (WAS) to the dewatering process |
50-1000 |
In the past, WAS feed rate has been regulated with throttling valves. Using a VFD to control feed rate saves energy and mechanical wear on the piping system. |
 |
75% |
Digester Centrifuge Pump
Regulate feed rate of sludge to the centrifuge dewatering step in the process |
50-200 |
In the past, sludge feed rate has been regulated with throttling valves. Using a VFD to control feed rate saves energy and mechanical wear on the piping system. |
 |
90% |
Effluent Pumps
Regulate the discharge flow rate per regulations in the region |
1000-1500 |
Each municipality in North America has different regulations on the discharging of treated wastewater. These regulations, coupled with the flow of water through the process, dictate the rate at which treated wastewater can be pumped into a river or lake. |
Table 2
Medium Voltage Versus Low Voltage
In 1995, using a medium voltage drive (2300/4160 Vac) would not have been considered for a 700 hp main lift pump application. Alternatively, a low voltage (480/575 Vac) motor and drive would have been selected due to several factors:
- Cost of the medium voltage drives was far greater than a low voltage drive
- Most maintenance personnel were not trained in medium voltage maintenance procedures
- Lack of installed base in medium voltage drives
Today, the increased reliability and advances in medium voltage drive technology have changed this decision process and the higher voltage drives are becoming popular in main lift pump applications. Consider the factors in Table 3.
|
Factor |
Winner |
Notes |
| Cost |
No clear distinction |
- If the user is willing to accept the utility harmonics created from a 6-pulse standard low voltage source, the low voltage is the lowest cost up to 1000 hp.
- If the user is concerned about utility harmonics and compares purchases the 18-pulse source commonly offered by low voltage drive vendors, the cost of medium voltage is competitive above 500 hp.
(See the "Cost of MV vs. LV AC Drives" chart (Figure 1) following this table.) |
| Cabling |
Medium Voltage |
The size of a motor cable correlates directly to the amps it is designed to convey. Also, for a given horsepower, the current required is proportional to its voltage. The table below illustrates the dramatic difference in current required for a low voltage and medium voltage motor:
|
700 hp AC Motor |
Full Load Amps |
|
480 V |
860 |
|
4160 V |
100 |
Assumes both motors have:
- Efficiency of 0.954
- Power Factor of 0.765
|
| Simplicity of hardware |
Low Voltage |
Medium voltage drive technology incorporates two significant elements of complexity that low voltage does not have:
- Fiber-based communications between the control circuit board and gate driver board in the power bridge
- Medium voltage switchgear
|
| Required training |
Low Voltage |
Most industrial plants have 480 Vac equipment, familiar to most technicians. A relatively small number of plants have medium voltage equipment and technicians may require a several days of special training to become familiar with it. |
Power system harmonics |
Medium Voltage |
Medium voltage sources are typically made up of an 18 or 24 diode rectifier that meets the IEEE 519-1992 standard (<5%) for power system harmonics. A standard low voltage source is a 6-diode rectifier that does not meet the IEEE 519-1992 standard. Low voltage drive vendors commonly offer 18-pulse sources as an option.
In the pricing chart below (Figure 1), these options are identified with a third choice (Mirus filter) to reduce power system harmonics. |
| Footprint |
Medium Voltage |
Drive capacity is a key element in its footprint. A medium voltage drive in the 500-1000 hp range can typically fit in half the space of an equivalent low voltage drive.
|
Table 3
Figure 1
John Hamby is the Manager of General Industries Engineering for GE Toshiba Automation Systems, LLC
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