Engineering of Water Systems

Published On: September 19, 2023By Categories: Pumps and Water Systems, The Water Works

Electrical System and Control Methods: Multiple Pump Control System

By Ed Butts, PE, CPI

Several columns of The Water Works have outlined some of the many ways to control a pumping plant. These have involved supervisory control and data acquisition (SCADA) systems, remote control using hardwire or radio transmission, and local control methods where the controlled unit is adjacent to the controller.

Although each method is fundamentally acceptable, reducing the complexity and thus the initial and operating costs of the control system is generally the preferred objective, particularly when the sites are remote from the master controller.

With this in mind, we will continue our discussion on control systems with an example of a real-world scenario for an industrial water system with multiple well sites. We’ll feature various control methodologies along with the advantages and disadvantages of each method. At the end of the column, I will disclose which system was chosen and the reasons for its selection.

Also know that I am not saying that one or any of the following control systems is the best choice for every scenario. The following option was simply deemed the best for this application.

 

System Background

Figure 1. Control option #1. Analog input pump activation and flowmeter decline analog input deactivation.

The actual system involved a feasibility study I performed a few years ago for a four-site deep well pumping system for a large food processor in the Midwest.

Due to schedule variations in the type, volume, and processing requirements of received crops, the daily water demand is subject to extreme ranges of demand and use. Each site is equipped with a 100 HP vertical turbine well pump with identical make and model, each designed for a rated condition of 1000 GPM at 87 psi (200 feet TDH) of delivery head at the wellhead.

The pumping lift to the surface was typically within 10% of each other well. As a standard element, each well pump motor is equipped with a pulse-width-modulated type of variable frequency drive with analog control input capability.

The VFDs are provided for slow ramping pump speed during starting and stopping to control line surges, plus an added control function if desired by the owner. A 480-volt, three-phase electrical service is present at each site with a four-wire control cable planned to be routed from each well to the main plant.

All four wells pump into a single 14-inch or 16-inch common transmission main with a 10-inch submain routed to each well. The wellfield is situated on a single 160-acre parcel with each well located within 1500 feet of each other and no more than 2000 feet from the central controller to be located at the main plant, which is adjacent to the water’s common entry point.

The client desires to use the least complex control system that will function accurately, flexibly, and reliably between a pressure range of 70 psi to 95 psi at a minimum flow rate of 250 GPM up to a maximum of 4500 GPM, but without the need for constant adjustment, maintenance, or service.

Figure 2. Control option #2. Discrete input pump activation and deactivation (on-off control).

The potential of adding two more identical sites within 10 years for a total of six sites must also be considered.

Control System Selection Process

For this proposed control system, following an initial cost-benefit and technical analysis and evaluation to eliminate the least favorable and unfeasible options while establishing the most viable options, four separate methodologies were ultimately selected and forwarded for the owner’s consideration.

Each viable control system included consideration of many factors, including:

  • Impact from blended operation (the use of multiple pumps at separate sites but in simultaneous service)
  • Transitional functions (pump starting and stopping sequencing and hand-off or exchange control procedure)
  • Logistics and impact of variable speed operation on each well pump
  • Hydraulic surge control at the wells and within the transmission system and plant
  • System and unit efficiency considerations and minimum flow variables
  • Available unit redundancy
  • Required operating flow ranges with economic considerations of individual units
  • Individual and system-wide (multiple units) wire-to-water efficiency, well efficiency, and power utilization variables if any (i.e., power factor penalties, demand factor, etc.).

Many of these considerations required the use of complex algorithms with computer modeling and analysis while others required basic statistical, hydraulic, and economic analyses.

Commonalities of each pump and proposed control method included consideration of the minimum safe continuous flow rate for the well pumps; the value of this parameter provided from the manufacturer at full speed (1800 RPM) is 450 GPM.

Following consideration of the impact from reduced speed, this value was subsequently lowered to 250 GPM at the proposed minimum pump speed. This flow rate was selected as the minimum design flow for each pump and the system.

Advantages and Disadvantages of Each Control System

Each of the proposed control methods has specific advantages and opposing disadvantages to the operation of the water system. All control methods utilize programmable logic controllers as the primary control mechanism as the operators have a rudimentary knowledge of PLCs from plant experience and hard-wired controls were felt to be too troublesome along with excessive degrees of needed attention. The following represent the primary advantages and disadvantages (pros and cons) associated with each method:

Control Option No. 1

Figure 3. Control option #3. Analog pump activation with flowmeter analog input and two-site VFD frequency deactivation.

The first control option (Table 1, Figure 1) uses a central PLC with a single pressure analog input from the pressure tank. Discrete output activation signals are directed to each well based on input pressures and programming with deactivation triggered by a low-flow analog signal from the flowmeter.

This option provides great flexibility and control redundancy in the event of a loss of a pumping site. The advantages and disadvantages associated with this option are shown in Table 1.

Control Option No. 2

The second control option (Table 2, Figure 2) reverts to a more conventional water system control logic by using a series of pressure switches for pump starting and shutdown. This option also includes use of a hydropneumatic pressure tank for surge control, pump control, and pressure range operation.

Strictly from an engineer’s perspective, this option is the least desirable from an efficiency and control standpoint. The sole use of pressure settings necessitates using high- and low-pressure settings only and negates many of the advantages of incorporating VFDs into the system. However, this option was prepared and forwarded for consideration at the plant operator’s request for a system that would not need to rely on any analog circuitry.

Control Option No. 3

The third control option (Table 3, Figure 3) is a control system option without the use of a hydropneumatic pressure vessel for control and surge abatement and presents the highest risk for pressure surges in the system. This system heavily relies on the use of VFDs for pressure control; thus, the well pumps could be required to rapidly decelerate as well as excessively vary speed to match system pressure settings.

Figure 4. Control option #4. Analog or discrete pump activation with VFD frequency decline deactivation.

This system would rely entirely on system pressure or flow increase for pump activation. Therefore, adequate buffering in the form of time delays would need to be added to prevent excessive pump cycling. The addition of feedback from two VFDs, though, will provide greater flexibility in pump deactivation settings and operating range.

Control Option No. 4

The fourth control option (Table 4, Figure 4) represents the simplest overall control option, but with the greatest risk. Starting of the well pumps is performed by either an analog input or pressure switch. The signal can be variable or a fixed setpoint. Run commands are transmitted through the remote cable to each site.

Once the well pump starts and is running, the frequency output at each VFD senses the decline in motor speed to coincide with the output flow rate. When the signal falls to a predetermined threshold, the pump deactivates.

As with option No. 3, this does not use a pressure vessel. Therefore, the potential for system surges and erratic operations remains high. This was another option requested by the operators.

Figure 5. Well pump control system for the four well pumps.

Selective Flow Ranges

The application of multiple pumping sites with a VFD can be extremely challenging for a designer. Blending in each drive into a pumping system can be even worse when the pumps are identical in model and duty point.

The misapplication of a VFD, the possible maladjustment of the speed reference, use of one pump with a VFD combined to a fixed speed unit, or varying conditions at each well (i.e., pumping water levels, frictional head loss, etc.) can result in one unit overpowering another unit.

During my early years working with VFDs, I ran into this problem several times and often had to improvise a solution. I developed a computer program and spreadsheet approximately 15 years ago to assist with the determination of these variables and multiple VFD coordination. I refer to this as “selective flow ranging.”

Using this process, I can plot multiple variable speed curves with overlapping performance. By incorporating the minimum safe continuous flow, we can determine the lowest safe flow rate for the VFD unit as well as transitional flow rates, or the flow rate when one unit is disabled with the remaining unit left in operation. Many similar tools are available from pump selection software providers to assist designers with these applications.

Figure 6. Pump hydraulic efficiency vs. head.

Selected Option

Ultimately, control option No. 1 was selected for the project. Although the client was not overly excited with the prospect of spending several thousand dollars for a pressure vessel, they were more concerned with the potential that a system-wide pressure surge could cause to their plant and underground piping—and the possibility of a plant-wide shutdown for several days and loss of revenue.

Although VFD feedback was not originally implemented, the selected VFDs possess this capability for possible future addition to the control system. The operational curve and  system head curve at the respective delivery pressures are illustrated in Figure 5. Figure 6 represents the full and variable speed curve for the four well pumps. Note how the efficiency changes with the flow rate at reduced speed. The use of this curve enabled the selection of transitional flows.

This one example of a multiple pump system with a wide range of flows is an illustration of the design power that VFDs and modern control systems now offer to system designers. By incorporating the advantages of variable speed gained from the affinity laws with the many available control components, water system designers can offer their clients almost unlimited water system control options.

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In the next installment of The Water Works, we will conclude this series on electrical and control systems with a discussion on backup power systems.

Until then, keep them pumping!

Learn How to Engineer Success for Your Business
 Engineering Your Business: A series of articles serving as a guide to the groundwater business is a compilation of works from long-time Water Well Journal columnist Ed Butts, PE, CPI. Click here for more information.

Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He can be reached at epbpe@juno.com.

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