Part 18(f)—Electrical Systems and Control Methods
By Ed Butts, PE, CPI
Numerous methods exist to control and operate a water system. Most depend on the type of system and pump/motor size, number of pumping units, needed redundancy and desired reliability, and specific operating criteria.
The criteria depend on the water system size, number of service levels, water storage availability and volume, flow patterns, and time and duration of peak demands.
There are three fundamental parameters of water system control: flowrate, pressure, or water level (with two used together in tandem on occasion). The control modes are generally configured for the system’s applicable operational parameter increase or decrease (inverse) functions using direct or pilot duty operation.
This column is dedicated to the discussion of the various methods and control logic used for the supervisory control of domestic, commercial/industrial, irrigation, and municipal water systems.
Basic Concepts of Water System Control
Before we delve into the various methods used to control a water system, it is incumbent to understand exactly what a water system is and what its control should entail. In this context, a system is defined as a “fixed quantity of matter (in this case, water) contained within a well-defined boundary.”
The boundary could be a pressurized pipeline, a pressure tank, or a water storage reservoir. The state of the system is defined as the “condition of the system, as determined by its properties.” Finally, a cycle is a “process that begins and ends at the same state.” Therefore, a pump cycle is the beginning and end of a process to transmit or pressurize water.
There are two typical properties that comprise and dictate the state of a water system. There is flowrate, expressed in units of gallons per minute (GPM) for imperial (U.S.) units or cubic meters per second (m3/sec) for metric (SI) units, and head, which is measured in feet and commonly referred to as pressure and expressed in pounds per square inch (psi) for imperial (U.S.) units or bars for metric (SI) units.
It is important to understand this long-winded definition because a water system by its nature must be adequately sized, operated, controlled, and managed to maintain its desired state within its boundaries.
This means the selected control system method must be capable of limiting the pressure and flow as required to prevent possible escape from its boundaries at the desired state, which may translate to either making the supply pump operate to maintain the desired state of flow and pressure or shut down once the so-called well-defined boundary is met.
In other words, water system designers must be cognizant that the control system must be obviously able to supply water to customers, but then reactivate or deactivate the supply source as needed to restrain it within its intended boundary and perpetuate its state.
This may consist of continuously reducing the downstream pressure through an inline restriction with pressure-reducing valves, using flow control with flow control valves, or increasing the pressure by activating pumping units which generate an instantaneous increase of the flowrate or established water volume using pumping techniques.
With gravity-supplied water systems, altitude or solenoid valves must open to allow water to freely flow into and refill water storage reservoirs. To summarize: In virtually all cases, an effective water system control must control both components of pressure and flow to keep the system properly functioning within its established and well-defined boundaries.
In some cases, this may require control of both elements, while in others, control of only one element is involved. Although this may appear redundant, it boils down to many electrical, mechanical, and hydraulic components of a water system can be potentially considered as control devices—including pump and driver supervisory controls; variable speed devices; inline pressure-reducing, solenoid, or altitude
valves; offline pressure relief and bypass valves; and assorted safety devices used as override controls to keep the system within well-defined boundaries.
Available Control Methods
There are two fundamental types of water delivery systems used, an open loop and closed loop.
An open-loop water system is the most common type and typically operates against a hydraulic gradeline, with the ultimate destination commonly consisting of an elevated, atmospherically pressure-influenced water storage reservoir, pond, or other water body. This generally includes a component of reliable static head, with the only variable being frictional head losses in the delivery system.
An open-loop system with a dedicated transmission main between the pump and destination will provide the most consistent total head value for determining the output flowrate. Generally, depending on the delivery system sizing and length, the static head comprises between 25% to 75% of the total system or dynamic head.
A closed-loop water system does not operate with an established or fixed hydraulic gradeline, common to systems with elevated water storage reservoirs.
Rather, closed-loop water systems are analogous to most electrical power distribution systems in that they operate with no fixed hydraulic boundary or degree of storage, but deliver water on demand by use of an arbitrary pressure gradient established by the parameters of the water system requirements in capacity (flowrate) and head (pressure) plotted against the same performance characteristics of the pumping plants.
Boiler feed and circulating pumps are common examples of closed-loop systems. They generally result in an inverse system head curve in which the defining operational setpoints are frequently determined by preestablished pump activation setpoints, using declining pressures and deactivation setpoints based on increasing pressures or declining flowrates.
The closed-loop concept is much less a determining factor when calculating the delivery head as variations to the system pressure will significantly impact the total dynamic head.
Closed-loop pumping systems are also greatly impacted from the frictional head associated with the transmission pipeline or distribution system. As water demand will often occur in specific localities or areas at differing times of the day, the frictional head associated with these local changes in flowrate will impact the total dynamic head of the pump, and therefore, the delivery flowrate.
Another potential disruption in flowrate can occur with deep well pumps. Extreme variations in the pumping water level will impact the total output head of the well pump. These changes can have a drastic impact on system and individual pump flowrates as pump performance will vary along its curve in relation to these head changes.
In a system using multiple pumping units, these head changes can result in seemingly minor variations all the way up to major variations in total system flow as each unit responds to the head changes based on their specific head-capacity curve characteristics.
The result of these flowrate reductions or increases can create a difficult control protocol, particularly in a conventional water system. In many cases, the use of automatic pressure-regulating valves or variable speed controllers are used to respond to these variations as well as smooth out the flowrate.
Hydraulic modeling is often used to predict these system head variations and to provide more accurate data for pump design. The following section summarizes the typical methods of control over an open- or closed-loop water system.
Basic Fixed On-Off Direct Pressure Control
The most common water system control mode is obviously the fundamental concept of pressure declining or dropping to close the switch and activate the pump’s starter or control circuit, and pressure increasing or rising to deactivate or open the pump’s starter or control circuit. This concept is also known as the simple “on-off” pressure control used with most pressure modulated types of switching systems.
This type of control mode is usually based on working within water system pressure ranges between 20 up to 125 psig with use of either a snap-action (i.e., make-and-break clearing contacts) pressure activated switch using a differential (i.e., span setting between the on and off settings) adjustable value between 15-40 psig or a mercury tilt pressure activated switch, generally used for tighter differentials between 1.0 up to 25 psig.
Snap-action switches generally function with definite preset settings that are typically not subject to severe hysteresis or flutter. Thus, this group can be used for either direct motor control or pilot duty, while mercury tilt switches are almost always exclusively used for pilot duty with a time delay relay installed in series due to their possible exposure to incomplete or intermittent circuit closure and accompanying flutter.
The snap-action switch predominates the majority of use for domestic and most small commercial water systems as it is simple to apply, install, adjust, and maintain.
Standard pressure switches used for domestic purposes are generally equipped with two-pole, single-throw contact arrangements that allow complete connection and disconnection of 120/240 VAC, single-phase electric motors at specific setpoints. Typically, this group of pressure control devices is rated for 2 or 3 HP at 230 VAC for direct operation or unlimited motor size as pilot duty switches.
Most smaller pressure switches are capable of proof pressures to 300 psig, an adjustable pressure range generally between 40 to 80 psig, and differential from 15 to 30 psig although other ranges and differentials are available.
Switches rated for larger motors are also available to operate up to 7.5 HP single-phase motors. Pressure switches are generally rated in horsepower and voltage, poles, and pressure range, but are typically limited to two-poles.
The horsepower and voltage rating ensures the switch has the electrical capacity needed to safely route the rated motor’s starting and running amperage through the contacts as with an industrially rated control. The number of poles refers to the number of individual electrical load contacts contained within the switch, while the pressure rating refers to the safe working pressure and proof (maximum) pressure of the switch.
A pressure switch’s operating setpoints are determined from two separate adjustments: the range and the differential. The range is used to set the switch’s cut-in point while the differential is used to determine and set the difference between the cut-in and cut-out points of the switch, essentially the cut-out setpoint.
Most pressure switches used for domestic water systems have an adjustable range between 20 to 80 psi and a differential between 10 to 40 psi.
Pressure switches used for pilot duty only need to handle the load and voltage required by the motor starter or intermediary relays used as interposing devices. In many cases, pilot duty pressure switches are used to initially start the pump, and once the pump has started, a parallel contact arrangement from a separate relay is frequently used to lock in the controller, irrespective of the increasing pressure value.
Other parallel control signals, such as a SCADA/telemetry signal, declining flowrates using analog values, or inline flow switches, are thereafter used to deactivate the pump.
Many pressure switches are available with optional features such as low-pressure cut-outs, run lights, replaceable contact blocks and contacts, and manual cut-out levers.
Fixed On-Off Direct Water Level Control
The next two types of control modes use water level control commonly used in an open-loop water system:
- Water level decline to activate/water level recovery to deactivate (pump up)
- Water level increase to activate/water level decrease to deactivate (pump down)
The float switch is another device used for water system control, often found on low-producing wells and cistern systems. A float switch is usually a tilting switch housed within an enclosed shell that either opens or closes when the switch rises or falls with an oscillating water level.
They are commonly used as direct operation motor controls for fractional horsepower motors (such as sump pumps) and for pilot duty applications.
Historically, mercury was used within the switch to engage the circuit. But the trend in recent years has been toward reed switches or other switch types not potentially injurious to the water or environment.
Another device commonly used for reservoir filling and well pump operation is the induction relay. These relays work on the principle of using the water as a conductor of a low amperage signal.
The relays are typically equipped with two coils. One is the primary coil used for the incoming line voltage, which induces power across a bar winding to a secondary coil that generates the low amperage secondary control voltage. Common voltage values are between 110 to 480 VAC.
For these relays to function properly, the conductance of the water, in micromhos, is critical. The water must contain adequate levels of dissolved minerals to conduct the secondary coil voltage through the electrodes, water, and back to the relay to complete the loop circuit.
For problematic installations, or those involving long transmission distances where the pump will either not reliably shut off or start with a typical induction relay, the use of a higher secondary coil voltage (360 to 480 volts) or a solid-state relay will usually correct the situation.
Fixed On-Off Volume Control Methodology
This type of control method is not commonly used with potable water systems, but is often used for industrial and agricultural systems with a dedicated storage volume or usage. This method involves a concept known as batch filling, which is generally reserved for systems with predictable usage and refill volumes, such as time-sensitive reservoir fill, controlled low-volume well tank fill, and chemical transfer and dilution.
Variable Pump Speed Tied to Pressure/Water Level or Flow Control
Water system control using individual parameters of flow and head (i.e., pressure or water level) are one of the most common water system control methods practiced today.
Although variable speed control for water pumps has been used for decades, the recent rise in popularity and advances in programming simplicity of variable frequency drives has enabled designers to employ these devices as an effective tool in generating more efficient pump operation, particularly as the affinity laws are observed.
Variable frequency drives are now used for all types of pumps, although the application of VFDs with well and booster pumps necessitate the added consideration of varying well lifts or inlet pressure to the calculations, which can complicate the speed setting and pump performance when these variable inlet heads are involved.
In addition, variable pump speed controls used with water systems without significant static head (i.e., closed-loop systems) are generally more efficient and predictable for overall control as the added component of a minimum fixed static head in the system is not included.
This permits the designer to factor the frictional head only against the operating flow and head conditions of the pump, which follows known hydraulic characteristics of the affinity laws with the head loss from established friction factors (C values) and pipe lengths. This enables programming of the drive to directly coincide with a system head curve developed from the beginning points of the horizontal component of flow at zero and the vertical component of head at zero with greater head loss linearly associated with increasing pump flow.
Programming the VFD speed reference setting is conducted to disable the pump and motor when the pump speed declines to a preset value. For example, a design may constitute shutdown of a particular unit when the motor speed lowers to 1467 RPM with a 1760 RPM motor, corresponding to a motor frequency of 50 hertz. This provides an easily calculated change in flowrate, head, and horsepower by using the
affinity laws as a guide and can be programmed into the VFD as the shutdown frequency.
Declining Pressure for Pump Activation with Declining Flow for Pump Deactivation
The use of declining pressure to start a pump with declining flow to stop the pump is also a well-accepted and often used control methodology. This control method provides pump activation at a preset low-pressure level and deactivation when the pump or system output flowrate reaches a predetermined low flow setpoint.
This method eliminates risk of rapid cycling by responding to actual system demand by providing water system control using pump activation on declining pressure and pump deactivation on declining flow.
It uses a two-step approach where a standard pressure switch or other similar device is generally used to activate the pump, and the pump is then held through a latching circuit to negate the pressure control aspect.
A low-flow off signal delivered from an inline flowmeter analog circuit, discrete signal from a valve stem actuated limit switch on a pressure-reducing valve, high pressure switch consistent with the desired low flow value, or an inline flow switch is used to deactivate or shut down the pump.
In some cases, pump stepping up is performed by adding pumps on increasing flowrates. This method uses the same analog flowmeter signal that is used for stepping down pumps. This method can also be adapted to SCADA control by using an analog pressure transducer to provide the pump start variable.
A programmable logic controller (PLC), pressure switch, analog sensing relay, or modular controller senses the declining pressure and activates the pump at the appropriate setpoint, generally after a predetermined time delay to ensure the pump activation pressure value is consistent and not subject to deviation, flutter, or temporary but recoverable pressure decline.
Once the pump has activated and is pumping online, primary pressure control is negated, and the pump continues to operate irrespective of system pressure. Pump deactivation is accomplished through one of the methods previously described. The methods will work on all types of pumping systems, especially booster pumping systems with flat curve pumps.
However, all systems should be equipped with a method to ensure protection is afforded for any potential overpressure events. To protect the system against possible overpressurization from pumps with steep curves, well pumps, or due to pilot failure on a pressure-reducing valve, a high-pressure cut-out switch, a pressure-relief valve in a strategic location to exhaust the excess pressure, or an override from the analog signal should be incorporated. This is a critical element when pressure vessels are used in the system.
Multiple Pump Control
Pumps that typically operate in sequence, commonly known as a lead-lag pumping system, is a popular way to meet varying but progressively higher water system demands as well as wastewater pumping requirements using two (duplex) pumping units.
There are two concepts of using primary lead-lag control. The first uses two matched units (i.e., akin to the Pictsweet system with Wells A-1 and B-1) and the second uses dissimilarly sized pumping units.
With matched units, cycling of the lead or first-on pump between the lead pump activation setpoint and deactivation setpoint provides the system’s principal operating range. Increased system demand results in further pressure decline to a setpoint below the lead pump activation value. At this point, activation of a matched lag or second-on pump provides added capacity to the system, typically 150% to 200% of the total plant’s capacity, with the actual percentage dependent on the system head and pump curves.
This type of system adds greater overall reliability in the form of redundancy while increasing the lifespan of the entire system by approximately evening the operating hours. Although redundancy and system reliability are increased by the use of multiple pumping units at the same site, system redundancy and reliability are often increased to a greater degree when the pumping units are situated at multiple sites due to the increase in power (utility), pumping header, and electrical reliability.
Dissimilarly sized pumping units are often designed using a pump capable of 25% to 75% of the total design flowrate as the lead pump. This unit operates as the jockey pump, and to satisfy the lower demands while the second or lag unit is frequently designed for 100% to 150% of the required total design flowrate.
This unit is intended to provide peak demands and can function in place of the lead pump for a short time, albeit inefficiently, in the event of its failure. The system can be further expanded in concept to include three units (triplex), four units (quadriplex), and even five or more parallel pumping units.
In a traditional three pump (lead-first, lag-second, lag) pumping up (pressure) configuration, the lead pump is the first pumping unit activated. It runs until the demand on the system is either satisfied or is too great for the single pump to meet. At that point, the first lag pump initiates, generally at a slightly lower starting pressure setpoint than the lead pump, until the demand is either met or pressure continues to fall. If the pressure continues to fall, the second lag pump (third pump) activates, increasing the system capacity up to 300%.
The proper use of system head curves plotted against pump performance curves can be invaluable in selecting the starting setpoints for the pumps as well as the deactivation flowrate.
The individual pumping plants can be configured to activate on declining pressure or increasing system flowrate and to deactivate simultaneously at individual preset pressures, low flow setpoints, combined (system) low flow setpoints, staggered reduced flow levels, or if VFDs are used under the proper circumstances, at predetermined settings of declining frequency (hertz) that correspond to a lower flowrate.
A lead-lag staggered pumping system can consist of any number of pumps, and they are often alternated to ensure even wear occurs between the units. This creates a round-robin scenario where up to 10 or more units can alternate to fill the lead pump position.
An extra pump added to the system strictly for the purposes of redundancy is known as a standby pump. If the pumps are alternated, however, the system will not have a single standby pump but rather each of the pumps in the run sequence will take a turn (i.e., rotation) as the standby pump.
Many applications require and function best using a leadlag configuration. Some configurations use across-the-line starters while others use solid-state soft starters or VFDs.
Two typical applications, a pressure tank and an irrigation system, exemplify how lead-lag configurations are used, the function of various motor controls in these types of systems, and the basic differences between starters and VFDs in leadlag applications.
A lead-lag alternating drive package will usually provide different alternating modes (such as fault alternation) and lead-lag control based on input signals from pressure transducers or switches and the VFD status outputs.
Adding alternation to a duplex lead-lag drive package will increase the cost of the unit, but in many cases, it is a worthwhile and necessary option. The main reason people choose
VFDs over across-the-line starters is their ability to control pump starting acceleration, reduce hydraulic surges, and vary speed. The ability to vary speed allows a VFD to provide constant pressure to precisely meet varying demands, crucial in many lead-lag applications. VFDs can provide significant energy savings, especially in higher horsepower applications. Thus a VFD will usually pay for itself in time.
This concludes this month’s edition of The Water Works. In the next installment in April, we will continue this topic with additional water system control methods and how to implement them into a real-world application.
Until then, keep them pumping!
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 email@example.com.