Part 18(g)—Electrical Systems and Control Methods 2
By Ed Butts, PE, CPI
Numerous methods exist to control and operate a water system with single or multiple pumps. In the January 2022 installment of The Water Works, we introduced the concept of basic water system control as it applies to single and multiple pumping units.
In this month’s second and final installment outlining water system control methods, we will discuss multi-variable measurement techniques and demand versus scheduling control logic along with control of common water system types such as irrigation, industrial, and municipal water systems.
Multi-variable instrumentation generally means the measurement of more than one independent process variable in a single stand-alone instrument. However, for purposes of water system control, the definition should be expanded to include multiple variables from one or more instruments.
The most common variables used for water system control are head (pressure) and flowrate. Typically, fixed water level, pressure, and flowrate setpoints are digitally read using a float switch, pressure switch, inline flow switch, or other similar device. Variable head (pressure) is measured using an analog pressure or water level transmitter.
Variable flowrates are measured using an inline type of flowmeter with an integral transmitter, which can include a propeller, magnetic, vortex, orifice or V-cone (differential pressure), ultrasonic (time of travel) meter for instantaneous flows, or Coriolis meters for mass flows.
There are often numerous advantages to using both pressure and flow as combined process variables for water system control as they usually mirror real-world water usage patterns.
Typically, increased demand of water is correlated to a decline in pressure and decreased demand is associated with both an increase in pressure and associated reduction in flowrate. Although the use of a single process variable, pressure for example, is generally used and works well for domestic and other small water systems, this value alone does not generally reflect an accurate representation of system demand.
This is where the use of flowrates—from either single or multiple sources or the use of variable frequency drives with low and high flowrate setpoints—can provide a viable alternative to pressure alone.
In fact, as the water system increases in size and diversity of demand, the use of both pressure and flowrate as control parameters is preferred.
An effective water system control design must not only consider the types of demands and water use patterns but must also include consideration of the minimum and maximum flowrates of the individual pumps, maximum allowable working pressure, NPSH (when relevant), and type of driver.
The driver, although usually an electric motor, can also consist of an internal combustion engine, particularly in a generator, backup, or standby role such as diesel, natural gas, or propane fueled engines.
In these instances, the system must include factors of minimum and maximum engine speed, engine starting, operating, and safety shutdown parameters, and how to interface control requirements with the engine (i.e., fuel controlled such as solenoid valve, servo or throttle controlled, or electrically controlled [ignition].)
Isolation of the DC to AC interface is particularly important to prevent any possible crossover of the voltage systems that could result in personnel shock, system failure, or equipment damage. This should be performed with physical isolation and appropriate color coding of the two systems.
As with most things, a control system should be designed with the most reliable but least number of components and simple circuitry to improve reliability and future troubleshooting and prevent possible system failure due to the loss of a single component that could disrupt or shut down the entire process.
In some cases, especially with critical need systems, use of redundant components or circuit paths is often warranted and will greatly increase the reliability of the system. This might include dual batteries and automatic switchover on a low DC voltage signal, backup starters and fuel supply, and redundant relays and other components that must reliably operate as elements of the engine starting cycle. In many cases, this same design philosophy can also be applied to AC driven equipment.
Demand Versus Scheduling
In addition to the open or closed loop control mode used to determine how to operate a pump, there are two distinct parameters used to determine when to operate a single or multiple pumping units: system demand and pump scheduling.
The demand method responds to changes in pressure, water level, flowrate, or other system variable, where the scheduling method only depends on the change of time. Obviously, this makes the demand method much more unpredictable for pump operation, as the system must immediately respond to changes occurring with the water demand to activate or deactivate pumping units.
The demand method is far and away the most common method of pump control with a water supply system. It is used for the majority of domestic, commercial, industrial, irrigation, and municipal water systems.
The scheduling method, although not as common, still has a place in water system control. The scheduling method can be used with water systems that use a predictable volume of water between a given timespan to provide the demand or replenish the depleted water storage.
The most common form of scheduling occurs with automatic irrigation systems. These systems use an automatic sprinkler controller that is programmed to activate control valves and a pump. These systems are generally programmed to operate the irrigation system during the early morning hours before the daytime heat can cause the evaporation of applied water.
Scheduling on this basis also benefits a municipal water utility as the irrigation demands do not coincide with the higher water system demands during the daylight and early evening hours—lowering the demand on the water infrastructure.
A quasi-scheduling method is commonly used with lower capacity wells to deliver the volume of water stored in the wellbore by calculating the differential between the inflow rate versus the stored volume in the wellbore. This is often used with water reservoir systems with marginal source capacity to provide slow refilling of the reservoir during the late evening and early morning hours for household use during the daytime hours.
Another application of scheduling occurs in municipal water systems where well or booster pumps operate during a predetermined interval to refill water storage vessels. This also often occurs during the late evening to early morning hours when water system and power demands are the lowest.
This method can provide dual benefits to the water utility by using power at lower use periods, which, depending on the rate schedule, can mean lower power rates and when frictional losses in the system are also the lowest, reducing the total dynamic head per gallon of water pumped, conserving pump and motor horsepower.
Scheduling can be performed manually using an interval timer or automatically by using a pre-programmed seven-day timer, such as an irrigation controller or cycle timer.
Finally, scheduling can be effectively used in wellfield applications when recharge to each well is limited, although predictable and verified. Operational rotation between the wells can be used to extract the stored water volume that occurred during well recovery. But the use of scheduling for this purpose requires a supplemental low-water-level-cutoff safety control to shut down and protect the operating well pump should a low-water-level condition occur.
Depending on the precise aquifer and well conditions, sequencing of the wells can occur in a radial, circular, or grid pattern. In some cases, seasonal changes to programming are possible to take advantage of removing greater volumes of stored water.
In wellfields with sufficient dimensions, simultaneous operation of two or more adequately spaced wells may also be possible. Typically, using this method for this application should follow accurate modeling of the aquifer and a period of well yield testing with a minimum operating reserve of 15%-20% of stored volume as a recommended safety factor.
Proper pump scheduling is another method of improving system reliability as well as system and unit efficiency. The purpose of pump scheduling is to plan the operation of pumps over a specific time period and at optimum delivery pressures to efficiently meet the current water consumer demands.
This is often ignored due to the perceived complexity involved in facilitating pumping unit operating changes and schedules. Optimizing this function has proven to be a practical and highly effective method in reducing operating costs without significantly altering the actual infrastructure or reprogramming of the entire pumping or control system.
Pump system scheduling should be oriented around the combined use of the most efficient units to meet current demands, but retaining enough flexibility to handle any conceivable operational scenarios or anomalies and demand changes. It should also ensure the system can functionally handle all potential operating conditions, such as inserting the use of pumps with variable speeds, constant speeds, and cycling units into an operating step, as well as examining potential anomalies.
Modern day SCADA systems and water systems controlled by programmable logic controllers (PLCs) generally possess the flexibility to easily make these types of control setpoint revisions.
Irrigation System Control
The control of an irrigation system can be divided into three principal methods: manual, semi-automatic, and fully automatic.
Manual control simply uses the discretion of the operator to determine the operating time and which sprinkler or zone must be watered. If multiple zones are used, the operator typically isolates the zones where watering is not needed and opens the valve to the zone needing water. Following a predetermined period of irrigation, the water supply is shut off, as with a gravity source or inline valve, or the pump is disabled. Depending on the specific system, lateral lines may be relocated to the next irrigation set or the valves controlling irrigation are swapped to the next set and the process repeats.
A semi-automatic irrigation uses an interval timer to operate the pump or control valve. This timer is positioned by the operator to a number of preset hours based on the required depth of watering or the computed time needed to perform a traveler run or solid set Big Gun full or part circle. Once the timer reaches the end of the timing sequence, the pump or valve shuts off and irrigation is suspended.
This type of system generally requires manual relocation of the sprinklers and laterals between sets and is most common with traveler and solid set systems, although systems are built that use manual valves to individual and established sprinkler zones.
A fully automatic irrigation system is commonly used with underground pivot, linear, landscape, and turf irrigation systems on areas such as large fields, golf courses, landscaped areas, and residential lawns. For turf watering, this type of system generally uses either automatic, low voltage solenoid-operated control valves that provide water to a specific zone or bank of sprinklers at the same time, or a valve-in-head sprinkler that uses a solenoid-operated valve just below the sprinkler head.
The zonal control valves are often used for smaller grass and landscaped areas, such as residential and commercial lawns and gardens. The valve-in-head method is most commonly used for larger landscaped areas such as parks, causeways, highway dividers, and golf course fairways, greens, and boundaries.
Both methods use a pre-programmed electronic automatic timer or controller that routinely rotates operation to the valve solenoids, commonly called stations, on a daily basis. The controller is programmed to provide operational time to each station, typically around 10 to 20 minutes per day on most systems. The controller steps through each station in progressive sequence to provide water to replace the previous day’s water loss from plant uptake and evaporation.
Although most landscape and turf irrigation systems derive water from municipal or other potable water systems, some obtain water from a separate water source using a pump. Most controllers are equipped with a master valve output that also operates concurrently during each station operation. The master valve output is generally used to operate the supply pump when one is used.
Many controllers are also equipped with rain gauges that permit skipping watering on days when adequate precipitation has offset the need for water.
Automatic control systems for agricultural irrigation using mechanized equipment, such as a pivot or linear, generally utilize a pump run command sent from the master control panel at the pivot point or power tower in the case of a linear. Command signals can be directed through a buried cable to the pump site or by using proprietary radio control systems.
Typically, the pump is programmed to operate over a predetermined interval based on the calculated time required to perform a circular run in the case of a center pivot, or at the end of a run for a linear. Some irrigation systems without remote control capability can shut down the pump using a high-pressure or low-flow signal at the pump site triggered by a valve on the irrigation machine’s inlet that partially closes when the irrigation requirement is met. This method must be carefully coordinated to ensure dangerous high-pressure conditions cannot occur which can damage pipelines.
Industrial Water System Control
Control of an industrial water system must be tailored to the specific water requirements, source capacity, and demand timing of the facility. There is no one-size-fits-all control philosophy that can be applied to most industrial facilities.
Facilities that employ uniform rates and volumes of water throughout the day or work shift are much more predictable than those that use irregular flowrates and volumes, requiring variable flow performance from pumping units or multiple pumps with various design flowrates.
Most large industrial facilities with an independent water supply choose interim elevated or ground level water storage to allow the sources to operate at a sustained flowrate and provide reserves for peak demands and possible interruption of source pumping that could necessitate a plant shutdown.
This type of system is particularly advantageous to canneries and food processing plants where water demands frequently vary with the type of crop and time of year (season).
Systems with variable demands but multiple well or booster pumps with uniform best efficiency point (BEP) flowrates can often be controlled by dedicating one or two of the pumps to a variable speed drive (such as a variable frequency drive), with the remaining units reserved for full rate delivery.
The variable speed units operate within a range of minimum flow up to just below full rate delivery, switching to a fixed speed, high-capacity unit when the flowrate exceeds the efficient capacity of the variable speed unit. This provides an efficient and effective use of all pumps in the system, reduces operating hours on the units, and avoids sustained operation at inefficient flowrates.
Municipal and Potable Water System Control
Controlling a municipal or other potable water system pumping plant requires consideration of several elements, including the type and size of the system, flowrate variances, number of pump stations and individual pumps, whether a booster or water well system is used, and allowable water levels for reservoir storage or pressure range for closed-loop systems.
Closed-loop water systems with adequate hydropneumatic (pressure tank) storage and only one or two properly sized pumps can usually be controlled by using standard pressure switches for on/off control. If two matched units are used, implementing pump rotation with an alternator is desirable, as it will cut operating hours for each unit roughly in half and greatly extend the operating life of each unit.
Large closed-loop water systems must incorporate multiple pumping units with various efficient flowrates or variable speed-controlled units. Potable water systems are unique in that the typical flowrate may vary by a multiple factor of 10 to 15 during a 24-hour day.
This means a water system may experience a minimum demand of 50 GPM during the early morning hours and increasing to 500 GPM or more during the late afternoon to early evening hours. This type of water system can benefit by using the same pump to flowrate matching scenario outlined earlier when we discussed industrial water system control.
Control of a potable water system must not only include consideration for the residential demands and timing, but any commercial and industrial demands that may be placed on the system. In some cases, a uniform daily water demand from industrial water users will flatten the daily use patterns, creating a more predictable control scenario.
This concludes this installment of The Water Works. The next column is in the July issue and we will continue our discussion of electrical and control systems for water systems with an introduction to programmable logic controllers, often referred to as PLCs.
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 firstname.lastname@example.org.