Part 11, Pumping Systems
By Ed Butts, PE, CPI In this month’s edition of Engineering your Business, we will head toward wrapping up this more than year-long series on irrigation fundamentals with a discussion on pumping system design for irrigation applications along with an overview on tailwater recovery systems.
Irrigation Pumping Systems
Pumping systems used for irrigation systems can be as diverse as the imagination will allow and the application requires. However, there are four basic classes of pumps used for most types of irrigation systems:
- Vertical turbine pumps
- Submersible pumps
- Centrifugal pumps
- Mixed flow and axial flow pumps.
Vertical turbine pumps and submersible pumps are most often used for deep well service. Centrifugal pumps, generally end-suction styles, are more frequently used for shallow well lift applications (less than 25 feet), generally for sprinkler and flood (surface) irrigation and as booster pumps when needed. Mixed flow and axial flow pumps are often used for tailwater pumping service or as alternates to vertical turbine pumps for high head or transfer service. Some applications have special pumping requirements, but there are many common considerations used in the selection of an appropriate pump for most irrigation applications. Some of these include:
- Required discharge (Q) flow rate and head (conditions of service or COS)
- Adjustable range of flow rate, efficiency, and/or total dynamic head (TDH) if desired or needed
- Suction lift, if any (for shallow well applications; usually dictates use of a centrifugal pump).
- Well lift (for deep well applications; usually dictates use of a submersible or vertical turbine pump)
- Suction, submergence, and NPSH limitations
- Well or sump diameter and/or depth limitations
- Irrigation frequency, seasonal and daily hours of operation
- Required reliability and permitted downtime for repair
- Type, local source, reliability, and unit cost of available energy
- Initial capital cost investment, depreciation, salvage (resale) value, and interest charges
- Physical constraints (Must the pump be set within a limited space such as in a well or small building?)
- Shelter needed or available (Will the pump and/or electric motor need a vented enclosure?)
- Potential for pump damage due to corrosion or scaling from inferior water quality and wear from abrasives (sand or grit)
- Labor and equipment requirement for system setup, operation, and movement between sets or sites
- Scope of available facilities, equipment, and labor for ongoing maintenance and repair.
You may have noticed that none of the preceding factors considers the actual method of irrigation. This is because a pump and whatever driver is used have no concept of where the water is going or how much flow and pressure are actually needed to do the job. Their only need is to deliver the flow rate at the required pressure or head. It’s up to the designer to make sure the pump is appropriate for the application and service conditions. This should generally start with the required flow rate or pump capacity. To that end, irrigation system designers must be aware of the available and most efficient types of pumps for the application and unique service condition—two separate but equally important factors. Each of the four types of pumps mentioned earlier will be outlined separately.
A centrifugal (radial) pump works by converting the applied energy of a rotating impeller to increase the velocity of a liquid by using centrifugal force to generate kinetic energy (velocity) of the liquid. Besides the shaft, the impeller is the only device in the pump that rotates, and depending on the specific type, is usually contained inside a volute/casing or diffuser.
A diffuser and volute are used to convert the value of kinetic energy velocity to the pressure head needed for the specific application. The impeller is usually connected to and driven from an electric motor or engine, which through the pump shaft provides the necessary energy that is transferred to the liquid.
The design procedure for a cold water (less than 75°F) centrifugal pump primarily depends on if the pump is used under a suction lift or suction head.
In the case of a suction lift, the pump must develop enough suction or negative pressure within the impeller eye—a pressure sufficiently below the amount of atmospheric pressure present at the pump’s altitude—so that whatever force of atmospheric pressure remains after inlet, piping, and fluid losses can push the liquid into the pump to relieve the negative pressure.
Suction lifts (ASL) are limited to the available atmospheric pressure present at the pump’s relevant altitude, and thus limited to a maximum theoretical value of 33.957 feet at sea level (equal to 2.31 feet per psi × 14.7 psi of atmospheric pressure). Depending on the specific pump model, this is generally reduced to between 15 feet to 25 feet as the maximum practical suction lift due to a pump’s inherent inability to develop a perfect vacuum as well as limiting factors related to the pumped water itself.
Suction or inlet heads are generally regarded as supply or suction pressures greater than 2 to 4 psi (5-10 feet of head) and typically applied to centrifugal pumps used as booster pumps or with a flooded suction from a pressurized or higher situated gravity water supply.
Limitations and Advantages of Centrifugal Pumps for Irrigation
- Unless a self-priming model is used, the pump requires complete and sustained priming before use
- At sea level and depending on the NPSHR, the pump and motor can be placed up to 25 feet above the water level
- Predictable and versatile performance under most cold-water applications
- Can be driven from various power sources including diesel, propane (LPG), natural gas, or gasoline engines, electric motor, inline turbine, or closed loop hydraulic fluid drive
- Available in various styles and mounting configurations including close-coupled, frame-mounted, power take-off (PTO), SAE bell housing, and belt drive, as well as in vertical or horizontal mounting configurations
- Efficient and wide range of available flow rates, speeds, impeller trims, heads, and pressure ratings
- Can be trailer- or skid-mounted for rapid installation or removal due to failure or danger of flooding
- As a portable pumping unit, a single unit can be transported and used at several different sites
- Typically are easier and less expensive to install and maintain than a vertical turbine or submersible pump
- Excellent versatility for use as either a source pump (within allowable suction (NPSH) limits) or booster pump.
Vertical Turbine Pumps
Vertical turbine pumps are used for many deep and shallow well irrigation direct use applications as well as for sumps, dewatering, raw water lift or transfer, tailwater recovery, and booster pumping.
They are versatile in design and application, with the capacity usually determined by the diameter of the impeller/diffuser assembly (bowl) and the head using multiple impellers (stages) in a stacked assembly to generate the required total pump head.
They are typically used on sets as shallow as 5 feet in sumps or caissons up to 1000 feet or more in deep well applications, capacities between 100 to 5000 GPM or more, heads up to 1000 feet or more, bowl diameters between 6 inches to 24 inches or more, and between 10 hp to 5000 hp.
Vertical turbine pumps are generally intended for 1800 or 1200 RPM operation and use either an open (water- or product-lubricated) or enclosed (oil-lubricated) type of construction. Open lineshaft construction is generally limited to physical pump settings of 500 feet or less while enclosed construction can exceed 1000 feet. In addition, open lineshaft constructions must receive a pre-lubrication of water whenever the static water level exceeds 50 feet before starting the unit to avoid burning or destroying the rubber lineshaft bearings.
Although oil-lubricated pumps are often restricted or banned for use on potable water supplies due to concern regarding possible water contamination, they are commonly used for irrigation service using petroleum products.
Limitations and Advantages of Vertical Turbine Pumps for Irrigation
- Must be vertically mounted and are more prone to vibration and failure from misalignment or a crooked well
- Self-priming and usable for both shallow and deep vertical boreholes, sumps, caissons, and water wells
- Under the appropriate circumstances and when initially primed, they can lift water below the bowl assembly
- Since water is pulled into the lowest level of the pump, they are suitable for a shallow submergence
- Failure of the surface-mounted driver does not require pulling and removal of the entire unit
- Can be driven from an offset PTO, engine, or electric motor by using a right-angle gear drive and driveline
- Less prone to damage from sand and grit than a higher speed submersible pump
- Generally provides a longer service life and is more efficient than a comparable submersible pump.
Submersible pumps are turbine or radial pump types directly attached to an electric motor underneath the pump. They are usually slightly lower in unit (wire-to-water) efficiency than a comparable size of centrifugal or vertical turbine pump, due mostly to the lower motor efficiency.
Many submersible pumps use an identical model as a vertical turbine pump bowl but rotate at twice the speed (3600 RPM vs. 1800 RPM) to generate the higher head needed from the smaller-diameter bowl.
The motor and pump are assembled into a single unit with the motor placed underneath the pump, which keeps the motor totally submerged. The motor depends on the water pumped to pass it for cooling. Thus, a failure of the water supply can result in rapid and serious damage to the unit.
The pump and motor are dimensioned for use and are longer in comparison to their diameter. Due to their lower initial cost and ease of installation versus a vertical turbine pump, they are a popular choice for most 6-inch, 8-inch, and 10-inch wells.
Limitations and Advantages of Submersible Pumps for Irrigation
- The motor is much more vulnerable to potential damage from a lack of cooling than surface-mounted motors and must be continually cooled by water flowing past the motor at an adequate velocity or by using allowable alternative methods
- The pump is more susceptible to damage, up to four times, from sand or grit due to the higher pump and motor speed
- Self-priming and without a long drive shaft, they are easier to install and cost less to purchase than a vertical turbine pump
- Wide flow range, from 2 GPM for drip/micro up to 5000 GPM or more for flood or sprinkler irrigation
- High head and installing depth capability due to high speed, multiple staging construction, and lack of lineshaft
- May be installed on a slanted angle or in a significantly misaligned well or borehole
- May be installed in rivers or areas subject to flooding. As the pump has no above-ground working parts, the electrical and utility equipment can be placed above and away from the flood level on an offset pole.
Mixed Flow and Axial Flow Pumps
Although most groundwater irrigation pumping applications use a centrifugal, vertical turbine, or submersible type of pump design, there are specific applications requiring higher capacity pumping rates at relatively low values of head.
These include those for irrigation supply from shallow wells or infiltration/intake galleries, field dewatering, and tailwater recovery, lift, and transfer where an axial or mixed flow style of pump is often the preferred unit. Although axial flow impellers are typically available in only one configuration, mixed flow impellers can be provided as open, closed, or with double entry designs.
For axial flow, commonly referred to as propeller pumps, these applications typically include and offer higher production rates (more than 1000 GPM) at relatively low and stable operating heads between 10 feet to 50 feet, very efficient performance, and shallow sets.
On the other hand, many mixed flow pumps, while often used for higher production rates, are capable of lower flow rates than many axial flow units but deliver higher head per stage along with multi-staging capability.
Axial flow pumps are available from several manufacturers in a wide range of flows that encompass a range of 1000 to more than 200,000 gallons per minute at heads up to 30 feet per stage while mixed flow units cover a range of flows from 500 to more than 100,000 gallons per minute at heads up to 100 feet per stage.
Both types of pumps can be built using multiple stages to increase the total head capability, but axial flow pumps are generally limited to two to three stages due to potential hydraulic instability.
Flow Rate and Pump Selection
The best pump for a specific application depends on the method and type of irrigation system since the system parameters and how it is operated will ultimately determine the pump’s performance. Descriptions of pumps and their performance will help designers select a suitable pump for the situation.
Centrifugal pumps are diverse machines with a wide range of capacity, up to 10,000 GPM and heads up to 500 feet from a single stage. Therefore, the range of service duties are virtually endless. However, their ability to lift water is extremely limited.
Each irrigation method uses a different amount of water depending on the specific method, application efficiency, applied area of coverage (i.e., flow rate and pressure or the sprinkler characteristics), hours of operation per day, coverage rotation days, and the crop’s daily consumptive use. Each of these individual factors plays a key role in deciding the volume of water needed to satisfy the irrigation needs of a crop.
For example, the hours of daily operation is just one critical factor. If a pump was chosen to deliver 500 GPM over a 24-hour period or continuous operation, the same system would require twice this flow rate or 1000 GPM for a 12-hour per day irrigation operation with all other factors remaining equal.
This alone can decide which type of pump is used. In order to protect against possibly falling behind in watering and risking crop damage, the amount of daily system downtime for repairs and for normal system teardown, movement, setup, and startup between sets means that irrigation systems are rarely designed for continuous operation—with 18 to 20 hours per day (75%-83%) the most common maximum daily period of irrigation operation.
Designers must also determine the amount of water to be applied and consumed during the crop’s peak consumptive use (CU) period by multiplying the size of the field in acres by the amount of water in inches that must be applied. The result is then converted to gallons per minute (GPM) which determines the capacity rating of the pump.
The other design factor—pressure or head—is determined by two variables: the static head or physical lift from the pumping water surface to the point of delivery combined with the dynamic head, which consists of the hydraulic frictional losses from conveying water in piping, valves, fittings, and any inline devices such as filters or screens, and the required pressure at the point of delivery.
Although the term of pressure in pounds per square inch (psi) is generally used to define the delivery pressure, for uniformity of terms, the value of head in feet is preferred. Head is obtained by multiplying gauge pressure (psig) by 2.31. Each of the irrigation methods cited in this series includes an equation to determine the flow rate for that specific method.
Determining the Required System Head
Unlike the process of figuring the required flow rate, the determination of the total system head is not as much of a judgment call. In fact, to ensure accuracy, the proper calculation of the head needed for an irrigation pumping system cannot substantially deviate from the generally accepted procedure I am about to show.
Although there are various terms and conversions used to equate the needed lift or pressure with the design of the pump, the most important term deals with the water flowing or in transition (dynamic state). This term is correctly called the total dynamic head (TDH) of a water pumping system.
The total dynamic head is a combination of several factors that include the well’s (vertical) lift from the pumping water level to an arbitrary elevation (usually the wellhead, ground level, or ultimate destination point); rise or fall in elevation (if any) from the reference level to the discharge point; valve, fitting, and piping frictional losses; the sum of any miscellaneous losses from inline devices such as sand separators, filters/screens, or inline turbines (for driving hard hose travelers); and any needed sprinkler operating pressure.
Once again, judgment enters the mix as the designer will need to ensure the piping has been adequately sized for the maximum anticipated flow rate to reduce friction loss, with the velocity generally kept below 5 feet per second (less than 5 FPS) to prevent potentially destructive water hammer damage. This value will typically be the peak demand or the highest instantaneous rate of the pump or irrigation system.
Since water hammer is a known destructive force in irrigation systems, it is important to control the velocity transition or value of instantaneous pressure surge by controlling and limiting the maximum pipeline velocity.
Although there are numerous formulas available to calculate the head loss in a water system, the Hazen-Williams equation is the simplest and best overall method in my opinion for determining the head loss in an irrigation system. When calculating the pumping head and head loss in a water system, unless you are developing or using a system head curve, it is important to use a uniform and single value of flow for the entire set of calculations. This is usually the highest projected flow rate anticipated, in gallons per minute (GPM).
There are up to nine components, A through I, that make up the sum of the TDH in an irrigation system:
AWL, Well Lift: Also referred to as the pumping water level (PWL). This is the vertical distance (in feet) between the centerline of the discharge tee or pitless adapter at the wellhead for submersible pumps or discharge head for vertical turbine pumps to the lowest known or projected operating water surface in the well during pumping (dynamic) conditions. Shown as a positive (+) input value for computations.
ASL, Suction Lift: This variable is used to describe the vertical distance between the suction (inlet) centerline of a centrifugal pump and the lowest anticipated water level during pumping conditions. Unlike a well lift, this factor must also include the dynamic elements of suction piping and valving frictional losses and velocity head (B + C + H) to verify the maximum suction lift does not exceed the allowable total dynamic suction lift (TDSL) or net positive suction head required (NPSHR) (as appropriate) of the specific centrifugal pump in use.
B, Riser, Suction, or Column Pipe Friction Losses: The frictional loss occurring from water flow within the column pipe and shaft assembly (vertical turbine pump), riser pipe (submersible pump), or suction pipe (centrifugal pump), stated in feet of head loss per hundred feet of pipe multiplied by the total length of pipe, in hundreds of feet (feet/c).
C, Riser Check Valve, Foot Valve, and Inlet Friction Losses: The frictional loss per individual riser check or foot valve multiplied by the total number of valves in the well or at the top of the well as a discharge check valve. Check valve losses are typically used for submersible and vertical turbine pumps, while a single foot valve (with or without a screen) is normally applied to the bottom end of a suction pipe for a centrifugal pump under a suction lift (measured in feet of head loss per valve and multiplied by the number of valves in use). It also includes the inlet losses for the foot valve or pump suction screen, when applicable.
D, Submersible Wellhead/Pitless Adapter or Vertical Turbine Discharge Head Friction Loss: The head loss incurred at the wellhead fittings or pitless adapter for a submersible pump or discharge head for a vertical turbine pump (in feet).
E, Discharge Elevation: The total (net) elevational difference in gain or fall between the wellhead or pump and the final delivery point. Shown as a positive or increasing (+) value for a higher net elevation or a negative or decreasing (–) value for a declining net elevation from the wellhead or pump elevation (in feet).
F, Main and Submain Pipe Frictional Losses: The friction head loss occurring from flow within the mainline or submain (if applicable) pipes between the wellhead or pump and the final delivery point or connecting lateral line as appropriate. Head loss is measured in feet of head per hundred feet of applicable pipe size multiplied by the total length of the same pipe size, in hundreds of feet (feet/c), with each separate run of pipe added together for a total sum of head loss (in feet/c × total length of pipe, in hundreds (c) per separate pipe (+) separate loss from each pipe section).
G, Lateral Line Friction Loss: The element of friction loss occurs due to the portion of flow or all of the total flow running through the individual lateral (branch) line originating from the main or submain to the last sprinklers on the line. Depending on the method of irrigation, this factor may occur from aluminum or PVC lateral line tubing or pipe for impact, rotor, or Big-Gun sprinkler irrigation, drip irrigation emitter lines, or soft or hard hose tubing friction losses for Big Guns in feet of head per hundred feet of pipe or tubing multiplied by the total length of pipe or tubing in hundreds of feet (in feet/c × total length of pipe, in hundreds [c]).
H, Miscellaneous Losses: This category includes the sum of all miscellaneous friction losses for all inline isolation and throttling valves, fittings, inline screens/filters/sand separators, pressure-regulating valves, maximum pressure drop through inline turbine or water drive needed to operate a traveler or pivot, velocity head, suction and discharge concentric and eccentric inlet and outlet reducers/increasers (primarily for centrifugal pumps), etc. (in feet). In the absence of a specific loss, a factor of 10 feet to 20 feet of head loss is often applied for this variable for submersible and vertical turbine pumps.
I, Operating Pressure: The design operating or discharge pressure at the final point of delivery or sprinkler as applicable. Typically measured at the sprinkler in pounds per square inch gauge (psig) and converted to the feet of head for design (1 psig × 2.31 = feet of head). This variable can range from a low of 5 to 20 psi for drip emitters, 15 to 40 psi for low pressure impact or spray sprinklers, 30 to 80 psi for medium and higher pressure impact and rotary sprinklers, and 80 to 125 psi for stationary or portable Big-Gun type of sprinklers.
Although many potential options exist for this variable, the best procedure for a typical irrigation system is to use the highest anticipated sprinkler operating pressure for the pump design. In all cases, verify that the highest required pressure and flow can be met by the selected pump and that the lowest operating pressure will not result in overpumping or taxing the source or well.
Figure 1 illustrates an example of a centrifugal pump installation with a suction lift for a conventional sprinkler irrigation system with impact sprinklers. Figure 2 shows a submersible pump installation in a deep well for a drip system. Figure 3 illustrates a vertical turbine deep well pump for a hard hose reel (traveler) application. Head factors are shown for each example.
Each of the above factors are added to create a sum, referred together as the total dynamic head (TDH):
For a well lift: (TDH, in feet of head) = AWL + B + C + D +/- E + F + G + H + I
For a suction lift: (TDH, in feet of head) = ASL + B + C +/- E + F + G + H + I
Finally, the required flow rate in gallons per minute (GPM) along with the total dynamic head (TDH) is used to create the conditions of service (COS), which equals ___GPM at ___ feet TDH. This COS is then evaluated against a pump curve for the specific pump type for the final pump selection.
Tailwater and Reuse Water Collection and Pumping Systems
I believe this series has done a commendable job on outlining the fundamental concepts of agricultural and ornamental irrigation; terms and definitions; typical soil, plant, and water characteristics and interrelationships; crop average and peak demands; and a review and system design for various types of irrigation methods.
However, there are many locations throughout the United States and the world at large where water resources are so precious, unavailable, or simply already overallocated and strained that prevent any considerable use in irrigation. In many of these sites, the need to collect and reuse water that has previously been applied may be necessary.
In these cases, the installation and application of an irrigation water reuse system using a specialized system of collection, transfer (tailwater pumping), treatment (if necessary), interim storage, and equipment to repressurize water to the irrigation system may be warranted.
Although this system is practical for almost all methods of irrigation (with a very efficient and minor waste system of drip irrigation being the only possible exception), these systems are extremely practical and virtually mandatory for irrigation systems that are either intentionally or unintentionally losing a considerable amount of applied water to excessive percolation, overland runoff, or overspray losses.
This is often the case for flood or furrow irrigation or sprinkler irrigation when water is overapplied for the crop needs or soil intake (acceptance) rate values; when irrigation water must compete with normal precipitation water storage in the soil horizon; or where there is deep percolation loss due to permeable soils or surface runoff losses from tight or impermeable soils.
Even with a well-designed system, these losses can easily amount to 20%-25% of the total (gross) volume of applied water. In these instances, water that is not otherwise directly used for crop consumption (transpiration) and unavoidable evaporation percolates below the root zone vertically or laterally by gravity forces to be drained and collected in a subsurface system of perforated laterals, all of which flows into a common main and subsequently into treatment (if needed), and ultimately ends up flowing by gravity or pumping into an atmospheric holding pond.
These separate elements, when combined, constitute a tailwater recovery system. The water is thereafter stored until repumped back into the irrigation system as replacement or supplemental water in contrast to using the new source water by using a pump specifically designed for this application as a tailwater reuse pump.
In order to be able to safely operate against the source or irrigation system pump, this pump will generally exhibit the same operational head curve as the irrigation pump, with the exception of the vertical lift that may be present from a well or other source. The flow rate is determined from the available water storage and percentage of reclaimed water, a ratio of reclaimed water to source water.
Tailwater recovery and reuse is a popular method of water conservation and is increasing in popularity and use as much of the world’s water supply becomes more strained and limited.
An example layout of a tailwater recovery system from surface (flood) irrigation is illustrated in Figure 4 and a tailwater pumping arrangement is illustrated in Figure 5.
This concludes Part 11 of our irrigation fundamentals series. Next month, we will continue with a discussion on some other common applications for the use of irrigation.
Until then, work safe and smart.
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.