Pump Sizing for Efficient Irrigation

Although selecting high-efficiency pumps for irrigation applications is important, pump sizing is equally critical to reduce energy consumption and costs. In fact, incorrect sizing is a common reason for low pumping efficiency. Optimal pump selection requires choosing the product that is best suited to the operating conditions required by the irrigation application. To ascertain whether the operating conditions are stable, users must calculate how much pump flow and head are required.

Calculating Flow

End users should determine the amount of water to be applied during the peak period by multiplying the size of the field by the amount of water in inches that must be applied. The result is then converted to gallons per minute (gpm), and this number determines the size of the pump. For example, to grow corn on a 50-acre field using water from a well, a pump with best efficiency at 524 gpm could pump for 24 hours per day. However, to allow for system downtime and power cost windows, a larger pump will always be chosen, and a runtime of between 12 and 18 hours per day is typical. If a 12-hour runtime per day is chosen, the pump must be able to pump double the amount—1,048 gpm. Using the curve in Figure 1, one submersible pump can provide 1,048 gpm, which is close to the maximum efficiency, and this pump was, therefore, selected. The calculation for water loss and water used by the crop is: ET_p × K_c = 0.5 × 1 = 0.5 inch per day Where: Evapotranspiration potential (ET_p) in the area for sweet corn = 0.5 inch per day Crop efficiency (K_c) = 1 Assume that irrigation efficiency is 90 percent. Then the required amount per day is a bit more: 0.5 / 90 × 100 = 0.56 inch per day With 1 acre inch of water equaling 27,154 gallons, the water needed per acre is: 27,154 × 0.56 inch = 15,086 gallons A 100-acre field, therefore, requires 1,508,600 gallons of water per day, or converted to gallons per minute, the amount required is: 1,508,600 / 24 x 60 =1,048 gpm

Calculating Head

An irrigation pump has to overcome the following four elements of pressure:

• Pressure required for the application devices (such as sprinklers, spray heads and drippers)
• Friction loss from the piping system, pipes, screens, valves, elbows and tees
• Elevation lift
• Suction lift

For a deep-well pump, such as a submersible or a vertical turbine pump, another consideration is the drawdown of the static water level. The static water level is defined as the depth to water when no water is being pumped from the well. As soon as the pump begins operation, the water level will start to go down. The water level will continue to lower until equilibrium is reached, and that is when the friction loss in the aquifer and the casing screen (feet of friction) is the same as the drawdown (feet of head). The dynamic water level is defined as the depth to water when the pump is running at its operating capacity. When the total head for a groundwater pump is calculated, two things are different from a surface pump:

• There is no suction lift.
• The drawdown has to be added to the elevation lift.

The other components in the calculation are unchanged. Returning to the calculation to determine the required head that the pump must produce, assume:

• The application device uses 10 psi of pressure, or 23 feet of head.
• The friction loss in the pipes, elbows, valves and tees has been calculated to 30 psi, or 69 feet of head.
• Assume that the elevation lift is only 58 feet of head.
• The static water level is 200 feet (this corresponds to suction lift for a surface pump).
• The drawdown in the well is 10 feet.

The total head requirement to the pump is: 23 + 69 + 58 + 200 + 10 = 370 feet The earlier calculation of the flow shows that the submersible pump meets this requirement. The pump performance curves show that a three-stage pump can meet the head requirement of 370 feet. The data curve (Figure 4) shows that a three-stage pump will give the exact flow and head required. The pump is submersible with three stages or bowls and a 125-horsepower motor. In this example, the performance requirement was on one of the curves. What if the head requirement had only been 355 feet? In that case, no curve in Figure 4 matches the duty point, which is between two curves. The solution is to select one of the two curves. If the upper curve is selected, a bit more reserve is in the pump. If the lower one is selected, the pump will run marginally for a longer time. It has no practical influence on the performance or the efficiency in either case, which can be seen from the efficiency curve.

Groundwater Wells

A well is an opening that stretches from the surface of the ground to an underground aquifer, where the groundwater is located. The depth of the well may vary from a few meters to several hundred meters. Wells are typically drilled with special drilling equipment that can penetrate the ground’s layers—such as sand, clay and bedrock. Inside the drilled hole, a casing (pipe) is typically installed, which prevents the well from collapsing around the pump. Below the casing and in line with the aquifer is another casing with fine slots. This is the well screen, where the slots allow the water to enter the well. It prevents sand and larger particles from entering the well. To improve the filtering function, the borehole typically features a diameter that is 2 to 3 inches larger than the casing. A fine sand gravel pack filter is placed between the casing and the aquifer (see Figure 5). Some casings come with a pre-made gravel pack filter. If made correctly, this filtering method prevents sand and silt from entering the well. The U.S. Environmental Protection Agency and American Water Works Association recommend the following sand limits in well water:

• 1 part per million (ppm) in water for drip and microspray applications
• 10 ppm in water for sprinkler irrigation systems
• 15 ppm in water for flood irrigation

Before the well can be put into operation, it must be developed. Initially, a new well will always produce some sand and silt, and well development is the process of pumping a new well free from sand and silt. It is accomplished by pumping with a very high flow, which draws the fine particles in the aquifer into the filter of the well. This slowly makes the filter more effective. After approximately one day of pumping, the well is normally pumped clean and is ready for normal operation. The pump used for well development wears out relatively quickly because of the high sand content, and it should always be replaced with a new pump as soon as the well does not produce any more sand. The submersible pump must always be installed above the screen area of the casing. This is to ensure that the water is forced past the motor, providing adequate motor cooling. If the pump cannot be installed above the screen filter, a cooling sleeve is recommended to create the necessary flow along the motor for proper cooling. The drawdown for each specific well can be tested using a test pump with the same flow rating as the production pump. The test pump is installed in the well, and the pump is started. The groundwater level is recorded, and the equilibrium point has been reached when the water level becomes constant. This level is the dynamic water level. The drawdown is the difference between the static and the dynamic water level. All the water that the well produces is pushed through the aquifer and the well screen by the pressure differential between the static and the dynamic water levels. The higher the flow is, the bigger the drawdown will be. For this reason—considering the operating costs—using two or more smaller pumps (and wells) instead of one large one is better. With smaller pumps, the drawdown is limited, and the elevation lift is reduced. Some aquifers have so much resistance that the water flow to one well is not enough for the irrigation need. A second and third well may be the only solution for the required amount.

Sizing for Efficiency

To reduce energy consumption and save energy costs, selecting the most efficient pump for the application is important. Following the simple guidelines described in this article will ensure the pump is best suited for the job, and that the irrigation efficiency is optimized.