About Pivots

About Pivots

The center pivot is an ideal system for agricultural irrigation because it requires so little labor and maintenance and is easy to operate, high performing and effective. When properly designed and equipped with high efficiency water applicators, a center pivot system conserves water, energy and time.

Manufacturers today have greatly improved center pivot drive mechanisms (motors, gears and shafts), control devices, pipe sizes and span lengths to work effectively in the fields. The first pivots produced in the 1950s were propelled by water motors. They operated at high pressures of 80 to 100 psi and were equipped with impact sprinklers and end guns that sprayed water toward the sky, resulting in significant evaporation losses and high energy use. Today, pivots are driven by electric or oil hydraulic motors located at each tower and guided by a central control panel. Pressures as low as 10 to 15 psi (at the pivot mainline) are usually adequate for properly designed LESA (low elevation spray application) and LEPA (low energy precision application) pivots that are 1/4 mile long operating on flat to moderately sloping fields. Water application efficiency with such systems is 85 to 98 percent.

Pivot Design Choice
When purchasing a center pivot system one must select:
1. Mainline size and outlet spacing;
2. Length, including the number of towers;
3. Drive mechanisms;
4. Application rate of the pivot;
5. Type of water applicator.

These choices affect your investment and operating costs, irrigation efficiency, and crop production. Smart decisions will result in responsible water management and conservation, flexibility for future changes, and low operating costs.

Wheel and Drive Options
The travel speed is determined by the wheel size in combination with the power drive mechanism, and is set at the central control panel. The speed of the pivot determines the amount of water applied.

The electric power drive has two gear reductions. One gear reduction is in the drive shafts connecting the electric motor to a gear box located at each of the two tower wheels. The second gear reduction is in the gear box driving each wheel. The maximum center pivot travel speed depends on the:
1. Electric motor speed or rotation in revolutions per minute (RPM);
2. Speed reduction ratios in both the center drive shafts and gear boxes;
3. Wheel size.

Design Printout
The computer printout of the design provides information about the center pivot and how it will perform on a particular tract of land. A typical design printout includes:
1. The pivot design flow rate (or system capacity) in GPM;
2. Irrigated acreage under the pivot;
3. Elevation changes in the field as measured from the pivot point;
4. Operating pressure and mainline friction losses;
5. The pressure regulator rating in psi (if used);
6. The type of water applicator, spacing and position from the mainline;
7. Nozzle size for each applicator;
8. Water applicator nozzle pressure;
9. Maximum travel speed;
10. The precipitation chart.

It is essential that correct information about the available water supply (in GPM) and changes in field elevation are used in designing the pivot so that accurate irrigation amounts, operating pressure requirements, and the need for pressure regulators can be determined. Give this information to your dealer, and then inspect the resulting design printout before placing your order to ensure that the system will accommodate your site conditions and perform as expected. Always check the design mainline operating pressure at the pad to determine if it is what you want. If not, inquire about ways to lower it.

System Capacity
System irrigation capacity is determined by the gallons per minute (GPM) and the number of acres irrigated. System capacity is expressed in terms of either the total flow rate in GPM or the application rate in GPM per acre. Knowing the capacity in GPM per acre helps in irrigation water management. These irrigation amounts apply for all irrigation systems with the same capacity in GPM per acre. The amounts do not include application losses, and are for systems operating 24 hours a day. To determine your system’s capacity, select the desired irrigation amounts in inches and multiply the corresponding GPM per acre by the number of acres you are irrigating. For example, if you irrigate 120 acres with 4 GPM per acre, 480 GPM (120 acres x 4 GPM per acre) are required to apply 0.21 inches per day, 1.50 inches per week, and 6.40 inches in 30 days.

Mainline Pipe Sizing
Mainline pipe size influences the total operating cost. Smaller pipe sizes, while less expensive, may have higher water flow friction pressure loss, resulting in higher energy costs. Plan new center pivots to operate at minimum operating pressure to minimize pumping cost.

Some dealers may undersize the mainline in order to reduce their bids, especially when pushed to give the best price. Check the proposed design printout. If the operating pressure appears high, ask the dealer to provide another design using proportional lengths, usually in spans, of larger pipes, or using telescope pipes to reduce operating pressure. Saving money on the initial purchase price often means paying more in energy costs over the life of the system.

Telescoping involves using larger mainline pipes at the beginning and then smaller sizes as the water flow rate (GPM) decreases away from the pivot point. Typical mainline sizes are 10, 8 1/2, 8, 6 5/8 and 6 inches. Mainline pipe size governs options in span length (the distance between adjoining towers).

Telescoping mainline pipe size is a method of planning a center pivot for minimum water flow friction loss and low operating pressure, and thus, lower pumping costs. Telescoping uses a combination of pipe sizes based on the amount of water (GPM) flowing through. Telescoping is usually accomplished in whole span lengths. Its importance increases with both higher flow rates (GPM) and longer center pivot lengths. Dealers use computer telescoping programs to select mainline pipe sizes for lowest purchase price and operating costs.

Pressure Regulators
Pressure regulators are "pressure killers." They reduce pressure at the water delivery nozzle so that the appropriate amount of water is applied by each applicator. Selection of nozzle size is based on the rated delivery psi of the pressure regulators. Nozzles used with 10 psi regulators are smaller than those used with 6 psi regulators. Low rated (psi) pressure regulators, if used, are designed for the center pivot design to work at minimum operating pressure.

Pressure regulators require energy to function properly. Water pressure losses within the regulator can be 3 psi or more. So, the entrance (or inlet) water pressure should be 3 psi more than the regulator rating. Six-psi regulators should have 9 psi at the inlet; 10-psi regulators, 13 psi; 15-psi regulators, 18 psi; and 20-psi regulators, 23 psi. Regulators do not function properly when operating pressure is less than their rating plus 3 psi. Pressure regulator operating inlet pressure should be monitored with a gauge installed upstream adjacent to the regulator in the last drop at the outer end, and should be checked when the machine is upslope. Another gauge located in the first drop in a span one will monitor operating pressure when the center pivot is located on downslope terrain.

Pressure regulator psi rating influences system design, appropriate operating pressure, total energy requirements, and cost of pivot irrigation.

As with other spray and sprinkler systems, pressure regulators are not necessarily needed for all sites.

Elevation changes in the field have the largest impact with lower design pressures. From the first to last drop on a pivot, the operating pressure at the nozzle should not vary more than 20 percent from the design operating pressure. Without regulators, operating pressure and pumping cost usually will not increase significantly if the elevation does not change more than 5 feet from the pad to the end of the pivot. Where elevation changes are greater than 5 feet, either increase the operating pressure (and probably pumping cost) or use pressure regulators. This decision is site specific and should be made by comparing the extra costs of pressure regulators to the increased pumping costs without them.

Where the water flow rate, and thus the operating pressure, vary significantly during the growing season, perhaps from seasonal variations in groundwater pumping levels, the design flow rate (or system capacity) and the use of pressure regulators should be evaluated carefully. If water pressure drops below that required to operate the regulators, then poor water application and uniformity will result. In contrast, if the design operating pressure is high, pumping costs will be unnecessarily high. When operating pressure decreases to less than required, the solution is to renozzle for the reduced gallons per minute. The amount of water flow in the mainline decreases or increases operating pressure for the nozzles installed.

Low-pressure Applicators

Very few center pivots are now equipped with impact sprinklers. There are improved applicators and design technology for more responsible irrigation water management. These new applicators operate with low water pressure and work well with current center pivot designs. Low-pressure applicators require less energy and, when appropriately positioned, ensure that most of the water pumped gets to the crop.

The choice is which low-pressure water applicator to use and how close to ground level the nozzles can be. Generally, the lower the operating pressure requirements the better. When applicators are spaced 60 to 80 inches apart, nozzle operating pressure can be as low as 6 psi, but more applicators are required than with wider spacings (15 to 30 feet). Water application is most efficient when applicators are positioned 16 to 18 inches above ground level, so that water is applied within the crop canopy. Spray, bubble or direct soil discharge modes can be used.

Field testing has shown that when there is no wind, low-pressure applicators positioned 5 to 7 feet above ground can apply water with up to 90 percent efficiency. However, as the wind speed increases, the amount of water lost to evaporation increases rapidly. Evaporation loss is significantly influenced by wind speed, relative humidity and temperature.

The following sections describe three types of low-pressure application systems that can significantly reduce operating pressure and deliver most of the water pumped for crop production.

MESA
With Mid-Elevation Spray Application (MESA), water applicators are located approximately midway between the mainline and ground level. Water is applied above the crop canopy, even on tall crops such as corn and sugar cane. Rigid drops or flexible drop hoses are attached to the mainline gooseneck or furrow arm and extend down to the water applicator. Weights should be used in combination with flexible drop hose. Nozzle pressure varies depending on the type of water applicator and pad arrangement selected. While some applicators require 20 to 30 psi operating pressure, improved designs require only 6 to 10 psi for conventional 8 1/2 to 10-foot mainline outlet and drop spacing. Operating pressures can be lowered to 6 psi or less when spray applicators are positioned 60 to 80 inches apart. With wider spacings, such as for wobbler and rotator applicators, manufacturers’ recommended nozzle operating pressure is greater.

LESA
Low Elevation Spray Application (LESA) applicators are positioned 12 to 18 inches above ground level, or high enough to allow space for wheel tracking. Less crop foliage is wet, especially when planted in a circle, and less water is lost to evaporation. LESA applicators are usually spaced 60 to 80 inches apart, corresponding to two crop rows. Each applicator is attached to a flexible drop hose, which is connected to a gooseneck or furrow arm on the mainline. Weights help stabilize the applicator in wind and allow it to work through plants in straight crop rows. Nozzle pressure as low as 6 psi is best with the correct choice of water applicator. Water application efficiency usually averages 85 to 90 percent, but may be less in more open, lower profile crops. LESA center pivots can be converted easily to LEPA with an applicator adapter that includes a connection to attach a drag sock or hose.

The optimal spacing for LESA drops is no wider than 80 inches. With appropriate installation and management, LESA drops spaced on earlier, conventional 8 1/2- to 10-foot spacing can be successful.

LEPA
Low Energy Precision Application (LEPA) irrigation discharges water between alternate crop rows planted in a circle. Water is applied with:
Applicators located 12 to 18 inches above ground level, which apply water in a "bubble" pattern; or drag socks or hoses that release water on the ground.

Socks help reduce furrow erosion; double-ended socks are designed to protect and maintain furrow dikes. Drag sock and hose adapters can be removed from the applicator and a spray or chemigation pad attached in its place when needed. Another product, the LEPA “quad” applicator, delivers a bubble water pattern that can be reset to optional spray for germination or chemigation.

LEPA applicators typically are placed 60 to 80 inches apart, corresponding to twice the row spacing. Thus, one row middle is wet and one is dry. Dry middles allow more rainfall to be stored. Applicators are arranged to maintain a dry row for the pivot wheels when the crop is planted in a circle. Research and field tests show that crop production is the same whether water is applied in every furrow or in alternate furrows. Applicator nozzle operating pressure is typically 6 psi.

Field tests show that with LEPA, 95 to 98 percent of the irrigation water pumped gets to the crop. Water application is precise and concentrated, which requires a higher degree of planning and management, especially with clay soil. Center pivots equipped with LEPA applicators provide maximum water application efficiency at minimum operating pressure. LEPA can be used successfully in circles or in straight rows. It is especially beneficial for low profile crops such as cotton and peanuts, and even more beneficial where water is limited.

FERTIGATION

The application of fertilizers with irrigation water, or fertigation, is often referred to as “spoon-feeding” the crop. Fertigation is very common and has many benefits. Most fertigation uses soluble or liquid formulations of nitrogen, phosphorus, potassium, magnesium, calcium, sulfur and boron. Nitrogen is most commonly applied because crops need large amounts of it. Keep in mind that nitrogen is highly soluble and has the potential to leach; it needs to be carefully managed.

There are several nitrogen formulations that can be used for fertigation. Be sure a solid formulation is completely dissolved in water before it is metered into the irrigation system. This may require agitating the mixture for several hours. Continue agitating throughout the injection process.

Advantages of Fertigation
Nutrients can be applied any time during the growing season based on crop need.
Mobile nutrients such as nitrogen can be carefully regulated in the soil profile by the amount of water applied so that they are available for rapid use by the crop.
Nutrients can be applied uniformly over the field if the irrigation system distributes water uniformly.
Some tillage operations may be eliminated, especially if fertilization coincides with the application of herbicides or insecticides. However, do not inject two chemicals simultaneously without knowing that they are compatible with each other and with the irrigation water.
Groundwater contamination is less likely with fertigation because less fertilizer is applied at any given time. Application can correspond to maximum crop needs.
There is minimal crop damage during fertilizer application.

NOTE: THE INFORMATION ABOVE IS FROM LEON NEW, PROFESSOR AND EXTENSION AGRICULTURAL AND IRRIGATION ENGINEER IRRIGATION & GUY FIPPS, PROFESSOR AND EXTENSION AGRICULTURAL ENGINEER AT THE TEXAS A&M SYSTEM.