Irrigation Pumps. Pumps are usually either of the centrifugal or turbine type. Centrifugal pumps (which may be self-priming or non-self-priming) are used to pump water from sur- face sources and shallow wells. Turbine pumps (either vertical shaft or submersible) are used to pump from deep wells. Features of each type of pump are discussed in Extension Service publication AG-389, Water Supplies for Subirrigation, and AG-452-6, Pumping Plant Performance Evaluation.
Some pumps are more efficient than others, as shown in Figure l . However, most types of pumps are available over a wide range of operating effciencies. Pump efficiency is the ratio (expressed as a percentage) of the water horsepower of the discharge water divided by the power delivered to the pump by the motor or engine. Normally, pump cost increases as effficiency increases (Figure 2), but savings in operating costs usually compensate for this difference after a few years of use. If the pump is likely to be used over a broad range of operating conditions, ensure that it will meet aU the required conditions. If possible, the pump should be operated in the midrange of its performance curve.
Figure 1. Typical efficiency ranges of irrigation pumps. Most irrigation dealers offer a range of pumps that span the entire efficiency range shown. Values shown are representative for most pumps, but there are exceptions in all categories. Consult pump characteristic curves to select a pump that provides the desired efficiency for a given set of operating conditions.
Figure 2. General relationship between the initial cost of an irrigation pump and its efficiency.
The effficiencies of individual pump types vary among models, manufacturers, and types. A pump efficiency in the range from 75 to 85 percent is preferable. Once the operating pressure (head) and system capacity (in gallons pff minute) have been determined, pump charactffistic curves (Figure 3) should be consulted to select the most effficient pump for the specific conditions.
Figure 3. Typical pump characteristic (performance) curve for a motor-driven pump.
Power Units. Power-unit effficiency is also important to pumping plant performance. Power sources are of two types: (1) internal combustion engines (either direct~rive engines used solely for irrigation or tractors equipped with a power takeoff to drive pumps) and (2) electric motors.
Power-unit efficiency is the efficiency with which chemical energy stored in the fuel (for internal combustion engines) or electrical energy (for electric motors) is converted to mechanical energy to drive the pump. Where electricity is available, it is the most effficient power source (Figure 4). As shown in Table 3, the effficiency of electric motors ranges from about 80 percent for motors under 7.5 horsepower to over 90 percent for motors of 75 horsepower or larger.
Figure 4. Relative energy cost for power sources most commonly used in North Carolina . All values are based on the recommended Nebraska Performance Standard for the power units shown. Energy costs are based on the following rates: electricity, 7 cents per kilowatt hour; diesel fuel, $1 per gallon; gasloine, $1.10 per gallon. For gasoline engines, the fuel cost has been increased by 15 percent to allow for annual engine tune-up and maintenance costs.
Internal combustion engines are ineffficient. Gasoline engines are only 20 to 26 percent efficient, whereas diesel engine effficiencies range from 25 to 37 percent. Performance standards for the different types of power units are listed in Table 2. Over the life of the pumping plant, the difference in operating costs among various types of power units and pumps must be weighed against any savings in initial investment.
Internal Combustion Engines. Each percentage decrease in engine efficiency increases the amount of fuel used by 3 to 5 percent. For example, fuel consumption increases by 19 percent for the gasoline engine shown in Figure 5 as the engine effficiency decreases from 23.8 percent (Nebraska Performance Standard) to 20 percent. For a 100 horsepower engine, the fuel consumption would increase from 8.7 gallons per hour to 10.3 gallons per hour, resulting in an increased operating expense of nearly $2 per hour.
Figure 5. Increase in fuel consumption of a gasoline engine resulting from poor engine efficiency.
Pump engines should be tuned up at the beginning of each irrigation season to ensure effficient performance. Air and fuel filters should be changed at manufacturers' recommended intervals.
Electric Motors. A buildup of dirt or oil, obstruction of cooling vents, worn or dragging motor bearings, and voltage surges caused by lightning can cause electric motors to overheat. Overheating often leads to shorted wires in the winding and is the most common cause of low motor effficiency. Voltage surges can also cause damage to or misalignment of phases in three-phase motors, resulting in low motor effficiency. Misaligned shafts between motor and pump and overtightened packing glands or seals can also reduce efficiency. Low effficiency caused by extended use (wear) can sometimes be improved by replacing the commutator (brushes) or having the motor rewound. In the case of small motors (20 to 25 horsepower or less) it is usually more practical to replace the entire motor. Electric motors should be protected from rain and direct sunlight.
Matching System Components. Often, irrigation systems are altered from their original design, resulting in mismatching of components. Alterations such as adding or deleting sprinklers or laterals are often made without making corresponding adjustments to the pumping plant. A change in the depth to water, particularly in some deep wells where the static head has declined, can alter the pumping conditions enough that the pump no longer operates in the recommended efficiency range.
In some cases, the pump can be adjusted to resume operation within the recommended range. On turbine pumps the bowls can be adjusted or impellers trimmed. If wear is excessive, bowls and impellers should be replaced. If an internal combustion engine is used, the system can some- times be adjusted by changing the operating speed. Other cases may require more costly adjustments, such as replacing the electric drive motor or the entire pump.
Minimizing Pressure Losses. Reduced irrigation efficiency can also result from poor maintenance and from wear of system components, leading to excessive pressure losses. With surface-water sources, intake hoses should be checked routinely for clogging and for air leaks or partial collapses of the suction hose. Couplers should be checked for leaks and adjusted or replaced when necessary. Check for misalignment of the rubber seal in quick-connect couplers. Sprinkler nozzle wear is a common cause of poor water use efficiency. Worn nozzles apply more water at lower pressure, leading to excessive water use and poor application uniformity. With most sprinklers it is easy to replace nozzles and other mechanical parts.
Using undersized pipe, especially for mainlines, can significantly reduce energy efficiency. This problem is common when the pumping plant has been adjusted or modified to increase system capacity. For a given flow rate, friction loss and thus pumping cost decrease as pipe size increases (Figure 6). For example, consider a pumping plant modified to increase system capacity by 50 percent from 800 to 1,200 gallons per minute (GPM). The typical mainline pipe for an 800-GPM system is 8 inches in diameter. But when the flow rate is increased to 1,200 GPM, the fuel cost required to overcome the additional friction losses in 2,000 feet of 8-inch mainline pipe compared to 10-inch pipe is nearly $1 per hour. In this case, the long-term energy savings should be weighed against the cost of installing larger pipe.
Figure 6. Friction losses in pipe relative to pumping rate and pipe size. Values shown are for Class 160 PVC pipe.
Adjusting or replacing system components can often improve overall efficiency. These adjustments are usually fairly inexpensive, and in many cases the cost is justifiable. In some cases, however, water- and energy-use efficiencies may remain relatively low. For example, older pumps, even when properly adjusted, are not as efficient as pumps available today. High-pressure systems are not as energy efficient as low-pressure systems. Therefore, it is sometimes necessary to replace costly system components to gain efficiency.,p>
Some systems can be converted to low pressure simply by replacing existing sprinlders with low-pressure sprinklers designed to reduce operating pressure by 25 to 50 percent. Low-pressure systems typically have high instantaneous application rates and are therefore most appropriate for use on sandy soils, which have a high infiltration rate.
Sprinkler nozzles are designed to operate within a narrow range of pressures. Correct nozzle pressure is necessary to ensure uniform water application. Thus, converting to low-pressure nozzles also requires adjusting the pump pressure. Pump replacement, a more expensive alternative, may be necessary in some cases. Be aware that reducing operating pressure may lead to one or more of the following problems:
These changes may cause:
Increased droplet size can cause soil surface compaction because the impact energy of larger drops is greater.
Irrigation scheduling is the process of evaluating these factors and determining when to irrigate and how much water to apply. The effect of soil, water, and crop factors on irrigation scheduling is discussed in Extension Service publication AG-452-1, Soil, Water, and Crop Characteristics Important to Irrigation Scheduling. Irrigation scheduling strategies are discussed in Extension Service publication AG-452-4, Irrigation Scheduling to Improve Water- and Energy-Use Efficiencies.
There are a number ways of to minimize peak electrical demands. Irrigators can voluntarily schedule irrigation at night during nonpeak periods (typically 10 p.m. to 10 a.m.), thus reducing power consumption during the peak demand period. Some power companies offer financial incentives to customers who allow the company to interrupt power to the pumping unit during high demand periods using a remote control system. See your power company agricultural representatives for information on available programs. As discussed earlier, some systems can be redesigned to reduce pressure and discharge. Improving pumping plant efficiency will also reduce power usage. Procedures for testing pumping plant perforrmance are discussed in Extension Service publication AG-452-6, Pumping Plant Performance Evaluation. Each alternative for conserving energy has a cost and will reduce peak demand by a different amount.
One option that requires no new equipment is to take advantage of time-of-use rates offered by electric power suppliers. These rates encourage customers to avoid using electrical power during the hours of peak demand (usually from 10 a.m. to 10 p.m.). To take advantage of off-peak rates, you need to know the capacity of your irrigation system and the time it takes to deliver the amount of water needed during periods of high moisture stress. To provide adequate amounts of water during off-peak times, it may be necessary to purchase additional equipment to pump greater quantities of water during the off-peak hours. Planned use of the water stored in the crop root zone is the key to effective off-peak irrigation scheduling. Soil moisture monitoring is a necessary safeguard to assure that soil moisture levels are adequate.
As energy costs increase, efficient irrigation systems can mean more money in your pocket. Consider having your system checked periodically to ensure that it is operating at peak performance. Your irrigation dealer can often recognize components that have become ineffficient as the system ages. In addition, your county Cooperative Extension Service and Soil Conservation Service offices are available to help. Their staff members have received training on irrigation design and can recommend management strategies for effficient, long-term operation of your system.