Testing pump efficiency
Most irrigation in Nebraska depends on groundwater as the water source and uses multi-stage, vertical turbine pumps. The University of Nebraska has field tested hundreds of pumping plants over the years. Based on these field tests and laboratory tests of engine efficiency, UNL has developed the Nebraska Pumping Plant Performance Criteria (NPC). This criteria states the amount of useful work (water horsepower-hours, whp-h) one should reasonably expect to achieve in the field for each unit of energy consumed by a pumping plant.
In a pumping plant test, the technician measures total head (lift plus system pressure), flow rate (gallons per minute), and rate of energy consumption. These four measurements are used to calculate the performance of the particular pumping plant. Finally, the performance of the pumping plant is compared to the Nebraska Performance Criteria and is given a performance rating, expressed as a percentage of the NPC. A rating of 100% indicates that the pumping plant is operating as expected. A rating below 100% indicates the pumping plant is using more energy for the work that it is doing than the criteria calls for. For example, a pumping plant operating at 70% of the NPC is only producing 70% of the useful work it should for the energy it is consuming.
Nebraska test results
UNL’s most recent statewide pumping plant efficiency study tested 165 pumping plants. As one might expect, the efficiency of the pumping plants tested by the university varied considerably. Some pumping plants achieved very good efficiency. In fact, 15% actually exceeded the NPC. (Performance ratings over 100% of the NPC are possible when a highly efficient motor is attached to a well-designed pump that is not worn or misadjusted). The fact that some pumping plants exceed the criteria indicates the NPC is a reasonable target for all pumping plants. The other 85% of the pumping plants were found to use more energy per unit of work than would be expected by the NPC with a few using about twice the energy they should.
When the performance ratings of all 165 pumping plant tests in the study were tallied, the average pumping plant in Nebraska was found to be using 130% as much energy as it would if it were operating at the NPC.
Causes
When the efficiency of a pumping plant is not what it should be, the problem is either in the power unit or in the pump or both. Internal combustion power units on irrigation pumps can have the same problems as those in cars and trucks. About the only thing that will cause poor electric motor efficiency is if the bearings are bad or if the motor is far larger than is needed for the job. Causes for poor pump performance include: pump designs that are poorly matched to the job they are currently doing (perhaps the operator has switched from gated pipe to a center pivot sprinkler or a high pressure to a lower pressure sprinkler package), pumps that had worn impeller vanes and/or internal seals as a result of pumping sand, or impellers that were not properly adjusted within the pump bowls.
Making adjustments
There are many pump manufacturers and each manufacturer can have dozens of pump designs in its catalog. A given impeller design operates on a head versus capacity curve for a given rotational speed. The greater the head the pump is working against, the lower the capacity (GPM) the impeller can produce (Figure 1). The efficiency (work produced versus energy consumed) changes along the operational curve. Each design will have a best efficiency point at a certain head/capacity condition, with lower efficiencies at higher and lower head/capacity conditions on either side of the best efficiency point.
In recent pumping plant tests, 58% were determined to potentially benefit from adjustments. Adjustments made with a wrench in the field either to the engine or pump or both resulted in 14% average savings in energy costs compared to the initial test results. An equally important result was inefficient pumping plants were identified and the feasibility of making repairs beyond the field adjustments were calculated. On some pumping plants, the potential savings in energy costs from major repair or even replacement of the pump would pay for itself in only a few years.
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| Figure 2. A cutaway view of one stage of a multi-stage enclosed impeller and bowl assembly as it would appear when first installed. |
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| Figure 3. The same assembly as in Figure 2, but after years of wear caused by sand in the irrigation water has worn down the surface of the impeller vanes. |
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| Figure 4. Worn impeller repositioned as low as possible in the pump bowl so it establishes a better seal. |
If there isn´t a water meter installed on the system, a short-term pumping plant test can be run using one of a variety of devices to measure the flow rate. Contact a reputable well driller and ask if they are equipped to run a short term pumping plant efficiency test. At current energy prices, identifying a pumping plant that needs adjustment or repair could result in hundreds or even thousands of dollars savings in energy costs per year.
If the producer has records of the total fuel used over a period of time, the total volume of water pumped (from water meter readings), the system pressure measured at the discharge head, and the water level (measured while the pump is running) an estimate of the performance rating can be calculated. This estimate can be used as an initial screen to help identify pumping plants that may require the services of an experienced technician. A one-page procedure for using long term records to estimate the performance rating of a pumping plant is found at: http://cropwatch.unl.edu/archives/2003/pumpingplantcalculation.pdf. An Excel worksheet has been developed to simplify the calculations. It can be found on the Irrigation Page of the University of Nebraska Extension in Lancaster County web site or accessed directly at http://lancaster.unl.edu/ag/crops/Long_Term_Pump.xls.
Why impeller wear reduces efficiency
Vertical turbine pump assemblies use multiple stages to create the total head (pressure) required to lift the irrigation water level in the well to the soil surface and to supply the pressure needed for the distribution system to function properly. Figure 2 is an illustration of a cutaway of one stage of a multistage enclosed impeller and bowl assembly as it would appear when first installed. Since the purpose of each stage of the pump assembly is to add pressure to the water, there must be a seal to prevent the higher pressure water that has passed through the impeller from leaking back into the lower pressure area at the inlet (eye) of the impeller. This seal is created by the close tolerance between the skirt of the rotating impeller and the wear ring area of the stationary bowl.
Figure 3 shows the same assembly after years of wear caused by sand in the irrigation water. This sand not only wears the surface of the impeller vanes, it causes wear to the seal area. As the seal deteriorates, more and more water that should be moving through the system, is merely recirculated within the pump bowls. This constant recirculation of a portion of the water being pumped adds to the work being performed by the pump but with no beneficial results.
Figure 4 shows the worn impeller repositioned as low as possible in the pump bowl so it establishes a better seal. This can reduce the volume of water recirculating within the pump bowls which results in a larger volume moving on through the pump and into the distribution system, increasing both volume delivered and efficiency.
Caution! Impeller adjustments to pick up a bottom seal must be performed by a qualified person who knows how to calculate the lineshaft elongation that occurs when the pump is operating under load. Great harm can be done to the pump if the impellers are improperly adjusted. Do not attempt to adjust the impellers yourself unless you know how to account for line shaft elongation based on your particular impeller model, lineshaft diameter and length and the total head the pump is producing.
Tom Dorn
Extension Educator, Lancaster County
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| Published by University of Nebraska-Lincoln Extension in the Institute of Agriculture and Natural Resources Cooperating with the counties and the U.S. Department of Agriculture. | ||
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