Field testing programs in Colorado, Wyoming, Nebraska, Texas, Louisiana and other states have shown that overall pumping plant or ‘wire-to-wire’ efficiencies for electrically driven pumps average 45 to 55 percent, as compared to a realistically achievable efficiency of 72 to 77 percent. This implies that around 25 percent of the electrical energy used for pumping is wasted due to poor pumping plant efficiencies alone. Therefore, farmers can reduce energy costs by raising pumping plant efficiencies from present average levels to potential efficiencies. Farmers are advised that pumping plants should attain at least 65 percent efficiency and every new pumping plant should be tested to determine the pumping plant overall efficiency.
Common Causes of Unsatisfactory Performance
There are many reasons for poor pumping plant efficiency. Some of the more common causes of unsatisfactory performance and their remedies are as follows:
- Impellers that are out of adjustment are the easiest and least expensive problem to correct. Both pumping rates and efficiency are reduced because energy is used to pump water that is recirculated around the impellers instead of being pumped into the irrigation system. Impeller adjustment is especially critical with semi-open impeller pumps. Impellers may be out of adjustment because of improper initial adjustment or because of wear. To avoid pump damage, only experienced pump people should attempt to make impeller adjustments.Field adjustments include: a) for semi-open impellers, all impellers in the bowl assembly must be running in close proximity (0.003 – 0.007 in) to the next lower bowl. Thus, careful adjustment in the field is required. Shaft stretch determines the final position of the impellers. Also, it directly varies with discharge head. Therefore, it has to be set to a proper specification to perform well at a given discharge head. Multistage units may require that the impellers be trimmed (reduction in diameter) to obtain proper fitting and clearance in the assembly bowl. For enclosed impellers, with a principal seal that is parallel to the centerline of the shaft, a close axial adjustment is not necessary. Therefore, this type of impeller is suited for operation under variable head conditions. Capacity (see terms definition in the “Useful Definition” section at the end of the document) and horsepower requirements can be controlled by raising the impeller until the skirts are out of the wear rings.
- Pump bowls designed for a higher pumping rate than the well can supply is one of the most common reasons for poor pumping plant efficiency. Overestimating well yield often results from poor testing of the well after drilling. If well testing was inadequate, the yield of the well may have been less than anticipated. In other cases, the pump supplier recommended oversize pump bowls in order to require fewer stages, thereby reducing initial cost. Furthermore, declining water tables in some areas have reduced well yields. In this situation, a pump may begin to cavitate (partial vacuum, low temperature boiling of pumped water that causes vibration and noise from water hammer) because it is being is forced to operate at a lower flow rate and higher lift than that for which it was designed. If for any of these reasons the pump capacity does not fit the well characteristics, a high pumping plant efficiency can be achieved only by replacing the bowls with new (not rebuilt) bowls that meet the well requirements.
- Damaged impellers also will result in poor performance. Three common causes of impeller damage are cavitation (also resulting in reduction of discharge and deterioration of other pump parts), sand pumping (due to well filter failure or design problems) and improper impeller adjustment. Sometimes only the impellers need to be changed, but more often the permanent solution is to replace the entire bowl assembly. If this is done, it is likely that a different model of pump bowls should be used to fit present well conditions.
- Incorrect power unit selection is another major cause of low efficiency. This is much more important for engines than for electric motors. While the efficiency of electric motors does not vary greatly with loading, it should be noted that over-loaded motors have shorter lives, are less dependable and are more expensive to maintain. On the other hand, because of graduated energy costs, underloaded motors often increase the cost per kilowatt of power used. Incorrect engine selection is a major cause of low efficiencies among the natural gas pumping plants. Many are overloaded. Automotive-type V-8 engines often are used for applications where heavy-duty industrial engines should be used. Operating speeds of the smaller engines are increased so that they will produce adequate power. As a result, they wear out rapidly and require much more fuel.
- Failure to perform required maintenance, including tune-ups, is often a cause of low efficiency in engine-driven pumping plants. Electric motors, on the other hand, usually operate efficiently. In the case of semi-open impellers, close adjustment is necessary for proper operation. Thus, if variation in required discharge head occurs then the pump could be damaged. The higher thrust requirement may affect the lift of the thrust bearings, therefore fast bearing wearing can be expected. Monitoring the pumping unit pressure head and flow discharge is critical to assure proper operation and a longer unit life span. Enclosed impellers, on the other hand, will have increased bearing life with up to 30 percent less thrust (large discharge head variation/demand). The lower thrust allows using smaller shafting, which affects the cost of the initial installation and on maintenance.
- Differences in operating conditions. A change in operating conditions from those for which a pumping plant was designed will result in a drop in efficiency. Three common situations that result in increased pumping lifts and total discharge head (Figure 1) or pressures are a drop in water table elevation, converting from open discharge to a pipeline, and changing from surface irrigation to sprinkler/trickle (pressurized) irrigation. On the other hand, a reduction in operating pressure results when center pivot sprinklers are converted from high pressure to low pressure in an attempt to save energy. Usually the pump will operate less efficiently under the new lower pressure conditions than it did under high pressure. As a result, anticipated savings in energy costs may not be realized.
- Poor plumbing. Horizontal axis/centrifugal pumps have a range window of pressure and flow rate conditions for the inlet and outlet of the pump for optimum efficiency. Some pumps require inlets constantly flooded, others need sufficient back pressure on the outlet. If a pump is not operating in optimum conditions water hammer and cavitation are common symptoms along with frequent replacement of impellers and seals. Consult with your pump vendor on pump suitability and always examine installation instructions carefully before purchasing accompanying pipework.
Impellers that are out of adjustment are the easiest and least expensive problem to correct. To avoid pump damage, only experienced pump people should attempt to make impeller adjustments. Incorrect power unit selection is another major cause of low efficiency. This is much more important for engines than for electric motors. Consult with your pump vendor on pump suitability and always examine installation instructions carefully before purchasing accompanying pipework. A pump test or energy audit can help you understand if pumps are performing adequately and can prescribe solutions.Last updated: October 19, 2017 at 14:59 pm