June 19, 2009

Improve Irrigation Efficiency, Save Energy Costs

Is your irrigation pumping plant operating at about the average efficiency level of all other pumping plants in Nebraska, or perhaps above or below that efficiency? Nebraska research indicates that if your irrigation pumping plant is operating at the average efficiency level of pumping plants in Nebraska, you could be using 30% more energy than necessary.

Most irrigation in Nebraska depends on groundwater as the water source so vertical turbine multi-stage pumps are used. The University of Nebraska has field tested hundreds of pumping plants over the years. Based on these field tests and on laboratory tests of engine efficiency, the University developed the Nebraska Pumping Plant Performance Criteria (NPC). This criteria states the amount of useful work (water horsepower - hours, whp-h) that you should reasonably expect to achieve in the field for each unit of energy consumed by a pumping plant.

Figure 2. A cutaway view of one stage of a multi-stage enclosed impeller and bowl assembly as it would appear when first installed. Figure 3. The same assembly as in Figure 2, but years of wear and sand in the irrigation water have worn down the surface of the impeller. Figure 4. Worn impeller repositioned as low as possible in the pump bowl so it establishes a better seal.

Testing Pumping Plant Efficiency

In a pumping plant test, the technician measures total head (lift plus system pressure), flow rate (gallons per minute), and rate of energy consumption. Performance of the pumping plant is stated in terms of water horsepower hours (whp-h) per unit of fuel. The performance rating is the performance of the particular pumping plant compared to the Nebraska Performance Criteria and is 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 than the criteria calls for to do the same level of work. 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.

The most recent statewide pumping plant efficiency study conducted by the University of Nebraska tested 130 farmer-owned pumping plants. As one might expect, the efficiency of the pumping plants tested by the University varied considerably. Some pumping plants achieved 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. The overall average pumping plant in Nebraska was found to be operating at only 77% of the NPC. That means the average pumping plant in the study was using 130% as much energy as it would if it were operating at the NPC (1.0/0.77 = 130%). Stated differently, the average pumping plant is using 30% more energy than necessary.

Identifying the Problem

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.

Poor pump performance may be caused by: 

1. pump designs that are poorly matched to the job (for example, this might occur when the operator switches from gated pipe to a center pivot sprinkler or from a high pressure sprinkler to a lower pressure package without changing the pump),
2. pumps that have worn impeller vanes and/or internal seals as a result of pumping sand, or
3. impellers that were not properly adjusted within the pump bowls.

There are many pump manufacturers and each manufacturer can have dozens of pump designs in their catalog. A given impeller design operates on a head versus capacity curve for a given rotational speed. The greater the head (feet) the pump is working against, the lower the capacity (GPM) the impeller can produce (see 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 different head/capacity conditions on either side of the best efficiency point.

Pump Adjustments That Reduce Energy, Costs

In the recent pumping plant tests, 58% were determined to potentially benefit from adjustments. Field adjustments made with a wrench 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 that 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.

If a water meter isn't 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. Use a reputable well driller who is equipped to run a short-term pumping plant efficiency test. At today's energy prices, identifying a pumping plant that needs adjustment or repair could save hundreds or even thousands of dollars per year.

If the producer has records of total fuel use over a given 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), the performance rating can be estimated. This estimate can be used as an initial screen to help identify pumping plants that may require an experienced technician.

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 illustrates 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, a seal must 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 wears down the surface of the impeller vanes and causes wear to the impeller skirt and to the bowl in the wear ring area. Wear opens up the seal area allowing some of the higher pressure water exiting the impeller to flow back into the lower pressure area below the impeller. This water not only disrupts the smooth flow of water into the eye of the impeller, it is repumped and repressurized by passing back through the impeller. This constant recirculation of a portion of the water adds to the work being performed by the pump 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.

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