Pumping System Efficiency Calculator Instructions and Tips
1. Introduction
This "Pumping System Efficiency Calculator" can be used to determine the overall "wire-to-water" efficiency of your pumping plant and to determine the mechanical efficiency of an electric motor-driven centrifugal pump served by a three-phase power supply. To use this calculator you must:
- Measure pump discharge pressure (in psig)
- Measure pump suction pressure (in psig) or sound your well to determine pumping water level and static lift (in feet)
- Measure pump flow (in gpm)
- Determine column friction head losses (in feet)
- Enter the specific gravity of the liquid being pumped
- Enter the horsepower, synchronous speed, full-load efficiency and enclosure type for your pump drive motor, and
- Measure the input power to your pump motor.
The wire-to-water efficiency of your pumping plant is the ratio of the water horsepower provided by your pumping plant---which is proportional to the amount of water or other fluid being delivered (in gpm) at your operating pumping head (in feet)---divided by the electrical power supplied to the pump drive motor (input kW are converted into input hp). The greater your overall efficiency is, the lower your overall pumping costs will be. Another useful performance indicator is the quantity of water delivered (in gallons) per kWh of electrical energy consumed.
Efficiency tests assist pumping plant operators to identify inefficient systems, to schedule pumps, determine energy efficiency improvement measures, and to estimate potential energy savings. Efficiency tests are usually scheduled on larger pumps that operate for extended periods of time.
2. Motor Load and Efficiency Estimation
Motor load and efficiency values are determined by entering the input power to your pump drive motor, by measuring the supply voltage and current on each phase, or by determining an input kW value through entering meter information and then counting revolutions over a known time increment. The equations used to determine the load on your pump motor are:
Kilowatt Ratio Motor Load Estimation Technique
Voltage Compensated Amperage Ratio Technique
Once the motor load is known, the motor efficiency at that load point must be determined so the pumping system efficiency calculator can isolate the pump efficiency from the overall pumping system efficiency. The pumping system efficiency calculator uses the entered full-load efficiency value for the pump drive motor to ascertain its efficiency class. Motors with no nameplate full-load efficiency are assumed to be standard efficiency motors. The calculator then refers to one of three part-load efficiency tables containing full, 75, 50, and 25 percent motor efficiency values. Full and part-load efficiency tables are available for:
- NEMA Premium Efficiency motor. With a full-load efficiency value equal to or exceeding the NEMA Premium EfficiencyTM motor standard (As defined in Table 12-12 of NEMA MG 1-2006, Rev 1).
- Energy Efficient motor. With a full-load efficiency value equal to or exceeding the NEMA Energy Efficient motor standard (In accordance with Table 12-11 of NEMA MG 1-2006, Rev 1).
- Old Standard Efficiency motor. Having a full-load efficiency below the minimum standards given in Table 12-6B of NEMA MG 1-1987.
The tables contain efficiency characteristics for motors of specified speed, horsepower, and enclosure classes. The motor full and part-load performance characteristics tables were derived by examining all motors in the manufacturer's database from the Department of Energy's MotorMaster+ software tool. Efficiency values at intermediate load points are obtained through linear interpolation. When the voltage compensated amperage ratio technique is selected, motor input kilowatts are determined from the calculated load and efficiency at load point values. Below the 25% load point, the motor efficiency is considered to be equal to the load served divided by the load served plus the fixed losses at the 25% load point.
3. Pumping Plant Test Data
The pumping plant test must be performed at one time when conditions are constant. The test should not begin until the water level in a well has stabilized so that the pump water level can be measured. Discharge pressures, flow rates, and input power measurements should be measured at the same time. Pump flow rates can be determined with portable instruments such as a sonic (Doppler-type) or "transit time" flow meters or by inserting a Pitot tube or manometer into the discharge pipe. To avoid turbulence, the ideal length of straight discharge pipe ahead of the Pitot tube access point is eight times the discharge pipe diameter.
Determine the pumping water level or required lift (for a well pump) or the suction pressure for a pump supplied by a booster pump or an elevated tank. In order to determine the pumping water level for well pumps, it is necessary to "sound" the well. Some pumps have sounding access holes in the pump head. Newer wells may include an "airline" that can provide a rapid determination of water level.
3a. Column Friction Head Losses
Friction or head losses occur when water flows through a pump column pipe. The head loss is greater in a pump column than in regular pipe of the same diameter because the pump column also houses the lineshaft and oil tube. Higher friction losses occur as flow velocity increases due to reduced cross-sectional area.
As the column friction head loss cannot be measured in the field, the Pumping System Efficiency Calculator provides default friction losses for columns between 4 and 10-inches in diameter. The friction losses are derived from the Western Area Power Administration's "Revised Irrigation Pumping Plant Test Procedure Manual". Figure 11 in this reference document gives the expected water head loss per 100 feet of pump column length as a function of the column diameter, oil tube size, and flow rate (in gpm). Also enter the friction losses (in feet) due to fluid flows through inlet and discharge valves and fittings and the height of the pressure gauge above the discharge line (in feet).
4. Interpretation of Results
Overall or "wire-to-water" pumping plant efficiency is equal to the product of the motor and pump efficiencies. Efficiency values for pumps and motors do not approach 100%, even when they are in excellent condition. The full-load efficiency for an electric motor is in the range of 85% to 96%, while the peak efficiency of a new pump is often in the range of 75% to 85%. The efficiency of larger pumping plants is expected to be higher than that for smaller plants. Pumping plant performance can be classified as "Low", "Fair", "Good", or "Excellent" by referring to the following table, which is based upon the results of thousands of pump tests conducted by Pacific Gas & Electric Company (see PG&E Application Notes "Agricultural Pumping Efficiency Improvements").
| Motor Hp | Low | Fair | Good | Excellent |
| 3 - 7.5 | < 44.0 | 44 - 49.9 | 50 - 54.9 | > 54.9 |
| 10 | < 46.0 | 46 - 52.9 | 53 - 57.9 | > 57.9 |
| 15 | < 47.1 | 48 - 53.9 | 54 - 59.9 | > 59.9 |
| 20 - 25 | < 48.0 | 50 - 56.9 | 57.0 - 60.9 | > 60.9 |
| 30 - 50 | < 52.1 | 52.1 - 58.9 | 59 - 61.9 | > 61.9 |
| 60 - 75 | < 56.0 | 56 - 60.9 | 61 - 65.9 | > 65.9 |
| 100 | < 57.3 | 57.3 - 62.9 | 63 - 66.9 | > 66.9 |
| 150 | < 58.1 | 58.1 - 63.4 | 63.5 - 68.9 | > 68.9 |
| 200 | < 59.1 | 59.1 - 63.8 | 63.9 - 69.4 | > 69.4 |
| 250 | < 59.1 | 59.1 - 63.8 | 63.9 - 69.4 | > 69.4 |
| 300 | < 60.0 | 60 - 64.0 | 64.1 - 69.9 | > 69.9 |
If your pumping plant efficiency is rated as "Excellent", no corrective action is necessary. If your plant performance is rated as "Good", consider adjusting your impeller clearances. If your performance is only "Fair", consider adjusting your impeller and repairing or replacing your pump if the impeller adjustment has little effect. Consider repairing or replacing your pump when the pumping plant performance is rated as "Low".
5. Improving Pumping System Performance
The University of California at Davis has identified a number of common causes for poor irrigation pumping plant performance including:
- Wear (often due to sand pumping)
- A pump that is improperly matched to irrigation (or process) requirements
- Changes in pumping conditions (irrigation system distribution piping, pressure requirement, or groundwater level changes)
- A clogged pump impeller
- Poor suction conditions, and
- Throttling the pump discharge
Once the total dynamic head (TDH in feet) and fluid flow (in gpm) have been determined, the actual operating point on the pump's performance or head/capacity curve is known. When the operating point falls below the original pump curve for the impeller size installed with your pump, it is a possible indication that pump efficiency has been reduced by worn or maladjusted parts or excessive impeller clearances. Corrective actions include restoring internal clearances, and replacing or refurbishing worn or damaged throat bushings, wear rings, impellers, or pump bowls.
A pump adjustment or repair often results in an increase in flow rate, total dynamic head, and input kilowatts supplied to the pump drive motor. This is not of concern for pumps that operate in an "on/off" mode controlled by a reservoir water level indicator as the running time is decreased. Energy consumption will increase for continuously operating booster pumps or circulating water pumps. For irrigation systems, a reduction in pump operating time is necessary to achieve energy savings and avoid over applications of water. This means changing you set times or increasing the number of irrigated acres per set.
A mismatched pump is one that is operating properly, but is not operating close to its Best Efficiency Point (BEP). Compare your delivered head and flow rate with your required head and flow. The PG&E Application Note "Agricultural Pumping Efficiency Improvements" states that "The single greatest contributor to pump inefficiency is an over-sized pump…:.it will deliver excess fluid at a higher head than is necessary..." A throttling valve on the pump discharge is often set to reduce the flow rate when the pump is over-sized. Pumps that differ from their BEP flow rate by more than 30% require detailed study.
Sometimes inefficient, low cost pumps with standard efficiency drive motors are purchased or acquired at an auction and installed. Changeouts to more efficient pumps with NEMA Premium Efficiency™ drive motors, impeller trimming, the use of adjustable speed drives, installation of a pony pump, and parallel pumping are strategies that can be employed to reduce the excess energy consumption associated with an over-sized or mismatched pump. Additional information is available from the Pumping System Energy Tips at the U.S. Department of Energy's Industrial Technologies Program download website: http://www.eere.energy.gov/industry/bestpractices/tip_sheets_pumps.html
Potential energy savings due to restoring a pump to its original operating condition are determined by comparing the actual wire-to-water pumping plant efficiency (ηa ) with an optimal operating efficiency (ηo ). The U.S. Department of Energy's Pumping System Assessment software tool (PSAT) provides estimates of optimal pumping efficiency.
The equation for calculating annual energy savings due to a pumping system efficiency improvement when the pumping system operating time does not change is:
Savings (kWh) = kW x t x ( 1 - ηa / ηo )
Where:
kW = Input power to the pumping system drive motor, and
t = Annual operating hours
When a change in pumping system operating time occurs due to a pump adjustment or repair, the energy savings are:
Savings (kWh) = kW1 x t1 - kW2 x t2
Where:
kW1 = Original or baseline input kilowatts to pump drive motor
kW2 = Input kilowatts to pump drive motor after a pump adjustment or repair has been made
t1 = Baseline annual operating hours
t2 = Adjusted annual operating hours
6. Further Resources
Agricultural Pumping Efficiency Improvements (pdf) - Pacific Gas & Electric
Irrigation Pumping Plant Efficiency - Colorado State University Extension
Irrigation Scheduling - Colorado State University Extension
Irrigation Pumping Plants (ppt) - Center for Irrigation Technology. California State University.
Best Management Practices For Irrigation Management (pdf) - USDA-SCS, Colorado Irrigation Guide
Handbook for Agricultural Water Use Efficiency (pdf) - Santa Clara Valley Water District
Energy Saving Tips for Irrigators - ATTRA - National Sustainable Agriculture Information Service