High Service Pump Efficiency Testing, Ranking Procedure
With the cost of petroleum, natural gas, and electricity on the rise, using energy wisely and efficiently is vital to keeping operating costs down. At Memphis Light, Gas and Water (MLGW), the same holds true. MLGW strives to provide its customers with electricity, natural gas, and water in the most efficient ways possible. On the water side of the company, the Water Energy Team (WET) is continually looking for ways to decrease the costs of providing customers with quality drinking water. WET is constantly investigating scenarios that would decrease the overall energy usage of MLGW’s water pumping stations while maintaining the demand required by the water system.
Memphis Light, Gas and Water is the nation’s largest three-service public utility. The utility serves more than 247,000 water customers and on a peak day supplies more than 250 mgd. MLGW uses one of the largest artesian well systems in the world to meet the water demands of Shelby County.
The Water Operations department of MLGW operates and maintains the 179 water wells and 10 water pumping stations that provide the water for MLGW’s distribution system.
In 1994, University of Memphis graduate student Kenneth Oliver conducted research on MLGW’s water well pumps and high service water pumps in order to devise a method to reduce the water pumping costs per volume of water delivered by MLGW. The pump efficiencies of the well and high service pumps were calculated and evaluated along with a system optimization simulation of the water pumping system. Oliver’s research and results were published in “Optimization of Pumping Costs for Well Fields and Distribution Systems.” The research published by Oliver is the basis for the current efficiency testing for MLGW’s high service pumps.
The purpose of the high service pump efficiency tests is to determine the pump efficiencies for each of the high service pumps at the eleven water pumping stations and to establish a ranking system from highest pump efficiency to lowest. Along with pump efficiency, cost of water for each high service pump will be evaluated and ranked from lowest cost to highest cost. The cost of water is evaluated in kilowatt-hours per million gallons of water per day (kWh/MGD).
Once the pumps are evaluated and ranked, Supervisory Control and Data Acquisition (SCADA) system water operators will incorporate the ranking information into the high service pump operating schedules. The operators will arrange pump schedules so that the pumps are operated in order of most efficient to least efficient. The results would be reduced energy consumption by the high service pumps and reduced electricity costs for all of the water pumping stations.
Another objective of the pump efficiency testing is preventative maintenance. By checking pump efficiencies semi-annually, maintenance crews will be able to monitor the performance of each high service pump. In the past, high service pumps would be regularly scheduled for removal from service for maintenance whether the pump was in need of maintenance or not. By testing the pump efficiencies, pumps showing a decrease in efficiency can be scheduled for maintenance, while pumps showing no decrease in efficiency may be left in service. This type of preventative maintenance would result in a decrease in labor hours and maintenance dollars spent on the servicing of high service pumps.
Due to the various types of pump arrangements, different calculations are used to determine pump efficiency and accompanying variables. Pumps are categorized as vertical and horizontal.
The total dynamic head (TDH) for each vertical pump is calculated based on the operating parameters of the water storage reservoir, discharge pressure gauge reading and various heights. Total dynamic head for vertical pumps is expressed in feet (ft.) and calculated according to the following equation.
TDH = Gauge Height (ft.) + [Distribution Main Centerline Elev. (ft.) - Reservoir Bottom Elev. (ft.) - Reservoir Level (ft.)] + [Discharge Pressure (psi) x 2.31 ft/psi]
For horizontal pumps, a vacuum system is necessary for priming the pumps prior to operation. The need for a vacuum suction for horizontal pumps adds another variable to total dynamic head equation. Total dynamic head for the horizontal pumps is identical to the TDH equation for vertical pumps with the addition of the vacuum suction variable. TDH for the horizontal pumps is also expressed in feet and calculated as shown below.
TDH = Gauge Height (ft.) + [Distribution Main Centerline Elev. (ft.) - Reservoir Bottom Elev. (ft.) - Reservoir Level (ft.)] + [Discharge Pressure (psi) x 2.31 ft/psi] + [Vacuum Reading (in. Hg) x 1.135 ft/in. Hg]
The pump efficiency (e) for the high service pumps is calculated according to the following:
where Q is the volumetric flow rate measured in gallons per minute (gpm), R is the number of revolutions made by the electric meter disk, Mc is the electric meter disk constant, and t is the amount of time, in seconds (sec), that R number of revolutions are made by the electric meter disk. This equation was used by Kenneth Oliver during the research of his thesis.
The cost of water for the high service pumps is calculated according to the following equation:
This equation was also utilized by Kenneth Oliver.
A data worksheet and spreadsheet have been created to collect data and calculate pump efficiencies, respectively, for the high service pumps at each of the water pumping stations. The data worksheets are designed to aid the maintenance crews in collecting the information necessary to calculate the pump efficiency for each pump. The worksheet is designed to collect information for vertical and horizontal pumps. The maintenance crews will fill out the appropriate section of the data worksheets with all the pertinent information related to that pump efficiency test.
Once the efficiency tests and data worksheets are completed, the test data will be input into the test spreadsheet. The spreadsheet is designed to perform all the necessary calculations to determine pump efficiency and cost of water for the high service pumps at each pumping station. Each pumping station has its own separate sheet in the spreadsheet containing the specific information for the high service pumps located at that station.
As data is collected and input into the spreadsheet, pumps are ranked according to pump efficiency and cost of water. Once the spreadsheet is complete, the information will be passed along to maintenance and SCADA for implementation.
As of the writing of this article, five of MLGW’s water pumping stations have had efficiency tests performed on their high service pumps. These tests have included 16 of the 42 high service pumps MLGW operates and maintains. Generally, a full day is reserved for maintenance crews to setup and perform an efficiency test for one pump. In the mornings, the crew will coordinate with SCADA to remove the pump from service.
Once isolated, crews begin installation of pressure gauges on the inlet and discharge of the pump. Crews will then wait until afternoon (when peak electrical demands are lowest) to begin the testing of the pump. The crew will test the pump at full flow (which the efficiency ranking is based) and varying percentages of full flow by adjusting the discharge line check valve.
Once the test is complete and all the data is collected, the crew will once again notify SCADA to remove the pump from service and disassemble the test equipment.
Table 1 shows the results from the 1994 tests conducted by University of Memphis graduate student Kenneth Oliver and the 2004 tests conducted by MLGW Water Operations Mechanical Maintenance crews.
As energy prices continue to rise, the Water Operations of MLGW has a procedure in place to monitor the energy consumption and cost to help provide the city of Memphis and Shelby County with high-quality drinking water at the lowest prices possible. This procedure also gives Water Operations a way to monitor the performance of their high service pumps which allows Water Operations to be proactive in the preventative maintenance of the high service pumps. By maintaining the high service pumps and utilizing the most efficient pumps, the electrical consumption and cost to pump water is vastly reduced, thus benefiting MLGW and its customers. With help from the Water Energy Team and Water Operations, MLGW can continue to lead the way in implementing new ideas in order to provide inexpensive, clean water to the people of Memphis. WW
About the Authors:
Kris Findley has been a Mechanical Engineer at Memphis Light, Gas and Water for over a year. He currently is a Energy Engineer in the Energy Resources Area. Findley obtained his BSME and MSME from Mississippi State University and is currently pursuing a MBA from the University of Memphis. Reginald Sisco has been employed at Memphis Light, Gas and Water Division for 23 years. Currently, his position for the last 13 years has been Supervisor, Plant Maintenance, Operations & Water Supply. He obtained his Associate of Engineering Technology degree in Instrumentation from State Technical Institute in 1982.