With the cost of petroleum, natural gas, and electricity on the rise, using energy wisely and efficiently is vital to keeping operating costs down.

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.

Source : ww.pennnet.com