The following case studies, performed by the author, paint a picture of the savings that can be achieved with properly executed pump field testing.
Deep Well Pump Wire-to-Water Efficiency Tests, Castle Rock Water Department (CRWD)
CRWD spends 93 to 99 percent of its total monthly energy operating cost on pumping water out of deep well aquifers that are 1,500 to 2,200 feet below ground. These wells also suffer from significant iron and manganese fouling that decrease the pipe cross sectional area and roughness coefficient. Evaluating the wire-to-water efficiencies of the top 10 of 68 highest energy consumption wells was a high priority. Field testing revealed that nine of the 10 variable frequency drive (VFD)-controlled, deep well pumps experienced wire-to-water efficiencies between 40 and 56 percent.
EEMs were developed that were estimated to save more than 1,240,000 kilowatt hours per year (kWh/yr) and 500 kW. EEMs ranged from de-staging the well pumps due to higher pumping water levels than the original design, to operating two of the 10 well pumps at higher speeds so greater than 62 percent wire-to- water can be achieved. This mode of operation resulted in an extra 400 gallons per minute (gpm) production from operating the well pumps at higher speeds while allowing CRWD to turn off two 200 gpm, 200-horsepower (hp) well pumps that operated at 40 percent efficiency, so they can be de-staged.
High Service and SW Pump Station Wire-to-Water Efficiency Tests, Fort Collins Water Department (FCWD)
Wire-to-water efficiency testing was performed on the city’s high service pump station and southwest pump station. Results of the testing revealed the city could reduce the kW demand charges by 18 percent with the implementation of the EEMs. Examples of recommended EEMs were:
- replacing a pump’s isolation butterfly valve, which was protruding into the cavity of the swing check valve immediately downstream, with a gate valve to allow the swing check to fully open
- installing a combination air valve on an air-locked pump discharge on end suction centrifugal pumps
- increasing the speed of the VFDs to a frequency that allows the pump system to overcome the backpressure of the system curve and provide flow at a substantially greater efficiency
Wire-to-Water Efficiency Tests, Paducah Water, Kentucky (PW)
A unique aspect of this project was incorporating the true wire-to-water efficiencies and field-verified system head curves from the testing into the extended period simulation hydraulic model, while also developing a punch-list of EEMs. The EEMs will be integrated into the annual Capital Improvement Program (CIP) based on priority. Results of the field testing revealed the majority of pumps tested experienced wire-to-water efficiencies between 0 and 50 percent, and only six of the 21 pumps tested exceeding 60 percent wire-to-water efficiency. Several immediate and long-term EEMs were identified such as:
- Increase the speed of a VFD so it pumps water and does not just spin in the casing.
- Increase pumping level in the clear well to reduce the kilowatts per millions of gallons per day (kW/mgd) by 15 percent.
- Add air release valves at top of split case pump casing and at several intermediate pipeline highpoints.
- Replace leaking check valve allowing 60 to 80 gpm to flow backwards through the pump.
- Install straightening vanes on split case pump suction to improve suction straight pipe lengths and balance impeller force load distribution.
- Lock out maximum hertz for certain pumps that cavitated at high speed.
Maple Grove Ultra-Filtration WTP Membrane Feed Pump Testing, CMWC
Wire-to-water efficiency testing was performed on the six 50-hp, VFD-controlled membrane feed pumps. Xcel Energy’s Demand Side Management (DSM) Rebate Program paid for $15,846 of the $21,128 fee to determine the wire-to-water efficiencies and develop EEMs that would result in lower kW/mgd. Testing revealed that five of the six pumps had wire-to-water efficiencies less than 30 percent during the low-flow operation totaling six months of the year.
The sixth pump had the membranes replaced with newer membranes that also offered 28 percent more surface area. Pump six also operated 45 percent more efficiently than the other pumps, while also having the ability to operate at two times the flow at high speed while remaining within the membrane flux requirement. The EEM for this project was to replace the other trains with the same amount of newer membranes, so an estimated annual energy savings of $14,400 can be achieved. CMWC already had the budget to replace the aging membranes. However, realizing the energy savings associated with the replacement helped them prioritize this project.