Pump Case Studies
Blower Use for an Industrial Application
Background
The town of Trumbull, CT was looking for a way to increase the operating performance of one of its ten sewage-pumping stations. The station consisted of two identical sewage-handling pumps (each with a 40-hp direct drive motor) vertically mounted below ground, handling 340,000 gallons of raw sewage per day. The system used one pump to handle the entire flow under normal operation, and used the second pump only in extreme conditions (heavy rainfall). To meet normal loads, each pump rarely operated more than 5 minutes at a time. The control system required two continuously running compressors. A constant pump speed of 1,320 rpm was obtained using a wound rotor and variable resistance circuit motor control system. The pumping system experienced frequent breakdowns, occasional flooding, and sewage spills.
After a thorough systems analysis, engineers installed an additional 10-hp pump with direct on-line motor starters and a passive level control system with float switches, replacing the old active control system. The new pump handles the same volume as the original 40-hp pumps during normal periods, but runs for longer periods of time. The lower outflow rate reduces friction and shock losses in the piping system, which lowers the required head pressure (and thus, the energy consumption).
In addition, the existing pump speed control was eliminated and the motors were wired for direct on-line start. Without the speed control, the motors powering the existing pumps run at 1,750 rpm instead of 1,320 rpm, so their impellers were trimmed to a smaller diameter. The existing pumps are still used for the infrequent peak flows that the new smaller pump cannot handle. Energy consumption was further reduced through the elimination of the two compressors for the active control system and the two circulating pumps for the old motor control system. The installed cost of all the added measures was $11,000.
Pump system use and savings
In addition to the annual 17,650 kWh of electricity savings from modifying the pump unit, significant energy savings also resulted from changes made to other energy use sources in the station. Annual energy consumption of the active level control (7,300 kWh/year) and the cooling water pumps (1,750 kWh/year) was entirely eliminated. In all, over 26,000 kWh is being saved annually, a reduction of almost 38%, resulting in $2,200 in annual energy savings.
This project also produced maintenance savings of $3,600. Maintenance staff no longer need to replace two mechanical seals each year. Other benefits of the project savings include extended equipment life due to reduced starting and stopping of the equipment, increased system capacity, and decreased noise. Most of the same measures can be utilized at the town's other pumping stations, as well.
The total annual savings from the project, due to lower energy costs as well as reduced maintenance and supplies, is $5,800 (see figure below), which is roughly half of the total retrofit cost of $11,000. Retrofit cost savings ($5,800 annually)
Retrofit cost savings ($5,800 annually)
Lessons Learned
Several key conclusions from Trumbull's experience are relevant for virtually any pumping systems project:
- Proper pump selection and careful attention to equipment operating schedules can yield substantial energy savings.
- In systems with static head, stepping of pump sizes for variable flow rate applications can decrease energy consumption.
- A "systems" approach can identify energy and cost savings opportunities beyond the pumps themselves.