Vfd Operational Efficiencies Analysis Results For City Owned Sewage Pump Stations

Nearly ten percent of the City’s 400+ sewage pumping stations feature pumps controlled with variable frequency drives. The VFD’s are typically installed toprovide reliable pump operation under highly variable discharge head conditions, preventing pump run-out under low discharge head conditions while accommodating pumping equipment capable of delivering peak flows under maximum discharge head conditions. As the City continues to prioritize and improve its sewage pumping stations, more and more variable speed pumping equipment is installed.The City’s current VFD control strategy increases pump speed based on incremental increases in wet well level. Wet well level is measured with traditional Murphy gauges and bubbler tubes. The City is preparing to upgrade its SCADA system with PLC based controls that can be utilized to enhance pump control options. An evaluation was performed to assess operational performance associated with alternate control strategies and develop the justification and control strategy for VFD’s at the sewage pumping stations. As part of the analysis, data including pump run time, starts per hour, discharge pressure and flow rate, and power consumption was collected to determine the most efficient and lowest operational cost alternative. The following control scenarios were evaluated:1. Existing wet well level based control2. Pump speed control to maintain a target wet well level3. Pump speed control based on discharge pressure4. Pump ramp to 30 Hz then adjust speed to maintain a target wet well level5. Pump ramp to 60 Hz then operate at 60 Hz until the pump off level is attained