A view of one of the mast-mounted mixers. The units can be adjusted for different angles of delivery, and raised up the pole for ease of maintenance.
"A three- to four-foot deep buildup of sediment would cover the floors of the 36' x 20' x 50' intake's two pump chambers and block the suction bells of the 1500-HP vertical submersible pumps in each chamber which are 16 inches above the floor," according to Ron Christian with CPS Energy's Field Operations.
"The configuration of the trash grate and traveling screens only aggravated the buildup and the originally designed back flushing system lacked enough force to clear the mud from the pump suction bells. This became a chronic and critical problem, especially during summer months," he said.
Without the makeup water, the level of the shallow lake declined daily.
A four-man crew often spent several days clearing the mud from the pits. Most often, their tool of choice was a large compressor unit that would power an air lance to loosen the sediment while flushing it out of the diversion structure. It was a tedious task inevitably repeated after nearly every high water event.
The problem was finally resolved when CPS Energy learned how another Texas utility's ingenuity had rectified a similar problem. During a meeting with that utility, CPS learned how it had dealt with an almost identical problem at the raw water pump station serving their power station. They stopped the silt settling problem by installing Flygt submersible shrouded mixers, manufactured by ITT Water & Wastewater. The compact mixing units reliably stopped the sediment from settling and blocking the intake to the pumps.
CPS Energy's Jason Wauson and Will Warnke worked closely with an ITT Water and Wastewater representative and the manufacturer's applications engineers to determine the correct mixer selection and integration into the pumping station. This led to purchasing four, mast-mounted ITT Flygt Model 4640 mixers that were installed by a local contractor.
"During the installation, the pump station's intake was closed off and the pits drained and cleared prior to receiving the shrouded axial impeller units," Christian said. "Two were mounted on masts with cables in each pit about one foot above the floor on opposite sides of the intake pump suction bells. The mixers can have their delivery angle reoriented 180 degrees off the wall line and the units raised up and down using their cables. The utility's personnel installed new water level sensors and mixer controls for automatic start/stop operation whenever river levels are 401- to 403-ASL and the river pumps not in service.
