San Antonio utility uses mixer technology to keep makeup water flowing to manmade lakes
For almost 50 years, CPS Energy of San Antonio has drawn water from the San Antonio River to recharge Braunig and Calaveras lakes, which the utility built in the 1960s to provide cooling water for its power plants.
As is often the case with flat-water streams, the San Antonio River can experience high water events that churn the meandering channel into a raging torrent of silty muck. These incidents left in their wake a thick buildup of mud on the floors of the 36-ft-by-20-ft-by-50-ft intake structure of the CPS pump station that draws from the river to recharge Calaveras Lake.
A 3- to 4-ft-deep sediment buildup would cover the floors of the two pump intake chambers, blocking the suction bells of the chambers’ 1,500-hp vertical submersible pumps, which are 16 in. above the floor.
“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,” said Ron Christian of CPS Energy’s field operations. “This became a chronic and critical problem, especially during summer months.”
Without the makeup water, the level of the shallow lake declined daily, due in part to evaporation caused by the unrelenting Texas sun.
A four-man crew often spent several days clearing the mud from the intake chambers with a large compressor unit that powered an air lance to loosen the sediment while flushing it out of the diversion structure. It was a tedious task that was repeated after nearly every high water event, costing man-hours and downtime to pump makeup water during this critical period.
During a meeting, CPS learned how another Texas utility had dealt with an almost identical problem at the raw-water pump station serving its power station by installing Flygt submersible shrouded mixers manufactured by Xylem. The compact mixing units reliably stopped sediment from settling and blocking the intake to the pumps. At that facility, four Flygt-brand model 4640 mixers immediately prevented silt from settling and building up.
Working closely with a Xylem representative and Flygt applications engineers, CPS Energy’s Jason Wauson and Will Warnke selected four Xylem Model 4640 mixers to be installed at the CPS pumping station.
The pump station’s intake was closed off and the pits were drained and cleared prior to installing the four Xylem Flygt mixers. Two mixers were mounted on masts on each side of the pit, allowing the delivery angle to be reoriented 180 degrees off the wall line and the units raised up and down for service and adjustment. Utility personnel installed new water level sensors for the Xylem Flygt-supplied controls for automatic operation during high water events, as well as cycle timers for silt migration during normal flows.
“As soon as the mixers were activated in the first pit, even the small amount of sediment remaining on the floor was immediately churned up and flushed down river,” said Christian said.
The mixers have performed during the many high water events that have followed, solving the problem caused by micron-size silt and saving the man-hours once needed to clear the pits to help ensure a reliable source of makeup water from the river.