Five years ago, we ran an article about a city of Dallas program to upgrade flood protection facilities.
Author Al Petrasek Jr., an HDR Inc. vice president, explained the solutions Trinity Watershed Management Department managers considered before deciding on something common in Europe but new to the U.S.: concrete volute pumps (CVPs).
Since then, two pump stations (New Pavaho and Baker No. 3) are in service, and a third (Able No. 3) is under construction.
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We asked Petrasek, who’s since retired, to tell us how the project’s working.
The first station completed, Pavaho has 56 feet total dynamic head. Two variable-frequency drives (VFDs) can switch between three CVPs; two primary circuits, one of which is redundant, provide power.
The drivers on Baker No. 3’s four CVPs are 3,000 hp electric motors. For motor starting, two VFDs can be switched between any of the pumps. Primary power is provided by two circuits, one of which is redundant.
Discharge piping is connected to existing box culverts, eliminating the need for a levee crossing, which was also done at Pavaho. However, to ensure column separation, a vertical loop referred to as “the candy canes” was installed. These keep the pump from going into siphon operation and running off of the pump curve.
Able No. 3’s four pumps will be powered by 3,750 hp induction motors and started by one of four VFDs. There are three primary circuits into the site, two of which are required to operate all four pumps. The third, redundant circuit can be switched to back up either of the other two circuits.
Epitome of Reliability
One reason the Trinity Watershed Management Department selected the pumps is their reliability.
A survey of 23 pump stations with 59 pumps and 1,461 pump years of operation revealed one unplanned mechanical failure. That was during start up and, after correction, the pump has operated with no problem. These data indicate that a mechanical failure in a four-CVP station will occur once every 346 years.
The pump’s low rotating speed, which results in low bearing velocities and low vibrational forces, is a major reason the design is so reliable.
Dallas had record-setting rain in spring 2015. The Pavaho and Baker CVPs saw a lot of run time but performed flawlessly, saving many homes from being inundated.
A local television station asked a resident in an area that used to flood frequently how he liked the new pump station. “Oh, it's working real wonderful,” he said. “We don't have the trouble no more.”
The assistant director for Flood Control and Citywide Interior Drainage had this to say: “They work as designed and are helping keep citizens safe. When it comes to flood fighting, the CVPs rock!”
