Challenge

The Searcy Water Treatment Plant in Arkansas serves approximately 50,000 people, processing 15 million gallons of water each day. The low alkalinity/buffering ability of the raw river water necessitates the addition of hydrated lime (2 to 6% strength) in order to achieve proper chemical reactions. After treatment and filtration of the water, lime solution is again added to increase pH to a level determined to prevent corrosion in distribution piping.

 In 1997, twin diaphragm pumps with maximum capacity of 1200 liters per minute and maximum working pressure of 45 psi were installed at the two lime injection sites. A by-pass valve was incorporated to allow either pump to discharge either point in case of a failure or out-of-service condition.

Unfortunately, this application proved to be a disaster. When the treatment plant filled to capacity and cycled off for a few hours each day, the lime in the pump suction lines would come out of solution. When the plant came back on line, the first stroke plugged the suction or discharge check valves. This condition not only stopped the discharge of the pumps by 50% or more, it also allowed a backflow of clean water to dilute the contents of the lime feeder tank, reducing its strength to an undeterminable amount.

Solution

The existing pumps were replaced with M-Series Chemtube® PPS from Wallace & Tiernan.  With the Chemtube PPS pumps, there are no valves nor seats required in the pumped fluid to foul or wear out, as with other pump types.

• In addition, premium, efficient roller technology eliminates use and disposal of the lubrication fluid typical of other peristaltic pumps using a sliding-shoe design.

Results

“I was searching for some type of positive displacement pump with no check valves,” says Water Treatment Division Manager Scott Boggs, “and could not think of a better approach than the W&T Chemtube PPS – M Series.” After years of spending upwards of $400 each month for replacement parts for the plants’ twin diaphragm pumps, ordering replacement hoses only once a year for the Chemtube PPS is a great relief.

• The initial design plan was to have two identical pumps to feed the two different points, in order to afford parts interchangeability and pump duplication for emergencies. “In retrospect this has not been a valid concern and I would size closer to the specific feed point need if I could go back,” says Boggs.
• The addition of intermittent run cycle timers allows the batch strength to be maxed out and the pumps still run proportional to plant flow--they now run 15 seconds per minute, then stop for 45 seconds and cycle again. “Cost savings on the hoses alone will be $1000 to $1500 per year,” says Boggs. We have had little go wrong with the W&T pumps and representative support is top notch,” says Boggs. I have little patience with inferior products and only the best get to stay. The W&T Chemtube PPS M-Series hose pumps are keepers. The simplicity of design makes maintenance almost nil and we really were able to gut the engineering folly previously in place.”