Florida coastal town resolves clogging pumps with special impeller
A Florida water reclamation plant's persistent problems with clogged pumps was resolved with a special type of impeller which can cut through fibrous material.
COCOA BEACH, Fla., Oct. 26, 2000—A Florida water reclamation plant's persistent problems with clogged pumps was resolved with a special type of impeller which can cut through fibrous material.
On Florida's Atlantic coast, Cocoa Beach Water Reclamation Department's (CBWR) prime mission is to convert sewage generated by its residents and tourists-plus nearby Patrick Air Force Base-into irrigation water for lawns at an adjacent golf course, and the base's public areas.
That mission, however, was threatened less than two years ago when a series of new pumps, installed to boost plant capacity as part of a $12 million upgrade, almost immediately started to fail. While four internal recycle axial flow Flygt model LL 3152 pumps were not impacted by the inflow of hair and rags, the four new RAS (return activated sludge) axial flow LL 3127 pumps were becoming so clogged they were barely functioning.
According to Darby Blanchard, Water Reclamation Administrator, "Within weeks of installation, the new Flygt model LL 3127 submersible pumps were continuously being snarled by masses of hair and rags that would clog the pump impellers and render the pumps useless."
Quarter-inch influent screening at the plant's headworks blocks coarse or heavy material, but hair and pieces of rags were still able to get through.
"Another factor," according to utility operator Don Hoover, "is the crumbling cement around the screening at the sewage water entry point. Some of that cement dates back to the plant's mid-1960's opening, and gaps definitely allow material to pass through.
As frequently as every other day, Blanchard's staff would have to pull and disassemble the pumps, then hack away at hair masses with a machete. Blanchard vividly remembers, "This was dirty and dangerous work, particularly when there was a strong possibility of the staff encountering sharp objects that could easily slice through a work glove."
Needing a solution, Blanchard turned to Ellis K. Phelps and Company, the Orlando-based Flygt distributor. One key factor, Blanchard stressed, was that he was certain that, hydraulically, the high flow, low head Flygt pumps were right for the job.
When the snarling first occurred, Blanchard recalls, "We thought it was an air binding problem. But that proved not to be the answer. Just when we thought we'd exhausted all possibilities, Flygt suggested replacing the impellers." And after replacing one with a Flygt model "N", and subsequently three others, since April, 1997, except for standard maintenance tasks such as checking the oil and motor, "The pumps have remained in place and on the job."
What proved to be necessary was a means of cutting fibrous stringy material before it got into the pumps. The design of the new "N" impeller features a base groove operating within a very tight configuration. The sharpened end of the high chrome impeller provides the needed shearing action. And, while the same volume of air and rags are likely still entering the Cocoa Beach water treatment system, their ability to foul critical pumps has essentially been diced.
Pump flow rates at Cocoa Beach vary with the volume of inbound material. Often high flow spikes throughout the day and throughout the season.
Of course, Blanchard notes, that shouldn't be surprising given the fact that tourism-dominated Cocoa Beach can see its 12,000 base population swell four-fold during the tourism season, and that the Air Force base tends to pump over its sewage in very high flows for short intervals at various times during the day. Therefore, variable speed pumps make sense for various reasons.
"Since electrical power is always a key cost factor, we achieve tremendous savings by using just the power we need, instead of continually operating at top speed," Blanchard said. "We definitely didn't want to have to go to maximum speed just to avoid pump fouling."
The N-Impeller equipped pumps allow the CBWR to do just that.
About Cocoa Beach's water reclamation
The Cocoa Beach Water Reclamation Department is the second Florida city-St. Petersburg was first-to provide its residents with a separate lawn irrigation system.
"Cocoa Beach homes have a colour coded pipe which enters each valve box," Blanchard said. "And each home must have installed a back flow preventer that blocks any mixing of irrigation with drinking water. Homeowners are also required to attend a seminar that details the dos and don'ts of water service, Cocoa Beach style."
Blanchard further stresses that Cocoa Beach's irrigation water system is a real cost saver to homeowners since much-more-expensive drinking water does not have to be squandered on lawns. While it can be costly to install new irrigation water lines, Blanchard stresses subsequent savings make those investments a very good deal.
And now that the plant's pumps are fulfilling their original mission, normal maintenance has again been achieved. That not only allows the hard-working Cocoa Beach Water Reclamation staff time to ponder new ways of turning sewage and sludge into profits; they've also been able to hang up their machetes.