Keeping Up With Demand
Following its selection as an alternative to rebuilding an existing pump station, a new, customized 288,000 gpd booster pump station in Columbia, Tenn., is reportedly working strong following its recent installation, according to the superintendent of the county water system.
The prefabricated unit, manufactured by ITT Industries company Flowtronex PSI Inc., was installed last year to help meet increased demand on the local potable water system. Last year’s installation followed the installation of a similar unit to meet comparable needs two years ago.
“When you start changing one thing, you have to change another and by the time you get through, you might as well buy a new pump station,” said Larry Chunn, superintendent of the B. Dodson and Scott Hollow Road Pump Stations, Maury County Board of Public Utilities.
Chunn said the previous B. Dodson station, installed in 1989, ran at about 90 gpm max. in. service to fewer than 200 residential units.
The new B. Dodson station, serving 450 units, can operate up to 178 gpm in a 12–13 hr. cycle, raising up to 182 ft of head through an elevation gain of 330 ft.
“The Tennessee Division of Water Supply looks at pump run time as 80% of the capacity of the pumps,” he noted. “When peak demand reaches that 80%, people are using more than we can supply, and at 100%, we’d be getting in real trouble. This became a problem in 1998, and we needed options to change capacity to meet the demand.”
“We considered increasing impeller size to upgrade the 15 hp pumps,” he continued, “but the pumps were 10 years old and we would have also needed to change the pump motors. That would have in turn raised the electrical demand, requiring a new electrical entrance to the motors and we saw that it was more feasible to buy a new station than rebuild the one we had.”
Chunn said he placed the need out on the market as a complete, packaged, 200 gpm pump station, with three-phase electric current to provide for lower power consumption and variable frequency drives (VFDs) to allow for adjustment of flow rate through the pumps at any given time.
“We didn’t have the capability to change the speed of water flow before,” he recalled, “and when demand would change, we could get complaints about pressure drop throughout the system. Now we can vary from 50–200 gpm by frequency conversions. That wasn’t an issue at Scott Hollow, but we took advantage of VFDs in that case just to convert single-phase electric current to three-phase and operate more efficiently.”
Chunn added that a special enclosure for each station was also important.
“We now have a marine-grade aluminum enclosure for the station, which not only provides a good appearance, but can also keep out cold air,” he said. “The previous fiberglass enclosure wasn’t tight enough to do that and the air gaps allowed pumps to actually freeze and burst. Mold was also a problem. We chose Flowtronex from among about five competitors not only because we had previous successful experience with them, but because they were the only one that could include this superior enclosure.”
Flowtronex set up the two stations with special needs in mind. The B. Dodson Station was sized for the ultimate buildout of the residential area, but was set up to run slower and at lower output in the meantime. The VFDs allow operators to throttle back as needed. At the Scott Hollow Station, there was an additional VFD benefit that is common to rural areas.
Station management there decided to use VFDs even though they didn’t need the capability to change station output. With most pump motors over 10 hp requiring three-phase current, this move not only enabled them to operate more efficiently, but also avoided the need to change the existing electrical service at the site, which is often a problem at rural locations.