Published June 2007

WATER: Reaching Out

By Scottie Dayton (page 14)

An innovative pumping solution helps the City of Commerce, Texas, resolve issues created by a drop in the level of its drinking water reservoir.

T­he City of Commerce, Texas, draws its drinking water from Lake Tawakoni, a manmade reservoir that covers 36,700 acres.

Ensuring a stable supply of high-quality water to the city is the job of the Public Utilities Department. City staff members deal with the usual challenges of underground pipe maintenance and replacement, as well as the occasional crisis. For example, when the level of Lake Tawakoni fell 12 feet in 2006, the city faced the prospect of inadequate fire protection and water rationing.

The upper bowls on the three vertical turbine water pumps that drew water from the lake were in danger of sucking in air, jeopardizing their reliability. Public Utilities needed a remedy before confronting summer’s high water demands.

The city solved the problem by building a steel platform on the end of an existing pier, then bolting a centrifugal pump to it. That pump was attached to a flexible, 8-inch HDPE suction pipe running to a spot in the lake 10 feet deeper than the water level at the end of the pier.

The pump surpassed the city’s requirements and prevented it from running out of water, pumping it 26 miles through a 14-inch force main while maintaining a consistent 1,000 gpm at 300 feet of head.

Dealing with events

The Lake Tawakoni project was a major issue, but not the first time the city’s Public Utilities Department faced challenges. Public Utilities is responsible for 55 miles of water lines ranging from 3/4-inch service lines to 16-inch cast-iron mains. The city’s population is 8,500 — not counting 3,500 students at Texas A&M University-Commerce.

Water pumped from Lake Tawakoni is stored in two 500,000-gallon water towers, and additional well water is stored in a one-million-gallon ground storage tank. From these facilities, freshwater is continuously pumped to the distribution system, which is maintained at 65 psi, says Tracy Lunceford, director of Public Works.

Most water mains were installed in 1967, and their age is responsible for the city’s two active water main break seasons. As the soil expands or contracts every spring and fall, its shifting creates stress fractures in the pipe.

“We experience a lot of water main breaks with major water loss, but the department’s rehabilitation program is hampered by its financial situation,” says Lunceford. “Our last major project was two years ago.”

One of the main feeds from a water tower runs beneath Pecan Street as it travels through residential and commercial areas. When this 12-inch pipe began experiencing regular breaks, the department acted decisively.

Long-range plans

Using a grant from the state Office of Rural Community Affairs (ORCA), the city hired Osage Aggregates Inc. in Denison, Texas, to replace the line. Working one block at a time to disrupt traffic as little as possible, they open-trenched the street and laid C900 PVC 12-inch water pipe. The old main, left undisturbed on the opposite side of the street, continued to supply customers with water in the interim.

Pre-washed sand was placed to a depth of one foot above the C900 pipe. The trench was backfilled with native material, topped with 18 inches of base rock, and covered with 2 inches of asphalt. The project took 120 days to complete.

The department plans eventually to replace a 26-mile long, 14-inch reinforced concrete cylinder pipe running from the treatment plant to Lake Tawakoni. The city will probably replace it with 20-inch pipe, but the material will depend on availability and pricing. Excavation will be open-trench.

Another objective is to install a new 12-inch water main around the south end of town to serve future development and to increase volume for existing customers. The department has applied for grants to finance these projects and will undertake them as soon as money is available.

Dealing with a crisis

The Lake Tawakoni project posed an immediate concern. The reservoir, built in 1960, provides water to surrounding communities, the City of Dallas, and the North Texas Municipal Water District (NTMWD).

Heavy spring rains usually fill the reservoir to capacity, but the region had been in drought conditions. That, combined with draws from Dallas, had reduced the lake’s depth from 437 feet to 425 feet.

To deal with the issue, city manager Bill Shipp, assistant city manager Mike Dunn, and the city council agreed to contact project manager Mike Tibbets of Hayter Engineering in Paris, Texas. Tibbets distributed proposals and selected Pierce Pump Co. Inc. of Dallas.

The city’s three connected vertical turbine pumps are on a concrete pier extending 300 yards into the lake. To reach deeper water, Pierce Pump representative Don Drake at first proposed mounting a 5,000-pound vertical turbine pump on a barge anchored in the middle of the reservoir. A pipe would connect the pump to the city’s force main.

The barge manufacturer, however, voiced concerns about anchoring the craft too far from shore. “Severe summer storms can create 5- to 6-foot waves, and the barge would be susceptible to them in open water,” says Tibbets. “Routine maintenance would be inconvenient and costly, too. Besides a barge-mounted crane to lift the turbine, other expenses included hose and piping, a power cable, and a pump house. We had arrived at a price for the system already and had to stay within the budget.”

After reconsidering, Drake suggested building a platform and bolting a Gorman-Rupp VS6 centrifugal pump to it. That way, operators could drive along the pier to service the pump, just as they did for the other three pumps.

“The VS6 can pass 3-inch solids such as fish, turtles, and debris,” says Drake. “It also had to lift water 8 or 10 feet while pumping it 26 miles from Lake Tawakoni to the Commerce treatment plant.”

Choosing the spot

The self-priming pump feeds a duplicate centrifugal pump close-coupled to its discharge. Each is powered by a 75-hp electric motor. “After drawing water from the lake, the low stage takes it to 65 psi,” says Drake. “Then the next stage kicks it up to 130 psi.” The Pierce Pump team found that the best location for the suction pipe was about 300 feet from the pier where the water was of suitable depth.

However, soon after the platform extension and pump were installed, severe vibrations occurred during pumping. Investigations by Gorman-Rupp’s engineering team revealed that a faulty 6-inch check valve on a 6-inch branch line connecting to the force main was allowing the main to drain partially back into the lake when the pumps were not running. Furthermore, whenever the discharge pipe had developed a leak, city crews had partially drained it to make repairs.

With the force main not completely full of water, the total dynamic head was less than expected. Lacking adequate backpressure, the new pump was pumping more liquid than it was designed to handle. If left unchecked, the physical effects could be devastating due to cavitation of the impeller and casing. The excess vibration could also cause the seal and bearing to fail with loss of lubrication. Once the leaking valve was replaced, however, the pump ran smoothly at its design point, delivering 325 feet of total head at 1,100 gpm.

“We came in on budget, and the VS6 is working like a charm,” says Tibbets. “In addition, the total cost of ownership at the end of its lifecycle will be lower than the original plan, and Commerce has a much better system.”