Home to the largest contiguous cotton-growing region in the world and encompassing more than 115 square miles, Lubbock is the economic, education and health care hub of the South Plains of Texas. With a population of approximately 212,000 in 2006-estimated to exceed 250,000 by 2060 according to the U.S. Census-Lubbock is also one of the fastest growing cities in west Texas.
A decade ago, forward thinking local officials realized Lubbock was expanding beyond the capabilities of its municipal water supply system. Lubbock's water resources included surface water from Lake Meredith and ground water from a regional well field. Two pressure planes, an east pressure plane and a west pressure plane, served the city, with the division generally occurring near the city center. (A pressure plane is a designated area for water distribution defined by water pressure in feet above sea level.) Eight pump stations and three elevated storage tanks supplied water to the distribution system in the two pressure planes.
The residential and commercial development in the city's southwest portion placed a tremendous burden on Lubbock's water storage and distribution system. To sustain reasonable water pressures throughout Lubbock and particularly into the expanding areas, the city decided to expand its water service capabilities in the southwest.
In 2001, Lubbock initiated a comprehensive Water Distribution System Study to determine the projected water demands through the year 2050 and the improvements necessary to meet those demands. Based on the results of a steady-state hydraulic model that projected the required water demands at peak conditions, the following recommendations were made:
- Create a new southwest pressure plane via the addition of a 5 mgd pump station and a 0.5 million gallon elevated storage tank. This pressure plane would allow the anticipated growth in southwest Lubbock to be served at pressures above minimum requirements.
Build a new 14 mgd pump station and a 5 million gallon ground storage tank to serve the existing west pressure plane.
Finding a Cost Effective Solution
In 2004, in response to the city's solicitation, Lockwood, Andrews & Newnam, Inc. (LAN), a planning, engineering and program management firm headquartered in Houston, and Enprotec/ Hibbs & Todd (eHT), a local civil, environmental and geotechnical engineering firm, proposed a number of solutions to implement the creation of the new southwest pressure plane and the pump station improvements necessary to meet the demands in the west and southwest pressure planes.
Convinced that the proposed solutions offered the advantages the city wanted, Lubbock officials awarded the contract to the team. LAN was chosen to provide design and construction management services, which included modeling and sizing of the new pump station, selection and sizing of pressure sustaining valves and design of the pump station and ground storage tank. eHT's services included forecasting future growth in the area, environmental studies on the pump station site and pipeline route, and design of the offsite pipelines and pressure sustaining valves.
The team thoroughly reviewed the city's existing distribution system. Through extensive data conversions, the H20Net water model was updated to a WaterCAD model and multiple scenarios were evaluated. The steady state hydraulic model used in the 2001 study was transformed into an extended period simulation and updated to include future growth. Steady state analysis takes a snapshot in time, at peak water usage and minimum water usage periods, whereas an extended period study analyzes the system hour by hour for a more thorough review. The model also analyzed the existing system, determined fill and draw capabilities using the existing system components, and the pump station site serving the west and southwest distribution areas.
"We not only looked at the near term conditions and growth projections, but also the improvements that should be made to the pump station in the next 15 to 20 years," said Graham Moore, LAN's project manager.
Based on this hydraulic modeling analysis, the team recommended the following solutions:
- Combining the facilities suggested in the 2001 Water Distribution System Study into a single facility to reduce construction costs. The team suggested designing and constructing a 14 mgd dual pump station that would take suction from a new 5 million gallon ground storage tank. This would serve the new southwest pressure plane and the existing west pressure plane.
Deferring the construction of the proposed elevated storage tank in the southwest pressure plane and instead installing two pressure sustaining valves coupled to the existing west pressure plane. In the event of a pump station failure, the pressure sustaining valves open, allowing water from the elevated storage tanks in the existing west pressure plane to back feed into the southwest pressure plane.
Flexible Design
Driving the project team's design of the new southwest pressure plane and associated pump station improvements were two major considerations: operational flexibility and ease of access. A number of features that would maximize the distribution system's usage and service capabilities, minimize operational impacts during maintenance periods and accommodate the future expansion of the site were incorporated.
The location of the pump station and ground storage facilities, just 400 feet west of Milwaukee Avenue-one of the major growth areas-provided a short and convenient route to the southwest pressure plane. Consequently, the transmission mains installed from the pump station site were much shorter than planned in the 2001 study. In addition, the transmission mains-30 and 24 in mains serving the southwest and west pressure planes, respectively-were less complicated to install with respect to conflicts with existing development and utilities. This resulted in cost savings of more than $1.5 million.
Four 2,725 gpm horizontal split-case booster pumps (at a pressure of 154 ft) were installed in the pump station building to provide the required water pressures. Two of the pumps used variable frequency drives (VFDs) to more efficiently match the demands and pressures required by the southwest pressure plane. During periods of low demand, the VFD decreases the motor speed, thereby controlling the pump performance in relation to system demand and avoiding the wasted energy resulting from using constant speed motors.
The team incorporated a number of valves and pipes outside and inside the pump station to improve system pressures and accommodate future expansion of the site with minimal service outages. The pressure sustaining valves were installed on the existing distribution mains at two streets on Milwaukee Avenue. In addition to serving the southwest pressure plane in the event of a pump station failure, the valves convey higher pressure water from the southwest pressure plane to the lower pressure west pressure plane during times of high demand. A 16 in water line was also upgraded to a 20 in water line from the pump station site to improve the water pressures in the far southern portion of the southwest pressure plane.
"Typically, you set up a new pressure plane using elevated storage tanks, but elevated storage tanks are expensive to build," said Moore. "Using pressure sustaining valves allowed us to defer building the elevated storage tank for many years. The result is a water distribution system that is less expensive to construct yet still meets the permitting requirements."
Deferring the elevated storage tank saved more than $1 million in construction costs, while still allowing Lubbock to provide water service at required pressures.
The 5 million gallon pre-stressed concrete ground storage tank was installed to serve Lubbock through its near future demands. Valves and stubs were placed on the ground storage tank fill and discharge pipes to allow for the future addition of a second 5 million gallon ground storage tank. The yard piping at the pump station was designed such that the pumps can take suction from the ground storage tank or from the fill line through an anti-cavitation valve, bypassing the ground storage tank.
Common pump station designs include major pipe fittings hidden under the floor slab. However, fittings historically fail, causing leaks or pressure loss in a water distribution system. The project team exposed the discharge headers in a pit in the pump station so that no fittings are located beneath the building slab but are still easily accessible to the city's maintenance staff. In addition, a bridge crane designed inside the pump station allows all major fittings, valves, pumps and motors to be easily accessed. The inline arrangements of the pumps in the stations further aided maintenance and repair and reduced the need to remove the pump station from service.
A portion of the pump station, including the pipe galley, falls beneath the 100-year flood plain. To construct a facility that could be insured, the team worked with the city to alter the foundation design. Changes to the site grading were introduced to reduce the risk of disruptive flood waters infiltrating the system.
Conclusion
In February 2007, the construction of the $7.8 million project was completed. The project has received a number of professional awards, including the South Plains (Lubbock) Chapter of the Texas Society of Professional Engineers' "Trailblazer Award" for excellence in 2006 and Texas Construction Magazine's 2007 Excellence Award in Public Design.
Lubbock has been able to meet the growing water demands throughout the city while maintaining water pressures above 50 psi. As demand increases, the city is planning to build a second 5 million gallon ground water storage tank, adding another 6.5 mgd pump along with room for chlorine and ammonia systems.
"City officials have stated that the southwest pump station is operating just like it was planned," said Moore.