Pump StationPressure Control StructuresPipeline RoutePipeline Material
The Lake Gaston Project Pump Station
The heart of the Lake Gaston project is the pump station and associated facilities located on the shores of Lake Gaston at Pea Hill Creek. The pump station has a footprint of approximately 7,000 square feet and contains six vertical-turbine centrifugal pumps. Five of the six pumps are dual-speed, 500/1250 horsepower pump and motor combinations operating at 4,000 volts AC (VAC). The nominal capacity of each of these five pumps is approximately 10 million gallons per day (MGD) at low speed (900 rpm) and 15 mgd at high speed (1200 rpm). The sixth pump is a 250 horsepower, 440 VAC pump and motor combination that can deliver 4 to 8 mgd. This pump is used for filling the line and for maintaining small flows when larger flows are not necessary.
The pump station is based upon a flooded wetwell design. Underneath the pump station is a large, deep basement (i.e., a wetwell), the bottom of which is some 35 feet below the normal pool level of the lake. Two 60-inch diameter pipelines connect the wetwell to a series of intake screens, several hundred feet offshore. Water flows through the intake screens and into the wetwell until the water elevation in the wetwell is the same elevation as the lake (hence the term "flooded wetwell"). Vertical-turbine centrifugal pumps, which extend down into the wetwell, pump the water from the wetwell into the pipeline.
The top of the intake screens are 15 feet below the normal pool level of the lake. The screen size is 1 mm, which is slightly less than the thickness of a dime. Maximum velocity at the screen surface is 0.5 feet per second (one-third of a mile per hour). One inch away from the screen surface the maximum velocity is 0.17 feet per second (one-tenth of a mile per hour). These are maximum velocities when only one intake line and one screen array are in service. Most of the time, velocities will be one-half to one-third of the maximum velocities stated above. The withdrawal of water through the intake screens will not disturb the water surface.
The pump station facilities are located on a small peninsula on Pea Hill Creek just north of the Virginia/North Carolina line. Lakeside development on Pea Hill Creek is upscale and similar to the development that occurs on the Lynnhaven River in Virginia Beach. Noise and aesthetics were identified early on as potential impacts from the project to those living near the pump station. In response to these concerns, the City made a serious commitment to design and construct a facility that would be both quiet and aesthetically pleasing.
The flooded wetwell design was the first step toward containing noise and maintaining a low visual impact on the lake. Much of the noise associated with large water pumping operations comes from water rushing through the impellers and pump casings at high velocity. The use of the submerged vertical-turbine pumps keeps much of the noise contained in an underground vault of concrete surrounded by tons of rock and earth. The wetwell design also keeps much of the pump station underground which allows for a relatively low visual profile from the Lake.
To further reduce noise, the City included maximum allowable noise levels in the motor specifications. The motors installed at the Pea Hill Creek pump station are among the quietest available for that size and type. Finally, the interior of the pump station is lined with a special sound reducing concrete block. Recent noise level tests have indicated that the City's efforts have been successful beyond its expectations. The pump station is so quiet, that even with interior sound levels exceeding those that will occur at maximum operation, no one outside of the pump station will be able to detect any noise.
The City also made an extensive effort to insure that the pump station and its associated facilities would be as attractive as they were quiet. Both the profile and the footprint of the pump station were kept as low as possible. Because the development around Pea Hill Creek involves small clusters of homes on the peninsulas which extend into the lake, the City laid the facilities out in the same manner. All exterior treatments (i.e., roofs, walls, doors, windows, flashing, decking, etc.) were upgraded so that they would 1) look very attractive, 2) require very little maintenance, 3) last a very long time, and 4) survive major storms with little or no damage. To further beautify the facility, a landscaping plan will be implemented.
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The Pressure Control Structures
The elevation of the water at Lake Gaston is 200 feet mean sea level (msl). Within a few miles from the pump station, the pipeline crosses a ridge which separates the Roanoke and Chowan river basins. The elevation at this point is approximately 330 feet msl, or a rise of about 130 feet above the lake surface. Although from this point on, the elevation of the pipeline route goes up and down as it travels over hills, valleys and rivers, the net elevation change is downward until the pipeline terminates at the Norfolk raw water system near Windsor, VA where the elevation is about 50 feet msl.
At flows below about 45 mgd, the hydraulic head required to move the water is less than the net elevation difference between the high point in the pipeline (330 feet msl) and the terminus structure (50 feet msl). Under this scenario, water would flow through the pipeline too fast causing pressure surges, water column separation, and water hammer which could damage the pipeline. The weight of the water in the 76-mile pipeline is 500 million pounds. The speed at which this weight is accelerated and decelerated is a critical engineering issue in the design and operation of the pipeline.
To address this issue, there are three pressure control structures located on the pipeline. These structures create a back pressure in the pipeline which throttles the water flow when flows of 45 mgd or less are being pumped. Two of the three structures are buried vaults which contain an array of automatic, pressure controlling valves. These valves sense the upstream pressure in the pipeline and close or open to maintain the necessary back pressure at low flows. The two valve vaults are located at roughly the one-third and two-thirds mark along the pipeline route.
The third structure is located near the end of the pipeline. It is an elevated weir which creates a back pressure in the pipeline by forcing the water to flow over the top of the weir which is approximately 30 feet above the pipeline grade. This structure is combined with a set of cascading steps which begin at the top of the weir. From the weir, the water flows over the steps where it mixes with air and is re-oxygenated after its long trip through the pipeline.
From the bottom of the weir/aeration structure, the water flows by gravity through two more miles of pipeline where it discharges through a concrete culvert and shallow trough into the Ennis Pond Channel.
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The Pipeline Route
The Lake Gaston pipeline is approximately 76-miles long, including six overhead river crossings which account for about one-half mile of the total length. Of the 76-mile pipeline route, 64 miles are within two existing, major right-of-way corridors, and six more miles parallel a third. Only the first six miles of right-of-way involves a new utility corridor. This made property acquisition easier (relatively speaking) and reduced environmental impacts from the pipeline construction.
Leaving the pump station site, the pipeline travels east for six miles through new right-of-way until the intersection with a 500,000 VAC power line owned by Virginia Power. From there, the pipeline travels north within the power line right-of-way for about 21 miles to a point near Purdy, VA where it intersects an abandoned railroad right-of-way. Virginia Beach has purchased the railroad right-of-way from the Norfolk Southern Corporation. From Purdy, the pipeline travels east within the former railroad right-of-way for approximately 43 miles to a point near Walters, VA. From there, the pipeline travels east for about six miles, parallel to existing water lines owned by the City of Norfolk, to the terminus structure near Windsor, VA.
Because the power line right-of-way and railroad right-of-way were already cleared, the total acreage of trees that had to be cut and permanently removed was very small relative to the total right-of-way. Additionally, the railroad right-of-way included five of the six major river crossings and most of the wetland areas. Although these wetland areas were extensive, the railroad embankment formed a large, earthen berm constructed almost a century ago. In these areas, pipeline construction was restricted to the railroad embankment with little or no impact to the wetlands or the environment. When the railroad abandoned the right-of-way, it removed the tracks, ballast, and railroad ties, but left behind concrete piers where the railroad crossed five major rivers. Virginia Beach used the existing railroad piers for these five river crossings eliminating any need to disturb the river bottom or the adjacent wetlands.
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Approximately 45% of the pipe is ductile iron and approximately 55% is pre-stressed concrete cylinder pipe. Both products have a nominal inside diameter of 60 inches and both are delivered in 20 foot long segments. There are about 20,000 individual pipe sections in the 76-mile pipeline. Both ductile iron and pre-stressed concrete cylinder pipe are assembled using slip joints in which a rubber gasket or o-ring seals the annular space between the bell end and the spigot end of the pipe. Pipe segments with working pressures ranging from 250 pounds per square inch (psi) to 150 psi were used in the 74 miles from the pump station to the weir/aeration structure. The two mile gravity flow section of pipe from the weir/aeration structure to the terminus has a working pressure of 40 psi.
Each 20-foot section of ductile iron pipe weighs approximately 8,000 pounds. The wall thickness is about five-eighths of an inch thick. The primary source of iron for ductile iron pipe is junk automobiles. The City's project alone has recycled about 27,000 automobiles. Ductile iron pipe is made in a casting process in which 8,000 pounds of molten iron is poured into a mold spinning at approximately 800 rpm in about 30 seconds. Ductile iron pipe derives its strength from the tensile and compressive strength of solid iron. To prevent internal corrosion, ductile iron pipe is lined with a high density mortar coating about one-fourth of an inch thick. To prevent external corrosion, ductile iron pipe is encased in a polyethylene wrap.
Each section of pre-stressed concrete cylinder pipe (PCCP) weighs approximately 24,000 pounds. The wall thickness is about 6 inches. In contrast to ductile iron pipe which is made in a very rapid casting process and derives its strength from solid iron, PCCP is made in several engineered steps, and derives its strength from the interaction of the multiple materials and steps which go into its fabrication.
PCCP is made by casting concrete on both sides of a thin, steel cylinder. After curing, the concrete pipe is pre-stressed by wrapping it with high tension steel wire. This places the concrete under compression. The steel pre-stressing wires are then coated with a thick layer of cement mortar to prevent corrosion. Concrete pipe derives its strength from the tensile strength of the pre-stressing wire and the compressive strength of concrete. PCCP does not need any internal coating. Some portions of the PCCP were given an external coating of coal-tar epoxy to provide additional corrosion protection, but in most soils, the PCCP is not subject to corrosion.
Both ductile iron pipe and pre-stressed concrete cylinder pipe were specified for all five pipeline contracts, along with a third material (welded steel pipe), in order to achieve maximum competition. Ductile iron pipe was the low bid for two of the five pipeline contracts and PCCP was the low bid for the other three.
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