Atlanta power plant rehabilitates failing pipe
Acoal-fired, electric power-generating plant near Atlanta was constructed in the 1960s to provide electricity to metropolitan Atlanta. The plant’s original two coal-fired units had a total generating capacity of 540 MW and were replaced in 2011 by three natural gas combined-cycle units capable of generating 2,520 MW. This included the decommissioning of the site’s older coal-fired plant, the installation of new cooling towers with plume abatement technology and a stream diversion culvert retrofit project. The projects will improve air quality emissions and preserve the local environment.
Bottom ash and fly ash are the two main byproducts of coal-fired electric utility generation. Bottom ash is a coarse, granular, incombustible material collected from the bottom of furnaces and conveyed to ponds or decant basins for dewatering, crushing, and stockpiling for disposal or recycled use. Fly ash, a fine-grained powdery particulate captured from flue gas during coal combustion, often is reused in portland cement or other construction and industrial applications, or stored in onsite ash ponds.
Site selection for the plant’s ash pond was a challenge because of the facility’s urban setting. Even in the mid-1960s, there was little undeveloped land around the plant site. As a result, one of the ash ponds was situated on a parcel bisected by a creek. The engineered solution was a 90-in., 1,630-ft-long asphalt-coated corrugated metal pipe (CMP) culvert installed to carry the stream and allow for the construction of the earthen embankment ash pond. The stream’s diversion culvert, which was installed along the original alignment of the creek, is buried beneath earthen dams and the ash pond, along the length of which it transitions.
Through its regular inspection program, plant management identified the need to complete a detailed evaluation of the culvert to assess the need for rehabilitation. A local engineering consultant was hired to perform the assessment and design a solution. Along its length, the pipe had deflected up to 7.5% under the maximum vertical load, was locally corroded in several areas, and had loose soil backfill conditions present in several areas.
The engineering consultant, with assistance from Hobas Pipe USA, devised a plan to rehabilitate the pipe by sliplining the existing CMP with centrifugally cast fiberglass-reinforced polymer mortar (CCFRPM) pipe. The liner pipe diameter was specified to maximize the final interior diameter of the rehabilitated line for the greatest hydraulic capacity. This included consideration of the minimum radial clearance for fit and annular grouting.
Because the stream bisects the ash pond, it was important that the liner pipe provided a leak-free joint to preserve the water quality in the stream. The Georgia Division of Water Resources (DWR) regulates wastewater discharged from coal ash ponds to state waters, streams and lakes. DWR also requires groundwater monitoring and storm water management.
“We were faced with many installation challenges, including the condition of the existing CMP,” said Dan Davis, Eastern division manager for Hobas Pipe USA. “Hobas supplied a flush joint pipe, which provided the most radial clearance to the existing pipe, as well as custom short sections to navigate the directional changes in the line. Since the original line was laid very near to an existing streambed, it followed the stream and was not straight, with 30-degree and 9-degree horizontal elbows. Fortunately, we were provided a very accurate survey to work with to custom-make the fittings.”
Because they were buried at great depth beneath the embankment dams and ash stack, the two horizontal bends were inaccessible from above. The fittings had to be carried to the proper location through the line.
Access to the outlet was limited due to the transition to two smaller box culverts at the buried outlet location under a rail crossing. As a result, the entire length of the 1,630-ft line was rehabilitated by carrying segments into the existing culvert, piece by piece, building from the downstream side toward the inlet structure. The location of the first elbow, a 30-degree horizontal bend, is 100 ft upstream of the outlet, meaning the lower 100 ft of line was accessible only after negotiating both bends. For this section, due to the reduced hydraulic cross-section required near the outlet, relatively short 6.5-ft-long pieces of smaller-diameter pipe were utilized to navigate the bend.
The middle portion of the line utilized longer sections of pipe since the 9-degree elbow was much more navigable than the 30-degree bend.
“Our field service department made several trips to the plant for installation support both during the installation of the pipe and during the grouting process,” said Randy Whiddon, field service manager for Hobas Pipe USA. “There was extensive communication about the means and methods related to the pipe installation procedures, blocking diagrams, grouting calculations and the logistics in general.”
The pipe was supplied in 72-, 78- and 84-in.-diameter sizes for sliplining within the 90-in. existing CMP. In addition to the custom fittings manufactured by Hobas to accommodate the vertical deflections and diameter transitions within the line, a custom intake structure was built. The intake structure consisted of extending the existing headwall by concrete-encasing a 110-by-84-in. eccentric reducer to provide increased hydraulic inlet efficiency.
A 72-psi pipe stiffness product was utilized for the majority of the line to withstand the large hydrostatic forces and ash and embankment overburdens.
“One of the many benefits of using Hobas CCFRPM on slipline and tunnel applications is the flush joint configuration and the high-strength composite,” Whiddon said. “The tight fit allowed by the flush joint, the oversized inside diameter and the superior hydraulics allow for optimum flow recovery. This project required a pipe that could support the load from the future overburden and hydrostatic pressure. Corrosion resistance was also of utmost importance. Hobas CCFRPM pipe was able to meet all of the demanding project requirements.”