Concor, a leading South African contractor, recently demonstrated its construction and engineering prowess by undertaking a critical project at Eskom’s Kusile power station in Mpumalanga. The project centered on the construction of three steel flue gas stacks, each incorporating 605 tons of steel. These stacks were built to restore operation to the power station’s boilers after the unexpected failure of the original flue gas duct, known as the West Chimney for Kusile Unit 1. This failure had severely impacted South Africa’s already strained electricity supply, making the prompt completion of the project vital.

Juan Passmore, Project Director, Kusile Chimney West – Repair & Related Works Project at Concor, explains that given the temporary nature of the structures and the urgent timeline, Concor’s technical team was tasked with ensuring rapid execution while considering the availability of raw materials and the capacity of subcontractors to meet the tight deadlines. The temporary stacks were designed to be dismantled once the West Chimney was repaired, adding complexity to the design and construction process.

“In collaboration with design engineers ICC Commonwealth, Concor explored various construction materials and methods during the early engineering stages, and steel was selected as the optimal choice due to its availability and suitability for the accelerated schedule,” Passmore says. The team quickly assessed the local market for steel plate availability, securing enough 2.4m by 10m plates to cover the majority of the project’s needs. “This early acquisition enabled the design process to begin promptly, focusing on the specific parameters of the available steel plates,” he adds.

After evaluating several design alternatives, a freestanding structure 116m tall with a 7m internal diameter was chosen as the most economical and feasible solution. The design adhered to the International Committee on Industrial Construction (CICIND) standards for chimneys and chimney flues, ensuring the stacks met all necessary safety and operational criteria.

Their location was strategically planned to be as close as possible to the horizontal permanent ducts from the power station’s turbine units. Wind impact was another critical consideration, leading to the inclusion of helical strakes (corkscrew fins) on the top 30m of the stacks to provide aerodynamic stability and prevent wind oscillation.

Passmore says that steel’s adaptability was further highlighted during the fabrication process. The stacks were fabricated under safe, controlled factory conditions in Vanderbijlpark, where Domento Mechanical Services employed bolted segment fabrication, significantly reducing the need for on-site welding.

This method was particularly advantageous given the brownfields nature of the site and the live environment of the Kusile power station. Domento worked across three workshops operating 24 hours a day, six days a week, to meet the project’s stringent deadlines.

Steel supplier Bekker Steel also played a crucial role in cutting and drilling the flanges and rolling the stack plate, while Domento fabricated and welded the bottom 19.6m section of the stacks. This welded section was composed of four cans with bolted flanges at each horizontal full penetration weld, ensuring a secure and stable foundation for the entire structure.

The remainder of each stack was fabricated in sets of three segments, each 2.4m high, with flanges on all sides. A total of 120 flanged segments were fabricated for each stack, with up to six segments transported to the site daily to maintain the construction schedule. Given the temporary nature of the stacks, a single coat of red oxide primer was applied to a blasted SA2.5 surface, as the design life was limited to just three years.

The foundation work required significant attention, with each stack supported by piled foundations consisting of 16 piles of 900mm diameter, varying in depth from 8m to 20m. Due to time constraints, Concor sourced 50mm diameter Dywidag thread bars from abroad by air freight to meet the construction program, as the originally specified anchor rods were unavailable in time.

“Site installation demanded meticulous planning and execution with the construction taking place in a brownfields environment,” Passmore says. “Preassembly was on done in a gravel-surfaced zone away from the power station’s operating areas.”

A dedicated crawler crane facilitated the preassembly of multiple cans and the associated bolting platform. The bottom 19.6m portion of each stack was assembled and welded directly on the foundation site, while the rest of the stack was constructed by assembling a 9.6m high can in the preassembly area before transporting it to the site for lifting and bolting.

Three high-capacity crawler cranes—450t, 650t, and 750t—were secured through effective forward planning, enabling the team to maintain a consistent installation pace of one can per week per stack. Due to the height of the stacks, wind posed a significant challenge, so lifting operations were generally conducted early in the morning when wind speeds were lower. Lightning risks also necessitated the halting of construction during electric storms.

Each stack required 4,776 galvanized pre-tensioned bolts, which were fitted and torqued with the aid of specialised indicator washers, ensuring rapid and accurate pre-tensioning. The design also incorporated safety features to facilitate fast installation, including a bolting platform a metre below the top flange of each 9.6m high can, providing easier access to the bolts between cans.

Eskom employees needed access to the 66m level for flue gas monitoring, necessitating longer platforms at this height, positioned at 90° to one another. The stack temperatures reached 150°C, so the face of the stack at the monitoring level was insulated to protect operating personnel. Access to this level was provided by a temporary scaffold staircase braced back to the stack.

The project also involved the fabrication and installation of horizontal ducts, comprising 120 tons of steel, to convey flue gas from the boilers to the stacks. These ducts were fabricated in segments and delivered to the site by abnormal load, directly to the relevant stack site for installation. The temporary bypass ducts were installed over seven weeks, with flanges sealed and bolted into position.

“Concor’s successful execution of this project underscores our capability to manage complex, fast track projects in challenging environments, further cementing our position as a leader in the South African construction industry,” Passmore concludes.

Concor played a key role in stabilising South Africa’s electricity supply by constructing three temporary steel flue gas stacks at Kusile Power Station.

Site installation demanded meticulous planning and execution with the construction taking place in a brownfields environment.

Each stack is supported by piled foundations consisting of 16 piles of 900mm diameter, varying in depth from 8m to 20m.

Each stack required 4,776 galvanized pre-tensioned bolts ensuring rapid and accurate pre-tensioning.

Given the temporary nature of the stacks, a single coat of red oxide primer was applied to a blasted SA2.5 surface, as the design life was limited to just three years.