Silos Strengthened with Carbon Fiber
As one of the most abundant domestically produced energy resources, coal has helped to power the growth and prosperity of the country for more than a century. Behind this growth have been coal-fired power plants, such as the PPL Brunner Island Steam Electric Station in Pennsylvania. The 1,490-megawatt generating facility has 240 full-time employees and provides reliable and affordable electricity to about 1 million homes in the region.
Focusing on the Assets
PPL Generation has made a significant investment to keep up with maintenance of this important asset. The coal silo strengthening project is a result of a continued commitment to improving the safety and reliability of this facility. Recently, PPL Generation focused their attention on a pair of concrete coal silos.
These silos were made of reinforced concrete and built in the 1960s and have conjoined walls. Measuring 41-feet in diameter and 80-feet tall, the silos are staged on a structural steel frame above grade and are top-fed from two trippers. Silo upgrades were performed for the following reasons:
- Age – the silos were constructed in the late 1960s
- Design codes – new and stricter codes for design
- Loading – additional axial loads and non-concentric coal loads induced greater strain on the silos
- Considerations to add additional equipment on the tripper floor
The project began with a single contract with PULLMAN and STRUCTURAL TECHNOLOGIES to perform a comprehensive inspection and evaluation. PULLMAN performed inspections at night, when coal was not being fed to other unit silos, and during scheduled individual outages to ensure power generation was not unnecessarily interrupted. Inspection services included: ground penetrating radar, echo impact, crack mapping, core sample testing and visual observation. STRUCTURAL TECHNOLOGIES provided some Non-Destructive Evaluations (NDE) to verify the as-built silo condition. PULLMAN then compared the as-built condition against the current codes to determine if a repair was required.
After an extensive review of the data, the team concluded that under reinforcement of silo walls, wall cracking, and bulging due to eccentric loading and the age of the concrete would all have to be addressed. The inspection data combined with structural analysis confirmed the need for a unique design-build repair solution to address all of the condition issues at once and during a planned outage so that the silos could be emptied. Rebuilding the silos was simply not a cost or schedule effective option, which left only one viable option: strengthen the existing structures.
Traditionally, a 360-degree fiber wrap is used for a strengthening solution of this nature, but due to the design using a common wall that traditional design would not work. It was necessary to transfer the loading from the outside of the wall to the inside of the wall to maintain continuity of the strengthening system.
As part of the design-build efforts, STRUCTURAL TECHNOLOGIES presented three repair options – CFRP, post-tensioning, and section enlargement were the solutions considered. Ultimately, the decision was made to use a combination of near-surface mounted (NSM) carbon fiber rods for the silos’ interior and V-Wrap™ FRP Composite Strengthening System for the exterior. This uniquely designed solution would solve the geometric shape challenge, allow for minimal downtime for the silo, and would add at least another 20 years of service life to the silo structure. The solution took up no additional space and adds no extra weight to the elevated deck; it posed little or no structural risk to the silos or deck and it was easier to implement from a constructability perspective.
Overcoming the Challenges
Due to the configuration of the silos and a desire by PPL Generation to attain a 20+ year life extension, innovative repair methods and materials were needed. The two silos were conjoined; they shared a common wall, forming a figure 8 shape. The challenge for PULLMAN was to strengthen the vessel and get consistent loading capacity and additional strength throughout. Additionally, access to the structure was challenging given one side of the silo was open, while the other was directly built adjacent to the boiler structure.
An additional challenge for PULLMAN was that the work had to be performed during a planned outage. Outside work was done while the plant was still operational, but the inside work was done throughout the 11-week outage. During this time, the adjacent units were operating, so coal was traveling above the silos. This involved close coordination with the coal handling system.
The Completed Repair
With minimal disruptions to plant operations, the design-build team of PULLMAN and STRUCTURAL TECHNOLOGIES safely completed the strengthening project at the PPL Brunner Island Steam Electric Station on schedule during a period of 3.5 months. The success in executing this project is largely attributable to the team’s initial inspection, pre-planning, and examination of cost and constructability completed with PPL Generation plant management before the work was initiated. Continuous coordination with the plant coal yard and cooperation with plant field services greatly contributed to the efficiency of the work. The innovative use of materials and methods allowed for successful repair of the silos. With the silos strengthened, PPL Generation can confidently continue to provide safe, economical, and reliable energy.