Structure: Conventionally reinforced precast concrete column and beam frame structure with cast-in-place grouted connections supporting horizontal fan radiators (fin fans) supported by cast-in-place spread footings
Location: U.S. Gulf Coast
Age: 25 years
Materials of Construction: Standard 60-ksi reinforcing steel bars with 4,500-psi concrete
Process Environment: The manufacturing process involves banks of very large fans (greater than 10 inches in diameter) and coil tubing assemblies rigidly fixed to concrete beams. The purpose of the equipment is to cool process fluids associated with the production of hydrocarbon-based resin plastics. Essentially, the equipment functions as a super-sized, horizontally mounted, “car radiator.” Process temperatures in contact with the open-frame structure are ambient; however, significant vibration is associated with equipment operation.
Loading Conditions: The banks of fin fans are made of lightweight structural materials (i.e., structural aluminum frames, thin walled alloy-steel tubing and carbon-fiber composite fan blades). Fan motors represent the highest loads but are a small portion of the structural assembly.
Owner’s Concern: Cracking, delamination and open spalls with exposed corroding reinforcing steel bars and numerous failed repair programs elevated concern about the functionality of this critical piece of equipment in the process stream. Additionally, falling concrete pieces from elevated regions of the structure created near-miss events for personnel who operated the unit.
Repair Approach: Reviewing many of the repair attempts and subsequent failures, it was clear that the root-cause deterioration mechanisms were not clearly understood. By touch, vibrations were noticeable and thought to be a significant contributor to the observed distress. The owner was eager to review the problem from another perspective, so a team was assembled to design-build an approach that could be implemented without placing the high revenue-generating unit into a T/A scenario.
Specialized expertise was contracted to assess the structural vibration phenomenon as well as conduct a forensic evaluation of the existing condition of the concrete frame and failure mechanisms associated with prior repair programs. One item noted, was the manufacturer’s recommendation that the fin fan blades be balanced at regular maintenance intervals.
Attaching recording accelerometers at strategic locations throughout the fin fan structures revealed the fans were unbalanced and imparting a vibration response to the structure. This response was close to the natural harmonic signature of the concrete support frame – causing the structures to experience a shudder when the fin fans ran up (start up) or ran down, (shutdown). Reviewing maintenance records for the unit revealed the fan blades had never been balanced in their 25-year history. It was calculated, based on an analysis of the structural design and reported fin fan operation records, that the structures were experiencing fatigue from the daily operational shudder.
The QA/QC during original fabrication ensured the production of a good quality precast concrete frame. In addition, forensic evidence revealed the mechanical and chemical properties of the precast concrete frame were of good quality. Cracking, delamination and open spalls with corroding reinforcing steel bars were determined to be secondary effects associated with the primary deterioration mechanism of structural vibration and fatigue. Vibration also created fatal flaws in prior repair programs attempted on-line as the repair products didn’t have an opportunity to develop bond with parent concrete substrates.
After much discussion, the repair team determined three scenarios that could reduce or mitigate vibration in the fin fan structures. These scenarios included:
- balancing the fin fan blades at recommended manufacturer intervals;
- adding mass to the structures to dampen mechanical vibration effects; or
- strengthening the structure by stiffening its members to resist mechanical vibration effects.
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After analyzing the various scenarios, the owner indicated balancing of fin fan blades would not be performed at recommended intervals, since outages were very rare in the unit and only after extreme conditions warranted the downtime (e.g., fire, explosion, chemical spill, etc.). Adding mass wasn’t feasible due to the tight footprint of the structures within the unit, leaving strengthening as the opportunity of choice.