Fin Fan Vibration Remediation

F2 - Vibrations and Failure Analysis

A major refinery has many fin fan coolers, in this case approximately 760 fans of similar structural design with significant variations. Approximately 30 of them experienced high vibrations and several had broken loose from their overhead supports. Plant personnel had monitored vibrations and attempted to stiffen some of the structures with bracing to reduce vibrations, but with only limited success. Amplitudes were not reduced to satisfactory levels.

A testing and analysis program was initiated to identify the vibration mode shapes, frequencies, and amplitudes. The differences among the various fan designs were evaluated, and 23 of the unsatisfactory fans were tested. The primary tests were operational deflection shapes (ODS) and impact modal tests on non-operating units. The field testing also identified that some of the fans were limited in their ability to push air through the radiators, apparently due to plugging or over design. The air was found to be recirculating back around the outer edge of the fan blades and exiting in a counterflow direction, generating turbulence to excite vibrations.

In addition, a finite element model was constructed of the primary fan design to identify the modes at the frequencies measured in the field. This model was also instrumental in identifying effective modifications to remove or control the identified modes of vibration.

Three problem modes were identified. Two of these were clear in the field data and were the modes addressed by the initial plant modifications. The finite element analysis, however, found a third frequency that was hidden among the other modes. Treatment of this mode made it possible to effectively and adequately control the vibrations of all problem fans by adding structural bracing between fan structures, both laterally and longitudinally.