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Induced Draft Fan


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Vibration Analysis of Induced Draft Fan


by Kevin R. Guy

Abstract


This paper follows the case history of an induced draft fan with significant vibration problems. The vibration problems began after the fan rotor was retrofitted with a new wheel of different design and the installation of turning vanes within the ducting. Identifying the vibration problem involved extensive testing such as transient testing, impact testing of the fan shaft & wheel as well as collection of data with both seismic & proximity probes. Thermal effects on the fan’s vibration were identified and knowledge of these effects were used in the solution which was balancing the fan using proximity probes, not seismic probes as well as removal of the turning vanes and alignment of the fan bearings.

Abstract

PREVIEW


“Background Information: The subject fan was retrofitted with a new fan wheel using the existing shaft. The number of fan blades was increased from twelve (12) blades to eighteen (18) blades. The fan wheel and shaft is also 650 pounds lighter than the original fan wheel and shaft. Vibration problems occur when the fan is cooling down from the 450 degree start up temperature to the 220 degree operating temperature. Initial data collected indicated the vibration increases during the fan cool down; however, the vibration eventually lowers to below 2.5 mils (pk-pk). This data showed a possible thermal vector and a very high directional vibration indicating a possible resonance problem. The initial testing was very limited due to the fact the plant was in full production and load variations or shutting the fan down for impact testing was not possible. This fan is installed as an induced draft fan on the bag house of a plant collecting dust.

The fan manufacturer was given the fan wheel design and told to build it to the specification of the fan engineering firm. The fan manufacturer did no engineering on the project. They just built the new fan wheel and installed it on an old shaft. They then were contracted to reinstall the new fan wheel and old shaft at the plant. Additionally; to increase the fan efficiency a turning vane was installed on the fan (Figure #1).

Immediately upon the initial runs of the new fan wheel vibration problems existed. The fan manufacturer had problems balancing the fan. Specifically: getting phase angle was almost impossible. Even though fan vibration was over six to eight mils the phase angle was erratic and wandering. The fan field rep decided on his own to cut back the turning vane. This modification allowed the fan to be balanced successfully below 1.0 mils (pk-pk). Within a few days the fan manufacturer had to be back on site and balance the fan a second time.

The second balance procedure kept the vibration below 3.0 mils (pk-pk) for several weeks and the plant personnel rebalanced the fan when levels exceeded 5.0 mils (pk-pk). Plant engineers were able to lower the vibration to below 2.0 mils (pk-pk) but within twenty-four hours vibration was above 5.0 mils again. Since the fan was balance sensitive, plant engineering requested the fan manufacturer get an outside analyst look into the recurrent balance problems of the fan. The engineering firm also was asked to call in their design consultant.”

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