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Double Suction Fan Vibration-FM

Is Your Fan Balanced?

Double suction draft fans are prone to problematic vibrations when operated under imbalanced conditions. Often when the topic of imbalance is brought up, it is conceived as mass imbalance. On the other hand, a thermal imbalance can cause similar and even more severe rotordynamic consequences when overlooked. KnightHawk Engineering (KHE) witnessed such a problem with a double suction draft fan.

Axial Vibration and Rotor Rub in Double Suction Fan

A double suction draft fan experienced large axial vibrations and difficult startups followed by light rubs. Bode plots suggested a typical imbalance problem while first critical speed was above the running speed with adequate margin. Primary investigations indicated that pedestal stiffness was neglected in the calculation of the first critical. The separation margin considering the pedestals effect lowered the critical speed close enough to the running speed. But the issue was beyond the pedestal stiffening.
    The thermal data collected at the fan inlets revealed a temperature difference of 150⁰ F between the two inlets, due to a nearby heat exchanger. The water-cooled elliptical bearing added more complexity to the thermal imbalance picture. KHE’s thermal simulations estimated the radial thermal gradients across the rotor to be large enough to induce synchronous eccentric forces and explain the measured field vibrations.

    Additionally, a draft fan is designed to operate under balanced flow conditions. The thermal imbalance between the two suctions means a higher mass flow through the cooler side. KHE’s simulations suggested a significant mixing of the discharge flow. Furthermore, the flow imbalance never allowed the fan to reach its design flow. This thermally imbalanced reduced flow caused axial shuttling due to thermal mixing and dynamic pressure pulsations at the outlet of the fan. As a result, this fan experienced abnormally large axial vibrations during startups and upset conditions. It should be noted that double suction impellers are inherently prone to axial shuttling which has disastrous impacts on the bearings and seals life.


Thermal gradients in the shaft and bearing regions in axial and radial directions


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