Fan Testing

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ITI was called to help correct excessive vibration on a 10,000 HP induced draft fan at a new fossil fuel power plant.

A coast-down analysis, performed as the fan was shut off and coasted down, revealed a structural mode at about 780 RPM (13 Hz). Test data identified a rocking motion of the fan and its foundation as well as torsional flexing between the inboard and outboard ends.

Impact Technique & Design

To adequately excite the basic modes of such a large structure, it was necessary to concentrate the excitation energy at lower frequencies. ITI devised a roving response impact test using a 1000 lb impact exciter as an energy source.

This allowed engineers to manipulate and measure the impact force focusing the input energy at lower frequencies.

Measurements and Windowing

The impacter was suspended from an overhead crane to strike near the top of the fan's concrete pier. For this test, tri-axial accelerations were collected at 65 structural points on the fan and foundation for 3 separate impact directions.

Results & Modifications

After correlating a finite element model based on the extracted model parameters, ITI used the model to determine structural modifications required to move the resonant frequencies out of the fan's operating range.

Modifications included:

• 36" caissons were set to a depth of 20' supporting the concrete piers
• The concrete slab area was increased

Fan vibration was reduced to an acceptable level. As a result, the plant can operate the fan at full capacity without fear of damage