Summary

A single-channel Datastick VSA™ Vibration Spectrum Analyzer can be used to measure vibration and perform balancing of an industrial cooling fan, even in a harsh environment.

Details

In the process of manufacturing carbon electrodes used in the Bessemer Process of making steel, the electrodes are loaded into a large furnace for final curing and to bake out all volatile organic compounds.

The furnace is gas fired with air introduced through burners. The extremely hot air is extracted through fans in the roof. At the end of the cycle, the fans are reversed and water spray is added to accelerate cooling. The unfortunate side effect of this is distortion and warping of the fans due to rapid temperature change.

In this particular example, fan vibration was excessive. The client had already made several unsuccessful attempts to balance the fans.

The project was to determine the actual root cause of problem and fix it.

Out on the roof, the fan was tested for vibration with a VSA handheld Vibration Spectrum Analyzer. Overall vibration was more than excessive, so the fan was shut down after just one reading. As the outside temperature was below freezing and snow was falling, the actual vibration analysis was done indoors after the read was recorded on the VSA handheld.

Vibration at 1 X rpm was less than 1/3 of the total vibration. The spectrum signature indicated looseness and lack of lubrication (harmonics of 1X rpm along with high levels of high frequency noise.) However, the bearings were damaged beyond the point where added lubrication would help.

The bearings were replaced and lubricated according to specification and the fan was run and tested again for vibration. Although the vibration level was high, the FFT showed 90% of vibration at 1X rpm, indicating imbalance.

Because the motor and bearings were on the outside of the roof while the fan itself was on the inside of the furnace, no phase information was available. This made it impractical to use dedicated balancing hardware for this application. Instead, balancing was done using the three-point trial-weight method. (See Datastick Application Note: Four-Run Balancing for details.) This method does not require a phase signal.

A balance procedure was done and the overall vibration was deemed acceptable. Since the fan blades warp on each heating/cooling cycle, there was no point in going for precision balance for this particular application.

Summary of Four-Run method for balancing.
(See Datastick Application Note 4: Four-Run Balancing for details.)

Run the fan and record overall vibration. Note results.
Add a test mass to the 0-degree point.
Run the fan again and record overall vibration. Note results.
Move the mass to the 120-degree point.
Run the fan again and record overall vibration. Note results.
Move the mass to the 240-degree point.
Run the fan a final time and record overall vibration. Note results.
Use a graph or calculation to determine the unbalance magnitude and angle in terms of the test mass.
Remove the test mass and add correction mass according to result, run fan, and record amount.
Condition: vibration should be acceptable.
Since these fan blades warp on each cycle, there is no point in going for precision balance.

Derek Norfield
Director of Applications
Datastick Systems, Inc.