Saturday, March 30, 2019

COOLING TOWER FAN CASE STUDY-2 - HIGH VIBRATION OF CT FAN-2


INTRODUCTION
Already we have discussed regarding the cooling towers and the unbalance problem. Similarly, there was a problem with similar model cooling tower, with a larger span. Noticed an increase in 1x vibrations from the motor readings at fan RPM.
On observation, it was found that the cause of high 1x was the increased clearance in gearbox output bearings.

How to conclude based on the findings?
First of all, thorough observation is required apart from vibration monitoring. Here, in this case, it was possible to view the top side of the fan from an adjacent platform. While viewing the fan, it was found that the whole rotor assembly was wobbling. This might happen due to two cases, either an extremely loose base or an increase in bearing clearance.
Based on that, scaffolding was arranged near the cooling tower and the gearbox base was observed for movement. Since there was no provision to collect the vibration data from fan gearbox, the only method was by visual inspection.
Visual inspection helped in confirming increased bearing clearances. Now, since it is a taper roller bearing, it might not be a bearing fault, but the shimming problem on the shaft. The cup and cone arrangement of the bearing has to sit properly to have good bearing clearances.

How it was solved?
The plant has to be stopped when the vibrations trend came in alarm zone. But everything was planned, thanks to condition monitoring tools that are helpful in prediction.
The bearings have been cleaned and the shaft has been brought down, reducing the shim thickness to maintain proper bearing clearances. The radial and axial play were checked. The maximum tolerance for these was kept up to 0.7 mm.

Availing the opportunity, all the base, structure joints and foundation bolts were tightened to improve the reliability. This was important because the high vibrations have caused some bolts to become loose.
Proper alignment was done followed by lube oil filling and cooling tower basin cleaning.
The issue was solved and the vibrations came down from 6mm/s to 2.4mm/s.
The fan is running smoothly for the past 8 months.
Vibration spectrum-initial
Vibration spectrum-horizontal- After correction
Vibration spectrum -Vertical- After correction

Points to ponder
1.    Check thoroughly before deciding with vibrations alone.
2.    If possible, inspect the fan rotor from outside, while in operation for wobbling or any other abnormalities.
3.    Confirm the Direction of Rotation.
4.    Study the equipment manual and drawings for understanding the origin of the problems.
5.    Follow the efficient method for solution. (Instead of changing bearings, check the condition of bearings and adjust the clearance as in the case with taper roller bearings. Go for replacement in case of ball bearings.)

6.    Inspect the rotor assembly and gearbox, availing the opportunity. Perform DP tests if any cracks are observed. If the DP test cannot be done, confirm based on visual inspection.

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