Report on Thaisen Fan (Scrubber)
After cleaning of the fan blades, vibration of the fan gradually increases during operation and in a span of 10 to 14 days vibration level reaches an unacceptable level, which necessitates the next cleaning cycle. However, for so long, this matched the scheduled production window provided by operation. However, after the recent changing of the rotor and the bearings, the fan now reaches unacceptable level of vibration within a short span of time that does not coincide with the scheduled “production window” of the operation, which causes “unplanned downtime.”
Goal of the investigation: To correct the imperfection in the system so that the fan cleaning cycle coincides with.the scheduled production window.
Result of the investigation:
1. The problem of rising vibration within a short period of time is an inherent problem (a birth defect) of the fan. The main reason is the Coriolis effect on the fan. Coriolis force is a force exerted by a moving fluid on the disc or impeller rotating in the fluid. If the rotation is CCW (counter clockwise) then the fluid moves to the right of the impeller and away from the centre. Similarly, when the impeller moves in the CW (clockwise direction) the fluid moves towards the left of the impeller and away from the centre.
In this case, with the fan moving in the CCW direction the Coriolis force moves toward the right of the impeller in the same direction as the damping force. This effect (the fan moves in the CCW direction) produces negative damping (since the two forces are in the same line of action).
Negative damping is a phenomenon, when damping force, which usually opposes the driving force, acts in the same direction as the driving force. In such a case the vibration of the system is amplified.
Combination of negative damping and Coriolis effect produces this phenomenon of gradually rising vibration of the fan in a short period of time, which goes away upon regular cleaning. In the present context nothing can be done to eliminate the phenomena of Coriolis Effect and Negative damping. However, if a similar system is to be installed in the future, we would be pleased to provide necessary suggestions and recommendations so that such phenomena are eliminated right from the start.
2. Present signatures indicate misalignment and dynamic imbalance
3. Weak foundations
Actions to be taken to increase the cleaning cycle to match scheduled “production window.”
1. Take care to align the rotor properly. Care to be taken while putting shims.
2. Dynamically balance the fan in two planes to eliminate the imbalance
3. Cleaning cycle can be initiated when vibration of the fan on the bearings reaches 7 mm/sec (rms). It is safe to run the fan upto this point.
4. Monitor the condition of the foundation by taking vibration measurements in displacement and acceleration modes. Displacement should be taken in the horizontal direction on the topmost accessible point of the columns and at the base. Acceleration should be taken in both vertical and horizontal directions. Displacement should not cross 50 microns in the horizontal direction or at the base of the columns. Similarly acceleration both in the vertical and horizontal directions must not cross 1.5 g. This would ensure safety of the equipment. In case it crosses corrective actions are to be taken to rectify the foundation.