Centrifugal fans operate by having a motor drive the impeller, which draws air into the impeller and accelerates it outward through the discharge outlet. This process inevitably results in varying degrees of vibration. Generally, the vibration amplitude of a centrifugal fan remains within standard limits, but in some cases, it can exceed the standard. So, what causes excessive vibration in centrifugal fans?
During the operation of a centrifugal fan, the impeller is subject to erosion by flue gases, leading to wear. Applying wear-resistant welding rods and anti-wear coatings on the airfoil blade surfaces can enhance their wear resistance. However, the uneven wear of each blade, uneven application of welding rods, and uneven application of anti-wear coatings can cause dynamic imbalance when the centrifugal fan starts.
Airfoil blades are generally hollow, and when the root area and webs are eroded by flue gases, dust can enter the blades. As dust accumulates, the center of the impeller shifts, causing dynamic imbalance and increased vibration.
The self-weight of the centrifugal fan rotor causes the shaft to sag, which must be considered during coupling alignment. The upward deflection of the coupling on the fan side and the gap at the top must be accounted for. The standard alignment should have the motor side lower by 0.2mm and the top gap at 0.15mm. Aligning as if it were a rigid rotor can result in significant vibration.
During the startup phase of the centrifugal fan unit, when one fan is running in a single series and the other is not in use, the rotor of the unused fan can bend due to heating by flue gases. Measurements show that even in the turning gear state, the rotor deformation remains significant, and the turning gear cannot eliminate this deformation. Thus, the rotor center shifts, causing excessive vibration upon the next startup, rendering the fan unusable.
Dynamic imbalance due to center shift can be corrected by trial weight addition. Mark 12 points corresponding to the blades around the rotor shaft near the thrust bearing side using white paint and Arabic numerals from 1 to 12.
After starting the centrifugal fan, use a stroboscopic vibration meter to measure the phase and overall vibration amplitude. Calculate the position and weight for trial weight addition using the vector method. Add weights at the corresponding imbalance positions on the rotor intermediate disk. Typically, two trial weight additions can solve the imbalance issue.
For example, if the overall vibration amplitude after starting the fan is 0.18mm, and the phase is at point 10, calculate the trial weight and phase. Add a trial weight of 1.5kg at point 11, measure the vibration amplitude as 0.10mm, with the phase at point 8. Then calculate the weight needed to eliminate the vibration based on the effect of the trial weight.
Activate the turning gear and measure the deflection of the centrifugal fan shaft at half-hour intervals, positioning the bulge of the shaft bend at the top. Perform thermal shaft alignment and monitor the deflection, repeating the process until the deflection is eliminated.
In conclusion, dynamic imbalance is a common cause of excessive vibration in centrifugal fans, and it can usually be resolved through dynamic balancing. The key is accurately determining the phase and weight to be added to achieve balance.