Generally speaking, vibration during fan operation is unavoidable. Especially in the later stage of fan operation, deterioration of various parameters aggravates fan vibration. This requires us to solve the fault before the fan condition further deteriorates, so as to ensure normal fan operation. The following is a brief discussion of common fault causes and treatments for vertical mill circulating fans in cement plants.

1. Treatment of Misalignment Between Fan and Motor Coupling

The fan and motor are connected by a coupling to transmit motion and torque. Misalignment is the most common fan fault, and 60% of fan faults are related to misalignment. Fan misalignment refers to the degree of inclination or offset between the axis lines of the two rotors of the fan and motor and the center line of the bearing.

Main causes of misalignment in the fan rotor system:

-Excessive bearing clearance or pitting of the balls. This condition is generally caused by insufficient lubrication or poor oil quality. Once it occurs, it easily causes misalignment and vibration. The bearing should be replaced as appropriate.

- Long-term micro-vibration of the bearing housing causes displacement. Since vibration of large fans is unavoidable, this easily causes slight loosening of the base fastening bolts, resulting in misalignment and vibration.

- Caused by the main motor itself. Large motors have high requirements for dynamic balance. After long-term operation, due to various reasons, the dynamic balance of the motor itself is destroyed, requiring misalignment, etc. After misalignment occurs in the fan rotor system, a series of dynamic effects unfavorable to equipment operation will be generated during rotation, causing deflection of the coupling, wear of bearings, instability of the oil film, and flexural deformation of the shaft, etc. The additional forces on the rotor and bearings lead to abnormal vibration of the fan and early damage to the bearings, which is extremely harmful.

For fan misalignment faults, we currently generally use conventional methods to deal with them:

- First, level the fan main shaft, and rotate the impeller to ensure that the levelness deviation of the fan main shaft at each point is within the specified range;

- Then align the fan and motor coupling. Generally, a dial indicator is used to detect runout at four points to ensure that the runout at the four points is within the technical range. In this way, the alignment is basically completed. An advanced method can use a laser alignment instrument, which is convenient and quick.

2. Treatment of Vibration Caused by Impeller Imbalance

The causes of impeller imbalance during use can be briefly divided into impeller wear and impeller scaling.

In the cement plant where I previously worked, this phenomenon often occurred in the raw meal vertical mill circulating fan. The function of the circulating fan is to collect raw meal powder through a cyclone dust collector, and the remaining small amount of raw meal powder is driven by the circulating fan to an electrostatic precipitator. In this way, many fine dust particles pass through the circulating fan together with high-speed gas, causing the blades to be continuously scoured. Over time, knife-edge wear forms at the blade outlet. Since this wear is irregular, it causes impeller imbalance. In addition, when many fine dust particles pass through the circulating fan, they are adsorbed on the non-working surface of the blades under the action of gas vortices, especially forming relatively serious dust scaling at the inlet and outlet of the non-working surface, which gradually thickens. When part of the ash scale falls off under the combined action of centrifugal force and vibration, the balance of the impeller is destroyed, and the entire fan will vibrate.

Generally, to deal with impeller scaling, after the fan is shut down, use a shovel or brush to clean the scaling. In the past, the method for dealing with impeller imbalance was the static balancing method. According to the different positions of the impeller when rotated, and based on experience summarized in actual work, the weight and position of the counterweight to be added were found, basically ensuring that the fan static balance was in an ideal state. At present, the method for dealing with impeller imbalance is to use a dynamic balancing instrument and add counterweights on site, so that the fan vibration parameters are controlled within the technical range.

3. Treatment of Vibration Caused by Connection Defects Between Fan Impeller and Hub

The connection methods generally used between the fan impeller and hub are rivet connection and reamed hole bolt connection. The circulating fan in the cement plant uses reamed hole bolt connection. Under normal circumstances, the fit between the fan impeller and hub uses an interference fit, and the pre-tightening force of the rivets or reamed hole bolts meets the technical requirements. However, there are still a small number of fans that do not meet the requirements in this respect, causing slight loosening and vibration. In this case, re-riveting or re-tightening is generally adopted. If the requirements are still not met, the impeller must be replaced. In the circulating fan of the cement plant, vibration occurred due to excessive wear of the nuts of the reamed hole bolts, resulting in insufficient pre-tightening force between the impeller and hub. Analysis showed that the main reason was that internal circulating air was generated inside the air duct, causing excessive wear of the nuts.

Treatment methods:

- Improve the air duct structure to eliminate internal circulating air;

- Replace the reamed hole bolts and use a torque wrench to tighten the bolts to achieve consistent pre-tightening force;

- Strengthen inspection to prevent further expansion of the fault.

Summary

Generally speaking, there are many causes of fan vibration and many treatment methods. The above is a superficial discussion based on what I have encountered. In fact, no matter what equipment fault is encountered, as long as the cause of the fault is clearly analyzed and the fault is eliminated in a targeted manner, the equipment can basically be ensured to operate within the technical range.

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A pillow block bearing (or plummer block) is a mounting used to support a rotating shaft with the use of bearings and various accessories. The assembly consists of a mounting block which houses a bearing. The block is mounted to a foundation, and a shaft is inserted, allowing the inner part of the bearing/shaft to rotate. The inside of the bearing is typically 0.025 millimetres (0.001 in) larger diameter than the shaft to ensure a tight fit. Set screws, locking collars, or set collars are commonly used to secure the shaft. Housing material for a pillow block is typically made of cast iron or cast steel.

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