【20090921 Key Topic】Failures of centrifugal fans

There are various reasons for vibration, such as rotor imbalance, misalignment of couplings, loose foundation bolts, etc. Oil film oscillation in large-scale units can also cause vibration due to the mechanical properties of the bearing shells.

The causes of unit vibration include: surge, oil film oscillation, static and dynamic friction resulting from too small a gap between the units, uneven thermal expansion of the units, low oil temperature, rotor bending, poor alignment of the couplings, disruption of the dynamic balance due to rotor deformation, rotor scaling, low oil pressure and insufficient oil volume, poor oil quality leading to emulsification, deformation of the unit’s foundation or loose bolts, liquid present in the compression medium, and inaccurate instrument readings.

Centrifugal fans are important process equipment in cement plants. Accidents in which the entire production line comes to a halt due to fan failures occur frequently in cement plants; therefore, this document summarizes the causes of the failures of the two fans in our company and the measures taken to address them. 　　1. Fault in the fan of one chamber of the grate cooler 　　1.1 Fan parameters and symptoms of the fault 　　The centrifugal fan is model Y5—48№12.5D; its air handling capacity is 22,400 m3/h, and its air pressure is 6,570 Pa. The motor associated with it is of type Y280M-4, with a power output of 132 kW and a rotational speed of 1,490 rpm. The transmission mechanism used is I)D. 　　Since it was put into operation, severe vibrations have occurred on multiple occasions; the vibration speed of the bearings exceeded 21 mm/s, while the vibration amplitude was 0.6 millimeters. The bearing housings shook violently, which led to damage to the bearings at the impeller end, forcing the machine to be shut down for maintenance. Bearings have a short service life; sometimes, newly replaced bearings get damaged again within less than 1 month, which severely impacts production. 　　1.2 Analysis and handling of fault causes 　　1) Dust accumulation on the impeller 　　This fan features backward-tilted blades, which tend to accumulate dust. At the same time, since the fan is installed below the kiln head, the flying sand that falls from the kiln opening can easily get attached to the blades, causing the impeller to lose its dynamic balance and leading to fan vibration. 　　First, the seal at the kiln head was modified, with the original single-layer graphite seal being replaced by a fish-scale-type contact seal, **which reduced the amount of sand flying out from the kiln head. At the same time, the kiln head platform is treated to prevent leaks; when a small amount of sand falls, it goes directly into the chute, and it is cleaned manually on a regular basis ； Secondly, regular cleaning of the dust accumulated on the blades is carried out through planned maintenance or shutdowns, to ensure the operating accuracy of the fan ； Another change is to switch the fan’s air intake from horizontal to vertical; although the air intake resistance increases slightly, the quality of air entering the fan improves significantly. After taking these measures, the vibration caused by dust accumulation on the fan impeller was significantly reduced, which contributes to the stable operation of the equipment. 　　2) The impeller shaft has an excessively long cantilever length, resulting in excessive bearing load. The distance between the center of the bearing seat at the impeller end and the center of the impeller is 540 mm; compared with similar domestic fans of type K, this means that the impeller shaft has an overly long cantilever length, which leads to excessive loading on the bearings in this area. This in turn causes local defects in the bearings or even their damage. Based on the structure of this fan and its installation location on site, we moved the bearing housing on the impeller side 100 mm toward the casing, reducing the distance between the impeller and that bearing housing, which significantly improved the loading conditions on the bearings. 　　3) The selected bearing is too small. Upon checking the bearing, it was found that the originally designated double-row deep groove ball bearing of model 1622 has an insufficient rated load, and it breaks down within no more than 3 months of use. Considering the dimensions of the bearing housing and the need to reduce capital investment, we have replaced the bearing here with a 3622 double-row radial spherical roller bearing; the technical specifications of both types of bearings are shown in Table 1. Table 1 Technical parameters of bearings 1622 and 3622. As can be seen from Table 1, the rated dynamic load of the 3622 bearing is higher. Although its maximum allowable speed is slightly lower than that required, it has operated normally over the years since the modification, so there is no need to worry about the impact of this factor on the bearing’s lifespan. 　　Through these modifications, the service life of the bearings has been extended to over 1 year; no such failures that cause shutdowns have occurred in the equipment over several years of operation. Additionally, we manufactured another set of fan rotors and replace them regularly, while maintaining the replaced rotors for use as spares, thereby ensuring the stable and reliable operation of the kiln. 　　2. Fault of the high-temperature fan at the kiln exit 　　2.1 Fan parameters and fault symptoms 　　The centrifugal fan is of model BB124; its air volume is 220,000 m3/h, air pressure is 7,845 Pa, and the operating temperature is 350°C. It is equipped with a motor of 800 kW, operating at 740 r/min, with a F-type drive mechanism. 　　During the initial stage of production, the high-temperature fans often experienced overheating in their free-end bearings, which led to increased vibration in the fans and eventual bearing failure, forcing the entire production line to shut down. After replacing the bearings of the fan, to ensure stable operation, the fan is tested for 2–4 hours in accordance with the procedures. During the testing phase, the fan operated smoothly, with the vibration speed generally remaining below 5 mm/s, and the bearing temperature staying around 400°C. However, after the rotary kiln was heated and production began with material feeding, the free-end bearings started to heat up after the fan had been running for some time, and the temperature rose rapidly. Even though cooling measures such as air cooling and pouring water on the bearing housings were taken, it was still not possible to control the rise in temperature, forcing a shutdown of the equipment. 　　2.2 Analysis of Fault Causes and Remedial Measures Through the handling and analysis of several faults, it was found that the main cause of bearing overheating was improper operation during installation; this allowed the bearing at the free end to adjust automatically as the shaft expanded due to heat, resulting in excessive axial load on both bearings and causing them to overheat and get damaged. 　　The normal operating temperature of the high-temperature fan is 350°C; therefore, the elongation of the shaft under operation is given by: △L = L·α ·△t. Here, L represents the length of the fan shaft inside the casing, which is 2500 mm ； 　　　　　α: Coefficient of linear expansion of the material. The material used for this bearing is 42CrMo, which belongs to the Cr steel category; therefore, a value of 13×10-6 is used. △t = t2 – t1. t2: Operating temperature of the fan shaft in °C. t1: Ambient temperature, 0°C in winter and 35°C in summer. From the calculations above, it can be seen that as the fan moves from a normal temperature state to its operating state, due to seasonal variations, the elongation during installation ranges between 10.2 and 11.4 mm. However, during the inspection of the records, the repair mechanic, due to a poor understanding of the structure in this area, used large work gloves and a lever to apply force when tightening the bearing cover; two people worked together to tighten the bolts on the cover. As a result, the bearing housing at the free end is pressed too tightly and cannot move freely as the shaft expands due to heat, which compromises the assembly accuracy of the bearing at that end and leads to overheating and damage of the bearing. 　　Experiments were conducted to verify the correctness of the analysis conclusions. The test was carried out without a bearing housing: first, the dimensions of the bearing’s inner hole were measured; then one person used a wrench to tighten the bolts on the bearing cover, and measurements were taken again; finally, two people used a tensioning rod to tighten them and took measurements once more.

There are various reasons for vibration, such as rotor imbalance, misalignment of couplings, loose foundation bolts, etc. Oil film oscillation in large-scale units can also cause vibration due to the mechanical properties of the bearing shells.