Introduction

Rotary kiln is the core thermal equipment of cement, metallurgy, building materials, chemical industry and so on. Its normal operation determines the quality, output and cost of products. Due to the high temperature of 1500℃ in the rotary kiln and the large temperature change gradient, there are high circumferential stress and thermal stress in the kiln, which have a significant impact on the operation of the rotary kiln. Therefore, it is of great significance to master the temperature gradient distribution of the cylinder when the kiln equipment is running, to guarantee the efficient and safe production of the kiln and to prolong the service life of the kiln. Based on the analysis and the final treatment results of an abnormal serious bending accident with 4.8m×72m rotary kiln of a company, it is shown that the circumferential temperature difference distribution data of the cylinder is a very important parameter in the operation of the kiln equipment.

01Bending accident of kiln

The rotary kiln has been in normal production since October 2016, but on October 20, abnormal vibration occurred at the big tooth ring of the kiln, and the vibration amplitude increased gradually. After the two supporting wheels of the kiln tail stop were moved into 2mm at the same time, the vibration did not decrease, and at this time, the kiln tail pier also appeared obvious shaking. On the afternoon of October 22, it was found that the kiln tail was seriously bent and the yaw rubbed against the sealing cover of the kiln tail, as shown in Figure 1.

Fig. 1 The kiln tail cylinder is seriously bent and rubbed to the sealing cover of the kiln tail

On October 23, the kiln was stopped because of the need to dig and repair the fired tube. The tip clearance of the big gear ring was immediately measured, and the data were abnormal, as shown in Table 1. The bolt of spring plate of big gear ring and its connecting bolt were checked, and no abnormal condition was found. Due to the failure to find the reason for the bending of the cylinder, we did not pay attention to check the thickness of the kiln skin. In the ignition and heating stage of kiln on October 26th, although the intermittent rotary kiln stopped kiln was corrected, the tooth tip clearance and the bending data of kiln tail barrel did not improve during the correction process. When there is no feeding, the minimum gap between the kiln tail bending and the sealing cover is about 20mm. After 24 hours of feeding, the vibration of the big gear ring increases with the increase of the cylinder temperature, and the kiln tail begins to rub against the sealing cover again.

Table 1 Measuring data of tooth tip clearance of big gear ring in hot state of kiln when kiln is stopped (mm)

Circumferential mean point of cylinder	1	2	3	4	5	6	7	8	9	10	11	12
Top clearance of tooth	19	17	17	16	13	11	9	11	12	17	19	22

02 Cause analysis and treatment measures

2.1 Cause analysis

During the ignition and heating of the kiln body, the author has dealt with the bending of the kiln for more than ten times due to the out-of-time rotation of the kiln, but it is the first time that the kiln was seriously bent in the normal operation. The investigation process for its causes is as follows:

(1) According to the temperature scanning data of the kiln body, the temperature of the cylinder at the end of the kiln is about 200℃ when bending occurs, which excludes the reason that the rigidity of the cylinder is distorted due to high temperature.

(2) Check that there is no crack in kiln tail cylinder after kiln stop, and check all bolts and connecting bolts of spring plate of big gear ring, and no looseness is found.

(3) Stop the kiln to check the tip clearance of the big and small gears. The big tooth modulus of the kiln is 40, and the theoretical calculation of tooth tip clearance should be 12 ~ 13mm. Considering the radial run out of the big tooth ring is about 2mm, the actual range of tooth tip clearance should be 14 ~ 15mm. Table 1 shows the measured data of tooth tip clearance of hot big-gear ring in kiln stop. At 7 point, the minimum tip clearance is 9mm, which is 3 ~ 4mm smaller than the theoretical value. At 12 o ‘clock, the maximum tip clearance is 22mm, 9 ~ 10mm larger than the theoretical value, and the maximum difference between the two is 13mm, which indicates that the thermal deformation of the cylinder at the big gear ring is great. This will cause vibration of the ring gear. In order to prevent the top tooth collision caused by the small top tooth clearance, the two supporting wheels at the end of the kiln were pushed inward at the same time by 2mm, but the vibration of the big tooth ring did not decrease after opening the kiln.

• Circumferential temperature difference and local thermal expansion of the cylinder at the big tooth ring. During the shutdown period from October 23 to 26, the thickness of the auxiliary kiln skin in the kiln with big tooth ring was not checked and cleaned, which led to the abnormal temperature distribution of the cylinder body in the kiln after opening the kiln. When adjusting the supporting wheel on October 28, abnormal circumferential temperature distribution of the cylinder was felt beside the big tooth ring. The infrared thermometer was used to measure and it was found that the maximum temperature difference of the cylinder in one circle was close to 180℃. The circumferential temperature data of the cylinder were shown in Table 2.

Table 2 Circumferential temperature measurement data of cylinder at big gear ring (℃)

Circumferential mean point	1	2	3	4	5	6	7	8	9	10	11	12
The surface temperature	144	159	250	311	306	294	297	274	277	261	144	138

In Table 2, the maximum temperature difference between point 4 and point 11 is 173℃. According to the related research on the thermal deformation of kiln barrel, the diameter increase of thermal expansion of cylinder temperature difference is S=фT(r)△t。

In the formula, the diameter of the cylinder is 4800mm, T(r) cylinder Q235 steel at about 300℃ range of thermal expansion coefficient is about 0.000012mm/℃, △ T cylinder temperature difference is 173℃. Put them into the above formula, and the increase of thermal expansion in the local diameter of the cylinder is: S=4800×0.000012mm/℃×173℃=9.96mm.

The circumferential temperature difference of the cylinder here is 173℃, and the increase of thermal expansion in the radial direction is nearly 10mm, which is the root cause of severe thermal bending of the cylinder at the end of the kiln.

2.2 Treatment measures

After the cause of bending fault was identified, considering the thickening data of kiln skin reflected by the infrared temperature scanner of kiln body, the related process parameters were adjusted to gradually burn down and fall off the thickened kiln skin in the kiln with large ring tooth, so as to remove the thickened kiln skin and reduce the circumferential temperature difference data of the cylinder body. After 1 ~ 2 days, the vibration at the ring of big tooth gradually decreases. On the third day, the circumferential temperature difference of the cylinder is less than 50℃. At this time, the vibration at the ring of big tooth is eliminated, and the bending of the cylinder at the end of the kiln returns to a normal straight line.

The accident caused 2 bolts to break and many other bolt joints to crack. Due to the timely treatment, the kiln avoided other more failures. The following reflections are reflected on the handling process of the accident:

(1) In the past, this kiln also had the event that the big tooth ring began to vibrate to gradually increase, and then disappeared by itself, and the kiln body was not obviously bent. Therefore, this time, only mechanical inspection of equipment was done, such as stopping kiln to check the tooth tip clearance of big and small gears and the connecting bolt of spring plate, etc., but the circumferential temperature difference data of cylinder body was ignored, and the root cause of the accident was not found out.

(2) Because it is not recognized that the circumferential temperature difference data of the cylinder is an important parameter for the operation of the kiln equipment, the temperature difference change data of the cylinder is not paid attention to. In fact, the temperature scanning data of the kiln has truly reflected the length and thickness of the kiln skin, and the vibration of the big tooth ring is gradually intensified, which also indirectly reflects the thermal bending deformation of the cylinder.

03 Conclusions

(1) Circumferential temperature of the cylinder, especially the temperature difference data of the cylinder near the kiln wheel belt and the big tooth ring, is an important parameter for the normal operation of the kiln equipment. The abnormal changes are related to the dynamic center line of kiln, the bending of kiln and the increase of dynamic load of supporting wheel, which will directly affect the normal production and operation of kiln equipment. Therefore, it must be included into one of the key monitoring parameters in daily management of kiln equipment.

(2) The accident reminds the equipment manager not only to find the cause of the bending of the kiln from the mechanical aspect, but also to consider the kiln is a thermal equipment. It is necessary to observe the abnormal change of temperature parameters of the kiln body and cooperate with the production process manager to jointly ensure the efficient production and operation of the kiln.

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