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Project background
1. Application status of thermal imaging technology in rotary kiln inspection
At present, rotary kiln is an important equipment in metallurgy, building materials, chemical plants and silicate industry in China. According to relevant statistics, more than 60% of the kiln stoppages are caused by the damage of the kiln lining. The lining of the rotary kiln is mainly composed of refractory bricks. The refractory bricks rub against the materials for a long time and are in a high temperature state, which will lead to the deformation, serious wear and tear of the refractory bricks and even the falling of the bricks. This not only affects normal production, but also wastes energy and increases production costs. The on-line monitoring technology of the surface temperature of the rotary kiln enables the on-site staff to timely understand the temperature in the kiln and the changes of refractory materials, and arrange maintenance personnel to repair according to the actual situation, which greatly improves the efficiency and accuracy of production equipment fault detection.
2. Application requirements of rotary kiln for thermal imaging technology
2.1 working characteristics of rotary kiln
The rotary kiln is composed of cylinder, transmission device, supporting device such as supporting wheel and runner, sealing device at kiln head and tail, kiln head cover and combustion device. The barrel of rotary kiln is a heated rotary part, and the barrel is equipped with temperature measurement and sampling hole devices. Some kilns also need to be specially equipped with infrared thermometer to monitor the temperature of the barrel surface. Infrared imaging technology has been applied to the temperature measurement of materials in rotary kiln and the surface temperature detection of rotary kiln.
2.2 Routine maintenance methods and means of rotary kiln
For the maintenance of rotary kiln, the current practice is to strictly implement the regular overhaul system. According to the operating regulations, the refractory bricks should be replaced every 150 days of operation, while the actual operation can reach about 210 days. For the inspection between the two intermediate repairs of the rotary kiln, the conventional method is that the inspection workers use the hand-held temperature measuring instrument, combined with their inspection experience, to measure the temperature of the part of the rotary kiln that may be damaged, and then record and repair the rotary kiln with abnormal temperature. This method mainly relies on the experience of the testing workers to measure the temperature at a limited point, rather than the overall measurement of the rotary kiln. Therefore, it has certain limitations, randomness and is easy to miss the inspection.
Maintenance difficulties
The volume of the rotary kiln is large and it is rotating all the time. Moreover, the inspection workload of maintenance workers is large, and the time of one inspection is also relatively long. Similarly, sometimes the failure to repair in time due to faults will also lead to accidents and serious consequences. In view of this situation, various online monitoring means are being strengthened, the regular overhaul system is being reformed, and the transition to predictive maintenance system is gradually underway. Infrared thermal imaging is one of the important online monitoring means.
2.2 Advantages of infrared thermography detection of rotary kiln
The infrared thermal imager can quickly and conveniently obtain the two-dimensional thermal image of the outer wall of the rotary kiln, that is, the surface temperature field of the outer wall. Due to the obvious difference in heat conduction between the outer metal and the inner non-metallic materials of the rotary kiln, the degree, type and distribution of lining damage are the direct reasons for the different distribution of temperature field on the outer wall surface of the rotary kiln. Therefore, the main defects of the inner lining can be reflected in different degrees in the infrared image.
The damage types of rotary kiln lining can be roughly divided into thinning, cracking, bulging and falling off. Although the strict theoretical relationship between each type of damage and infrared thermogram has not been established, through the analysis of infrared thermogram and the comparison of lining inspection, the infrared spectrum of each type of defect has its own characteristics. For example, in terms of shape, for the crack type damage corresponding to the general long strip or slender overtemperature zone, when the maximum temperature of the overtemperature zone is greater than a certain value, it can be diagnosed as through crack damage; When the temperature is moderate, it may be a general crack. If the average temperature in the overtemperature zone exceeds a certain value, it can generally be diagnosed as falling off damage; If the average temperature in the overtemperature zone is lower than a certain value and the temperature gradient is small, it can generally be diagnosed as thinning.
It is a scientific and effective means to analyze, judge and inspect the working state of the internal lining through the on-line non-contact infrared thermal image test of the temperature field on the outer surface of the rotary kiln by using the infrared thermal image, which can provide a practical basis for ensuring the long-term operation of the rotary kiln.
2、 Current situation of infrared thermography and temperature measurement of rotary kiln
1. Use the hand-held thermal imager to regularly detect the rotary kiln
At present, the handheld thermal imager has been widely used in the steel industry. For example, the hand-held thermal imager is used to identify the blockage of steam pipes and hot water pipes and find the blockage point; Conduct regular maintenance of high-voltage electrical equipment to detect whether the contacts of such equipment are overheated; It can also detect the parts of some rotating (moving) equipment that are heated due to friction. Production equipment in iron and steel enterprises, including blast furnaces, hot blast stoves, hot blast pipes, heating furnaces, soaking pits, various steel-making furnaces, etc., are high-temperature service equipment with refractory lining, just like rotary kilns. Handheld thermal imagers have also been widely used in the maintenance of such equipment to judge the damage degree of refractory materials and play the role of preventive maintenance.
The infrared and visible light pictures taken by the hand-held thermal imager use the hand-held infrared thermal imager to detect the rotary kiln, mainly for the regular maintenance of the rotary kiln. The maintenance personnel regularly go to the site to observe the rotary kiln. First, store the thermal image in the memory card of the thermal imager, and then use the computer to read and analyze the collected pictures to form a report. This report is also mainly used for auxiliary positioning during intermediate repair of rotary kiln. Therefore, it is limited to use the hand-held thermal imager to regularly detect the rotary kiln:
1) When analyzing the infrared thermal image of the rotary kiln, the general analysis software is used, which does not combine the infrared spectrum characteristics of the lining damage of the rotary kiln. The judgment of the defect points mainly depends on the experience of the maintenance personnel. Therefore, the defects that can be confirmed by the maintenance personnel are often more serious damage, and usually they cannot give more specific information about the type and degree of damage;
2) Because it is a periodic inspection, the discovery of defects also has a certain contingency, which can not completely avoid the occurrence of major accidents;
3) The work of manual maintenance is complex, and the workload of using infrared thermal imager to overhaul all rotary kilns in use is very large. If it is overhauled in stages and batches, the overhaul interval will be extended, which is not conducive to the timely treatment of major damage to the lining of the rotary kiln.
2. Scanning rotary kiln infrared thermal image detection system
The infrared thermal imager is installed on the tracks on both sides of the rotary kiln, and the infrared scanner reciprocates in the x-axis direction at the same time during the scanning process. From the working principle of scanning imaging, it can be seen that the infrared scanner needs to move back and forth before the system can detect the complete imaging of the rotary kiln. Moreover, when the rotary kiln and infrared scanner move at a uniform speed as far as possible, the proportion of the infrared thermal image obtained is consistent with the physical proportion of the rotary kiln, that is, the imaging does not deform.
From this, we can see the limitations of scanning imaging. Without the intuition and reality of focal plane imaging, the image details are not clear enough, that is, the resolution of temperature is not enough.
Infrared thermographic detection system for rotary kiln
After site measurement, the site dimensions are as follows:
Rotary kiln size: 70000 ×Φ 4200mm；
Site installation distance: 15000mm;
Parameters of infrared thermal imager are as follows:
It is calculated that the maximum length of the observation object of the infrared thermal imager is 17000mm, and the length of the rotary kiln to be monitored is about 62000mm. To monitor the whole rotary kiln, it is necessary to take periodic infrared photos of the rotary kiln at least at four preset positions. The detection system has two schemes, which are introduced in detail below.
Programme1 Install an infrared thermal imager, configure the track for the infrared thermal imager, and use the track control system to stop the infrared thermal imager at the specified position and take photos periodically. The site installation diagram and system structure diagram are shown in the following pages.
Installation diagram
System structure diagram
If this scheme is adopted, the whole system is composed of infrared image acquisition system, slide motion control system and infrared back-end analysis system. The infrared image acquisition system is composed of an infrared thermal imager and a network access system to realize the infrared image acquisition and transmission function of the rotary kiln; The slide rail motion control system is composed of motor, motor driver, sensor, slide rail and motion controller to realize the positioning function of the rail car; The infrared back-end analysis system is composed of server and analysis software to complete the storage and analysis of surface temperature field data of rotary kiln.
Programme2Three infrared thermal imagers with pan tilt are installed. Each infrared thermal imager is equipped with three prefabrication positions, and the three infrared thermal imagers monitor the surface of the whole rotary kiln. The site installation diagram and system structure diagram are as follows.
If this scheme is adopted, the whole system is composed of infrared image acquisition system, pan tilt control system and infrared back-end analysis system. The infrared image acquisition system is composed of three infrared thermal imagers, PTZ and network access system. The infrared image acquisition system and the infrared back-end analysis system have the same functions and scheme I.
The thermal imagers of the two schemes are installed on the side of the rotary kiln. Since the rotary kiln itself rotates continuously (generally, the rotation speed is 0.5~2 RPM), the thermal imager can collect the temperature field data of the rotary kiln regularly (such as every 5~10 seconds), which can cover all areas of the rotary kiln.
In addition, the infrared thermal imager is installed near the rotary kiln, which has to withstand the test of wind, rain, snow and other adverse weather, as well as high temperature and dust. Therefore, the protective shell of the infrared thermal imager must have the ability to prevent dust and rain. When the ambient temperature is high, it must be able to cool the thermal imager to ensure the reliability of work. Additional lens protection components, detector protection components and dust protection devices are provided.
Scheme 1 is difficult to install on site, which requires a new slide rail. In addition, the railcar runs for a long time, and its service life is limited, so it needs regular maintenance. In addition, the stability of the slide rail will also affect the accuracy of the rotary kiln infrared monitoring system; Scheme 2 can use the existing supports around the rotary kiln for fixed installation, which is convenient and reliable for construction and installation. The communication control technology between the rotary kiln and the pan tilt is also relatively mature, so we recommend scheme 2.
Technical means and function realization
1. Infrared focal plane imaging technology
The infrared thermal imager adopts an advanced uncooled focal plane detector with 384 pixels × 288. Each time the thermal imager images the target, there are 110592 sampling points. Therefore, the thermal imager can form a plane image of the temperature characteristics of the whole rotary kiln surface. The detector has high sensitivity and temperature resolution of 0.12 ℃. Small temperature changes on the surface of the rotary kiln can also be reflected. In addition, with focal plane imaging, there is no distortion in the thermal image, and the proportion of the thermal image is completely consistent with that of the rotary kiln entity. The corresponding points on the rotary kiln body can be found directly according to the temperature abnormal points on the thermal image, which is convenient for the location of damage points.
1.1 Infrared real-time temperature measurement technology
Infrared thermography uses computer computing technology to calculate the temperature data of the object surface according to the original infrared data. And store the temperature data for statistical analysis.
2. Rotary kiln surface temperature analysis function
2.1 Statistics of surface temperature data of rotary kiln
Combined with the experience of Baosteel's ladle monitoring project, the following functions are set for the rotary kiln monitoring system: according to the statistical temperature field data, the maximum temperature trend analysis chart is established according to the zoning of the rotary kiln, and the alarm value threshold of different areas on the surface of the rotary kiln is established in combination with the maintenance experience. Through the pre alarm analysis, the service life of the rotary kiln is prolonged and the refractory is saved on the premise of safe use of the rotary kiln; Judge the cinder blockage fault according to the average temperature, and find the cinder blockage fault in time.
2.2 Rotary kiln lining damage alarm
The following figure shows the maximum temperature history curve of multiple surface temperature field data collection in a rotary block. Set a reasonable alarm value according to the statistical ladle surface temperature field data and empirical value. When the maximum temperature of the ladle surface reaches the alarm value, remind the staff to arrange maintenance work in real time, so that the rotary kiln can prolong its service life on the premise of ensuring safe production, so as to reduce production costs. The infrared imaging system itself adopts focal plane imaging technology. When a fault is found, the actual position of the fault point can be calculated according to the infrared picture of the alarm point.
2.3 Rotary kiln cinder blockage alarm
After long-term operation of the rotary kiln, there may be cinders attached to the inner wall in some areas, which may cause cinder blockage if not cleaned in time. The surface temperature of this area is different from that of other areas due to the different heat-resistant layer. Using this feature, we have measured the surface temperature of each block of the rotary kiln for many times during the trial operation, and recorded and counted the average temperature and maximum temperature of these temperature data. The average temperature alarm threshold of cinder blockage can be inferred from the statistics of this historical curve and experience. Then, the surface temperature of the rotary kiln is measured in real time and compared with the average temperature alarm threshold of cinder blockage, and then the cinder blockage fault in the rotary kiln can be judged.
3. Feasibility of rotary kiln infrared thermal image detection system scheme
Wuhan huajingkang Photoelectric Technology Co., Ltd. is a high-tech company specializing in providing infrared thermal imager products and industrial application solutions. At present, the company's infrared thermal imager product series mainly covers the fields of power system, iron and steel metallurgy, security monitoring and so on. Because the company's products have the characteristics of online real-time monitoring, high temperature measurement accuracy, customized customer application functions, etc., we successfully won the bid for the power monitoring system of a substation of the State Grid. In this system, we successfully used the PTZ control system, thermal imager imaging and accurate temperature measurement.
In the application of iron and steel metallurgy, our ladle infrared thermal image detection system has been successfully applied to an ironmaking plant of Baosteel. In this system, we have accumulated the infrared thermographic data of the ladle and improved the temperature measurement accuracy of high temperature metallurgy. In this system, we customized and developed rich functions for specific applications in the metallurgical industry, which is convenient for operators to use. The rotary kiln infrared thermal image detection system will integrate the user interface functions of the pan tilt control system, the thermal imager imaging system and the ladle infrared thermal image detection system in the substation power monitoring system. The main system components have been successfully applied and experienced by customers, so the scheme has high feasibility.
