Premature wear of radial bearings in large centrifugal compressors and preventive measures

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Premature Wear of Radial Bearings in Large Centrifugal Compressors and Preventive Measures Premature Wear of Radial Bearings in Large Centrifugal Compressors and Preventive Measures Ruan Xin1, Ruan Xiaoyu2, Zhao Guolong3, Fu Shuanghong3 3. Hebei Jinshi Petrochemical Fertilizer Plant, Shijiazhuang 050031) Abstract: Based on the practices in compressor design and manufacturing in China over the past few years, this paper analyzes the characteristics, causes, and preventive measures for the premature wear of radial sliding bearings in large centrifugal compressors used in the chemical industry. It also presents several targeted suggestions based on the actual conditions of some manufacturers in China. 　　Large centrifugal compressors are the core equipment in chemical plants, and the premature wear of their radial sliding bearings is one of the major faults that prevent the compressors from operating properly. Since chemical plants typically operate on a integrated production basis with closely interconnected units, this often leads to disastrous consequences. For many years, scientists and technologists have been tracking down and exploring its origins, in search of ways to prevent and address it. Bearing wear and scoring can be classified into normal shaft gripping scoring and abnormal shaft gripping scoring (i.e., sudden excessive shaft gripping scoring that occurs early on), depending on the operating time of the components used. This paper mainly explores the formation characteristics, causes, and preventive measures of the latter type of failure. 1 Reasons for premature wear and damage of bearings 1.1 Analysis of the basic characteristics of prematurely worn bearings Here, only two examples are given. A nitrogen-hydrogen compressor at a fertilizer plant (put into operation in August 2005) had to be shut down due to severe vibration; inspections revealed that the bearing alloy layer was damaged. Based on an analysis of the on-site conditions, the damaged bearings were primarily caused by overly tight installation (bearing 2 on the shaft journal). Upon inspection, it was found that the scuffing area on the lining of the bearing bush accounted for 23%, with a tearing depth of 0.46 mm. If the damaged area is black in color, it indicates that the lubricant has deteriorated due to high temperatures. In another case, a domestic large-scale compressor used by a chemical company (for use after the expansion of its production facility) stopped operating just 13 days after installation due to smoking at the bearing area. Inspection revealed severe damage to the bearing caused by binding; measurements showed that the depth of the scratches on the alloy layer was 0.53 mm. This was indeed the first major accident to occur at that factory in many years, causing severe disruptions to production. 1. 2 The material properties and mechanical characteristics fail to meet the specific requirements for use. Bearings are components that are subject to significant stress and experience complex, variable loads during compressor operation; therefore, attention must first be paid to the quality of the materials used. Currently, the bearing materials (alloy layers) for large compressors used in the domestic chemical industry are mainly ZCH2SnSb1126, ZCHPb1621622, ZCuPb30, etc. Although the strength of these materials is acceptable, they have some shortcomings, mainly in terms of low plasticity and toughness, as well as insufficient wear resistance. Especially at the several key stress points of the bearing – the junction between the spindle journal and the bearing housing (where extremely high standards are required for the contact surfaces with other parts as well as for any defects there) – even the slightest defect can easily lead to wear and scratches when significant forces are applied during operation. Even with better bearing alloy grades, they sometimes fail to meet the required physical and chemical standards after testing. If, after damage to a compressor bearing in a factory, testing reveals that its mechanical properties and chemical composition do not meet the required standards. Recent research has also shown that the material properties of bearings are not constant; the degree of variation is related to environmental stability. Therefore, it is necessary to continuously monitor and adjust the operating conditions of the bearings. Furthermore, there is also an optimal ratio between the material of the bearing and that of the matching components. All of this can only be determined through repeated experiments. Table 1 shows the mechanical properties of the material used for the compressor spindle in a certain factory (material grade: 38CrMoAl), while Table 2 lists the materials of the bearing pairs and their main operating conditions. Table 1 Mechanical properties of materials for compressor main shafts
Property index Rm /MPa ReL /MPa δ5 /% Ψ /% HRC
Required value 850 700 15 45 27–29
Measured value 810 592 19.6 59.6 23–24

Table 2 Materials of bearing assemblies and main operating conditions
Name Diameter Bearing bush Bushing body Bearing assembly Operating temperature/°C Lubricating oil
Material 35CrMo Chsnsb1126 ZG25 < 65 Turbine oil No. 30
Diameter or width d = 80, L = 70 < 65 Turbine oil No. 30
Surface roughness (Ra) 0.4 0.8 < 65 Turbine oil No. 30
1.3 The operating conditions are relatively harsh and unstable.
It is well known that, apart from inherent factors, the variability of operating conditions plays a significant role in the premature wear and scoring of compressor bearings. Compressors used in chemical plants have strict requirements regarding operating conditions. However, due to various reasons – technical, human-related, equipment-related, or process-related – process parameters such as gas impurities, temperature, and pressure fluctuate significantly, which increases the load on the rotor shaft and leads to premature damage. The quality of the environmental conditions within the entire process system (such as lubricating oil, cooling water, feed gas, etc.) also determines the length of the normal operating cycle of the machines or components used. 1. 4 The design structure is inadequate, and the precision in manufacturing and installation is not high enough; the compressor bearings get damaged after operating for only a short time, which indicates that there are serious problems in this regard. Based on an analysis of the various components of the compressor in a factory that stopped operating due to certain issues, it was found that its design structure was unreasonable in some aspects. For example, if the source of lubricant for the machine bearings is far from the main lubrication point, it severely affects the lubrication effect. The lubrication system and oil circuit design of bearings should be made more rational and scientific, to ensure that the most stress-intensive areas of the bearings have favorable conditions for fluid friction. Furthermore, insufficient precision in compressor manufacturing and installation is also a cause of early accidents. Centrifugal compressors fall into the category of precision machinery in chemical plant installations; stringent requirements apply to every aspect, from design and manufacturing to installation and positioning, with each step having to be carried out in accordance with **standards and technical specifications. After an accident occurred at a factory, when the compressor casing and main bearing positions were inspected using the wire-tensioning method, slight deviations beyond the specified limits were detected; of course, this could also be caused by premature wear of the compressor as well as vibrations resulting from damage to the components. Recent research findings indicate that the bearing structure and installation positioning, as well as appropriate flexibility, are also important factors in extending the service life of the \"spindle 2 bearings\" assembly. 2 　Several measures to extend the service life of compressor bearings 2.1 　Ensuring the quality of materials used and the precision of manufacturing For bearings, in addition to designing them in a rational and scientific manner, it is also important to ensure high quality in the selection of materials. The bearing materials used must come with certificates of conformity, and they have to meet the relevant inspection standards for chemical composition and mechanical properties. In terms of processing and manufacturing, extra care must be taken; every stage, from the production of the raw material, the casting of the alloy layer, to mechanical processing and lining grinding, must be carried out in accordance with technical specifications. Each step must pass quality inspection before moving on to the next stage. Since the \"main shaft 2 bearing\" is the most crucial component of a compressor, any premature damage to it can have disastrous consequences. Bearings support the operation of the rotor; when wear is inevitable, it is necessary to ensure that the shaft journals experience minimal, slowest, and most uniform wear, thanks to the material used for the bearings. If the bearings wear and get scratched prematurely, it indicates that the shaft journals have also suffered corresponding damage. To this end, bearing alloys should possess high compressive strength and fatigue strength, as well as comprehensive properties such as sufficient plasticity and toughness. According to the requirements for bearing alloys, their ideal microstructure should consist of hard particles dispersed within a soft matrix. When the machine is in operation, the soft substrate wears down quickly and becomes indented, while the harder particles, being more resistant to wear, bulge out and bear the pressure from the shaft and rotor. At the same time, the soft matrix also offers impact resistance, vibration resistance, and good running-in properties. To fully meet the requirements of bearings (especially large-scale precision bearings) in compressors, the dimensions of the main bearings are typically manufactured to IT7 accuracy, with a surface roughness Ra value not exceeding 1.6–0.8 μm. Furthermore, since bearings are also subjected to significant dynamic and alternating loads (as rotors have a certain degree of imbalance), in order to avoid stress concentration and improve fatigue strength, the bearing surfaces must be free from defects such as dents, scratches, burrs, cracks, and inclusions. Ultrasonic and magnetic particle testing are required after rough and finish machining, respectively, and it is also necessary that all transition corners of the bearings be smooth and polished. 2.2 Improving the installation quality and assembly precision of the machine: It is particularly important to pay attention to the overall installation precision of the compressor as well as the assembly precision of its various components, as this directly affects the proper operation of the bearings and, by extension, the compressor itself. Based on on-site investigations and tests, it was found that the installation accuracy of the compressors used in several factories did not meet the standards. At present, domestic manufacturers generally use the \"thin wire pulling method\" for alignment during compressor installation, and this method results in significant installation errors. Based on the experience of experienced manufacturers in installing machines, it is best to use the laser collimator alignment method; for compressors of the DA350261 model, this method achieves a position accuracy of ±2 mm. The vertical and horizontal level deviations of the frame joint surface are measured and adjusted using a box level with a precision of 0.01 mm/m, achieving an accuracy of no more than 0.03 mm/m. In addition, the assembly precision between various components of the compressor must also be ensured, especially those related to the \"main shaft 2 and bearing 2 housing\" (such as impellers, oil seals, diaphragms, balance discs, return devices, diffusers, couplings, etc.). The condition of their assembly affects the quality of the stress conditions under which the bearings operate. Before installation, cleaning inspections and necessary repairs must be carried out; special attention must be paid to ensuring compliance in those assembly areas with stringent technical requirements. The interference fit between the bearing bush, the bearing housing, and the bearing cover shall comply with the specifications in Table 3. Table 3 Reference data for the interference amount of compressor shaft bearings in mm Sequence Number Name Interference Amount 1 Cylindrical bearing 0.03–0.07 2 Elliptical bearing 0.02–0.05 3 Multi-oil- wedge cylindrical bearing 0.02–0.04 4 Tilting bearing 0.01–0.03 5 Spherical thrust bearing 0.03–0.05 6 Spherical bearing –0.06––0.02 2.3 Improving the purification efficiency of compressed gas and the quality of sealing, as well as optimizing the system’s operational conditions, are important aspects that cannot be ignored. Some compressors operate under harsh conditions; for example, the nitrogen-hydrogen compressors used in fertilizer plants have low efficiency. The media that is circulated and processed does not undergo sufficient purification, resulting in many impurities entering the compressor, and the bearings are also corroded or eroded by the gases leaking from various sources. This also forces some moving components under high stress to wear out prematurely. Theoretically, the gaseous medium in a fertilizer plant after the refining section must meet the design requirements. Following the author’s suggestions, a factory improved its production process by adding a small re-purification device to the existing system, which effectively increased the efficiency of the refining section. The service life of the compressors also increased; whereas the bearings in the original machines would become damaged after only a short time of use, the bearings of the compressors now have remained intact even after several months of operation. Practice has shown that in the initial stages, it is necessary to remove from the feed gas those toxic components such as CO, CO2, O2, and H2S that can harm the catalyst, or to keep their levels below certain allowable values (the same applies to the moisture and oil content in the gas). Additionally, improving the sealing quality of the shaft seals helps not only to enhance the economic efficiency of the plant and improve the working environment but also to ensure the proper operation of the compressors. 2.4 Improve lubrication conditions and optimize the operating environment. During operation, the bearing alloy layer is subjected to significant compressive stress and torsional torque. If the bearing lubrication conditions are poor or unstable, it will inevitably affect the proper operation and service life of the bearings on the spindle 2. Furthermore, the rotor is constantly subjected to immense friction forces during startup, shutdown, and operation. Therefore, it is necessary to select an appropriate grade of lubricating oil (such as grades 220 and HU255 being commonly used for bearings), and to ensure that the oil passages, oil grooves, and oil holes are unobstructed. In addition, the quality, temperature, pressure, and cleanliness of the lubricating oil must also be maintained to ensure proper fluid sliding in the bearing assembly of shaft journal 2. After repairing the premature wear and damage to the two bearing journals of the compressor in winter, a certain factory improved the oil filter and added an oil circuit insulator; since the plant was restarted after operation, such faults have not occurred again. 2.5 Strengthen operational monitoring and improve maintenance standards. During the operation of the compressor, it is necessary to enhance monitoring, including ensuring the quality and quantity of lubricating oil, as well as checking the gap sizes of various moving parts and the stability of connections between fasteners. Furthermore, scientific local technical modifications can also be carried out; for example, one factory modified the originally used cylindrical tiles into elliptical tiles through lapping (model EI35029/0.97), **which eliminated premature bearing damage caused by oil film oscillations. Sufficient attention should also be paid to the quality of maintenance. Issues such as improper tightening of the connection threads, uneven mating surfaces, or insufficient surface roughness can all cause the machine to operate inflexibly, generate noise and vibration, and accelerate the wear and damage of its components. During operation, if large compressor bearings suffer from severe wear (with a wear amount greater than 0.13–0.15 mm), abrasion or scratches (resulting in deep longitudinal grooves), or significant bending deformation, they should be repaired promptly and properly; in cases where the defects are too severe, the bearings should be replaced. If the main journal is worn, it should be addressed promptly; it can be repaired by chromium plating or by turning its outer surface. After being abraded or scratched, the surface of the journal should first be cleaned, and then the longitudinal grooves can be removed either through manual grinding or by using machine tools for turning operations. If severe scarring of the bearing lining is detected, it can be repaired by welding a new layer of alloy onto its surface and then machining it to the specified requirements; however, when using this method, care must be taken to avoid new deviations caused by thermal deformation of the bearing. 3 Other points to note: (1) Pay attention to drawing on new research findings from home and abroad, enhance the monitoring of the machine’s operating conditions, adjust them in a timely manner, and understand the characteristics and patterns of the machine’s operation (by drawing reference curves for operating conditions). (2) Whenever a machine is installed or overhauled, the bearings and all components that interact with them must be carefully inspected and tested; purchased parts and those that have been repaired externally should ensure good internal quality and high machining precision. It also pays attention to changes and regulation of dynamic accuracy. (3) When transporting, storing, and lifting a large spindle rotor, it should be handled with care to avoid scratching or damaging the bearings. (4) Continuously improve the theoretical and technical skills of operators, maintenance personnel, and managers to create favorable conditions for the correct and scientific use, maintenance, and management of machinery. (5) Theoretical calculations and empirical assessments are carried out based on conditions such as bearing wear, corrosion, and scratches, in order to determine the maintenance cycle appropriately; this approach also offers a certain degree of flexibility in response to changes in actual conditions. (6) During the disassembly and assembly of the compressor, it is advisable to keep the shaft rotor as horizontal as possible. The steel wire rope on the crane should be placed at the center of gravity of the component, in front of and behind it (avoiding the sensitive areas that are prone to wear, and using soft padding). 4 Conclusion: A chemical plant owns two large centrifugal compressors. By implementing the measures mentioned above in recent years, the frequency of failures in these compressors has **decreased, and problems such as premature excessive wear and damage to the bearings have been effectively resolved. It can be seen from this that: (1) Normal wear and scratches on compressor bearings are an allowable natural phenomenon and pattern, representing the most common form of damage; whereas premature excessive wear and scoring fall under the category of accidental damage. (2) Through artificial and technical control, it is entirely possible to create a favorable operating environment and conditions for the machine, and the service life of the bearings can also reach the designed operational period. (3) By employing advanced technical modifications to make its structure more rational (by adding an appropriate degree of flexibility to the areas where stress is concentrated and by significantly improving the lubrication conditions of the bearing linings), along with ensuring high material quality and precision in processing and installation, the performance of bearings in large compressors can be further enhanced