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Combined with the practice of compressor design and manufacture in China in recent years, this paper analyzes and studies the characteristics, causes and preventive measures of premature pulling of radial sliding bearings of large centrifugal compressors used in chemical industry, and puts forward some pertinent opinions and suggestions according to the actual situation of some domestic manufacturers.
Large centrifugal compressor is the heart equipment of chemical plants, and the premature strain of its radial sliding bearing is one of the major failures of the compressor that cannot operate normally. As most chemical plants are produced in a single process, each unit is closely connected, which often leads to unimaginable consequences. Over the years, scientific and technological workers have tracked and explored its roots, seeking ways to prevent and solve it.
Bearing wear and strain can be divided into normal axle holding strain and abnormal axle holding strain (i.e. early sudden excessive axle holding strain) according to the length of running time of the parts used.
This paper mainly explores the formation characteristics, causes and preventive measures of the latter fault.
1 Causes of premature bearing strain and damage
1.1 Analysis of basic characteristics of early loss bearing strain
Here are just two examples. A nitrogen hydrogen compressor (put into use in August 2005) in a chemical fertilizer plant was forced to stop due to severe vibration, and it was found that the bearing alloy layer was strained. According to the field analysis, the damaged bearing is mainly caused by the installation (journal 2 bearing) being too tight. It was found through inspection that the scratch area of the lining of the bearing bush was 23%, and the scratch depth was 0.46mm. The damaged part is black, indicating that the lubricating oil has deteriorated under high temperature. Another example is a large domestic compressor (used after the expansion of the production workshop) used by a chemical company. It only took 13 days after the installation of the new machine and it was found that the bearing was smoking and the machine was shut down. It was also found that the bearing was severely strained by holding the shaft. It was found that the scratch depth of the alloy layer was 0.53mm. This is indeed the first major accident that the factory has encountered in many years, causing serious losses to production.
1.2 Materials and mechanical properties cannot meet special requirements
Bearing is a part with large force and complex and changeable load when the compressor is running, so the quality of material selection should be checked first. At present, the bearing materials (alloy layer) of large chemical compressors in China are mainly ZCH2SnSb1126, ZCHPb1621622, ZCuPb30, etc. Although the strength of these materials is acceptable, there are also some shortcomings, mainly due to the lack of plasticity, toughness and running in ability. In particular, there are slight defects in the joint between the main journal and the bearing seat (where the contact surface and spots with other parts are required to be extremely high), which is very easy to cause wear and scratches when a large force is applied during work. Even better bearing alloy grades sometimes fail to meet the required physical and chemical standards after inspection. For example, the mechanical performance and chemical composition of the compressor bearing in a factory do not meet the required indicators after inspection.
Recently, it is also found that the bearing material performance is not a fixed number, and its change degree is related to the environmental stability. Therefore, it is necessary to observe and control the working conditions of the bearing at any time. In addition, the material of the bearing has an optimal ratio with that of the matching parts. This can only be determined by repeated tests.
1.3 The working conditions are harsh and unstable
As we all know, for the early wear and tear of compressor bearings, in addition to its own factors, the variable working conditions can not be ignored. For example, the compressors used in chemical plants have strict requirements for working conditions, but due to various reasons (technical, man-made, equipment and process), the impurities, temperature, pressure and other process indicators of the gas fluctuate greatly, resulting in the external load borne by the rotor spindle and early damage. The environmental conditions of the whole process system (such as lubricating oil, cooling water, feed gas, etc.) also determine the length of the normal working cycle of the machine or parts used.
1.4 Inadequate design structure
The manufacturing and installation accuracy is not high enough. The compressor bearing is damaged after a short normal operation time, indicating that there is a major problem here. According to the analysis of the compressor parts stopped for some reason in a factory, it is found that the design structure is not reasonable in some aspects. For example, the distance between the lubricating oil source of the machine bearing and the main lubrication point is far, which seriously affects the lubrication effect. The lubrication system and oil circuit structure of the bearing should be designed more reasonably and scientifically to ensure that the local area with the most complex bearing force has a good liquid friction situation. In addition, the low accuracy of compressor manufacturing and installation is also the reason for the early occurrence of the accident. Centrifugal compressor belongs to the category of precision machine in chemical plant devices. Whether it is design, manufacturing or installation positioning, each link has quite strict requirements, and each step should be done according to national standards and technical requirements. After an accident occurred in a factory, when checking the position of the disassembled compressor body and main bearing with the wire drawing method, it was found that there was a small deviation exceeding the standard. Of course, this may also be caused by premature wear and strain excitation of the compressor.
The latest research results show that the bearing structure, installation and positioning, and appropriate flexibility are also important conditions to improve the service life of the "Spindle 2 Bearing" assembly.
2. Several measures to improve the service life of compressor bearings
2.1 Ensure the material selection quality and manufacturing accuracy of bearings
In addition to making the bearing structure design reasonable and scientific as far as possible, we should also control the quality in the selection of materials. The bearing materials used shall have quality certificates, and must conform to relevant inspection standards for chemical composition and mechanical properties.
In terms of processing and manufacturing, we need to carefully check, from billet production, alloy layer casting to mechanical processing and lining grinding and other links should be handled according to the technical requirements, and each step should be qualified by quality inspection before entering the next process. As the "main shaft bearing" is the most important heart component of the compressor, the consequences of premature damage in one aspect are unimaginable. The bearing supports the work of the rotor. When wear is unavoidable, the journal shall be guaranteed to suffer the minimum, slowest and uniform wear from the bearing material.
If the bearing is worn and strained prematurely, it means that the journal is also damaged to some extent. Therefore, bearing alloy should have high compressive strength, fatigue strength and sufficient plasticity and toughness. According to the application requirements of bearing alloy, the ideal structure should be the distribution of hard particles on the soft matrix. When the machine is running, the soft substrate is quickly worn and concave, and the hard particles become convex when they are relatively wear-resistant, and bear the pressure of the shaft and rotor. At the same time, the soft substrate has impact resistance, vibration resistance and good running in ability. In order to fully meet the function of bearings (especially large precision ones) in compressors, the dimensions of main bearings are usually machined according to IT7 accuracy, and the surface roughness Ra value is not higher than 1.6-0.8 μ m。
In addition, since the bearing also bears large dynamic and alternating loads (the rotor has a certain degree of imbalance), in order to avoid stress concentration and improve fatigue strength, it is required that the bearing surface should be free of defects such as dents, bruises, burrs, cracks and inclusions, and ultrasonic and magnetic particle flaw detection should be carried out after rough and finish machining, and each transition fillet of the bearing should be smooth and smooth.
2.2 Improve the installation quality and assembly accuracy of the machine
It is particularly important to pay attention to the installation accuracy of the compressor as a whole and the assembly accuracy of its various parts, because it directly affects the normal operation of the bearing and even the subsequent operation of the compressor.
According to the field survey and inspection, it was found that the installation accuracy of compressors used in several factories did not meet the standard. At present, in the process of installing compressors, most domestic manufacturers use the "thin wire method" for alignment, which has a large installation error. According to the experience of advanced manufacturers in installing machines, it is better to use the laser collimator alignment method, such as the DA350261 compressor, and the position accuracy can reach ± 2mm after alignment. The junction surface of the engine base shall be measured and adjusted with a square level with an accuracy of 0.01mm/m, and the vertical and horizontal horizontal deviation shall be no more than 0.03mm/m.
In addition, the assembly accuracy of compressor components must also be guaranteed, especially for some related components (such as impeller, oil seal, diaphragm, balance plate, reflux device, diffuser, coupling, etc.) of "main shaft 2, bearing 2, bearing seat", the assembly condition between them will affect the stress state of the bearing during operation. Cleaning inspection and necessary repair work must be done well before installation, especially for some assembly parts with strict technical requirements. For example, the interference between the bearing pad and the bearing seat and the bearing cover shall comply with the provisions in Table 1.
Table 1 Reference Data of Compressor Bearing Bush Interference (mm)
2.3 It is an important link that can not be ignored to improve the purification efficiency of compressed gas medium and the sealing quality to improve the system process. Some compressors are in poor working conditions. For example, nitrogen and hydrogen compressors used in chemical fertilizer plants have low equipment use efficiency, and the circulating and treated media have insufficient purification efficiency (more impurities), so they enter the compressor. The bearings are also corroded or corroded by the gas at the leakage. Therefore, it also forces some moving parts with large force to be damaged prematurely. Theoretically, the gas medium process after the refining section of the fertilizer plant must meet the design requirements. According to the author's suggestion, a factory improved the production process flow, added a small reprocessing equipment to the original system, effectively improved the efficiency of the refining section, and greatly improved the service life of the compressor. The original machine bearings were only used for a short time, and the compressor bearings that have been running for several months are also intact.
Practice has proved that the CO, CO2, O2, H2S and other toxic components in the feed gas that are toxic to the catalyst must be removed in the early stage section, or less than a certain allowable value (the same is true for the water and oil content in the gas), and then the seal quality of the shaft seal must be improved. This not only improves the economic benefits of the plant, improves the working environment, but also effectively protects the normal operation of the compressor.
2.4 Improve the lubrication conditions and optimize the operating environment During the operation of the compressor, the bearing alloy layer bears large compressive stress and torsional torque. If the bearing lubrication condition is poor or unstable, the normal operation and service life of spindle 2 bearing will be affected. In addition, the rotor bears great friction at all times during startup, shutdown and operation. Therefore, it is necessary to select the lubricating oil with appropriate brand and make the oil path, oil groove and oil hole smooth. In addition, certain oil quality, temperature, pressure and cleanliness of lubricating oil shall be maintained to ensure good liquid sliding of journal 2 bearing assembly. After repairing the early wear and damage of the journal of compressor bearing 2 in winter, a factory improved the oil filter and added an oil circuit insulator. After starting up and putting into production again, no such failure occurred again.
2.5 Strengthen the operation monitoring and improve the maintenance level. During the operation of the compressor, the monitoring should be strengthened, such as the guarantee of the oil quality and quantity of lubricating oil, the clearance size of each fitting piece and the connection stability between fasteners. In addition, scientific local technical transformation can also be carried out. For example, a factory changed the original cylindrical pad into an elliptical pad by scraping and grinding, which greatly eliminated the premature damage of the bearing caused by oil film oscillation. Attention should also be paid to the maintenance quality. If the connecting thread is not tightened correctly, the mating surface is uneven, and the roughness is not up to the requirements, the machine will not operate flexibly, and noise and vibration will be generated, which will aggravate the damage speed of the machine parts. During the use of large compressor bearings, if there are serious wear (wear amount is more than 0.13-0.15mm), abrasion or scratches (deep longitudinal grooves), and large bending deformation, they should be repaired timely and reasonably, and if the defects are too serious, they should be replaced. If the main journal is worn, it should also be handled in time, and it can be repaired by chrome plating or turning the outer circle; After abrasion or scratch, the journal surface can be cleaned first, and then the longitudinal grooves can be eliminated by manual grinding or lathe turning. If it is found that the bearing lining is seriously strained, it can be repaired by surfacing a new layer of alloy on its surface and machining to the specified requirements, but pay attention to the new deviation caused by the bearing thermal deformation when using this method.
3 Other precautions
(1) Pay attention to drawing on new research achievements at home and abroad, strengthen the detection of machine operation state, adjust the operation conditions in time, and grasp the operation characteristics and laws of the machine (draw the reference operation state curve).
(2) During the installation and overhaul of the machine, the bearing and its matching parts shall be carefully inspected and accepted, and the purchased parts and repaired parts shall ensure the internal quality and processing accuracy. And pay attention to the change and regulation of dynamic accuracy.
(3) The large spindle rotor should be handled with care during transportation, storage, assembly, disassembly and hoisting. Do not scratch or impact the bearing.
(4) Constantly improve the theoretical and technical level of operators, maintenance personnel and management personnel, and create favorable conditions for correct and scientific use, maintenance and management of machines.
(5) Theoretical calculation and empirical evaluation shall be carried out according to the wear, corrosion, scratch and other conditions of the bearing, so as to reasonably determine the maintenance cycle, and have certain flexibility according to the changes of the actual situation.
(6) During the disassembly and assembly of the compressor, the shaft rotor should be kept as horizontal as possible, and the wire rope on the crown block should be * * located at the center of gravity of the front and back of the parts (avoid the precision area being worn and pad with soft pads).
4 Conclusion
There are two large centrifugal compressors in a chemical plant. In recent years, the above measures have been taken, so the frequency of compressor failures has been greatly reduced, and the problems of early excessive wear and tear of bearings have been basically effectively solved. It can be seen that:
(1) The normal wear and scratch of compressor bearing is an allowable natural phenomenon and rule, which is the main damage form, while the early excessive wear and strain belong to the category of accidental damage.
(2) It is entirely possible to create a good operating environment and conditions for the machine through manual technical regulation, and the service life of the bearing can also reach the designed working life.
(3) If advanced technical transformation is adopted to make its structure more reasonable (moderately increase the flexibility of the bearing back area with concentrated stress and fully improve the lubrication conditions of the bearing liner), and ensure higher material quality and processing and installation accuracy, the use of large compressor bearings can also be further improved.
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2023-01-03