Thread Content
Analysis of the Causes and Solutions for Broken Cylinder Head Bolts in 4M45 Compressors Zhang Jingzhong, Liu Lin, Zhu Yonghua (Yankuang Lunan Fertilizer Factory, Tengzhou, Shandong 277527) 1 Main technical parameters of the compressor: The second nitrogen fertilizer synthesis workshop at Yankuang Group Lunan Fertilizer Factory is equipped with 3 4M45-19/17-320 type nitrogen-hydrogen compressors. These compressors are of the symmetrically balanced type with an M-shaped arrangement; they feature four cylinders, four rows, and four stages of compression. The motor rotor is separate and directly connected to the crankshaft. These compressors were manufactured using proprietary technology from German company Borgwardz, imported by Shenyang Gas Compressor Factory, and represent a relatively advanced type of compressor today. Compressor speed: 300 r/min, shaft power 2450 kW, stroke 400 mm ； Motor model: TK2500-20/2600, speed 300 r/min, power 2500 kW ； Transmission method: Direct connection ； Inlet flow rate: 19 m3/min ； Air volume adjustment range: 100%, 85% ； Main unit weight: 50547kg ； The technical parameters of the compressor are shown in Table 1. http://yf116.cn/JISHUWANG/upload/0605151511025871.jpg 2 Accident Overview: At around 21:10 on April 19, 1999, the compressor operator discovered, during a routine inspection, that there was a broken bolt under the cylinder head of Stage 1 of Compressor B#. Upon further inspection, two more bolts were found to be broken but still in place. An immediate emergency shutdown was carried out for inspection, and three more bolts were found to be on the verge of breaking; in total, 6 out of 12 bolts had broken. After emergency repairs, the compressor was brought back online. The fracture condition is shown in Figure 1. Based on the theoretical calculations for the cylinder head bolts of the compressor as well as the actual operating parameters, the cause of the accident is analyzed as follows: http://yf116.cn/JISHUWANG/upload/0605151512071300.jpg The cylinder head bolt for the 4M45 compressor is of type M33×120, made of 45-grade steel; there are a total of 12 such bolts. The preload force for these bolts is calculated as follows: The maximum pressure on the cylinder head is given by Q = (πD2)／4·p = (π·3702)／4×4.218 = 453,294 N. Here, D represents the diameter of the first-stage cylinder, with D = 370 mm ； p is the primary exhaust pressure, p = 4.218 MPa. The working load per bolt is F = Q/Z = 453,294/12 = 37,775 N. Here, Z represents the number of cylinder head bolts, which is 12. The remaining preload force is F′ = 1.5F = 56,662 N. The maximum tensile force on the bolt is F0 = F + F′ = 94,436 N. The relative stiffness coefficient c1/(c1+c2) is set to 0.8 when asbestos gaskets are used. The preload force is therefore F′ = F0 – c1/(c1+c2)F; thus, F = 94,436 – 0.8 × 64,217 = 64,217 N. The preload torque is T = 0.2F′d, where d = 0.2 × 64,217 × 16.5 ≈ 2.12 × 105 N·mm. Here, d represents the radius of the cylinder head bolt, and its value is 16.5 mm. According to the requirements for normal design preload, the bolt preload is 64217 N, and the preload torque is 2.12×105 (N·mm). When a 12-pound sledgehammer is used to strike a ratchet with a lever arm of 300 mm, the pre-tensioning force at this time is: Torque M = Q × S = ma² × S = 12 × 0.454 × 202 × 300 ≈ 6.54×105 (N·mm). Here, a represents acceleration; it has been determined that a = 20 m/s². Through calculation and comparative analysis, it is found that the preload at this time is much greater than the designed preload. 3 Analysis results: (1) During the equipment’s maintenance, the technicians did not use torque wrenches according to the designed preload values; instead, they used hammers to apply force to the wrenches and controlled the preload based on experience. As a result, the bolt preload was higher in areas where it was easier to use the hammer, and lower in areas where it was more difficult. Since the cylinder head bolts are subjected to alternating loads, the maximum pressure exerted on the cylinder head after compression is calculated as follows: Q = π/4·D²p = 0.785×4.8 = 515,839.2 N. Due to the uneven preload, only 8 bolts are actually under stress. Therefore, the working load on the bolt under stress is: F′2 = F2 – c1/(c1+c2)F2 = 0.2F2 = 0.2×64,480 = 12,896 N. The relative stiffness coefficient is c1/(c1+c2) = 0.8. Through calculation and analysis, it can be concluded that bolts with a higher pre-tensioning force endure greater alternating loads, while those with lower pre-tensioning force endure smaller alternating loads. By the time the cylinder head bolts broke, the compressor had been in operation for 46,800 hours, and it was operating under poor conditions with a pressure of 0.322 MPa above the design value, which increased the working load on the bolts and accelerated their fatigue failure. Based on the distribution of cylinder head fractures in the first stage of the compressor, it can be seen that the bolts located in positions where hammering is convenient are mostly fractured, which is consistent with the results of theoretical analysis. (2) The bolts were not inspected for defects in strict accordance with the equipment maintenance procedures. In accordance with the 4M45 compressor maintenance procedures, flaw detection testing of stress-bearing components such as bolts should be carried out for both major and minor repairs. Before this fracture occurred, a mid-term maintenance session had just been carried out, and no flaw detection was performed on the cylinder head bolts during that maintenance, thus missing the opportunity to prevent bolt failure. 4 Conclusion (1) The preload value must be strictly controlled, and this should be done using a torque wrench based on the actual preload of the cylinder head bolts. A torque wrench with a torque range of 80–300 N·m is suitable for this purpose; it is essential to avoid using hammers to tighten the bolts. (2) Strictly carry out the flaw detection inspections of the cylinder head bolts as specified in the maintenance procedures, keep records promptly, and establish files for each cylinder head bolt, ensuring strict monitoring from quality inspection upon arrival at the factory, during use, up to maintenance. (3) The material of the bolt was changed from 45# steel to 35GrMoA, in order to increase the bolt’s yield strength and fatigue strength under symmetric bending. (4) During maintenance work, increase the emphasis on scientific and civilized maintenance practices, strictly follow the maintenance procedures, ensure strict control over each step of the maintenance process, keep accurate records, and improve the quality of maintenance. After the implementation of the above measures, the compressor head bolts have been in safe operation since April 1999. Non-destructive testing of these bolts during the major maintenance in March 2001 showed that they were in good condition, achieving the intended effects of the improvements and effectively preventing bolt breakage incidents.