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The booster expander (booster/expander) for the synthetic ammonia unit of the fertilizer plant at CNPC Lanzhou Petrochemical Company was manufactured by the German company ATLAS COPCO; it is model ETB150MS. This device serves as a key component of the air separation unit, generating cooling capacity by compressing nitrogen and then expanding it to produce work. On October 19, 2007, the production unit experienced a power fluctuation that caused it to stop operating, resulting in severe damage to the booster expander. After analyzing the causes of equipment damage, the rotor was repaired, dynamic balancing was performed, and the damaged parts were replaced; subsequent reinstallation and testing showed good performance. 1 Process Principle: The process of the pressure-boosting expansion engine involves taking nitrogen gas coming from the second-stage outlet of the nitrogen compressor (at 40°C and 3.4 MPa), pressuring it using the booster, cooling it afterward (to 40°C and 4.8 MPa), and then sending it to the circulating heat exchanger where its temperature is reduced to –114.7°C. It subsequently passes through a pre-filter, a quick-close valve, and a nozzle control valve; finally, the nitrogen is pushed by the nozzle to rotate the impeller of the expansion engine, thereby doing work on the booster. As a result, the temperature and pressure of the nitrogen decrease to –175.8°C and 0.67 MPa, with 6%–7% of the nitrogen being liquefied in the process ； After passing through the separation tank, the liquid nitrogen is sent to the low-pressure tower as a small amount of reflux fluid; the gaseous portion merges with the low-pressure nitrogen and enters the circulation heat exchanger for reheating, before finally entering the medium-pressure nitrogen main pipe. The process is illustrated in Figure 1. 2 Analysis of the accident causes 2.1 Causes of the accident On the noon of October 19, 2007, the entire production system came to a stop due to voltage fluctuations. During the resumption of operation, after the pressurization expansion machine in the air separation unit was started, there was an abnormal change in the temperature of the thrust bearing on the expansion side; this temperature subsequently stabilized, after which the equipment experienced a low-speed interlock shutdown. After the process treatment, the equipment was disassembled for maintenance; inspections revealed severe wear on the thrust bearings on the expansion side, damage to the rotor shaft shoulder, as well as damage to the oil seals and gas seals. The damage is shown in Figures 2 and 3. The booster expander is equipped with an oil storage tank and an air bag to ensure a steady supply of oil in the event of a power failure or mechanical malfunction of the oil pump. After checking that the supporting facilities of the oil system were in good condition, wear caused by a lack of oil was ruled out. Upon careful examination of the instrument records, it was found that the anti-surge valve did not close in a timely manner during startup. Due to the delay in the operation of the anti-surge solenoid valve, when the expander is operating under load, the booster has no load, resulting in excessive axial force. Excessive axial force in the direction of the expansion side causes an immediate imbalance in the lubrication system between the thrust bearing surface and the shaft shoulder; as a result, the rigidity of the oil film is disrupted, leading to severe wear between the thrust bearing and the shaft shoulder. Wear caused the instrument wire to break, which led to abnormal temperature readings that later stabilized. The wear also reduced the speed, eventually resulting in a low-speed shutdown due to interlock mechanisms, and this caused severe damage to the equipment. 2.2 Calculation of axial force: Factors such as variations in operating procedures and equipment performance have a direct impact on the axial force. Therefore, the axial force on the equipment is a force that changes as the equipment operates. The calculation of this axial force is based on theoretical methods, without taking into account any changes in the axial force caused by external factors. The theoretical axial force on the rotor consists of two parts, with the main part being generated by the pressure difference between the gases on either side of the impeller ； In another case, when the airflow flows axially into the impeller and then radially into the flow channels between the blades, or when it flows radially from the blades and exits axially, the sudden change in axial momentum becomes an axial thrust acting on the impeller. The main axial forces on the booster expander are generated by the expander impeller and the booster impeller; these forces act in opposite directions to each other. A small amount of axial force is also reserved to prevent the rotor from shifting, and the forces acting on the impellers are shown in Figure 4. As can be seen from Figure 4, the formula for calculating the axial force is: where F is the total axial force, in N ； F1—Axial force induced by the impeller on the pressurized side, N ； F2—Axial force induced by the impeller on the expansion side, N ； D1—rotor diameter, m ； D2—inner diameter at the impeller inlet, m ； D3—Outer diameter at the impeller inlet, m ； D4—outer diameter of the impeller, m ； G—mass flow rate of gas entering/leaving the impeller, kg/s; G = Vρm/3600 ； Axial velocity of gas entering/leaving the CO impeller, m/s, CO = 4V/π(D32 – D22) ； V—the expansion volume of the booster expander, V＝9.166m3/s ； ρm—is the density of nitrogen, ρm=1.25kg/m3 ； u2—is the speed of the impeller, u2=29300 r/min. By measuring the expansion impeller, the pressure-increasing impeller, and the rotor, the respective D1–D4 values were obtained. Using the aforementioned formula, the magnitude of the axial force generated by the impeller on the pressure-increasing side was calculated to be F1≈26245 N ； The magnitude of the axial force generated by the impeller on the expansion side is F2≈28112N ； The total axial force is F = F2 – F1 = 1867 N ； In the fault condition, F2/F≈15. Based on the above calculations, it can be seen that the axial force generated by the expansion impeller is slightly greater than that caused by the compression impeller; the overall axial force acts in the expansion direction and is supported by the thrust bearing on the expansion side. When the booster impeller has no load, that is, when /1 is approximately zero, the axial force acting on the rotor is about 15 times that under normal conditions. From the above analysis, it can be seen that the cause of damage to this equipment was that while the impeller of the booster did not generate any axial force, the impeller of the expander did produce an axial force. The suddenly high axial force caused severe friction between the shaft shoulder and the thrust bearing, leading to serious damage. 3 Maintenance and Operation 3.1 Maintenance Measures Based on the disassembly of the equipment, the main damaged component is the shaft shoulder on the expansion side of the rotor, which is severely worn – approximately 2 mm has worn away, as shown in Figure 2 ； The expansion side thrust bearing shell is completely damaged, as shown in Figure 3 ； The instrument cable is burned out, and the impeller on the pressurized side shows ring-shaped wear. The damaged shoulder of this rotor was sent to a factory in Shenyang for repair using laser fusion welding; the worn areas of the impeller were rewelded and polished, followed by separate dynamic and static balancing. Finally, the impellers at both ends as well as the mass balance disk are assembled with the shaft, and high-speed dynamic balance testing is conducted. After comparing the main installation data with those of the original factory spare parts, and in line with the principle of reducing maintenance costs, it was decided to continue using the old rotor. Due to the significant vibration caused by the wear process, the clearances between the rotor and the gas seals as well as the oil seals all exceeded the allowable limits by a large margin; it was decided to replace all the inter-stage gas seals on the rotor of the booster expander, as well as the oil seals at both ends. The radial thrust bearing on the expansion side was replaced due to complete damage, while the radial thrust bearing on the pressurization side showed normal wear and therefore did not require replacement. Table 1 shows the basic measurement data before and after the maintenance; all the gap values after maintenance were within the allowable range, meeting the requirements for maintenance. 3.2 Operation after maintenance: After the maintenance, the booster expander was started successfully once, operated well, was able to achieve the required level of expansion and pressure, with minimal gas seal leakage. During a certain period, the data displayed by the instruments were within the normal range as shown in Table 2. 4 Summary: Through an analysis of the causes of equipment damage, it was found that the main reason was the failure to close the anti-surge valve in a timely manner, which led to and exacerbated equipment wear during operation. This requires very thorough inspections of the equipment before it is put into operation, in order to identify and resolve problems promptly. In the event of any abnormalities during operation, it is necessary to organize teams from areas such as equipment, process engineering, instrumentation, and electrical systems to analyze the possible causes; rushing to start up or conduct tests should be avoided in order to minimize damage to the equipment. After this maintenance, backup temperature measurement points were added to the temperature sensor wires for the radial and thrust bearings, in order to prevent false readings and misjudgments caused by broken wires. The drive mechanism of the anti-surge valve was also replaced. The technical complexity of this maintenance task was high; after numerous discussions, it was decided to use an old rotor, thereby saving over 1.4 million yuan in spare parts costs. After the maintenance, the equipment operated well; all process parameters were within the required ranges and in optimal condition, which provides valuable guidance for future maintenance efforts.