【Haichuan’s Excellent Articles Recommendation】Fault Diagnosis and Optimization of Misalignment in Ethylene Heat Pump Compressors

1 Introduction The ethylene heat pump compressor is a key device in the ethylene distillation process, and the stability of the high-speed shaft of its gearbox directly affects the safe operation of the unit. After maintenance, when the heat pump compressor unit of a certain company was started up, the vibration value at the interlock terminal of the high-speed shaft of the speed increase gearbox (XE4.01A/B) rose sharply from 26.96 μm to 63.4 μm within 2 minutes (the interlock threshold being 51 μm), which triggered an emergency shutdown. Upon inspection, both the bearing clearance (0.17 mm, with a standard range of 0.15–0.20 mm) and the lubricating oil pressure (0.13 MPa, with a standard range of 0.09–0.13 MPa) were within the specified limits. However, the vibration spectrum showed that 63% of the energy came from the second harmonic, and the axis trajectory was in the form of an “inner 8” shape (with an ellipticity of 0.07 mm, which exceeds the ISO 7919 limit of 0.05 mm). This indicates that shaft misalignment is the main cause. By integrating thermodynamic models and the three-table method, this paper systematically addresses the issue of the disconnect between cold-state alignment standards and hot-state operating conditions, providing theoretical support for the optimization of similar equipment. 2 Introduction to Ethylene Heat Pump Compressors Ethylene heat pump compressors are the core equipment in the ethylene distillation process, and they consist of a drive unit, a speed increase transmission system, and a two-stage centrifugal compression unit. The drive unit uses a 2000 kW motor, which is connected to the speed increase gearbox via a diaphragm coupling ； The speed increase system consists of a high-speed shaft and a low-speed shaft with a speed of 13,871 r/min. It utilizes an involute gear pair (with a diameter of 123.958 mm, a speed ratio of 4.65:1, and a center distance of 350 mm) to increase the rotational speed. The system is equipped with sliding bearings (with a preload of 0.01–0.03 mm) as well as a forced lubrication system (with an oil pressure of 0.13–0.30 MPa), ensuring that the oil film thickness on the gear meshing surfaces is at least 20 μm, while also maintaining dynamic stability within allowable thermal expansion tolerances of ±0.02 mm ； The compression unit raises the pressure of the gaseous ethylene from 1.6 MPa to 3.6 MPa using a two-stage impeller; thereafter, it is condensed into a liquid phase in a heat exchanger and returned to the top of the distillation tower, thereby reducing energy consumption and improving distillation efficiency.

3 Characteristics of interlock shutdown faults 3.1 Event sequence: After maintenance, the ethylene heat pump compressor was started under no-load conditions, and severe vibration abnormalities occurred within 2 minutes. The vibration value at the interlock terminal of the high-speed shaft of the speed increase gearbox (XE4.01A/B) rose sharply from an initial value of 26.96 μm to 63.4 μm, exceeding the interlock threshold of 51 μm ； The vibration values at the compressor interlock terminals (XE4.02A/B) also increased to 39.5 μm simultaneously. The “two-out-of-two” interlock logic is triggered, resulting in an emergency shutdown of the system for protection. 3.2 Fault Characteristics The vibration data show that during interlock shutdowns, the vibration energy of the high-speed shaft of the gearbox is concentrated at the 2nd harmonic, accounting for 63%; moreover, the trajectory of the shaft center follows a typical \"inner 8\" pattern, indicating that the rotor system is subjected to periodic radial forces during operation, resulting in a significant deterioration of its dynamic stability. Furthermore, the shaft elevation at the coupling end of the high-speed shaft is 203 μm, which is 50 μm higher than the historical value of 153 μm; this elevation increase is positively correlated with the vibration amplitude. Although the vibration at the compressor coupling end did not exceed the interlock threshold, the proportion of energy in its second harmonic was 61%, and its dynamic characteristics were consistent with those at the gearbox end, further confirming the systematic nature of the fault. Conclusion: The current interlock shutdown incident exhibited the following key characteristics. Sudden vibration excess: A rapid increase in vibration levels within 2 minutes indicates that the fault is caused by dynamic imbalance ； Spectrum and trajectory consistency: Dominance of the second harmonic and an “inner figure-8” trajectory indicate axis misalignment or thermal deformation. 4 Analysis of Interlock Shutdown Fault 4.1 Analysis of On-site Conditions After the interlock shutdown, it was checked that the actual value of the bearing clearance was 0.17 mm (design standard: 0.15–0.20 mm), and the lubricating oil pressure was 0.13 MPa (standard range: 0.09–0.13 MPa); both values met the design requirements. During the no-load start of the unit, the process parameters (suction flow rate of 54 t/h, inlet temperature of 16°C) remained stable and met the startup conditions, eliminating the influence of process fluctuations. However, the vibration exceeding limits still indicates a potential dynamic imbalance in the equipment. By reviewing the maintenance records, the last alignment adjustment was carried out based solely on the original cold-state alignment standards (outer diameter tolerance of –0.04175 to +0.02225 mm), without taking into account the upward movement of the axis caused by the operating temperature of 100°C (the thermal expansion value being 0.1 mm). 4.2 Condition Monitoring Analysis 4.2.1 Vibration Spectrum Characteristic Analysis According to the API617 standard, a typical characteristic of alignment deviation is that the energy of the second harmonic is significantly dominant. In this measurement, the vibration spectrum at the high-speed shaft coupling end of the gearbox (XE4.01A/B) showed that the 2nd harmonic (corresponding to a rotational speed of 13,871 r/min × 2 = 462.4 Hz) accounted for 63% of the total energy, which is well above the standard limit of ≤40%; the energy associated with the fundamental frequency (1X) was only 18.99 μm, indicating that the fault-related vibrations are caused by secondary harmonics resulting from alignment errors. This characteristic is consistent with the spectral pattern of \"poor alignment of rigid couplings\" in ISO10816-3, verifying the root cause of the dynamic imbalance of the shafting. 4.2.2 Diagnosis of the axis trajectory shape: When the axis trajectory takes an “inner 8-shaped” form, its formation mechanism is the difference in stiffness in orthogonal directions resulting from alignment errors, which induces periodic alternating radial forces. Through quantitative analysis of the trajectory ellipse, the ratio of the major axis to the minor axis is 1.8 (the standard value is ≤ 1.3), and the ellipticity is 0.07 mm (the ISO 7919 limit is ≤ 0.05 mm). Trajectory phase analysis showed that the amplitude difference between the 90° and 270° directions was 28 μm, indicating that the rotor was subjected to asymmetric loads, resulting in a significant deterioration of its dynamic stability. 4.2.3 Dynamic correlation of shaft elevation The position of the shaft axis at the high-speed coupling end was monitored using eddy current sensors; the data showed that the shaft elevation during operation reached 203 μm, an increase of 50 μm compared to the historical baseline value of 153 μm ； Vibration correlation: A linear regression analysis of the lift amount and vibration amplitude showed a correlation coefficient of R² = 0.92 (p < 0.01), indicating that thermal shaft alignment deviation is a direct cause of increased vibrations. Conclusion: The original cold-state calibration standard does not cover the axial displacement of 0.044 mm under hot operating conditions (100°C), resulting in excessive stress on the coupling diaphragm and dynamic imbalance. The bearing bush and oil pressure parameters meet the standards, further confirming that alignment deviation is the main cause of the interlocked shutdown; therefore, the alignment criteria need to be revised based on thermodynamic models.

5. Change in alignment standards: Based on condition monitoring and maintenance analysis, both the bearing clearance and oil pressure parameters are within standard limits; however, excessive vibration and abnormal shaft center trajectory indicate that alignment errors are the main cause of the interlocked shutdown. The original cold-state alignment standard did not take into account the thermal expansion effect at the actual operating temperature of 100°C (an axial shift of 0.044 mm), resulting in dynamic imbalance. Thermodynamic models and dynamic simulations are required to scientifically revise the alignment standards, providing theoretical support for precise adjustments. 5.1 Thermal expansion compensation calculation: Analysis of interlock failures shows that the original alignment standards did not take into account shaft alignment variations under hot operating conditions, resulting in dynamic imbalance. To scientifically adjust the alignment parameters at low temperatures, it is necessary to accurately calculate the amount of thermal expansion based on the temperature difference between the actual operating temperature (100°C) and the ambient temperature (25°C), namely ΔT = 75°C, taking into account the geometric characteristics of the equipment. This provides a quantitative basis for revising the alignment standards. Formula: ΔL = α·L·ΔT. Parameter: Coefficient of linear expansion (α): 11.7×10–6/°C for alloy steel (ASTM A322 standard) ； Shaft length (L): Effective length of the compressor rotor is 1200 mm ； Temperature difference (ΔT): Actual operating temperature – Ambient temperature = 100°C – 25°C = 75℃ ； Calculation result: ΔL = 11.7×10⁻⁶/°C × 1200 mm × 75°C = 1.053 mm ； Radial lift amount (ΔR): Based on the geometric constraints of the shaft system (shaft diameter D = 100 mm), ΔR ≈ ΔL × LD/2 = 0.044 mm. Conclusion: The axial upward displacement caused by actual thermal expansion is 0.044 mm, and this compensation amount must be taken into account during alignment in the cold state. 5.2 Standard Revision Plan: Based on thermal expansion compensation and dynamic stability requirements, the alignment standards are revised as shown in Table 1.

End face tolerance: full circumference control ≤ 0.03 mm, to ensure uniform stress distribution in the coupling diaphragm. Conclusion: The revised standard, verified through dynamic simulation, can effectively compensate for thermal expansion and reduce the risk of diaphragm failure. 6. Three-table method for alignment: Based on the revised alignment standards, high-precision alignment techniques are required to achieve accurate compensation for cold-state deviations. The three-gauge method eliminates the measurement errors associated with the single-gauge method by simultaneously monitoring radial and axial deviations, ensuring a uniform distribution of stress in the coupling diaphragm under hot operating conditions, thereby providing technical support for successful startup. 6.1 Alignment principle: The three-gauge method eliminates the measurement blind spots of the single-gauge method by simultaneously measuring the deviations of the outer circle (radial) and end face (axial) of the coupling. The core formulas are: outer circle deviation = (A0 – A180)/2, and end face deviation = [(B0 + B180) – (B90 + B270)]/4. Here, A0, A90, A180, and A270 are the angle readings at various positions on the outer circle, while B0, B90, B180, and B270 are the angle readings at various positions on the end face. 6.2 Alignment Procedure The alignment of couplings must follow a standardized process: first, when removing the intermediate jumper, M6×50 bolts should be used to symmetrically pre-tighten the diaphragm assembly by 2–3 mm, ensuring even stress distribution across the assembly; forced knocking is strictly prohibited as a method for removal. Next, install the gauge holder; fix magnetic gauge bases at the shaft ends of the compressor and the speed increase gearbox, respectively. Adjust the position of the dial indicator so that its probe points vertically at the shaft surface, with the inclination angle of the gauge rod being ≤10° to ensure measurement accuracy. During the data collection phase, it is necessary to manually rotate the shafts to ensure their synchronous rotation. Data recordings are taken at four angular positions: 0°, 90°, 180°, and 270°. After each rotation, the radial and axial deviation values are read once the pointer has stabilized, and these values are recorded in the data table (Table 2). Deviation adjustment is carried out using a progressive correction method: the vertical deviation is adjusted via the cat’s-claw set screws on the compressor side, while the horizontal displacement is adjusted with base shims. The amount of adjustment per step shall not exceed 0.05 mm. After each adjustment, the data must be rechecked until the axial deviation is ≤0.05 mm and the radial deviation is ≤0.03 mm, as required by API 671 standards. The final alignment data should retain the cold-state compensation value to ensure a good alignment condition during the unit’s hot operation. 7 Conclusion The main reason for the interlocked shutdown of ethylene heat pump compressors is that the cold-state alignment standards fail to account for thermal expansion effects, resulting in excessive stress on the coupling diaphragm and dynamic imbalance. The centering tolerance was revised through thermodynamic calculations (outer diameter: –0.03 to +0.02 mm), and the three-table centering technique was employed to achieve an adjustment accuracy of outer diameter deviation ≤ 0.015 mm, successfully eliminating the risk of vibration. In the future, it is necessary to develop a dynamic compensation model for thermal alignment, combining real-time monitoring of the shaft position with a spectral early-warning system, in order to optimize the whole-life cycle management of high-speed units and provide a technical framework for the long-term operation of petrochemical plants.

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