Analysis of the Causes of Water Intrusion in the Compressor Oil System and Prevention Measures

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1. Basic situation: The ethylene cracking gas compressor kept experiencing shaft vibration interlocks, resulting in 4 unplanned shutdowns of the compressor within two months. Two of these shutdowns led to a complete shutdown of the entire ethylene plant, causing significant economic losses. A comprehensive analysis and inspection were carried out on the process flow and equipment condition of the compressor unit. During the analysis of the lubricating oil sample, it was found that there was a significant amount of free water in the oil tank, with the water content in the oil far exceeding the allowable value of 0.1%. The ethylene cracking gas compressor is driven by an E—GBT201 turbine (manufactured by SIEMENS) and is equipped with a dedicated lubricating oil station with a capacity of 8000 L. The oil used is MOBIL DTE OIL MEDIUM turbine oil, which has a dynamic viscosity of 46 mm2/s at 40°C and a minimum ignition temperature of 201°C. This turbine oil serves as the sealing oil and lubricating oil for both turbines and compressors, as well as the control oil for the turbine control system. 2. Cause analysis: By comparing the daily drainage volume from the bottom of the tank, it was found that approximately 10–20 L of free water is discharged each day. This indicates that water from the outside is continuously entering the compressor oil system. There are three possible reasons for water to enter the oil system: ① a leak in the oil cooler. ②The heating coil in the fuel tank or degassing tank is leaking, and the partition is also leaking. ③Lubricating oil water ingress on the steam turbine side. Since the cooling water pressure in the tube side of the oil cooler is only 0.45–0.48 MPa, while the lubricating oil pressure in the shell side reaches 2.4 MPa during normal operation, the pressure difference between the two sides is nearly 2 MPa; therefore, in the event of a leak, the return cooling water will definitely contain oil. By checking the COD value of the return cooling water, no oil was detected. Therefore, the cause of the oil cooler leakage can be ruled out. By analyzing the oil quality in the heating layers of the fuel tank and degassing tank and comparing it with that of the lubricating oil, it can be determined that the heating coils are functioning properly. An inspection of the turbine seal revealed significant leakage. The seal drain water is discharged through a hole located at the lower part of the seal body; however, if the steam leakage is excessive or the drainage system is inadequate, steam or condensed water may pass through, and the leaked steam will enter the lubrication system via the bearing seals. 3. Control measures: First, the drainage holes at the bottom were cleared, reducing the daily drainage volume of the fuel tank to 5L, which improved the situation somewhat; however, the problem of lubricant containing water was not completely resolved. To this end, a nitrogen seal line was added outside the oil seal of the bearing housing; the oil pressure inside the bearing housing is 1.8 MPa. If the pressure of the nitrogen gas applied is too high, on one hand, the nitrogen will enter the bearing housing through the oil seal, affecting the proper functioning of the lubricating oil, and on the other hand, it will result in significant waste. Therefore, in actual production, the nitrogen pressure applied is kept at around 0.05~0.07 MPa, thereby maintaining a certain positive pressure outside the insulation panel and the oil seal. This prevents the steam leaking from the turbine from entering the insulation panel, thus effectively avoiding the entry of condensate water into the lubrication system. After treatment, the daily water discharge from the tank is close to zero. After further observation for a month, no water was found to be discharged from the bottom of the fuel tank. Due to the limitations of production processes and process conditions, it is very difficult to achieve a completely moisture-free oil system in machinery; therefore, it is essential to remove moisture from the oil system promptly. The YJG type oil purifier can be used to regularly separate water from the lubricating oil of the unit and filter out impurities. For high-quality compressors, it is recommended to use imported compressor oil, such as the synthetic oils produced by CPI Engineering Services in the United States: www.cpihualai.178b2b.com