1 Overview
The cracked gas compressor and driving steam turbine of the 100Kt/a ethylene production department of Bora LyondellBasell Petrochemical Co., Ltd. are all equipped with equipment from Japan’s Mitsubishi Heavy Industries.
The pyrolysis gas compressor is a three-cylinder five-stage 16-stage impeller centrifugal compressor with 6 suction ports and 5 discharge ports. The main performance parameters are as follows; the rated speed is 4056r/min, the rated power is 53567KW, the discharge pressure of the compressor is 3.908Mpa, the discharge temperature is 77.5°C, and the flow rate is 474521kg/h. The driving steam turbine thrust bearing of the unit is a Kingsbury type thrust bearing with 6 pads. These bearings are equipped with 6 groups of lubricating oil inlets for lubrication, and each group of oil inlets has 4 3.0mm and 5 A 1.5mm oil inlet hole, the axial clearance between the thrust bearing and the thrust plate is 0.46-0.56mm. Adopt the forced lubrication method of centralized oil supply at the lubricating oil station.
Its axis diagram is as follows:
2, Unit problem
Since the start of the compressor unit on August 5, 2020, the temperature of the thrust bearing TI31061B of the steam turbine has fluctuated frequently, and has gradually increased. As of 16:43 on December 14, 2020, the temperature of the TI31061B reached 118°C, which is only 2 minutes away from the alarm value. ℃.
Figure 1: Trend of steam turbine thrust bearing temperature TI31061B
3. Cause analysis and treatment measures
3.1 Causes of temperature fluctuation of steam turbine thrust bearing TI31061B
After checking and analyzing the temperature fluctuation trend of the thrust bearing of the steam turbine TI31061B, and excluding the on-site instrument display problems, process fluctuations, steam turbine brush wear, equipment speed fluctuations, and parts quality, the main reasons for the shaft temperature fluctuations are:
3.1.1 The lubricating oil used in this compressor is SHELL TURBO T32, which is mineral oil. When the temperature is high, the lubricating oil in use is oxidized, and the oxidation products gather on the surface of the bearing bush to form a varnish. Mineral lubricating oil is mainly composed of hydrocarbons, which are relatively stable at room temperature and low temperature. However, if some (even a very small number) of hydrocarbon molecules undergo oxidation reactions at high temperatures, other hydrocarbon molecules will also undergo chain reactions, which is a characteristic of hydrocarbon chain reactions.
3.1.2 When the lubricating oil is added to the equipment, the working condition becomes a state of high temperature and high pressure, so this process is accompanied by the acceleration of the oxidation reaction. During the operation of the equipment, because the turbine thrust bearing is close to the ultra-high pressure steam, the heat generated by heat conduction is relatively large. At the same time, the axial displacement of the compressor has been too large since it was started, reaching 0.49mm at one time, while the alarm value was ±0.5mm. The axial thrust of the steam turbine rotor is too large, so the oxidation rate of this thrust bearing part may be twice as high as the oxidation rate of other parts. In this process, the oxidation product will exist in a soluble state, and the oxidation product will be precipitated when the saturated state is reached.
3.1.3 Soluble varnish precipitates to form insoluble varnish. Lubricating oil forms a soluble varnish in the high-temperature and high-pressure area. When the oil flows from the high-temperature area to the low-temperature area, the temperature decreases and the solubility decreases, and the varnish particles are separated from the lubricating oil and begin to deposit.
3.1.4 Deposition of varnish occurs. After the varnish particles are formed, they begin to agglomerate and form deposits that preferentially deposit on hot metal surfaces. At the same time, since the temperature of the thrust bearing has been high since the start of operation, the temperature of the bearing pad here has risen rapidly while the temperature of other bearings has changed slowly.
3.2 Solve the temperature rise problem of steam turbine thrust bearing TI31061B
3.2.1 After finding that the temperature of the thrust bearing TI31061B rose slowly, the temperature of the lubricating oil was lowered from 40.5°C to 38°C, and the pressure of the lubricating oil was raised from 0.15Mpa to 0.176Mpa to ease the slow rise of the bearing bush temperature.
3.2.2 The steam turbine rotor has 15 stages of impellers, the first 12 stages of impellers have balance holes, and the last 3 stages are not designed with balance holes. The axial thrust margin designed by Mitsubishi is too small, so adjust the steam turbine extraction to adjust the axial thrust. As shown in Figure 2 1279ZI31001C, the shaft displacement of the steam turbine is 0.44mm. After consulting the compressor manufacturer, the shaft displacement is positive, which means that the rotor is shifting to the compressor side relative to the original design rotor, so it is decided to reduce the intermediate air extraction from 300T/h Reduce to 210T/h, increase the load on the low-pressure side of the steam turbine, increase the thrust on the high-pressure side, and reduce the axial thrust on the thrust bearing, thereby slowing down the rising trend of the thrust bearing temperature.
Figure 2 Relationship between shaft displacement of steam turbine and thrust bearing
3.2.3 On November 23, 2020, the lubricating oil sample of the unit was sent to the testing institute of Guangzhou Institute of Mechanical Science Co., Ltd. for testing and analysis. The results are shown in Figure 3. The analysis results found that the MPC value was high, which can determine the occurrence of oil oxidation. The varnish is one of the reasons for the high temperature of the steam turbine thrust bearing TI31061B. When there is a varnish in the lubricating oil system, the dissolution and precipitation of the varnish particles in the oil is a dynamic equilibrium system due to the limited ability of the lubricating oil to dissolve the varnish particles. When it reaches a saturated state, the varnish will hanging on the bearing or bearing pad, causing the bearing pad temperature to fluctuate. It is a major hidden danger to safe operation.
Through research, we chose Kunshan Winsonda, which has better use effect and market reputation, to produce WVD electrostatic adsorption + resin adsorption, which is a composite varnish removal equipment to eliminate varnish.
varnish is a product formed by the degradation of oil, which exists in the oil in a dissolved or suspended state under certain chemical conditions and temperature. When the sludge exceeds the solubility of the lubricating oil, the sludge will precipitate and form a varnish on the surface of the component.
WVD-II series oil purifier effectively combines electrostatic adsorption purification technology and ion exchange technology, which can effectively remove and prevent the soluble and insoluble sludge generated during the normal operation of the steam turbine, so that the varnish cannot be produced.
The goal of WVD-II series oil purifiers is to eliminate the cause of varnish formation. This technology can minimize the content of sludge in a short period of time, and restore the original lubricating system with a large amount of sludge/varnish to the optimal operating state within a few days, thus completely solving the problem of slow temperature rise of thrust bearings caused by the varnish .
Figure 3 The test and analysis results before installing the varnish removal unit
One-time clean oil: electrostatic adsorption to remove non-soluble sludge/varnish Principle: electrostatic adsorption technology removes pollutants, the oil is in the action of a circular high-voltage electrostatic field, so that the polluted particles show positive and negative charges respectively, and under the action of a trapezoidal electric field Push the positively and negatively charged particles to swim towards the negative and positive electrodes respectively, and the neutral particles are squeezed and moved by the flow of charged particles, and finally all the particles are adsorbed on the collector to completely remove the pollutants in the oil.
Secondary clean oil: Ion exchange resin adsorption to remove dissolved colloids Principle: The charge adsorption technology alone cannot solve the dissolved varnish, while the ion resin contains billions of polar sites, which can absorb soluble varnish and potential varnish, to ensure that degradation products do not accumulate in the lubricating oil, and can improve the solvency of the lubricating oil, so that the system is in an optimal operating state.
Figure 5. Schematic diagram of secondary clean oil
3.3 Effect of removing varnish
The varnish unit was installed and operated on December 14, 2020, and the temperature of the steam turbine thrust bearing TI31061B dropped to about 92°C on December 19, 2020 (as shown in Figure 6).
Fig.6 Temperature trend of thrust bearing TI31061B of steam turbine
After more than a month of operation of the varnish removal unit, the quality of the lubricating oil of the unit has improved significantly. Through the detection and analysis of the Guangyan Research Institute, the varnish tendency index of oil products has been reduced from 10.2 to 6.2, and the pollution level has been reduced from >12 to 7 Grade, there is no loss of any additives in the lubricating oil (see Figure 7 for the detection and analysis results after the varnish removal unit is installed).
FIG. 7 Test and analysis results after the unit is installed
4 Economic benefits generated
Through the installation and operation of the varnish removal unit, the problem of the slow temperature rise of the thrust bearing TI31061B of the steam turbine caused by the varnish is completely solved, and the huge loss caused by the shutdown of the pyrolysis gas compressor unit is avoided (at least 3 days, the loss is at least 4 million RMB; the replacement of the thrust bearing of the steam turbine takes 1 day, the loss is 1 million), and the loss of spare parts to the rotating and sealing parts after the temperature of the thrust bearing increases slowly (the loss is between 500,000 and 8 million yuan between).
The unit was filled with a total of 160 barrels of oil products, and the oil products completely reached the qualified index after the high-precision filtration of the varnish removal unit, saving 500,000 RMB in oil product replacement costs.
5 Conclusion
Due to long-term high-temperature, high-pressure, and high-speed operating conditions in the lubrication system of large units, the oil oxidation speed is accelerated and the varnish index is increased. The hidden danger of bush burning in the push bearing ensures the long-term stable operation of the unit, which proves that the above measures are effective.
Post time: Dec-28-2022