Innovative application of varnish filtration technology in syngas compressor

Abstract: Analyze the causes of main bearing shell temperature fluctuation of centrifugal compressor unit, put forward specific solutions, and master the risk points of operation and preventive measures.

Key words: centrifugal compressor group varnish bearing bush temperature

1 summary

 Syngas compressor unit K04401 of CNOOC Huahui Coal Chemical Co., LTD is designed and manufactured by Mitsubishi, Japan. Its shape layout is installed as follows:

Syngas compressor unit K04401 high 3V-7S (Hp), low pressure cylinder 3V-7 (Lp) shell is a barrel structure, the barrel body bottom toward the driver, the free end side open, easy insertion into the inner cylinder.

Table 1: Performance parameters of the K04401 low and high pressure cylinder 3V-7 (Lp) / 3V-7S (Hp) equipment

2. Unit 2 has problems

In May 2020, the axle shell temperature of the unit fluctuated, and the temperature of some temperature points could not return to the original operating value. Among them, the radial main bearing shell temperature of the steam turbine exhaust end TI-04457B has reached 82℃ and has an upward trend.

Figure 1: Trend of unit bearing bush temperature point TI04457B

3. Cause Analysis and treatment measures

3.1 The rise of rising temperature

By testing the oil index of the unit operating oil, it is found that the varnish tendency index 22.4 is high, and the pollution degree is also high (see Table 2). And high varnish tendency index, varnish may cause the formation of varnish on the shaft adhesion accumulation, thus reducing the oil film gap, increase friction, serious lead to the shaft of poor heat dissipation, shaft temperature rise, oil oxidation acceleration. At the same time, due to the high pollution in the oil, the varnish will adhere to other contaminated particles, forming a grinding effect to aggravate the equipment wear.

 Analyzing the fluctuation of the bearing bush, it may be the varnish produced in the unit lubricant, the varnish is finally concentrated on the bearing bush,

 Causes the main bearing shell temperature to fluctuate and rise.

The cause of varnish: the first is the natural oxidation of oil products. Hydrocarbon oil oxidation follow the free radical chain reaction mechanism, oxidation of carboxylic acid, esters, alcohol peroxide, these peroxides further condensation reaction of high molecular weight polymer, dissolved in the oil state, when exceeds the dissolution degree of the lubricating oil, lubricating oil saturated, excessive degradation products will form a varnish. Secondly, the oil “micro-combustion” will also accelerate the formation of varnish. Under normal conditions, a certain amount of air (<8%) is dissolved in the lubricating oil. When the dissolution limit is exceeded, the air entering the oil exists in the oil in suspension. Once the lubricating oil is pumped into the high pressure area from the low pressure area, these small bubbles suspended in the oil are sharply compressed, resulting in a rapid increase in temperature in the oil microarea, sometimes up to 1100℃, resulting in adiabatic “microcombustion” in the oil microarea, generating a very small insoluble material. These insoluble materials are polar, extremely unstable, and also easy to adhere to the metal surface to form a varnish. Again “electric spark” in the oil is also an important reason for the formation of varnish, in large unit high temperature, high pressure, high speed, environment, when the oil after very small gap such as valve core, precision filter, molecular friction between static electricity, suddenly discharge, accumulated after thousands of degrees of high temperature, is also easy to generate varnish. Generally speaking, the oil product oxidation is a slow process, and the oil product adiabatic “micro-combustion” generation of varnish speed is much faster. Finally, such as the insufficient amount of lubricating oil, the unit itself installation clearance is too small, the uneven axle shell load distribution will also accelerate the generation of varnish. When the concentration of polar oxide in these lubricants reaches saturation at a specific temperature pressure, precipitation on the inner surface of the metal, to some extent, it will affect the heat dissipation of the bearing bush and lead to the bearing bush temperature fluctuation or rise.

 3.2 Solve the problem of shaft temperature increase

For shell temperature fluctuations, avoid unplanned shutdown, within the group and coal chemical industry enterprises investigated the electrostatic adsorption, balanced charge, resin adsorption, low temperature precipitation, mechanical filtration and several varnish filter effect and market reputation, finally chose the W VD electrostatic adsorption + resin adsorption this composite varnish equipment. Through electrostatic adsorption to solve the precipitated varnish, through the resin adsorption to solve the dissolved varnish, so as to completely solve the bearing bush temperature fluctuation rise problem caused by the varnish, in addition, in the removal of varnish abnormal, but also solve the problem of oil pollution.

3.2.1 Working principle and schematic diagram of electrostatic adsorption technology-Remove the precipitated state varnish

The electrostatic adsorption principle uses the electrophoresis and the dielectrophoresis force generated by the high-voltage electrostatic field, Polarize the contaminated particles in the oil and show the positive and negative electricity, respectively, Positive and negative electric particles swim in the direction of negative and positive electrodes under the action of ultra-high voltage electric field, Neutral particles are moved by a flow of charged particles, Finally, all the particles were adsorbed to the collector attached to the electrode, Thoroughly remove pollutants from oil products, The principle of electrostatic adsorption technology is used to make the weak polarity of the oil product after purification, Constantly stripping of contaminants attached to the tank walls, piping, valve parts, To the clean pipe system, To improve the cleanliness of the entire oil system, Provide a reliable guarantee for the stable operation of the unit.

3.2.2 Working principle and schematic diagram of ion resin adsorption technology-removal of dissolved varnish

Ionic resin technology can remove the soluble varnish. When the unit is running, because the oil temperature is high, the dissolved varnish (also called varnish embryo) is highly tolerable, not easy to remove with electrostatic adsorption technology, and ion resin adsorption technology can remove soluble pollutants in the oil. Ion exchange resin is mainly composed of two parts: polymer skeleton and ion exchange group. The adsorption principle is shown in the figure below. The exchange group is divided into fixed part and active part, which is bound to the polymer matrix and cannot move freely to become fixed ions; the active part and fixed part are combined by ion bond to become exchangeable ions. Fixed ions and active ions have opposite charges, respectively. In the solution, the active part dissociates into free moving ions, exchanging with other degradation products with the same charge in the solution, which are combined with the fixed ions and firmly adsorbed on the exchange group, so as to remove the soluble varnish in the solution and reduce the MPC value.

3.3 Remove the varnish effect

The varnish filter has been installed and put into use. At present, the color degree of oil has been significantly improved after one month of filtering. Through analysis and external detection data analysis, the tendency index of varnish of oil is reduced from 22.4 to 2.5, the pollution level is reduced from NAS9 to 7, and the acid value index is reduced from 0.064 to 0.048.

Table 2: M PC and cleanliness index before filtration

Table 3: filtered M PC and cleanliness index

Table 4: acid value index before filtration

Table 5: Filtered acid value index

Figure 2: Color contrast before and after filtering

Figure 3: Trend of temperature after unit lubricating oil filtration (temperature drops to 67.1℃)

4. Economic benefits generated

 Through electrostatic adsorption precipitation of state varnish, through resin adsorption dissolved varnish, so as to completely solve the bearing bush temperature and vibration fluctuation caused by varnish, to avoid the huge output loss (daily loss of urea output 1700 tons, 3 million yuan; if it is open cylinder replacement rotor, time at least 3 days, 9 million), and shell temperature vibration increased rotation and sealing parts caused by the spare parts loss (loss between 10-5 million yuan).

The WSD varnish removal unit has been installed and put into use. At present, the color degree of oil has been significantly improved after one month of filtering. Through analysis and external detection data analysis, the tendency index of varnish of oil is reduced from 22.4 to 2.5, the pollution level is reduced from NAS9 to 7, and the acid value index is reduced from 0.064 to 0.048. In addition, the unit contains about 150 barrels of oil products, through the varnish removal  high fine filtration the oil fully reached the qualified index, saving the oil replacement costs and waste oil disposal costs, a total of 400,000 RMB.

5 Conclusion

Due to the long-term high temperature, high pressure and high speed working conditions of the lubrication system of the large unit, the oil oxidation speed accelerates, the varnish index increases, and the gelatin content increases. Soft impurities accumulate in the large unit system, which affects the accuracy of the speed regulation system and the normal operation of the unit. It is easy to lead to the fluctuation of the unit or even unplanned shutdown. The varnish glue deposited on the bearing bush surface will also increase the bearing bush temperature, and the adhesion of the varnish and solid particles will also aggravate the wear of the equipment. The varnish removal unit can continuously improve the quality of lubricating oil of the unit, ensure the long-term stable operation of large units, prolong the service cycle of lubricating oil, improve the operating environment of the system, and reduce the purchase cost of lubricating oil.