Resolving Abnormal Operation of a PTA Turboexpander via Varnish Removal Filtration Technology
Equipment Management Department, SINOPEC Yizheng Chemical Fibre Co., Ltd., 211900
Gong Tianshou
Abstract: This paper analyzes the root causes of operational abnormalities in a large-scale turboexpander train, proposes a series of corrective measures, and identifies key operational risk points and preventive strategies. Through the application of varnish removal filtration technology, potential safety hazards were eliminated, ensuring the intrinsic safety and long-term reliability of the unit.
1. Overview
The air compressor train of the 600,000 t/a (60t/a in original text) PTA unit at SINOPEC Yizheng Chemical Fibre Co., Ltd. utilizes equipment manufactured by MAN Turbo (Germany). It is a three-in-one compressor train consisting of a multi-shaft five-stage centrifugal air compressor, a condensing steam turbine serving as the main driver, and a turboexpander serving as the auxiliary driver. The turboexpander features two-stage (high-pressure and low-pressure) expansion, each equipped with a suction inlet and an exhaust outlet, and utilizes three-dimensional (3D) impellers (see Figure 1).
Figure 1: Cross-Sectional View of the Turboexpander Unit (Left: High-Pressure Side; Right: Low-Pressure Side)
The primary performance parameters of the turboexpander are as follows:
The high-pressure (HP) shaft speed is 16,583 r/min, and the low-pressure (LP) shaft speed is 9,045 r/min; the rated total power output of the expander is 7,990 kW, with a mass flow rate of 12,700–150,450 kg/h; the inlet pressure is 1.3 MPa, and the exhaust pressure is 0.003 MPa. The HP inlet temperature is 175°C with an exhaust temperature of 80°C; the LP inlet temperature is 175°C with an exhaust temperature of 45°C. Both ends of the HP and LP pinion shafts are supported by tilting-pad journal bearings, each comprising 5 bearing pads. Oil can be supplied via dual oil supply lines; each bearing has one oil inlet hole feeding 3 groups of oil injection nozzles (15 nozzles in total) with a nozzle orifice diameter of 1.8 mm. Each bearing has 9 oil drain holes, normally configured with 5 open and 4 plugged. The entire three-in-one train adopts a centralized forced-lubrication system supplied by the main lube oil station.
2. Existing Operational Issues of the Unit
In 2018, to comply with VOC emission regulations, a new VOC treatment unit was added to treat the tail gas from the oxidation reactor before injecting the treated tail gas back into the turboexpander. Because bromine salts in the raw tail gas generate bromide ions after high-temperature oxidation, both the inlet and exhaust temperatures on the HP and LP sides of the turboexpander had to be increased (see Table 1) to prevent bromide ions from condensing and precipitating during expansion work, which would otherwise cause pitting corrosion in the turboexpander and downstream equipment.
Table 1: Summary of Turboexpander Inlet and Exhaust Operating Temperatures Before and After the VOC Retrofit
Prior to the VOC retrofit, the non-drive (non-impeller) end bearing temperature on the LP side of the turboexpander remained stable at around 80°C (the High Alarm threshold for this bearing is 110°C, and the High-High Alarm threshold is 120°C). After the unit was restarted following the VOC retrofit on January 6, 2019, the LP non-drive end bearing temperature rose slowly, peaking near the High-High Alarm limit of 120°C, although shaft vibration parameters showed no significant changes during this period (see Figure 2).
Figure 2: Turboexpander Flow Rate and Non-Drive End Shaft Vibration and Bearing Temperature Trends
1 – Flow Rate Curve; 2 – Non-Drive End Bearing Temperature Curve; 3 – Non-Drive End Shaft Vibration Curve
3. Root Cause Analysis and Corrective Measures
After investigating and analyzing the bearing temperature fluctuation trends—and ruling out field instrumentation errors, process fluctuations, electrostatic discharge from steam turbine grounding brush wear, rotational speed fluctuations, and spare parts quality—the primary causes of the bearing temperature fluctuations were identified as follows:
3.1.0 Causes of Temperature Rise at the LP Non-Drive End Bearing
3.1.1 Disassembly inspection revealed that both the bearing-to-shaft clearances and gear tooth meshing clearances were within normal tolerances. Apart from suspected varnish deposits observed on the LP non-drive end bearing surface (see Figure 3), no abnormalities were found on any other bearings.
Figure 3: Photograph of the Turboexpander Non-Drive End Bearing and Moving Components
3.1.2 Because the lubricating oil had been in service for less than one year and had passed pre-startup quality testing, the company sent oil samples to an independent third-party laboratory for diagnostic analysis to eliminate any doubts. The laboratory confirmed that the deposits on the bearing surface were early-stage varnish, and evaluated the MPC (Membrane Patch Colorimetry / Varnish Potential) index (see Figure 4).
Figure 4: Technical Oil Analysis Report Issued by the Professional Fluid Monitoring Laboratory
3.1.3 The lubricating oil used in this turboexpander is Shell Turbo T 46 turbine oil (mineral oil). When mineral oil is exposed to elevated temperatures, oxidation occurs and oxidation byproducts accumulate on the bearing pad surfaces to form varnish. Mineral lubricating oils are primarily composed of hydrocarbons, which remain relatively stable at ambient and lower temperatures. However, under high-temperature conditions, once even a tiny fraction of hydrocarbon molecules undergoes oxidation, adjacent hydrocarbon molecules follow in a chain reaction—a characteristic feature of hydrocarbon free-radical chain oxidation.
3.1.4 Equipment engineers thoroughly inspected the machine casing supports, cold-state piping stresses on the inlet and outlet lines, lube oil system leakage, and RTD temperature probe integrity. They also replaced the LP non-drive end bearing assembly. However, just one month after startup, the bearing temperature reached 110°C again, followed by significant fluctuations in both vibration and temperature. Although process engineers made multiple adjustments to the LP side flow rate, pressure, and temperature in an effort to approximate pre-retrofit operating conditions, these adjustments had virtually no effect (see Figure 5).
Figure 5: Trend Chart of Relevant Operating Parameters from February 13 to March 29
By recalculating the actual operating duty, it was found that under the current operating point, when the inlet gas flow stabilizes at 120 t/h, the power output is 8,000 kW, which is close to the original design output of 7,990 kW; when the inlet gas flow reaches 130 t/h, the power output rises to 8,680 kW; and at 146 t/h, the power output reaches 9,660 kW. Because the LP stage accounts for two-thirds of the total work performed by the turboexpander, the LP side was operating under potential overload conditions. Whenever the bearing temperature exceeded 110°C, shaft vibration changed drastically, indicating that newly formed varnish on the shaft journal and bearing pad surfaces was causing intermittent rubbing during those periods (see Figure 6).
Figure 6: Turboexpander Train Power Balance Sheet
3.2 Mechanism Analysis of the Existing Problem
3.2.1 As shown in Figure 7, within the coordinate system, the angle between the micro-vibration direction of the pad pivot and the horizontal coordinate axis is \beta, and the pad tilt angle is \phi. In a 5-pad tilting-pad bearing system, when a bearing pad is subjected to hydrodynamic oil film pressure, the pad pivot is not an absolute rigid body; under compressive deformation, pivot stiffness causes a slight displacement of the pivot position along the direction of geometric preload, thereby altering the bearing clearance and oil film thickness [1].
Figure 7: Single-Pad Coordinate System of the Tilting-Pad Journal Bearing
3.2.2 As seen in Figure 1, the rotor is a cantilevered overhang structure in which the impeller is the primary working element. Because the impeller end is the driven end, aerodynamic damping during gas expansion keeps the shaft journal at the impeller end in an ideal position within its bearing pads, maintaining normal oil clearances. However, during torque transmission through the bull-gear and pinion mesh—which acts as a fulcrum—the radial freedom of the non-impeller end shaft is restricted under overload conditions. Consequently, the hydrodynamic oil film pressure at the LP non-drive end bearing exceeds that of the other bearings, increasing oil film stiffness, slowing down the oil film renewal rate, increasing frictional heating, and promoting varnish formation.
3.2.3 Varnish in lubricating oil is primarily generated via three mechanisms: thermal oxidation, oil "micro-dieseling" (adiabatic micro-combustion), and localized high-temperature electrostatic discharge. In this case, the varnish was primarily induced by micro-dieseling. The mechanism is as follows: lubricating oil inevitably dissolves a certain amount of air (typically less than 8%). When the solubility limit is exceeded, excess air exists in the oil as suspended micro-bubbles. Upon entering the high-pressure zone of the bearing, these entrained air bubbles undergo rapid adiabatic compression, causing localized fluid temperatures to spike instantaneously and triggering adiabatic "micro-dieseling" of the oil. This generates sub-micron insoluble degradation products. Because these insoluble byproducts are polar, they readily adhere to metal surfaces to form varnish. The higher the local pressure, the lower the solubility of these degradation products, making them precipitate and deposit even more easily as varnish.
3.2.4 As varnish builds up, the effective hydrodynamic oil film thickness in the loaded zone is encroached upon by the varnish layer, further reducing the oil film renewal rate. Rising temperatures increase friction between the bearing Babbitt surface and the rotating shaft. Simultaneously, the deposited varnish acts as a thermal insulator, impairing heat dissipation and raising both oil temperature and bearing metal temperature. Ultimately, the shaft journal rubs directly against the varnish layer, manifesting as violent fluctuations in shaft vibration.
3.2.5 Although laboratory testing indicated that the bulk oil MPC value of the turboexpander lube oil was not exceptionally high, a dynamic equilibrium exists between the dissolution and precipitation of varnish precursors because lubricating oil has a finite solvency for polar varnish molecules. Once local saturation is reached—or when solubility drops—varnish precipitates onto the bearing pads, causing bearing temperature fluctuations and posing a severe threat to safe unit operation. Its adhesion to the bearing pads was a direct driver of the persistent temperature rise.
4. Countermeasures and Solutions
Eliminating varnish accumulation at the bearings ensures that the unit bearings operate within a controllable temperature range. Following technical evaluation, we selected the WVD-II Hybrid Electrostatic Adsorption + Ion-Exchange Resin Varnish Removal Purifier to eliminate the varnish problem.
The WVD-II series oil purifier effectively combines electrostatic adsorption purification technology with ion-exchange resin technology—removing soluble varnish via resin adsorption and insoluble/precipitated varnish via electrostatic adsorption. This hybrid process rapidly minimizes sludge and varnish levels within a short period, restoring heavily varnished lubrication systems to optimal operating condition and resolving slow temperature creep in thrust and journal bearings. It effectively removes and prevents both soluble and insoluble sludge generated during continuous turbine operation.
Its primary working principles are as follows:
4.1 Removal of Soluble Varnish via Ion-Exchange Resin
Ion-exchange resin consists primarily of a polymer matrix backbone and functional ion-exchange groups. The adsorption principle is illustrated in Figure 8.
Figure 8: Adsorption Principle of Ion-Exchange Resin
The exchange groups comprise fixed ions and mobile counter-ions. The fixed ions are covalently bound to the polymer matrix and cannot move freely; the mobile ions are attached to the fixed ions via ionic bonds and act as exchangeable ions. The fixed and mobile ions carry opposite electrical charges. Within the purifier, the mobile ions dissociate and exchange with polar oil degradation byproducts carrying the same charge, binding those soluble varnish molecules firmly onto the functional groups of the resin beads so they are permanently removed from the circulating oil stream.
4.2 Removal of Suspended Insoluble Varnish via Electrostatic Adsorption
Electrostatic adsorption technology utilizes a high-voltage generator to establish a high-voltage electrostatic field, polarizing contaminant particles in the oil so that they carry positive or negative charges. Under the influence of the intense electrostatic field, positively and negatively charged particles migrate toward the negative and positive collecting electrodes respectively, while neutral particles are swept along by the charged particle stream and captured on the collector media (see Figure 9).
Figure 9: Principle of Electrostatic Adsorption Technology
Electrostatic oil purification removes all insoluble contaminants, including hard wear particles and suspended sub-micron varnish generated by oil degradation—whereas conventional mechanical filter elements can only capture larger particles above their pore rating and cannot trap sub-micron suspended varnish.
This combined system thoroughly removes existing varnish precipitated and deposited on the bearing pads, completely eliminating bearing temperature fluctuations and shaft vibration spikes caused by varnish rubbing, and enabling long-term, stable unit operation.
5. Conclusion
Since commissioning the WVD-II Varnish Removal Oil Purifier, the dual-action combination of electrostatic adsorption and ion-exchange resin stabilized the bearing temperature rapidly. Throughout 2 years of continuous operation, the LP non-drive end bearing temperature has remained consistently stable at around 90°C (see Figure 11), and the compressor train has operated normally. Subsequent overhaul disassembly confirmed that the bearing Babbitt surfaces and shaft journals were virtually free of varnish deposits (see Figure 10).
Figure 10: Photograph of the Disassembled Bearing After Installing the Varnish Removal Unit
Figure 10: Photograph of the Disassembled Bearing After Installing the Varnish Removal Unit
Figure 11: Turboexpander Flow Rate and Non-Drive End Shaft Vibration and Temperature Trends After Installing the Varnish Purifier
1 – Flow Rate Curve; 2 – Non-Drive End Bearing Temperature Curve; 3 – Non-Drive End Shaft Vibration Curve
6. Economic Benefits Achieved
The installation and operation of the varnish removal oil purifier successfully resolved the slow temperature rise at the LP non-drive end bearing of the PTA turboexpander, avoiding massive financial losses from unplanned unit shutdowns (a unit shutdown and restart takes at least 3 days, incurring a production loss of at least 4.0 million RMB; bearing replacement takes 1 day, incurring a loss of 1.0 million RMB), as well as preventing secondary spare-parts damage to rotors, bearings, and labyrinth seals caused by prolonged overheating (estimated between 500,000 and 8.0 million RMB).
Furthermore, the lubrication system holds a total of 160 drums of turbine oil. After high-precision varnish removal filtration, every oil quality indicator returned to full compliance, saving 500,000 RMB in new oil replacement costs.
References:
[1] Liu Siyong, Xiao Zhonghui, Yan Zhiyong, Chen Zhujie. Numerical Simulation and Experimental Study on Dynamic Characteristics of Tilting-Pad Bearings with Pivot Elasticity and Damping [J]. Journal of Mechanical Engineering, October 2014, 50(19): 88.
Author Biography: Gong Tianshou, male, born in 1965, Bachelor's degree, Senior Engineer, currently responsible for rotating and mechanical equipment technical management in the Equipment Management Department of SINOPEC Yizheng Chemical Fibre Co., Ltd.
Contact: +86-13952522762; [email protected].
