1. Unit Overview
The syngas compressor is critical long-term operating equipment for the methanol plant. It pressurizes feed gas and delivers syngas to the methanol synthesis reactor for reaction. This unit is a manufactured centrifugal compressor driven by a condensing steam turbine. The compressor adopts an 8-stage compression configuration, including 5 compression stages and 3 recycle stages. The compressor inlet pressure is 3.3 MPa, outlet pressure is 7.3 MPa, and the maximum rotating speed is 11,754 r/min. It features a radial (vertical) horizontally split casing. The rotor is of straight-through design; all impellers face the same direction, and a balance piston is arranged at the discharge end.
2. Main Maintenance Scope of the Unit
2.1 Existing Problems of the Syngas Compressor
Since the emergency repair carried out in February, severe corrosion and damage have been found in the interstage seals and internal cavity seals. After temporary restoration and restart-up, although the unit operates relatively stably, it fails to reach the designed compression capacity, restricting long-term full-load stable production of the plant.
2.2 Main Maintenance Contents
2.2.1 Compressor Section
(1) Complete replacement of the compressor rotor bundle
This overhaul mainly focuses on replacing the compressor rotor bundle, covering the following work items:
(2) Replacement of coupling with domestic spare parts
(3) Realignment of the coupling
(4) Replacement of dry gas seals
2.2.2 Inspection and Maintenance of Auxiliary Systems
(1) Lubricating oil system:
Replace lubricating oil according to oil analysis reports, eliminate leakage in the oil circuit system, replace oil filters, clean the inlet strainer of oil pumps, calibrate safety valves on oil pump outlet pipelines; remove and open the end cover of the oil cooler to clean the cooling water side.
(2) Inspection and calibration of instruments, valves and control systems for lubricating oil and control oil pipelines.
2.2.3 Repair of Damaged Thermal Insulation and Anti-corrosion Coating after Overhaul
3. Overhaul of the Syngas Compressor
3.1 Pre-overhaul Preparation
(1) Propose maintenance plans, formulate overhaul schemes and maintenance network schedules, and confirm maintenance items combined with unit operating conditions and fault characteristics;
(2) Discuss and review maintenance plans and overhaul schemes;
(3) Prepare maintenance materials, spare parts, maintenance tools, special tooling and overhead lifting equipment;
(4) Review and prepare equipment technical files and drawings;
(5) Conduct on-site technical disclosure for maintenance personnel; isolate and purge power, gas, air and process media to satisfy safety overhaul conditions of the unit;
(6) Ensure the construction site meets safety and health standards; complete maintenance work permits, verify isolation of water, gas and medium pipelines between the unit and external systems;
(7) Auxiliary materials: ethanol, plastic film, sealant, various pipeline gaskets, white cloth, envelopes;
(8) Tool preparation: special hydraulic disassembly tools, torque wrenches, 2 dial indicators, 1 micrometer, 1 vernier caliper, 1 depth gauge, one set of sleeves, one set of laser alignment instrument, guide screws and jackscrews for casing lifting, other general tools and lifting appliances.
3.2 Disassembly Procedures of the Unit
(1) Remove wiring for temperature, vibration and other measuring instruments, detach accessories, oil & gas pipelines and guards, and seal all pipeline openings;
(2) Disassemble the coupling and record alignment data:
① Mark all removed coupling bolts;
② Remove coupling guard and coupling spacer, push the compressor rotor towards the main thrust side of the thrust bearing, align the turbine output shaft and compressor rotor on the same side, measure and record the distance between the two shaft ends. The data serves as reference for judging proper inner barrel positioning and coupling alignment during assembly;
③ Check shaft axial float and shaft alignment. After coupling removal, measure compressor axial float by the shaft pushing method, inspect and record shaft alignment data as original disassembly records.
(3) Remove shear rings. The shear rings and positioning rings inside the casing bore resist internal pressure and maintain the position of the inner barrel rotor during compressor head maintenance.
(4) Pull out the complete inner barrel bundle assembly
Observe the overlapping arrangement of shear rings before disassembly and mark the position of each piece. Remove the top piece first, then left and right pieces, and finally the bottom piece. Jackscrews are arranged at the middle of each shear ring for ejection.
① Mark the fit between casing bore and bundle assembly to ensure accurate alignment when reinstalling the bundle into the casing;
② Use depth micrometers to measure the distance from the casing end face to the suction end head at four uniformly distributed circumferential positions and record data. The positioning dimension (0.0762–0.1270 mm) is required for reinstallation so that shear rings and positioning ring segments can be fitted into the annular sleeve of the casing;
③ Use special pulling modules and screws to evenly pull the bundle assembly out of the casing bore;
④ Install special bundle support tools, slowly pull the bundle assembly out of the compressor casing by means of turnbuckles on the support and place it onto the support frame;
⑤ Mount special bundle lifting devices and lift out the bundle assembly.
(5) Disassemble thrust bearing and thrust disc
Procedures:
① Remove front and rear covers, measure thrust pad clearance (axial float) by the shaft pushing method;
② Dismount bearing temperature measuring probes;
③ Remove upper half of thrust bearing, measure thrust disc face runout, then disassemble the thrust bearing;
④ Use special hydraulic tools to remove the thrust disc.
Key Operation Points:
① Note the position of two O-rings and the outlet hole for instrument wiring before removing thrust pads. Adjusting shims are fixed on the back of thrust pads by screws. Axial float can be adjusted by changing shim thickness;
② Mark main/auxiliary thrust pads and position of each pad segment for error-free reassembly;
③ Measure total axial float of the rotor inside the inner barrel after removing thrust pads. This data is the reference for mechanical seal positioning (lock the mechanical seal when the rotor is centered inside the inner barrel);
④ Measure dimension C, the distance from the thrust disc to the shaft end plane before disassembly. This reading judges whether the thrust disc is seated properly during reassembly.
(6) Disassemble radial bearings
Measure radial bearing clearance by the shaft lifting method (tilting pad bearing) combined with feeler gauges before removal. No barring is allowed after radial bearing removal, otherwise all stage gas seals will be damaged.
(7) Disassemble dry gas seals
Remove dry gas seals at both ends with special dry gas seal pullers.
(8) Replace rotor bundle
This overhaul adopts complete replacement of the inner barrel bundle. Lift the new bundle shell onto the installation platform, lift out the upper shell of the new rotor bundle, inspect diaphragms, gas seals and oil seals inside the shell, and clean anti-rust oil on components with cleaning agent.
3.3 Unit Reassembly
Reassembly follows the reverse sequence of disassembly. The workflow is shown as below:
Inspection of diaphragm and gas seal clearance → rotor installation → shaft seal installation → installation of radial bearing & thrust bearing → reinstall temperature probes → fit upper cover of bundle → reinstall inner barrel bundle assembly → reinstall vibration and displacement probes → fit coupling → restore instruments and pipelines & complete alignment.
Quality control for key assembly components shall be strictly implemented during reassembly.
4. Main Quality Control Points for Syngas Compressor Overhaul
(1) Fully inspect all disassembled or replaced components. No casting sand, particles, weld burrs, metal chips, oil stains, dirt, scratches or collision damage are permitted, especially the vibration measuring area on the rotor shaft.
(2) Thoroughly clean all components; vibration measuring areas and shaft journals must be well protected.
(3) Quality control of assembly clearances for all parts
Strictly control assembly clearances of all components to guarantee assembly quality. Key clearances include impeller eye labyrinth seal clearance, interstage labyrinth seal clearance, balance disc labyrinth seal clearance, oil seal clearance, radial bearing clearance and thrust bearing axial clearance. These shall be adjusted and controlled as priority quality checkpoints during overhaul.
① Measure clearances of impeller eye rings and interstage labyrinth seals on upper and lower casings to meet specification requirements.
Use long feeler gauges to measure seal clearances on lower casing and ensure uniform clearance.
Two methods are commonly used for upper gas seal clearance measurement: lead wire impression method and medical adhesive tape marking method.
A. Lead wire impression method: Select lead wire with diameter slightly larger than the seal clearance, confirm clear impression after compression.
B. Adhesive tape marking method: Apply adhesive tape to check upper casing eye ring seal and shaft seal clearance. Bar the rotor for one full revolution; only slight contact trace is acceptable. Avoid relative scraping between seals/diaphragms and rotor when lowering the upper casing; stagger adhesive tape positions to prevent overlapping.
② Diaphragm inspection: Use feeler gauges to measure clearance at positioning surfaces of each stage diaphragm to comply with standards.
③ Measure horizontal reading at split faces of seal-diaphragm and diaphragm-inner casing with dial indicators. Positive algebraic sum of readings will cause diaphragm deformation, resulting in internal gas leakage and inability to close casing properly. Ensure split face dimensions meet requirements.
(4) Quality control points for dry gas seal reassembly
Dry gas seal assembly is a critical checkpoint directly related to long-term stable operation of the unit. Key requirements:
① Confirm matching between shaft rotation direction and seal structure before installation, as seal performance is highly direction-dependent;
② Check rotor shaft and seal cavity surface for burrs or notches. Thoroughly clean the fitting area and perform trimming if necessary;
③ Measure axial relative position between rotor and compressor casing as the installation reference for seals;
④ Rotate the seal cartridge manually before installation to avoid suspended secondary seal assembly which leads to high initial leakage;
⑤ Lightly apply silicone-based grease to inner O-rings and anti-seize compound on shaft and sleeve bore;
⑥ Place the seal cartridge into the seal cavity, push it evenly into position with special pressing plates until metal-to-metal contact forms between positioning ring and compressor casing;
⑦ Install positioning blocks.
(5) Quality control for clearance measurement and adjustment of radial bearings and thrust bearings
Bearing clearance measurement is essential during disassembly and reassembly.
Radial bearing clearance measurement
Adopt shaft lifting method (tilting pad bearing) or lead wire method with feeler gauges; clearance must comply with design values.
Thrust bearing clearance measurement
Adopt shaft pushing method. Fix dial indicator base on pump head end face with the probe perpendicular to the drive-end shaft end. Push the rotor fully toward the thrust bearing side and zero the dial indicator. Then push the rotor fully toward the drive end. The final reading represents total axial float, which shall meet design specifications.
Determination of thickness for thrust bearing adjusting shims
Since the inner barrel bundle (rotor) is completely replaced, the thickness of thrust bearing adjusting shims shall be reconfirmed to ensure thrust bearing float complies with design and maintain proper operating clearance.
Key steps:
① Impeller-throat passage overlap and axial clearance
Without thrust bearings installed, adjust overlap of each impeller and axially center the rotor to optimize overlap value.
② Determination of adjusting shim thickness
A. Outer thrust bearing shim: Install lower half outer thrust bearing without shims until thrust shoes fully contact the thrust disc. Measure clearance 1 between back of base ring and thrust bearing housing. Clearance 1 equals the required thickness of outer adjusting shim, then fit the shim and reassemble outer thrust bearing.
B. Inner thrust bearing shim: Push thrust disc against outer thrust shoes, install lower half inner thrust bearing without shims until thrust shoes contact the thrust disc. Measure clearance 2 between inner thrust bearing base ring and housing mating surface. The thickness of inner adjusting shim = clearance 2 minus designed total operating thrust bearing clearance.
(6) Coupling alignment
Coupling alignment is a vital quality checkpoint after unit assembly. Laser alignment instrument is adopted for cold alignment in this overhaul.
5. Conclusion
In summary, the overhaul procedure and key quality control points of centrifugal compressor units are summarized based on the overhaul practice of the syngas compressor. This paper elaborates pre-overhaul preparation, formulation & approval of overhaul schemes, technical disclosure, disassembly procedures and whole-process quality control measures, providing practical guidance for major overhauls of centrifugal compressor units.