Cryogenic air separation units (ASUs) are critical industrial equipment, widely used in the steel, chemical, and electronics industries. As a professional industrial gas technology provider, Shenger Gas has accumulated extensive operational and maintenance experience in this field. Given the continuous long-term operation of these units, any failure can not only disrupt production but also pose safety risks. Therefore, mastering scientific maintenance methods and fault prevention techniques is crucial for ensuring stable operation.
Operational Characteristics and Maintenance Requirements of Cryogenic ASUs
Cryogenic ASUs separate oxygen, nitrogen, and argon from the air through low-temperature rectification. The unit mainly consists of compression, purification, heat exchange, and rectification systems. With operating temperatures as low as -190°C and continuous operation, high demands are placed on material properties and equipment reliability.
Daily maintenance must focus on several key areas: the lubrication status of rotating equipment, insulation effectiveness of static equipment, stress changes in piping, and instrument accuracy. These seemingly routine maintenance points can trigger chain reactions in a cryogenic environment. For example, a minor valve leak can cause the perlite insulation in the cold box to freeze, subsequently damaging pipes.
Key Daily Maintenance Points for Critical Equipment
- Compressor Maintenance Management: The compressor is the heart of the ASU. Maintenance priorities differ between piston and centrifugal compressors. For centrifugal compressors, critical parameters include shaft vibration, axial displacement, and lube oil pressure and temperature. Basic daily inspections involve listening for unusual sounds, measuring temperature, and checking oil levels. It's recommended to analyze lubricating oil quarterly and determine oil changes based on results rather than a fixed schedule.
- Purification System Regeneration Control: The molecular sieve purification system removes moisture, carbon dioxide, and acetylene from the air. Regeneration effectiveness directly impacts the operational safety of the downstream cold box. Operators should monitor changes in the regeneration temperature curve. A decrease or earlier occurrence of the peak regeneration temperature may indicate reduced adsorption capacity or damaged molecular sieves. Additionally, internal leakage of switching valves is easily overlooked; an annual infrared thermography check of valve closure is advisable.
- Cold Box and Heat Exchanger Insulation Maintenance: Ice formation within the perlite insulation layer of the cold box is difficult to manage. Daily monitoring should focus on temperature changes in the cold box foundation and casing. Localized icing or temperature anomalies require immediate analysis and potentially localized sand removal for inspection. Channel blockage in plate-fin heat exchangers is common; monitoring changes in resistance and temperature differentials can help predict blockages early.
Common Fault Types and Preventive Measures
- Distillation Column Pressure Fluctuations: Pressure fluctuations are often related to feed rate, cooling balance, or product withdrawal. Experience shows that many fluctuations stem from false instrument signals or sticking control valves. Prevention involves regular inspection of control valves, checking diaphragm condition, positioner, and stem movement. It's also recommended to calibrate critical instruments during every scheduled maintenance shutdown.
- Acetylene Accumulation Risk: Acetylene accumulation in liquid oxygen poses a significant safety hazard. Prevention relies on ensuring the purification system's adsorption efficiency. Furthermore, a portion of liquid oxygen must be continuously drained from the main condenser to prevent impurity concentration. The accuracy of online acetylene analyzers is critical; monthly calibration with standard gas is recommended.
- Rising Bearing Temperature in Rotating Equipment: Increased bearing temperature is typically linked to lubrication or alignment issues. Sometimes, it's caused by degraded oil or restricted oil flow. Preventive measures include regularly cleaning oil coolers, monitoring oil filter differential pressure, and maintaining proper oil levels. For long-running units, annual vibration and oil analysis are recommended to track equipment condition trends.
Condition-Based Maintenance Strategies
With advances in sensor technology and data analysis, condition-based maintenance is replacing traditional scheduled maintenance. For cryogenic ASUs, a trend management system for key parameters can be established.
- Temperature Trends: Focus on changes in compressor stage discharge temperatures, expander inlet/outlet temperatures, and the warm end temperature difference of the main heat exchanger.
- Pressure Trends: Monitor changes in compressor inter-stage pressure drop, purification system resistance, and distillation column pressure drop.
- Vibration Trends: Establish vibration history records for high-speed rotating equipment like expanders and compressors.
When a parameter shows a trending change, technical analysis should be initiated to develop countermeasures. For instance, a gradual increase in compressor inter-stage pressure drop might indicate a fouled intercooler, allowing for online cleaning to avoid a shutdown.
Planning and Execution of Turnaround Maintenance
Even with excellent daily maintenance, cryogenic ASUs require periodic scheduled shutdowns for comprehensive inspection. The maintenance interval should consider the unit's operating condition, failure history, and production schedule.
Prior to a turnaround, problems identified during the previous operating cycle should be reviewed, and a detailed maintenance task list created. Key tasks include inspecting piping supports inside the cold box, repairing valve sealing surfaces, fully calibrating instruments, performing offline safety valve checks, and cleaning heat exchangers.
Cold box sand removal and inspection is a major overhaul task. Adequate preparation is essential, including erecting protective shelters, securing sufficient perlite bags, and arranging adequate personnel. During sand removal, carefully inspect piping supports for loosening, the cold box interior for abnormal wear, and the foundation for settling. When backfilling perlite, ensure proper compaction to prevent localized settling during operation.
Management Measures for Fault Prevention
Beyond technical measures, management practices are equally important.
- Establish comprehensive equipment files containing design data, installation records, maintenance history, and failure analysis reports. This data is fundamental for analyzing failure patterns and developing preventive strategies.
- Operator competence and diligence directly impact equipment performance. Conduct regular technical training on equipment principles, common fault diagnosis, and emergency procedures. Encourage operators to document even minor anomalies during operation to avoid overlooking potential issues.
- Spare parts management is a crucial element of fault prevention. Maintain a reasonable inventory of wear-prone parts for critical equipment, such as compressor bearings, seals, and valve diaphragms. Ensure spare parts quality, as substandard parts can lead to failures.
Maintenance and fault prevention for cryogenic air separation units constitute a systematic undertaking that necessitates an integrated approach combining both technical and managerial measures. Through rigorous daily maintenance, condition-based monitoring, scientifically planned overhauls, and standardized management protocols, equipment operational cycles can be significantly extended while effectively reducing failure rates. Each unit possesses distinct operational characteristics. Maintenance personnel must not only master fundamental principles but also continuously optimize their strategies based on their specific unit's actual operating conditions. Only through meticulous attention to detail can the safe and stable operation of the unit be guaranteed, thereby maximizing value creation for the enterprise.
