Centrifugal compressor preventive maintenance, what should be repaired? The case is comprehensive and focused.

A centrifugal compressor is a high-speed, high-precision, and highly reliable mechanical device. Its daily maintenance requirements are minimal, but if a malfunction occurs, it can cause low compressed air pressure in the pipeline. If the on-duty personnel cannot arrive at the scene to inspect it in time, it may lead to production line downtime. Therefore, it is crucial to strengthen the maintenance and preventive maintenance of the air compressor and its supporting equipment, and only personnel who have received professional training can perform on-site repairs and inspections.

Working principle of centrifugal compressor

A centrifugal compressor, also known as a turbo compressor, is a type of turbomachinery. Its main goal is to increase the number of gas molecules per unit volume, which in turn increases gas pressure and shortens the distance between gas molecules. The working principle of a centrifugal compressor involves using an impeller to drive gas in high-speed rotation, causing the gas to experience centrifugal force and increase its flow velocity. As the gas flows through the impeller, it undergoes an expansion process that further increases its pressure. The gas then passes through an expansion chamber and diffuser, where its velocity decreases and its pressure increases, converting kinetic energy into static pressure. Centrifugal compressors have several advantages, including a wide range of displacement, continuous gas output, high-quality gas supply, compact structure, and high efficiency at large gas volumes. However, they are not suitable for very small gas flows and have a low single-stage compression ratio.

Centrifugal compressor preventive maintenance requirements

Centrifugal compressors are widely used in various industries due to their ability to provide higher flow rates and lower maintenance frequencies compared to similar-sized positive displacement compressors. With fewer moving parts and a more energy-efficient design, maintenance is simpler and the relative cost over the machine’s lifecycle is lower. However, environmental conditions, wear and tear, or inadvertent neglect can affect the performance of the centrifugal compressor. Therefore, preventive maintenance is necessary to extend the unit’s lifespan and improve system performance.

For this preventive maintenance of the centrifugal air compressor, a comprehensive cleaning of the equipment’s flow path and testing and adjustment of the related control system are required. This includes cleaning and spraying of the air duct, impeller, diffuser, and cooler, maintenance inspection of each working valve body, inspection of the internal structural components of the gearbox, and alignment of the motor.

The preventive maintenance requirements for a centrifugal compressor include the following items:

Inlet piping and head: ① Check and adjust intake and relief valves; ② Maintain and inspect the first, second, and third stage impellers and diffusers; ③ Clean and polish the first, second, and third stage guide vanes, and apply corrosion-resistant high-temperature coatings; ④ Clean the first, second, and third stage coolers, polish and spray the chamber, and clean the oil cooler water circuit; ⑤ Replace parts, align the main motor, and perform interlock testing.
Lubrication system: ① Check the lubrication system piping connections; ② Check the lubrication system one-way valve; ③ Check and adjust the oil pressure regulating valve of the lubrication oil system; ④ Check the main oil pump and replace the oil pump shaft seal; ⑤ Check the auxiliary oil pump and replace the shaft seal; ⑥ Check the oil filter and replace the filter element; ⑦ Check the oil heater; ⑧ Check the oil cooler, clean the water side, and replace the sealing gasket; ⑨ Check the instrument air filter and regulator valve; ⑩ Check the breathing apparatus and replace the filter element; Check and adjust the negative pressure gauge of the oil tank.
Air cooling system: ① Disassembly, rust removal, anti-rust coating, and assembly of the first, second, and third stage inlet and outlet pipes; ② Inspection and cleaning of the air side of the first, second, and third stage coolers; ③ Inspection, cleaning, rust prevention treatment, and gasket replacement of the front cover of the first, second, and third stages; ④ Inspection, cleaning, rust removal treatment, and coating of the first, second, and third stage cooling boxes; ⑤ Inspection of the first, second, and third stage drain valves.
Instrumentation and control system: ① Calibration of the low-speed rotor vibration detection system; ② Inspection of the shaft vibration sensing and indication alarm system; ③ Inspection of the pressure sensing and indication alarm system; ④ Calibration of the instrumentation air pressure switch; ⑤ Testing of the main oil pump, auxiliary oil pump, and oil level switch; ⑥ Inspection of the temperature sensing and indication alarm system; ⑦ Inspection of the control system and panel; ⑧ Inspection of the set values; ⑨ Analysis of operating data; ⑩ Testing of the control system interlock device; Inspection, dust removal, tightening, and checking of relays, wiring terminals, and PLC power supply CPU I/O modules and communication modules.

Rotor and cooler components require special attention

The rotor and cooler components are crucial to the centrifugal air compressor, and regular inspection and proper maintenance are key to maintaining system reliability and high performance. In addition to the large gears and drive equipment, other moving parts in the centrifugal compressor include the rotor – one of the most important components in the entire system. During operation, the rotor rotates at high speed and is composed of components such as impellers, spindles, balance discs, and thrust discs. In daily maintenance and normal operation, solid impurities may inevitably accumulate on the surface of the impeller, causing increased vibration of the rotor at all levels and reducing the efficiency of the compressor. Therefore, careful inspection, cleaning of surface impurities, and probing to identify hidden dangers are necessary to reduce machine wear, maintain the compressor in optimal working condition, and extend the life of the rotor. Dynamic balance correction can also effectively reduce machine wear, and this step can be performed on-site or in a professional dynamic balance testing room.

Cooler components are typically made of copper or stainless steel. Copper has a higher heat exchange efficiency than stainless steel, but is slightly less corrosion-resistant and more expensive. Therefore, the appropriate material should be chosen based on the actual situation. Proper maintenance can improve the performance and lifespan of the cooler. If properly maintained, the heat exchanger can work for a longer period of time. When using it, attention should be paid to the heat exchange temperature difference, which is equal to the air outlet temperature minus the cooling water inlet temperature. In general, it will be higher than 8℃ and it is difficult to achieve lower than 6℃, so it should not exceed 15℃. The cleaning time of the unit depends on the quality of the water source and the operating conditions of the equipment. The heat exchange temperature difference can also be used as an indicator for determining the cleaning cycle of the cooler. The cooler is generally cleaned using measures such as soaking in chemicals and washing with a water gun. When cleaning, care should be taken not to damage the cooler body.

Improvement effect

A centrifugal compressor can operate more efficiently and safely with preventative maintenance. In this case, preventative maintenance was performed on the compressor, resulting in improved performance. Prior to maintenance, the specific power was 6.5 kW/(m3/min) and the airflow was 112.55 m3/min. After maintenance, the specific power decreased to 6.3 kW/(m3/min) and the airflow increased to 115.45 m3/min. This resulted in a 0.2 kW/(m3/min) decrease in specific power and a 2.9 m3/min increase in airflow.

Epilogue

To ensure the safe and efficient operation of equipment, regular maintenance inspections of critical components are necessary. One aspect of this is to regularly check the functionality of the compressor and use equipment debugging methods to detect potential faults and prepare for troubleshooting. Additionally, it is important to disassemble and inspect major components such as rotors and cooling systems to identify any wear or lubrication issues and address them promptly to improve system efficiency.