Operation mechanism and common faults of centrifugal compressors

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I. Overview of Centrifugal Compressors 1. Working Principle Generally speaking, the main goal of increasing gas pressure is to increase the number of gas molecules per unit volume, that is, to reduce the distance between gas molecules. Methods that can be used to achieve this goal include: 1. Positive displacement compression methods that use squeezing elements to compress gas (such as piston-type) ； 2. Using gas dynamics principles, that is, by having the machine’s working elements (high-speed rotating impellers) do work on the gas, the pressure of the gas is increased within the centrifugal force field; at the same time, its kinetic energy also increases significantly. Subsequently, as the gas flows through the diffuser channel, this kinetic energy is converted back into static pressure energy, thereby further raising the gas pressure. This is the operating principle or pressurization mechanism of centrifugal compressors. By applying the principles of gas dynamics, once the medium enters the impeller channel, the rapidly rotating blades cause the medium to rotate as well, generating centrifugal force. Under this force, the medium is ejected from the channel, acquiring kinetic and pressure energy, thereby achieving the conversion of mechanical energy. Subsequently, the medium enters the diffuser, where its flow velocity decreases, causing some of the kinetic energy to be converted into pressure energy, thereby achieving an increase in pressure. 2. Characteristics of centrifugal compressors. Why are centrifugal compressors widely used? (1) It has a large gas discharge volume; the gas flows continuously through the centrifugal compressor, the flow cross-sectional area is large, and the impeller rotates at high speed, resulting in a high gas flow velocity and thus a large flow rate. (2) Compact structure and small size. It is much smaller than piston-type ones with the same volume ； (3) Smooth and reliable operation, long continuous operating time, low maintenance costs, and minimal requirement for operators ； (4) It does not contaminate the compressed gas, which is very important for chemical production ； (5) The rotational speed is relatively high, making it suitable for direct drive by a steam turbine or gas turbine. (6) There is no lubrication inside the cylinder, so the process gas is not contaminated by lubricating oil. II. Failures in centrifugal compressors 1. Fault analysis: Failures can occur in the lubrication system, filters, sealing devices, coolers, and bearings of centrifugal compressors. (1) Compressor surging (2) Sudden drop in oil pressure ； (3) Decrease in gas outlet flow rate ； (4) Increase in bearing temperature. 2. Causes of centrifugal compressor failures and corrective measures. 2.1 Compressor surge: The internal reasons for surge are closely related to the impeller design as well as the gas present within the flow channels. When the flow rate of the incoming gas decreases suddenly and falls below the allowable minimum level, there is a significant deviation in the direction of airflow within the compressor as well as in the installation angle of the blades at their inlets. This results in severe \"rotational separation\" of the airflow within the blade channels, causing the gas to stagnate there and leading to a sudden drop in compressor pressure. However, the pressure in the outlet system does not drop instantly; as a result, the high-pressure gas in the exhaust pipe flows back into the compressor, replenishing the flow rate within the blade channels and allowing normal operation to resume. When the flow rate inside the compressor decreases again, gas flow reverses in the system, and this process repeats itself. Consequently, periodic oscillations of the airflow occur within the system, accompanied by intense noise – this is what constitutes compressor surge. Solution: The operator should have the compressor performance curve with the surge line marked, so as to always know the position of the compressor’s operating point on the performance curve. For the sake of operational safety, a surge prevention line can be added at a point where the flow rate is 5%–10% higher than that on the surge line, to alert the operator. Reducing the operating speed can decrease the flow rate without causing surge, but the outlet pressure decreases as a result. Guide vane rotating mechanisms are installed at the primary stage or at various stages to adjust the angle of the guide vanes, so that the inlet shock angle does not become too large when the flow rate decreases, thereby preventing surge. A bypass pipe is installed at the compressor outlet; when it is necessary to reduce the flow rate delivered by the compressor during operation, the excess gas is either vented or sent back into the intake pipe after being depressurized. It is preferable to consume more flow rate and power rather than allow the compressor to operate with an insufficient flow rate, in order to prevent it from entering a surge condition. 2.2 Sudden drop in oil pressure: The reasons for a sudden drop in oil pressure are related to the lubrication system, such as the oil pump and oil pipes. When the oil pump fails, the pressure drops; the oil pump should be inspected promptly to identify the cause of the fault. When the fault is related to the oil tube, it is caused by a rupture in the tube resulting in a leak; the solution is to replace it with a new one. 2.3 Decrease in gas outlet flow rate The main reasons for the decrease in gas outlet flow rate are the filter and the sealing device. A reduction in air intake caused by a clogged gas filter can be resolved by cleaning the gas filter. The reason for the failure of the sealing device is an excessive sealing gap, which causes leakage. The best solution is to replace the seal, or adjust it as specified. 2.4 Increased bearing temperature: An increase in bearing temperature is related to the quality of the lubricating oil and the performance of the bearing bushes. When the lubricant temperature rises, the oil cooler should be turned up to bring the lubricant back to an appropriate temperature. If the oil pressure drops or decreases, the oil pump should be checked and its pressure adjusted as required. The issue with the bearing pads is their poor performance; the bearing should be inspected and replaced if necessary. Another reason is an increase in axial thrust or improper assembly of the thrust bearing. At this point, the process parameters should be adjusted to reduce the axial thrust ； Check the thrust bearings as necessary and adjust the sealing clearances. III. Conclusion: To extend the service life of centrifugal compressor units and ensure their safe and reliable operation, those responsible for troubleshooting mechanical issues should keep these units free from dirt and oil, and maintain them regularly through inspections and upkeep. Additionally, a system of designated operators should be implemented, along with detailed maintenance plans. To ensure the service life of the entire centrifugal compressor unit as well as its safe operation, it is necessary to strengthen the maintenance and repair of the sealing devices, filters, lubrication system coolers, and bearings. The causes of failures in centrifugal compressors are closely related to their maintenance, repair, operation, installation and commissioning, as well as manufacturing. To effectively prevent failures, addressing them should primarily involve repair; repair helps to reduce the amount of spare parts needed, minimize the processing of components, and save materials, thereby shortening repair times and reducing repair costs.