Q&A on the operation of centrifugal compressors

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1. What components make up the lubricating oil system of a centrifugal compressor? The lubricating oil system consists of a lubricating oil station, a high-level oil tank, intermediate connection pipelines, as well as control valves and monitoring instruments. The lubricating oil station consists of an oil tank, an oil pump, an oil cooler, an oil filter, a pressure control valve, various monitoring instruments, as well as oil pipelines and valves. 2. What is the function of a high-position fuel tank? The high-level oil tank is one of the safety measures for the machine unit. During normal operation, the lubricating oil enters from the bottom and exits from the top to return directly to the oil tank. In the event of a power outage that causes the machine to stop, if the auxiliary oil pump cannot start in time to supply oil, the lubricating oil in the high-level oil tank will flow through various lubrication points via the inlet pipelines before returning to the oil tank, thereby ensuring an adequate supply of lubricating oil during the machine’s coasting phase. 3. What are the characteristics of centrifugal compressors? A centrifugal compressor is a type of turbine compressor that features large gas handling capacity, compact size, simple structure, stable operation, easy maintenance, and gas that remains free from oil contamination; it also allows for the use of various drive methods. 4. What is the working principle of a centrifugal compressor? Generally speaking, the main goal of increasing gas pressure is to raise the number of gas molecules per unit volume, that is, to reduce the distance between gas molecules. To achieve this goal, gas dynamics methods are employed, which involve the use of mechanical working elements such as high-speed rotating impellers. Work is done on the gas, causing its pressure to increase due to centrifugal forces; at the same time, its kinetic energy also increases significantly. Later, in the diffuser section, this kinetic energy is converted back into static pressure energy, further raising the gas pressure. This is the working principle of a centrifugal compressor. 5. What are the common prime movers for centrifugal compressors? Common prime movers for centrifugal compressors include electric motors, steam turbines, gas-fired turbines, etc. 6. What are the auxiliary equipment of centrifugal compressors? The operation of the centrifugal compressor unit is contingent on the proper functioning of the auxiliary equipment, which includes the following aspects: (1) Lubrication system. (2) Cooling system. (3) Condensate water system. (4) The electrical instrumentation system is the control system. (5) Dry gas seal system. 7. What types of centrifugal compressors are there based on their structural characteristics? Based on their structural features, centrifugal compressors can be classified into types such as horizontally split, vertically split, isothermal compression, and combined types. 8. What components does the rotor consist of? The rotor includes a main shaft, impeller, shaft sleeve, shaft nut, spacer sleeve, balance disc, and thrust disc. 9. What is surge in a centrifugal compressor? During operation, centrifugal compressors can sometimes experience sudden intense vibrations. The flow rate and pressure of the gas medium also fluctuate significantly, accompanied by periodic dull \"humming\" sounds, as well as loud \"whooshing\" noises caused by the fluctuations in airflow within the piping system. This phenomenon is known as surge condition in centrifugal compressors. The compressor cannot operate for long periods under surge conditions; once it enters such conditions, the operator must immediately take corrective actions by reducing the outlet pressure, or increasing the inlet or outlet flow rate, in order to bring the compressor out of the surge zone and ensure its stable operation. 10. What are the characteristics of surge phenomenon? Once surge occurs during the operation of a centrifugal compressor, the operation of the unit and the piping system exhibits the following characteristics: (1) The outlet pressure and inlet flow rate of the gas medium change significantly, and sometimes gas backflow may also occur. It is a dangerous operating condition when the gas medium flows from the compressor outlet back toward the inlet. (2) The pipeline network experiences periodic vibrations with large amplitudes and low frequencies, accompanied by periodic \"roaring\" sounds. (3) The compressor casing vibrates intensely; both the casing and bearings experience severe vibration, along with a loud, periodic airflow noise. Due to these intense vibrations, the lubrication conditions of the bearings are compromised, the bearing shells can be damaged, and even the shaft may break. Friction and collisions between the rotor and stator occur, and the sealing elements suffer serious damage. 11. How to perform anti-surge adjustment? The hazards of surge are severe, but it has not yet been possible to eliminate it through design; instead, efforts must be made during operation to prevent the unit from entering a surge condition. The principle of surge prevention involves addressing the causes of surge, and by increasing the compressor’s flow rate just before surge occurs, ensuring that the unit operates outside the surge zone. There are specifically three methods to prevent surge: (1) partial gas venting method. (2) Partial gas reflux method. (3) Method of changing the compressor operating speed. 12. What are the reasons for liquid present at the compressor inlet? (1) The process gas delivered from the previous system is at a high temperature, and it is not completely condensed; furthermore, the gas delivery pipes are too long, resulting in liquid present in the gas after condensation within the pipes. (2) The temperature in the process system is high, causing the components with lower boiling points in the gaseous medium to condense into a liquid. (3) The liquid level in the separator is too high, resulting in gas-liquid entrainment. 13. What are the reasons why the compressor operates below the surge limit? (1) The outlet backpressure is too high. (2) The valves in the inlet pipeline are throttled. (3) The valves in the export pipeline are throttled. (4) The anti-surge valve is defective or not adjusted properly. 14. What are the methods for adjusting the operating conditions of centrifugal compressors? Since process parameters in production inevitably change, it is often necessary to manually or automatically adjust the compressor so that it can operate under varying conditions to meet production requirements and maintain the stability of the production system. There are generally two types of regulation for centrifugal compressors: one is isobaric regulation, which involves adjusting the flow rate while keeping the back pressure constant ； Another method is constant-flow regulation, which involves adjusting the exhaust pressure of the compressor while maintaining a constant flow rate. Specifically, there are five such adjustment methods: (1) Outlet flow regulation. (2) Import flow regulation. (3) Change the speed regulation. (4) Adjust by rotating the inlet guide vanes. (5) Partial venting or backflow regulation. 15. What do equal pressure control, equal flow control, and proportional control mean? (1) Isobaric regulation refers to a type of regulation in which the exhaust pressure of the compressor is kept constant, while only the gas flow rate is changed. (2) Constant-flow regulation refers to a type of regulation that maintains the flow rate of the gas medium delivered by the compressor constant, while only changing the discharge pressure. (3) Proportional control refers to control that maintains a constant pressure ratio (such as anti-stall control), or maintains a constant percentage of volumetric flow rate for the two gas media. 16. What is a pipeline network? What are its components? The pipeline network is the piping system used by centrifugal compressors to transport gaseous media. The pipe located before the compressor inlet is called the suction pipe, while the pipe located after the compressor outlet is called the discharge pipe. The combination of the suction pipe and the discharge pipe constitutes a complete piping system, which is commonly referred to as the pipeline network. Piping systems generally consist of four elements: pipes, fittings, valves, and equipment. 17. What are the hazards of axial force? High-speed rotating rotor. An axial force directed from the high-pressure side to the low-pressure side is always present. Under the action of axial force, the rotor will undergo axial displacement in the direction of that force. This axial displacement of the rotor causes relative sliding between the shaft journal and the bearing shells. Therefore, it is possible for the journal or bearing shells to be damaged. More seriously, due to rotor displacement, friction and collision between rotor components and stator components can occur, leading to mechanical damage. The axial force exerted by the rotor poses a risk of friction, wear, collision, and even machine destruction. Hence, effective measures should be taken to achieve balance in order to improve the operational reliability of the unit. 18. What are the methods for balancing axial forces? The balance of axial forces is a crucial issue that must be taken into account during the design of multi-stage centrifugal compressors. Currently, two main methods are generally used: (1) Opposite arrangement of impellers (with the high-pressure side and low-pressure side of the impellers facing each other). The axial force generated by a single-stage impeller points toward the inlet of the impeller, that is, from the high-pressure side to the low-pressure side. If multiple impellers are arranged in this sequential manner, the total axial force on the rotor will be the sum of the axial forces of each individual impeller; obviously, such an arrangement results in a very large axial force on the rotor. If multi-stage impellers are arranged in opposition, the impellers on opposite sides of the inlet generate axial forces in opposite directions, which can balance each other out; therefore, this opposed arrangement is the most common method for balancing axial forces in multi-stage centrifugal compressors. (2) Installation of a balance disk: The balance disk is an axial force balancing device commonly used in multi-stage centrifugal compressors. It is usually installed on the high-pressure side, and a labyrinth seal is provided between its outer edge and the cylinder, thereby maintaining a certain pressure difference between the high-pressure side and the low-pressure side connected to the compressor inlet. The axial force generated by this pressure difference acts in the opposite direction to the axial force produced by the impeller, thus compensating for the axial force caused by the impeller. 19. What is the purpose of balancing the axial force on the rotor? The purpose of rotor balancing is mainly to reduce axial thrust and lessen the load on the thrust bearings. In general, 70% of the axial force is eliminated by the balance disk, while the remaining 30% is borne by the thrust bearings. Production experience has shown that maintaining a certain amount of axial force is an effective measure to ensure the smooth operation of the rotor. 20. What are the reasons for the increase in the temperature of thrust bearings? (1) The structural design is unreasonable; the bearing area of the thrust pads is small, resulting in an excessive load per unit area. (2) Failure of the inter-stage seal allows the gas exiting the impeller of the subsequent stage to leak into the preceding stage, increasing the pressure difference on both sides of the impeller and thereby generating a large thrust force. (3) The balance tube is blocked, preventing the pressure in the balance disc’s secondary pressure chamber from being released, and thus the balance disc cannot function properly. (4) Failure of the balance disk seal results in the working chamber pressure not being maintained at normal levels; the balancing capacity decreases, and part of the increased load is transmitted to the thrust bearings, causing them to operate under excessive load. (5) The throttle aperture for oil supply to the thrust bearing is small, resulting in insufficient flow of cooling oil; thus, the heat generated by friction cannot be completely removed. (6) If the lubricating oil contains water or other impurities, a complete liquid lubrication layer cannot be formed on the thrust bearings. (7) The oil inlet temperature of the bearing is too high, resulting in a poor working environment for the thrust bearings. 21. How to deal with excessively high thrust bearing temperatures? (1) Check the compressive pressure on the thrust bearing pads, and appropriately increase their load-bearing area to keep the thrust load within the standard range. (2) Inspect the inter-stage seals for damage and replace any damaged parts. (3) Inspect the balance tube, remove any blockages, so that the pressure in the pressure chamber of the balance disc can be released in a timely manner, thereby ensuring the effective functioning of the balance disc’s balancing capability. (4) Replace the balance disk seal strip to improve its sealing performance, maintain the pressure in the balance disk’s working chamber, and ensure that the axial thrust is properly balanced. (5) Increase the diameter of the oil inlet hole in the bearing to boost the amount of lubricating oil, thereby allowing the heat generated by friction to be removed promptly. (6) Replace it with new, qualified lubricant to maintain its lubricating properties. (7) Open the inlet and outlet valves of the cooling coil in the Kaidai unit to increase the amount of cooling water and reduce the oil supply temperature. 22. How to deal with liquid at the compressor inlet? (1) Connect to the previous system and adjust the process operations. (2) This system appropriately increases the number of liquid discharge cycles of the separator. (3) Reduce the liquid level in the separator to prevent gas-liquid entrainment. 23. What are the reasons for the decline in the performance of compressor units driven by steam turbines? (1) The inter-stage seal of the compressor is severely damaged, resulting in reduced sealing performance and increased backflow of the gas medium. (2) The impeller is severely worn, resulting in a reduced performance of the rotor, and the gas medium does not receive sufficient kinetic energy. (3) The steam filter in the turbine is clogged, which hinders steam flow; as a result, the flow rate is low and the pressure difference is high. This affects the turbine’s output power and reduces the performance of the unit. (4) The vacuum level is below the specified requirement, causing obstruction in the turbine exhaust. (5) The steam temperature and pressure parameters are below the operational specifications; the internal energy of the steam is low, failing to meet the requirements for the unit’s operation. (6) Surge condition occurs. 24. What are the main performance parameters of centrifugal compressors? The main performance parameters of a centrifugal compressor include: flow rate, outlet pressure or compression ratio, power, efficiency, speed, etc. The main performance parameters of a device are the basic data that characterize its structural features, operating capacity, operating environment, etc.; they serve as important guiding materials for users when selecting devices and formulating plans. 25. What is the meaning of efficiency? Efficiency is a measure of the degree to which the energy transmitted by a centrifugal compressor to the gas is utilized; the higher this degree of utilization, the greater the efficiency of the compressor. Since gas compression involves three processes: polytropic compression, adiabatic compression, and isothermal compression, the efficiency of compressors is also divided into polytropic efficiency, adiabatic efficiency, and isothermal efficiency. 26. What does the compression ratio mean? The compression ratio we are referring to is the ratio of the pressure of the gas discharged by the compressor to the inlet pressure; therefore, it is sometimes also called the pressure ratio. 27. What is the function of sealing? To achieve good operational performance, a centrifugal compressor must maintain a certain gap between the rotor and the stator, in order to prevent friction, wear, as well as collisions and other types of damage. At the same time, the presence of gaps inevitably leads to leakage between stages and at the shaft ends. Such leakage not only reduces the compressor’s efficiency but also causes environmental pollution and may even result in explosion accidents. Therefore, leakage must not occur. Sealing is an effective measure to prevent leakage between compressor stages and at the shaft ends, while maintaining an appropriate gap between the rotor and the stator. 28. What are the different types of sealing devices based on their structural characteristics? What are the selection principles? Depending on factors such as the operating temperature and pressure of the compressor, as well as whether the gas medium is hazardous or not, different structural forms of seals are used, which are collectively referred to as sealing devices.