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This post was last edited by Zhongyuanren on 2023-5-25 at 15:55. Introduction to Centrifugal Steam Compressors I. Working Principle The main components of a centrifugal compressor used for compressing gases are the high-speed rotating impeller and the diffuser, whose flow area increases gradually. In short, the operating principle of a centrifugal compressor is that the impeller does work on the gas; within the flow channels of the impeller and diffuser, centrifugal pressure rise and deceleration-induced diffusion processes are utilized to convert mechanical energy into the pressure energy of the gas. A turbine or electric motor drives the impeller of the centrifugal compressor’s main shaft to rotate at high speed; the centrifugal force generated by this rotation throws the gas drawn in from the center of the impeller toward the back of the working wheel, where it enters the diffuser, which has an increasingly larger airflow passage. At the center of the working wheel, a region of thin gas is formed; the gas that needs to be compressed flows continuously from the inlet at the center of the working wheel into the impeller. As the working wheel keeps rotating, the gas is continuously thrown outward (similar to a centrifugal pump), thereby maintaining a continuous flow of gas within the compressor. The gas is pressurized and accelerated due to the centrifugal force, and exits the working wheel at high speed. However, after passing through the diffuser, its speed decreases gradually; part of its kinetic energy is converted into static pressure energy, resulting in an even further increase in pressure. If the pressure generated by a single impeller still does not meet the requirements, it is possible to achieve the desired outlet pressure by connecting multiple impellers in series (similar to multi-stage pumps). For centrifugal compressors, the number of stages can be as many as ten or more; the inter-stage connection is accomplished through bends and return channels. When a centrifugal compressor is in operation, gas enters the compressor body through the inlet. After being compressed within the first impeller, it is drawn from the outlet of that impeller to the center of the second impeller. This process continues through all the impellers, and finally the gas is discharged through the exhaust outlet. This is the working principle of a centrifugal compressor. As the gas pressure increases step by step, the gas volume decreases accordingly; consequently, the impellers also become smaller step by step. After being compressed several times, the temperature of gas 0 rises significantly. Excessively high gas temperatures can compromise the safety of the compressor; therefore, compressors that require multiple compression stages to reach the desired outlet pressure are divided into several sections. In this way, each section comprises several stages, and the stages within a given section can be either single-cylinder or multi-cylinder. Intermediate condensers and gas-liquid separators must be installed between sections in order to reduce the temperature of the gas entering the next section and to remove the condensable components from the gas. The greater the compression ratio of a gas, the higher its temperature rise becomes. The thermal stress generated by this temperature rise increases as well, which in turn has a greater impact on the safety of the compressor equipment. Therefore, it is necessary to install intercoolers between stages, and these intercoolers form an important part of the compression system – the cooling system. II. Problems encountered with lubricating oil during operation and their solutions: During operation, steam compressors require lubricating oil to lubricate and cool the bearings and gears. After surveying numerous MVR users, we found that after the compressor operates continuously for 2 to 3 months, lubricant emulsification often occurs – with a paste-like, milky sediment at the bottom of the oil tank and a large amount of foam at the top. Lubricating oil was taken for monitoring and analysis; the emulsion water content was as high as 10–15%. 1. The hazards of emulsified oil: If the oil does not have the ability to quickly and completely separate water that has mixed into it, the oil will become emulsified. This reduces or even eliminates the oil’s lubricating properties, accelerates its oxidation and deterioration, and increases wear on mechanical components as well as corrosion of equipment. 1) High corrosivity, reducing the service life of various components ； 2) The adhesion decreases, the bearing oil film is damaged, resulting in dry friction between the bearing and the rotor, which damages the bearing bushes ； 3) The acid value increases, the anti-emulsification capacity declines, and the oil sludge formed tends to clog the filter before the pump ； 4) Fluidity decreases, reducing the ability to remove heat from the bearings, which increases the risk of bearing wear. 2. Reasons for the production of emulsified oil: The fundamental cause of lubricant emulsification is water vapor entering the lubricant system from the gearbox cavity. 1) Isolation gas – The saturated steam pressure is too high, and a large amount of steam is supplied, causing water vapor to enter the gearbox cavity ； 2) Excessively large gas seal gap ； 3) When the steam compressor is in operation, there is a pressure difference between the compressor chamber, the isolation gas area, the atmosphere, and the gearbox chamber; the pressure of the isolation gas is higher than that in the gearbox chamber. Moreover, the exhaust space on the side connected to the atmosphere is too small, preventing the water vapor discharged from that side from being removed in a timely manner ； 4) When the steam compressor is in operation, the water generated by the condensation of saturated steam moves along with the rotation of the main shaft and enters the gearbox chamber ； 5) Before the steam compressor starts operating, the gearbox chamber is under negative pressure, while the pressure of the inert gas introduced is 0.98 MPa.G; water vapor then enters the gearbox chamber. 3. Regeneration of emulsified oil: When the water content in emulsified oil is less than 15%, the lubricating oil can be regenerated and reused. The emulsified oil is fed by a lubricating oil pump into the cyclone separator-heat exchanger for oil-vapor separation. The emulsified oil is heated to 70–80°C using saturated steam. The cyclone separator maintains a vacuum level of -0.07 to -0.05 MPa.G, thanks to the vacuum pump (which is already part of the MVR system). The steam present in the emulsified oil evaporates through centrifugal force under negative pressure, and the lubricating oil that results from this process is filtered before being returned to the oil tank. When the water content in the emulsified oil exceeds 15%, the lubricating oil is demulsified and regenerated through physical separation combined with chemical refining, namely acid-alumination separation and sulfuric acid refining. The emulsified oil is discharged through the waste discharge valve into the heating reaction tank (equipped with a stirrer), where it is stirred and heated to 70–80°C. After adjusting the pH value with sulfuric acid, aluminum sulfate and sodium hydroxide are added to cause flocculation and precipitation. Oil separation follows, after which concentrated sulfuric acid is used for oxidation. Finally, the mixture is washed and allowed to settle; after adding additives, the oil meets the standards of new oil. The lubricant level in the steam compressor oil tank is approximately 400–1200 L; therefore, an integrated regeneration unit can be added to the MVR system to enable the regeneration of emulsified oil. 4. Methods to prevent lubricant emulsification 1) Optimizing the sealing device: The sealing of steam compressors can be divided into gas seals and oil seals. Gas seals can be dry gas seals, inflated labyrinth seals, honeycomb seals, or abradable seals, etc. Oil seals can be labyrinth seals, oil slinger rings, helical seals, etc. Considering both equipment cost and sealing performance, it is recommended that: A. An inflatable labyrinth seal be used for the gas seal, the sealing teeth should be replaced with spiral teeth, and the isolation gas should be discharged through a volute; two exhaust chambers of appropriate size should be established based on the pressure of the isolation gas, atmospheric pressure, and the pressure inside the gearbox chamber ； B. The oil seal uses a labyrinth seal, and a spiral seal as well as an oil-scattering ring of special shape are installed on the main shaft. 2) Add sealing gas: If site utility conditions permit, introduce a stream of instrument air or nitrogen on the lube oil side for sealing and isolation. 3) Optimize the system startup: before the steam compressor starts up, switch from isolation gas to instrument air to prevent lubricating oil from entering the compressor chamber. Once the exhaust pressure of the compressor exceeds the pressure in the gearbox chamber, switch the isolation gas to the volute exhaust, and activate the instrument air on the lubricating oil side. 5. Countermeasures for problems: Steam compressors often experience lubricant emulsification during operation, which is mainly caused by water vapor entering the gearbox cavity. By taking appropriate measures, the recurrence of lubricant emulsification can be prevented. Even in the event of operational errors on-site, the device enables the regeneration of emulsified oil, thereby extending the service life of lubricants, reducing operating costs, and ensuring the stable and reliable operation of steam compressors. 1) An integrated regeneration unit—a cyclone separator-cooler—is added to the MVR system to remove water vapor from the emulsified oil. Shutong Database