The lubricant for steam compressors tends to emulsify – is it due to the oil or a problem with the equipment?

Thread Content

Steam compressors MVR (Mechanical Vapor Recompression) are primarily used in energy-saving and environmental protection projects, as well as in achieving zero discharge of industrial wastewater, in industries such as hydrometallurgy, chemicals, pharmaceuticals, food, and beverages. MVR increases the pressure and temperature of all the secondary steam generated through mechanical compression, returning it to the evaporator to replace raw steam as a heat source, thereby achieving energy savings. 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 Hazards of emulsified oil: If an oil cannot effectively separate water that has mixed into it quickly and completely, the oil will become emulsified, which reduces or even eliminates its 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 bushings ； 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 introduced, causing water vapor to enter the gearbox chamber ； 2) Excessive air 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 Emulsion oil regeneration: When the water content of the emulsion oil is less than 15%, the lubricating oil can be regenerated and reused. The emulsified oil is pumped by a lubrication pump into a cyclone separator-cooler for oil-gas 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 separated lubricating oil 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 drain 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 left to stand; after adding additives, the oil meets the standards of new oil. The lubricating oil level in the steam compressor 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) Optimize the sealing device. Steam compressor seals can be divided into gas seals and oil seals. Gas seals can be dry gas seals, pneumatic labyrinth seals, honeycomb seals, or abradable seals, while oil seals can be labyrinth seals, slinger rings, spiral seals, etc. Taking into account the 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 provided, based on the pressure of the isolation gas, atmospheric pressure, and the pressure within the gearbox chamber ； B. The oil seal uses a labyrinth seal, and a spiral seal as well as an oil-scattering ring of special design are installed on the main shaft. 2) Increase seal gas: If the on-site utility conditions permit, add an additional stream of instrument air on the lubricating oil side for sealing purposes. 3) Optimize system startup: Before starting the steam compressor, 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 Conclusion: During operation, steam compressors often experience lubricant emulsification, which is mainly caused by water vapor entering the gearbox cavity. By taking appropriate measures, the emulsification of lubricating oil can be prevented from occurring again. 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 in order to remove water vapor from the emulsified oil. 2) Optimizing the sealing structure of the steam compressor and the system startup process, as well as increasing the supply of instrument air, can effectively prevent steam from entering the gearbox cavity. If you have anything to say, feel free to leave a message to communicate!