Lubricant and the safe operation of compressors

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Lubricating Oil and the Safe Operation of Compressors I. Compressor Oil and Fire/Explosion Risks - ©Air Separation Home -- ----A resource for air separation operators and managers. 　　Based on the analysis of the causes of fires and explosions in compressor systems, the most dangerous factor is the spontaneous combustion resulting from oil carbon deposition. Oil carbon deposits can not only cause flammable mixtures to ignite spontaneously, but also cause oil to evaporate from these deposits, continuously forming flammable hydrocarbon mixtures. Thus, compressed air containing a lubricating oil in gas and liquid phases creates the conditions for an explosion. ^2 © Air Separation Home -- ----A platform for air separation operators and managers. 　　The dynamic properties of Ux lubricant within the compressor’s cylinders and piping systems depend to a large extent on the quality of the lubricant. Therefore, choosing the right lubricant is of great importance for the proper operation of the compressor. dFgHa © Air Separation Home -- ----A community for air separation operators and managers. 　　It is a mistake to solely pursue the use of lubricants with a high flash point, as there is no relationship between flash point and auto-ignition temperature. The temperature of compressed air usually does not exceed 180–200°C, and under such conditions, lubricant vapor or carbon deposits do not necessarily catch fire on their own. The flash point is determined by the vapor pressure of oil that poses an explosion risk, that is, a physical property of the fuel ； The ignition and auto-ignition temperatures of fuel depend on its thermal stability. For hydrocarbons with the same chemical structure, complex molecular substances have higher boiling points than simple molecular substances, while their vapor pressures are lower; in other words, their flash points are higher than those of the simpler substances. However, as the molecular structure becomes more complex, both its thermal stability and ignition temperature decrease. ':sy8{ ©Air Separation Home -- ----A community for air separation operators and managers. 　　j Lubricants with a high flash point tend to produce more carbon deposits; those with a lower flash point result in fewer deposits and greater stability. When it is difficult to predict the dynamic behavior of lubrication in compressors and carbon deposits form under certain operating conditions, special attention should be paid to minimizing the thickness of these carbon deposit layers. Therefore, do not use lubricants with a too high flash point. ^/^(^ ©Air Separation Home -- ----A forum for air separation operators and managers. 　　E Based on studies of the oxidation rate and chemical composition of oils, naphthenic oils produce less carbon deposition compared to commonly used paraffinic oils. To delay the oxidation process and reduce carbon deposition, adding antioxidant additives to compressor oil is effective. Common antioxidant additives include phenolic compounds and aromatic amino organic sulfides, among others. Jt\; ©Air Separation Home -- ----A community for air separation operators and managers. 　　B. From the perspective of the formation of oil carbon deposits, the most essential properties of compressor lubricating oil are its evaporation characteristics and viscosity related to the distillate. A good lubricant should not contain wide ranges of low and high viscosities, but rather be a homogeneous narrow-range substance. Because the thermal effects inside the cylinder are very intense, it is desired that the lubricating oil injected into the cylinder leave it quickly after instantly lubricating the friction surfaces of the cylinder. Otherwise, due to the effect of temperature, the lubricating oil will oxidize excessively, leading to an increase in carbon deposits. This requires the lubricating oil inside the cylinder to evaporate rapidly and be carried along with the airflow to the cold area of the compressor unit. @e< ©Air Separation Home -- ----A platform for air separation operators and managers. 　　In an air flow, both the movement of the liquid fraction of the oil and the evaporation that occurs during this process are influenced by the viscosity of the oil. The higher the viscosity, the higher the evaporation temperature. In other words, the higher the viscosity, the longer the oil stays in the hot areas of the compressor unit, resulting in more carbon deposits. :jFrYe © Air Separation Home -- ----A platform for air separation operators and managers. 　　Ums3 II. Measures to Prevent Fires and Explosions UE © Air Separation Home – A forum for air separation operators and managers. 　　UX completely solves the safety issue of lubricating oil in compressors by eliminating all combustible substances within the system, that is, by using oil-free lubrication compressors. Due to constraints related to cost, service life, and operational reliability, it is neither economical nor reasonable to use oil-free lubrication compressors in certain applications. To address the safety issues related to lubricating oil in oil-lubricated compressors, other approaches must be found. >XiE~e © Air Separation Home -- ----A community for air separation operators and managers. 　　\In compressor systems in which cylinders are lubricated with oil, the measures to ensure their safe operation are: {`{` ©Air Separation Home -- ----A platform for air separation operators and managers. 　　sGwA (1) Improving the physicochemical properties of oil © Air Separation Home – A forum for air separation operators and managers. 　　gX}>: Under the condition of ensuring normal friction and wear between the cylinder and piston assembly, it is necessary to use lubricants with good thermal stability and low viscosity, even if their flash point is slightly lower. qCy © Air Separation Home -- ----A platform for air separation operators and managers. 　　During the refining of lubricating oil, qY is used to ensure an appropriate amount of aromatic pitch components in the oil, thereby preventing its oxidation. cj~`Rc © Air Separation Home -- ----A community for air separation operators and managers. 　　f@") Additives that prevent oxidation and carbon deposition are incorporated into the lubricant. S © Air Separation Home -- ---- A platform for air separation operators and managers. 　　Fx@e (2) Improving the working conditions of oil b"2ixa © Air Separation Home – A forum for air separation operators and managers. 　　7 The working temperature in the compressor cylinders should be reduced as much as possible, in order to limit the area and duration to which high temperatures affect the lubricating oil. q © Air Separation Home -- ----A platform for air separation operators and managers. 　　s};2` By using high-quality lubricant, and when the operating temperature is no higher than 150°C or 28°C below the flash point of the lubricant, it can be assumed that the compressor will operate safely. However, limiting the cylinder temperature does not guarantee absolute safety, as even for the same compressor, local overheating can occur at normal temperatures. Under certain conditions, the oxidation reaction that occurs when oil carbon deposits absorb lubricant is an exothermic reaction. It itself becomes a source of combustion that raises the temperature, and it has no direct relationship with the temperature inside the cylinder. < ©Air Separation Home -- ----A platform for air separation operators and managers. 　　@ {. It is very important to prevent lubricant from accumulating in the areas where compressed air flows. If carbon deposits and rust penetrate the lubricating oil, it will severely prevent the flow of the lubricant. By using stainless steel materials or applying a special coating to the inner surface of the pipes, it becomes difficult for oil carbon deposits to form in the hot areas of the pipes. 7XXl* © Air Separation Home -- ----A community for air separation operators and managers. 　　#wqzd enables the proper selection of lubricant consumption, which can **reduce oil carbon deposits. According to relevant information, if the consumption of lubricating oil is reduced by half, the oil carbon deposits can be reduced to 1/20–1/30 of their original level. Generally, the oil consumption for cylinder lubrication specified by compressor manufacturers is on the high side; therefore, under normal circumstances, the actual oil consumption can be determined through operational testing. (?\^!v ©Air Separation Home -- ----A forum for air separation operators and managers. 　　To reduce oil carbon deposits in the pipes, the cooler should be installed as close as possible to the cylinders. An efficient oil-gas separator should be considered for installation behind the compressor. In compressed air piping systems, structural defects that can lead to the accumulation of oil vapor or carbon deposits should be avoided, such as dead zones in containers, dead ends, and areas where the pipe diameter changes suddenly. 0$n& © Air Separation Home -- ----A community for air separation operators and managers. 　　q0P (3) Remove oil carbon deposits 1s/Bdd ©Air Separation Home -- ----A platform for air separation operators and managers. 　　It is impossible to completely prevent the oxidation and decomposition of oil using F8. Therefore, the aged lubricating oil and its decomposition products in the compressed air system should be removed in a timely and regular manner. When purging a container, in addition to removing oil-water emulsions or compressed condensate, another function is to ventilate and clean the dead zones within the container. V{k%1) © Air Separation Home -- ----A platform for air separation operators and managers. 　　~3 China’s \"Safety Rules and Operating Procedures for Fixed Air Compressors\" stipulate that any carbon deposits in oil-lubricated air compressors must be removed effectively on a regular basis. For general power air compressors, the frequency of inspection and cleaning should be such that the thickness of the carbon deposit layer does not exceed 3 mm. This thickness is considered a safe thickness. !}U © Air Separation Home -- ----A platform for air separation operators and managers. 　　}. (4) Prevent the formation of an explosive concentration of oil and air mixture *u/? © Air Separation Home – A resource for those involved in air separation operations and management. 　　B To enable the compressed airflow to effectively carry away oil carbon deposits, it is necessary, from a safety perspective, to properly select the airflow velocity in each section of the compressor system. According to Swedish Atlas Copco o] © Air Separation Home -- ----A platform for air separation operators and managers. 　　^ Company research: When the air flow velocity exceeds 8 m/s, oil moves more rapidly along the inner wall of the exhaust pipe. When designing the compressor cooler, the air flow rate in that company is approximately 10 m/s. y:o © Air Separation Home -- ----A community for air separation operators and managers. 　　" When the compressor runs idle or other methods are used to adjust the exhaust volume, the adjustment time for reducing the exhaust volume should not be too long, as the oil supply volume will increase relatively sharply at this time, thereby raising the concentration of the explosive components in the mixture. L_ © Air Separation Home -- ----A platform for air separation operators and managers. 　　ce,3 (5) Eliminate the source of combustion l;/2& © Air Separation Home -- ----A forum for air separation operators and managers. 　　'To prevent sparks caused by static electricity buildup, the compressor unit should be grounded, and special attention should be paid to this in mobile compressor units. Both impacts and abnormal seizure of components can cause high temperatures; therefore, looseness in container and pipeline components is not allowed, nor is abnormal friction or seizure of moving parts inside the crankcase. Flammable sealing materials should not be used. +g>n7. © Air Separation Home -- ----A platform for air separation operators and managers. 　　$! Eliminate the faults causing the increase in compressed air temperature. If the air valve leaks, or if there is leakage through the piston rings in double-acting or stepped cylinders, this will lead to an increase in the intake air temperature, which in turn causes the exhaust temperature to rise as well. When cutting off the intake air to adjust the air volume, the temperature at the end of compression in the cylinder increases sharply; therefore, the adjustment time should not be too long. i © Air Separation Home -- ----A platform for air separation operators and managers. 　　g Of course, special attention should be paid to removing oil carbon deposits, as they themselves are a source of combustion that can catch fire due to exothermic oxidation reactions. K;1Ms © Air Separation Home -- ----A community for air separation operators and managers. 　　?Yowp (6) Liquid Cooling VR © Air Separation Home -- ---- A forum for air separation operators and managers. 　　'By spraying water into the compressor cylinders or intake pipes, the evaporation process is utilized to significantly reduce the temperature inside the compressor cylinders as well as in the exhaust pipes. This leads to a substantial reduction in the amount of carbon deposits. Moreover, the oil-based carbon deposits formed under these conditions are soft in structure, making them easy to flake off due to air flow fluctuations and be carried away by the airflow. According to relevant tests, even regular spraying of water can yield quite satisfactory results. Abroad, research on this topic was conducted about 40 years ago, with the aim of reducing exhaust temperature and compressive power consumption while enhancing the safety of compressed air systems. I