Multiple precautions and safety guidelines for the use of compressor lubricants

Multiple precautions and safety guidelines for the use of compressor lubricants

Lubricants of different brands cannot be mixed. Mixing oils of different quality and grades will reduce the performance of the oil, and produce viscous deposits and lacquer deposits, which will block the oil supply system and affect the pressure delivery of the oil. Under normal circumstances, the oil in the oil cooler, oil and gas separator and oil system should be replaced every 2000h, and the filter should be replaced or cleaned at the same time. If working in a dusty or high humidity environment, the oil change interval should be shortened. At present, the lubricating oil is generally sampled and analyzed regularly, and whether to change the oil is determined according to the sampling results.

The factors affecting the service life of the oil are: 1. The inhaled air contains dust, iron oxide, carbon dioxide and other substances that can accelerate the deterioration of the oil; 2. The unclean discharge of used oil will shorten the service life of the newly added oil; 3. Compression The high operating temperature of the machine can accelerate the oxidation and deterioration of the oil; 4. Water leaking from the oil cooler enters the oil. Reasonable and safe application of compressor lubricating oil The basic task of compressor lubrication is to allow the lubricating oil to form a liquid layer (oil film) between the opposed friction surfaces, which is used to reduce friction, reduce the power consumption of the friction surface, and also cool the movement mechanism The friction surface seals the working volume of compressed gas. At the same time, lubricating oil also has secondary functions such as noise absorption, cleaning, anti-rust, anti-corrosion, shock absorption and buffering. The choice of compressor oil depends on factors such as compressor structure, compressed gas composition, exhaust temperature and exhaust pressure. As shown in Table 1, the discharge temperature of different types of compressors is also different.

The correct choice of lubricating oil According to the analysis of the reasons for the fire and explosion of the compressor unit, the most dangerous factor is the spontaneous combustion of oil carbon deposits. Part of the lubricating oil of the air compressor forms an oil film on the inner surface of the cylinder, and the molecules of other lubricating oils are suspended in the end surface of the cylinder piston, piston ring groove, exhaust pipe, valve, wind bag and separator. It is oxidized under the action of hot air to form carbides, which gradually increase and become carbon deposits, which are flammable. Oil carbon deposits can not only promote the spontaneous combustion of combustible mixtures, but also evaporate oil from the deposits, and continue to form combustible hydrocarbon mixtures. Therefore, the compressed air containing gas and liquid two-phase lubricating oil has the conditions for explosion.

The dynamic characteristics of the lubricating oil in the cylinder and piping of the compressor largely depend on the quality of the lubricating oil. Therefore, the correct selection of lubricating oil is of great significance to the normal operation of the compressor.

It is wrong to blindly pursue the use of high flash point lubricants, because the flash point and the auto-ignition temperature have nothing to do with each other. The temperature of compressed air is usually not higher than 180-200°C. Under such conditions, spontaneous combustion of lubricating oil vapor or carbon deposits may not occur. The flash point is determined by the oil vapor pressure with explosion hazard, that is, the physical properties of the fuel; the ignition and spontaneous combustion temperature of the fuel depends on its thermal stability. For hydrocarbons with the same chemical structure, the boiling point of complex molecular substances is higher than that of simple molecular substances, and the vapor pressure of the former is lower than the latter, or the flash point of the former is higher than the latter. However, as the molecular structure becomes more complex, its thermal stability and auto-ignition temperature will both decrease.

Lubricants with a high flash point are prone to produce more carbon deposits. When the flash point is lower, not only the carbon deposits are less but also stable. When it is difficult to predict the dynamic characteristics of lubrication in the compressor and the formation of carbon deposits under certain process conditions, special attention should be paid to minimize the carbon deposit. Therefore, do not use lubricants with a high flash point.

　According to the research on the oxidation rate and chemical composition of the oil, the carbon deposit of naphthenic oil is less than that of commonly used paraffinic oil. In order to delay the oxidation process and reduce carbon deposits, it is effective to add antioxidant additives to the compressor oil. Commonly used antioxidant additives include phenolic compounds and aromatic ammonia organic sulfides.

From the perspective of the formation of oil carbon deposits, the most essential characteristics of compressor lubricants are the evaporation performance and viscosity related to the fraction. A better lubricating oil should not contain low-viscosity and high-viscosity wide fractions, but should be a homogeneous narrow fraction. Because the heat effect in the cylinder is very strong, it is hoped that the lubricating oil injected into the cylinder will leave the cylinder quickly after lubricating the friction surface of the cylinder in an instant. Otherwise, the lubricating oil will be excessively oxidized due to the effect of temperature, resulting in an increase in carbon deposits. This requires the lubricating oil in the cylinder to evaporate quickly and be brought to the cold zone in the compressor unit along with the air flow.

In the air flow, the movement of the liquid fraction of the oil and the evaporation in the process are affected by the viscosity of the oil. The higher the viscosity, the higher the evaporation temperature. In other words, the greater the viscosity, the longer the oil stays in the hot zone of the compressor unit, and the more carbon deposits there will be.

Lubricant fire and explosion prevention measures

To completely solve the safety problem of lubricating oil in compressors is to completely eliminate combustible substances in the system, that is, to use oil-free lubrication compressors. Due to the limitations of cost, service life and operational reliability, it is neither economical nor reasonable to use oil-free compressors in certain applications. In the compressor with oil lubrication, how to solve the safety problem of lubricating oil must find another way.

For compressors whose cylinders are lubricated by lubricating oil, the measures to ensure their safe operation are:

(1) Improve the physical and chemical properties of oil. Under the condition of ensuring the normal friction and wear of the cylinder and piston components, even if the flash point is lower, lubricating oil with better thermal stability and lower viscosity should be used.

In the refining process of lubricating oil, it is necessary to ensure that the oil has an appropriate amount of pitch aromatic components to prevent oxidation of the lubricating oil. Additives to prevent oxidation and carbon deposition can be added to the lubricating oil.

(5) Improve oil working conditions. The working temperature in the compressor cylinder should be reduced as much as possible, and the area and time for the high temperature to act on the lubricant should be limited.

(6) The use of high-quality lubricating oil, when the working temperature is not higher than 150℃ or lower than the flash point of the lubricant 28℃, can be considered to ensure the safe operation of the compressor. But limiting the cylinder temperature does not guarantee absolute safety, because even for the same compressor, local overheating may occur at normal temperatures. Under certain conditions, the oxidation reaction that occurs after the oil carbon deposits absorb lubricating oil is an exothermic reaction. It itself becomes the combustion source of temperature increase and has no direct relationship with the temperature in the cylinder. It is very important to avoid the accumulation of lubricating oil in the compressed air circulation area. If carbon deposits and rust are immersed in the lubricating oil, the circulation of the lubricating oil will be strongly prevented. With stainless steel material or a special coating on the inner surface of the pipeline, it is difficult for the hot zone of the pipeline to form oily carbon deposits. Normal selection of lubricating oil consumption can greatly 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 the original. Generally speaking, the cylinder lubricating oil consumption specified by the compressor manufacturer is relatively high, so under normal circumstances, the lubricating oil consumption can be determined by operating tests.

In order to reduce the oil carbon deposit in the pipeline, the cooler should be installed as close to the cylinder as possible. An efficient oil and gas separator should be considered behind the compressor. In the compressed air piping system, structural defects that promote the accumulation of oil vapor or carbon deposits, such as dead zones in the container, blind pipes and sudden changes in pipe diameters, should be avoided.

(7) Eliminate oily carbon deposits. It is impossible to completely avoid the oxidation and decomposition of oil, so the aging lubricating oil and its decomposition products in the compressed air system should be removed timely and regularly. When the container is blown off, in addition to blowing off the oil-water emulsion and compressed condensate, another function is to ventilate and purify the dead zone of the container.

my country's "Safety Rules and Operating Regulations for Stationary Air Compressors" stipulates that any carbon deposits on oil-lubricated air compressors should be effectively removed regularly. For general power air compressors, the number of inspections and removals should be such that the thickness of the carbon layer does not exceed 3mm. This thickness is considered a safe thickness. (4) Prevent the formation of explosive concentrations of oil and air mixtures. In order to enable the compressed airflow to effectively take away the oily carbon deposits, from a safety point of view, the airflow velocity of each section of the compressor unit should be correctly selected.

According to Copco's research: When the air velocity exceeds 8m/s, the oil moves faster along the inner wall of the exhaust pipe. When the company designs the compressor cooler, the air velocity is about 10m/s.

When the compressor is running dry or performing other methods of displacement adjustment, the adjustment time to reduce the displacement should not be too long, because the oil supply will increase relatively sharply, which will increase the concentration of the explosive components of the mixture.

(8) Eliminate combustion sources. In order to avoid sparks caused by static electricity accumulation, the compressor unit should be grounded, which should be paid special attention to in the mobile compressor unit. Impact and abnormal seizure of parts may cause high temperature. Therefore, the container and pipeline parts are not allowed to loosen, and abnormal friction and seizure of moving parts in the crankcase are not allowed. Flammable sealing materials should not be used.

Eliminate the fault that causes the temperature of the compressed air to rise. Such as air valve leakage, air leakage through the piston ring in a double-acting or step-differential cylinder, will cause the temperature of the stage intake to increase, thereby increasing the exhaust temperature accordingly. When cutting off the intake air to adjust the air volume, the compression end temperature in the cylinder will increase sharply, so the adjustment time should not be too long.

Special attention should be paid to timely removal of oil carbon deposits, because it is itself a combustion source for spontaneous combustion due to the exothermic reaction of oxidation.

(6) Spray liquid cooling. Inject water into the compressor cylinder or intake pipe, and use the effect of evaporation to greatly reduce the temperature in the compressor cylinder and the temperature in the exhaust pipe, so that the amount of carbon deposits is greatly reduced. The oily carbon deposit formed under the conditions has a soft structure, and is easy to peel off under the pulsation of the airflow and be carried away by the airflow. According to relevant tests, even as long as regular water spraying, quite satisfactory results can be obtained. Research on this subject was carried out abroad about 40 years ago. On the premise of increasing the safety of the compressed air system, it can not only reduce the exhaust temperature but also reduce the compression power consumption.