1 Introduction
Reciprocating air compressors are used for lubrication of working parts such as cylinders, pistons, piston rings, packings and other direct contact with compressed air. They are called cylinder lubrication [1-2], and most of them use pressure lubrication. The principle mainly relies on special oil filling. The cylinder is forced to be lubricated under pressure, and the oil injection point and the oil injection amount can be controlled. Each lubrication point is supplied by a separate oil pipe.
The lubrication of the reciprocating air compressor, in addition to the self-lubricating material, requires the injection of a proper amount of lubricating oil on the surface of the relative motion to form a certain thickness of the oil film to reduce friction, wear and reduce power loss. In addition, the lubricant can also act as a blockage, seal gas, and wash cooling. Therefore, the correct selection of the grade and oil injection of the lubricating oil is an important basis for ensuring the safe and reliable operation of the reciprocating air compressor.
Lubricating oils vary greatly with the development of base oils and additives, and the types of compressors. In actual application, lubricating oils suitable for compressors should be selected according to the working conditions of the compressors, combined with the physical and chemical indicators of lubricating oils.
2 Lubricating oil development outline
Reciprocating air compressor oil is mainly divided into mineral oil type and synthetic oil type. China began to research and produce mineral oil type lubricating oil in the 1960s against the Soviet Union OCT1861-54 standard [3], and released the Ministry of Petrochemical Industry. Standard SY-1216-66, there are only two grades of HY-13 and HY-19 in this standard, HY-13 has no additive, HS-19 has added anti-oxidant preservative, and the oil at that time could not meet the high pressure and high temperature work. In other cases, it is easy to produce carbon deposits and damage the piston and air valve. After that, the SY-1216-77 version of the standard was revised and improved on a sub-base, but the number is still small.
In 1981, ISO began to release the ISO6743 lubricant standard family. In 1987, the first national standard for air compressor lubricants, ISO6743-3A/3B (3B for refrigeration compressors) was released. In 2003, ISO released the 6743-3:2003 standard, eliminating the distinction between 3A and 3B [4].
China officially implemented GB 12691-90 in 1991 and officially replaced SY-1216-77 in 1992. This standard mainly specifies the technical conditions of mineral oil type air compressor oil, which is divided into light load (DAA) and medium load (DAB) 2 oil products according to exhaust pressure, temperature, stage pressure ratio and final exhaust pressure. It is suitable for oil-lubricated reciprocating air compressors, and is divided into 32#, 46#, 68#, 100#, 150#5 grades according to the 40°C kinematic viscosity, which is increased by 3 compared with the original standard.
In 1992, China officially released GB/T 7631.9-92. In 1997, it was first revised in accordance with ISO 6743-3A:1987 and ISO6743-3B:1988, and the GB/T7631.9-1997 standard was issued [ 5]. The standard is divided into three categories: light load (DAA), medium load (DAB), and heavy load (DAC). There are 15 types of 5 grades. The standard is commonly known as DAA and DAB as mineral oils, and DAC is synthetic oil. In 2014, after the second revision, the GB/T7631.9-2014 standard was issued. The standard is equivalent to the ISO 6743-3:2003 standard, and the DAC classification is eliminated. The DAA and DAB contain mineral oil and synthesis in addition to the load distinction. Oil [4], DAA mainly includes deep refined mineral oil, semi-synthetic and fully synthetic oil. DAB mainly includes special formulated mineral oil, semi-synthetic and fully synthetic oil. According to the research, this has a great relationship with the screw type in the air compressor market.
At present, the working conditions of reciprocating air compressors are gradually developing to “three highs”, namely high pressure, high pressure difference and high discharge temperature [6]. The development trend of air compressor lubricants is high reliability and long life. With the increasingly harsh conditions of air compressors, the requirements for lubricating oils are increasing, and synthetic lubricants with high temperature oxidation stability, low carbon deposition tendency, good viscosity and temperature performance, low volatility and long service life are obtained. Rapid development [7-8].
3 Air compressor oil physical and chemical indicators
3.1 Viscosity
Viscosity is the most important indicator of lubricating oil, defined as the ratio of internal friction coefficient to density at a certain temperature. A large viscosity means that the internal frictional resistance during lubrication is large, which will increase the lubrication power loss; if the viscosity is too small, an oil film of a suitable thickness cannot be formed, which affects the lubrication performance and increases the friction work and wear.
The viscosity of the lubricating oil also changes with temperature. Generally, the lubricating oil changes little when it is low temperature, and it changes greatly when it exceeds 60 °C. The degree of viscosity change with temperature is the viscosity index, defined as the ratio of viscosity at 40 ° C to 100 ° C. When the viscosity index is above 80, it is often considered to be a high viscosity index lubricant. Since the compressor is repeatedly cooled and heated during use, it is often required to properly lubricate the viscosity index.
The oil, ie the viscosity, does not change much with temperature.
3.2 Flash point
When the lubricating oil is heated, the surface begins to form oil vapor. When the vapor pressure of the oil is heated under atmospheric pressure to reach the lower limit concentration ignited by the open flame, the temperature at which the transient combustion occurs is the flash point.
The flash point is too high, indicating that the oil fraction is heavy, the viscosity is high, the asphaltene content is high, and it is easy to deposit carbon when used; the flash point is too low, when the temperature inside the cylinder is too high, it is prone to combustion explosion, causing personal injury and equipment damage. . As an important indicator of oil safety, flash point must be suitable. The general literature suggests that the flash point should be higher than the operating temperature of 20~30 °C.
3.3 Oxidation stability
Oxidation stability means the ability of an oil to maintain its properties against the action of the atmosphere or oxygen. In an air compressor, the oil film formed on the surface of the cylinder metal is catalyzed by metal and air.
The oxygen in the chemical reaction occurs to produce products such as fatty acid, tar, and bitumen, which makes the color of the lubricating oil black, the viscosity and acid value increase, and the colloidal deposits can be precipitated to block the oil passage, the oil filter, the gas valve, and the like.
Poor oxidation stability will shorten the life cycle of the lubricant and increase the probability of compressor failure or damage. It is an indicator that needs to be controlled. The appropriate oil can be prepared by selecting suitable base oil and adding antioxidants.
3.4 Carbon deposition tendency
The carbon deposition tendency is measured by the percentage of carbon deposits produced by the metal wall. A part of the lubricating oil discharged from the cylinder and the compressed air is attached to the surface of the exhaust valve and the exhaust pipe in a wet state, at high pressure and high temperature, in the metal. The surface is prone to generate carbon deposits on the surface, which affects the safe operation of the equipment. Practice has proved that high-viscosity oil is prone to carbon deposition, and low-viscosity naphthenic oil sometimes produces a large amount of carbon deposits [9]. In order to prevent carbon deposition, lubrication of suitable viscosity should be selected according to compressor load and pressure temperature conditions. oil.
3.5 other
Anti-wear and anti-emulsification in reciprocating air compressor oil are also important indicators. Anti-wear performance represents the anti-wear performance of the compressor in use. The laboratory uses a friction and wear tester. The small wear marks indicate that the compressor oil has good wear resistance and lubricity. The anti-emulsification property multi-characterizes the separation performance of the oil from liquid water, and the shorter the separation time, the better the demulsibility.
