For new oils, regardless of the influence of additives, the higher the degree of refinement of petroleum products, the less acidic substances. For old oils, acid substances increase due to oxidative deterioration or hydrolysis reaction during the use process. These acidic substances not only reduce oil products. Lubricity also accelerates equipment wear and corrosion, increasing equipment failure rates. Therefore, in the later use stage of industrial lubricating oil, the change of its acid value should be paid attention to. Hazards of Acidic Substances in Industrial Lubricants The acidic substances of industrial lubricating oil have a corrosive effect on the copper, iron, aluminum and other metal materials used in the equipment components, which will affect the normal operation of the equipment and reduce the service life of the equipment; in addition, the generated metal salts accumulate into sludge and other sediments, It will improve the conductivity of the oil, reduce the insulation performance of the oil, and also affect the heat dissipation performance of the equipment. In the hydraulic system, it will cause the hydraulic control valve spool to block and accelerate the wear of the spool.
Sources of Acids in Industrial Lubricants Naturally occurring acids in crude oil Some non-hydrocarbon compounds contained in crude oil, such as fatty acid, naphthenic acid, aromatic acid, phenol, etc., are collectively referred to as petroleum acid, accounting for about 1% to 2% of the mass of crude oil, of which the mass of naphthenic acid accounts for the total amount of petroleum acid. About 90% of the mass, it is the most important acidic oxygenate in the Base Oil. According to the requirements of China National Petroleum Corporation's enterprise standard Q/SY 44-2009 "General Lubricant Base Oil Standard", the acid value of class I base oil is 0.02~0.05 mgKOH/g, and the acid value of class II and III base oil does not exceed 0.01 mgKOH/g . Additives in oil It is difficult for the base oil to meet the requirements of equipment use. Various additives such as antioxidants, extreme pressure anti-wear agents, cleaning and dispersing agents, and anti-foaming agents are added according to the operating environment of the equipment to improve the comprehensive performance of oil products. In extreme pressure antiwear agents, alkyl acid phosphate, zinc dialkyl dithiophosphate (ZDDP), etc. are acidic additives. For gear oils and hydraulic oils that require extreme pressure and anti-wear properties, the new oil acid value is generally around 0.5 mgKOH/g, while the new oil acid value of turbine oil does not exceed 0.2 mgKOH/g. Oxidation in the use of oil produces acidic substances The oxidation of oil products is divided into two stages: low temperature oxidation and high temperature oxidation. Low temperature oxidation forms peroxides, alcohols, aldehydes, ketones and water. When the temperature exceeds 120 °C, high temperature oxidation forms acidic substances. The formation of acidic substances is through alkylation. Reaction of oxygen radicals with aldehydes. The oxidation of oil is not only related to its own chemical composition, but also closely related to the conditions of use, such as temperature, air pressure or oxygen partial pressure, and the catalytic effect of metal ions in contact. Acidic substances produced by hydrolysis of fire-resistant hydraulic oil Fire-resistant hydraulic oil (mainly refers to polyester and phosphate ester fire-resistant hydraulic oil, no water glycol hydraulic oil) is prone to hydrolysis when it encounters water, and the hydrolysis products mainly include acid phosphoric acid diester, acid phosphoric acid monoester, phosphoric acid and phenols Substances and other acidic substances. Control of Acidic Substances in Industrial Lubricants The acid value of industrial lubricating oils contributed by base oils and additives can be controlled through production deacidification and formulation design. Oil temperature control ● Electric heater manufacturers or system complete equipment manufacturers generally recommend suitable electric heating power according to the installed capacity of oil products and use experience. There is no relevant theoretical argument for the influence of oil viscosity and flow rate, and the country has no relevant selection standards. Therefore, when replacing the electric heater during the later equipment maintenance, do not easily increase the surface area power, especially for high-viscosity oil, so as not to cause the local oil temperature to be too high and lead to premature aging of the oil. ● The design and layout of the oil pipeline is reasonable, and the high temperature area should be avoided as much as possible. If it cannot be avoided, the thermal insulation layer should be added. ● Reasonable selection of oil cooler, and control the oil temperature of the oil tank within the range required by the system. Generally, for hydraulic oil, the oil temperature of the oil tank should be controlled at 40~50 °C, and the maximum temperature should not exceed 55 °C; for turbine oil, it is advisable to control the inlet oil temperature of the bearing (bearing bush) not to exceed 45 °C. Moisture control It is mainly aimed at the problem of hydrolysis of fire-resistant hydraulic oil when it encounters water. It is required that the moisture should be controlled below 0.1% (mass fraction), and when the moisture content exceeds the standard, a dehydration filter device should be used to remove the moisture in time. In addition, an air filter with drying function can be installed on the top of the fuel tank to reduce the influence of water vapor in the environment and condensed water inside the fuel tank. Periodic acid value testing It is recommended that if the oil temperature of the equipment is lower than 50 ℃, the acid value should be tested once every six months; when it exceeds 50 ℃, the acid value should be tested once a quarter; when it exceeds 70 ℃, the acid value should be tested once a month. According to the experience of use, it is recommended to refer to the oil manufacturer's recommended value for the control range of the oil acid value increase value. Oxidation or hydrolysis will inevitably occur during the use of industrial lubricants, resulting in acidic substances. It is suggested that the anti-oxidation performance of the oil should be reasonably considered according to the operating conditions and temperature of the equipment. Periodic detection and tracking of the acid value of the oil should be done during the operation of the equipment. Slow down the wear and corrosion of equipment caused by acidic substances and prolong the service life of oil products.
