Change in acid value of industrial lubricating oil, eliminate potential equipment failures!
June 09, 2023
For new oil, without considering the influence of additives, the higher the degree of refining of petroleum products, the less acidic substances there are For old oil, due to oxidation deterioration or hydrolysis reaction during use, acidic substances increase. These acidic substances not only reduce the lubricity of the oil, but also accelerate equipment wear and corrosion, increasing equipment failure rate. Therefore, in the later stage of use of industrial lubricating oils, it is important to focus on the changes in their acid value.
1. Hazards of acidic substances in industrial lubricating oils The acidic substances in industrial lubricating oils have a corrosive effect on metal materials such as copper, iron, and aluminum used in equipment components, which will affect the normal operation of the equipment and reduce its service life; In addition, the generated metal salts aggregate into sediment such as oil sludge, which will improve the conductivity of the oil product, reduce the insulation performance of the oil product, and also affect the heat dissipation performance of the equipment. In the hydraulic system, it will cause problems such as hydraulic control valve core jamming and accelerated valve core wear. 2. Sources of acidic substances in industrial lubricating oils 1) Acidic substances naturally present in crude oil Some non hydrocarbon compounds contained in crude oil, such as fatty acids, naphthenic acids, aromatic acids, phenols, etc., are collectively referred to as petroleum acids, accounting for about 1% to 2% of the total mass of crude oil. Among them, naphthenic acids account for about 90% of the total mass of petroleum acids, making them the most important acidic oxidizing compounds in Base Oil. According to the enterprise standard Q/SY 44-2009 "General Lubricant Base Oil Standard" of China National Petroleum Corporation, 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 oils does not exceed 0.01 mgKOH/g. 2) Additives in oil products It is difficult for the base oil to meet the requirements of equipment use. Various additives such as antioxidants, extreme pressure anti wear agents, detergents and dispersants, and anti foaming agents are added according to the operating environment of the equipment to improve the comprehensive performance of the oil product. Alkyl acid phosphate ester and zinc dialkyldithiophosphate (ZDDP) are acidic additives in extreme pressure anti wear agents. For Gear Oil and hydraulic oil with requirements for extreme pressure and anti-wear performance, the new oil acid value is generally about 0.5mgKOH/g, while the new oil acid value of turbine oil is not more than 0.2 mgKOH/g. 3) Acidic substances generated by oxidation during the use of oil products The oxidation of oil products can be 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 ℃, high-temperature oxidation forms acidic substances. The formation of acidic substances is through the reaction of alkyl peroxide radicals with aldehydes. The oxidation of oil is not only related to its own chemical composition, but also closely related to usage conditions, such as temperature, air pressure or oxygen partial pressure, and the catalytic effect of metal ions in contact. 4) Acidic substances generated by the hydrolysis reaction of fire-resistant hydraulic oil Flame resistant hydraulic oil (mainly referring to polyester and phosphate ester flame resistant hydraulic oil, without water containing ethylene glycol hydraulic oil) is prone to hydrolysis reaction when it comes into contact with water, and the hydrolysis products mainly include acid phosphate diester, acid phosphate monoester, phosphoric acid and phenol.
3. Control of acidic substances in industrial lubricating oils
The acid value of industrial lubricating oil products contributed by base oil and additives can be controlled through production deacidification and formula design.
1) Oil temperature control
Manufacturers of electric heaters or complete system equipment generally recommend suitable electric heating power based on the installed capacity and usage experience of oil products. There is no relevant theoretical demonstration on the influence of oil viscosity and flow rate, and there is no relevant national selection standard.
So when replacing the electric heater during later equipment maintenance, do not easily increase the surface area power, especially for high viscosity oil, to avoid causing local oil temperature to be too high and causing premature aging of the oil.
The design and layout of the oil pipeline should be reasonable, avoiding high temperature areas as much as possible. If it is impossible to avoid it, an insulation layer should be added.
Reasonably select an oil cooler and control the oil temperature in the tank within the system requirements. Generally, for hydraulic oil, the oil temperature in the oil tank is controlled between 40~50 ℃, with a maximum temperature not exceeding 55 ℃; For steam turbine oil, it is advisable to control the inlet oil temperature of bearings (bearing shells) to not exceed 45 ℃.
2) Moisture control
Mainly aimed at the problem of water hydrolysis of fire-resistant hydraulic oil, it is required to control the moisture content below 0.1% (mass fraction). When the moisture content exceeds the standard, a dehydration and filtration device should be used in a timely manner to remove the moisture. In addition, an air filter with drying function can be installed on the top of the fuel tank to reduce the impact of water vapor in the environment and condensation inside the fuel tank.
3) Regular acid value testing
It is recommended to conduct an acid value test every six months for equipment with an oil temperature below 50 ℃; When the temperature exceeds 50 ℃, conduct an acid value test every quarter; When the temperature exceeds 70 ℃, conduct a monthly acid value test. Based on usage experience, it is recommended to refer to the recommended values of the oil manufacturer for the control range of the added acid value of the oil product.
The use of industrial lubricating oils inevitably leads to oxidation or hydrolysis, resulting in the production of acidic substances. It is recommended that when selecting oil products, their antioxidant performance should be reasonably considered based on the operating temperature of the equipment.
During the operation of the equipment, periodic detection and tracking of the acid value of oil products should be carried out, and the system oil temperature and moisture content of fire-resistant hydraulic oil should be reasonably controlled to slow down equipment wear and corrosion caused by acidic substances and extend the service life of the oil products.
