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Classification, performance, and selection of turbine oils, as explained by a lubricant tester from Corsaid. Turbines, also known as turbojets, include steam turbines and gas turbines; they are rotary thermal power machines that use steam or gas as the working medium. They offer advantages such as high individual power output, high efficiency, stable operation, and a long service life. The main use of steam turbines is as prime movers for power generation, accounting for about 80% of the total electricity produced. Since turbines can operate at variable speeds, they can be used to directly drive various pumps, fans, compressors, and ship propellers. Based on their applications, turbines can be classified into power station turbines, industrial turbines, marine turbines, etc. A steam turbine must work in coordination with a steam generator (boiler), driving machinery such as generators, as well as condensers, heaters, pumps, etc., to form a complete set of equipment. A gas turbine is also a complete power unit composed of three main parts – the compressor, the combustion chamber, and the turbine – along with corresponding auxiliary equipment. Turbines are widely used in industries such as the power sector, petrochemicals, steel manufacturing, and large ships. 1) Functions of turbine oil The role of turbine oil in steam turbines and gas turbine units is the same; it primarily serves for lubrication, cooling, and speed control. (1) Lubrication function The turbine oil is delivered to the space between the main shaft and bearing shells of the turbine unit’s sliding bearings by a lubricating oil pump, where an oil wedge is formed to provide fluid lubrication. In addition, turbine oil is also used to lubricate frictional components such as gear reducers and speed control mechanisms. (2) Cooling and heat dissipation During operation, the turbine unit can reach a speed of 3000 rpm; the internal friction of the shaft and lubricating oil generates a large amount of heat. Whether it is steam or gas that is used in the turbine, the heat generated by these substances is also transferred to the bearings through the impeller. If this heat is not removed in a timely manner, it can severely affect the safe operation of the unit, and may even lead to accidents such as the melting of the main shaft. Therefore, the turbine oil must circulate continuously within the lubrication system to remove heat from the bearings, thereby providing cooling and ensuring that the normal operating temperature of the bearings remains below 60°C. (3) Speed control function The turbine oil used in the turbine speed control system actually acts as a hydraulic medium, transmitting the pressure generated by the control mechanism to enable speed control of the turbine. 2) Properties of turbine oil Appropriate viscosity is a key factor in ensuring proper lubrication of the turbine unit. The requirements for the viscosity of lubricating oil in turbines vary depending on the turbine’s design. Turbines that use pressure cycling require turbine oil with a low viscosity ； For small steam turbines lubricated by oil rings, heat transfer from the rotating shaft affects the adhesion of the oil film on the shaft, so oils with higher viscosity are required ； Small turbo-generator sets and marine turbines equipped with speed reduction devices also require oils with higher viscosity to ensure proper lubrication of the gears. To ensure proper lubrication of the turbine unit at different temperatures, the turbine oil is required to have good viscosity-temperature properties, with a value generally above 80–90. ①Good oxidation stability: Although the operating temperature of turbine oil is not high, it is used in large quantities over extended periods of time. Moreover, due to exposure to air, moisture, and metals, oxidation reactions still occur, resulting in the formation of acidic substances and precipitates. The accumulation of acidic substances causes metal components to corrode, forms salts, and accelerates the oxidation of oils as well as reduces their emulsification resistance ； Oxides dissolved in oil increase the viscosity of the oil, thereby reducing its effectiveness in terms of lubrication, cooling, and power transmission ； The precipitated oxides can contaminate and clog the lubrication system, reducing cooling efficiency and causing abnormal oil supply. Therefore, turbine oil is required to have good oxidation stability, to age very slowly during use, and to have a service life of at least 5 to 15 years. ②Good anti-emulsification properties: During the operation of steam turbines, steam and water inevitably leak into the turbine oil from shaft seals or other areas. If the turbine oil lacks good anti-emulsification properties, it will not only result in the formation of emulsions that reduce lubrication performance, but it also accelerates the oxidation of the oil and causes corrosion of metals. Especially when lubricating oil is supplied using a pressure circulation method, the volume of turbine oil in circulation is large and it remains in a turbulent state; it tends to emulsify upon contact with water. Therefore, the lack of emulsification tendency is an important property of turbine oil. To ensure that turbine oil has good emulsification resistance, the base oil must be highly refined to minimize the presence of naphthenic acids, gums, and polycyclic aromatic hydrocarbons in it. ③Good rust and corrosion resistance: When water enters the lubrication system of steam turbine units, it not only causes the oil to emulsify but also leads to rust and corrosion of the metals. This is especially true for steam turbine units used on ocean-going ships, where seawater is used as the cooling medium in the lubricating oil coolers. Due to the high salt content in seawater, any leakage in the coolers can cause severe rusting of the metal components in the lubrication system. Therefore, turbine oil, especially that used in ocean-going vessels, must have good rust-resistant properties. Rust-resistant turbine oil is typically composed of highly refined mineral base oil to which additives such as antioxidants, rust inhibitors, metal deactivators, and antifoam agents are added. ④Good antifoaming properties. During cyclic lubrication, turbine oil can absorb air for the following reasons: (1) Air leakage from the oil pump ； (2) The oil level is too low, causing the oil pump to be exposed above the oil surface ； (3) Poor ventilation of the lubrication system ； (4) Excessive return oil in the lubricating oil tank ； (5) Excessive return flow on the return oil pipeline ； (6) The oil discharge rate of the pressure control valve is too fast ； (7) Impurities in the oil ； (8) The oil pump delivers excessive oil. When the air drawn into the turbine cannot be released in time, foaming occurs, which creates air blockages in the oil circuit. As a result, the oil supply is insufficient, the lubrication effect declines, and the cooling efficiency decreases. In severe cases, this can even cause the oil pump to run dry and disrupt the control of the speed regulation system. To prevent foaming in turbine oil, in addition to operating in accordance with the turbine specifications and carrying out proper maintenance to minimize air absorption into the oil, it is also necessary for the turbine oil to have good antifoaming properties, enabling it to release any absorbed air promptly. ⑤Special properties of turbine oil Turbine oil used in a common lubrication system for compressors and turbines that operate with ammonia as the compression medium must possess ammonia resistance (ammonia-resistant turbine oil). To ensure the lubrication and safety of the high-pressure, high-speed systems as well as the hydraulic systems in large generator sets, turbine oil with extreme pressure and anti-wear properties as well as flame resistance is required. Such turbine oil contains extreme pressure and anti-wear additives, which grant it strong load-carrying capacity.