【Daily Question No. 334】2018.12.06

The flash point of lubricating oil indirectly reflects its thermal stability; lubricants with a low flash point are prone to thermal decomposition, and in the presence of flammable substances, they can easily lead to accidents. The thermal pyrolysis of lubricating oil destroys its lubricating properties, thereby shortening the service life of the equipment.

The flash point is an indicator of the volatility of lubricating oils. Lubricants with a low flash point have high volatility, are prone to catching fire, and pose a lower level of safety. Lubricating oil has a high volatility and tends to evaporate and be lost during operation; in severe cases, this can lead to an increase in the viscosity of the lubricating oil, affecting its effectiveness. If the flash point of the lubricating oil drops suddenly, it may indicate the presence of light oils mixed in. It is generally required that the flash point of the lubricating oil be 20 to 30 degrees Celsius higher than the operating temperature to ensure safety.

The flash point primarily affects safety performance; a low flash point indicates poor safety performance

Oils with a high flash point and high viscosity are more likely to cause carbon buildup, as well as increase frictional resistance and waste power. Practice has shown that the flash point increases as pressure rises, which means that lubricants under pressure have an increased safety factor.

The flash point of lubricating oil is the lowest temperature at which the vaporized oil on its surface ignites under specified test conditions. When the vaporized oil on the oil surface mixes with oxygen in the air, combustion can occur more easily if the flash point is low. Therefore, the main problem associated with a decrease in the flash point of lubricating oil is that it can lead to combustion or even fires. In addition, combustion can also damage seals and cause leaks. The higher the flash point of lubricating oil, the better, but a high flash point does not necessarily mean that the lubricant has excellent high-temperature performance.

Flash point: Under specified conditions, the sample is heated; when it reaches a certain temperature, the mixture of vapor from the sample and the surrounding air will catch fire upon contact with a flame. This temperature represents the lowest temperature at which flash ignition occurs in the sample. The methods for testing flash point are divided into open flash point and closed flash point; generally, the open flash point is used to determine the flash point of lubricants. For the same type of oil, its open flash point is 20–30°C higher than its closed flash point. 　When determining the open-cup flash point according to the standard method of GB/T 267-88, the sample is placed in the inner crucible up to the specified mark. First, the temperature of the sample is raised rapidly, after which the heating is done slowly. As the temperature approaches the flash point, heating continues at a constant rate. At specified temperature intervals, a small flame from an igniter is passed over the surface of the sample at a predetermined speed. The lowest temperature at which the flame causes the vapor on the surface of the sample to ignite is the flash point of that lubricant. Compressor lubricants operate in high-temperature and high-pressure environments, so higher requirements are placed on their flash points.

The flash point of lubricating oil indirectly reflects its thermal stability; lubricants with a low flash point are prone to thermal decomposition, and in the presence of flammable substances, they can easily lead to accidents. The thermal pyrolysis of lubricating oil destroys its lubricating properties, thereby shortening the service life of the equipment.

The flash point of lubricating oil indirectly reflects its thermal stability; lubricants with a low flash point are prone to thermal decomposition, and in the presence of flammable substances, they can easily lead to accidents. The thermal pyrolysis of lubricating oil destroys its lubricating properties, thereby shortening the service life of the equipment.

The flash point is a safety indicator for compressors. Since the temperature of compressed air can exceed the flash point of oils, the flash point of the lubricating oil used should be 30–40°C higher than the exhaust temperature. The flash point is different from the ignition point, and the flash point increases under high-pressure air. For example, an oil with a flash point of 216°C at atmospheric pressure has a flash point of 243°C at 0.8 Mpa (8 atmospheres). Moreover, the exhaust temperature of most compressors does not reach 243°C; using oils with a high flash point does not completely solve the problem of explosions. Explosions occur when the oil decomposes at the ignition point of overheated carbon deposits, producing volatile hydrocarbons. Oils with a very high flash point and high viscosity are more likely to cause carbon deposits, as well as increasing frictional resistance and wasting power. Practice has shown that the flash point increases as pressure rises, which means that lubricants under pressure have an increased safety factor. Lubricants with a high flash point tend to produce more carbon deposits, while those with a lower flash point result in fewer deposits and greater stability. When it is difficult to predict the dynamic behavior of lubrication in compressors and carbon deposits form under certain operating conditions, special attention should be paid to minimizing the thickness of these carbon deposit layers. Therefore, do not use lubricants with a too high flash point.

The flash point of lubricating oil indirectly reflects its thermal stability; lubricants with a low flash point are prone to thermal decomposition, and in the presence of flammable substances, they can easily lead to accidents. The thermal pyrolysis of lubricating oil destroys its lubricating properties, thereby shortening the service life of the equipment.