The flash point of an oil product refers to the oil temperature when the oil vapor and air reach the lower explosive limit under normal pressure. Generally, the flash point of high boiling oil is the oil temperature at its lower explosion limit. For low boiling point oil products, such as gasoline and volatile liquid petroleum products, the oil gas concentration at room temperature has greatly exceeded its lower explosion limit, and its flash point is actually the oil temperature of its upper explosion limit. The flash point of oil products is an indicator to indicate the degree of fire and explosion hazards, and is an indicator to evaluate the safety of petroleum products. Liquid with flash point lower than 45oC is called flammable liquid, and liquid with flash point higher than 45oC is called flammable liquid.
The ignition point is the lowest temperature at which the temperature continues to rise after the flash point of the open cup of the oil is measured, and the combustible mixture can be ignited by the external flame and continuously combust for not less than 5s under specified conditions. The ignition point is also called the open cup ignition point. The principle is consistent with the open flash point of the tester.
The self ignition point is the lowest oil temperature at which the oil is heated to a very high temperature and then contacted with the air without ignition.
The flash point and spontaneous ignition point of oil products have opposite changing laws. The lighter the distillate, the lower the boiling point, the lower the flash point and the higher the spontaneous ignition point. The autoignition point of gasoline is 510-530oC, that of kerosene is 380-425oC, and that of diesel is 350-380oC. The autoignition point of lubricating oil varies with the weight and chemical composition of the distillate, mainly depending on whether it is easy to oxidize. Alkanes are easier to oxidize than aromatic hydrocarbons, so oil products contain more alkanes and lower spontaneous ignition point, but their flash point is higher than that of oil products with the same viscosity and more cycloalkanes and aromatic hydrocarbons. In similar hydrocarbons, with the increase of the relative molecular weight, the spontaneous ignition point decreases, while the flash point and ignition point increase. For hydrocarbons with the same carbon atom number, the order of spontaneous ignition point is: alkanes