1. Test purpose
Flash point is an essential item for identifying operating oil. The decrease of flash point indicates that volatile and combustible gases are produced in the oil. These gases are often produced by the cracking of insulating oil at high temperature caused by overheating and discharge of oil filled electrical equipment. The failure of equipment can be found in time through the determination of flash point. At the same time, for the transformer oil newly filled into the equipment, measuring the flash point can also prevent or find out whether it is mixed with light distillate oil, so as to ensure the safe operation of the equipment.
2. Test method (closed cup method)
The determination of oil flash point needs to be carried out under strict conditions. For example, the heating rate of oil products, the size of evaporation space, the height of oil level and the type of instrument must be strictly regulated. In this way, the flash points of various oil products can be compared with each other to further evaluate the quality of oil products.
Closed flash point measurement is to add fuel in the closed flash point instrument to make the space on the oil surface form a mixture of oil vapor and air. When the oil gas concentration reaches a certain limit, it will produce flash fire in case of open fire and extinguish it immediately. At this time, fill the thermometer immediately to read out the temperature as the measurement result of flash point.
3. Key test points
(1) The oil test quantity shall be strictly controlled. Because the amount of oil is related to the volume of space on the liquid level, it affects the oil vapor and controls the mixture concentration. The result is low due to the large amount of oil test; On the contrary, the result is high.
(2) The heating speed shall be strictly controlled. If it is fast, it will advance the concentration of oil vapor and air mixture on the oil level.
(3) The diameter of ignition flame, the height of flame from oil and the length of residence time all affect the flash point test results, which shall be carried out according to the requirements of the test method.
4. Factors affecting test results
(1) Chemical composition
The flash point of oil is related to its chemical composition. The oil with high boiling point components and the oil with more alkanes have higher flash point, while the oil with more volatile components has lower flash point. In addition, due to the effect of battery and temperature, the oil in operation will crack out combustible gas and dissolve in the oil, reducing the flash point.
(2) Atmospheric pressure
The flash point of oil products also increases with the increase of atmospheric pressure and decreases with the decrease of atmospheric pressure. The flash point of oil products is 101.33pa.
(3) Physical conditions
Physical conditions include instruments, methods, temperature, pressure and other external conditions for measuring flash point. For the same oil product, the flash point is measured with open cup and closed cup respectively, and the values vary greatly. The control of test conditions is not strict, and the results are very different. For example, the higher the oil level of the closed cup, the smaller the evaporation space, the easier it is to reach the flash concentration and the lower the flash point.