LNG is a flammable gas, mainly used as fuel. The main purpose of purging the pipeline before loading is to exhaust the air in the pipeline, so as to avoid mixing air into LNG to form explosive mixture, which may cause explosion when meeting with dangerous sources. Purging after loading is to purge the remaining LNG and CNG in the pipeline back to the storage tank, causing waste on the one hand. Nitrogen is chosen because it is an inert gas and does not burn or support combustion. In this LNG, there is nitrogen in it, which will not change the gas composition and affect the quality of LNG.