Nitrogen purging step for chemical plant systems

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In most chemical processing plants, especially petrochemical facilities, the raw materials, fuels, as well as the semi-finished and finished products generated during the production process are all flammable and explosive. Therefore, before introducing such flammable and explosive substances into the system, the air within the system must be replaced with nitrogen, reducing its oxygen content to below 0.5% (by volume), in order to prevent the formation of explosive mixtures when these substances come into contact with air. In some cases, nitrogen purging is also an important measure to prevent corrosion on the surfaces of equipment within the system caused by oxidation by humid air. 1. Conditions required for nitrogen purging and preparatory work before purging: a The system must have passed the airtightness test, and any systems that require to be dried must have been properly dried ； For purging with nitrogen, the oxygen content must be less than 0.1% (on a volume basis); the pressure in the inlet area should typically be between 0.6 and 0.7 MPa. In low-temperature systems, the dew point of the nitrogen used for purging should be between -60 and -70 degrees Celsius ； c The systems related to public utilities are operating normally ； d Therefore, the instruments, control systems, and safety devices (including interlocks) have been properly tested and are in good standby condition ； It involves the formulation and preparation of the nitrogen purging process for the system, which includes determining the introduction of nitrogen, its flow direction, discharge points, and analysis sampling points, as well as the necessary equipment ； Nitrogen is an asphyxiant gas, so it is very important to work safely; the area must have good ventilation as well as the necessary safety equipment to prevent asphyxiation. 2. Purging method: The nitrogen purging method is generally carried out in an intermittent manner. First, the system is filled with nitrogen to a pressure of ~0.5 MPa; then the nitrogen filling valve is closed and the nitrogen discharge valve is opened to reduce the pressure in the system to around 0.01 MPa. After that, the discharge valve is closed again and nitrogen is filled in, repeating this process several times until the oxygen content in the gases taken from various sampling points is less than 0.5%, and the oxygen level that could cause catalyst oxidation is less than 0.2%. At this point, the system is considered to be properly purged. However, for low-temperature systems, it is also necessary to measure the dew point at each sampling point and ensure that it meets the required standards. 3. Post-permutation handling and precautions: a Once the permutation is complete, the systems that have undergone it should be marked on the flow diagram, and the pressures and frequencies of pressurization and depressurization, as well as the analysis results and the time when compliance is achieved, should be recorded ； After the permutation is successful, close all discharge valves and sampling valves. The system is then filled with nitrogen again to a pressure of 0.5 MPa for a leak test; once this test is successful, the system’s pressure is reduced to around 0.05 MPa, and it is held at that pressure for subsequent use.