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General Procedure Guidelines for Nitrogen Purging Protection of Pressure Vessels 1. Scope of Application These guidelines shall be applied when, as required by the contract and the technical documents provided by the design institute, the equipment needs to be protected by nitrogen gas prior to shipment. The pressure for nitrogen filling, nitrogen filling markings, and related requirements shall comply with the provisions in the drawings and technical specifications. Unless otherwise specified, nitrogen at 0.05~0.1 Mpa is generally used for filling. II. The necessity of nitrogen protection 1. Equipment needs nitrogen protection during sea transport; maintaining a positive pressure can effectively prevent seawater from entering the equipment ； 2. Prevent external oxidizing gases, corrosive gases, and moisture from entering the equipment, thereby protecting its internal components and materials so that they remain in their condition as at the time of manufacture (stainless steel equipment can prevent chloride-induced stress corrosion) ； 3. If the equipment is not installed and tested on site immediately, it may need to be stored for a while. Nitrogen purging provides a way to determine whether the equipment has been damaged during transportation and installation, and to check for any leaks in certain areas. III. Reference Documents 1: Design Institute, User Technical Documents ； 2. HG/T20584-2020 \"Technical Requirements for the Manufacturing of Steel Chemical Storage Vessels\" ； IV. Preparation Work 1. Tools, fixtures, and auxiliary materials for nitrogen filling 1.1 Nitrogen storage tank (pressure cylinder) and pipelines with valves ； 1.2 Three-way piping, with one end being the intake piping equipped with a valve ； 1.3 Pressure gauge and its piping connected to valves ； 1.4 Pure nitrogen (nitrogen purity of 99.99% or higher) ； 1.5 Portable oxygen content detector ； 1.6 Soapy water. 2. Preparations before nitrogen filling: 2.1 Equipment, instruments, and meters must be calibrated to meet the required standards by the relevant authorities and must be within their valid warranty period. Two pressure gauges with the same range and calibrated must be used. The range of the pressure gauge should be twice the nitrogen filling pressure; however, it must not be less than 1.5 times nor more than 3 times the test pressure. The accuracy class of the pressure gauge must be at least 2.5 grade ； 2.2 Operators must receive specialized technical training before they can take up operations. Comply with relevant regulations, rules, and systems regarding labor protection and safety to ensure safe production, and follow the operating procedures when using equipment and tools ； 2.3 Check the reliability of various relevant pipelines, connectors, valves, and product connections ； 2.4 The pressure gauge is installed at a convenient, visible position on the top of the product. V. Nitrogen filling process: 1. After the equipment is manufactured and passes the inspection, it must be dried out using compressed air before nitrogen filling, especially for equipment that has just undergone a hydrostatic test; such equipment needs to be dried and verified to be satisfactory before proceeding ； 2. Install and weld the flange covers (plug covers), plugs, manhole covers, etc., for each pipe outlet; the flange covers (plug covers) must pass strength verification. The fastening bolts at all connection points must be fully installed and properly tightened ； 3. The connection pipes for the nitrogen filling port of the equipment should be welded firmly to prevent loosening due to vibrations during transportation, ensuring that the nitrogen pressure gauge still shows a proper reading once the equipment arrives at the installation site, so that it can fulfill its protective function properly. 4. Nitrogen displacement: 4.1 Nitrogen enters from above and exits from below. To save nitrogen, intermittent displacement is used ； 4.2 Specific procedures: First, close all valves at the bottom (including the vent valve). Then, purge nitrogen into the upper part of the container; once a certain pressure is reached inside the container (0.1 MPa), stop the nitrogen supply. Open the vent valve to allow exhaust at maximum flow, and close the vent valve when the pressure drops to 0.01 MPa ； 4.3 Recharge with nitrogen, repeat this process 3 times; use a portable oxygen content detector to check the level at the vent – the process is complete when the oxygen content is below 5%. Then stop the gas exchange, close all valves, and seal the container ； 4.4 If it fails, replace it again until it passes ； 4.5 The positions of the intake valve and the vent valve should be arranged as far apart as possible, not on the same plane; this helps to save nitrogen and makes it easier to clean out any dead corners. 5. After nitrogen filling is complete, close the valve on the nitrogen filling device, remove the nitrogen filling tube, and tighten the valve on that device. Use soapy water to check whether the connections, plugs, manholes, and pipes are properly sealed; if there are any leaks, tighten them again before checking once more. Install the pressure gauge guard. VI. Safety considerations: Nitrogen is not toxic in itself, but an environment with a nitrogen concentration of over 78% can affect the oxygen level in the air. When the oxygen level drops below 18%, people may experience symptoms such as dizziness, nausea, and chest tightness; in severe cases, there is a risk of suffocation.