Why are gate valves prohibited for use in oxygen pipelines?

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Why are gate valves prohibited for use in oxygen pipelines? In recent years, as the demand for oxygen has increased, major oxygen users have adopted oxygen pipelines for delivery. Due to the long lengths and wide distribution of the pipelines, coupled with valves that open or close rapidly, accidents involving combustion and explosion in oxygen pipelines and valves occur from time to time. Therefore, it is crucial to conduct a thorough analysis of the potential hazards and risks associated with oxygen pipelines and valves, and to take appropriate measures. I. Analysis of the causes of combustion and explosion in several common oxygen pipelines and valves 1. Rust, dust, and slag inside the pipeline rub against the inner wall of the pipeline or the valve openings, generating high temperatures that lead to combustion. This situation is related to the type and particle size of the impurities as well as the airflow velocity; iron powder tends to burn in reaction with oxygen, and the finer the particle size, the lower the ignition point ； The faster the gas flow rate, the easier it is for combustion to occur. 2. Substances with a low ignition point such as grease or rubber are present inside the pipes or valves, and they catch fire under local high temperatures. Ignition points of several combustibles in oxygen (at normal pressure): 3. The high temperature resulting from adiabatic compression causes the combustibles to burn. For example, if the pressure before the valve is 15 MPa and the temperature is 20°C, while the pressure after the valve is 0.1 MPa at normal pressure, opening the valve quickly can raise the oxygen temperature after the valve to 553°C according to the adiabatic compression formula – a value that reaches or exceeds the ignition point of certain substances. 4. The reduced ignition point of combustible materials in high-pressure pure oxygen is a cause of combustion in oxygen pipeline valves. Oxygen pipelines and valves pose a great risk when exposed to high-pressure pure oxygen; experiments have shown that the energy required to initiate a fire is inversely proportional to the square of the pressure, which represents a serious threat to such pipelines and valves. II. Preventive Measures 1. The design shall comply with relevant regulations and standards; it must meet the requirements set out in the \"Several Provisions on Oxygen Pipeline Networks in Steel Enterprises\" issued by the Ministry of Metallurgy in 1981, as well as other regulations and standards such as the \"Safety Technical Regulations for Oxygen and Related Gases\" (GB16912-1997) and the \"Design Code for Oxygen Stations\" (GB50030-91). (1) The maximum oxygen flow rate in carbon steel pipes shall comply with the table below. Maximum oxygen flow rate in carbon steel pipes: (2) To prevent ignition, a pipe made of copper-based alloy or stainless steel with a length of at least 5 times the pipe diameter and at least 1.5 m long should be connected after the oxygen valve. 　　(3) Oxygen pipelines should have as few elbows and branch connections as possible; for oxygen pipelines operating at a pressure higher than 0.1 MPa, the elbows should be manufactured using stamped valve-type flanges. The airflow direction at the branch point should form an angle of 45° to 60° with the airflow direction of the main pipe. 　　(4) In butt-welded male and female flanges, using copper welding wire as the O-ring provides a reliable sealing method that ensures the flame resistance of flanges used for oxygen. 　　(5) The oxygen pipeline shall be equipped with good conductive devices; the grounding resistance should be less than 10Ω, and the resistance between flanges should be less than 0.03Ω. 　　(6) Bleed pipes should be installed at the ends of the main oxygen pipelines inside the workshop to facilitate purging and replacement of these pipelines. Filters should be placed before longer oxygen pipelines reach the control valves in the workshop. 2. Installation precautions: (1) All areas in contact with oxygen must be thoroughly degreased, and after degreasing, they should be cleaned using oil-free dry air or nitrogen. (2) Tungsten inert gas welding or arc welding should be used for welding. 3. Precautions for operation: (1) The oxygen valve should be opened and closed slowly; the operator should stand to the side of the valve, and it should be opened completely in one motion. (2) It is strictly prohibited to use oxygen to clean pipes or to conduct leak tests or pressure tests with oxygen. (3) Implement an operation ticket system, providing detailed explanations and specifications in advance regarding the purpose, methods, and conditions of the operations. (4) For manual oxygen valves with a diameter greater than 70 mm, operation is permitted only when the pressure difference before and after the valve is reduced to within 0.3 MPa. 4. Precautions for maintenance (1) Oxygen pipelines should be regularly inspected and maintained, including rust removal and repainting, every 3 to 5 years. (2) The safety valves and pressure gauges on the pipelines must be calibrated regularly, once a year. (3) Improve the grounding system. (4) Before carrying out hot work, displacement and purging should be performed; the oxygen content in the blown-out gas is considered acceptable when it is between 18% and 23%. (5) The selection of valves, flanges, gaskets, as well as pipes and pipe fittings, shall comply with the relevant provisions of the \"Safety Technical Regulations for Oxygen and Related Gases\" (GB16912-1997). (6) Establish technical records, and train operators, maintenance, and repair personnel. 5. Other safety measures (1) Raise the awareness of safety among construction, maintenance, and operating personnel. (2) Raise the awareness of management personnel. (3) Improve the level of science and technology. (4) Continuously improve the oxygen delivery plan. In summary, the real reason for banning gate valves is that their sealing surfaces suffer wear and damage due to friction during relative movement (i.e., when the valve is opened or closed). Once damaged, \"iron particles\" break off from these sealing surfaces, and such tiny iron particles can easily catch fire and burn – and this is where the real danger lies. In fact, gate valves are prohibited for use on oxygen pipelines; accidents can still occur with other type of stop valves as well. The sealing surfaces of these stop valves can be damaged, and danger can still arise. The experience of many companies shows that copper-based alloy valves should be used throughout oxygen pipelines, rather than carbon steel or stainless steel valves. Copper-based alloy valves have advantages such as high mechanical strength, wear resistance, and good safety (no static electricity generation). The real reason, however, is that it is the iron shavings produced by the easy wear of the seal surface in gate valves that are the main problem; whether the sealing performance declines or not is not the key factor. In fact, many oxygen pipelines that do not use gate valves have also experienced explosion accidents, usually occurring at the moment when there is a large pressure difference on either side of the valve and it is opened quickly. Multiple accidents have shown that the source of ignition and combustible materials are the ultimate causes; banning the use of gate valves is merely a means to control combustible materials, just as regular removal of rust, degreasing, and prevention of oil accumulation serve the same purpose. As for controlling flow rates and ensuring proper static grounding, these measures are aimed at eliminating sources of ignition. In my opinion, the material of the valve is the most important factor. Similar problems have occurred in hydrogen pipelines as well. The fact that the new regulations no longer include the clause prohibiting the use of gate valves is clear evidence of this. What’s crucial is to identify the root causes. Many companies simply ignore the operating pressure and insist on using copper-based alloy valves, yet explosions still occur. Therefore, controlling ignition sources and combustible materials, maintaining them properly, and keeping safety a top priority are what truly matter.