Combustion accidents of high-pressure oxygen pipelines and preventive measures

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1 Causes of combustion accidents in high-pressure oxygen pipelines High-pressure oxygen pipelines generally use steel pipes. However, during the operation of the valve, there have been many accidents in which the combustion of the pipe itself caused high-pressure oxygen to be ejected. There are also ejection accidents caused by burning of high-pressure oxygen pipes seen in high-pressure oxygen bottle filling factories, and bottle valves flying out and injuring people, etc. Since there are many cases, we will not list them one by one here. For accident cases, please refer to "Cryogenic Technology". This kind of accident is not a simple gas ejection accident, but an accident in which steel pipes or valves transporting high-pressure oxygen are ignited and burned in the high-pressure oxygen medium inside, causing holes in the pipe wall and ejecting high-pressure gas. The immediate cause is a metal fire based on the chemical reaction between iron and oxygen. 2 The combustion reaction of iron in oxygen is in the oxygen medium. If you want to burn metal, you must first heat the metal to above a certain ignition point temperature. Under normal pressure oxygen, the ignition point of stainless steel is 1380°C, the ignition point of steel is 1290°C, the ignition point of about 10g of iron blocks drops to 930°C, and the ignition point of about 200 mesh iron powder is 315°C. Regardless of this, the ignition point of iron in oxygen under normal pressure is relatively low when it is in powder form, and is high when it is in block form. That is, the finer the iron powder particles, the lower the ignition point temperature. If the oxygen pressure is increased to 3MPa, the ignition point in oxygen is approximately 842°C. Therefore, under the conditions of pressurized oxygen, the ignition temperature will be lowered by dozens of degrees or even about 100°C. Once iron is ignited in oxygen, its combustion heat is very large. Therefore, the sharp increase in temperature presents a scorching state, which can generate iron oxide and be impacted by the air flow after being melted. Combustion can be continued under the condition of continuous supply of oxygen. The chemical reaction between oxygen and iron and its combustion heat can be expressed by the following formula: Fe+ (3/4)02→(1/2)Fe2O3+408.3kJ/mal. The calorific value per gram of iron is: 408.3/55.85=7.31kJ/g Oxygen required to burn each gram of iron (3/4)×22.4/55.85=0.3L/G. That is, the amount of oxygen required to burn each gram of iron at normal temperature and pressure is approximately 300mL. Therefore, if iron continues to burn, it requires 2360 times more oxygen than the volume of iron. For this reason, when a combustion accident occurs on an oxygen pipe, the burning direction of the pipe is towards the direction where oxygen is provided. That is, it propagates in the opposite direction to the oxygen flow. Therefore, as long as the valve of the pipeline is closed and the supply of oxygen is cut off, the burning of iron can be easily eliminated. 3. Ignition principle of oxygen pipeline. The oxygen pipeline ignites on its own. First, the pipe wall is heated to at least the block ignition point temperature of 800~900℃ or above. Secondly, is it possible to heat the oxygen pipeline to such a high ignition temperature? 1． Friction of rust scale in oxygen flow Oxygen flows at high speed in the pipe, and rust scale particles also flow together, generating heat after friction with the pipe wall. Due to the small mass of rust particles, the amount of heat generated is also very small. But the pipe wall is infinite, so the friction heat of the pipe wall disappears quickly, so the temperature rise in the pipe wall itself is not obvious. For this reason it can be concluded that the pipe wall cannot be heated to a temperature above the ignition point. 2． Combustion of combustibles in pipelines When there are combustibles in oxygen pipelines, they can burn if they encounter some kind of ignition source. There may be combustible materials in the pipeline: (1) Mist lubricating oil. Liquid oxygen often contains a very small amount of lubricating oil, which becomes oil mist, mixes into the oxygen flow, and condenses in dead ends such as pipe connections or valve parts. There are also cases where rust and oil mist are mixed together to form a viscous substance. (2) When installing or cleaning pipelines, flammable detergents may remain inside the pipelines. (3) Use flammable lining materials. Such as fiber, rubber, etc. as cushioning materials, such as the wrong use of this cushioning material. The above are three situations to consider when there may be flammable materials in the oxygen pipeline. If these combustibles are ignited in oxygen, their combustion heat may cause the pipeline to burn. 3． The iron powder produced during pipeline friction causes rust to form on the inner wall of the oxygen pipeline, especially on the inner wall of the bend. Due to impact wear and tear, the incomplete iron oxide powder in the rust is peeled off, and there are even iron particles, which may cause the pipeline to burn in oxygen. 4. Red hot iron powder ignites combustible materials in high-pressure oxygen. When the iron powder flows with the oxygen, it rubs against the pipe wall and can easily reach the ignition temperature and spontaneously ignite. At this time, the iron powder is surrounded by the oxygen flow with very small thermal conductivity, so it accumulates combustion heat and becomes high-temperature red-hot particles (about 1000°C or above) flowing at high speed in the oxygen flow. On the other hand, there may be combustible materials (oil mist, detergent, gasket, etc.) in the pipeline. When the red-hot iron particles collide with these combustibles, their temperature is enough to ignite the combustibles instantly. It can also be said that iron powder becomes a source of ignition for combustible materials. It will also directly cause the pipe wall to be burned and perforated, and high-pressure oxygen will be ejected. 5. Heating caused by adiabatic compression If the valve in the high-pressure oxygen pipeline or the oxygen bottle valve is opened sharply, heat generation due to adiabatic compression may become the cause of the accident. 4 Preventive measures are to prevent combustion accidents of oxygen pipelines. The following measures must be considered: (1) The surfaces of the inner walls, valves, joints, etc. of high-pressure oxygen pipelines should be smooth and have no protruding parts, and should not cause dead corners to the air flow in the pipes. (2) It is strictly prohibited to use combustible materials for gaskets (especially fibers). (3) Oxygen pipelines should use straight pipes as much as possible and avoid elbows to avoid wear. Dangerous iron particles are generated, and the collision of red-hot particles may cause a fire hazard. (4) There is no oil in the pipe or no detergent is allowed to remain in the pipe. (5) Try your best to remove rust and adsorbent particles in the pipeline. The moisture in the oxygen can promote the production of rust, so the moisture in the pipe must be removed. It is required that very dry oxygen passes through, and moist oxygen is not allowed to pass through at this time. This is to avoid passing through wet oxygen and then dry oxygen, which will cause rust and scale to fall off, causing trouble. (6) The pipes are made of stainless steel or some pipes are made of copper, which can inhibit the production of oxidized rust and abrasive iron particles. However, if these particles or other combustibles are still produced in other parts, even if stainless steel or copper is used, it cannot prevent pipeline combustion accidents. (7) The oxygen flow rate in the pipeline is within 3MPa and limited to a speed below 8m/s. If various factors are restricted and handled properly, the flow rate in the pipe can be below 25m/s. (8) The opening and closing operation of the valve should be slow to avoid being too hasty.