First, the replacement of the fire and the pressure does not replace the hot fire
Welding repair of chemical and fuel containers and pipelines currently has two methods of replacing dynamic fire and pressure-free replacement of dynamic fire. Any welding or cutting operation using an electric arc or flame is a hot fire or a hot work.
(1) Displacement of dynamic fires is to replace the flammable gas in the vessel or pipeline with water and non-combustible gas before welding, or to replace the toxic and harmful gases in the vessel or pipeline with air, so that the flammable gas in the vessel or pipeline is toxic or toxic. The content of harmful gases meets the specified requirements, thus ensuring the safety of welding repair operations.
Displacement of hot fire is a safe and proper method, and is widely used in the production and maintenance of containers and pipelines. However, when the replacement method is used, the container and the pipe need to be suspended, and the other medium is used for replacement. In the process of replacement, it is necessary to continuously sample and analyze, until it is qualified, it can only be fired. After the fire, it needs to be replaced again, which is time consuming and troublesome. In addition, if there are many dead ends in the pipe, it is often difficult to replace and leave a hidden danger.
(2) The pressure does not replace the dynamic fire. The pressure does not replace the dynamic fire, that is, the oxygen content is strictly controlled, so that the concentration of the combustible gas greatly exceeds the upper limit of the explosion, and then it is ejected from the pipe mouth at a steady speed, and The combustion is ignited to form a combustion system with the surrounding air and to maintain a steady continuous combustion. Then, you can perform the welding repair work.
The pressure-non-replacement method does not require replacement of the original gas, and may be carried out while the equipment is operating. The procedure is small and the operation time is short, which is advantageous for production. This method is mainly suitable for external welding of containers and pipes of flammable gas. Since this method can only be carried out while maintaining a certain positive pressure continuously, the control is difficult, and it cannot be used without a certain pressure, and has a large limitation. Therefore, the current application is not extensive.
Second, the cause of the explosion fire
(1) The sampling analysis of the gas inside or outside the vessel or pipeline before welding is inaccurate, or the sampling site is not suitable. As a result, there is an explosive mixture around the vessel, the pipeline or the hot point.
(2) During the welding repair process, the surrounding conditions have changed.
(3) The container being repaired is not isolated from the system being produced, and the explosive gas collides with each other, enters the welding repair area, or the production system discharges the exhaust gas and encounters a spark.
(4) Welding repair work in workshops, warehouses, etc. with dangers of combustion and explosion.
(5) Sealed containers that have not been safely treated or have not been opened.
Third, the safety technical measures for replacement welding
(1) Fixed hot fire area In order to concentrate the welding repair work and facilitate the management, a fixed hot fire zone can be demarcated in the factory and in the workshop. Any item that is detachable and conditionally moved to the fixed hot-fire zone must be moved to the fixed hot-fire zone to reduce the hot work in the explosion-proof workshop or workshop. The fixed hot zone must meet the following requirements.
(1) No gas pipelines and equipment, and more than 10m from the pipeline of inflammable and explosive equipment.
(2) The indoor fixed fire zone should be separated from the explosion-proof production site, and there should be no collusion such as doors, windows and trenches.
(3) The flammable gas or vapor cannot be diffused into the hot fire area when the equipment in production is normally vented or in the event of an accident.
(4) Always keep a sufficient number of fire-fighting tools and equipment.
(5) It is forbidden to use various flammable substances in the fixed hot fire zone.
(6) Delimitation should be made around the work area and fire safety signs should be hung.
(2) To implement reliable and isolated on-site maintenance, it is necessary to stop the work of the equipment or pipeline to be overhauled, and then take reliable isolation measures so that the equipment to be repaired and repaired is completely isolated from other equipment (especially the equipment in the production part). Ensure that combustible materials, etc. cannot diffuse into and around the weld repair equipment. A reliable method of isolation is to install a blind plate or remove a connecting line. The technical conditions of the materials, specifications and processing precision of the blind plate must conform to the national standards. It must not be abused and assembled correctly. It must ensure that the blind plate has sufficient strength to withstand the working pressure of the pipeline and is not leaky. A venting tube or pressure gauge should be added between the blind plate and the valve, and a special person should be assigned to guard. For the removal of the pipeline, pay attention to the blocking plate on the production system or the side where the material is stored. Blocking plates must also meet the technical requirements of national standards. At the same time, attention should be paid to the space isolation of atmospheric pressure open equipment to ensure that Mars cannot come into contact with combustible materials that escape from the container mouth. For some short-time welding repairs, the water source can be cut off with a water seal, but a special person must guard the overflow of the water seal overflow pipe to prevent the water seal from failing. In short, it is not allowed to do a good job of isolation.
(3) Implementing a complete replacement After the isolation work, the equipment itself must drain the materials and completely replace the flammable or toxic medium in the container and the pipeline. During the replacement process, samples should be continuously sampled and analyzed until the flammable and toxic substances in the container pipes meet the safety requirements.
Commonly used replacement media are nitrogen, water vapor or water. The method of replacement depends on the specific gravity of the displaced medium and the replacement medium. When the displacement medium is larger than the medium to be replaced, the replacement medium should be fed from the lowest point of the container or pipeline and discharged from the highest point. When the gas is used as the replacement medium, the amount required is generally three times or more the volume of the medium to be replaced. Some of the replaced combustible gases have the property of being retained, or the specific gravity of the replacement gas is not much different. At this time, attention should be paid to the incomplete replacement or the mixing of the two. Therefore, the thoroughness of the replacement cannot be based only on the amount of the replacement medium, but on the results of the analysis of the gas composition. When water is used as the replacement medium, fill the equipment pipeline.
(4) Correct cleaning of the container After the container and the pipe are replaced, they must be carefully cleaned inside and outside. Because some flammable and explosive media are adsorbed on the dirt on the inner wall of the equipment and pipes or the insulation material on the outer surface, liquid combustibles will adhere to the inner walls of the container and the pipe. If it is not thoroughly cleaned, the combustibles will gradually be released due to the influence of temperature and pressure changes, which will cause the unqualified hot fire conditions to become unqualified, resulting in fire and explosion accidents.
The cleaning can be carried out by hot water cooking, pickling, caustic washing or solvent cleaning to soften and dissolve the scale and the like on the inner wall of the equipment and the pipe. The method used for cleaning should be determined on a case-by-case basis. The alkaline washing is washed with an aqueous solution of sodium hydroxide (caustic soda). The cleaning process is to first add the required amount of water to the container, and then gradually add the quantitative alkali tablets in batches, while slowly stirring, waiting for all the alkali tablets. After being added to dissolve, it can be boiled by steam. The end of the steam tube must extend to the bottom of the liquid to prevent splashing of lye foam after the water vapor is introduced. It is forbidden to add water (especially hot water) after the alkali tablet is placed first, because a large amount of heat is generated when the caustic soda is dissolved, and the container pipe is poured out to burn the operator.
For the scale that cannot be removed by the cleaning method, the operator wears protective equipment and enters the inside of the equipment to remove it with a tool that does not ignite, such as wood, brass (including copper less than 70%) or aluminum knife, brush, etc. It can also be removed by suction, sand and other methods using hydraulic, pneumatic and electro-mechanical methods. Displacement and cleaning must be careful not to leave a dead angle.
(V) Air Analysis and Monitoring Dynamic fire analysis is the sampling and analysis of equipment and pipelines and the surrounding environment. The hot fire analysis can not only ensure that the fire conditions are met when the fire starts, but also the changes in the fire conditions during the welding repair process. During the replacement operation and before the hot work, gas samples should be taken from different parts inside and outside the container and the pipeline for analysis to check the content of flammable and explosive gases and toxic and vocal gases. After the inspection is passed, the welding repair should be carried out as soon as possible. The analysis data is valid within half an hour before the fire, otherwise it should be resampled and analyzed. Sampling should pay attention to the representativeness of the sampling to make the data accurate and reliable. The work site environment needs to be analyzed at regular intervals after the start of welding repair. The time interval of the hot fire analysis is determined according to the site conditions. If the relevant gas content exceeds the specified requirements, the weld repair should be stopped immediately, cleaned again and sampled and analyzed until it is qualified.
The qualification requirements for gas analysis are:
(1) The content of flammable gas or flammable vapor: if the lower explosion limit is greater than 4%, the concentration should be less than 0.5%; if the lower explosion limit is less than 4%, the concentration should be less than 0.2%;
(2) The content of toxic and harmful gases shall comply with the provisions of the "Design Industrial Hygiene Standards for Industrial Enterprises";
(3) For equipment and pipelines where the operator needs to enter the internal welding repair, the oxygen content should be 18% to 21%.
(6) It is strictly forbidden to weld the sealed container without holes. Before welding, the manhole, hand hole, cleaning hole and material hole of the container should be opened, and good ventilation should be maintained. It is strictly forbidden to weld the sealed container without holes.
When gas welding or gas cutting is required in the container and pipeline, the ignition and flameout of the welding and torch should be carried out outside the container to prevent excessive acetylene gas from accumulating in the container and the pipeline.
(VII) Safety organization measures (1) The fire approval procedures must be handled in accordance with the prescribed requirements and procedures. The aim is to develop security measures and clarify the responsibilities of the leader. The welder who undertakes the welding repair work shall be specially trained and obtained the corresponding qualification certificate after examination.
(2) Before the work, it is necessary to formulate detailed and practical plans, including welding procedures and specifications, safety measures and construction drawings, and notify the relevant fire brigade, first aid station, production workshop and other aspects to make emergency arrangements.
(3) Other fire work should be stopped within 10m around the working point, and flammable and explosive materials should be moved to a safe place.
(4) The workplace should have sufficient lighting, and the portable lighting should be 12V safe voltage and have a good protective cover.
(5) Guardians must be provided for hot work in the fire-fighting area and for welding work in containers and pipes. The purpose of guardianship is to ensure the careful implementation of safety measures. The guardian should be an experienced person. The guardian should clarify his duties and stick to his position.
(6) When entering the container or pipeline for welding repair work, the risk of electric shock is the greatest. It is necessary to strictly implement the regulations on safe use of electricity and take necessary protective measures.
Fourth, the safety technical measures of pressure replacement without replacement welding
(I) Strict control of oxygen content At present, some departments stipulate that the limit oxygen content of hydrogen, carbon monoxide, acetylene and furnace gas should not exceed 1% as a safe value, and it has a certain safety factor. The limit oxygen content of hydrogen at normal temperature and pressure is about 5.2%, but it is 1% considering the difference in high pressure and high temperature conditions, as well as the error of instrumentation and detection. Before the pressure is not replaced, the weld must be tested for oxygen content in the vessel or pipeline. When it is found that the oxygen content in the system is increased, the cause should be found out as soon as possible, otherwise the welding should be stopped.
(2) Positive pressure operation In the entire welding repair. In the process, the container and the pipeline must maintain a stable positive pressure continuously, which is the key to the safety of the pressure without replacement. Once a negative pressure occurs, the air enters the container or pipe being welded and is prone to explosion.
The pressure is generally controlled at 0.015 to 0.049 MPa (150 to 500 mm water column). If the pressure is too high, the airflow speed will increase, causing violent fire, which will cause difficulties in the welding operation, and even cause the melt hole to expand, causing an accident; the pressure is too small, which may cause pressure fluctuations, and the air will penetrate into the container or the pipe during welding. Explosive gas mixture.
(3) Strictly control the content of flammable gas around the working point When the pressure is not replaced by welding repair work indoors or outdoors, the content of flammable gas in the surrounding residual space is preferably less than 0.5%. The sampling part of the analysis gas should be correctly selected according to the nature of the gas and the structural characteristics of the house to ensure the correctness and reliability of the test results.
When welding indoors, doors and windows should be opened for natural ventilation. If necessary, mechanical ventilation should also be adopted to prevent the formation of explosive mixed gas.
(IV) Safety requirements for welding repair operations Refer to (1) to (4) for safety organization measures and safety organization measures for replacement welding.
(1) During the operation of the welder, the ignited flame should be avoided to prevent burns.
(2) The welding specification should be pre-adjusted according to the specified process. If the welding current is too large or improperly operated, it will easily cause burn-through under the action of the medium pressure, resulting in an accident.
(3) If there are changes in the surrounding conditions, if the pressure in the system drops sharply or the oxygen content exceeds the safe value, the welding repair should be stopped immediately. After the corresponding countermeasures are taken, the welding can be continued.
(4) In the process of welding repair, if a violent fire is observed, fire-fighting measures should be taken immediately. Do not cut off the source of flammable gas before the fire is extinguished, nor reduce or eliminate the pressure of the container or pipe to prevent the container or pipe from drawing in air to form an explosive mixture.
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