What are the requirements for working with fire under pressure?

Hot work is carried out under pressure without gas displacement, primarily by strictly controlling the oxygen level so that the concentration of flammable gases **does not exceed the upper explosive limit**, thereby preventing the formation of an explosive mixture； Under normal conditions, the combustible gas diffuses out of the cracks in the container at a steady and constant rate, forming a combustion system with the surrounding air, thereby igniting the combustible gas. As long as this diffusion combustion system is maintained under stable conditions, the safety of the welding repair work can be ensured. ⑴Strictly control the oxygen content. Before performing welding or repair work under pressure, it is necessary to analyze the gas composition inside the container to ensure that the oxygen content is below the maximum allowable level. The oxygen content in the system must be kept stably below safe levels at all times; this requires a balanced production load, improved coordination between different processing stages, dedicated personnel to oversee critical areas, and enhanced analysis of gas composition (an automatic oxygen analyzer can be installed for this purpose). When an increase in oxygen content in the system is detected, the cause should be identified as soon as possible and eliminated promptly; welding must be stopped immediately if the oxygen level exceeds the safe limit. ⑵Positive pressure operation: Before starting the welding process and throughout the entire welding operation, the container must maintain a stable positive pressure at all times. The pressure level should be kept at a level that prevents intense spraying of flame; generally, it should be between 2×104 and 6.7×104 Pa, so as to ensure a positive pressure without intense flame spraying. ⑶The concentration of combustible gases around the welding area must be strictly controlled. Whether indoors or outdoors, when performing welding or repair work on containers under pressure without gas displacement, it is necessary to analyze the concentration of combustible gases remaining in the space around the welding area; a level of 1/3 to 1/4 of the lower explosive limit is considered acceptable. The sampling location should take into account the properties of the combustible gas (such as specific gravity and volatility) as well as the characteristics of the factory building. Attention must be paid to the accuracy and reliability of the test data; welding repairs should only be carried out after it has been confirmed to be safe and reliable. ⑷Safety requirements for welding operations: ① Before carrying out welding repairs, the flammable gases escaping from cracks must be ignited. During the operation, the welder should not face the area where the fire is being generated to avoid burns. ②The current level of the welding machine should be adjusted in advance, especially for containers with a pressure of over 0.1 MPa and thin steel plates; an excessive welding current can easily cause melting and penetration, and under the pressure of the medium, larger holes and cracks will form, increasing the risk of accidents. ③If there are changes in the conditions for performing hot work, such as a sharp drop in pressure within the system to the specified limit, or if the oxygen level exceeds the safe value, the work must be stopped immediately. The cause must be identified and appropriate measures taken before welding repairs can be carried out. ④If intense sparking occurs during the welding process, fire-fighting measures must be taken immediately. The source of combustible gases must not be shut off, nor should the pressure in the system be reduced, to prevent air from entering the container and forming an explosive mixture. ⑤Welders must possess a high level of skill and practical experience; welding operations need to be carried out evenly and quickly. Only after receiving specialized training can they proceed with welding repairs under pressure.

Working with fire under pressure is not advisable; safety comes first. It is important to determine what type of gas or liquid is contained within the pipes and equipment that are to be welded, such as water, oil, hydrogen, nitrogen, or oxygen. Take the appropriate measures. In short, performing hot work under pressure is not advisable. (Personal opinion)

I posted a thread in the similar-topic discussion section of the Japanese version on April 2nd; the original poster can take a look.

In the production of dry synthetic ammonia, has anyone never carried out pressure welding? As long as the oxygen content in the medium is well controlled and the welders are careful during the operation, it’s fine.

Safety precautions for hot work under pressure without displacement 1. Hot work under pressure without displacement is not allowed in the following situations: unstable production. The equipment, pipes, etc. are severely corroded. 2. A construction safety plan must be developed, and safety and fire prevention measures must be implemented. During hot work, the workshop supervisor, the safety officers from the unit performing the hot work as well as those from the unit affected by it, the personnel from the factory’s safety and fire prevention department, and the factory manager or chief engineer must be present on site. If necessary, a professional fire brigade can be called in to oversee the operation. 3. It is necessary to ensure that the oxygen content in the hot work environment is ≤0.5%. 4. Before carrying out hot work, the production unit must inform the factory’s production scheduling department and relevant units, so that appropriate emergency measures can be taken promptly in case of any abnormalities. 5. During hot work, a dedicated person must be assigned to monitor pressure changes within the production system, ensuring that the system maintains a positive pressure of not less than 980.665 Pa (100 mm water column). Work involving open flames should be stopped when the pressure is below 980.665 Pa (100 mm water column). The cause must be identified and measures taken before work can resume; working with open flames under negative pressure is strictly prohibited. Good ventilation at the site where hot work is carried out is necessary to ensure that any leaked gases can be discharged smoothly.

Our factory’s approach: Ensure that space fire sampling and analysis meet the required standards； Ensure that the pressure inside the pipeline remains between 20 and 50 mm of water column; use a U-tube for monitoring. A dedicated person shall be responsible for this, and if the pressure exceeds this range, the fire source must be extinguished immediately and any welding work must be stopped ； The safety supervisors ensure that the fire extinguishing equipment and protective gear at the site where welding is carried out are complete and in good working condition. For reference only.