Micro-positive pressure welding repair of liquid ammonia pipelines

For flammable and explosive media, plugging leaks under pressure is generally the only option; it’s the first time I’ve heard of welding repairs for gas pipelines using a slight positive pressure! ! Is there still gas in the pipe? What safety measures have been taken? Can it be shared? !

What boldness, I’ve seen it! ! !

1. First, analyze the cause of the leakage: is it due to long-term corrosion or some sudden external factor such as a collision? In the case of corrosion, it is necessary to check whether other parts are also severely corroded; if the service life of the component has expired, then replacing it entirely should be considered (of course, this should be done at an appropriate time). If it is only localized corrosion and not very severe, welding under pressure can be considered. 2. In accordance with relevant regulations, obtain a strict permit for welding under pressure. 3. Make the necessary preparations. Construction workers, supervisors, tools, fire-fighting equipment, protective equipment. Construction workers must wear long-tube respirators, and it is advisable to wear raincoats as well (since the liquid ammonia evaporating from the liquid ammonia pipes needs to be absorbed with water in order to create a suitable working environment for the workers). 4. Drain all liquid ammonia from the liquid ammonia pipeline to maintain positive pressure. 5. Use hoses to bring water to the site to flush and absorb any leaked ammonia; remove them during the operation. 6. Start the assignment.

It’s possible. But before starting the work involving fire, pay attention to the following: 1. The sampling analysis must be satisfactory; 2. A slight positive pressure must be maintained.

I don’t quite understand the situation on the 5th floor. Generally, micro-positive pressure welding is used when, due to production requirements or various other reasons, it’s difficult to completely purge the system or pipes. In such cases, it’s simply impossible to carry out proper analysis and sampling. In fact, the principle behind micro-positive pressure welding also relies on making full use of the three conditions necessary for combustion; as long as any one of these conditions is missing, it is safe. Therefore, when micro-pressure welding is performed on ammonia pipes, it is necessary to maintain a slight positive pressure inside the pipe, and to have a backflow or exhaust system in place to prevent the welding heat from increasing the pressure inside the pipe, which could lead to welding failure or more serious accidents. Second, oxygen-containing gases such as substandard nitrogen must not be used to purge the pipelines.

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, pipelines, 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 area 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 they can take appropriate emergency measures in case of any abnormalities. 5. During hot work, a dedicated person must be assigned to monitor the 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). Hot work must be stopped when the pressure is below 980.665 Pa (100 mm water column). Hot work can resume only after the cause has been identified and appropriate measures taken; hot work 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. Conditions required for performing hot work under pressure without gas displacement: When carrying out hot work under pressure without gas displacement, it is essential to strictly control the oxygen level, so that the concentration of flammable gases does not exceed the upper limit for explosion, 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 coordinated control of various processes, dedicated personnel to oversee critical positions, 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 threshold. ⑵Positive pressure operation: Before starting welding work and throughout the entire welding process, 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 may form, leading to 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 done evenly and quickly. Only after receiving specialized training can they carry out welding repairs under pressure.

Our factory’s approach: Ensure that sampling and analysis for work in confined spaces are successful； 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 task, 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.

My approach is as follows: (I often deal with cracks in liquid ammonia pipelines, and it works well; I’m bold yet careful.) 1. Use a chisel to seal the crack so that no leakage occurs. 2. Perform welding under positive pressure, typically at 0.005 MPa; prepare a steam pipeline on-site for use during welding, and control the heat output while welding. These implementation conditions are for obtaining a special-grade hot work permit, with the working pressure generally being below 1.6 MPa. The pipe thickness must be checked