Experience in emergency repair of the omega seal ring for a hydrogenation heat exchanger at a certain company

Thread Content

I. Incident overview: On May 30, 2022, crew members working in a workshop of a certain company discovered oil and gas escaping from the insulation area of the flange of a high-pressure heat exchanger. They immediately reported this to the workshop supervisors and the company’s management. Upon learning of the situation, the company’s management decided to shut down the equipment for repairs, taking into account the structure of the equipment and the properties of the materials at the site of the leak. II. Cause analysis: The sealing structure at the flange of the heat exchanger housing and the tube box flange is an “Ω” ring made of material NO6625, with a thickness of 2.5 mm. During the manufacturing of this equipment, the “Ω” ring required to be welded together along its edges; due to the limited space available, welding defects such as pores occurred during welding. As a result of corrosion by the operating medium and mechanical stresses, leakage occurred. 【Reasoning for judgment: The leakage is manifested as two small pinholes in the heat-affected zone along the edge of the weld seam; the distance between these pinholes is approximately 1 cm, and no cracks were detected between them during PT inspection.】 III. Preliminary countermeasures: After the leakage occurred, a repair plan was put on the agenda. The discussions among the company’s engineering technicians, together with the repair suggestions from the equipment manufacturer, all agreed on the following approach: the welds at the leakage sites should be cut open, the cut areas should be treated to remove hydrogen, and once no defects are found upon inspection of those areas, they should be cleaned thoroughly before welding. After welding, another inspection is conducted, and the process is considered successful if no welding defects are detected. IV. Preparation work: Materials, construction equipment, and nickel alloy welders must be secured. Materials: The “Ω” rings are made from special materials, and welding wires for repair work are extremely scarce; this issue was resolved by contacting Petrochemical Construction Company. Construction equipment: Some M76 main bolts need to be removed for maintenance, which requires hydraulic wrenches that shall be provided by the construction party. Welder: The construction unit was contacted, and a certified senior welding technician with expertise in special materials was assigned to carry out the emergency repair work. Reconsideration of countermeasures: After the senior welding technician arrived and inspected the site, enthusiasm for carrying out emergency repairs waned, as it was deemed difficult to ensure repair quality under the existing conditions. Since the material had already been in use, it was not possible to use argon gas for protection during welding; therefore, the material could not be repaired in the manner specified by the manufacturer. NO6625 is a high-nickel alloy, and without argon protection, slag would form inside the weld seam, making it impossible to guarantee the quality of the welds – the task is extremely challenging. Since the senior welding technicians were reluctant to carry out welding repairs, the client tried to persuade them. Taking into account the requirement for argon shielding, another solution was proposed: instead of thoroughly cleaning the area to avoid the need for argon shielding inside, external argon protection could be used, with the pressure inside the equipment reduced to normal levels, thus preventing bubbles from forming and interfering with the quality of the newly welded joint. A new weld layer could then be applied over the original weld. A high-frequency pulse welder is used for pulse melting welding on the outer surface of thin walls, thereby preventing new problems caused by the high welding temperatures that could melt through the material. Based on practical engineering experience and the certainty of repair quality, the repairs are ultimately carried out according to a plan discussed further between the two parties. After the plan was finalized, a high-frequency pulse welding machine was obtained again, and a welding technician carried out the repairs. During the process, the surface welds were cleaned using an electric grinder to reveal the natural color of the metal. Pulse TIG welding was performed in three stages with low current; after each welding pass, a PT inspection was conducted, and only when no defects were found would the next welding step proceed. Finally, the three welds were successful, and the surface cladding to eliminate leaks was accomplished in one attempt. After the weld repair was completed, the high-pressure bolts were reinstalled, and the facility resumed production overnight. This weld has been in stable operation for over 2 years without any new issues arising. Some thoughts: 1. The high-pressure heat exchanger features an “Ω”-shaped seal structure, which has certain limitations. Firstly, from a structural perspective, once a defect appears in the form of a crack, it is extremely difficult to repair such defects, and the costs involved are very high ； Second, seals are replaced during regular major maintenance of the equipment; however, on-site construction does not have the same conditions as those in a factory, making it difficult to ensure quality. 2. The company only has one set of “Ω” ring accessories available, so proper maintenance and replacement tasks will be hindered by a lack of replacement parts ； When problems arise, one can only rely on repairs; there is actually no opportunity for replacement, and the risk of long-term operation is high ； 3. There are many large-scale and high-voltage devices in the company; essential maintenance tools such as hydraulic wrenches and hydraulic stretchers also need to be taken into consideration. 4. During the construction of equipment projects, great attention must be paid to details in quality inspection. In this case, hydraulic wrenches were used for the high-pressure bolts, but it was not possible to tighten them at all. The reason was that no anti-high-temperature seizure agent was applied to the bolts during installation; as a result, the bolts rusted when exposed to high temperatures, creating excessive friction with the nuts, which forced the use of fire to remove the bolts ； It is necessary to ensure and verify the quality of every step from the very beginning of equipment installation.