First, Analysis of the Causes of Flange Leakage
For flange seals, leakage is related to many factors, including the physical properties of the sealed medium, operating conditions, the roughness of the flange sealing surface, the clamping force, the quality of the flange itself, and the basic conditions, dimensions, and loading/unloading process of the gasket. In a certain plant, the dissimilar steel welded joint on the outer side of the outlet pipeline flange of the DC102 hydrocracking reactor developed cracks and leaked under high temperature, high pressure, and hydrogen-exposed conditions.
Since the metal annular gasket was replaced in the K201 pre-hydrogenation reactor of the reforming unit after the last overhaul operated for two and a half years without leakage, and the sealed process medium and flange remained unchanged, the influence of operating conditions can be considered alone.
01: Statistical analysis shows that in the period between the last reactor inspection and the occurrence of leakage, there were approximately 20 start-up and shutdown cycles. Under certain pressure and temperature operating conditions, the gasket experienced fatigue failure, leading to leakage.
02: Prolonged high temperatures cause creep and stress relaxation in bolts and gaskets, leading to a decrease in gasket sealing pressure. Bolts are stretched, and flanges deform. If the residual sealing pressure is lower than the minimum working sealing pressure, leakage will occur.
03: Once a leak occurs, high-pressure material overflows from the leak point, eroding the sealing surface and causing radial pitting.
Second, Leak Prevention Measures, Flange Repair, and Assembly
1. Avoid abnormal temperature fluctuations and excessively rapid heating and cooling rates during start-up and shutdown.
Minimize unplanned shutdowns and avoid drastic temperature changes in the reactor, mitigating the temperature gradient.
01: During shutdowns and material stoppages, the introduction of flushing hydrogen into fresh feed and circulating oil pipelines should be done slowly to prevent leakage from high-temperature, high-pressure flanges due to rapid cooling. If a flange leaks due to cooling, steam should be used to purge the oil and gas promptly to prevent fire.
02: Thermal expansion and stress during equipment heating can cause small leaks at flange and gasket joints. When such a leak occurs, a steam hose should be placed at the leak point to disperse the oil and gas, preventing a fire before the connection is tightened. To minimize the hazards of thermal expansion, the heating rate should generally not exceed 25℃/h.
03: After maintenance or during the first start-up, bolts on equipment pipelines may have been replaced or disassembled. These were tightened at room temperature (cold state), whereas during normal production, they are tightened at higher temperatures.
2. Flange Sealing Surface Repair
Due to uneven erosion on the ring contact surface of the flange, repairs must be carried out in accordance with the “Pressure Vessel Safety Technical Supervision Regulations.” The equipment connection flange is connected to the equipment body and can be repaired on-site using specialized tools. These specialized tools consist of a support plate, a central shaft, and a tool holder.
3. Selection of High-Performance Gaskets and Digital Technology
For reforming hydrogenation reactors operating within a relatively wide temperature range, the selection of high-performance gaskets is crucial. Simultaneously, combining high-precision constant torque tightening technology, digital on-site flange assembly quality assurance process control technology, and online monitoring and prevention technology for sealing status can further improve the sealing performance and reliability of the flanges. Furthermore, selecting high-temperature resistant bolts is one of the key measures to ensure low or minimal leakage levels during a major overhaul cycle after the inlet and outlet flanges of the hydrogenation reactor are insulated. This effectively reduces energy consumption and environmental pollution, while improving production efficiency and safety.
4. Flange Sealing Assembly: Online continuous monitoring data and leakage trends of the flange sealing status help companies understand the equipment’s operating status promptly and prevent potential safety hazards. Real-time monitoring and analysis of flange sealing data can provide customers with scientific and reasonable recommendations on when to perform hot tightening, thereby reducing the risk of equipment failure and ensuring production safety.
Third, Post-Repair Operational Results.
After flange repair, a new elliptical metal ring gasket was installed. After on-site grinding, pressure testing, and airtightness testing, it was put into operation. No leakage occurred during a major overhaul cycle. During periodic maintenance, the flange sealing surface was disassembled and inspected, and found to be in good condition.First, Analysis of the Causes of Flange Leakage
For flange seals, leakage is related to many factors, including the physical properties of the sealed medium, operating conditions, the roughness of the flange sealing surface, the clamping force, the quality of the flange itself, and the basic conditions, dimensions, and loading/unloading process of the gasket. In a certain plant, the dissimilar steel welded joint on the outer side of the outlet pipeline flange of the DC102 hydrocracking reactor developed cracks and leaked under high temperature, high pressure, and hydrogen-exposed conditions.
Since the metal annular gasket was replaced in the K201 pre-hydrogenation reactor of the reforming unit after the last overhaul operated for two and a half years without leakage, and the sealed process medium and flange remained unchanged, the influence of operating conditions can be considered alone.
01: Statistical analysis shows that in the period between the last reactor inspection and the occurrence of leakage, there were approximately 20 start-up and shutdown cycles. Under certain pressure and temperature operating conditions, the gasket experienced fatigue failure, leading to leakage.
02: Prolonged high temperatures cause creep and stress relaxation in bolts and gaskets, leading to a decrease in gasket sealing pressure. Bolts are stretched, and flanges deform. If the residual sealing pressure is lower than the minimum working sealing pressure, leakage will occur.
03: Once a leak occurs, high-pressure material overflows from the leak point, eroding the sealing surface and causing radial pitting.
Second, Leak Prevention Measures, Flange Repair, and Assembly
1. Avoid abnormal temperature fluctuations and excessively rapid heating and cooling rates during start-up and shutdown.
Minimize unplanned shutdowns and avoid drastic temperature changes in the reactor, mitigating the temperature gradient.
01: During shutdowns and material stoppages, the introduction of flushing hydrogen into fresh feed and circulating oil pipelines should be done slowly to prevent leakage from high-temperature, high-pressure flanges due to rapid cooling. If a flange leaks due to cooling, steam should be used to purge the oil and gas promptly to prevent fire.
02: Thermal expansion and stress during equipment heating can cause small leaks at flange and gasket joints. When such a leak occurs, a steam hose should be placed at the leak point to disperse the oil and gas, preventing a fire before the connection is tightened. To minimize the hazards of thermal expansion, the heating rate should generally not exceed 25℃/h.
03: After maintenance or during the first start-up, bolts on equipment pipelines may have been replaced or disassembled. These were tightened at room temperature (cold state), whereas during normal production, they are tightened at higher temperatures.
2. Flange Sealing Surface Repair
Due to uneven erosion on the ring contact surface of the flange, repairs must be carried out in accordance with the “Pressure Vessel Safety Technical Supervision Regulations.” The equipment connection flange is connected to the equipment body and can be repaired on-site using specialized tools. These specialized tools consist of a support plate, a central shaft, and a tool holder.
3. Selection of High-Performance Gaskets and Digital Technology
For reforming hydrogenation reactors operating within a relatively wide temperature range, the selection of high-performance gaskets is crucial. Simultaneously, combining high-precision constant torque tightening technology, digital on-site flange assembly quality assurance process control technology, and online monitoring and prevention technology for sealing status can further improve the sealing performance and reliability of the flanges. Furthermore, selecting high-temperature resistant bolts is one of the key measures to ensure low or minimal leakage levels during a major overhaul cycle after the inlet and outlet flanges of the hydrogenation reactor are insulated. This effectively reduces energy consumption and environmental pollution, while improving production efficiency and safety.
4. Flange Sealing Assembly: Online continuous monitoring data and leakage trends of the flange sealing status help companies understand the equipment’s operating status promptly and prevent potential safety hazards. Real-time monitoring and analysis of flange sealing data can provide customers with scientific and reasonable recommendations on when to perform hot tightening, thereby reducing the risk of equipment failure and ensuring production safety.
Third, Post-Repair Operational Results.
After flange repair, a new elliptical metal ring gasket was installed. After on-site grinding, pressure testing, and airtightness testing, it was put into operation. No leakage occurred during a major overhaul cycle. During periodic maintenance, the flange sealing surface was disassembled and inspected, and found to be in good condition.
Post time: Nov-28-2025