Analysis of the main causes of hydrogenation reactor flange leakage and treatment measures

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PART 01 In the K201 pre-hydrogenation first reactor of a company's reforming unit, after two and a half years of continuous operation, the feed flange on the top of the equipment leaked, and the amount of leakage gradually increased. The operating pressure of the K201 pre-hydrogenation first reactor is 2.45MPa, the working pressure is 2.23MPa, the design temperature is 385℃, the working temperature is 370℃, the working medium is hydrogenated oil (mainly composed of C6~C9), and the corrosion margin is 2mm/a ； The main component materials are shown in Table 1. Table 1 K201 Pre-hydrogenation No. 1 Reactor Materials Table 2. Feed flange leakage PART 02 1. Flange leakage: The leakage point is the feed flange on the top of the equipment. Fastening bolt specification M33x260mm, material 35CrMoA/35CrMo. Figure 1 is a photo of the reactor feed flange. Because the contact between the oval metal ring gasket and the ring connection surface is discontinuous, breakpoints are formed in local areas, resulting in gaps in the seal between the metal ring gasket and the ring connection surface, forming leakage channels. After measurement, the longest discontinuous sealing chord is about 17mm, and the deepest radial groove formed by material erosion is 0.8mm. Figure 1 Photo of reactor feed flange 2. Leakage Cause Analysis For flange seals, the leakage situation is related to many factors such as the physical properties of the sealed medium and working conditions, the roughness of the flange sealing surface, the pressing force, the quality of the flange itself, the basic conditions and dimensions of the gasket, and the loading and unloading process. For example, the A182-F347 material of the outlet pipeline flange of a hydrocracking reactor in a 3.6 million t/a hydrocracking unit of a natural gas company in Dalian Petrochemical cracked and leaked during use due to the existence of structural defects such as coarse austenite grains and brittle precipitate phases. The dissimilar steel welded joint on the outer side of the DC102 outlet pipeline flange of the hydrocracking reactor of Yangzi Petrochemical Aromatics Plant cracked and leaked under high temperature and high pressure hydrogen conditions. Since the metal annular gasket replaced in the first pre-hydrogenation reactor of the reforming unit K201 has been running for two and a half years without leakage since the last overhaul, and the sealed process medium and flange have not changed, so only the influence of working conditions can be considered. 1 According to statistics, between the last inspection and the occurrence of leakage, the reactor was started and stopped about 20 times. Under the pressure of 2.45MPa and the operating condition of 370°C, the gasket suffered fatigue failure and leakage occurred. 2 Long-term high temperature causes creep and stress relaxation of bolts and gaskets, and the gasket sealing specific pressure decreases ； The bolts are stretched and the flange is deformed. At this time, if the residual sealing specific pressure is less than the minimum working sealing specific pressure, leakage will occur. 3. Once a leak occurs, high-pressure material overflows from the leak point, and the sealing surface at the leak point is eroded, resulting in radial pits. 3. Leakage prevention measures and flange repair and assembly PART 03 1. Avoid abnormal fluctuations in operating temperature and excessive temperature rise and fall rates during startup and shutdown. Minimize unplanned shutdowns, avoid sharp changes in reactor temperature, and slow down the temperature gradient of the reactor. Generally, the hydrogenation reaction is a strongly exothermic reaction, and the reaction rate is strongly affected by temperature. Improper control of the reaction temperature will cause the hydrocracking reactor to "overheat" in a short period of time. Therefore, attention should be paid to controlling the temperature rise and fall rate at all times when starting and stopping work. 1. During the shutdown process, the introduction of flushing hydrogen into the fresh feed and circulating oil pipelines should be done slowly to prevent the high-temperature and high-pressure flanges from leaking when the cooling rate is too fast. ； When the flange leaks due to cooling, the oil and gas should be purged with steam in time to prevent fire. 2. Thermal expansion and thermal stress during the heating of the equipment will cause small leaks at the flange and gasket joints. When such a leak occurs, a steam hose should be placed over the leak to blow away the oil and gas. This will prevent a fire until the connection points are tightened. ； In order to minimize the harm of thermal expansion, the general heating rate should not exceed 25°C/h. 3. After maintenance or when starting work for the first time, the bolts of the equipment pipelines have been replaced or disassembled. These are tightened at normal temperature, that is, cold. When the equipment is in normal production, it is done at a higher temperature. As the temperature rises, pipelines, flanges, and bolts will undergo thermal expansion. The thermal expansion coefficient of bolts is different from that of equipment or pipeline flanges. The fastening bolts of some flanges will loosen or the bolts will not be tightened enough due to thermal expansion. The flange seal will leak even if the air tightness test at normal temperature passes. In order to ensure no leakage and safe production of the device, online monitoring technology of thermal tightness or high-temperature oil and gas sealing points when the reactor temperature rises to 250°C can be used. The flange sealing condition monitoring system can continuously monitor online the health status of inaccessible flange sealing points under the high-risk and leak-proof insulation layer, achieve predictive maintenance, and reduce the long-term operation and maintenance costs of important equipment such as reactors or their components. Figure 2 Online continuous monitoring and prevention of high-temperature oil and gas flange seal leakage 2. Flange sealing surface repair Due to the uneven erosion of the ring contact surface on the flange, repairs need to be carried out in accordance with the "Pressure Vessel Safety Technical Supervision Regulations". The equipment take-over flange is connected to the equipment body and can be repaired at the installation site using special tools. Figure 2 is a photo of the repair site. The special tool consists of three parts: support plate, central shaft and tool holder. Fix the support plate on the flange pipe, and distribute eight jackscrews symmetrically on the support plate to align and adjust the tool holder. ； After installing the central axis and the tool holder on the support plate and aligning them, install the turning tool on the tool holder, push the tool holder manually to make a circular motion on the connecting flange to cut the ring joint surface, correct it with a 230 angle template, and finally use grinding method to make the surface roughness of the cut surface reach: The groove slope is Ra1.6 and the groove bottom is Ra6.3. Figure 3 Photo of equipment flange repair site 3. Use high-performance gaskets and digital technology. For reforming hydrogenation reactors with operating temperatures no higher than 400°C, use high-performance gaskets that are resistant to high and low temperature alternations and creep relaxation. Combined with high-precision fixed-torque fastening technology, digital on-site flange assembly quality assurance process control technology and sealing status online monitoring and prevention technology, high-temperature-resistant bolts can also ensure that the hydrogenation reactor inlet and outlet flanges are insulated and run for an overhaul cycle to maintain low leakage or small leakage levels, saving energy, reducing consumption, and being safe and environmentally friendly. 4. The flange seal assembly uses high-precision constant torque fastening technology and a non-reaction force hydraulic wrench to assemble flanges, gaskets and bolts at ambient temperature, that is, in a cold state. Determine the timing of heat tightening based on online continuous monitoring data of flange sealing status and leakage trends, prevent leakage through technology, and promote prosperity through technology. 4. Operation results after repair PART 04 After the flange was repaired, a new oval metal ring gasket was replaced. It was put into production after on-site grinding, pressure test, and air tightness test. After a period of overhaul, no leakage occurred. During periodic maintenance, the flange was disassembled to check the flange sealing surface, which was in good condition.