How can we prevent the recurrence of flange leakage accidents in refining and petrochemical plants?

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At around 21:07 on November 27, 2021, during the inspection, maintenance, and commissioning of the S-Zorb (coking hydrogenation) unit in the Fourth Refining Department of Sinopec’s Guangzhou branch, leakage of coker gasoline from the flange at the top of the reactor led to a fire. The hydroprocessing unit for coker gasoline at Guangzhou Petrochemical uses the foreign S-Zorb patented process technology. The schematic diagram of its operational principle is as follows: The key equipment in this process unit is the hydrogenation reactor shown in the diagram, with an operating temperature of around 420° and an operating pressure of around 3.0 MPa. The head flange of the reactor refers to the high-temperature oil and gas flange at the outlet at the very top of the reactor; its pressure rating is usually CLASS 600 or higher. The nominal diameter of such flanges ranges from DN450 to DN900, and their thickness is generally over 80 mm. In this context, the sealing performance of the flange connections becomes the weak point of the entire system ； If the flange seal is not tight, hydrogen under high temperature and pressure will leak out first. When a mixture of hydrogen and light hydrocarbons leaks into an oxygen-rich atmospheric environment, it can catch fire spontaneously at temperatures of 300°C to 400°C, even in the absence of an ignition source. So how can we ensure the sealing of the flange? Figure 1: High-temperature hydrogen-containing oil and gas flange with the insulation layer removed. At 7 p.m. on June 9, 2021, Hong Weidong, who was working outdoors at the LTAG complex of Yangzi Petrochemical, became alert when he noticed a drop of \"rainwater\" on site. He immediately used a walkie-talkie to call the chief operator Wang Zhiyong; the two of them divided their tasks and checked layer by layer – layers 1 through 6 were normal, layer 7 was also normal, and there was no leakage on layer 8… It was not until they reached layer 12 (at a height of approximately 60 to 72 meters) that they discovered gasoline leaking from the flange located in front of a high-pressure valve in the absorber tower. They promptly reported this to the workshop, which took immediate action to address the issue. Yangzi Petrochemical’s top management awarded substantial bonuses ranging from ¥3,000 to ¥10,000 to the frontline employees who detected the severe leakage in high-risk equipment and the violations promptly. Figure 2 shows a leak at the flange on the high platform of the stabilizer unit in Yangzi Petrochemical’s LTAG complex. However, if the flanges for high-temperature oil and gas at locations such as the top of the tower are equipped with insulation layers, then after 2–3 years of operation under normal conditions, due to creep stress relaxation, reduced resilience of the gaskets, or aging of the sealing materials, minor leaks that are not visible – meaning no oil droplets fall – can occur, and these leaks cannot be detected in time by operators ； Under high-pressure and high-temperature operating conditions, minor leaks can develop into moderate leaks (≥2000–5000 μmol/mol) or even severe leaks (≥10000 μmol/mol) within 3–4 weeks. Even if a fire caused by hydrogen-containing light hydrocarbon gases is detected through visual inspection, the leaking high-pressure gases have already caused visible radial grooves in the sealing materials (usually 0.1–0.5 mm in width). What should be done? At this point, thermal tightening is usually ineffective; should leakage be sealed under pressure online, or should an unplanned partial shutdown be carried out to fix the issue? Incorrect flange connection materials, design, and stress design, as well as quality defects in gaskets and issues with installation, can all cause leakage at the flange seal. However, since the aforementioned reasons are essentially hidden in nature – that is, after the flange bolts are tightened, it is not possible to visually determine the sealing condition of the flange connection – it becomes impossible to know whether the installation is successful or not. In such cases, a leak test is necessary. The traditional approach to leak testing involves conducting nitrogen pressure tests at room temperature in accordance with standard requirements, and then spraying soapy water on the flanges to determine whether there are leaks. However, this method has certain limitations, as there is a lack of systematic experimental research on the long-term sealing performance of flanges under high-temperature conditions. There is also a lack of data on the reduction in resilience and creep aging of gaskets after long-term use under high temperatures, which makes it impossible to design and optimize parameters such as the structural dimensions and stress levels of various components in flange connections according to the actual operating conditions. In many cases, even though the torque tightening quality checks were successful, and the flange bolt connections passed the airtightness tests and hydrostatic tests, leaks still occurred under high temperature and pressure after they were put into use. What should be done in this situation? The common practice among companies is to perform secondary tightening of the flanges in stages after commissioning, based on the temperature rise of the materials and industry experience; this is commonly referred to as \"thermal tightening\". If no leaks are detected after this secondary tightening, it can generally be considered that the system is satisfactory. In the event of leaks similar to those at Guangzhou Petrochemical and Yangzi Petrochemical, if such leaks occur in areas beneath insulation layers or other locations that cannot be accessed through regular inspections, what measures can be taken to prevent fires caused by high-temperature oil and gas leaks, or to address the extremely high risks associated with attempting to seal such leaks under pressure? Firstly, there is online monitoring and prevention of flange sealing conditions. This is not a detection and alarm system for toxic, harmful, flammable, and explosive gas concentrations as specified in the \"GB/T 50493-2019 Design Standards for Detection and Alarm of Flammable and Toxic Gases in Petrochemical Industries.\" Gas concentration detectors are generally installed at a distance of 0.5 – 1.0 meter from the sealed points; they can only detect leaks when they are quite large (LDAR can detect leaks at levels of 20,000 – 30,000 μmol/mol, while at a distance of 0.5 – 1.0 meter from the leak site, the concentration is only a few or dozens of PPMV or μmol/mol) ； Secondly, the wind is strong at higher elevations, and the prevailing and secondary wind directions vary depending on the season or month. The density of high-temperature hydrogen-rich C1-C2 light hydrocarbon mixtures is generally lower than that of air, so they drift upward with the wind in case of a leak; therefore, installing only 1-2 concentration detectors is often not sufficient for detecting and warning of leaks at flanges. Furthermore, the explanations in clauses 4.1.4, 4.2.1, 4.2.2, 4.2.3, and 6.1.2 of standard GB/T 50493-2019 state that for release sources located in open and elevated areas, if the amount of leakage is low and the scale of such sources is small, resulting in no hazardous environment, it may not be necessary to install sensors – is this because the cost of wiring such old devices in use is too high, or because the detection rate is too low? An online continuous monitoring and prevention system for the condition of critical sealing points, unaffected by weather conditions, is capable of detecting minor leaks at flange seals that are prone to leakage on the top of towers or tanks or beneath insulation layers. It automatically sends yellow or orange alarm messages to those responsible for static seal maintenance or equipment integrity management, informing them promptly: \"Owner, I’m starting to leak; please pay attention to me😢.\" Figure 3: Online monitoring and prevention system for flange sealing status (note the droplets at the bottom). Next, what should be done in the event of a flange leak? Similar to vibration monitoring of pumps or compressors, the purpose of seal condition monitoring is to enable predictive maintenance. In the past, the timing of thermal tightening for flanges used with high-temperature oil and gas was determined mainly based on experience. If leakage persisted even after a second tightening attempt (the sealing surface having been damaged by the high-speed flow of oil and gas), further tightening only made the leakage worse, until the gasket was crushed and no longer functional; in such cases, alternative temporary measures such as replacing the gasket or performing plugging under pressure were required, forcing the equipment to operate despite its defects. When dealing with leaks, it is important to note that as soon as a leak is detected, a decision must be made at the site of the leak (in a safe area) regarding whether to take action and how to do so, as quickly as possible. There should be no unnecessary demands on frontline employees to prepare numerous forms and documents in meeting rooms, such as those requiring an analysis of the causes or identification of responsibilities, followed by cross-departmental approval processes. What matters is to address the leak promptly, rather than trying to create evidence to prove that one has fulfilled their duties. Otherwise, it may lead to minor leaks that could be handled locally not being reported even after they have been dealt with (due to the requirement to fill out many forms), and reports are only made when they can no longer be handled on site (which may result in missing the optimal time for handling them). Predictive maintenance is far cheaper than reactive pressure sealing repairs and conditional preventive maintenance. Maintenance costs for oil refining and chemical enterprises typically account for 15% of production costs ； In the United States, companies spend over $200 billion each year on maintenance costs, of which one-third is wasted. China’s refining and petrochemical production scale has been the largest in the world since 2022; will the amount of waste generated be less than that of its American counterparts? Figure 4 shows that the proportion of predictive maintenance used by top chemical companies is more than 400% that of ordinary companies. Figure 5 illustrates the predicted leakage trend through time-series analysis (orange line); this corresponds to the situation described in Figure 3, where the flanges leak once every 3–4 minutes, and since thermal tightening safety measures are not in place, thermal tightening can be postponed for now. Thirdly, this applies to the high-temperature oil and gas flange seals of critical equipment. High-performance gaskets capable of withstanding alternating high and low temperatures as well as creep relaxation should be used, in combination with high-precision torque-controlled fastening and online continuous monitoring of the sealing condition to implement predictive maintenance, thereby avoiding emergency repairs under pressure or unplanned partial shutdowns to address issues. For those high-temperature oil and gas pipeline flange seals with excellent stress distribution and stress level design, insulation layers can also be installed as per design requirements, which is energy-saving and environmentally friendly, reduces energy consumption – achieving two benefits at once! In short, for important high-temperature hydrogen-handling, high-temperature oil and gas equipment and pipelines, it is necessary to minimize leakage sources such as flange and valve connections during the planning and design phase ； During the inspection and maintenance phase of the equipment, it is essential to ensure the integrity of the flange seals and gaskets; hiring experienced installers can often yield twice the results with half the effort ； Most importantly, while strengthening human-based prevention measures, it is also necessary to enhance technical prevention methods. In particular, for those high-risk, leak-prone sealing points that are inaccessible through manual inspections, such as those located under the insulation layers on the tops of towers and tanks, online continuous monitoring systems for flange sealing conditions should be installed to detect leaks early and prevent problems before they occur!