Application scenarios for sealing points to address the safety risk of valve and flange leaks in chemical, petrochemical, and refining plants——Detect leaks early to prevent problems before they occur

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Difficulties and challenges in detecting leaks at the sealing points of valve flanges: In high-temperature and high-pressure systems, as well as those with alternating high and low temperatures, where the process medium consists of highly toxic or highly reactive volatile organic compounds (HRVOCS), the static and dynamic sealing points of the equipment pipes and their components are extremely prone to leakage； If these high-risk sealing points are located at high elevations or in physically or chemically inaccessible areas such as confined spaces, they pose a risk of leakage. (1) Alternating high and low temperatures can easily lead to leaks. When equipment and pipeline flanges are exposed to high temperatures as well as alternating high and low temperature conditions over a long period of time, the bolt connecting those flanges is prone to stress relaxation or creep, which can result in leaks very easily ； If leaks are not detected early, the flange joint will gradually develop from a minor leak that poses a safety risk into a major leak, eventually leading to safety and environmental accidents. (2) Challenges in detecting toxic and harmful gases. Toxic and harmful gases such as benzene, butadiene, carbon monoxide, and hydrogen sulfide can cause physical harm to personnel carrying out inspections on site if they leak; in the presence of high temperatures, explosions may also occur. (3) Unreachable sealing points pose high safety risks. These leak-prone, unreachable sealing points remain isolated due to specific physical or chemical factors, making it impossible to know whether they are leaking or not. There are no effective human-mediated methods for detecting and repairing such leaks, resulting in monitoring blind spots. Given the widespread presence of such unreachable sealing points in oil refining and chemical processing facilities, these blind spots can turn into full-blown monitoring gaps, creating vulnerabilities in safety management and weakening the integrity of mechanical structures ； And the areas that cause leakage accidents in refining and petrochemical plants often stem from these weak points. (4) Limitations of handheld gas detectors: Current LDAR technologies typically rely on handheld gas detectors for operation, and these detectors are constrained by factors such as sensor principles, sensor power consumption, battery level, and manual inspection. Firstly, the PID and FID detectors need to be placed close to the detection point, which makes the detection process difficult ； Although infrared sensors can detect leaks from a distance, there are limitations regarding range, they are highly affected by heat sources, and they can only detect large leaks. Points that are beyond range or out of line of sight cannot be detected. Therefore, handheld gas detectors struggle to detect leaks of toxic and harmful gases in areas that are difficult for personnel to reach (such as very high heights, environments with highly toxic substances, or areas with extreme temperature fluctuations). Leaks in such locations cannot be detected, resulting in numerous blind spots. Areas that are blind spots generally include: sealing points or welds beneath the insulation layer in high areas, locations that cannot be accessed through manual inspection, and areas that cannot be detected by portable infrared thermal cameras. (5) In accordance with the relevant provisions of the Work Safety Law and the Measures for the Safe Management of Hazardous Chemical Leaks in China’s petrochemical industry, and based on the newly revised Work Safety Law of 2021, production and business operations entities shall establish, improve, and implement a system for hierarchical control of safety risks. They shall take corresponding technical and managerial measures in line with these hierarchical control measures to promptly identify and eliminate potential accident hazards. In accordance with the \"Safety Management Measures for Hazardous Chemical Leaks at Sinopec\", a leak constitutes an accident; therefore, during the design and construction phases, it is necessary to fully identify and assess leakage risks, and take measures at the source to control the hazards associated with leaks. Leaks in blind spots and areas with limited visibility are difficult to detect, representing weak points in the management of mechanical structure integrity. If such areas remain outside the scope of manual inspections and conventional leakage detection methods for an extended period, their safety status remains unknown, and they can easily turn into vulnerabilities in safety management – just as a dam thousands of miles long can be destroyed by tiny holes caused by ants” ； Therefore, the blind spots of manual inspections, LDAR, and GDS leakage detection must also be taken into account. Based on the duration and frequency of leaks, common leak-prone sealing points in refining and petrochemical plants can be classified into continuous leaks, periodic short-term leaks (primary release sources), and occasional short-term leaks (secondary release sources). According to the \"GB/T 50493-2019 Design Standards for Detection and Alarm of Flammable and Toxic Gases in Petrochemical Industries\", common locations for short-term leaks include: 1. The dynamic seals or mechanical seals of gas compressors and liquid pumps ； 2. Frequently used liquid sampling ports and gas sampling ports ； 3. Frequently used liquid/gas discharge (water) ports and vent ports ； 4. Flanges that need to be disassembled daily or per shift according to process requirements, and valve assemblies that are used frequently. The main targets detected by combustible gas and toxic gas detectors are devices or locations that belong to category 2 release sources. Sudden leaks and fugitive leaks are generally continuous leaks. Sudden leaks require immediate handling, with the duration usually not exceeding 1 to 2 hours ； Diffusive leaks that exceed certain thresholds, such as 2,000 MMOL/MOL or 20,000 MMOL/MOL, require prompt treatment. Sudden leaks often occur at locations where the material of the equipment has deteriorated (such as graphitization, hydrogen bubbling, creep, and aging of sealing materials), at points of geometric discontinuities and defects (such as slag in welds, incomplete welding, cracks, tees, elbows, etc.), due to discontinuities in stress and strain (such as excessive settlement of the equipment pipeline foundations, failed pipeline supports, cracks, or design flaws), or as a result of stress corrosion or localized corrosion and perforation. Such locations include air coolers, welds, elbows, tees, valves, multiphase flow sections, the gas-liquid interface of the equipment, and flange threaded connections. High-risk areas for sudden leaks are commonly found in units such as atmospheric and vacuum distillation, catalytic cracking, gasoline and diesel hydrogenation, hydrocracking (reaction, separation, desulfurization units), continuous reforming (pre-treatment, reforming units), desulfurization units, ethylene production (pyrolysis, quenching units), polyethylene and polypropylene production (polymerization, refining units), aromatic compound production (benzene manufacturing, extraction units), synthetic rubber production, syngas production, and ammonia synthesis (purification, gasification units), as well as in storage and transportation equipment that is beyond its service life or contains defects. Diffuse leaks are common in flange connections, threaded connections, valve packing seals, pipe welds, equipment manholes, and dynamic seals of pumps and machinery. The level of leakage risk can be comprehensively assessed based on the probability of leakage occurring (L), the severity of the hazards resulting from the leakage (C), and the detectability of the leakage (D). The probability of leakage in moving equipment, as well as the severity of the consequences it causes, are much higher than those in stationary equipment; therefore, such equipment receives far more attention and monitoring ； However, the total number of stationary equipment is usually about 100 times that of moving equipment. The total number of leakage points and potential leakage areas is often higher in stationary equipment as well. The technologies available for real-time monitoring of its condition are far less sophisticated than those used for moving equipment, resulting in a lower detectability level; yet the number of safety accidents caused by stationary equipment is not fewer than those caused by moving equipment. The safety risk level of fugitive leaks from many stationary equipment is also high. The most prone to leakage in static equipment are various flanges and the static sealing points of valves; the leakage risk level can be assessed using the product R = L × C × D, or according to the informational appendix in Appendix A of the \"Technical Specifications for the Installation of Flange Sealing Structures\" ； You can also refer to the diagram below for a quick assessment. For example: if a leakage site is located on the equipment pipeline at the intersection of attributes I, II, and III as shown in Figure I below (i.e., I∩II∩III), it can be quickly assessed as a high-risk leakage site ； If it is located on the equipment piping at the intersection of any two of the attributes shown in Figures I, II, and III below (i.e., I∩II, II∩III, or I∩III), it can be quickly assessed as a leakage site with a medium risk level. Class A and Class B sealing points account for less than 20% of all such points, yet they are responsible for 80% of leaks in sealing structures. Whether it is a sudden leak or a gradual one, if it is not detected in time or if it is not dealt with promptly and scientifically, it can have serious consequences for the production safety of refining and chemical processing plants or units.