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When operating under its design conditions, bolted flange joints are expected to achieve their maximum service life. Whether before or after the expected lifespan, a flange joint is considered failed once its leakage exceeds the legally mandated environmental limits or the limits specified at the plant site. The failure of flange joints can endanger the proper operation of equipment or pipelines, leading to the failure of the entire system and forced shutdown. Reports indicate that in the total volatile emissions from some factories, those originating from flanges account for 28%. The failure modes that can arise from the various components of a simple bolted flange joint manifest themselves as leakage of the sealing medium along the surface of the gasket or as internal leakage. The amount of leakage in these cases can be small or minimal, but when the gasket ruptures due to blowing out, it can lead to significant leakage. Studies show that the main causes of leakage failures in most flange joints are not gaskets but other factors, including improper installation (26%), damaged flanges (25%), missing bolts (15%), and misalignment of the flanges (12%). Only a small proportion (26%) of the failures are directly related to gaskets. Early failures manifest as the gaskets not being damaged, which is mostly due to insufficient, excessive, or uneven pre-tensioning, damage to the flange sealing surfaces (scratches, warping, corrosion, etc.), and installation errors of the flanges (non-parallelism, misalignment, etc.). By observing the damage and variations around the gasket, especially in the case of soft gaskets, it is possible to determine the causes of gasket failure, such as improper gasket thickness, inappropriate selection, excessive compression, insufficient pre-tightening force, improper storage, gasket degradation, or repeated use of the gasket. Depending on the time period when the leakage occurred, there are several reasons for the failure of bolted flange joints. 1. Pressure testing: To detect leaks, pressure vessels and equipment must undergo strength tests (hydraulic or pneumatic) after completion of manufacturing or before being put back into use again; these are pressure tests carried out at pressures higher than the operating pressure. Since no testing is conducted or temperature cycling loads are applied, if a leak occurs at the flange joint in this test, it indicates significant problems with the installation or bolt loading, including: (1) improper installation and uneven gasket loading ； (1) Excessive installation flange error ； (1) The material grade of the bolts and nuts is incorrect; the threads are corroded, the bolts yield or deform excessively. Stainless steel bolts with a lower strength grade should be used, such as ASME SA-193 B8 C1 (304), which is equivalent to 06Cr19Ni10 (0Cr18Ni9) in China ； (1) Wear on the bearing surfaces of certain bolts or nuts ； (1) The gasket is not placed in the center of the sealing surface ； (1) Damage to the flange sealing surface, warping or deformation of the flange ring. 2. Leakage occurs when pressurization starts: (1) Improper installation, insufficient compression force on the gasket, or sliding due to inadequate friction between the gasket and the flange, or the gasket being crushed or broken as a result of excessive compression ； (1) Fastener failure, including bolt breakage, chemical corrosion, stress corrosion cracking, fatigue and relaxation, nut loosening, etc ； (1) The flanges are not aligned ； (1) The flange sealing surface is damaged, creating a leakage path ； (1) Improper selection of gaskets, with insufficient pressure-bearing capacity, including gaskets that are too thick and exceed the allowable pressure range ； (1) Damage to the gasket caused by improper pipe supports or restraints ； (1) Use old gaskets. 3. Leakage during heating or initial operation: (1) Excessive initial creep of the gasket during heating causes loss of bolt load or extrusion of the gasket ； (1) Differences in thermal expansion among different components lead to loss of bolt load ； (1) Improper installation, uneven gasket loading, or excessive compression ； (1) The wound gasket bends inward or the outer ring arches upward ； (1) Wear on the bearing surfaces of certain bolts or nuts ； (1) Excessive bending moment caused by improper pipe supports or restraints ； (1) The gasket is not placed in the center of the sealing surface ； (1) The gasket was selected improperly, exceeding its allowable pressure and temperature range. 4. Leakage occurs due to multiple thermal cycles or pressure fluctuations (1) Temperature and pressure fluctuations cause loss of bolt load ； (1) Sudden changes in climate, such as heavy rain. 5. Leaks that occur after prolonged operation: (1) Chemical degradation of the gasket (decomposition, oxidation, corrosion, etc.); (1) Physical degradation of the gasket (the adhesive swells due to exposure to steam, organic solvents, or hot oil) ； (1) Bolt load failure caused by long-term creep of gaskets and bolts ； (1) The gasket is not suitable for the operating temperature (rubber hardens when exposed to heat) ； (1) Different radial movements of the flange cause the gasket to be cut or misalignment between the gasket and the flange ； (1) Excessive bending moment caused by improper pipe supports or restraints ； (1) Changes in system operating conditions, fluid content, heat/pressure fluctuations, vibrations, and the external environment (such as insulation failure or other factors causing external heating/cooling) ； (1) Sudden pressure changes, temperature fluctuations, thermal shocks, or vibrations ； (1) Improper bolt pre-tightening, resulting in the gasket not achieving the desired level of compression ； (1) The flanges are not aligned. 6. Leakage during parking: (1) Loss of bolt load caused by long-term creep of gaskets and differences in cooling rates among various components ； (1) The gasket is not suitable for the operating temperature ； (1) Degradation of gaskets ； (1) Different radial movements of the flange cause damage to the gasket. Among the above reasons, insufficient bolt load or gasket stress is the main cause of flange joint failure. Statistics show that 65% of the 100 cases of gasket failure were due to insufficient load, and among the factors leading to insufficient load, improper installation accounted for 36%.