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Leakage accidents caused by flange connections in petrochemical plants are on the rise, posing significant risks to the operation of these plants. By analyzing the reasons for flange leaks, this paper introduces the concept of flange management, proposes specific process control measures as well as a reasonable method for calculating bolt tightening loads, in order to improve the quality of flange installation and reduce the incidence of leaks. In petrochemical plants, process pipelines are akin to the blood vessels in the human body – complex in structure – and the fluids flowing through them are often toxic and harmful. Therefore, ensuring the quality of pipeline construction becomes a key aspect of project quality control. When it comes to the quality of pipeline construction, the first thing that comes to mind is likely the quality of pipeline welding. However, in petrochemical plants, in addition to welding, flange connections are also widely used for joining pressure pipelines. There are a large number of flange bolts involved, and the operating conditions are complex. In recent years, the number of leakage accidents caused by flange connections has been increasing; approximately 18% of all leakage accidents are due to flaws in these flange connections. Therefore, it is particularly important to control the quality of flange installation and to improve the management of flanges. 1 Main causes of flange leakage There are many reasons for flange leakage, and during the construction phase, the main causes include the following: 1) Damage to the sealing surfaces such as the flange and gaskets ； 2) Incorrect use of materials such as gaskets or bolts, which fails to meet the operating conditions required for pipeline operation ； 3) Flange misalignment causes leakage ； 4) Improper tightening causes damage to the gasket ； 5) Due to insufficient tightening load, the gasket cannot achieve a sealing performance ； 6) The gasket was damaged due to excessive tightening load. 2 Control measures to prevent flange leakage. As discussed above, given the reasons that lead to flange leakage, to ensure that flanges do not leak it is necessary to implement strict process control procedures for flanges, to guarantee the quality of their installation as well as to select an appropriate tightening force so that the gaskets can achieve good sealing performance. The control measures will be introduced below from two aspects: flange process management and fastening load calculation. 2.1 Flange Process Management 2.1.1 Personnel Training and Management All personnel involved in flange management must receive training and work with valid certificates; operating without such certificates is strictly prohibited. Training consists of theoretical training and practical training. The theoretical training is conducted by professional engineers who teach the operators the theoretical knowledge related to flange management. Practical training primarily involves providing on-site, systematic hands-on training for dedicated operators, covering the overall quality control process as well as equipment usage skills. 2.1.2 Material Management Material management includes the management of flanges, fasteners, and gaskets. All materials must undergo acceptance inspection upon arrival to ensure they meet the required quality standards. The acceptance requirements for materials are as follows: 1) All materials must come with quality certification documents, and the contents of these documents must comply with the design and specification requirements. 2) Upon arrival of the flanges, each one should be inspected for its sealing surface; the sealing surface must be intact, without any defects such as rust or radial scratches. 3) The outer edge of the flange shall have the specified markings, and they shall correspond to the quality certification documents. 4) Flanges that have passed the inspection should be stored indoors and protected from rust. 5) During maintenance work, after the flange is opened, the condition of the flange sealing surface should be inspected, and any impurities that could affect sealing should be removed ； 6) Upon receipt of the fasteners, ensure that their threads are intact, free from defects such as scratches, burrs, or rust ； If the threads are incomplete, it is strictly prohibited to use it. 7) After the fasteners pass the inspection, a thread protector should be applied to the thread surfaces, and it must be applied evenly over the threaded areas. 8) Upon receipt, the wound gaskets shall be inspected; there should be no signs of looseness or warping, and their surfaces must not have any defects that could affect their sealing performance. 9) For the bolts to be used after flanges are removed in maintenance work, the principle of first cleaning them collectively, then inspecting them, and finally using them should be followed. During cleaning, check whether the threads are damaged and whether the nut can be screwed onto the bolt at any position smoothly; bolts that do not meet these standards must not be used. 2.1.3 Equipment Selection: Depending on the bolt specifications, different tightening equipment is used, with manual torque wrenches, electric torque wrenches, and hydraulic wrenches being the most common ones. Bolts with a torque of ≤1000 N·m can be tightened using a manual torque wrench, while bolts with a torque greater than 1000 N·m are recommended to be tightened with an electric torque wrench or a hydraulic wrench. An impact wrench can be used for initial tightening to improve efficiency. 2.1.4 Flange Installation 1) Before installing the flange, it is necessary to inspect the flange sealing surface and gaskets to ensure that there are no defects that could affect the sealing performance, and the protective grease on the flange sealing surface must be removed. 2) The bolts connecting the flanges should be able to pass through freely. 3) The installation direction and exposed length of the flange bolts should be consistent. 4) Hand-tighten the nut to ensure it rotates smoothly on the stud. 5) The flange must not be installed at an angle, and the parallelism of the flange sealing surfaces must meet the specification requirements. 2.1.5 Bolt Tightening: The flange connection bolts should be tightened in a symmetrical sequence, with the tightening torque being increased in stages until the desired final torque is achieved. The specific steps are as follows: Step 1: Hand-tighten the nuts, and use a manual wrench to apply initial tension, while ensuring proper parallelism of the flanges. Step 2: Tighten the bolts to a final torque value of 30%, using a symmetrical tightening method, and verify the flange spacing to ensure consistency. Step 3: Tighten the bolts to a final torque value of 60%, using a symmetrical tightening method, and verify the flange spacing to ensure consistency. Step 4: Tighten the bolts to the target torque value using a symmetric tightening method, and verify the flange spacing to ensure consistency. Step 5: Continue to tighten the bolts to the target torque value, tightening them in sequence, and finally verify the flange spacing to ensure consistency. After tightening the flange bolts following the steps above, use a marker to draw crosshairs on the end face of the studs/nuts. If possible, attach a label to the flange, indicating the pipeline number, flange identifier, target torque value, the person who performed the operation, and the time of execution. 2.2 Calculation of bolt tightening load Selecting an appropriate tightening load is also a key aspect of flange management. Currently, there is no unified or standard calculation method specified in domestic codes and literature; most of the information provided refers to a fixed value for the tightening torque. However, the tightening torque should not be a fixed value but rather fall within a certain range. This is necessary to ensure that the flange connection remains sealed, as well as to prevent the gaskets from being damaged and the bolts from breaking or becoming ineffective ； Furthermore, the bolt tightening torque is not either a very precise value. According to the torque calculation formula: T = KFd, where T represents the torque, K is the torque coefficient (usually ranging from 0.1 to 0.2), F is the preload force, and d is the nominal diameter of the bolt. The torque coefficient K is a variable value that depends on the smoothness of the thread contact surface, the smoothness of the nut and flange surfaces, as well as whether lubrication is used. Changes in the torque coefficient have a significant impact on the torque value; therefore, to reduce errors, it is necessary to ensure accurate calculation of the preload force. 2.2.1 Calculating the pre-tightening force based on gasket performance 1) Under operating conditions, the minimum load required to tighten a single bolt is calculated using the minimum compression force of the gasket, according to the formula: Fo = (F + Fp)/n, where F represents the total axial force resulting from internal pressure under operating conditions, Fp represents the minimum compression force of the gasket under those conditions, and n is the number of bolts. The methods for calculating F and Fp are specified in GB150.3. 2) Under the pre-tensioned condition, the minimum bolt load Fg1 required to tighten a single bolt is calculated based on the minimum compression force of the gasket, using the formula: Fg1 = Fa/n, where Fa represents the minimum compression force of the gasket under pre-tensioned conditions; the method for calculating Fa is specified in GB150.3. 3) Based on the maximum allowable stress of the gasket, calculate the maximum bolt load Fg2 required for bolting. The formula is: Fg2 = (Sg × Ag) / n, where Sg represents the maximum allowable stress of the gasket, and Ag denotes the sealing area of the gasket. 2.2.2 Calculating the pre-tightening force based on the allowable stress of bolts: Using the allowable stress of bolts, the minimum bolt load Fb1 and the maximum bolt load Fb2 required for tightening each individual bolt are calculated respectively ； The formulas are: Fb1 = 0.25 × Rel × Ab, Fb2 = 0.7 × Rel × Ab ； In the formula: Rel is the yield strength of the bolt, and Ab is the stress cross-sectional area of the bolt. 2.2.3 Comprehensive comparison: Determine the minimum load Fmin and maximum load Fmax required for tightening each bolt respectively: Fmin = max(Fo, Fg1, Fb1); Fmax = min(Fb2, Fg2). 2.2.4 Calculate the range of bolt tightening torques, Tmin and Tmax: Tmin = k × Fmin × d; Tmax = k × Fmax × d. 2.3 Key points for quality process control 2.3.1 Pre-installation inspection: Check carefully whether the inside of the pipe is clean, whether there are any damages to the flange surfaces and gaskets, and verify that the material and specifications of the bolts and gaskets match those specified in the design drawings. For corroded flange sealing surfaces, manual wire brushes or other manual tools can be used to remove rust, and cleaning agents should be employed to eliminate stains. The condition of the sealing surface should be checked; flange sealing surfaces that are severely damaged should not be used. For inspection and maintenance equipment, after removing the flange, check its sealing surface; if it is damaged, it must be replaced. 2.3.2 Quality inspection after flange bolts are tightened: After the flange bolts have been tightened, the project quality inspection personnel conduct random checks on the bolt torque and flange parallelism based on the inspection report; once they confirm that everything is satisfactory, they sign to approve it. 2.3.3 Pressure testing for verification Pipe pressure testing is an important method for checking the sealing performance of flanges; during this process, it is essential to examine the sealing performance of each pair of flanges to ensure that there are no leaks ； To verify the sealing performance of the flanges during the pressure testing process, the following steps must be followed: 1) If a leak is detected in the flanges, it is strictly prohibited to tighten them while under pressure; the location of the leak must be identified and marked, and repairs should be carried out after the pressure is released ； 2) After pressure release, check the parallelism of the flanges and the tightening torque of the bolts. If the parallelism is not correct, loosen the bolts on the opposite side of the open end, adjust the parallelism of the flanges again, and then retighten them following the bolt tightening procedure ； If it is only the bolt torque that does not meet the requirements, it can be tightened directly to the target torque ； 3) After adjusting the flange, conduct a pressure test again; the test is considered successful if there is no leakage, and the flange bolts should be marked once more ； If there is still a leak, the pressure must be relieved before the flange is opened; the flange’s sealing surface and gaskets should be inspected, and new gaskets should be installed. If the flange’s sealing surface is damaged, it must be replaced or repaired on-site. After the defect has been resolved, pressure testing should be conducted again until the results are satisfactory, after which the flange bolts should be marked. 3 Conclusion Based on the aforementioned theories and data analysis, it can be seen that the management of pipeline flanges is a systematic task that requires taking into account all the factors related to the sealing performance of the flanges. These factors include flange alignment, the condition of the sealing surfaces, the quality of the sealing gaskets, proper use of bolts and control of torque values, as well as changes in the conditions under which the processing is carried out. Only through strict process control, enhanced material management, and strict control over fastening loads can the potential risk of leaks in flanges be effectively prevented.