Pipe flange

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Leakage incidents caused by flange connections in petrochemical plants are on the rise, posing significant risks to plant operations. By analyzing the causes of flange leaks, this paper introduces the concept of flange management. It also proposes specific process control measures and a reasonable method for calculating bolt tightening loads, aiming to improve the quality of flange installation and reduce the incidence of flange leaks. In petrochemical plants, process pipelines are as intricate as the blood vessels in a human body. Moreover, most of the media transported through them are toxic and harmful; therefore, ensuring the quality of pipeline construction is a key aspect of overall project quality control. When it comes to the quality of pipeline construction, what often comes to mind first is the quality of pipeline welding. However, in petrochemical plants, apart from welding, flange connections are also widely used for joining pressure pipelines. The number of flange bolts is substantial, and the operating conditions are complex. In recent years, there has been a growing number of leakage incidents caused by flange joints. Approximately 18% of all leakage incidents are attributed to such flange-related leaks. Therefore, it is crucial to control the quality of flange installation and strengthen flange management. I. Main causes of flange leakage There are many reasons that can lead to flange leakage, and during the construction phase, the main causes include the following: 1. Damage to the sealing surfaces of the flange, gaskets, etc ； 2. Incorrect use of materials such as gaskets or bolts, which fails to meet the operating conditions required for pipeline operation ； 3. Leakage caused by flange skew ； 4. Improper tightening causes damage to the gasket ； 5. Due to insufficient tightening load, the gasket cannot achieve a sealing effect ； 6. The gasket was damaged due to excessive tightening load. II. Control measures to prevent flange leakage As discussed above, given the reasons for flange leakage, to ensure that flanges do not leak it is necessary to implement strict procedures for managing the flange installation process, to guarantee the quality of the flange and gasket installation, and to select an appropriate tightening force so that the gasket can achieve good sealing performance. The control measures will be introduced below from two aspects: flange process management and fastening load calculation. 1. Flange process management
1) Personnel training and management
All personnel involved in flange management must undergo training and obtain certifications before performing their duties. Unauthorized operation without a certificate is strictly prohibited. The training consists of theoretical instruction 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) Material management: Material management includes the management of flanges, fasteners, and gaskets. All materials must undergo acceptance checks upon arrival to ensure that they meet the required quality standards. The acceptance requirements for materials are as follows: A. All materials must come with quality certification documents, and the contents of these documents must meet the requirements of the design and relevant specifications. B. 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. C. The outer edge of the flange shall have the prescribed markings, which must correspond to the quality certification documents. D. Flanges that have passed the inspection should be stored indoors and protected from rust. E. During maintenance work, after the flange is opened, the condition of the flange’s sealing surface should be inspected, and any impurities that could affect sealing should be removed ； F. After the fasteners arrive, ensure that their threads are intact, with no scratches, burrs, rust, or other defects ； If the threads are incomplete, it is strictly prohibited to use it. G. After the fasteners have passed inspection, a thread protectant must be applied to the thread surfaces, ensuring even coverage on the threaded areas. H. 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. I. For the bolts to be used after flanges are removed in inspection, maintenance, and repair work, it is necessary to follow the principle of first cleaning them collectively, then inspecting them, and finally using them. 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. 3) Equipment selection: Depending on the bolt specifications, different tightening devices are 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. 4) Flange installation: A. Before installing the flange, it is necessary to inspect the flange’s sealing surface and gaskets to ensure that there are no defects that could affect the sealing performance; moreover, the protective grease on the flange’s sealing surface must be removed. B. The bolts connecting the flanges should be able to pass through freely. C. The installation direction and exposed length of the flange bolts should be consistent. D. Tighten the nut by hand to ensure it can rotate smoothly on the stud. E. The flange installation must not be skewed; the parallelism of the flange sealing surfaces must meet the requirements of the specifications. 5) Bolt tightening: The bolts for flange connections should be tightened in a symmetrical and sequential manner. The tightening torque should be increased in stages until the required final torque value is achieved. The specific steps are as follows: Step 1: Hand-tighten the nuts, and use a manual wrench to perform a preliminary tightening while ensuring that the flanges remain parallel to each other. 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 operation. 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 tightening torque value. 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 achieves proper sealing, as well as to prevent the gaskets from being damaged and the bolts from breaking or becoming ineffective ； Additionally, the bolt tightening torque is not a very precise value. According to the torque calculation formula: T = KFd, where T represents the torque, K is the torque coefficient (typically 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; it depends on factors such as the smoothness of the threaded surfaces, the smoothness of the nut and flange end faces, as well as whether lubrication is used or not. Variations in the torque coefficient have a significant impact on the torque value. Therefore, to minimize errors, it’s essential to ensure accurate calculation of the preload force. 1) Calculate the pre-tightening force A based on the performance of the gaskets. Under operating conditions, determine the minimum load Fo required to tighten each bolt, using the formula: Fo = (F + Fp) / n. Here, F represents the total axial force resulting from internal pressure under operating conditions, Fp represents the minimum compression force of the gaskets under those conditions, and n is the number of bolts. The methods for calculating F and Fp are specified in GB150.3. B. 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. C. Based on the maximum allowable stress of the gasket, calculate the maximum bolt load Fg2 required for tightening a single bolt. The formula is: Fg2 = (Sg × Ag) / n, where Sg represents the maximum allowable stress of the gasket, and Ag is the sealing area of the gasket. 2) Calculate the pre-tightening force based on the allowable stress of the bolts. Using the allowable stress of the bolts, determine the minimum bolt load Fb1 and the maximum bolt load Fb2 required for tightening each individual bolt ； The formulas are: Fb1 = 0.25 × Rel × Ab, Fb2 = 0.7 × Rel × Ab ； Where: Rel is the yield strength of the bolt, and Ab is the stress cross-sectional area of the bolt. 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. Click to view – Summary of Chemical Engineering Skill Training Courses for 2023. 3. Key points for quality process control: 1) Inspection before installation: Check that the inside of the pipe is clean, that the flange surfaces and gaskets are undamaged, and verify that the materials 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. Cleaners should be employed to eliminate any stains. The condition of the sealing surfaces must be inspected; flanges with severely damaged sealing surfaces must 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) Quality inspection after flange bolt tightening: After the flange bolts have been tightened, the project’s quality inspection personnel conduct random checks on the bolt torque and flange parallelism in accordance with the inspection records. Once they meet the requirements, they sign to confirm compliance. 3) Pressure testing and verification: Pipe pressure testing is an important method for evaluating the sealing performance of flanges. During the testing process, it is essential to check the sealing performance of each pair of flanges to ensure that there are no leaks ； During the pressure testing process, the inspection of flange sealing performance must adhere to the following requirements: A. Should any leaks be detected on the flanges, it is strictly prohibited to tighten them under pressure. Instead, the leak locations must be identified and marked, and repairs carried out only after the pressure has been released ； B. After pressure relief, recheck the flange parallelism and the torque of the bolts. If the parallelism is not within specifications, loosen the bolts opposite the open end, readjust the flange parallelism, and then retighten the bolts according to the proper bolting procedure ； If only the bolt torque does not meet the requirements, it can be tightened directly to the target torque ； C. After adjusting the flanges, conduct the pressure test again. The test is considered successful if there are no leaks; thereafter, re-mark the flange bolts ； If there is still any leakage, the pressure must be relieved before opening the flange. Then, inspect the flange sealing surface and gasket; replace the gasket if necessary. If the flange sealing surface is damaged, it should be replaced or repaired on-site. After all defects have been eliminated, conduct another pressure test. Only after the test proves successful should the flange bolts be marked. III. 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 proper flange alignment, the condition of the sealing surfaces, the quality of the sealing gaskets, correct use of bolts along with proper 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 careful control of tightening loads can the risk of leakage in flanges be effectively avoided.