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Click the blue text to follow us. In the previous issue, we learned that to ensure no leaks at the flanges, it is necessary to implement strict control procedures during the flange installation process, to guarantee the quality of the flanges and gaskets, and to select an appropriate tightening force so that the gaskets can provide good sealing performance. Today, we will introduce control measures from the perspective of fastening load calculation. Choosing the appropriate tightening load is also a key aspect of flange management. Currently, there is no unified standard calculation method in domestic regulations 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 a 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 ； 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 factors such as the smoothness of the threaded joint surface, the smoothness of the nut and flange surfaces, as well as whether lubrication is used; changes in this torque coefficient have a significant impact on the torque value. Therefore, to reduce errors, it is necessary to ensure accurate calculation of the preload. 1. Calculate the pre-tightening force based on the performance of the gaskets: 1) Under operating conditions, determine the minimum load Fo required to tighten each bolt, using the minimum compression force of the gaskets. The formula is: 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 gaskets under those same conditions, and n is the number of bolts. The methods for calculating F and Fp are specified in GB150.3. 2) Under the pre-tightened 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. Here, Fa represents the minimum compression force of the gasket under pre-tightened conditions; the method for calculating this value is specified in GB150.3. 3) Based on the maximum allowable stress of the gasket, the maximum bolt load Fg2 required to tighten a single bolt is calculated using the formula: Fg2 = (Sg × Ag) / n, where Sg is the maximum allowable stress of the gasket and Ag is the sealing area of the gasket. 2. Calculate the pre-tensioning 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 ； In the formula: 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). 4. Calculate the range of bolt tightening torques, Tmin and Tmax: Tmin = k × Fmin × d; Tmax = k × Fmax × d. At the same time, it is also necessary to: conduct inspections before installation, paying special attention to the cleanliness inside the pipes, as well as checking whether there are any damages on the flange surfaces and gaskets; furthermore, 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, and cleaning agents should be employed to eliminate stains. The condition of the sealing surface must be checked; flange sealing surfaces that are severely damaged should not be used. For inspection and maintenance devices, after removing the flange, check the condition of its sealing surface; if it is damaged, it must be replaced. Quality inspection after flange bolt tightening: After the flange bolts have been tightened, the project quality inspection personnel conduct random checks on the bolt torque and the parallelism of the flanges based on the inspection records; once everything is satisfactory, they sign to confirm it. Pressure testing is an important method for checking the sealing performance of pipes. During this process, it is essential to examine the sealing quality of each pair of flanges to ensure that there are no leaks ； The inspection of the flange sealing performance during the pressure testing process must adhere to the following points: 1) If a leak is detected in the flange, it is strictly prohibited to tighten it while under pressure; the leak location 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 flange 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 flange’s parallelism again, and then retighten the bolts following the proper 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 another pressure test; the test is considered successful if there is no leakage, and the flange bolts should be marked again ； If there is still a leak, the pressure must be relieved before the flange is opened; the flange sealing surface and gaskets should be inspected, and new gaskets installed. If the flange sealing surface is damaged, it must be replaced or repaired on-site. After the defects are eliminated, a pressure test should be conducted again until the results are satisfactory, after which the flange bolts should be marked. Based on the theoretical analysis and data from these two periods, it can be seen that pipeline flange management is a systematic endeavor that requires taking into account all the factors related to the sealing performance of flanges, including flange alignment, the condition of the sealing surfaces, the quality of gaskets, proper use of bolts and control of torque values, as well as changes in process operating conditions. Only through strict process control, improved material management, and careful control of tightening loads can the risk of leakage in flanges be effectively avoided. That’s all for today’s sharing; see you next time! Disclaimer: The content contained herein is sourced from public sources such as the Internet and WeChat official accounts. We maintain a neutral stance regarding the views expressed in it; these views do not represent those of this official account. All articles are provided for reference and informational purposes only. 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