Causes and countermeasures for flange leaks in petrochemical plants

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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, what often comes to mind first is the quality of pipeline welding. However, in petrochemical plants, apart from welded joints, 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. 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 flanges and gaskets ； 2) Incorrect materials such as gaskets or bolts are used, failing to meet the operating conditions required for pipeline operation ； 3) Leakage caused by flange misalignment ； 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. 2 Control measures to prevent flange leaks. In summary, based on the causes of flange leaks, to ensure that no leaks occur at flanges, it is necessary to implement strict control procedures during the flange installation process. This ensures the quality of flange and gasket installation, as well as the selection of appropriate tightening loads so that the gaskets can achieve optimal sealing performance. Below, the control measures will be introduced from two main aspects: flange process management and calculation of fastening loads. 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 meet the requirements of the design and relevant specifications. 2) After the flanges arrive, each flange’s sealing surface must be inspected. The sealing surface should be intact, without any defects such as rust or radial scratches. 3) The outer edge of the flange shall have the specified markings and 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, it is necessary to inspect the condition of the flange sealing surface and remove any impurities that may affect the seal ； 6) 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. 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 bolts required after flange removal in maintenance and repair work, the principle of first cleaning them collectively, then inspecting and reusing 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 criteria must not be used. 2.1.3 Selection of equipment: Depending on the bolt specifications, different tightening equipment is used; primarily manual torque wrenches, electric torque wrenches, and hydraulic wrenches. 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, inspect and verify the flange sealing surface and gasket to ensure there are no defects that could affect the sealing performance; also, remove any protective grease from the flange sealing surface. 2) The bolts for 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 installation must not be skewed; the parallelism of the flange sealing surfaces must meet the requirements of the specifications. 2.1.5 Bolt tightening
The bolts used for flange connections should be tightened in a symmetrical and sequential manner. The tightening torque must 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. At the same time, ensure 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. 2.2 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 maintains its sealing properties, 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, and 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. 2.2.1 Calculation of pre-tightening force based on gasket performance 1) Under operating conditions, the minimum load required to tighten a single bolt is calculated based on the minimum compression force of the gasket, using 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-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, where Fa represents the minimum compression force of the gasket under pre-tightened 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 tightening a single bolt. 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 Calculation of preload based on allowable bolt stress: Based on the allowable bolt stress, calculate separately 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. 2.2.3 Comprehensive comparison
Determine separately the minimum load Fmin and maximum load Fmax required for tightening a single bolt:
Fmin = max(Fo, Fg1, Fb1)
Fmax = min(Fb2, Fg2)

2.2.4 Calculation of the bolt tightening torque range Tmin, Tmax
Tmin = k × Fmin × d
Tmax = k × Fmax × d

2.3 Key points for quality process control
2.3.1 Pre-installation inspection
Pay special attention to checking the cleanliness inside the pipe, as well as whether the flange surfaces and gaskets are damaged. Also 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 used 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 records; once everything is satisfactory, they sign to confirm 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 only the bolt torque 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 released before the flange can be 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. Once the defects have 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 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 sealing gaskets, proper use of bolts and control of torque values, as well as changes in process operating conditions. Only through strict process control, enhanced material management, and careful control of tightening loads can the risk of leakage in the flanges be effectively avoided.