In petrochemical plants, process pipelines are like blood vessels in the human body, which are intricate and intricate, and most of the media are toxic and harmful. Ensuring the quality of pipeline construction has become the focus of project quality control. When it comes to the quality of pipeline construction, the first thing that comes to mind is the quality of pipeline welding. However, in addition to welding connection, the connection method of pressure pipelines in petrochemical plants also adopts a large number of flange connections. The number of flange bolts is large and the working conditions are complex. In recent years, the leakage accidents caused by flange joints are also increasing. About 18% of all leakage accidents are caused by flange joint leakage. Therefore, control the installation quality of flanges and strengthen flanges. Management is especially important.
1 The main cause of flange leakage
There are many reasons for flange leakage, and the main reasons for flange leakage during construction are as follows: 1) Damage to sealing surfaces such as flanges and gaskets; 2) The wrong materials such as gaskets or bolts are used, Can not meet the operating conditions of pipeline operation; 3) leakage caused by flange deflection; 4) gasket damage caused by irregular tightening; 5) due to insufficient tightening load, the gasket cannot achieve sealing performance; 6) due to tightening load Too large, causing the gasket to fester.
2 Control measures to prevent flange leakage
In summary, it can be seen from the reasons for flange leakage that to ensure that the flange does not leak, it is necessary to pass strict flange process control procedures to ensure the installation quality of flanges and gaskets, and to select the appropriate tightening load to make the gaskets. Can achieve good sealing performance. The following will introduce the control measures from the two aspects of flange process management and tightening load calculation.
2.1 Flange process management
2.1.1 Personnel training and management
All personnel involved in flange management must go to work with a certificate after training, and operation without a certificate is strictly prohibited. The training is divided into theoretical training and practical training. Theoretical training is taught by professional engineers to operators, teaching the theoretical knowledge of flange management. Practical training is mainly to carry out systematic practical training for full-time operators on site, including the overall quality control process and equipment use skills.
2.1.2 Material Management
Material management includes the management of flanges, fasteners and gaskets. All materials must be checked and accepted on arrival to ensure that the material is essentially qualified. The acceptance requirements for materials are as follows:
1) All materials must have quality certification documents, and the contents of the documents meet the design and specification requirements.
2) After the flange arrives, the flange sealing surface should be checked piece by piece, the sealing surface should be complete, and there should be no defects such as rust and radial scratches.
3) The outer edge of the flange shall be marked with the specified marking and shall be consistent with the quality certification documents.
4) After the inspection, the flange should be kept indoors for storage, and should be treated with anti-corrosion.
5) In the maintenance project, after the flange is opened, the condition of the flange sealing surface should be checked by components, and the impurities affecting the sealing should be cleaned up;
6) After the fasteners arrive, ensure that the threads of the fasteners should be complete, without scratches, burrs, rust and other defects; if the threads are incomplete, it is strictly prohibited to use.
7) After the fasteners have passed the inspection, the thread surface needs to be coated with thread protective agent, and the thread parts should be coated evenly.
8) Upon arrival and acceptance of the wound gasket, there should be no looseness and warpage, and the surface should not have defects that affect the sealing performance.
9) For the bolts that need to be used after the flange is disassembled in the inspection and maintenance project, the principle of centralized cleaning first, inspection and reuse should be adopted. When cleaning, check whether the thread is damaged or not, and whether the nut can be smoothly screwed to any position of the stud. Unqualified bolts are not allowed.
2.1.3 Equipment selection
According to different bolt specifications, use different tightening equipment, mainly manual torque wrench, electric torque wrench and hydraulic wrench. Bolts with a torque less than or equal to 1000N·m can be tightened with a manual torque wrench. Bolts with a torque greater than 1000N·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 the initial tightening efficiency.
2.1.4 Flange installation
1) Before installing the flange, check the flange sealing surface and gasket to ensure that there are no defects affecting the sealing performance, and remove the protective grease on the flange sealing surface.
2) The bolts connecting the flanges should be able to penetrate freely.
3) The installation direction and exposed length of flange bolts should be the same.
4) Tighten the nut by hand to ensure that the nut can rotate smoothly on the stud.
5) The flange installation cannot be skewed, and the parallelism of the flange sealing surface must meet the requirements of the specification.
2.1.5 Bolt tightening
The flange connection bolts should be tightened in a symmetrical sequence, and the tightening torque value needs to be increased in stages until the final required torque is obtained. The specific steps are as follows:
Step 1: Tighten the nut by hand, and preliminarily tighten it with a manual wrench, while controlling the parallelism of the flange.
Step 2: Tighten the bolts at 30% of the final torque value, tighten them symmetrically, and verify the flange spacing to ensure consistency.
Step 3: Tighten the bolts at 60% of the final torque value, tighten them symmetrically, and verify the flange spacing to ensure consistency.
Step 4: Tighten the bolts at the target torque value, tighten them symmetrically, and verify the flange spacing to ensure consistency.
Step 5: Continue to tighten the bolts at the target torque value, in sequence, and finally verify the flange spacing to ensure consistency. After the flange bolts are tightened according to the above steps, use a marker to draw a cross line along the end face of the stud/nut. Flange number, target torque value, operator and operation time, etc.
2.2 Calculation of bolt tightening load
Selecting the appropriate tightening load is also the focus of flange management. At present, there is no unified and standard calculation method in domestic codes and literature, and most of them are given a fixed tightening torque value. However, the tightening torque should not be It is a fixed value but an interval value. One is to ensure the sealing performance of the flange connection port, and the other is to ensure that the gasket is not damaged and the bolt is not broken or failed; in addition, the bolt tightening torque is not very accurate. From the torque calculation formula: T=KFd, where: T is the torque, K is the torque coefficient (generally 0.1~0.2), F is the preload, d is the nominal diameter of the bolt, and the torque coefficient K is a change This is related to the smoothness of the threaded joint surface, the smoothness of the nut and the flange end face, and whether lubrication is used. The change of the torque coefficient has a great influence on the torque value. Therefore, in order to reduce the error, the calculation of the preload should be guaranteed. precise.
2.2.1 Calculate the preload according to the gasket performance
1) In the operating state, according to the minimum compression force of the gasket, calculate the minimum load Fo required for the fastening of a single bolt, the formula is: Fo=(F+ Fp)/n, where: F is the total shaft caused by the internal pressure in the operating state To force, Fp is the minimum gasket pressing force under the operating state, n is the number of bolts, and the calculation method of F and Fp refers to GB150.3.
2) Calculate the minimum bolt load Fg1 required to fasten a single bolt according to the minimum compression force of the gasket in the preloaded state. The formula is: Fg1=Fa/n, where Fa is the minimum gasket compression force in the preloaded state , the calculation method refers to GB150.3.
3) According to the maximum allowable stress of the gasket, calculate the maximum bolt load Fg2 required to fasten a single bolt, the formula is: Fg2=(Sg×Ag)/n, where: Sg is the maximum allowable stress of the gasket, Ag is the gasket seal area.
2.2.2 Calculate the preload according to the allowable stress of the bolt
Calculate the minimum bolt load Fb1 and the maximum bolt load Fb2 required for the fastening of a single bolt according to the allowable stress of the bolt; the formula is: Fb1 =0.25 ×Rel ×Ab, Fb2 =0.7 ×Rel ×Ab; where Rel is the yield strength of the bolt , Ab is the stress cross-sectional area of the bolt.
2.2.3 Comprehensive comparison
Determine the minimum load Fmin and the maximum load Fmax required for the tightening of a single bolt, respectively:
2.2.4 Calculate the interval value of bolt tightening torque Tmin, Tmax
2.3 Key points of quality process control
2.3.1 Inspection before installation
Focus on checking the cleanliness of the inside of the pipe, whether the flange surface and gasket are damaged, and check whether the material and specifications of the bolts and gaskets are consistent with the design drawings. For the rusted flange sealing surface, use a manual wire brush or other hand tools to remove the rust, and use a cleaning agent to remove the stains, check the condition of the sealing surface, and do not use the severely damaged flange sealing surface. For inspection and maintenance devices, check the flange sealing surface after the flange is disassembled, and replace it if it is damaged.
2.3.2 Quality inspection after flange bolts are tightened
After the flange bolts are tightened, the project quality inspectors will spot-check the bolt torque and flange parallelism according to the inspection report, and sign for confirmation if they are qualified.
2.3.3 Pressure test verification
Pipeline pressure test is an important means to test the sealing performance of flanges. During the pressure test process, the sealing performance of each pair of flanges should be checked to ensure that there is no leakage of the flanges; the flange sealing performance inspection during the pressure test process should do the following: point:
1) If the flange is found to be leaking, it is strictly forbidden to fasten it under pressure, check the leak point, make a mark, and deal with it after the pressure is released;
2) After the pressure is released, check the parallelism of the flange and the tightening torque of the bolts. If the parallelism is inconsistent, loosen the bolts opposite the opening end, adjust the parallelism of the flanges again, and re-tighten them according to the bolt tightening steps; If only the bolt torque does not meet the requirements, it can be tightened directly according to the target torque;
3) After the flange is adjusted, re-test the pressure test. The test is again qualified if there is no leakage, and the flange bolts are re-marked; Gaskets, replace with new gaskets. If the flange sealing surface is damaged, replace it or repair it online. After the defect is eliminated, test the pressure again and mark the flange bolts until the verification is passed.
According to the above theory and data analysis, it can be seen that pipeline flange management is a systematic project, which comprehensively considers the relevant factors related to the flange sealing effect, including flange pairing, sealing surface condition, sealing gasket quality, correct use of bolts and Torque value control, process operating condition changes, etc. Only through strict process control, enhanced material management and strict control of tightening loads can the possible leakage risks of flanges be effectively avoided.