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How to solve the leakage of flange interface

Leakage accidents caused by flange interfaces in petrochemical plants are also increasing, causing great harm to plant operation. In this paper, we analyze the causes of flange leakage, introduce the concept of flange management, propose specific process control measures and reasonable bolt tightening load calculation methods to improve the quality of flange installation and reduce the incidence of flange leakage.

In petrochemical plants, process pipelines are as intricate and complex as the blood vessels in human body, and most of the media are toxic and harmful, so ensuring the quality of pipeline construction becomes the focus of engineering quality control. Mention of pipeline construction quality may be the first thought is the quality of pipeline welding, however, in the petrochemical plant pressure pipeline connection in addition to welding connections, but also a large number of flange connections, flange bolts are huge and complex working conditions, in recent years by the flange interface caused by the leakage accident is also increasing, about 18% of all leakage accidents are due to flange interface leakage caused by, therefore, control the quality of flange installation, strengthen the management of flange is particularly important.

20221031232024 94355 - How to solve the leakage of flange interface

1. The main causes of flange leakage

Flange leakage, the more common are the following seven causes.
Off-port
Eccentric mouth, refers to the pipe and flange is not perpendicular, not concentric, flange surface is not parallel. When the internal medium pressure exceeds the load pressure of the gasket, flange leakage occurs. This situation is mainly caused during the installation construction or overhaul, and is easier to be found. As long as the project is completed with a true check, you can avoid such accidents.
Mismatch
Mismatch, refers to the pipe and flange vertical, but the two flanges are not the same center. The flange is not centered, causing the surrounding bolts are not free to penetrate into the bolt hole. In the absence of other methods, only reaming or use a small bolt through the bolt hole, and the method will reduce the tension of the two flanges. Moreover, the sealing surface line of the sealing surface is also deviated, which makes it very easy for leakage to occur.
Open mouth
Open mouth, refers to the flange gap is too large. When the gap of the flange is too large and causes external load, such as axial or bending load, the gasket will be shocked or vibrated and lose compression force, thus gradually losing sealing kinetic energy and leading to failure.
Wrong Hole
Wrong hole means that the pipe and flange are concentric, but the distance between the two flange relative bolt holes is deviated. Wrong hole will make the bolt produce stress, the force is not eliminated, will cause shear force on the flange bolt, time will cut off the flange bolt, resulting in seal failure.
Stress impact
In the flange installation, the two flange butt are relatively standard, but in the system production, the pipeline into the medium, resulting in pipeline temperature changes, so that the pipeline expansion or deformation, so that the flange is bending load or shear force, easy to cause gasket failure.
Corrosion influence
Chemical changes in the gasket occur due to the erosion of the corrosive medium for a long time. The corrosive medium penetrates into the gasket and the gasket starts to become soft and lose compression force, causing the flange to leak.
Thermal expansion and contraction
Due to the effect of thermal expansion and contraction of fluid medium, causing the flange bolt to expand or contract, so that the gasket will create a gap and the medium will leak through the pressure.
There are many reasons for flange leakage, and the main reasons for flange leakage in the construction phase are as follows.

  • (1) Flange, gaskets and other sealing surface damage.
  • (2) Gaskets or bolts and other materials with the wrong, can not meet the operating conditions of the pipeline operation.
  • 3) Leakage caused by flange deflection.
  • 4) Damage to gaskets caused by irregular fastening.
  • 5) Insufficient tightening load to make the gasket achieve sealing performance.
  • 6) Gasket collapse due to excessive tightening load.

2. Control measures to prevent flange leakage

How to solve the leakage of flange interface? In summary, from the causes of flange leakage, to ensure that the flange does not leak, we must ensure the installation quality of flange and gasket through strict flange process control procedures and select the appropriate fastening load so that the gasket can achieve good sealing performance. The following will introduce the control measures from two aspects of 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 be trained and licensed to work, and operation without license is strictly prohibited. Theoretical training is taught by professional engineers, who are responsible for teaching theoretical knowledge of flange management to operators. Practical training is mainly to provide systematic practical operation training to full-time operators on site, including overall quality control process and equipment use skills.

2.1.2 Material management

Material management includes the management of flanges, fasteners and gaskets, and all materials should be well accepted on arrival to ensure that the materials are qualified in nature. The acceptance requirements of metal materials are as follows.

  • (1) All metal materials should have quality certification documents and the content of the documents should meet the design and specification requirements.
  • (2)After the flange arrives, the sealing surface of the flange should be checked piece by piece, the sealing surface should be intact, and there should be no rust and radial scratches and other defects.
  • (3) The outer edge of the flange should have the required identification and match the quality certification documents.
  • (4) check the flange after passing, should be placed indoors for storage, and do a good job of rust and corrosion prevention treatment.
  • (5)After the flange is opened in the maintenance project, the component should check the condition of the flange sealing surface and clean up the impurities affecting the seal.
  • (6)After the arrival of fasteners to ensure that the threads of fasteners should be complete, no scratches, no burrs, no rust and other defects; if the threads are not complete, it is strictly prohibited to use.
  • (7)After the fastener is inspected and qualified, the thread surface needs to be coated with thread protectant and the threaded part is evenly coated.
  • (8)Winding gasket shall not be loose or warped upon arrival and acceptance, and its surface shall not have defects that affect the sealing performance.
  • (9)The bolts that need to be used after flange disassembly in inspection and maintenance works should be cleaned centrally first, then checked and then used. When cleaning, check whether the threads are damaged, whether the nut can be smoothly screwed to any position of the stud, and prohibit the use of unqualified bolts.

2.1.3 Equipment selection

According to different specifications of bolts, different tightening equipment is used, mainly manual torque wrench, electric torque wrench and hydraulic wrench. Bolts with torque ≤1000N-m can be tightened with manual torque wrenches, and bolts with torque greater than 1000N-m are recommended to be tightened with electric torque wrenches or hydraulic wrenches, which can be matched with impact wrenches for initial tightening to improve the efficiency of initial tightening.

2.1.4 Flange installation

  • (1) Before flange installation, the flange sealing surface and gasket should be checked and confirmed to ensure that there are no defects affecting the sealing performance, and the protective grease of the flange sealing surface should be cleared.
  • (2) The bolts connecting the flange should be able to penetrate freely.
  • (3)The flange bolts should be installed in the same direction and exposed length.
  • (4)Tighten the nut by hand to ensure that the nut can turn smoothly on the stud.
  • (5) The flange installation should not be skewed, and the parallelism of the flange sealing surface should meet the specification requirements.

2.1.5 Bolt tightening

The flange connection bolts should be tightened symmetrically in sequence, and the tightening torque value needs to be increased in a graded manner until the final required torque is obtained, and the specific steps are as follows.

  • Step 1: Tighten the nuts by hand with a manual wrench for initial tightening, while controlling the flange parallelism.
  • Step 2: Tighten the bolt by 30% of the final torque value, using the symmetrical method to tighten and verify the flange spacing to ensure consistency.
  • Step 3: Tighten the bolts at 60% of the final torque value and use the symmetrical method to tighten them, verifying the flange spacing to ensure consistency.
  • Step 4: Tighten the bolts at the target torque value, using the symmetrical method, and verify the flange spacing to ensure consistency.
  • Step 5: Continue tightening the bolts with the target torque value, using sequential tightening, and finally verifying the flange spacing to ensure consistency. After tightening the flange bolts according to the above steps, use a marker to draw a crosshair along the end face of the stud/nut, and hang a signboard on the flange if available. The signboard is marked with the pipe number, flange number, target torque value, operator and operation time.

2.2 Bolt fastening load calculation

Select the appropriate tightening load is also the focus of flange management, the current domestic norms and literature does not have a unified, standard calculation method, given mostly a fixed tightening torque value, however, the tightening torque should not be a fixed value but an interval value, one is to ensure that the flange connection to achieve sealing performance, the second is to ensure that the gasket is not damaged and the bolt is not pulled off or failure; In addition, the The bolt tightening torque is not a precise value, from the torque calculation formula: T=KFd, In formula: T is the torque, K is the torque coefficient (generally taken as 0.1-0.2), F is the preload force, d is the nominal diameter of the bolt, the torque coefficient K is a variable value, which is related to the smoothness of the thread bonding surface, the smoothness of the nut and flange end face and whether to use lubrication, the change of the torque coefficient has a great impact on the torque The variation of the torque coefficient has a great influence on the torque value, therefore, in order to reduce the error, the calculation of the preload force should be accurate.

2.2.1 Calculation of preload force according to gasket performance

  • (1) Calculate the minimum load Fo for single bolt tightening according to the minimum gasket tightening force in the operating condition, the formula is: Fo=(F+Fp)/n, In formula: F is the total axial force caused by internal pressure in the operating condition, Fp is the minimum gasket tightening force in the operating condition, n is the number of bolts, the calculation method of F and Fp is referred to GB150.3.
  • (2) Calculate the minimum bolt load Fg1 required for a single bolt fastening in accordance with the minimum gasket compressive force in the pre-tightened state, the formula is: Fg1=Fa/n, In formula: Fa is the minimum gasket compressive force in the pre-tightened state, the calculation method refers to GB150.3.
  • (3) Calculate the maximum bolt load Fg2 required for a single bolt fastening according to the maximum allowable stress of the gasket, the formula is: Fg2=(Sg×Ag)/n, In formula: Sg is the maximum allowable stress of the gasket, Ag is the sealing area of the gasket.

2.2.2 Calculate the preload force according to the allowable stress of the bolt

According to the allowable bolt stress, the minimum bolt load Fb1 and the maximum bolt load Fb2 are calculated for a single bolt fastening; the formula is: Fb1=0.25×Rel×Ab, Fb2=0.7×Rel×Ab; in the formula: Rel is the bolt yield strength, Ab is the bolt stress cross-sectional area.

2.2.3 Comprehensive comparison

Determine the minimum load Fmin and maximum load Fmax required for fastening a single bolt, respectively.

Fmin=max(Fo,Fg1,Fb1)
Fmax=min(Fb2,Fg2)

2.2.4 Calculate the bolt tightening torque interval value Tmin, Tmax

Tmin=k×Fmin×d
Tmax=k×Fmax×d

2.3 Quality process control points

2.3.1 Pre-installation inspection
Focus on checking whether the pipe is clean, the flange surface and gasket are damaged, and checking whether the bolt and gasket materials and specifications are consistent with the design drawings. For the rusted flange sealing surface, manual wire brush or other hand tools can be used to remove rust, and remove stains with cleaning agents, check the condition of its sealing surface, serious damage to the flange sealing surface is not used. For inspection and maintenance devices, check the condition of the flange sealing surface after flange disassembly, and replace it if it is damaged.
2.3.2 Quality inspection after tightening of flange bolts
After the tightening of flange bolts is completed, the project quality inspection personnel will conduct random checks on the bolt torque and flange parallelism according to the inspection records and sign for confirmation after passing.
2.3.3 Pressure test calibration
Pipeline test pressure is an important means to test the sealing performance of the flange, in the test pressure process to focus on checking the sealing performance of each pair of flanges to ensure that the flange does not leak; test pressure process of flange sealing performance inspection to do the following.

  • (1) If the flange is found to have a leak, it is strictly prohibited to tighten with pressure, check the leak point, mark it well and handle it after pressure relief.
  • (2) After pressure relief, review the flange parallelism and bolt tightening torque, if the parallelism does not match, loosen the bolt opposite the opening end, re-adjust the flange parallelism, and according to the bolt tightening steps to tighten again; if only the bolt torque does not meet the requirements, can be directly in accordance with the target torque for tightening;.
  • (3) Flange adjustment, and then re-test pressure test, and again test to not leak as qualified, and re-mark the flange bolts; if there is still leakage, you must open the flange after pressure relief, check the flange sealing surface and gasket, replace the new gasket, flange sealing surface if damaged, replace or online repair, defects eliminated, test pressure test again, until after passing the calibration, the flange bolts are marked.

3. Conclusion

According to the above theoretical and data analysis can be seen that the pipeline flange management is a systematic project, will involve the flange sealing effect of the relevant factors to consider, including flange grouping, sealing surface condition, sealing gasket quality, bolt correct use and torque value control, process operating conditions change. Only through strict process control, strengthen material management and strict control of tightening load, can we effectively avoid the risk of possible leakage of flanges.

Source: China Flange Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

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