2.2 Operating conditions
The different physical and chemical properties of the medium in the pipe such as corrosiveness and permeability have different impacts on sealing of flanges. In the case of the same sealing connection forms and working conditions, the leakage rate of gas is greater than that of liquid, and the leakage rate of strong corrosive media is greater than that of weakly corrosive media. The leakage rate of strong permeable media is greater than that of weakly permeable media.
 
The influence of working conditions such as medium pressure and temperature on flange sealing performance is also an important factor that cannot be ignored. The greater the pressure of the medium and the higher the temperature are, the easier the flange seal leaks. Under high pressure and high temperature conditions, the physical and chemical properties of flanges, bolts and gasket materials will be changed. For example, the resilience and stress of gaskets will decrease at high temperatures, and the gasket material will age. Therefore, compared with room temperature sealing, high pressure and high temperature sealing is much more difficult.
 
The external load is also an important factor affecting sealing performance of flanges. Because of the suspended support of the pipeline, the force on the flange connection is more complicated. Under high temperature and high pressure conditions, the pipeline is thermally expanded and deformed, and additional thermal stress is generated due to the constraints of the external support. The stress distribution of the pipeline is uneven due to uneven flow velocity, vibration and quality of the pipeline itself at the connecting part and external loads can be generated at the same time. When the external load is too great, it can cause the flange seal to fail and leak. Under a certain condition, an appropriate external load can play a role in strengthening the seal.
 
In addition, corrosion of the external environment of the pipeline such as water corrosion, atmospheric corrosion and soil corrosion has a great influence on the sealing performance of flanges, bolts and gaskets. With the prolongation of use time, flanges and bolts will produce rust stains due to corrosion, resulting in a serious decline in their sealing performance and even breakage. For the above-mentioned fasteners, atmospheric corrosion, especially severe corrosion of industrial atmosphere is the main factor.
 
2.3 Human factors
The failure of the flange seal caused by bad professional quality of the on-site operators, careless operation, improper use of tools, etc. is more prominent than the above two reasons. Common mistakes are as follows:

1. During installation, the sealing surfaces of the two flanges connected to the flange are not parallel or have different axis, and roughness of the flange surface is not appropriate. The boss of the flange surface is not aligned, and the gap between the two flange surfaces is too big. The flange sealing surface is damaged, severely corroded, not clean and smooth, and has mechanical damages and scratches. There is foreign matter between the flange sealing surface and gasket, resulting in false firmness.

 

2. The use of rough threaded bolts instead of fine threaded bolts or big gaps between bolts and bolt holes will cause the bolts to loosen or break. Thicken the nut. The thicker the nut is, the more uneven the load distribution will become, which will cause the connecting part to loosen. There are burrs on the bolts and nuts, and there is a shortage of threads. The bolts are bent, unevenly tight, too tight or not tight, causing the bolts, gaskets or flanges to undergo plastic deformation and lose their elasticity.

 

3. The bolts are too long and many gaskets are installed, resulting in uneven force of gaskets, lower bolt pre-tightening force and eccentric load, which reduces connection performance of the bolt. Gaskets are not used properly. The use of big gaskets will cause pressure of the gasket to drop and loosen the bolt. When compressing the gasket, the worker doesn't understand the designed pre-tightening force value and tightening procedure, and consciously increase the tightening force in fear of poor sealing, resulting in damaging the bolt. When tightening special pipe flanges, adjustable wrenches should be used instead of torque wrenches for the operator's convenience. Hydraulic tightening machines or hydraulic tensioners are not used for large bolts and bolts with high strength.

 

3. Conclusion

In conclusion, the failure of the flange seal is caused by many reasons. To further improve the sealing of flanges, first, professional quality of relevant personnel must be improved, for example, improving understanding of the correct operation of flange sealing. Second, correct flanges, gaskets and bolts must be selected according to the nature of the medium in the pipeline. Third, flanges, bolts and gaskets should be daily maintained. Newly installed or replaced flange bolts must be anticorrosive, and flange bolts with severe corrosion and rust must be replaced in time.