This Article Challenges Your Understanding of Flange Sealing Faces
What are the common forms of flange sealing faces, and what are their respective characteristics?
1. Flat Face (FF) Sealing Face: The flange surface is smooth, featuring a simple structural design and ease of processing. It is suitable for applications with relatively low pressure and temperature requirements. However, it has a large contact area with gaskets, requiring substantial compressive force. During installation, gaskets should not be placed, and after pre-tightening, gaskets tend to extend or move to the sides. Non-metallic lining or multiple raised faces should be used to ensure that the FF flange surface does not rupture.
2. Non-Regular Surface (MFM): It consists of both concave and convex surfaces, and gaskets are placed on the concave surface. Unlike flat face flanges, raised face gaskets are not easily compressed, making assembly easier. MFM flanges are suitable for applications with higher working environment pressures and stringent sealing requirements.
3. Ring Joint (TG): This flange sealing face consists of a groove surface and a groove face where the gasket is placed. Similar to the raised face, ring joint flanges do not experience compression within the groove, have a smaller compression area, and provide even gasket compression. Since the gasket does not directly contact the medium, it minimizes permeation pressure and corrosion mechanisms. It is suitable for high-pressure applications involving flammable, explosive, toxic media with stringent sealing requirements. Aligning components during installation benefits from surface gaskets, but processing the sealing face and gasket replacement can be challenging.
So, what is the general method for selecting a sealing face in industry? What should we be aware of?
Presently, some design institutions in the industry still strictly follow the provisions of HG/T 20583-1998 and HG/T 20583-2011, such as choosing TG surfaces for flanges with design pressures greater than 1 for highly toxic media and selecting MFM surfaces for inert gas media. For pressure ratings of PN 40 and PN 63, MFM surfaces are employed, while hydrogen gas is addressed with MFM or TG surfaces based on pressure ratings. However, in recent years, more design institutes have been moving away from this approach.
Recent experiences from numerous large-scale coal and petrochemical projects demonstrate that selecting RF sealing faces for less demanding work environments and RJ sealing faces for harsh conditions adequately meet sealing requirements. Many projects and major design institutions have set unified regulations, typically limiting the use of RF/RJ surfaces to pressure classes up to CL 600 or CL 900, with FF surfaces reserved for non-metal or lined flanges in low-pressure pipelines. Regardless of installation, maintenance, use, or cost optimization and scheduling, the results are favorable, with few issues concerning the compression of RJ gaskets (the extent of compression primarily depends on the level of installation expertise). Therefore, we believe the times are changing, and our technical perspectives should evolve. Designers should not adhere to conventions but should proactively learn and adapt.
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