Selecting the correct flange finish is critical to ensuring optimum gasket performance. A fundamental aspect of effective sealing is the level of friction between the flange and gasket surfaces. In fact, flange surface preparation can have a dramatic effect on several key performance factors, including bolted joint tightness, burst resistance, and gasket creep relaxation.
Flange finish options range from smooth to jagged and everything in between. As you consider the different options, keep in mind that there is no single best surface finish for all types of gaskets. Different finishes are better suited for different gasket types.
How to calculate surface finish?
Before choosing a surface finish, it is important to understand two different methods of calculating surface roughness:
Arithmetic Average Roughness Height (AARH)
Root Mean Square (RMS)
AARH is also known as Roughness Average (Ra). This is the most common method of measuring surface roughness: by calculating the average surface height over the entire surface.
In contrast, RMS is calculated as the root mean square of surface microscopic peaks and valleys. RMS is more sensitive to large peaks and valleys - even a single large peak or defect can raise the RMS value beyond the AARH value. Although RMS is currently not as commonly used as AARH, it may appear in older drawings. Note that there is no reliable formula for converting RMS and AARH values as they follow different calculation methods.
Correct Surface Treatment of Metal Gaskets
Most metal gaskets are creep resistant and structurally stable, which means they form a very tight and reliable seal. To achieve this best result, you must ensure alignment, parallelism and flange finish are maintained within the specified ranges. Metal gaskets, including solid metal and jacketed designs, have little forgiveness for bolting and assembly imperfections or flange finish.
As a rule, metal gaskets and those with poorer surfaces require a very smooth surface finish. Typical surface finish recommendations for these types of gaskets are 64 AARH/RMS or smoother (1.6 Ra or smoother).
Recommended Finishes for Non-Metallic Gaskets
Soft or non-metallic gaskets are more compressible than metal gaskets. The surface finish of non-metallic gaskets plays a critical role in their service life and long-term reliability. To learn about the process of choosing the right gasket type and finish, read our blog post on how to choose the right gasket for your application.
For non-metallic gaskets, the smooth surface does not create the necessary friction between mating surfaces. This results in creep under load, loss of bolted joint tightness, and potential leakage. General surface finish recommendations for soft washers vary between 125-500 AARH/RMS (3.2-13 Ra).
Optimum finish for semi-metallic and spiral wound gaskets
Like non-metallic gaskets, semi-metallic gaskets tend to perform better and have a higher surface roughness. An extremely smooth surface finish, such as an AARH/RMS of 8-16, will not allow the sealing material to seal properly. Recommended surface finish is 125-250 AARH/RMS (3.2-6.4 Ra) for spiral wound, Kammprofile, CorruKamm and similar gasket styles.
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