maintenance factor "m" & minimum design seating stress

Good (very early) morning, in my part of the world.

For in-depth details you should look to Appendix 2 in ASME Section VIII Div 1.

But in general, the flange thickness will need to increase with increasing "m" and "y" values. I say "in general", because it depends on 2 conditions: 1. Operating 2. Gasket seating. Whereas the operating condition is usual the larger value of the two (refer W_m1 in App.2 VIII-1) and governs the joint and only depends on "m" and not "y"; however, this detail is most likely a moot point because an increase in either "m" or "y" almost always means an increase in the other.

The reason is due to the moment created across the gasket surface:

(picture just for illustration taken from another source, so the nomenclature does not match VIII-1)

So you can see that when you increase "m" and "y", you require a larger bolt load, which causes a greater moment, and thusly requires a thicker flange to keep the stress levels within allowable.

• The thickness is more simple with a blind flange, see UG-34:
• t = d(CP/SE + 1.9Wh_G/SEd³)^1/2
• where the value of 'W' is directly related to "m" and "y", so you can see that if you increase "m" and "y", you increase W, and consequently 't' must be increased.
• The thickness of regular flanges is a little more involved so you should refer to App. 2 VIII-1; but the general principle is the same:
• increase "m" and "y" → require a larger bolt load → create a larger moment → require a thicker flange