I think this is an old detail that is no longer in common use. Every customer for which we are providing replacement equipment is asking that we remove the nubbin. Gasket technology has changed...
We have used something like a nubbin, but it's more of a triangular profile rather than rectangular, for metallic ultra-high vacuum seals. Based on what I've read, ultra vacuum might be one of the few exceptions to the rule.
TTFN
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faq731-376 forum1529
1) Gasket type. Solid metal gaskets (seldom used now) generally require nubbins in order to properly seat the gasket.
2) Cost. Per Appendix 2 of ASME Section VIII Div. 1, machining a nubbin onto the flange gasket surface usually reduces the required bolting area so that fewer (or smaller diameter) bolts can be used.
Personally I'd recommend against using nubbins if you have a choice.
