I know a little of what is common practice in process piping, though there are many types of flanges the basic hierarchy is as follows; gasket materials vary widely according to industry, fluid and temperature….

Flat faced: the lowest pressure, cheapest and most common (150 – 300 psi), uses common gaskets

Raised Face: the flat area is reduced to within the bolt P.C.D to increase the pressure on a smaller area of flange most common to 600psi, uses common gaskets

Ring Jointed:there is a groove in both flanges to take a metal ring as a gasket pressures beyond 600 psi to mega

Tongue and groove: often used to ensure piping is assembled the right way around (and lots of other reasons)

Quite a nice description here http://www.wermac.org/flanges/flanges_raised-face_flat-face_ring-type-joint.html

There are lot's of variation, especially from industry to industry, but I think the above is a fair simplification: we shall see

I have never heard of a flat face flange being put together without a gasket, or managed to make one seal without some form of gasket material or compound (boss white or stag) on the gasket faces!

The ones with grooves are hellish to clean up after they have been sealed for a year or two, with steam passing through them.

There are 'tables' for flange sizes and specifications.

http://www.hattersley.com/documents/13a.pdf

Bri