Welding Neck Flange

In (1) the 28.7mm is the flange thickness, the 12.7mm is probably the pipe thickness, and there is no reason they should be equal.

In (2) the 24.2mm is also the flange thickness.

I don't know how NACE relates to these thicknesses, unless something to do with external corrosion. (Or unless NACE means something other than National Association of Corrosion Engineers.)

30 - 30" (Inch is a US Customary unit)

28.7mm and 24.2mm are the thickness of the flanges

FLG - Flange

A694-F65 - ASTM A 694 Grade F65

WN - Weld Neck

RTJ - Ring type joint (ie, A metallic ring comes in between both RTJ flange. RTJ flange normally used for high pressure fluid service)

RF - Raised Face (ie, in this case the flange face is little bit raised.most probably a gasket will come in between two flanges during bolting)

CL-900 & CL-600 - It is the pressure class rating of the flanges

B16.47 - ASTM B16.47 standard for large flanges above 24" (ie, NPS 26 through NPS 60).

NACE - National Association of Corrosion Engineers issued lot of standards. The major two are NACE MR 0175 and NACE MR 0103.

NACE MR0175 - It deals with the materials quality requirement used in oil producing environment (ie, oil wells, crude oil transportation pipeline system etc. details available in NACE MR 0175)

NACE MR0103 - It deals with the materials quality requirement used in oil refining environment (ie, details available in NACE MR 0103)