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Currently at Engen Refinery we use API 550 guide lines for the instrument orifice runs installed. The flange cross sectional bore of the D/P flanges is 2" = 1/4", 3" =3/8", 4" and above = 1/2". Our standards have been revised to use PTS (Shell Standard). My query is that the 2" is now 4 mm, 3" is now 6 mm, 4" is 8 mm and 6" is 11.5 mm. I would like to know the technical reason for the different sizes used or could one use one standard bore, say 11.5 mm. If their are Shell spec users out there, I would like to know the MTBF of these small bore orifice flanges in a fairly low quality crude process.
