Technical guide

Flange Standards, Pressure Classes and Ring Gaskets

Updated 1 November 2026 · Safinat Lulua

Short answer: Oil and gas flanges follow three main standards. ASME B16.5 covers NPS 1/2 to 24 in Classes 150 to 2500. ASME B16.47 covers NPS 26 to 60 in Series A and B. API 6A covers wellhead flanges rated 2,000 to 20,000 psi, as type 6B (with R or RX ring gaskets) or 6BX (with BX gaskets). The gasket must match both the flange standard and the facing: raised-face (RF) flanges take spiral-wound gaskets, and ring-type joint (RTJ) flanges take metal ring gaskets.

ASME flange classes

ASME B16.5 and B16.47 flanges are rated by class (150, 300, 400, 600, 900, 1500, 2500). The allowable working pressure depends on the material group and temperature, as tabulated in ASME B16.5. For example, a Class 600 A105 flange is rated about 1,480 psig at ambient temperature.

API 6A flanges

API 6A flanges are rated directly in psi. Type 6B flanges (2,000 to 5,000 psi) have a standoff between faces and use R or RX gaskets. Type 6BX flanges (all sizes at 10,000 psi and above, and large bores at lower pressures) are made up face to face with BX pressure-energized gaskets.

Choosing a gasket

Facing / type	Gasket	Standard
ASME RF	Spiral-wound with inner and outer rings, or kammprofile	ASME B16.20
ASME RTJ	R (oval or octagonal)	ASME B16.20
API 6B	R or RX	API 6A
API 6BX	BX	API 6A
CP isolation	Insulating gasket kit (GRE)	—

Ring gasket hardness must be lower than the flange groove hardness. Soft iron, low-carbon steel, 316 and Alloy 625 are common, chosen by flange material and service.

Frequently asked questions

Are ASME B16.47 Series A and Series B flanges interchangeable?

No. Series A flanges are heavier, with fewer and larger bolts. Series B flanges are more compact, with more and smaller bolts. Bolt circles differ, so the series must always be specified.

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