BRIEF HISTORY OF FLANGE
Today API Spec 6A and MSS SP-44 specify all flanges commonly used in surface drilling and production. These flanges have a long history and many flanges used in the past have been discontinued, or obsoleted and replaced by more modern configurations. Originally, all flanges related to pipe and became standard as weld neck, threaded and blind configurations. As equipment with integral flanges came to the oilfield, standardization became necessary for functional interchangeability.
In the late 1930’s, the American Petroleum Institute (API) adopted the then ASA B16.5 flanges. For 720 psi and 960 psi applications API specified no changes to the ASA B16.5 Standard, using the 300 lb. and 400 lb. flanges respectively. For 2000, 3000, and 5000 psi applications, API specified an increase in material yield strength to the respective ASA B16.5 flanges: 600, 900, and 1500 lb.
API signify differences with ASA terms of "lb." to the API term "SERIES", (e.g. ASA 4" 900 lb. flange, 2160 psi working pressure, compared to API 4" SERIES 900 flanges, 3000 psi working pressure), dimensionally the same but differing in name, material strength, and working pressure. API issued the first specification for wellhead flanges as 6B, later incorporated into API 6A. The original flanges specified, retained the designation "6B". All API flanges adopted for wellhead use have Ring Type Joint (RTJ) facing. API flanges may have Full Face or Raised Face geometry, with either case having the specified ring groove. Most wellhead manufacturers make their 6B flanges full face.
Note: The ASA (now ANSI) terms 300 lb. through 1500 lb. represented working pressures of flanges in elevated temperature service. API assumed the ambient temperature working pressure for the 300 lb. and 400 lb. CLASS and assumed ambient temperatures for 2000, 3000, and 5000 psi flanges also. ANSI has since dropped the previous ASA term "lb." and now refers to flanges by "CLASS". Today, API Spec 6A specifies standard service temperatures of -75° F to 250° F. Current API Spec 6A flanges classed as 6B by API (easily identified by the fact that they have ring grooves that accept R or RX ring gaskets) have identical dimensions with ANSI B16.5 flanges.
Caution: A connection between mating ANSI and API flanges requires a rating of the connection no higher than the ANSI pressure rating.
In 1945 API introduced flanges rated at 7500 psi, and by 1949 these flanges were given new material specifications, a 10,000 psi working pressure, and the classification "SERIES" 2900. Then API included them with the earlier flanges under the new specification designation STANDARD 6B. By 1957 API adopted new flanges from the Association of Well Head Equipment Manufacturers (AWHEM, See Related Links) designs to replace the "SERIES" 2900. These new 10,000 psi flanges were specified in API STANDARD 6BX. In 1961, STANDARD 6B and STANDARD 6BX were combined into STANDARD 6A, now designated Spec 6A. API included 15,000 psi and 20,000 psi flanges in more recent editions of API Spec 6A. API classes all flanges that accept BX ring gaskets as 6BX flanges. API 6BX open face flanges must have raised faces, while API 6BX Studded Face flanges may have their raised faces omitted. Most manufacturers make API 6BX studded face flanges without raised faces.
API Spec 6AB, introduced in 1983, specified 30,000 psi flanged equipment. Spec 6AB has failed to achieve reaffirmation by API so it has the current status of "withdrawn".
API Spec 17D, 1st edition, appeared in 1992. This specification introduced 5,000 psi working pressure Type 17SS and 17SV swivel flanges and 10,000 psi swivel flanges that connect to API Spec 6BX flanges of the same working pressure. These flanges for subsea applications have dimensions to accept BX ring gaskets. Spec 17D requires all flanges used for subsea applications to have ring grooves manufactured from, or inlayed with, corrosion resistant alloy. This specification also introduced SBX and SRX ring gaskets designed to allow reliable make-up of flanges underwater. For more information specific to API Spec 17D connectors, see About API 17D SS and SV Swivel Flanges on this Web Site.
Low pressure Drilling Diverter Systems use MSS SP-44 flanges for 30" 500 psi and 30" 1,000 psi service.
Piping Class Ratings based on ASME B16.5 and corresponding PN (Pression Nominal*):
Flange Class 150 300 400 600 900 1500 2500 Flange Pressure Nominal, PN 20 50 68 100 150 250 420 * Pression Nominal is the French equivalent of Pressure Nominal
Pression Nominal is the rating designator followed by a designation number indicating the approximate pressure rating in bars.
Hydrostatic test pressure for flanged fittings in some common piping materials are indicated in the table below: