Difference between ASME B16.47 Series A and Series B flanges(RF)

ASME B16.47 is a standard for large diameter steel pipe flanges, from NPS 26 through NPS 60, and pressure classes 150 through 900.

ASME B16.47 flanges can be subdivided into welding neck and blind flanges, and two Series are available for both, namely:

ASME B16.47 Series A flanges
ASME B16.47 Series B flanges

Series A flanges were formerly known as MSS SP 44, while Series B flanges are also known as API 605 flanges.

The differences

In general, ASME B16 47 Series A flanges are thicker, heavier, and stronger than Series B flanges of the same size and pressure class, allowing them to withstand higher external loads.

ASME B16.47 Series B flanges require more, but fewer, bolts and nuts than Series A flanges and have a smaller bolt circle diameter. The smaller bolt circle diameter results in less flange movement after installation.

Both Series A and Series B define Raised Face (RF) welding neck flanges and blind flanges, but only Series A defines Ring Type Joint (RTJ) flanges from pressure class 300 through class 900 within this standard.

Dimensions ASME B16.47 Flanges incl. Stud Bolts

Key differences between ASME B16.47 Series A and Series B flanges in table form

Table item	ASME B16.47 Series A Flanges	ASME B16.47 Series B Flanges
Origin	Earlier Series A flanges were known as MSS SP 44 flanges.	These flanges were known as API 605 flanges.
Dimension	For A Series flanges with the same pressure class, the flange outer diameter, bolt circle diameter, flange thickness, bolt diameter, etc., are larger.	Flanges of the B16.47 Series B are more compact than the flanges of the Series A.
Strength	ASME B16.47 Series A flanges are relatively stronger, allowing them to withstand a higher external load.	The load bearing capacity is relatively lower for the same pressure class.
Fasteners	The quantities are smaller, but the dimensions are often larger; for example: a NPS 26 Pressure Class 300 Series A flange needs 28 bolts of 1.5/8 inch diameter.	The quantity of fasteners is more; for example: a NPS 26 Pressure Class 300 Series B flange requires 32 bolts with a diameter of 1.1/4 inch diameter.
Ring Type Joint (RTJ)	Available from Pressure Class 300 through Class 900 in NPS 26 through NPS 36.	Not available in these Series.
Pressure Class 75	Not available for Series A flanges	Available for Series B flanges
Flange Weight	ASME B16.47 Series A flanges are heavier, causing them to exert a greater load on pipe supports.	The weight is relatively lower.
Cost	More expensive.	Cheaper.
Industry Use	Used more frequently, and primarily for critical applications.	Less frequently used, and normally for non critical applications.