1. Series A or series B

When selecting American standard flanges, the diameter is greater than DN600, it is necessary to consider using larger diameter flanges. American standard flanges are in accordance with ASME B16.47-2020 standard.

You may have encountered a customer who chose a large diameter flange, but did not specify whether the flange should be produced according to series A or B, and ultimately purchased the wrong flange, causing losses.

Forgings are expensive, while large forgings are even more expensive. But why do large-diameter flanges need to be divided into series A or series B? Why not unify?

The American National Standards (ANSI) B16 Committee developed flanges ranging from 26 inch to 60 inch in December 1982, with flange dimensions based on the manufacturer's standardization association's standard practice MSS SP 44 "Steel Pipe flanges.". The subsequent B16.47 update includes flange dimensions for API 605 "Large Diameter Carbon Steel flanges". Used for Class150 and 300, as well as NPS36 or smaller sizes, API 605 and MSS SP-44 flanges are not compatible and cannot be used together. For ease of differentiation, MSS SP-44 flanges are designated as A-series. API 605 flanges are designated as B-series.

2. Differences between series A and series B

What is the difference between the series A and the series B?

Generally speaking, under the same size and pressure rating, ASME B16 47 series A flanges are heavier, thicker, and stronger than series B. So, A-series flanges are generally more expensive than B-series flanges.

Referring to the following figure, with the same pressure rating and nominal diameter, it can be intuitively felt that the series A is thicker and the series B is thinner.

B-series flanges require more but smaller fasteners (bolts and nuts) than A-series flanges, and the center circle diameter of the bolts is smaller than A-series flanges.

Both the A series and B series define raised face (RF) butt welding flanges and blind flange, but only the A series has ring connection face (RTJ) flanges ranging from Class300 to Class900.

The connecting pipe size of the A series is completely different from that of the B series. The outer diameter of the A series nozzle for DN650 Class150 is 660.4, and the B series nozzle is 661.9.

It should be noted that the outer diameter of the flange welding end (connecting pipe outer diameter) of the A series is not affected by changes in pressure level. However, the outer diameter of the flange welding end in the B series varies according to different pressure levels.

For example, for the B series flange of DN800, the outer diameter of Class150 is 814.3, the outer diameter of Class300 is 819.2, and the outer diameter of Class600/900 is 812.8. For DN1500, the connecting pipe of Class150 is 1527, and the outer diameter of the connecting pipe of Class300 is 1557.3, with a difference of 30mm.

3.How to choose?

Generally, customers will provide flange requirements for either A series or B series, as agreed upon by the customer or contract.

Due to the uniform size of A-series pipelines and the addition of RJ sealing surfaces, most pipeline users or designers may choose A-series flanges for new pipelines or equipment.

In general, the B-series is affordable. Previous devices used API 605 for refurbishment or replacement, or the use of B-series flanges or manholes can save construction costs.

However, the B series may not always be cheaper than the A series, for example, for Class300 flanges larger than DN950, the B series flanges are heavier than the A series.

Not all high pressure levels are heavier than low pressure levels. For example, the A series DN950 weighs 326.9kg for Class150 and 315.6kg for Class300.

Not all large diameter ones are heavier than small diameter ones, for example, the DN900 of the A-series Class300 is 578.9kg, and the DN950 is 315.6kg. The weight of DN650 for Class150 and B series is 54.5kg, and the weight of WN pipe flange for DN600 is 121.7kg.

So we need to analyze based on actual usage.