Large bore flanges (above NPS 24") to ASME B16.47-Serie A or MSS-SP 44 are heavier/stronger than the ASME B16.47-Series B in the same size and pressure rating.
In most end-users piping specifications the Serie A is specified, which can whithstand more external loadings than the Serie B type.
For both series the dimensions are different. From a commercial point of view, the serie B type may be selected for piping with a non critical application.
Generally speaking, newer equipment almost always uses series A, but if you are connecting to a nozzle on older or existing equipment, get someone to verify the flange type physically.
Insist on drawings from vendors that are specific about the type of flanged connections over 24"NPS. With many firms employing newbie drafters, this subtle issue is often neglected.
My opinion only
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