Flange series (A or B)

There are two Series for Flanges as per B16.5(Series A & .
Could you please tell me which one to use or conditions when a particular series is to be used.
For DN600 we have to use series A
Thanks

Sorryy,
The std is ASME B16.47
Series A & B are applicable for size greater than DN600.

Thanks

can you explain , where to use A & B type flanges

Ramoorthy
If you follow Jop's link it provides explanation about Asme B16.47 and where to use Class A or B flanges and API 605 and MSS SP44 and the differences between.
Just one note to remember when using Class B/API 605 flanges - check your valve for compatability to see if the bolt pattern matches. Usually the valve is manufactured to MSS SP44/Class A.