Larger Diameter Flanges are covered in ASME B16.47. B16.47 has a Series A and a Series B and they are not compatible with each other.
The Series A Flanges were previously known as MSS SP-44 flanges and the Series B Flanges were known as API-605 flanges.
You need to be very careful to buy flanges that match your equipment and valve flanges. It's not enough to specify one flange standard and forget about it. Often valves or equipment is only offered with one flange standard and if it's not the one you requested they may not bring it to your attention. Check and double check. It's not pretty when you get a call from the field telling you your 42" alloy pipe flange doesn't match the vessel flange.
If you are matching existing piping or equipment, check the flange you are mating to, there are some other flange classes as well, like the Taylor Forge CL175 flanges that are common in existing refineries.
B 16.47 forward indicates that once upon a time 1500# were also part of the standards.
I am in need of 36" 1500# Flange rating standard dimensions. As i see there are no standards that offer dimensions for 38" 1500 # rating.
In which year publication of API 605 or MSSP 44 or B 16.47 can i find 1500# rating flange (36"). I checked 1990and 1998 of 16.47 and could not get any further.
Waterworks applications quite often use flanges over 24", if they are something you can use in the application. AWWA C207 is the spec for steel ring flanges in various classes.