I'm reviewing a fabrication drawing and it is calling for 2"-3000# couplings and 4" API couplings. I was wondering why the 4" is not called out as a 3000# designation? Is there a difference between the two and if so what?
Check the API-5L standard for the 4" coupling detail which may fit the service condition you have.
The different pressure rating for 2" and 4" fitting may come from the different standards, such as ANSI vs API.