[timstorey]

Thank you so much.
So a vessel nozzle and a flange with different rating I can connect , but can I connect, for example, a 150# flange with a 300# flange?
I’ve checked flange tables and I realized the outside diameter and number of bolts for a certain diameter are different. Then, I conclude we cannot connect them. Is that right?

Thank you again.

That is correct, a Class 150 flange will not match up with a Class 300. You need to match the tank nozzle with a Class 300 flange. It is usually allowable to go up in flange rating, but I would be curious on why the vessel has a rating requiring Class 300 and the connecting piping is only rated for Class 150.