My general ? is can I design a 12' tall x 12' dia x 3/16" thk stainless steel tank to 650 code using a 20" manway flange and blind flange burned out of 3/4" SS plate instead of a standard nozzle/flange with bolt on blind flange?

Reason would be for less expense.

Appreciate your help,
Robert P.

Yes you can use plate for manway flanges
Refer to table 3.3 for manway flange and cover plate minimum thickness requirements as the same depends on liquid level (equivalent pressure) inside the tank.

Thank you!