Selection of Pipe Material & Its Grade

All,

Design conditions are

Temperature: 950F & Max Pr: 320psig.

Pipe class is 300 RF

Service is Extraction steam

Pipe material is selected as A335 but what is the criterion to select its grade such as P-11/21/22.

Also tell me how to select A335 and why not other alloy material.

Many Thanks for comments………