Double Block on a process line designated as B31.1 standard.

[Weegie]

I'm working on a sulph acid plant project (instrumentation group)and on one of the P & I.D's, a section of process line is called out as B31.1. This line is hp steam. The other ends of the process line is tagged as B31.3. (Typical of all our P & ID's) On this B31.1 line, I've noticed that drain valves are double block and one pressure gauge is also shown as double block.
There is an annubar for flow in this line and also an orifice plate for flow too. My question is this - are all root valves and/or instrument isolation valves double block (and bleed)on B31.1 piping?
Should the orifice flanges have double blocks on it ? Would the annubar have double blocks too ?
Please excuse the vague questions but, the B31.1 application is new to me.

Thanks in advance.