I have also used the redlion data station, you can stack a analog card on and data log the info to the data station, it would then be available over tcp/ip or they have a built in web server the opens up in internet explorer. Pretty solid hardware, software is free.
Typically all emergency valves would be ball valves as they provide "bubble tight" sealing qualities, and typically a fire safe valve. I personally use WOM valves. Max DP would normally be your MAOP of the line.
I apologize i didn't mean the function block needed to be per say "certified" (as Dan pointed out i don't think that would be feasible), as long as it abides buy the AGA8 standard calcs then you logic is fine. A meter that only measures static, diff and temp would not satisfy the AGA8...
I also cover this work under a hot work permit. You would do a initial LEL check for gas, depending on your situation having a meter at the motor where you may be expecting a possible spark. Typically having someone man the detector that is in the hazardous area if your are testing from a remote...
As long as the meter your using is a hart smart device i have used a HIM module made by Moore Industries. Similar to a rosemount tri-loop it would go in your control panel and give you an additional analog out of your preference. They have proven to be super reliable for me.
If your doing some kind of custody transfer then i would say that yes the program would have to be certified. A typical AGA8 block that i'm used to using comes in a library with my Modicon M340, the AGA8 block is already certified. I'm using ultra sonic meters so for that particular function...
