Please could you let me know your opinions or experience on the following task?

I want to install a 12" class 600 Double Block and Bleed system on a natural gas pipeline at the plant inlet. This line is going to be commissioned in 10 months. I want to know what valves type to use as main isolating valves?

Typically two flanged gate valves with a short spool between them which has a 3/4" bleeder (gate valve) in between them. Less often seen (in my mostly downstream experience) would be to put the line valves back to back and the bleeder in one of the line valves in the space between gates. See B16.34 Fig. 1 (page 13 in the 2004 ed). Use location C or D for the bleeder - just be sure its installed correctly in the field...

There are several existing threads with more info on double block and bleed setups. Use the search feature to find them if you want more backgound.

Gate or ball, whatever works for you. I'm used to seeing gate valves in most services, especially on lines over 8" or so.

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