Posted By George Wedgwood Robert, the HSE's latest edition of their Guide HSG253 to "The safe isolation of plant and equipment" will guide you here. It's all based on a competent assessment of the isolation required but basically, involves two stages: an initial isolation and a final or full isolation. There may be several old BS standards that talk about such things but this booklet is the latest and best way of understanding a definitive approach to isolation techniques and standards of isolation. It's well though through, aimed at the process and energy industries. I remember the CEGB fondly as well, being an old 'oil station man' and there were always lots of ways to isolate part of a system but the newer CCGT stations are less encrusted with valves and cross-overs so often you simply have one point of isolation and if that passes, you have to depressurise and drain. In the gas side there is a standard called 'double block and bleed', that is widely accepted but that relays mainly on a good vented section before the final isolation point. Let me know if you need more as I may be able to email some stuff to you. Regards, George