No lady am I, just an engineer who's getting easier to live with as he ages. Glad you like the motto; I'll be honest with you - I don't know if I made it up or I heard it some time ago. But I do think it has a lot of truth. The other motto I would use is to not trust everything you read! The more you learn, the less you are certain of.
As an addendum to the above - for those who must be able to maintain and inspect a relief valve while the process is online - relief valve manufacturers (eg Crosby {Tyco}) can supply bespoke units that overcome some of the problems indicated above by using linked 3-way, 100% open valves either side of two relief valves. Crosby have a brochure on what they call 'safety selector valves', which may be worth a look. I am not sure how the costs compare with a conventional set-up.
I have not personally used one of these systems before, but am considering doing so for a future project, so would be interested in any feedback from anyone who has used this system before...
Isolation valves are permitted, but only in very special circumstances.
The isolation valves must be mechanically linked to prevent simultaneous shut-off.
The old rules simply relied on administrative control of lockable valves, but this was determined to be un-safe.
There are special valves available from safety valve manufactuers (described above). They are not cheap, but less expensive than separately mounted isolation valves, and certainly much safer.
Dear,
you can have 2 PSVs with a 3 way diverter valve before the PSVs, this way ,you can allways have the needed protection while calibrating one of the PSVs.
Regards