Had a disagreement today with a colleague. The situation we discussed was something like this:
"Is it ok to use an instrument from a SIF to control the process?"
I said it depends. If it's 1oo1 and an undetectable fault could cause the process control to do what the SIF is supposed to protect against then no. He thought otherwise. I don't know if this is in any standard but I'd say it's bad design anyway.
If I have a SIF with 2oo3 (or more) instruments, then I'd control the process with one of those or an average/median. An undetected fault in one of those instruments that could cause process control to take the process beyond limits would be detected by the other 2 instruments anyway.
What's your opinion?
"Is it ok to use an instrument from a SIF to control the process?"
I said it depends. If it's 1oo1 and an undetectable fault could cause the process control to do what the SIF is supposed to protect against then no. He thought otherwise. I don't know if this is in any standard but I'd say it's bad design anyway.
If I have a SIF with 2oo3 (or more) instruments, then I'd control the process with one of those or an average/median. An undetected fault in one of those instruments that could cause process control to take the process beyond limits would be detected by the other 2 instruments anyway.
What's your opinion?