Process automation and safety philosophy

Dirt

Member
Join Date
Apr 2008
Location
Stockholm
Posts
292
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?
 
I'd agree.

In water treatment the minimum i will now accept for anything other than tiny treatment plants is 2 instruments for the critical control points. 1oo2, if one looks out of spec we shut the plant down until an operator can assess and mark the faulty instrument out of service, then restart the plant. you then do run without protection but that has been deemed acceptable for short periods.

Triple redundancy is best but that's only cost effective for larger plants. 2oo3 and you can still run the plant.
 
Thanks Saffa,

Found the appropriate standard as well for this situation

IEC61511-1 11.2.10
A device used to perform part of a safety instrumented function shall not be used for basic process control purposes, where a failure of that device results in a failure of the basic process control function which causes a demand on the safety instrumented function, unless an analysis has been carried out to confirm that the overall risk is acceptable.

Also described in IEC61508-2 7.4.2.3
 

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