in ISA 84.01 standard,

sensor section:

"B 1.5.3 For SIL 3, identical or diverse separation between BPCS and SIS is typically needed to meet the required safety integrity."

But I don't understand what is difference between "identical or diverse separation"

Anyone can tell me? I'll appreciate your reply.
 
 

Identical means using the same technology for both the BPCS and the SIS. Diverse means using different technologies from the same manufacturer or technologies from two different manufacturers. Separation generally means physical separation, i.e., the same instrument/actuator/logic solver/wiring is not used for both the BPCS and the SIS.

Diverse separation helps reduce common failures and systematic faults in addition to reducing random failures. Because the components are the same in identical separation, common or systematic faults may affect the performance of both the BPCS and the SIS.

By the way, it would probably be better to post this in forum196: ISA (control systems engineering) Code Issues.

xnuke
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Mr.xnuke

Thank you very much!
Your explanation is very detailed and convincing.
Also you are very kind.
I'm newcomer ,not familiar with forums here.
But I think Eng-Tips is very good forum, I'm luck to meet Eng-Tips and you!

Little activity exists in the ISA forum.  This is as good a place as any for the topic.

xnuke described the situation well.  Perhaps some companies want to use the same hardware for BPCS and SIS.  However different technologies are recommended to reduce the common mode issues.  This applies to different measurement technologies too.