In Safety Instrumented System (SIS), Instrument sharing may be considered based on the system configuration and available sensors or final control elements with due caution to ensure that all protection layers are independent.
If the protection layers are not independent, then the shared components must be made stronger to make up for the lack of independence.
The below picture shows the Independent protection layer (IPL) credits with
Safety Instrumented System – 1 credit
Safety Relief Protection System (Ex. Rupture disk) – 1 credit
Basic Process Control System – 1 credit
Pressure Safety Valve – 2 credits
Total – 5 credits
In this below example, two Temperature transmitters are shared for BPCS and SIS.
The Sharing basics as below
1. Sum of the LOPA credits
Design as SIL-2 (SIL-1 + BPCS LOPA credits).
2. Increased diagnostics
Shared instruments need 90% diagnostics.
70% for smart NAMUR instruments,
20% for “open/short circuit” detection plus Deviation Alarm between redundant sensors.
3. Independent usage of sensors
BPCS must select the signal (high or low) that will drive the process to a safe state.
If shared components “could be” the cause of the initiating event, then it’s not allowed to share.
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View Comments
Hello, I have a doubt in the sharing.
What norm allows the sharing sensors between BPCS and SIS, as well as grants each credit either for BPCS and SIS?
Though these were defined as IPLs.
I'll be perfect if you would mention the clause as well.
BR
Taek