Safety Instrumented System (SIS) – Verification & Validation are very critical for the safe implementation & functioning of SIS.
The performance of any new Safety Instrumented Systems is to be verified and validated before the system is taken into service. Certain steps in the SIS Work Process comply with the requirements of SIS Verification & Validation.
It is necessary to use written loop verification procedures for the loop check and commissioning of the SIF sensors, logic solver, and final elements.
Validation procedures shall functionally test the entire system during initial validation Each element in a SIL loop from the measuring element through the logic solver to the final control device shall be validated as defined in the validation procedure to fulfill the Safety Instrumented Function (SIF).
Detailed approach:
SIS Verification is the activity of confirming by examination checking of the SIS instrument – Sensor, Logic solver & Final control element meets the SIS design requirements of the safety function have been fulfilled.
SIS Validation is the activity of demonstrating by inspection and testing that the safety-related system under consideration, after installation, meets in all respects the safety requirements specification (SRS) for that safety-related system.
The performance of Safety Instrumented Systems shall be both verified and validated before being placed into service.
Validation is the one-time process in the entire SIS Lifecycle which is performed before the commissioning of a particular system/loop.
SIS Validation procedure shall be developed by a team of the Instrument Design Lead, and an Operations representative.
Define all activities required to test the SIS loop including timing for specific tasks with responsible personnel and the expected results.
Validation involves bringing all available sensors (for the SIS Loop under commissioning) to the alarm condition, verifying activation of the appropriate alarm, and ensuring the moving of the final elements to their failsafe position within the specified time.
Confirming the logic solver activate the required outputs and operator interfaces, and the field verifies the movement of the final elements to their failsafe position within the specified time. This full-loop validation applies to new or modified SIS and for BPCS with LOPA scenario.
Validation of the application program shall determine whether:
All of the specified application program safety requirements are correctly performed;
The application program does not jeopardize the safety requirements under SIS fault conditions and in degraded modes of operation and for BPCS fault conditions for any interfaces between the SIS and BPCS
The application program does not jeopardize the safety requirements by executing ’unused’ software functionality.
Quite often the SIS bypasses are forgotten to be restored to normalcy due to the urgency of equipment/system/plant startup. This is very vital and proper care shall be taken to restore the bypasses and timer/constant settings to the actual running conditions.
Reference:
IEC 61511: “Functional safety – Safety instrumented systems for the process industry sector”
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