Safety Integrity Level (SIL) assessment is a crucial part of ensuring process safety in industrial environments. Typically conducted in a dedicated workshop, this assessment helps determine the required SIL targets for various Safety Instrumented Functions (SIFs). One of the most critical aspects of this process is the proper documentation of findings, as this ensures that the rationale behind decisions is clear and traceable.
Importance of Proper Documentation
The documentation of a SIL assessment study is essential for several reasons:
1. Traceability
Industry standards mandate that all data must be traceable to its source. Proper documentation ensures that future reviewers can understand the origins of the information used in the study.
2. Review During the Operational Phase
Operational experience may provide insights that challenge initial assumptions made during the SIL assessment. Well-documented records enable teams to revisit these assumptions and confirm their validity.
3. Management of Change
When modifications to the Safety Instrumented System (SIS) are considered, the SIL assessment study report serves as a vital reference. This ensures that changes are made with full knowledge of the initial safety analysis.
Roles and Responsibilities in Documentation
During the workshop, a dedicated scribe is typically assigned the responsibility of recording the assessment in a structured worksheet. However, in some cases, the chairman or an engineering specialist can take on this role, provided they have strong typing skills and are proficient in using the relevant recording software.
Structure of the SIL Assessment Worksheet
The assessment worksheet is usually formatted as a table, with each row representing a unique SIF. Below is a suggested minimum set of columns to include:
- SIF Name, ID, and/or Tag Number: Unique identifiers for tracking.
- Sensor Tag Numbers: Identifies the sensors involved in the function.
- Initiating Event Description: Should cross-reference the corresponding HAZOP scenario number.
- Initiating Event Frequency or Likelihood Category: Provides a quantified estimate of the event occurrence.
- Outcome in the Event of SIF Failure (Consequence of Failure on Demand, CFD): Describes what happens if the SIF does not function as intended.
- Outcome Severity Category (and/or Tolerable Frequency): Helps assess risk levels.
- Other Independent Protection Layers (IPLs): Lists additional safeguards beyond the SIF.
- Risk Reduction Factor (RRF) Assigned to Each IPL: Either as a frequency reduction factor (e.g., 10) or an order of magnitude reduction (e.g., 1).
- SIL Target Assigned to the SIF: Defines the required safety integrity level.
- RRF or PFDavg Target Assigned to the SIF: Especially important for methodologies like Layer of Protection Analysis (LOPA) and Fault Tree Analysis (FTA).
- Notes, Comments, and Justifications: provide additional context for decision-making.
Capturing and Storing the Data
The worksheet can be maintained using generic office software such as word processors and spreadsheets or through specialized tools like PHA-Pro. Dedicated software provides additional features for structured risk analysis and record-keeping.
For SIL assessment methodologies involving calculations such as LOPA and FTA, an additional output should be prepared that details the computational process. This ensures transparency and ease of verification.
References
Functional Safety from Scratch by Peter Clarke
