SIL Determination
Worksheet (Excel)

A SIL determination worksheet is the document that assigns a Safety Integrity Level target (SIL 1, SIL 2, or SIL 3 per IEC 61511) to each Safety Instrumented Function on a process plant. It records the hazard scenario, the initiating event and its frequency, the credits taken for each Independent Protection Layer (IPL) such as the BPCS, operator action, and relief valve, the mitigated event frequency, the tolerable risk frequency, the required risk reduction factor, and the resulting SIL target. The Layer of Protection Analysis (LOPA) is the most common SIL determination method.

Required columns: worksheet reference, unit and node, P&ID reference, HAZOP action reference, hazard scenario, consequence category and severity, initiating event description, initiating event frequency per year, conditional modifiers (occupancy, ignition probability for fire scenarios), IPL descriptions and Probability of Failure on Demand (PFD) for each layer (BPCS PFD typically 0.1, operator PFD 0.1 with effective alarm, relief valve PFD 0.01 if properly designed), mitigated event frequency, tolerable risk frequency per project basis, required Risk Reduction Factor (RRF), SIL target per IEC 61511, SIF identifier, assigned final element and sensor. See the LOPA standard reference.

Pathnovo, an engineering document intelligence platform, builds the SIL determination worksheet by extracting HAZOP findings from the HAZOP register, cross-referencing the P&IDs for IPL identification (BPCS loops, relief valves, restrictive orifices, vents), and applying the project basis of design for IPL PFD credits and tolerable risk frequency. The platform produces a draft LOPA per HAZOP action and computes the SIL target. Drafts are then reviewed and approved by the SIL determination team and chair. See the HAZOP safety intelligence workflow.

IEC 61511 (Functional Safety: Safety Instrumented Systems for the Process Industry) defines four Safety Integrity Levels. SIL 1 corresponds to Risk Reduction Factor (RRF) 10 to 100 and Probability of Failure on Demand (PFD) 0.1 to 0.01. SIL 2 corresponds to RRF 100 to 1000 and PFD 0.01 to 0.001. SIL 3 corresponds to RRF 1000 to 10000 and PFD 0.001 to 0.0001. SIL 4 is rarely used in the process industry and is typically avoided by redesigning the SIF to require lower integrity. The worksheet computes RRF from the LOPA columns and assigns the SIL band. See the SIL standard reference.

SIL determination assigns the SIL target (what integrity the SIF must achieve). SIL verification proves the design meets the target (what integrity the chosen sensors, logic solver, and final elements actually deliver per their certified PFD and architecture). Determination is performed during HAZOP and LOPA workshops in FEED. Verification is performed during detailed design once vendor data is available, using the SIL verification calculation (RRF and PFD per IEC 61511 Annex B). The two are separate worksheets and both must be closed before SIS commissioning.

Each row of the SIL determination worksheet that yields a SIL 1, SIL 2, or SIL 3 target becomes a Safety Instrumented Function on the cause and effect matrix. The SIF Identifier column on both documents must match. The C&E matrix Voting and SIL Classification columns are driven by the worksheet output. Pathnovo cross-validates the C&E matrix SIF list against the SIL determination worksheet to ensure every SIF on the matrix has a closed LOPA, and every LOPA SIF is realised on the matrix. See the cause and effect matrix template.

On a revamp project, the existing SIL determination worksheet is the baseline. Each SIF is reviewed against the revamp scope: changes to feedstock, throughput, operating conditions, or IPL credits may change the mitigated event frequency and trigger a SIL re-assessment. New equipment introduces new SIFs requiring fresh LOPA. Removed equipment retires SIFs and their associated worksheet rows. Particularly important on Indian PSU and GCC brownfield projects where the original LOPA may pre-date IEC 61511 (2016 edition) and require a full re-baseline against the current standard.