Functional Safety Engineering

SIL Verification (PFD and PFH Calculation)

IEC 61511 Clause 11 lifecycle verification built on FMEDA grade reliability data, PFD and PFH calculation, and a full FSA Stage 2 dossier

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Technical overview

SIL Verification (PFD
and PFH Calculation)

SIL verification is the engineering proof that a Safety Instrumented Function, as designed, actually achieves the SIL that the hazard study allocated to it. Our work begins where SIL allocation ends. We take the SIL Allocation Register from your hazard study team and produce the probabilistic, architectural, and systematic capability evidence that satisfies IEC 61511 Clause 11. Verification comes down to three honest measurements that our team makes carefully. We source FMEDA grade reliability data covering lambda DD, lambda DU, SFF, and DC from TUV, exida, and SIRA certificates. We calculate PFDavg or PFH to IEC 61508 Part 6 Annex B with a calibrated common cause beta factor. We run the architectural constraint check under Route 1H or Route 2H. The audit failures we see across industry cluster around optimistic beta factor scoring, partial stroke test coverage claims that exceed manufacturer guidance, and prior use justifications offered without statistical evidence, and each of these collapses under FSA Stage 2 examination.

Engineering process

SIL Verification (PFD and PFH Calculation) workflow

SIF Scope & Boundary Definition

Scope each Safety Instrumented Function from HAZOP/LOPA hazard scenarios, define SIF boundary, initiator(s), logic solver, final element(s), document operating context, demand mode (low / high / continuous), and pre existing IPL credits.

Risk Reduction Factor & SIL Target

Calculate required risk reduction factor (RRF) per LOPA tolerable frequency target, allocate SIL band per IEC 61511 Table 4 (SIL 1 RRF 10, 100, SIL 2 100, 1000, SIL 3 1000, 10000, SIL 4 not used in process industry), document SIL allocation register.

FMEDA & Component Reliability Data

Compile component FMEDA data (λDD, λDU, SFF, DC) from exida / TÜV / SIRA certificates, validate prior use justification per IEC 61511 Cl.11.5.3 where vendor FMEDA unavailable, verify architectural constraints (Route 1H / 2H).

PFD / PFH Verification Calculation

Calculate PFDavg (low demand) or PFH (high demand) per IEC 61508 6 Annex B, apply 1oo1, 1oo2, 2oo2, 2oo3 voting formulas, include common cause β factor per IEC 61508 6 Annex D, compare against SIL band target.

Proof Test Coverage & Interval Optimisation

Specify proof test procedure with PTC (proof test coverage) calculation, optimise proof test interval to achieve SIL target while balancing operational disruption, specify partial stroke test for SIL ≥2 SDV where applicable.

SIL Verification Report & FSA Readiness

Issue SIL verification report per IEC 61511 Cl.11, prepare FSA Stage 2 dossier (design validation), maintain SIL register with revalidation triggers (component change, demand rate change, modification).

SIL Verification (PFD and PFH Calculation) Scope

Scope of work

Every deliverable from basis to handover

Complete SIL Verification (PFD and PFH Calculation) scope covering every calculation, drawing, specification, and construction support activity.

SIF boundary verification against the SIL Allocation Register and the SRS

Component FMEDA data sourcing for lambda DD, lambda DU, SFF, and DC from exida, TUV, and SIRA certificates

Prior use justification under IEC 61511 Cl.11.5.3 where vendor FMEDA is unavailable

Architectural constraint check across Route 1H using HFT and SFF and Route 2H using reliability data

PFDavg for low demand or PFH for high demand calculated to IEC 61508 Part 6 Annex B

Voting architecture treatment with 1oo1, 1oo1D, 1oo2, 2oo2, and 2oo3 formulas and their diagnostics

Common cause beta factor calibration to IEC 61508 Part 6 Annex D scoring, typically one to ten percent

Realistic proof test coverage that accounts for unrevealed failure modes

Partial stroke test coverage credit taken in line with manufacturer guidance, typically sixty to eighty percent

An FSA Stage 2 dossier that brings together the verification calculations, the FMEDA pack, and the design narrative

Engineering outcomes

Outcomes of SIL Verification (PFD and PFH Calculation)

SIL Target and SIF Integrity Calibration

Confirms each SIF achieves its target integrity
Identifies the SIFs that need an upgrade
Reinforces the demand rate analysis
Supports proof test interval optimisation

IEC 61511 SIL Determination Defence

Aligns with the IEC 61511 lifecycle
Documents SIL verification in line with ISA TR84
Holds up under an SIS audit
Supports the defence of your SIL claim to the regulator

SIL Verification and Proof Test Clarity

Optimises proof test intervals
Strengthens your SIS asset management
Identifies weaknesses in the SIL claim
Supports MOC for SIS modifications

SIL Over Specification Prevention

Sizes the SIL claim and SIF complexity correctly
Avoids unnecessary 1oo2 and 2oo3 voting
Optimises the cost of the proof test schedule
Reduces insurance loadings on your SIL claims

Standards & references

Codes & standards we work to

IEC 61511 Ed.2 Cl.11 (Verification)IEC 61508 6 Annex B (PFDavg / PFH) + Annex D (beta factor)ISA TR84.00.02 (SIL Verification Tools)exida CFSE Body of KnowledgeTUV Functional Safety Engineer ProgramManufacturer Safety Manuals (FMEDA data)OISD GDN 178 (Instrumented Safety)ANSI/ISA 84.91.01 to 2020 (Process Safety Critical Instrumentation)

When to engage

Triggers that signal the need

The hand off to detailed design after allocationA brownfield SIL revalidation under IEC 61511 Cl.11A component or vendor change that needs the FMEDA re verifiedA demand rate change that reclassifies low demand as continuousAn FSA Stage 2 examination by TUV, exida, or a third partyA spurious trip programme review that reworks the architectureAn SIS upgrade or a logic solver migration

Industries served

Where SIL Verification (PFD and PFH Calculation) applies

Oil & Gas, Upstream

Wellheads, separators, gas compression, FPSO topsides, produced water systems.

UpstreamOffshoreFPSO

Refineries & Petrochemicals

Distillation columns, reactors, heat exchangers, storage spheres, LPG handling.

RefiningPetrochemical

LNG & Gas Processing

Cryogenic exchangers, liquefaction trains, BOG compressors, storage and sendout.

LNGCryogenic

Specialty Chemicals

Reactive systems, batch reactors, solvent handling, runaway reaction scenarios.

ReactiveBatch

Power Generation

Boilers, HRSGs, steam headers, hydrogen systems, ammonia SCR units.

PowerHydrogen

Pharma & Food

Sterile vessels, CIP/SIP, pressure fermenters, solvent recovery, spray dryers.

PharmaFood & Bev

What we deliver

Tangible deliverables

A SIL Verification Report for each SIF with the PFDavg or PFH calculation
An FMEDA data pack with lambda DD, lambda DU, SFF, and DC per component
An architectural constraint check record for Route 1H or 2H
A beta factor scoring worksheet to IEC 61508 Part 6 Annex D
Voting architecture and diagnostic coverage analysis
A proof test coverage and interval optimisation report
A partial stroke test specification for SIL 2 and above SDV and BDV
An FSA Stage 2 design validation dossier
A SIL Register with its revalidation triggers

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