SIL Verification

Calculation of Random Hardware Reliability and Architectural Assessments for Safety Instrumented Functions (SIFs)

SIL Verification


Safety Integrity Level; or SIL Verification demonstrates the capability of a Safety Instrumented Function (SIF) in accordance with IEC 61508 and IEC 61511 against the following requirements:

– Quantifying the effect of random hardware failures (Probability of Failure on Demand (PFD) or the Average Frequency of Dangerous Failures (PFH);

– Hardware safety integrity architectural constraints (Safe Failure Fraction (SFF), Hardware Fault Tolerance (HFT), Element Type A or B);

– Systematic capability;

– Common Cause Failure (CCF).


Methods of SIL Verification


SIL Verification can be carried out using various modelling techniques including Reliability Block Diagram (RBD), Fault tree Analysis (FTA) and Failure Mode and Effect Analysis (FMEA). Where the SIF fails to achieve the target failure measure or SIL, a sensitivity analysis can be carried out to demonstrate the effect of modifying certain factors. This includes the following factors:

– Maintenance strategy (repair time and proof test frequency);

– System architecture;

– Implementing diagnostic mechanisms;

– Minimising CCF.


Example of a RBD


The following provides and example of a Reliability Block Diagram used for SIL Verification.




What inputs are required by FSES in order to conduct the SIL Verification Study?

As a minimum the following information would be required in order to conduct the workshop:

 – Existing SIL Determination report

 – P&ID’s

 – Cause and Effects Diagrams

 – Interlock Lists and configurations

 – Equipment Data Sheets and Reliability Data

What is the expected output of the SIL Verification Study?

FSES will prepare a report describing the facility, the scope of work, a detailed methodology, the SIF’s being assessed along with their configurations and the data utilised, the results of the analysis (i.e. SIL achieved for each SIF), sensitivity analysis (if required) and any recommendations required.

Based on the results of the SIL Verification study, FSES are also able to assist in writing detailed Safety Requirement Specifications (SRS) for each of the SIFs in accordance with the requirements of IEC 61508 and IEC 61511.




FSES have worked with clients all over the world conducting SIL Verification studies. Through our consultants, we have verified over 1000 SIF loops against their target requirements. FSES have managed to gather a vast amount of failure rate data of various devices, which they have available to use at all times.

For more information, or to discuss your needs for conducting a SIL Verification study, please contact us using the form below or email [email protected] .


Our Services at FSES

Compliance to the international Standards such as IEC 61508 and IEC 61511 are imperative to meeting the Functional Safety requirements for many processes and have formed part of the quality management systems of many companies. Provision of Functional Safety Services in line with the Phases and Activities detailed in the Safety Life-cycle is an area in which our consultants are able to demonstrated experience and expertise.

Learn more about our services

PHA, HAZID and HAZOP Studies

FSES can provide Process Hazard Analysis (PHA) and HAZard and OPerability (HAZOP) study chairpersons. Our expert PHA facilitators hold certification in conducing HAZOP, HAZAN and CHAZOP.

Functional Safety Training

FSES provide various Functional Training services for all of specific parts of IEC 61508 and its sector derivatives

Consequence Modelling

Discharge and Dispersion is often required to evaluate the consequence of identified hazardous events.

Design Engineering

Functional safety activities are most effective by integration into a well-established and proven quality management system.

Building Risk Assessment

Building Risk Assessment (BRA) is a tool used to assess the risks to people in occupied buildings in the facility.

Escape, Evacuation and Rescue Analysis

Escape, Evacuation and Rescue Analysis ensures that the design of a facility is such that the risk to personnel is kept to within the ALARP region.

Functional Safety Assessment

Independent Management of Functional Safety Assessments, Audits & Support.

Functional Safety Lifecycle and Management

Functional Safety Lifecycle and Management is a key requirement demonstrating how Functional Safety is to be implemented and achieved.

Human Factors

Human factor assessment is a key area in identifying potential hazards which may be instigated by human errors.

Legacy Systems

Legacy Systems Review and Compliance.

Prior Use

Prior Use Assessment.

Proof Testing Procedures

Proof Testing is one of the main requirements to ensure that Functional Safety is maintained throughout the operational phase.

Operation and Maintenance

FSES can carry out review studies and provide consultancy to ensure Functional Safety requirements are maintained during Operation and Maintenance.

Quantitative Risk Assessment

Quantitative of Risk in terms of IRPA, PLL and graphical demonstrations such as FN Plots.

Reliability, Availability and Maintainability Study (RAMS)

Reliability, Availability and Maintainability Study (RAMS) is used as a decision making tool to increase the availability of the system, and thus increase the overall profit as well as reducing the life cycle costs.

Safety Requirements Specification

The Safety Requirements Specification (SRS) is a core document, which is used as the main reference to be followed by designers, installers, and operators of a Safety Instrumented Function (SIF).

SIL Determination

FSES are able to us various methods in identifying SIL targets; qualitatively by Risk Graphs, semi-quantitatively by Layers of Protection Analysis (LOPA) or fully quantitatively by Fault Tree Analysis (FTA).

SIL Verification

Calculation of Random Hardware Reliability and Architectural Assessments for Safety Instrumented Functions (SIFs)

SIS Conceptual Design

Safety Instrumented System (SIS) Conceptual Design