Safety Integrity Level (SIL) Assessment Study

Governing standards

Majority of the Operators developed on the basis of methodologies and values based on the following accepted methods that are laid out in:

• • IEC 61508 Functional Safety of Electrical/Electronic/Programmable Electronic Safety-Related Systems
• • IEC 61511 Functional Safety: Safety Instrumented Systems for the Process Industry Sector

However, client’s order of precedence in the project will be governing in the end and if any client specific standards available will be followed, based on client’s advise.

What are layers of protection in process industries ?

Before we dive in about SIL determination, we ought to understand the layers of protection in process industries.

Each and every layer of protection are designed to prevent a postulated accident sequence from proceeding to a defined, undesirable endpoint (e.g., runaway reaction, release of toxic material, loss of contaminant, or fire).

The performance of any particular protection layer shall not be affected by failure of another protection layer or by conditions that caused another protection layer to fail. All Protection layers are also independent of initiating cause. Hence these are called Independent Protection layers (IPLs).

The LOPA study provides a consistent basis for judging whether there are sufficient independent protection layers against hazardous events to achieve the required risk reduction target.

One such protection layer is Safety Instrumented Function (SIF) achieved through Safety Instrumented System (SIS). Depending on the hazard that is brainstormed in Hazard and Operability (HAZOP) study a scenario may have one or more (the better) protection layers of various types, depending on complexity of process and potential severity of consequence.

Sometimes a scenario may or may not warrant for a SIF in the protection layer. The inherent hazard could be managed by a simple Basic Process control system (BPCS). Entirely depends on the design.

The assessment of a SIF’s reliability that is the level that is required is done in terms of calculating its Probability of Failure on Demand (PFD) or the Risk Reduction Factor (RRF) required of / achieved by the safety function. The International Standard for Safety Instrumented Systems, IEC 61508/61511, establishes four SIL classification bands as shown in following Table.

Which industries are employing SIL as an integral part in risk management?

• Oil and Gas
• Nuclear
• Pharmaceutical
• Chemical
• Manufacturing

Different types of method to determine SIL level.

The commonly used methods for SIL determination are:

• Risk Matrices
• Risk Graphs
• Layer of Protection Analysis (LOPA)

Among these, LOPA is widely used in large industrial facilities.

What are the Objectives?

The SIL determination study will be undertaken with following objective:

• To determine whether a Safety Instrumented Function (SIF) was needed or not, and if the other independent protection layers were sufficient for controlling risk to As Low As Reasonably Practicable (ALARP), and then classify the SIL rating for each SIF

When SIL Assessment shall be performed or What would warrant a SIL Assessment study?

SIL Assessment is performed after Hazard and Operability (HAZOP) study in design process.

It should preferably be carried out as early in the design phase be the phase in FEED, Detailed design or before performing the modifications in a running facility once the design documents are available to have influence on the design.

• Front-End Engineering Design (FEED) stage
• Detailed Design Stage
• Any modification during operation as a revalidation study

How it is performed?

The SIL assessment follows these steps:

1. Identify the safety instrumented function (SIF) from Hazard and Operability (HAZOP) report;
2. Define the design intent of this SIF
3. Decide the Target Mitigated Event Likelihood (TMEL) for this SIF with respect to personnel safety (S), environment (E), asset (A) and Reputation (R)
4. Identify all demand scenarios (i.e. initiating events) where the SIF needed to be activated
5. Identify all major consequences occurring involving the particular SIF
6. Estimate the frequency of each scenario – the frequency assessment in line with the agreed Risk Matrix and conditional modifiers
7. Estimate the frequencies of all IPLs that guards against the initiating causes
8. Calculate the PFD and RRF, that is assigned to the SIF in study

After completed SIL classification, SIL Verification calculations will be performed within in-house excel based calculation tool or any commercially available software.

SIL study will be executed in following phases,

How Aura Veritas can help achieve your objective?

Aura Veritas engineers and technologists are an independent and experienced functional safety certified experts, who understands your business and culture, shall provide expertise to lead/facilitate SIL study as a third party auditor or HSE specialists to achieve the aim of improving safety and ensuring compliance to relevant laws, codes and standards.