SIL Determination Methods — A Step-by-Step Guide

What Is SIL Determination Study?

A SIL Determination study is a critical risk assessment method used to evaluate if Safety Integrity Level (SIL) requirements are necessary for Safety Instrumented Functions (SIFs). The process involves:

• Assessing the likely occurrence and impact of an undesirable event that triggers a particular hazard;
• Calculating the risk reduction achieved through existing protective measures and identifying any residual risk;
• Assigning the appropriate SIL requirements for SIFs to bridge any risk discrepancies, in compliance with standards such as IEC 61508 or sector-specific standards like IEC 61511.

Step-by-Step Procedure of SIL Determination (The SIL Assessment Process)

The Safety Integrity Level (SIL) assessment process is a systematic approach used to determine the level of safety integrity required for safety instrumented functions (SIFs) in industrial systems. This process ensures that the risk associated with hazardous events is reduced to an acceptable level. Here is a detailed step-by-step procedure for conducting a SIL assessment.

1. Hazard Identification and Risk Assessment

Identify Hazards: The first step involves identifying potential hazards in the process or system. This can be done using techniques such as Hazard and Operability Study (HAZOP), Failure Modes and Effects Analysis (FMEA), or Preliminary Hazard Analysis.

Assess Risks: Evaluate the risk associated with each identified hazard. Determine the severity of potential consequences and the likelihood of occurrences. For this purpose, tools like risk matrices or risk graphs are used.

2. Determine Risk Reduction Requirements

Reduce Risks: Based on the risk assessment, determine the amount of risk reduction required to bring the risk to an acceptable level. Compare the existing risk level with the acceptable risk criteria set by the organization.

Set SIL Targets: After determining the required risk reduction, you need to define the appropriate Safety Integrity Level (SIL) for each Safety Instrumented Function (SIF). The target SIL is essentially a benchmark that indicates how robust a safety system needs to be to manage identified risks effectively.

Read: SIL Classification Levels

3. How to set SIL targets? The SIL Determination Methods

SIF SIL targets are determined by using different SIL determination methods.

➢ Risk Graphs (Qualitative Method)

Use methods such as Risk Graphs for an initial screening of safety functions. Risk graphs help determine the SIL by evaluating factors such as consequence severity, frequency of exposure, the possibility of avoiding the hazard, and the probability of the unwanted occurrence.

Risk graphs consider four primary factors,

1. Consequence Severity ©: This factor evaluates the potential impact of a hazardous event on people, the environment, and the system. Consequences are typically categorized as minor, significant, major, or catastrophic.
2. Frequency of Exposure (F): This factor assesses how often people, equipment, or the environment are exposed to the hazard. Categories range from rare exposure to continuous exposure.
3. Possibility of Avoidance (P): This factor considers the likelihood that the hazardous event can be avoided or mitigated if it occurs. Categories include impossible, rarely possible, and easily avoidable.
4. Probability of Occurrence (W): This factor evaluates the likelihood that the hazardous event will occur. Categories can range from very unlikely to highly likely.

Also Read: Qualitative Vs Quantitative Risk Assessment

Process of Using Risk Graphs for SIL Determination

Identify and Categorize Risk Factors: For each safety function, identify and categorize the relevant risk factors (consequence severity, frequency of exposure, possibility of avoidance, and probability of occurrence).

Assign Categories to Risk Factors: Assign a specific category to each risk factor based on the characteristics of the hazardous event. For example:

• Consequence Severity ©: Moderate (SA), Serious (SB), Very Serious (SC), Catastrophic (SD), Disastrous (SE)
• Occupancy (F): High Occupancy (FA), Low Occupancy (FB)
• Possibility of Avoidance (P): High Avoidance (PA), Low Avoidance (PB)
• Demand Rate (W): Rare (W0), Low (W1), Medium (W2), High (W3), Frequent (W4)

Plot the Risk Factors on the Graph: Using the assigned categories, plot the risk factors on the risk graph. The risk graph is typically a matrix with consequence severity on one axis and a combination of occupancy, possibility of avoidance, and demand rate on the other axis.

Determine the Intersection Point: The intersection point of the plotted categories on the risk graph indicates the required SIL. Each intersection point corresponds to a specific SIL level, from SIL 1 to SIL 4.

Example of a Risk Graph

Risk graphs serve as a valuable tool in the initial stages of SIL determination by providing a structured approach to evaluate and categorize risk factors. While they offer simplicity and efficiency, it is essential to complement them with more detailed quantitative methods, such as Layer of Protection Analysis (LOPA) or Fault Tree Analysis (FTA), for comprehensive SIL assessment and verification.

➢ LOPA — Layer of Protocol Analysis (Semi-Quantitative Method)

LOPA offers a more detailed and accurate assessment compared to qualitative methods and it is the most common method for SIL determination. This method involves identifying initiating events, intermediate events, and independent protection layers (IPLs). The cumulative effect of all IPLs is evaluated to determine if additional risk reduction is needed, and thereby establish the target SIL.

The components of LOPA assessment are,

1. Initiating Events (IE): These are events that can lead to hazardous situations. Initiating events can include equipment failures, human errors, or external events like natural disasters.
2. Intermediate Events: These are sequences of events that occur between the initiating event and the hazardous outcome. They are critical in understanding how a hazard propagates.
3. Independent Protection Layers (IPLs): These are safeguards that prevent the propagation of an initiating event to a hazardous outcome. Each IPL must be independent of the initiating event and other protection layers. Examples of IPLs include relief valves, alarms, automatic shutdown systems, and operator intervention.
4. Consequence Severity ©: This measures the potential impact of a hazardous event in terms of safety, environmental damage, or financial loss. Consequences are categorized based on severity, from minor to catastrophic.
5. Frequency of Initiating Event (FIE): This quantifies how often the initiating event occurs. It is usually expressed in events per year.
6. Probability of Failure on Demand (PFD): This measures the likelihood that an IPL will fail to perform its intended function when required. PFD is a critical factor in evaluating the effectiveness of each IPL.

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