What is the Safety Integrity Level (SIL) in process safety? And how is it important for operations risk management?

What is the Safety Integrity Level (SIL)?

SIL is a safety management technique that is used to measure the performance of a process in terms of risk reduction. It categorizes the reliability and performance requirements of safety functions, with higher SILs indicating greater safety and a lower likelihood of dangerous failures.

The International Electrotechnical Commission's (IEC) standard 61508 and IEC 61511 defines four SILs, ranging from SIL1 to SIL4, with SIL1 being the lowest level of risk reduction and SIL4 being the highest.

SIL determination

Various methods are used for the assignment of target SILs. One of the most effective Methods for determining SIL is Layer of Protection Analysis (LOPA), which is a risk management method that can provide a semi-quantitative assessment of the risks and layers of protection associated with hazard scenarios.

For successful process safety, LOPA consists of mechanical devices, process controls, shutdown systems and external response measures that prevent or mitigate a hazardous event. If one protection layer fails, successive layers will be available to take the process to a safe state. As the number of protection layers and their reliabilities increase, the safety of the process increases. One of the most important layers is SIS, which is indicated by its safety integrity level (SIL) therefore it will be explained below.

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LOPA has two types of layers: prevention layers and mitigation layers.

1.?The prevention layers contain:

· Process Design: This level comprises basic controls, alarms, and operator supervision.

· Critical Alarms: This layer of protection provides critical alarms to alert operators to a condition that a measurement has exceeded its specified limits and may require intervention.

· Automatic SIS: The SIS operates independently of the BPCS to provide safety rather than process control. The Safety Instrumented System (SIS) plays a vital role in providing a protective layer around industrial process systems. It performs shutdown actions when previous layers cannot resolve an emergency.

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A SIS is comprised of Safety Instrumented Functions (SIF) with sensors, logic solvers, actuators, and final elements acting in concert to detect a hazard and bring the process to a safe state. Every SIS has one or more safety functions (SIFs) and each affords a measure of risk reduction indicated by its safety integrity level (SIL).

For example: SIS → (SIF1)(SIL2) & (SIF2)(SIL2) & (SIF3)(SIL2)

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All of the SIS design, operation and maintenance choices must then be verified against the target SIL. This ensures that the SIS can mitigate the assigned process risk.

· Relief Devices: This active protection layer employs valves, pressure relief devices, or a flare system to prevent uncontrolled release.

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2. The mitigation layers contain:

·?Plant Response: This passive protection layer contains containment barriers for fire or explosions and evacuation procedures.

·?Community Response: The final level of protection is the emergency response action taken by the community and consists of firefighting and other emergency services.

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Steps for Calculating SIL

1. Risk Assessment: Identify hazards and assess their consequences.

2. Determine Required SIL: Based on the risk assessment, determine the necessary SIL level.

3. Safety Instrumented Functions (SIFs): Define SIFs that mitigate specific risks.

4. PFD Calculation: Calculate the PFD (Probability of Failure on Demand) for each SIF using reliability data and failure rates.

5. SIL Verification: Verify that the achieved SIL meets the required level. To achieve a given SIL, the device must have less than the specified probability of dangerous failure and have greater than the specified safe failure fraction. These failure probabilities are calculated by performing a Failure Modes and Effects Analysis (FMEA).

PFD (Probability of Failure on Demand) and RRF (Risk Reduction Factor) for SIL Levels as defined in IEC 61508 are as follows:

Note that (SIL1) is the lowest level of risk reduction and (SIL4) being the highest. The higher the safety integrity level, the higher the associated safety level and the lower the risk of a safety failure.

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Why is SIL Important?

SIL ensures that safety functions effectively reduce risk. Higher SIL levels correspond to greater risk reduction, making it a verifiable way of risk reduction to an acceptable level. On the other hand, SIL directly impacts process safety by minimizing the likelihood of hazardous events and minimum nuisance shutdowns or disturbances in the plant.

Finally, Understanding and applying SIL principles effectively can significantly enhance the safety of operations, making it an indispensable part of the industry's safety culture. By applying SIL standards, companies can successfully achieve an overall process safety management that positively impacts the sustainability of the industry.