What are SIS and SIL in functional safety

Organizations involved in industrial activities, such as the energy, pharmaceutical, and petroleum sectors, among others, must consider functional safety as one of their fundamental pillars. To comprehend this discipline, it is essential to delve into the various concepts that comprise it, both individually and in terms of their interconnections. SIS, SIL, and SIF are some of the crucial terms in this regard.

Understanding the Concepts to Grasp Their Industry Relevance

Functional safety is a discipline that encompasses all industrial protection systems based on various regulations, classifications, and implementations. Safety Instrumented Systems (SIS), Safety Integrity Levels (SIL), and Safety Instrumented Functions (SIF) form the foundation of these processes. Let’s delve into each of them.

Functional Safety

To define the concepts of Safety Instrumented System, Safety Integrity Level, and Safety Instrumented Function, it is necessary to explain that they are part of functional safety. This area of engineering is dedicated to ensuring the proper functioning of safety systems and mitigating the impact in case of incidents.

Safety Instrumented System (SIS)

An SIS, or Safety Instrumented System, consists of multiple safety functions grouped under this umbrella. In this sense, the Safety Instrumented System encompasses everything required for its operation, including electrical connections, physical storage, logistics, and other essential services.

Safety Integrity Level (SIL)

Safety Integrity Level (SIL) is the methodology for classifying protection in safety instrumented functions. It includes a scale from SIL 1 to SIL 4, representing decreasing to increasing safety integrity and, consequently, risk reduction in the physical elements of the system.

Safety Instrumented Function (SIF)

A Safety Instrumented Function is a part of a system and must have various processes to be considered as such. This is the core of functional safety, as it involves the integration of elements for detection, logical execution, automated incident resolution, and the connection between all these subsystems.

Key Considerations for SIS, SIL, and SIF

Functional safety wouldn’t make sense without these three terms, which, as we can see, are meaningful when analyzed together. Thus, SIS, SIL, and SIF are interrelated, and it is essential to consider some aspects that explain the relationship between these concepts and the need to address them collectively.

The Relationship between SIS and Safety Instrumented Functions

As previously explained, an SIS gathers multiple SIF and provides them with the necessary elements for their operation. Safety Instrumented Systems are independent of each other and with respect to other elements, so they must integrate their protection mechanisms, such as prevention and diagnosis of potential incidents. In contrast, Safety Instrumented Functions are supported by the SIS.

Classification of Different Levels of Safety Instrumentation SIL

The mentioned classification of safety instrumented functions through the SIL system involves evaluating various layers of protection simultaneously, specifically, all those that are part of the mitigation of the same risk. Safety levels are assigned to each SIF, not their individual elements, processes, or the entire instrumented system, which is essential to consider in the analysis.

Different Types of Safety Instrumented Functions

Within safety instrumented functions, different typologies can be found based on their application. Depending on the type of protection offered in each process:

  • Continuous safety instrumented functions consist of more complex equipment and provide permanent protection to devices, acting instantly in case of an incident.
  • On-demand safety instrumented functions are activated only in specific danger events and can be categorized as low demand or high demand based on the annual number of function activations.

Analytical Systems for Establishing SIL in Each SIS

Considering the importance of classifying the level of protection against risk using the SIL system, it is advisable to employ specific and highly effective evaluation processes. In this regard, the LOPA analysis, which can be included in HAZOP evaluation, is the most suitable methodology to accomplish this with assurance and compliance with established standards.

Essential Regulations in SIS Implementation

Both the design of safety instrumented systems and their implementation, use, and maintenance must adhere to international regulations. While some cases may involve state standards, they generally align with the norms developed by the IEC (International Electrotechnical Commission).

IEC 61508 Standard for Electrical Systems

IEC 61508 is the primary global standard for addressing the use of safety instrumented systems. It primarily provides specific instructions for electrical, electronic, and programmable electronic systems for functional safety, although it is often applicable to other areas within this field.

IEC 61511 Standard for Industrial Processes

Although developed later, the IEC 61511 standard is also widely adopted internationally in various industries with inherent risks. Its creation was necessitated by the limitations of the previous regulations when addressing functional safety processes beyond the scope of electrical systems.

Functional Safety with Engineering Experts

The analysis of safety instrumented systems and their constituent safety instrumented functions is crucial for protecting employees, facilities, and processes in the industry. At Positive Engineering, we conduct the necessary analyses to establish the SIL of each function and design suitable SIS based on the characteristics and needs of each company.