What is SIS, SIF, and SIL ?

Importance of Safety

Safety is increasingly important for the high-risk industry like oil, gas, chemicals and petrochemicals to adequately implement and follow safety programs. This can be achieved by adhering to regulations, which ensure safe processes and higher productivity.

Risk Involvement

The high-risk industry is one, where processes handle unstable reactions that can get out of control quickly and frequently.

A series of unforeseen conditions or events can trigger incidents with serious consequences for personnel, facilities and production.

Due to the above, the need is created to implement equipment and systems (layers of protection) that minimize the probability of an unwanted event or that minimize the risk.

Regarding these layers of protection, we can find those that involve both hardware and software such as:

  • Alarm Systems
  • Instrumented Security Systems


(Safety Instrumented System)

Instrumented system used to perform one or more Safety Instrumented Functions (IEC 61511-1: 2016 – 3.2.67).

Safety Instrumented System (SIS)

The SIS, in general, is made up of several Security Instrumented Functions (SIF) and all the common services necessary for its operation. It provides the basic common support services to host them, that is, the

  • Physical housing (for example, chassis for I/O cards or common processor housing).
  • Logical housing (for example, application program housing), common service such as power supply, etc.


(Safety Instrumented Function)

Safety function to be implemented by a Safety Instrumented System (SIS) (IEC 61511-1: 2016 – 3.2.66).

The SIF is considered a system formed by at least three (3) sub-systems:

Safety Instrumented Function (SIF)
  • The detection sub-system (One or more measurement or detection elements).
  • The logical sub-system (commonly a Programmable Safety Logic Controller).
  • The final element (which can be made up of one or more valves, motors, etc.).

In short, there must be someone who measures the variable, who executes the logic and who acts to bring the process to a safe state.


(Safety Integrity Level)

It is a discrete value from 1 to 4. SIL defines the safety integrity requirements that must be met by each SIF executed by the SIS (IEC 61511-1: 2016 – 3.2.69).

Security integrity comprises hardware security integrity and systematic security integrity. The value 1 to 4 defines the order of magnitude of the risk reduction provided by the SIF.

Safety Integrity Level (SIL)
  • Level 4 has the highest level of security integrity and indicates a risk reduction of the order of 4 zeros (more than 10,000);
  • Level 1 security integrity has the lowest and indicates a risk reduction of the order of 1 zero (more than 10).

What does “SIL” mean?

Let us first by understanding the safety Integrity Level (SIL) that it is always associated exclusively with a Safety Instrumented Function (SIF). So, if we start from the fact that a Safety Instrumented System (SIS) is made up of different SIFs. We can understand that in a SIS there will be as many SIL‘s as SIF’s coexist.

Trying to assign a SIL to an entire plant or installation would be a conceptual error. It would also be to think that a controller (for example, one capable of achieving SIL 3), is the one that determines the SIL of a SIF or worse still of a full SIS.

This leads us to specify that SIL is a specific measure of performance related to safety and this is determined for a specific SIF. Therefore, by the time a SIF is put on demand (its operation is necessary), its behavior, from the point of view of its success or failure, will be escalated at 4 different levels according to the IEC-61511 standard.

However, a SIF with SIL 4 will have a probability of failure between 0.00001 and 0.0001 of the time, which is obviously much lower.

The Risk Reduction Factor (which is the inverse of the PFD) avoids the use of scientific notations to define the behavior of a SIF.

Returning to the example of a SIF with SIL 4, this would be interpreted in l / PFD as the fact that one in 10,000 to 100,000 would fail within a year. Safety Availability is the interpretation of the behavior of the SIF from the point of view of its success.

So then, a SIL 4 is successful 99.99% to 99.999 of the times it is required.

Implementation in risk reduction

First of all, we must bear in mind that the IEC-61508 standard defines security as “free from unacceptable risks”. Therefore, absolute safety can never be achieved. In practice, Risk can only be scale down to an acceptable level.

Generally, the security methods used to reduce risk are

  • Change the process or mechanical design including plant and equipment arrangements.
  • Increase the mechanical integrity of the equipment.
  • Improve basic process control systems.
  • Increase the frequency of testing critical process components and protection elements.
  • Apply a Safety Instrumented System (SIS).
  • Install a system with equipment and systems to reduce the consequences in the event of an accident.

Safety management requires a verification that the SIS will truly give the Required Risk reduction or will meet the specified performance value (SIL) for each SIF during its operation.

Author: Abraham Moses

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