Safety and Availability in a SIF

Dec 20, 2018 / by GM International

Topics: Functional safety

If you’re a regular on the GMI blog, then you already know that we are big proponents of putting safety above all else. Irrespective of the industry you are in, compromising on your safety could result in catastrophes.

However, we also know that the safety industry is constantly improving – as it should be. Part of this improvement is addressing customer needs. And those needs are more than safety.

Availability in a SIF plays an important role in any safety plan. Today, we are exploring why it’s so important.

Let’s start with the basics.


What is SIF?

SIF stands for Safety Instrumented Function. It refers to equipment designed to prevent or mitigate the risks of a specific hazard. In other words, it’s a safety loop.

SIF serves a three-pronged purpose:

  • First off, when certain conditions aren’t met, it takes the industrial process back to a safe state.
  • Secondly, due to permissive functions, when specific conditions are met, it allows the process to move forward safely.
  • Lastly, it takes specific actions to minimise the consequences of the industrial hazard.

Simply put, SIF detects the imminence of an incident, decides to take a specific action and acts to bring the process back to a safe state – all abilities which are designated by the function’s SIL (Safety Integrity Level).

Sounds pretty safe so far, right?

Right, but there’s a small problem.


Availability in a SIF

In case of an internal failure, functional safety describes the behavior of a safety device (both at a hardware and a software level). The main target is reaching a safe state.

In order to get there, the device itself should detect its own internal faults and signal them.

But here’s the problem: the only requirement in the standard is safety, not availability.

Think about it for a bit: a safety device is capable of shutting down an entire production line that it monitors or even an entire plant in case it detects even the smallest probability of a potential failure. This way, the device is safe, it complies with the standards and it even reaches a high SIL.

But is this precisely what you, the client, need?

The standards require safety and the consumer requires safety AND availability. Availability doesn’t mean compromising on safety.

This is where SIS comes into play.

SIS stands for Safety Instrumented Systems. They are used to monitor the values’ condition, as well as the relevant parameters of a plant. When risk conditions are met, they trigger alarms and act to place the entire plant (or the section that is affected) in a safe condition. Sometimes, to achieve this safe condition a full shutdown must happen.

SISs have been widely used in process industries, which can automatically implement safety instrumented functions (SIFs) to prevent or mitigate the consequences of potentially hazard events

Safety and availability are the two important aspects to evaluate the performance of the SIS. Safety is the essential attribute which indicates the ability of the SIS to prevent against unwanted accidents.

The SIS with high safety can sense hazardous conditions effectively and take actions to move the process into safe states in time. Availability is the basic requirement for SISs indicating the assurance for the normal running.

Without intense availability, the system may be led to operate without an actual demand; i.e. spurious activation. Spurious activations of SISs may contribute to partial or the whole process shutdown, which will affect normal operations and bring out large economic loss.

In conclusion, you should never consider safety and availability mutually exclusive. Choosing the right SIS can ensure that your personnel and your plant are perfectly safe without you having to suffer any economic loss.

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