IEC 61511 is the standard that set the requirements for Safety Instrumented Systems (SIF). As part of this standard, a SIF is required to act within adequate response time to bring the process or a system to a safe state. This is called response time of a SIF and the time available to act before anything dangerous happens is called process safety time. (See part 1, part 2, part 4  part 5, part 6, part 7 and part 8 for more information).
PST calculation and definition is a mandatory requirement for all the instrumented functions to which IEC 61511 is applicable.

Important point to remember and it is also resulting from definition of a SIF is:
In Layer of Protection Assessment (LOPA, see here), there are layer of protection that reduce the chance of main even happening and there are layers that try to reduce the effects/severity of dangerous that has happened. The first type of layers before a dangerous event are prevention barriers and the layers of protection after a hazardous event are mitigation barrier.
See figure below for graphical explanation:

Process safety time layers of protectiokn SIF

Choosing a proper method for PST calculation
As I mentioned in part 2 of this article series, there are three ways to choose for PST calculation:
• Qualitative Analysis (QA)
• Quantitative – Simplified Analysis (QSA)
• Quantitative – Dynamic analysis (QDA)

At this stage, you might wonder which method shall be used for PST calculation. This is not a simple question to answer. As a process/ process engineer, one needs to look into list of SIFs and decide which method is suitable. Following table could be consulted for selection.

Proces Safety time calculation method

Table above shows only a guideline for selection of the calculation method. In any case, you need to select a method for calculation and discuss it with your client and agree upon it. Finally the Notified Body, who will review and certify safety systems, shall review your calculation and agree upon your selected method and your outcome.

When selecting your calculation method remember following points:

1- SIFs with a high Safety Integrity Level (SIL) level, need more accurate calculation. This is because they are providing a higher risk reduction factor (RRF).
2- Some parameters rate of change is faster or slower than other parameters. For example a level increase is faster to detect than a temperature increase. Therefore, a quantitative dynamic analysis (QDA) with help of simulation programs might be more accurate for a temperature protection switch than level protection switch.
3- Help/Advise of an experienced operator or vendor in defining PST is invaluable, after all he/she has been working with the system or equipment for many years and could help much better than any calculation.
4- If you are having the same system inside your company but not at the same location, it would be a good idea to look to the other system that is running and see what the PST is or how it is calculated. For example, a dynamic simulation for a high pressure switch at the outlet of compressor might be time-consuming and very difficult to set up in the simulation software like HYSYS. If you can find a similar compressor within your asset who has the same pressure switch, you might use it. Of course the Notified Body or Authority having Jurisdiction shall agree with you.
5- You might need to ask vendor’s help you to set up a dynamic analysis when a SIF is acting in start up or shut down situations. Imagine a high pump temperature switch that protects the pump during a dry run scenario.

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