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 (PST). (See part 1, part 2, part 3, part 4 part 5, part 6 and part 7 for more information).
PST calculation and definition is a mandatory requirement for all the instrumented functions to which IEC 61511 is applicable.
In the previous parts (See part 1, 2, 3, 4, 5, 6 and 7), I tried to define PST and calculation methodology, how to assess and explain the results and a sample calculation as well as notes and thoughts on for a terms of reference.
PST Calculation report contents
This part suggests contents of a typical PST calculation report.
At this stage, we have gathered lots of information and fata, and calculating the PST(s) for different SIFs in the project. For reporting of your findings try to cover the followings subjects as a minimum:
- Present a summary of SIFs with relevant PST that you did calculation for.
- If possible, summarize in the previous bullet’s table which method you used: qualitative, quantitative simplified or quantitative dynamic analysis (See part 3).
- A simplified sketch of the system and SIF that is protecting it. You need to provide one sketch per SIF.
- Define your terms and abbreviations for SIF response time, Safe design limits, etc.
- Clearly write down your assumptions and give reference to the document that you took the data from.
- Write down in detail how you calculated the PST, including formulas and input data. It is worth to note that using Excel program makes it easier if PST calculation is done by hand and not using a simulation program. For the result from a simulation program, print the report out, highlight the relevant sections and attach it.
- Capture the SIF response time and SIF PST in a table, this makes it easier for the reader to see in a glance and conclude.
- Write a conclusion if based on agreed criteria with your client, the PST calculated is acceptable or not.
- Attach all documents that you used as reference to your report and list it. If you are attaching a pdf file, it would be great help for the reader to highlight where the data are taken from.
- Of course your report might have several revision, make sure that you indicate the differences between two consecutive revisions.
- Important point is to make sure that your report and calculation is checked internally inside your organization or by others, before you discuss it with your client.
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