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 and part 4 for more information).

PST calculation and definition is a mandatory requirement for all the instrumented functions to which IEC 61511 is applicable.
This is the part contains notes on gathering data and calculation procedure for a PST calculation.

Calculation procedures

In the previous parts (See part 1, part 2, part 3 and part 4), I tried to define PST and calculation methodology and at the end how to assess and explain the results.
In this part, I will try to summarize how to make a calculation procedure for yourself or your company.

It is very important that you make a uniform calculation procedure/calculation sheet for yourself or your company. My advice is to make excel calculation sheet for PST calculations that are straightforward and could be made using simple heat and mass balance equations.

For the other calculations that need dynamic simulation, an agreement with the client or authority that will review and finally approve these calculation is a better option.

When making your calculation in Excel program sheet, one must take care that following points are taken into account:

1. Set a clear approach on the calculation.
2. Establish and define the safe design limit.
3. Define key variables in a PST scenario calculation.
4. Suggest possible opportunities for refining the PST estimate.

Define key variables

It is very important before starting any PST calculation to prepare a heat and mass balance. This heat and mass balance will be used to get physical data like operating conditions, physical properties, and flowrates.

If the flowrate or heat flow is estimated, it is very important to write down any basis, assumption and source of data used in the flow or heat flow estimation. This way your PST calculation is clear and easier to check.

A very important point to remember is when a PST calculation is being carried out for an existing plant, then Heat and Mass balance document shall be checked against actual data from the plant. This is because through the years, existing plants might often operate in a different conditions.

For instrumentation like control valve care must be taken to obtain the correct Cv of control valve. This is to specify the fail open scenario flow rate. For the case of existing plant, a control valve might have been replaced or its internals might have been changed over the years.

Even contacting the vendor for latest data seems to be a good idea, if the valve was modified between start-up of plant and the PST calculation is taking place.

When looking for the operating parameters of a system, after finding the H&MB, a chat with the operating personnel is a great idea. Many times the plant might be operating at a different condition that it is designed for. For example, the normal liquid level in the vessel was designed to be 50% of the total volume, but at the time of PST calculation, the level is maintained at 40% of total volume. This has a direct impact on your PST calculation.

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