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 3 part 4 and part 5 for more information).

PST calculation and definition is a mandatory requirement for all the instrumented functions to which IEC 61511 is applicable.

Timeframe within a project to do a PST time calculation

Of course like any other activity within a project, PST calculation shall fit within planning and shall not jeopardize the planning.

IEC 61511 also asks for a Safety Requirement Specification (SRS) for a SIF and its associated Safety Integrity Level (SIL). This is done when a decision is made to have a Safety Instrumented Function and a Safety Integrity Level is allocated to it, either by a simple decision making in a team or using a risk assessment method like (LOPA). For information on LOPA see here.

At this stage, a safety requirement specification is drafted. As part of its input, a process safety time calculation/decision making is to be carried out.

Process safery time calculation

Terms of reference

Once a decision is made to proceed with the PST study/calculation, the first step is to develop a PST calculation/study terms of reference. This document will define the scope as well as how the study is to be executed.

One must realize that because PST calculation and definition is a mandatory requirement of IEC 61511, all SIFs identified in a project shall be reviewed for PST calculation and at least for SIFs that have a SIL level of 1 and higher in relation to Health & Safety (H&S) and Environment (E).

Of course the project or the owner/business may add and include all or some SIFs with lower SIL level or SIL level 1, because of asset or reputation damage.

Contents of Terms of Reference

A terms of reference should include at least the following information:

-Activity plan and responsibilities between parties involved.
– Start/Finish dates and
– Data collection and data validation
– Agreement on selecting the calculation method for PST at least per parameters (SIFs that are protecting for high/low pressure or high low/levels)

Calculation method selection

Calculating or Estimating a PST is not an easy task. Therefore, it is very important to have an experienced operator in your team to ask his opinion when the PST calculation is very difficult, or if the team can’t agree on the way it has been calculated.

Based on the amount of information at hand, the design information available as well as an established method to calculate a specific PST, the time required and quality of calculation differs from case to case.

In general PST calculation could be grouped into following categories:

Qualitative analysis

As mentioned above, for certain PST calculation, numerical calculation is either time consuming and different methods are available for it with some degree of simplification. This means that the discussions like how the calculation shall be done, parameters or boundary conditions used, will be never ending.

In such situations, the only way forward is Qualitative PST estimation:

This means using engineering judgment to estimate the PST time and hence verify if a SIF’s response time is adequate. Then help of a specialist could be used, for example a compressor specialist, process/equipment/package manufacturer or an experienced operator.

Quantitative Simplified

Some systems are relatively easy for a quantitative- simplified PST calculation approach. The math is easy to do and there is no need for sophisticated simulation programs such as HYSYS. Of course assumptions and boundary limits shall be gathered and carefully documented.

In this case there is a semi-transient method, an example is the time that an overflow happens in an atmospheric tank if the inlet valve fails open. For this calculation one needs to divide the volume available between High-High liquid level (high level trip switch) and overflow outlet height.

By dividing this volume by the volumetric inlet flow to the tank, the time between high level switch activation and overflow (the consequence we intended to prevent) or in other words, PST, can be calculated.

Quantitative Dynamic
It is clear that this method is for modelling scenarios that are complicated and the behaviour of the process system needs to be simulated with a simulation package like HYSYS.

The simulation program is used to simulate/analyse the behaviour of a system after initiation of protection layer (a switch). A good model can predict the system behaviour realistically or close to reality.

Imagine that a pressure switch is protecting downstream of a control valve that is letting down a high pressure gas to a different pressure. The temperature drop after the control valve is depending on many parameters such as the flow through the valve, the amount of metal downstream the control valve, etc.

A good simulation program can calculate with good margin when the low temperature happens and at what distance relative to the control valve.
However, a dynamic modelling is relatively complex exercise that needs multiple variables and needs a lot of data and design information collection, before a result can be achieved.

In some cases the steady state solution within the simulation program shall be reached before dynamic model could be set up, run and reach the result.
This is where help of experts and peer colleagues or even a third party assistance will come handy.

See part 1 for PST definition and part 3 for PST calculation methodology.

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