Process Systems Enterprise Limited
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Model-Based Safety

The importance of rigorous modelling

Gas flare 

It is essential that models have a high degree of predictive accuracy

 

 

 

CO2 liquid evaporating in vessel - metal temperatures

3-D wall temperature modelling for evaporating CO2 pool safety case

Until now, much safety design and analysis has been done using steady-state or "pseudo-dynamic" trial-and-error modelling.

Some of the shortcomings of this approach are:

  • the assumption of equilibrium, where this is not a conservative assumption (or is highly over-conservative, resulting in unnecessary capital expenditure)
  • use of ‘off-the-shelf’ models that do not capture process complexity – for example, depressuring of ‘vessels’ that in fact represent multiple vessels and pipework
  • inaccurate calculation of temperatures of metal in contact with rapidly-evaporating liquid

or in general attempting to ‘bend’ flowsheeting packages into performing calculations they were not designed to do.

Fidelity is paramount

It goes without saying that the models used for safety design and analysis need to be of the highest quality. Here are 5 key requirements:

  • It is essential that models have a high degree of thermodynamic predictive accuracy. This requires at a minimum:
  • Spatially distributed models (2-D or 3-D). These are essential in order to capture geometric variations – for example, of metal temperatures in pipe walls.
  • Dynamic or transient capability: as most safety work deals with transient conditions, it is essential to use full dynamic models with proper modelling of:
    • pressure-flow behaviour, within a flowsheet context capable of representing all necessary equipment, from process vessels to flare tip.
    • reversible flow through pipes and vessels during pressure transients, in order to account for volume effects correctly.
  • Advanced numerical solution techniques: in particular it is essential to be able to:
    • predict peak pressures and flows accurately, as these are used to establish key design criteria. Certain fixed-step integrator implementations can simply ‘step over’ a pressure peak, missing crucial safety information.
    • model irreversible events such as bursting disk or pipe rupture correctly.
  • A custom modelling capability is important in order to deal with non-standard equipment (for example, water seals in flare headers) or abnormal situations (for example, complete flooding of distillation column trays).

As many accidents occur from a combination of conditions, the ability to use stoichastic optimisation techniques is an advantage.

PSE's gFLARE platform (including its Depressurisation option) include many of the above features as standard. Others – for example custom modelling capabilities – can be addressed using gPROMS ModelBuilder, PSE's advanced custom modelling environment.