Consequence Modelling
Dispersion and consequence modelling can be utilised to assess the potential effects of releases of toxic and/or flammable materials. Armed with a knowledge of atmospheric concentrations of the hazardous substances or mixtures, will enable many safety decisions to be made. Typical uses of such modelling include:
- As a plant design tool: for example, for determining storage options, siting of plant and associated control buildings, carrying out cost-benefit analysis on safety systems in conjunction with LOPA and other recognised risk assessment techniques, and assessing the extent of hazardous areas.
- To identify major hazards and prioritise risk reduction strategies on existing plants and processes.
- To help develop site emergency plans.
- To assist sites in complying with Seveso II (COMAH), ATEX/CAD (DSEAR) and other chemical-related safety regulations.
One example of where this modelling capability might be utilised is in the assessment of the consequences of a reactor relief scenario whereby gases and / or vapours from the relief stream are discharged to atmosphere via a high level relief stack. In this case, it is critical to gain an understanding of the toxicity or flammability of the resulting cloud. The questions that can be answered in this case are:
- Is a flammable concentration of the gas likely to impinge on un-zoned plant areas (or even worse, neighbouring off-site areas) where ignition sources may be readily available?
- Will toxic concentrations of the gas exist at ground level where personnel may be present?
Another example would be in the assessment of the consequences of an ignition. In this case the questions that can be answered are:
- What pressure profiles develop in the affected plant area?
- What are the thermal radiation levels at key plant areas?
- What structural damage may be caused for a specific scenario?
Within the Chilworth Group, a range of software is available for assessing the consequences or fires, explosions and toxic releases. Our highly skilled consultants are available to provide consequence analysis using the following industry standard methods:
PHASTTM
This is an integrated package (from DNV) that allows the user to model the ultimate consequences of a release based on source term conditions; be this exposure to toxic gases, an explosion (BLEVE, vapour cloud explosion etc.) or a fire. The package includes recent refinements to jet fire and vapour cloud explosion modelling. In certain cases the user has the ability to use a default model, or choose the model to be used. The programme produces both tabular and graphical output; with the ability to produce reports in Microsoft Word and plot output on maps, images or plans. PHASTTM is recognised by authorities such as the HSE in the UK for Seveso II (COMAH) report predictive work.
EFFECTSTM
This is a suite of models (from TNO) designed to estimate the physical effects caused by the release of materials; be this dispersion of a toxic gas or vapour, thermal radiation from a fire or BLEVE, and overpressure from a vapour cloud explosion. EFFECTSTM can also model the effects of a pressure vessel rupture. Recognised TNO models such as the multi-energy method for vapour cloud explosion modelling and the TNO jet fire model are included. The models are less integrated than PHASTTM, which allows greater flexibility in the manipulation of data where applicable. Output is presented in Microsoft Word and graphical format.
DAMAGETM
This is a suite of models marketed by TNO that are designed to estimate the likely harm to people of toxic releases, fires and explosions based on the results from EFFECTSTM modelling. The models make use of developed correlations for harmful effects such as Probit equations. In the case of explosions, DAMAGETM can also model the damage caused to buildings. Output is presented in Microsoft Word format.
ALOHATM
This is the consequence modelling package from the US EPA, which allows users to model dispersions of vapours or gases if certain source term information (e.g. the pool size) is known. Modelling of gas and two-phase releases, and liquid releases direct from tanks, can be carried out, along with tank BLEVEs, pool fires and vapour cloud explosions. However, modelling of liquid and two-phase releases from pipework, as well as jet fires, is difficult with the software.
AIChE models
The consequence models described by the AIChE in their publication "Guidelines for Chemical Process Quantitative Risk Analysis- second edition" (2000) are available as individual Microsoft Excel spreadsheets. The models are not integrated, which allows flexibility in the choice of models. However, they tend to be less sophisticated than those used by other software, so are more suitable for simple work such as calculating source term information and for screening hazardous events than for detailed consequence modelling.
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