A robust understanding of the possible outcomes - effect type, magnitude, duration - associated with a release of a hazardous material is central to the planning for emergencies process and developing an effective response strategy: Emergency Response with Phast.
Adequate planning for an emergency is central to developing an effective response strategy. Planning for an emergency requires a detailed understanding of the hazards of concern. In the particular case of the chemical process/oil and gas industries, a robust understanding of the possible outcomes (fire, explosion, toxic cloud etc.) associated with a release of a hazardous material is central to the planning process. Key questions need to be answered:
- How large the impact zone can be?
- What areas/population groups are at risk of exposure and how long for?
- What is the influence of the prevailing weather conditions e.g. day-time versus night-time?
The asset-based hierarchy in Phast 7 allows equipment items to be easily identified. These equipment items can be placed in a map which may also include Geographical Information System (GIS) – layers of information with, for example, the location of population area, control-room, accommodations, and specific areas of the plan where expensive equipment items are located. This provides a powerful visualisation tool which can be used for both planning but also communication of risks in the case of hazardous event.
The following map shows an example of how the analyst can easily identify equipment items on the map – by clicking in one point in the map, the data related to that point shows and, in this case, we are looking at the chlorine tank.
Vessel identification on Map
By double-clicking on it, the properties on this pressurised vessel are displayed:
Pressurised chlorine vessel
Ideally the analyst would like to run a vast array of scenarios for a specific equipment item. The rule of thumb for Emergency Response planning is: the more scenarios you run, the more complete understanding of the potential impact you would get. Upon defining the equipment item in Phast 7, analysts can define what kind of scenarios will be associated with that particular asset:
Asset-based: Study tree
The table below gives an overview of the different scenarios available (some of these are obviously limited to a specific asset e.g. Spill would work if the vessel is pressurised):
| Scenario | Description |
| Catastrophic rupture | An incident in which the vessel is destroyed by an impact, a crack, or some other failure which propagates very quickly. |
| Leak | A hole in the body of a vessel, or a small hole in a large pipe, modelled assuming that there are no frictional losses as the fluid flows through the vessel or pipe towards the hole.
The conditions at the start of the release are assumed to apply throughout the duration of the release, giving a constant release rate. |
| Fixed duration release | A release in the event of a rupture disc bursting. In the discharge calculations, the disk-seat is not considered as a restricting orifice so the flow-rate is dependent only on the pressure-drop through the tailpipe.
The conditions at the start of the release are assumed to apply throughout the duration of the release. |
| Short pipe | A full-bore release from a short length of pipework attached to a vessel, taking into account the pressure-drop through the line based on the frequency of bends, couplings and junctions and the valve velocity head losses, and the effect of any restricting orifices. You can use this scenario to model a line rupture, the lifting of a relief value, or the bursting of a rupture disc.
The conditions at the start of the release are assumed to apply throughout the duration of the release. |
| Vent from vapour space | This is used for the venting of material from the vapour space of an unpressurized or refrigerated vessel, typically during a filling operation.
The conditions at the start of the release are assumed to apply throughout the duration of the release. |
| Time varying leak | A hole in the body of a vessel, or a small hole in a large pipe, modelled assuming that there are no frictional losses as the fluid flows through the vessel or pipe towards the hole.
The effect of the release on the storage conditions is modelled, giving a release rate that changes with time. |
| Time varying short pipe release | A full-bore release from a short length of pipework attached to a vessel, taking into account the pressure-drop through the line.
The time-dependent effect of the release on the storage conditions is modelled. |
| User-defined source | Discharge calculations are not performed for this scenario, and the input data for the scenario includes a description of the state of the material after it has already been released and has expanded down to atmospheric pressure. You would normally use this scenario to model a situation that is not covered by the discharge calculations for the other scenarios, and use an external discharge model to obtain the state of the material after expansion to atmospheric pressure.
You can define multiple release segments, with different conditions for each segment, to represent a release that changes with time. |
| Spill | A liquid spill from an atmospheric tank, for which the entire released mass, is assumed to spill on the ground. Models the pool spreading and vaporisation, and the dispersion and toxic effects of the cloud that evaporates from the pool. |
| Breach | A breach in a long pipeline. Models the time-dependent release of material from the pipeline, through all the stages in its dispersion to a harmless concentration. The calculations include the effects of shutdown by modelling the closure of valves on the pipeline. |
For this case, a number of scenarios have been defined. In this example the two-phase vessel releases material from the vapour side via a hole in the body of the vessel. After the discharge calculations the cloud moves downwind. It is modelled until the cloud concentration drops below harmful toxic thresholds. The concentration in the cloud is converted to lethality levels using Probit values stored in the materials database.
The aforementioned makes sense right?
However, the ability of visualising the results is fundamental for a good Emergency Response plan:
Impact of the consequence
The above, taken together results in significant overall benefits in terms of adding value to the emergency planning process. Powerful visualization tools that allow the impact range to be imposed on location maps, thus allowing for clearer elucidation of the impact zones. In addition to the graphical format, the result outputs can be presented in various formats (graphs, tables, commentary).
This analysis can be extended to a detailed analysis of a range of hazardous outcomes (flammable, explosive and toxic) associated with a hazardous event taking into account various factors that impact on the development (e.g. variations in weather conditions).
Authors: Kenny Shaba and Colin Hickey