In an area where flammable liquids and gases are processed, handled or stored it is often possible to predict the physical effects of fires and explosions to assess the threat to personnel and to consider whether incident escalation is possible.
Recent advances in fire, explosion and gas dispersion modelling techniques enable fire protection engineers to determine with some confidence the potential effects of accidental releases of flammable fluid through the use of sophisticated computer programs or simulations. However, fire and explosion modelling alone cannot act as a substitute for an overall FEHM approach, in which incident experience, fire engineering and process awareness all play a significant part.
Fire and explosion modelling can be used to:
— Quantify the physical effects associated with fire and explosion such as heat radiation, explosion overpressure and flame shape or length. These calculations can be used to assess whether personnel and fire responders will be placed at risk in the immediate or surrounding environment.
— Determine the response of plant and equipment to heat radiation and blast loadings and estimate the likelihood of incident escalation due to factors such as the erosion or failure of vessels and piping/equipment by flame or heat radiation.
— Determine the response of buildings to heat radiation and blast loadings, and estimate what the consequences may be for the occupants, if either they remain in the building or attempt to escape.
— Highlight the need for fire protection or mitigation measures such as PFP or water spray for cooling purposes. Additionally, analyses can be used to underline the requirement for additional fire-fighting resources.
Results of modelling can be included in scenario-specific ERPs to provide guidance for technicians and fire responders in the early stages of an incident.
Information such as heat radiation or overpressure contours can be superimposed on installation plot plans to assist incident response.
2.7.2 Types of model 2.7.2.1 Pool fire
For the purposes of assessing risk to personnel, plant and equipment it is most often the heat radiation component that is modelled although the amount and toxicity of smoke can also be addressed. Most models express levels of heat radiation in terms of kW/m2, representing these as contours in the final output. Also, the degree of flame tilt and drag due to wind effects can be shown, since this can bring the fire closer to downwind objects and engulf them. A typical pool fire model output might appear as shown in Figure 2.1, with the results of an analysis being used in an ERP, shown opposite. In this example, the contours produced by a pool fire model have been superimposed on a storage tank in order to represent the levels of heat radiation and their distances, from a full surface fire. (It is worth noting that this type of analysis or 'firemap' could equally be used to show heat radiation emanating from pool fires beneath vessels and other process equipment).
2.7.2.2 Jet fire
From a modelling perspective factors such as flame length and fire duration should be addressed, since they determine the degree of flame impingement, subsequent heat transfer and therefore escalation potential. Jet flames tend to be extremely erosive due to their significant momentum, and so modelling jet fire behaviour can assess the likelihood of PFP damage. A typical jet fire model gives similar contours to the pool fire model, enabling risk to personnel and equipment to be considered. Recently, more sophisticated computat-ional fluid dynamics (CFD) models have evolved allowing more in-depth calculations of flame temperature in specific regions, and detailed breakdowns of convective and radiative heat transfer.
A typical jet fire analysis also requires modelling of fuel release rates. These should be found by using a separate 'source' model, which may be part of the fire-modelling package. Release rates invariably have a bearing on fire duration and flame length, and should be estimated from credible scenarios, e.g. as a result of small-bore pipe work, pump seal ruptures and larger equipment failures. Also, it is possible to model jet fires (and subsequent pool fires if liquid 'rains' out of the plume) whilst taking into account a plant’s blowdown strategy.
2.7.2.3 Gas dispersion
It is also possible to estimate the likely size, composition and flammability characteristics of accidental gas releases by modelling release rates. This should be carried out if the gas release may threaten large areas of process plant and personnel due to the risk of a VCE. Gas dispersion models are especially
Figure 2.1: Typical pool fire analysis and fire-map aspect of scenario-specific ERP
100
Pool fire: horizontal plane at 15 m
Material: Unconfirmed spillage on land
Down wind
This fire map is provided for guidance only and should not be regarded as a definitive map of any fire that may occur. Radiation contours as at top of tank.
6 kW/m
Rev Date Description By
Title
Tank Full surface fire DRG BT/T21 F.S.F Do not scale FH
Pits
2
21 2
useful when specifying and planning the location of flammable or toxic gas detection, since it is possible to determine potential gas concentrations at specific locations, and hence select and position detectors able to respond at a point well before the LFL or toxic threshold. Also, this type of model can be used to determine the extents of the flammable range, whether or not gas will accumulate at low points if heavier than air, or indeed whether pockets of potentially explosive gas/air mixtures might exist at a particular point.
Modelling can therefore help to define a significant gas hazard in terms of risk to personnel and assets. From a fire response perspective, the results can be used to track gas movement and provide guidance relating to the deployment of water curtains and other barriers to gas dispersion. More sophisticated models may even be able to portray the degree of mixing within congested areas and allow these results to be fed into further explosion severity analyses.
2.7.2.4 Explosion models
Regardless of model type, the approach is usually to calculate or specify maximum potential explosion overpressures upon the ignition of gas/air (in some cases fine droplet/air) mixtures. The results can be fed into the design of blast-resistant buildings in petroleum refineries, or to study the effect of plant design modifications in reducing explosion overpressures (See CIA Guidance for the location and design of occupied buildings on chemical manufacturing sites). The technique can also be used with very good effect for emergency response purposes and can aid the production of ERPs by indicating evacuation requirements.
Historically, explosion models such as the TNO multi-energy model have been used to determine potential hazard consequences. However, this method is not always appropriate for all VCEs and new approaches such as congestion assessment, exceedance and other CFD-based models are typically used.