Fire Scenarios

Fires range in size and consequence from those that are small, easily controlled, and result in minor damage to those that are large, difficult to control, and create a major loss. 

Fires in process facilities usually follow a loss of containment. While the consequences of such fires are dependent upon a number of factors (weather, wind, leak orientation, etc.), the most significant are the  

In theory, there are an infinite number of leak sizes, ranging from a tiny pinhole to a full rupture of piping or equipment. It is clearly impractical to investigate them all. Thus, some practical guidance is necessary in selecting leak sizes that will allow a reasonable range of fire scenarios to be evaluated. 

The process hazard analysis can be a starting point for the selection of fire scenarios. The process hazard analysis can be reviewed to develop a list of scenarios that result in fire as a consequence. Generic release sizes for small, medium, and large releases have been proposed as shown in Table 5-1 (Spouge, 1999). This saves time by eliminating the need to develop a detailed scenario. The analyst can use these release sizes to perform fire modeling calculations and determine the impact by moving the release point locations. The release criteria are considered to be representative of scenarios that could reasonably be expected to occur. 

The worst-case scenario is typically taken to be full-bore rupture of gas or liquid lines. There are circumstances where full-bore rupture is not the worst-case, particularly when there is a restricted inventory to consider. For example. 

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Fire test methods attempt to provide correct information on the fire contribution of a product by exposing a small sample to conditions expected in a fire scenario. Methods can be viewed in two ways the first entails the strategy of the fire test, ignition resistance or low flammabiUty once ignited the second addresses the test specimen, a sample representative of the product or a sample of a material that might be used in the product. Fire science has progressed markedly since the older test methods were developed and it is known that the basis for many of these tests is doubthil. Results from older tests must be used with great care. 

In this evaluation SCREEN was applied to calculating Ground Level Concentrations (GLCs). In this analysis, we assume an average emission rate over the length of the fire incident. Literature information supports that a fire of this magnitude and under uncontrolled conditions, would consume anywhere from 70 to 90 % of the products. The entire fire scenario is... 

Assess the effects of these fire scenarios on accident sequences in event trees for fire-induced initiating events. 

OTI586 Representative range of blast and fire scenarios... 

It has already been stated that the principal toxicant in a fire scenario is carbon monoxide, generated when all carbonaceous materials burn. Moreover, the carbon monoxide concentration in full scale fire scenarios depends heavily on fire load (i.e. how much material is burning, per unit volume) and on geometrical arrangements, including ventilation, while the dependence on materials is of a lower order. 

Thus, smoke toxicity is often very closely associated simply with the mass loss rate, since the toxicity in a fire scenario will be primarily a function of the mass ofsmoke per unit volume and per unit time being emitted into the ambient atmosphere. 

The traditional way of measuring fire properties is to determine each property individually by carrying out small scale tests on materials, in isolation of the fire scenario of interest. A crude means of fire hazard assessment would then be to establish minimal "passing" standards for each test and require all materials to meet them. 

Smoke obscuration is an essential parameter related to fire hazard, because it may cause visual impairment both of the occupants of a fire scenario and of the rescue team, creating a potential danger. 

Calculations were made, for many fire scenarios, in which the fire hazard model F.A.S.T. was used to simulate hazard to occupants of a standard room following a fire starting in a plenum above it. A total of 400 m of PVC wire coating was assumed to be present in the plenum. Its decomposition was made a function of the plenum... 

One accident mitigation procedure is called emergency material transfer, in which the material is transported away from the accident site before it becomes involved. We plan on mitigating a crude oil tank fire scenario by pumping the tank empty in 1 hr total time. 

As mentioned previously, two-phase flow discharges for fire scenarios are possible but not likely. To size the relief for fire and a single-vapor phase, use the heat input determined from Equations 9-36 to 9-38, and determine the vapor mass flow rate through the relief by dividing the heat input by the heat of vaporization of the liquid. This assumes that all the heat input from the fire is used to vaporize the liquid. The relief area is then determined using Equations 9-3 to 9-12. 

Determine the relief vent areas for the following two-phase fire scenarios. Assume ... 

We have already mentioned that practical data under fire conditions are commonly presented in terms of mass loss of the fuel package (i.e. Equations (9.3), (9.4) and (9.6)). Effectively, this means for fuels in natural fire scenarios, such as furnishings, composites, plastic commodities, etc., we must interpret the steady burning theory in the following manner ... 

Principally, conservation of energy for the compartment provides the important relationship to establish the extent of thermal feedback to the fuel. Conservation of mass and oxygen provide additional support equations. The process relationships, given previously, establish the important transport rates of mass and energy. These constitutive relationships may not always be complete enough to describe all fire scenarios. 

Strege, S. M., The use of saltwater and computer field modeling techniques to determine plume entrainment within an enclosure for various fire scenarios, including inclined floor fires, MS Thesis, Department of Fire Protection Engineering, University of Maryland, College Park, Maryland, 2000. 

Identify the fire scenario for the level of potential loss. 

These approaches essentially identify fire scenarios for all units and the consequences of those fires. If escalation is deemed possible, then additional damage is determined. Once the total damage is determined, a cost for replacement can be calculated. 

Figure 5-1 shows how the FHA is integrated into an overall risk assessment. A process hazard analysis is required to identify likely fire scenarios that are carried forward to the FHA. An FHA provides the tools to characterize the hazards and evaluate consequences. The results are incorporated into an overall risk assessment. See Chapter 6 for more information on fire risk assessment. 

Guidance on how to develop different types of fire scenarios common to process facilities. 

The release of a flammable gas or liquid can lead to different types of fire scenarios. These are dependent on the material released, the mechanism of release, the temperature and pressure of the material, ambient conditions, and the point of ignition. Types of fires include ... 

Determining fire water demand and duration of worst-case fire scenarios. 

The approach used in an FHA is to assume ignition of releases. In reality, not all releases result in afire. The likelihood of ignition can be addressed in the quantitative risk assessment process. However, in an FHA it is important to identify if ignition sources are present for the fire scenarios to occur. In some instances, fire scenarios can be eliminated from analysis because of the lack of a credible ignition source. 

Each identified hazard will have a range of possible scenarios it may not be reasonable to evaluate every scenario. Therefore, representative fire scenarios should be chosen to cover a range of foreseeable scenarios. The scenarios to evaluate are those where the initial release and ignition characteristics are likely to cause the most extensive damage, loss of production, and the greatest risk to personnel. The fire scenarios selected should have a sufficient inventory that will burn long enough to cause failure of equipment and/or the structure. 

Smaller fire scenarios that can escalate and less severe but very likely scenarios should also be considered. 

A consequence-only basis for installing fire protection equipment may not be cost effective. In reality, the likelihood of the fire scenario should be taken into consideration. There are several issues that need to be considered ... 

The initiating frequency estimate is derived from the cause of the fire scenario. This may initially be obtained from historical data (or more specific data if available) and modified where necessary to take account of any site-specific considerations that may affect the frequency. See Section 6.2.4.6 for additional... 

The calculation of individual risk at a geographical location near a facility assumes that the contributions of all incident outcome cases are additive (IChemE, 1985). Thus, the total individual risk at each point is equal to the sum of the individual risks, at a specific point, for all fire scenarios that can impact that point. 

Developing fire scenarios, including potential release rates and determining the dimensions of fire-scenario envelopes. 

Determining fireproofing needs based on the probability of an incident considering industry experience, the potential impact of damage for each fire-scenario envelope (see 7.3.2.2), and technical, economic, environmental, regulatory, and human risk factors. 

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