Impact Models

The source models generate a variety of outcomes that are caused by release of hazardous material or energy. The dispersion models estimate concentrations and/or doses of dispersed vapour. The explosion models estimate shock wave overpressures and fragment velocities, and fire models predict radiant flux generated from the outcomes. These models rely on the general principle that the severity of the outcome is a function of distance from the source of release. The next step in the QRA is to assess the consequences of these outcomes on human beings, expressed as deaths or injuries.

One method of assessing the consequence of an outcome is the direct effect model, which predicts effects on people or structures based on predetermined criteria (e.g., death is assumed to result if an individual is exposed to a certain thermal radiation level). In reality, the consequences may not take the form of discrete functions (i.e., a fixed input yields a singular output) but may instead conform to probability distribution functions. Therefore, a statistical method of assessing a consequence (called dose-response method) may be appropriate. This method is coupled with a probit equation to linearize the response. The probit (probability unit) method described by Finney (1971) reflects a generalized time-dependent relationship

Radiation Level for Fatalities

Fig. 4.13 The Fatal effect zone for a pool fire [203]

Radiation Level for Fatalities

Fig. 4.13 The Fatal effect zone for a pool fire [203]

for any variable that has a probabilistic outcome that can be defined by a normal distribution. The probit variable Y is related to the probability P by [160]:

where P is the probability or percentage, Y is the probit variable, and u is an integration variable. The probit variable is normally distributed and has a mean value of 5 and a standard deviation of 1. For spreadsheet computations, a more useful expression for performing the conversion from probit to percentage is given by

4.5.4.1 Thermal Impacts

The purpose of the thermal impact models is to estimate the likely injury or damage to people and objects from the thermal radiation of incidents. Thermal impacts of fire on humans depend on the rate at which heat is transferred from the fire to the person, and the time the person is exposed to the fire [43]. Even short-term exposure to high heat flux levels may be fatal. This situation could occur to persons wearing ordinary clothes who are inside a flammable vapour cloud (defined by the lower flammability limit) when it is ignited. In the study, it is assumed that all persons inside a flammable cloud at the time of ignition are killed and those outside the flammable zone are not.

Moan 50% ' fslsNtius

\ 17. Fatalities

injury \ \ threshold \

\

Mixlor

(1964)

x

1 10 100 1000 INCIDENT THERMAL FLUX, kW/M2

1 10 100 1000 INCIDENT THERMAL FLUX, kW/M2

Fig. 4.14 Serious injury/fatality levels for thermal radiation [2]

API (1996a) RP 521 and World Bank (1985) [2] provides a short review of the effects of thermal radiation on people. The thermal radiation impacts suggested by World Bank (1985) are shown in Table 4-10. Furthermore, Mudan (1984) summarized the data of Eisenberg et al. (1975) for a range of burn injuries, including fatalities, and of Mixter (1954) for second-degree burns (Fig. 4.14).

Thermal Radiation (kW/mA2)

Fig. 4.15 Effect of thermal radiation on man

Thermal Radiation (kW/mA2)

Fig. 4.15 Effect of thermal radiation on man

Eisenberg et al. (1975) develop a probit model to estimate fatality levels for a given thermal dose from pool and flash fires, based on nuclear explosion data [2], and shown in Fig. 4.15:

where Y is the probit, t is the duration of exposure (sec), and I is the thermal radiation intensity (W/m2).

Lethality levels (%) of the thermal radiation impacts (such as pool fire, jet fire and fireball) to people can be calculated from the Eq.4-11. For example, thermal radiation impact from jet fires with exposure time of 20s (flammable) is shown in Table 4-10. It should be noted that the time exposure (ts) in fireball is equal to the fireball duration, while in jet fire and pool fire is set to 18.7s (flammable).

_Table 4-10 Thermal Radiation impact from Jet fires (duration 20s)

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