Vulnerability parameters

Vulnerability parameters define how a specific physical effect is translated towards damage.

For toxic materials, this is derived from their toxic probits which are stored in the chemical database, but for flame contact, heat radiation and overpressure, dedicated damage translation can be defined..

Lethal fraction flash fire

This is the fraction of mortality used within the footprint of a flame envelope determined by a LEL footprint. Again, the height of the flammable cloud is not taken into account. Leave it to fraction 1 (100%) unless you have good reasons to change it.

Lethal fraction flame contour

This is the fraction of mortality used within the footprint of a poolfire or jetfire (no matter the height)

Leave it to fraction 1 (100%) unless you have good reasons to change it.

Heat radiation level total destruction

This parameter defines the heat radiation level that will be associated with total destruction:

100% lethality inside and outside and is used in both individual risk FX and societal risk FN calculations

Heat Radiation Exposure Duration

This value determines the maximum duration of exposure to heat load, as used in consequence calculations. Default is set to: 20.0 seconds

Protection factor clothing

The protection factor applied for clothing, used for societal risk calculations on heat radiation.

A probit calculation will be applied on heat radiation, leading to a lethality. This lethality is corrected with this factor to obtain the damage in case of societal (protected) calculations Heat radiation damage probits

By default, the vulnerability model (probit function) as described in the Green Book [4] has been used for the exposure to heat radiation:

with q = the heat radiation level in [W/m2] and t = the exposure duration in [sec], which is assumed to be maximum 20 sec (defined by parameter max heat radiation exposure duration). The probit value is transferred to a fraction of mortality (0..1) afterwards. This implies a probit A of -36.38, Probit B = 2.56, and probit N = 4/3

Because some countries are accustomed to use other probits, these A, B and N values can be modified.

The methodology described above is valid for individual and societal risk, but for inside population a protection of 100% is assumed, as long as the level is lower than the heat radiation total destruction level .

How to convert a probit to a fraction of mortality

The probit value Pr as mentioned several times in the chapters before varies between 2 and 9. To convert the probit value to a percentage of mortality, the table below is used.

The probit values are listed within the table itself. From the side and the top of the table, the percentage of mortality can be read. For example: A probit value of 4.01 (second row) corresponds with a value of 16% mortality.

% 0 1 2 3 4 5 6 7 8 9

Pressure level total destruction

This value is used to define the peak pressure level at which inside and outside lethality is assumed to be 100% (total destruction zone). Default value is 300 mBar (0.3 Bar)

Lethal fraction pressure total destruction zone

Defines the lethality within the total destruction pressure level zone. By default 100% (fraction 1)

Pressure damage based on

The translation to damage by overpressure can be defined by

1. Using two pressure levels: total destruction and inside (glas) fragments.

2. Using a probit based on Peak pressure: Pr = A + B * ln(PeakPressure^N)

3. Using a probit based on exposed pressure impulse Pr = A + B * ln(Pressure Impulse^N) In case 3, the pressure impulse is calculated as (0.5 * peakpressure * positive phase duration). Method 3 cannot be applied when using the TNT overpressure calculation, because that method does not provide a positive phase duration answer; one needs to use the Multi Energy method for method 3.

Peak pressure inside damage

This pressure defines the minimum pressure level for inside damage. All areas with pressure between "total destruction" and "inside damage" levels, will be treated with the corresponding inside damage lethality level. The lethality fraction will only be applied in societal risk calculations, on inside population. The pressure level will also be used as a treshold level for pressure contours presented by TNT or Multi Energy models.

Lethal fraction Pressure inside damage

All areas with pressure between "total destruction" and "inside damage" levels, will be treated with this corresponding inside damage lethality level. The lethality fraction will only be applied in societal risk calculations, on inside population.

Perform toxic indoors calculation

The toxic exposure inside can be calculated based on the actual concentration time profile and ventilation rate. This calculation is invoked by selecting "Yes" in this setting. The calculation is performed inside the Dispersion Toxic dose models which will also present a Inside lethality grid (expert parameter).

The inside lethality is strongly influenced by passage time of the cloud, and ventilation ratio.

Fixed indoors outdoors toxic ratio

By default, the QRA calculation uses a ratio of 1/10: lethality inside is one tenth of lethality outside. For long release durations, high exposures, or high ventilation ratios, this may be a very optimistic assumption: even an outside dose which is much higher than 100% lethality still has maximum 100% lethality, thus 10% lethality inside.

Indoor Ventilation ratio

This parameter is used in the calculation of inside lethality by toxic exposure. The ventilation ratio highly affects inside toxic exposure. The default value is 1 times per hour, representing natural ventilation. Note that for mechanical ventilation situation values ranging from 2.5 (living room) to 10 (bathrooms, moist environment) are common.

Toxic Exposure duration

The exposure duration is used to calculate a toxic dose, integrating the concentration (modified including the probit constants) as function of time over that period. (see inclined lines in graph at start of exposure).

The duration of exposure is needed as the dose increases the longer one is exposed to an effect. Normally, a default value of 30min (1800s) is used.

If in a given location the effect duration is lower than the exposure duration (the passage time of the toxic cloud is around 60s and the user chose an exposure duration of 1800s) EFFECTS will internally rearrange the exposure duration so there is not a loss of accuracy in the result of the integration process.

Example

The exposure duration is a powerful tool to model evacuation or sheltering. Say, a release happens and people can find shelter after 10 minutes. If we assume that people can find 100% shelter inside houses we can model this as follows:

1. Set the start of exposure to zero

2. Set the exposure duration to the time that people can find shelter (600s)

In this case the model starts the exposure at t = zero, which means that people close to the source of release will suffer from the effects but people further away from the release will be exposed to lower concentrations because the cloud has not reached them yet. All these are taken into account by the model.

NOTE 1: Different methods of applying this exposure duration are possible, see "exposure duration based on" parameter.

NOTE 2: By using the option "perform toxic indoors calculation" the dispersion model can take into account that people inside houses will still be exposed to (lower) concentrations.

NOTE 3: For heat radiation, a dedicated "heat exposure duration" parameter is used, which is default 20 seconds, because the human reaction to intensive heat radiation is much quicker.