Known limitations

Ignition points

RISKCURVES is not yet capable of working with location dependant ignition propabilities or ignition points. This implies that the so-called "free field" method is used for both individual risk and societal risk calculations.

The free field method assumes explosion occurring at the point where the LEL cloud has reached its maximum size. (Time is tMac as reported by dispersion explosives model).

Mixtures of chemicals

Our chemical database currently does not support the definition of mixtures. For well known

"real world" mixtures such as natural gas and LPG are usually modelled as Methane and Propane, but for other mixtures the following table is often used to derive the closest chemical.

Spray release model

1) The model is not feasible for use with e.g. CO2, which has a solid-vapor equilibrium at atmospheric conditions. (normal boiling point unknown or lower than melting point.

2) The model does not take solidification (snow) into account: the energy balance doesn't include solid phase transition enthalpy changes.

Neutral gas dispersion

For instantaneous or semi-continuous release the passage time of the cloud is not reported in the results. (artifact 35647)

Dense gas dispersion

The dense gas dispersion calculation model, currently used in EFFECTS, is based upon the SLAB code. At the moment we are aware of a number of issues with SLAB, which may result in unreliable results. We are working on improving the dense gas calculation engine. Some situations, potentially providing unreliable results, are listed below.

1) In SLAB, there is a transition between different models at the moment when the release stops (i.e. after the “duration of the release”). This may cause differences between results just before and just after the end of the release. In a “concentration vs. time”-graph, there may be a discontinuity at this moment. (artifact 30165)

2) For evaporating pool sources, the model sometimes cannot do a computation for certain combinations of mass flow rate, pool area, roughness length, stability class and wind speed. (artifact 37421)

3) The input parameter “diameter of expanded jet” has a substantial influence on the results, but can be hard to estimate by the user. Some release models, such as “spray release”

can be used to compute this diameter. (artifact 30682) Pool evaporation

1) When evaporation is higher than the mass flow into the pool, the pool thickness decreases until the minimum pool thickness (surface roughness) is reached. After that, the pool will shrink in size.

For pools in bunds on water, that model has not been implemented completely; for those pools, after the maximum area (bund area) has been reached, the pool thickness keeps decreasing until it is zero, while the pool area is constant. (artifact 35366)

2) The transition from non-boiling to boiling for cryogenic substances works for spills on land, but not on water. The calculation stops when the boiling temperature is reached.

Workaround: set the initial temperature of the spilled substance equal (or slightly higher than) to the boiling temperature. (artifact 32938)

3) Volatile liquids on water.

When a volatile liquid is released, its temperature usually decreases. This causes the water to also cool down. In the model for heat transfer that is implemented in EFFECTS, there is no heat transfer by convection (i.e. the water behaves like a rigid body). Therefore, the user may choose to set a maximum temperature difference between the pool and the water to limit the temperature decrease of the pool.

When the maximum temperature difference is reached, the chart of “heat flux from subsoil versus time” is not correctly presented: the heat flux from subsoil as calculated is not sufficient to keep the temperature constant. An additional heat flux term, stemming from heat transfer by convection of the water, would keep the pool at a constant temperature.

Combined models

1) For continuous release of liquefied gases, the heavy gas dispersion model always uses the ‘horizontal jet’ as source mode, even if the pool evaporation is dominant relative to the spray release.

2) Combined models do not handle vertical jet releases correctly.

For this to work, the neutral gas dispersion model should be able to handle vertical jet releases. Also, the heavy gas dispersion model would need to be able to interpret an outflow angle and decide if it is a horizontal or vertical outflow.

Index

A

-A new GUI 21

Accuracy parameters 113 Accuracy settings 94 Air relative humidity 120 Ambient temperature 120 Atmospheric pressure 120

C

-Calculation Set 22, 54, 113 Calculation Settings 22, 113 CalculationSet definition 54 Cell size Risk grids 113 Chemical database editor 96 Chemical Databases 95 Command button panel 65 Comparison set 22, 123

Concentrating averaging time flammables 89 Concentrating averaging time toxics 89 Cumulation set 22, 123

CurveNumber for Multi energy explosion method 89

D

-Damage definition 124 Default mixingheight 89

DIERS top venting (vessel only) 137 Display units 85

E

-Editing constant properties 99 Editing properties of chemicals 99

Editing temperature-dependent properties 99 Environment parameters 120

Environment settings 93 Equipment 22, 122 Error messages 158

Expert Parameter settings 89

F

-Fixed indoors outdoors toxic ratio 116

Formula 99 Formula ID 99

Fraction confined mass in Multi energy explosion method 89

Ground / Surface/ Bund temperature 120 Group Risk 125

H

-Heat radiation damage probits 116 Heat Radiation Exposure Duration 116 Heat radiation level total destruction 116 Hole contraction coefficient 89

How to use the built-in GIS system 71

I

-Individual Risk 125

Indoor Ventilation ratio 116

Installation of the software drivers for the dongle 8 Inter accident distance 113

Inter accident distance FN 113 Introduction 7

K

-Known limitations 160

L

-Lethal fraction flame contour 116 Lethal fraction flash fire 116

Liquefied gas from long pipeline 142 Liquefied gas release 136

List of chemicals 157 LOC scenario 22, 123 Log Panel 78

Lowest_significant_frequency 113

M

-Map display panel 69

Mass and volume calculator 105 Maxium number of accident points 113 menu bar 49

Meteorological data 22, 121 Meteorological distribution 91

Minimum valid and maximum valid temperatures 99 Modelset 22, 123

Mortality Probit calculator 106

N

-Peak pressure inside damage 116 Perform toxic indoors calculation 116 Pipe contraction coefficient 89 Population 22, 121

Presentation settings 87 Pressure damage based on 116 Pressure level total destruction 116 Probabilty FlashAndExplosion 89 Project file 128

Project tree 52

Protection factor clothing 116

Q

T

-The user interface in detail 47 TNO software products 7 toolbar 50

Toxic Exposure duration 116 Toxic Inhalation Heigth 89 Transport equipment 22, 122 Tree nodes 22

U

-Uninstalling the software 8 Upgrading 8

V

-Vapour release from vessel or pipe 138 Viewing graphs of the toxicity parameters 99 Vulnerability parameters 116

Vulnerability settings 92

W