An important step of the hazard definition is the consequence analysis. The size of exposed area, is the first parameter that has to be estimated, and such estimation depends on the following factors: physical-chemical properties of the hazardous material and its volume, design and characteristics of the container, pressure and temperature of the substance, conditions during accident and release mechanism and ambient meteorological conditions (wind direction and strength, temperature, humidity). Various scenarios should be analysed, depending on the dangerous material – the most important ones are: fire, BLEVE (Boiling liquid expanding vapour explosion), VCE (Vapour cloud explosion), explosions, flammable vapour cloud, asphyxiating cloud, chemical, biological or radioactive contamination. Event tree methodology can be applied to characterize the events leading to the accident, and finally to determine probabilities of the releases resulting in potentially high consequences. These consequences stand as the basis for further analysis for translating hazard into initiating event of NPP. Therefore consequence analysis of transportation accident have to be performed carefully taking into account all possible release mechanisms and scenario developments.
In case of fire and flammability hazards, the ignition of the material can lead to the formation of pool fire, flash fire or jet fire. This depends on the properties of the material and conditions during the release. In order to determine the impact of thermal radiation for objects and people, a number of parameters have to be known as: quantity released, physical-chemical properties of the substance, flame surface, meteorological conditions, distance to exposed objects and possibility of sheltering.
The area of a pool fire depends on whether the spill is confined or not. In the first case, it is naturally limited, while in the second case it depends on the volume of the liquid and its burning rate. When pressurized material is released and ignited at once, typically a jet fire is formed. Instantaneous release with immediate ignition can lead to fireball, while delayed ignition of pressure flammable material leads to flash fire or explosion. The latter case is related to VCE (vapour cloud expansion), which is the effect of rapid combustion with flame speed close to the sonic velocity. In consequence a blast wave can be produced. The combustion energy and the energy of the ignition source are the parameters determining the potential explosion. The characteristic of the material is a deciding factor for calculating how big the fraction of combustion energy is converted to the explosive one. In order to produce blast overpressure the turbulence is required, otherwise the flame front will not be accelerated enough, and finally hazard will be limited to thermal radiation caused by a burning cloud. The turbulence is usually due to the interaction between the flame front and obstacles; hence the localization plays an important role. Thus, the effects depend on flame speed, location and the type of the material – in this respect it should be mentioned that highly reactive substances are more likely to lead to VCE than the ones having lower reactivity.
While the consequences of fire and explosion can be significant for objects, in case of the release of toxic substances, essentially people are exposed. Inhalation, skin burn, blindness and also carcinogenic hazards are the main types, but the latter one can have negative effects only later in life. The most important are immediate incapacitating effects. In order to determine toxic dispersion zones the following parameters should be known:
-
physical-chemical properties of the substance;
-
quantity released;
-
condition of the release: duration, elevation, surrounding terrain;
-
atmospheric conditions: wind, humidity (rain), temperature, stability of atmosphere;
-
limiting concentration: while defining receptors it is important to distinguish between concentration with serious effect and concentration with just observable effect;
As far as the meteorological data are considered, in the consequence analysis it is important to provide a series of calculations with different sets of parameters. Often a conservative approach is applied, for example, stable conditions of the atmosphere are used, resulting in an increased zone.
Chemical Process Quantitative Risk Analysis (CPQRA) is probably one of the most adequate methods that can be applied to risk analysis [7] [8]. Analogous methodology has been proposed by TNO [9], [10]. The Dutch guidelines based on these reports include a computer program, mandatory to use for all hazardous activities. Table 3 below contains the summary of CPQRA hazards, event sequences, incident outcomes, and consequences. It is related both to stationary and transportation accidents with dangerous chemicals.
Table 3: CPQRA hazards, event sequences, incident outcomes, and consequences [11]
Process hazards
|
Event Sequences
|
Incident outcomes
|
Initiating events
|
Intermediate events
|
Significant inventories of:
Flammable materials
Combustible materials
Unstable materials
Corrosive materials
Asphyxiants
Shock sensitive materials
Highly reactive materials
Toxic materials
Inerting gases
Combustible dusts
Pyrophoric materials
Extreme physical conditions:
High temperatures
Cryogenic temperatures
High pressures
Vacuum
Pressure cycling
Temperature cycling
Vibration/liquid hammering
|
Process upsets
Process deviations
Pressure
Temperature
Flow rate
Concentration
Phase/state change
Impurities
Reaction rate/heat of
reaction
Spontaneous reaction
Polymerization
Runaway reaction
Internal explosion
Decomposition
Containment failures
Pipes, tanks, vessels,
gaskets/seals
Equipment malfunctions
Pumps, valves, instruments,
sensors, interlock failures
Loss of utilities
Electrical, nitrogen, water,
refrigeration, air heat
transfer, fluids, steam,
ventilation
Management systems failure Human error
Design
Construction
Operations
Maintenance
Testing and inspection
External events
Extreme weather conditions
Earthquakes
Nearby accidents’ impacts
Vandalism/sabotage
|
Propagating factors
Equipment failure
safety system failure
Ignition sources
Furnaces, flares, incinerators
Vehicles
Electrical switches
Static electricity
Hot surfaces
Cigarettes
Management systems failure
Human errors
Omission
Commission
Fault diagnosis
Decision-making
Domino effects
Other containment failures Other material release
External conditions
Meteorology
Visibility
|
Risk reduction factors
Control/operator responses
Alarms
Control system response
Manual and automatic ESD
Fire/gas detection system
Safety system responses
Relief valves
Depressurization systems
Isolation systems
High reliability trips
Back-up systems
Mitigation system responses
Dikes and drainage
Flares
Fire protection systems (active and passive)
Explosion vents
Toxic gas absorption
Emergency plan responses
Sirens/warnings
Emergency procedures Personnel safety equipment
Sheltering
Escape and evacuation
External events
Early detection
Early warning
Specially designed structures
Training
Other management systems
|
Analysis
Discharge
Flash and evaporation
Dispersion
Neutral or positively
buoyant gas
Dense gas
Fires
Pool fires
Jet fires
BLEVES
Flash fires
Explosions
Confined explosions
Unconfined vapour cloud
explosions (UVCE)
Physical explosions (PV)
Dust explosions
Detonations
Condensed phase
detonations
Missiles
Consequences
Effect analysis
Toxic effects
Thermal effects
Overpressure effects
Damage assessments
Community
Workforce
Environment
Company assets
|
The results of CPQRA are usually presented depending on applied risk measure (risk indices, individual risk or societal risk) in the form of risk contours, F-N curves or similar graphs (as this is a case of mandatory Dutch QRA program).
Share with your friends: |