Major Hazard Modelling -BLEVE (Boiling Liquid Expanding Vapour Explosion)

Major Hazard Modelling Explosions.................................................................................................................................. 10.0. Shock, Impact, Collapse .............................................................................................................. 10.1. Seismic ..................................................................................................................................... 10.2. Fire............................................................................................................................................ 10.3. 10.0 Explosions Out with electrical and nuclear explosions, there are generally only two main types of explosion, these are mechanical and chemical with several subsets within these categories. Explosions are generally differentiated by the source or mechanism by which the explosive overpressures are produced.  Mechanical explosions are those in which a high pressure gas produces a physical reaction, in which, the Nature of the Fuel does not change , and the vessel failure or rupture of the container is generally in the form of a BLEVE or (Boiling Liquid Expanding Vapour Explosion). If the material that is stored in the container, is flammable, then in many instances a resultant fire occurs as long as there is an ignition source or the temperature of the product is above its autogenous ignition temperature. Key to any resultant fire is the mixing of the fuel with air or an oxygen source. BLEVE (Boiling Liquid Expanding Vapour Explosion). If a vessel is ruptured, or fails under pressure, the vapour portion may rapidly leak, dropping the pressure inside the container and releasing a wave of overpressure from the point of rupture. This sudden drop in pressure inside the container causes violent boiling of the liquid, which rapidly liberates large amounts of vapour in the process. The pressure of this vapour can be extremely high, causing a second, much more significant wave of overpressure (i.e., an explosion) which may completely destroy the storage vessel and project it as shrapnel over the immediate surrounding area. Chemical explosions are those in which the Nature of fuel changes generating a high pressure gas wave front driven by an exothermic reaction resulting from the initiation of chemical explosives or fuel gases. The rate of reaction will vary, and when explosives are present, an outside oxidizer is not required. Assessment methodologies There are two main methodologies that exist for modelling explosions, these are: TNT Equivalency methods Methods based on the fuel-air charge blast The explosion models include the following widely accepted approaches: U.K. HSE TNT Equivalency was based on the work of the U.K. Health and Safety Executive (HSE). This model uses a proportional relationship between the flammable mass in the cloud and an equivalent weight of TNT. It assumes that the entire flammable. U.S. Army TNT Equivalency was based on the work of the U.S. Army. This model uses a proportional relationship between the flammable mass in the cloud and an equivalent weight of TNT and assumes that the entire flammable mass is involved in the explosion and that the explosion is cantered at a single location. The model uses one of two blast curves, depending upon whether the explosion being modelled is a surface burst or a free-air burst. High Explosives:  High Explosive Damage Assessment modelling has been conducted to evaluate the damage caused to structures within a facility as a result of a primary explosion and any accompanying secondary explosions.  High explosive damage assessment models can also predict injury to an unlimited number of personnel in a facility. The principal use of such software is for site analysis of explosive storage and manufacturing facilities; however, the software can also be used to evaluate terrorism and sabotage threats to an industrial or military facility.  Peak pressure effects of explosions wave fronts Glass Shatters: 3-7 kPa Person blown over: 6-7. kPa Concrete shatters 10-38 kPa Brick shears apart 20-60 kPa Lung damage > 69 kPa 50% chance of eardrum rupture 90-130 kPa 1% chance of death 160-230 kPa 50% chance of death 230-400 kPa 100% chance of death 400+ One atmosphere = 101325 pascals | 1013.25 hPa | 101.325 kPa | 0.0146959psi kPa = kilopascal or 1000 pascals. One pascal is equal to one newton of force applied over an area of one square metre. Velocity-related definitions The speed of sound is about 340.3m/s (1115ft/s) at sea level at one atmosphere pressure and 15 degrees Celsius. Low velocity< 304.8 m/s (1000 ft/s) Medium velocity 304.8 to 762 m/s (1000-2500 ft/s) High Velocity >762 m/s (2500 ft/s) Hypervelocity >1524 m/s (5000 ft/s) Cavitation and fragmentation effects predominate at higher energies (medium to high velocity). For reference: Objects moving faster than 150ft/sec (45.72m/s) will penetrate skin. Objects moving faster than 195ft/sec (59.44m/s) will fracture bone. This is regardless of surface area or shape. Hypervelocity missiles produce crater-like wounds. 10.1 Shock, Impact and Collapse this section comming soon. Calculate impact speed and mean energy of an object in freefall. 10.2 Seismic The calculations for seismic hazard can be quite complex. First, the regional geology and seismology is examined for patterns (using historical data, seismometers and earthquake location mapping) including any zones of similar potential for seismicity. Each zone is given properties associated with source potential: how many earthquakes per year, the maximum size of earthquakes (maximum magnitude), etc. Finally, the calculations require formulae that give the required hazard indicators for a given earthquake size and distance. For example, some districts prefer to use peak acceleration, others use peak velocity, and more sophisticated uses require response spectral ordinates. The computer program then integrates over all the zones and produces probability curves for the key ground motion parameter. The final result gives you a 'chance' of exceeding a given value over a specified amount of time. Standard building codes for homeowners might be concerned with a 1 in 500 years chance, while nuclear plants look at the 10,000 year time frame. A longer-term seismic history can be obtained through paleoseismology. The results may in the form of a ground response spectrum for use in seismic analysis. More elaborate variations on the theme also look at the soil conditions. If you build on a soft soil, you are likely to experience many times the ground motions than on solid rock. The standard seismic hazard calculations become adjusted upwards if you are postulating characteristic earthquakes. Areas with high ground motion due to soil conditions are also often subject to soil failure due to liquefaction in saturated soils. Soil failure can also occur due to earthquake-induced landslides in steep terrain. Large area land sliding can also occur on rather gentle slopes. 10.3 Fire The Fire protection of structures is becoming an increasingly important aspect of design. The public quite rightly expect their buildings and place of work to be designed to allow effective evacuation in the event of fire. Certain industrial structures have tighter fire protection requirements, such as oil and gas production and processing, nuclear facilities and chemical process and storage. Fire protection falls into two main categories, “prevention and protection”. Preventative measures will include control of flammable inventories, control of ignition sources, monitoring of environmental conditions leading to initiation of alarms and automatic process control, and fire detection systems designed to extinguish fires immediately on detection. Further to this, the protection measures fall into two categories, “passive and active”. Passive measures include fire barriers, fire resistant enclosures, fire doors, fire retardant coatings and fire protective coatings. Active measures include water and chemical sprays or deluges, foam dispersion and inert gas dispersal. Fire protection can be provided as an all encompassing scheme, or it can be functionally designed to optimise on cost, weight and maintenance.  Work carried out by various bodies has shown that pressure vessels subjected to fire, but which are successfully blown-down can resist the fire without rupture, without the application of fire protection.  Evidently the resulting vessel is not usable following the fire, but there is no guarantee that a fire protected vessel could be re-used. Using well defined techniques, the analysis can take a definition of a postulated fire and carry out a time domain thermal analysis of the structure. This model will include the basic thermal transmission phenomena, radiation, conduction and convection, the material temperature dependent properties of materials and the location and nature of fire protection measures. The resulting thermal histories are then applied to the structure to predict time to collapse, or to demonstrate the degree of collapse. From this, the structure can be economically protected to meet the safety requirements.