Fireballs

Fireballs Giant hazardous fireballs result from large BLEX s. Several formulas for BLE physical parameters and thermal radiation hazards have been summarized by the Center for Chemical Process Safety (CCPS) of the American Institute of Chemical Engineers and by Pruffh. (See AlChE/CCPS, 1989 Prugh, 1994.) For the maximum fireball diameter, in meters, CCPS has selected... 

Design of explosion suppression systems is clearly complex, since the effectiveness of an explosion suppression system is dependent on a large number of parameters. One Hypothesis of suppression system design identifies a limiting combustion wave adiabatic flame temperature, below which combustion reactions are not sustained. Suppression is thus attained, provided that sufficient thermal quenching results in depression of the combustion wave temperature below this critical value. This hypothesis identifies the need to deliver greater than a critical mass of suppressant into the enveloping fireball to effect suppression (see Fig. 26-43). 

Boiling liquid expanding vapour explosion follows failure of a pressurized eontainer of flaimnable liquid, e.g. LPG, or a sealed vessel eontaining volatile flammable liquids, under fire eonditions. Ignition results in a fireball and missiles. 

Figure 7.2 Diameter of fireball versus quantity of explosive...

The fireball resulting from ignition of a cloud of flammable vapor may be relatively long lasting (2-5 seconds), and represents a thermal radiation hazard to those close to the cloud CCPS (1994b). 

Useful formulas for BLEVE fireballs (CeSP, 1989) are given by equations 9.1-27 thru 9.1-30, where M = initial mass of flammable liquid (kg). The initial diameter describes the short duration initial ground level hemispherical flaming-volume before buoyancy lifts it to an equilibrium height. 

The thermal radiation received from the fireball on a target is given by equation 9.1-31, where Q is the radiation received by a black body target (kW/m ) r is the atmospheric transmissivity (dimensionless), E = surface emitted flux in kW/m", and f is a dimensionless view factor. 

The heat flux, E, from BLEVEs is in the range 200 to 350 kW/m is much higher than in pool fires because the flame is not smoky. Roberts (1981) and Hymes (1983) estimate the surface heat flux as the radiative fraction of the total heat of combustion according to equation 9.1-32, where E is the surface emitted flux (kW/m ), M is the mass of LPG in the BLEVE (kg) h, is the heat of combustion (kJ/kg), is the maximum fireball diameter (m) f is the radiation fraction, (typically 0.25-0.4). t is the fireball duration (s). The view factor is approximated by equation 9.1-34. where D is the fireball diameter (m), and x is the distance from the sphere center to the target (m). At this point the radiation flux may be calculated (equation 9.1-30). 

When the reboiler was brought back on line, the water was swept into the heat transfer oil lines and immediately vaporized. This set up a liquid hammer, which burst the surge tank. It was estimated that this required a gauge pressure of 450 psi (30 bar). The top of the vessel was blown off in one piece, and the rest of the vessel was split into 20 pieces. The hot oil formed a cloud of fine mist, which ignited immediately, forming a fireball 35 m in diameter. (Mists can explode at temperatures below the flash point of the bulk liquid see Section 19.5.)... 

When deflagration venting is nsed, a major hazard of concern is the fireball (flame clond consisting of bnrning gases and/or dnst) discharged from the vent. This can cause harm to personnel or process eqnipment and... 

BLEVEs are more commonly associated with releases of flammable liquids from vessels as a consequence of external fires. Such BLEVEs produce, in addition to blast and fragmentation ejects, buoyant fireballs whose radiant energy can bum exposed skin and ignite nearby combustible materials. A vessel may rupture for a... 

A BLEVE involving a container of flammable liquid will be accompanied by a fireball if the BLEVE is fire-induced. The rapid vaporization and expansion following loss of containment results in a cloud of almost pure vapor and mist. After ignition, this cloud starts to bum at its surface, where mixing with air is possible. In the buoyancy stage, combustion propagates to the center of the cloud causing a massive fireball. 

The distance of the fireball to targets and the atmospheric transmissivity will determine the consequences of radiation. 

During the test, hydrogen flow rate was raised to a maximum of approximately 55 kg/s (120 Ib/s). About 23 seconds into the experiment, a reduction in flow rate began. Three seconds later, the hydrogen exploded. Electrostatic discharges and mechanical sparks were proposed as probable ignition sources. The explosion was preceded by a fire observed at the nozzle shortly after flow rate reduction began. The fire developed into a fireball of modest luminosity, and an explosion followed immediately. 

Crescent City, Illinois, USA Several Fireballs from Rail Cars... 

At 6 30 A.M. on June 21, 1970, fifteen railroad cars, including nine cars carrying liquefied petroleum gas (LPG), derailed in the town of Crescent City, Illinois. The derailment caused one of the tanks to be punctured, then release LPG. The ensuing fire, fed by operating safety valves on other cars, resulted in ruptures of tank cars, followed by projectiles and fireballs. No fatalities occurred, but 66 people were injured. There was extensive property damage. 

At 8 30 A M., water pumped from a canal became available. It was used to cool other exposed spheres, but the sphere from which the initial spill of propane occurred was not protected. At 8 40 the sphere ruptured into five large fragments, producing a large fireball, killing or injuring nearly 100 people in the vicinity. 

At 5 45 A.M., a flash fire resulted. The vapor cloud is assumed to have penetrated houses, which were subsequently destroyed by internal explosions. A violent explosion, probably involving the BLEVE of several storage tanks, occurred 1 minute after the flash fire. It resulted in a fireball and the propulsion of one or two cylindrical tanks. Heat and fragments resulted in additional BLEVEs. 

Fireball dimensions seemed to be smaller than those predicted by models. However, the moment of the initial vessel failure was not captured by either video or still cameras. 

Very rapidly expanding ground-level fireballs occurred whenever vessels failed. 

At 8 4S the tank of the truck failed it had no safety valve. A fireball resulted, but no concussion was felt. The front end of the tank was propelled up for a distance... 

Investigations revealed that the initial fire was due to a small, continuous release from the transfer lines. The leakage was ignited by hot surfaces of the truck s engine. The fireball was found to have a maximum diameter of approximately 40 m (130 ft). It rose to 25 m (80 ft) above ground level. 

Wooden sticks affected by radiation from the fireball permitted an estimate of the radiation levels emitted. It was thus established that the emissive power of the LPG cloud was approximately 180 kW/m (16 BTU/s/ft ). 

A fireball then developed. Several ensuing explosions, fireballs, and BLEVEs destroyed the refinery almost completely, causing the deaths of seven people and injuries of ten. 

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