Explosion venting, gases/vapors

Explosion calculations, 499-504 Estimating destruction, 501 Overpressure, 502 Pressure piling, 501, 504 Relief sizing, 505 Scaled distance, 502, 503 Schock from velocity, 503 TNT equivalent, 499-504 Explosion characteristics of dusts, 515 Explosion suppression, 518 Explosion venting, gases/vapors, 504 Bleves, 504 Explosions, 482 Blast pressure. 496 Combustion, 482 Confined, 482 Damage, 498-501 Deflagration, 482 Detonation, 483... 

Most LNG vehicles provide a manifold to capture natural gas vented from the LNG tank and direct it to a safe location outside of the vehicle, usually near the top of the vehicle where the vented natural gas will rise and dissipate safely. For vehicle maintenance facilities, exhaust systems with explosion-proof blowers and motors should be provided. When the vehicle is brought inside for maintenance, the exhaust system hose is placed over the LNG vent and the system is turned on. Any vented natural gas vapor is then safely removed from the building. For vehicle parking facilities, this is usually not a practical solution fc - cost and implementation reasons. 

The European philosophy on area classification varies from that of the United. States and Canada. Specifically, in Europe and most other inter national areas, the Zone concept is utilized. An area in which an expio sive gas-air mixture is continuously present, or present for long perioiK of time, is referred to as Zone 0. The vapor space of a closed, but vented, process vessel or storage tank is an example. An area in which an explosive gas-air mixture is likely to occur in normal operations is designated Zone 1. An area in which an explosive gas-air mixture is less likely to occur, and if it does occur will exist only for a short time, is designated Zone 2. Zone 0 and Zone 1 correspond to Division 1 in the U.S. and Canada System. Zone 2 is equivalent to Division 2. 

As vent collection systems normally contain vapor/air mixtures, they are inherently unsafe. They normally operate outside the flammable range, and precautions are taken to prevent them from entering it, but it is difficult to think of everything that might go wrong. For example, an explosion occurred in a system that collected flarmnable vapor and air from the vents on a number of tanks and fed the mixture into a furnace. The system was designed to run at 10% of the lower explosion limit, but when the system was isolated in error, the vapor concentration rose. When the flow was restored, a plug of rich gas was fed into the furnace, where it mixed with air and exploded [17]. Reference 34 describes ten other incidents. 

Sizing of vent areas The empirical equation (23-12) can be used to calculate the required vent area for flammable gas or solvent vapor explosions. The equation is valid for flammable gas-air mixtures which have been ignited in a quiescent state (nonturbulent) with an ignition source of E = 10 J. 

The outside surfaces of refrigerated storage vessels are thermally insulated to assist in maintaining the desired storage temperature. Thermal excursions increase product vaporization, raise vessel pressure, and increase the amount of gas relieved to atmosphere orto a relief vent collection system. Fire exposure of a refrigerated vessel can increase product temperature and vessel pressure, possibly exceeding the capacity of relief valves or relief vent collection system, and could result in vessel rupture with major fire or explosion consequences. 

Potential hydrocarbon losses from the overpressuring of operating vessels are controlled first by staged computer alerts and/or manual alarms to provide for correction of the condition. If the overpressure exceeds a second set point, pressure relief valves vent the vessel contents to a flare release system. The flare system provides a means of controlled burning of hydrocarbon vapors at a nonhazardous point to avoid fire or explosion risks. Smoke problems from flares are avoided by more efficient designs that use multiple nozzles and low pressure operation to promote clean combustion [57]. Greenhouse gas concerns should more frequently stimulate an interest in energy recovery options from flared hydrocarbons. 

A large part of the vapors stripped by the residual air is cooled and condensed. Traces of toluene remaining in the incondensables are retained on activated charcoal, while part of the remaining vent gases may be recycled, after recompression, to increase the content of inerts in the gas holdup present in the reactors and thus reduce the explosion hazard. 

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