Hydrocarbon vapor release

Figure 13. Safety valve orifice area required for hydrocarbon vapor release.

Facility electrical systems and components provide a convenient source of ignition within a hydrocarbon or ordinary occupancies whenever the design, installation or maintenance is substandard. Electrical systems or components may short, overheat, operate incorrectly, etc. These failures will present themselves as available ignition sources for hydrocarbon vapor releases. All electrical installation should be according to recognized electrical industry standards such as API RP 540 and the NEC (NFPA 70). 

The probabihty of a hydrocarbon vapor release near the engine. 

Note 7—Cooling the assembly prior to disconnecting the gage will focihtate disassembly and reduce tte amount of hydrocarbon vapors released into the room. 

The primary weakness of the approach is that it does not apply to dense vapor releases, a categoiy which includes most hydrocarbon materials. Furthermore, the concentrations predicted are time-weighted averages, with instantaneous values potentially exceeding the average. Finally, the range of apphcabdity is typically from 0.1 to 10 km downwind from the release. 

Onboard Refueling Vapor Recovery (OR ) regulations were fust proposed m 1987 but were met with a litany of technical and safety issues that delayed the requirement. The 1990 CAA amendments required the implementation of ORVR and the EPA regulation requires passenger cars to first have the systems starting in 1998. The ORVR test will be performed in a SHED and will require that not more than 0.2 grams of hydrocarbon vapor per gallon of dispensed fuel be released from the vehicle. 

It is important to note that even if the blowdown is effective in disengaging liquid and vapor, further condensation could occur downstream especially if the vented vapor exits the drum at a temperature above ambient conditions. A proportion of such condensible materials in the blowdown drum vapor release may condense as a result of cooling in the flare header and contact with seal water, and then disengage in the flare seal drum while condensible vapors which are not condensed out at this stage may condense in the flare stack or its inlet line, thus creating the potential for hazardous fallout of burning liquid from the flare. Condensed hydrocarbon in the seal drum can be entrained out with the... 

They are effective as a means of removing heavy hydrocarbon vapors from emergency release streams, thus minimizing condensation problems in downstream equipment. 

Hydrocarbon materials must first be in a vapor condition before combustion processes can occur. For any gaseous material this is an inherent property. Liquids however must have significant vapor emissions in order for flammable concentrations to be present for combustion processes to occur. Therefore hydrocarbon liquid releases are nominally less dangerous than a gas release. 

It could be argued that vapor cloud explosions for hydrocarbon facilities need only be calculated for those facilities that contain large volumes of volatile hydrocarbon gases that can be accidentally released and where some degree of confinement or congestion exist. The most probable amount for an incident to occur is taken as 4,536 kgs (10,000 lbs ), however incidents have been recorded where only 907 kgs (2,000 lbs.) has been released. Additionally, an actual calculation of worst case releases to produce 0.2 bar (3 psio) at say 46 meters (150 ft.), indicates a minimum of 907 kgs (2,000 lbs.) of material is needed to cause that amount of overpressure. A limit of 907 kgs (2,000 lbs.) release of hydrocarbon vapor is considered a prudent and conservative approach. 

Industry literature typically cites concern with open air explosions when 4,536 kgs (10,000 lbs.) or more of flammable gas is released, however, open air explosions at lower amounts of materials are not unheard of. When the release quantity is less than 4,536 kgs (10,000 lbs.), a flash fire is usually the result. The resulting fire or explosion damage can cripple a hydrocarbon processing facility. Extreme care must be taken to prevent the release of hydrocarbon from vessels resulting in vapor releases and resultant blast overpressure. Measures such as hydrotesting, weld inspections, pressure control valves, adequate pressure safety valves, etc., should all be prudently applied. 

Any combination of vessels and piping that has the potential to release a total volume containing more than 907 kgs (2,000 lbs.) of hydrocarbon vapors. 

The underground tanks at this site were located in a tank hold that extended into the shale. When the release occurred, gasoline migrated downward into the shale and groundwater. Groundwater in the shallow aquifer in this area is not used, due to limited quantity and poor quality, although a significant concern existed due to elevated hydrocarbon vapors. A local basement fire had been attributed to the presence of the vapors. 

The Biocube aerobic biofilter is an ex situ off-gas filtration system that is commercially available. The technology utilizes microbes to biologically oxidize volatile organic compounds (VOCs) and complex odors. It can be used in conjunction with vapor-vacuum-extraction (VVE), a process that draws gases from subsurface soil. These gases often require further treatment before being released into the atmosphere. Biocube has been field tested and has been implemented at over 100 sites for the treatment of hydrocarbon vapors. The technology has also been successfully used for odor control at a variety of sites. In addition, the Biocube system can treat odor and VOC emissions simultaneously. The units are modular, so additional stacks can be added as needed for increased flow and/or removal rates. 

Thus, rather surprisingly, the key to reducing tropospheric ozone pollution is to minimize the release of hydrocarbon vapors from such sources as unburnt fuel in automobile exhausts, vaporization of fuel at service station pumps (modern pumps recover fuel vapors from automobile fuel tanks while refilling them), kitchen exhausts from fast-food restaurants and, as in Mexico City, leakages of liquefied petroleum gas (mainly butane) used for domestic heating and cooking.26... 

VOCs are the most well-known air contaminants released by chemical, petrochemical, and other industries. Benzene, toluene, xylenes, hexane, cyclohexane, thiophene, diethylamine, acetone, and acetaldehyde are examples of VOCs [77,78], Possibly, presently the most relevant technology for VOC control is adsorption on activated carbon [135-145], It is a recognized technology, largely applied in industrial processes for the elimination and recovery of hydrocarbon vapors from gaseous streams [77,136], Additionally, it offers several benefits over the others, that is, the opportunity of pure product retrieval for reuse, high removal efficiency at low inlet concentrations, and low fuel/ energy costs [135],... 

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. 

The National Research Council Subcommittee on Permissible Exposure Levels for Military Fuels reviewed studies on the toxic effects of hydrocarbon vapors on the cardiovascular system (NRC 1996 see Appendix A). Intentional or accidental inhalation of high concentrations of hydrocarbons has the potential to induce cardiac arrhythmias that can result in death. However, for the arrhythmias to occur, epinephrine must be released simultaneously with inhalation (Garb and Chenoweth 1948). No human studies that examined the car-... 

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