UVCE

Frank T. Bodurtha/ Sc D / E. I. du Pont de Nemours and Co., Inc., (retired) Consultant, Frank T. Bodui tha, Inc. (Gas Explosions Unconfined Vapor Cloud Explosions [UVCE.s] and Boiling Liquid Expanding Vapor Explosions [BLEVE.s])... 

Uucoufiued Vapor Cloud Explosions (UVCEs) and Boiling Liquid... 

UNCONFINED VAPOR CLOUD EXPLOSIONS (UVCEs) AND BOILING LIQUID EXPANDING VAPOR EXPLOSIONS (BLEVEs)... 

Numerous tests on dispersion of heavy gases and on causes of UVCEs have been performed in recent years. Dispersion tests and computer models based on them may not be representative of all... 

In assessing the hazard of a UVCE or in investigating a UVCE it is often necessary to (1) estimate the maximum distance to the lower flammable hmit (LFL) and (2) determine the amount of gas in a vapor cloud above the LFL. Figure 26-31 shows the maximum distance to the lower flammable limit, i.e., in the centerline of the cloud, based on the previous method from Bodurtha (1980) for wind speeds of 1 iti/s (2.2 mi/h) and 5 m/s (11 mi/h). Maximum concentrations probably occur near 1 m/s. The volume of fuel from the LFL up to 100 percent may be estimated by... 

Pressure Development Overpressure in a UVCE results from turbulence that promotes a sudden release of energy. Tests in the open without obstacles or confining structures do not produce damaging overpressure. Nevertheless, combustion in a vapor cloud within a partially confined space or around turbulence-producing obstacles may generate damaging overpressure. Also, turbulence in a jet release, such as may occur with compressed natural gas discharged from a ruptured pipehne, may result in blast pressure. 

Example The combustion process in large vapor clouds is not known completely and studies are in progress to improve understanding of this important subject. Special study is usually needed to assess the hazard of a large vapor release or to investigate a UVCE. The TNT equivalent method is used in this example other methods have been proposed. Whatever the method used for dispersion and pressure development, a check should be made to determine if any govern-mentaf unit requires a specific type of analysis. 

Flammable Vapor Detectors These should be installed to warn of leaks, although such devices do not effectively control UVCEs with sudden, massive releases. 

Large Fans These could be used to dilute a vapor cloud below its LFL with ambient air (see, for example, Whiting and Shaffer, Feasi-bihty Study of Hazardous Vapor Amelioration Techniques, Proc. 1978 Nat. Conf. on Control of Hazardous Material Spills, USEPA, Miami Beach, April 1978). But caution must be exercised because the turbulence produced by fans will likely promote rapid combustion and a resulting UVCE unless vapors are diluted below the LFL. Nevertheless, in new plants, strategic placement of air coolers may provide enough air flow to reduce the risk of a UVCE. 

Release of a pressurized, hquefied gas to the atmosphere will cause the gas to cool and condense water vapor in ambient air, forming a visible vapor cloud. Firefighters and operators who attempt to move such a cloud away from furnaces and the hke with fire hoses and water jet guns are at risk, because of the possibility of a UVCE near them. Plants and governmental agencies who recommend such practices need to reexamine their pohcies. 

Explosion a confined vapour cloud explosion (CVCE) can result from ignition of vapour within a building or equipment a boiling liquid expanding vapour explosion (BLEVE) can result when unvented containers of flammable chemicals burst with explosive violence as a result of the build-up of internal pressure unconfmed vapour cloud explosion (UVCE) can result from ignition of a very large vapour or gas/air cloud. 

If a large amount of a volatile flammable material is rapidly dispersed to the atmo vapor cloud forms. If this cloud is ignited before the cloud is diluted below its lower flammability limit, a UVCE occurs which can damage by overpressure or by thermal radiation. Rarely are UVCEs detonations it is believed that obstacles, turbulence, and possibly a critical cloud size are needed to transition from deflagration to detonation. 

Even if a flammable vapor cloud is formed, ignition is necessary for a UVCE. Hov----------- ,... 

UVe are modeled by equivalence of the flammable material to TNT by correlations with observed UVCEs (TNO model), or by computer modeling. Only the simple TNT model is discussed here. 

UVCEs were used in Vietnam, rather unsuccessfully, to send an overpressure into the underground tunnels. 

The Flixborough accident (Section 7.1.2.3) is an often-cited example of a UVCE. 

Exxon (unpublished). Damage estimates from BLEVEs, UVCEs and spill fires. 

Fire is more likely tlian an explosion where tliere is a loss of contaimiient of a flammable material from a railroad car, barge, ship tank, or from a pipeline. However, both unconfmed vapor cloud explosions (UVCES) and boiling liquid-e.xpanding vapor e.xplosions (BEEVES) can occur as a result of transport accidents, (see Section 7.5)... 

Davenport has listed more tlian 60 major leaks of flammable materials, most, of which resulted in serious fires or unconfined vapor cloud explosions (UVCEs)." Table 16.3.1 classifies tlie leaks by point of origin and shows that if transport containers are excluded, pipe failures accounted for more than lialf tlie accidents. The biggest cause of tliese failures lias been shown to be poor construction due to use of wrong specifications or failure to follow specifications established. 

The mineralization of 0.2 mM Indigo Carmine was conducted in a batch photo reactor (0.5 L) in the presence of 8W UVC light and 10 mM H2O2 at different initial solution pHs and 30 C, as described in our previous study [4]. The total organic carbon (TOC) of the... 

Fig. 2. displays the TOC removal of 0.2 mM Indigo Carmine as a fimction of time under different initial solution pHs in the presence of 10 mM H2O2, 1.0 g Fe-B/L, and 8W UVC. As the initial solution pH increases from 3.0 to 7.0, the mineralization kinetics becomes slower, indicating that the Fe-B nanocompsoite showed a decreased photo catalytic activity. For example, the difference between the TOC removal at initial solution pH of 3.0 and 7.0 is about 25 /o, which agrees well with previous studies [1-3]. 

O Exposure to ultraviolet radiation from the sun is recognized as one of the primary triggers for skin carcinogenesis. Based on their wavelengths, UV radiation is divided into three components UVA (320 00 nm), UVB (280-320 nm), and UVC (200-280 nm).15 UVB accounts for only 5% of the solar radiation that reaches the earth, but it is the primary carcinogenic component in the UV spectrum.15 The following sequence of events describes the process in which UV radiation causes skin cancer (1) UV radiation reaches the earth, and on the skin, it reaches the cells in the epidermal layer (i.e., squamous cells, basal cells, and melanocytes),16 (2) the UV radiation (specifically... 

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