Vapor boxes

Another type of box tested had flow spoilers mounted at the top, a configuration that would force the vapor back down into the box and toward the release points. 

Assessing the effects of enclosures or vapor boxes is similar to assessing the effects of vapor fences. Consider the case of a typical dike sized to hold 110 percent of the liquid spilled. For small releases, the dike walls would act as containment or storage for the vapor generated shortly after release occurred. The vapor that would be generated by evaporation or boiling from the dike floor would displace the air in the dike for a given period of time and then overflow the dike walls. The vapor holdup duration is easily estimated  

Equation (5.2) indicates that the time vapor is advected downwind will increase as zt (height of vapor containment box) increases, as this increases the time to fill the vapor box. This additional time allows for the effects of decreasing rates of conductive heat transfer from the dike floor for cryogenic materials, or decreasing convective mass transfer for materials with boiling points that are higher than ambient temperatures, to take effect. 

Equation (5.2) also indicates that the cloud s arrival time would be delayed at least by the time required for the vapor to overflow the vapor containment walls. Also, higher dilution in the near field should be exhibited, since the wind speed is higher at zd + zt than at grade level. 

Although a vapor box or containment may be effective for concentration reduction, it may increase the explosion hazards. Higher explosion peak overpressure may be realized in the near field because of cloud geometry and partial blockage (Melhem and Croce, 1994). 

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Recent work done on physical vapor barriers was performed by the Industrial Cooperative HF Mitigation/Assessment Program (ICHMAP) (Petersen and Radcliff, 1989). This program studied the effects of vapor fences and vapor boxes. The primary objective of the study was to determine the effectiveness of these devices to retard the transport and to dilute heavier-than-air (HTA) releases of a toxic material like hydrofluoric acid (HF). Because vapor barriers could also see releases of flammable materials, an effort was made to determine their impact on the consequences of a vapor cloud explosion. 

For cryogenics, vapor boxes may be more effective. Wind tunnel data indicate that vapor boxes have the potential to reduce near-field concentrations by a factor of 4 to 15 (Arthur D. Little, Inc., 1974). Additionally, far-field concentrations are lowered by the reduction in the rate the gas is discharged from the vapor box. This downwind reduction, when compared to a no-vapor-box case, can vary by a factor of 1 to 4. The degree of reduction is dependent on the release rate, the duration of the release, and the volume of the vapor box. 

Unintentional leaks of volatile liquid or vents of vapor (Box 4.1). 

The most popular method to study phase equilibria, however, is the Gibbs ensemble [101, 102, 103]. In this method, one simulates two boxes 1 and 2, where the total number of particles n = ni + H2 and the total volume F = Fi - - V2 are kept constant. One considers moves where particles are exchanged between the boxes and the volume is redistributed (Vi = V — AV, V2 V = V2 + AV). If the total density p = / V is chosen such that it falls in between the two branches of the coexistence curve, the system will evolve into an equilibrium state such that one box contains only the fluid with a density according to the liquid branch of the coexistence curve, and the other box contains only the fluid with a density according to the vapor branch. Of course, there is no physical reason which of the boxes should contain the liquid and which should contain the vapor, and in fact, if one simulates long enough, one will observe transitions where the boxes switch the identity of the phases they contain, i.e. the liquid box will become the vapor box and vice... 

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