Flash fractions

Since pipe flow is more nearly isenthalpic, the flash fraction x is found from an enthalpy balance between the stagnation point and a point z downstream. Accounting for changes in potential energy, kinetic energy, and heat added or removed from the pipe Q, x is given by ... 

Determine the flash fraction on the basis of actual thermodynamic data. 

The cloud inventory is equal to the flash fraction times the amount of fuel released. To allow for spray and aerosol formation, the cloud inventory should be multiplied by 2. This number may not, of course, exceed the total amount of fuel released. 

The quantity of fuel in a cloud is calculated by use of release and (flash) vaporization models that have been extensively described by Hanna and Drivas (1987). To account for aerosol formation during vaporization, the flash fraction should be doubled up to, but not exceeding, a value of unity. Pool vaporization is also considered. 

Determine the flash fraction of fuel on the basis of actual thermodynamic data. Equation (7.1) provides a method of estimating the flash fraction. 

Since the mass fraction of solids is constant, the vapor flash fraction is... 

The nonequilibrium flash fraction % is interpolated nonlinearly between and x, by ... 

If tabulated thermodynamic data are available, the isenthalpic (irreversible adiabatic) flashing fraction can be calculated from [10]... 

A similar isentropic (reversible adiabatic) calculation yields a smaller flashing fraction. 

If thermodynamic data are not available, the flashing fraction can be approximated by [14]... 

Some of the escaping liquid is carried with the flashing vapor as nust or aerosol, which then vaporizes as it mixes with warmer air and enters the cloud as vapor. It is usually assumed that, if the pressure in the container is the vapor pressure of the liquid, the amount of mist or aerosol is equal to that of the amount of vapor that results from flashing, except that if the flashing fraction exceeds 33%, the amount of mist or aerosol would be half of the remaiiung liquid [16], with rain-out of the rest of the cold liquid. If, however, the pressure in the vapor space above the liquid is substantially above the vapor pressure, discharge of the contents may result in entrainment of all of the liquid, with no rain-out or deposition of liquid [16]. 

For alternative evaluations, the weight of flammable material in a vapor cloud can be calculated as the gas-release or vapor-release rate multiplied by the estimated time required to stop the release. For liquid releases, the vapor-release rate would be calculated from the liquid-release rate multiphed by twice the flashing fraction (to account for aerosol vaporization) or, for hquids released below the boiling point, as the rate of vaporization from a pool multiplied by the estimated time required to cover or dilute the pool. Also, a lower energy-conversion efficiency (such as 0.03) can be used in the calculation. 

Fireball characteristics—size, duration and surface emissive power (SEP)—should therefore be functions not only of the mass of the liquid involved (Roberts, 1982) but also the time delay from the LBB initiator to the final LOC and whether the LOC occurs with the contents still increasing in pressure and prior to the liquid contents becoming homogeneous. If vessel LOC occurs with a stratified liquid layer and a subcooled core under increasing pressure, the flreball should be less buoyant and have an appreciable flash fraction and/or rainout and thus a lower SEP than in a case with dropping pressure and therefore homogeneous boiling. 

The flash fraction is a useful concept in interpreting results on two-phase flow. Flashing can be defined as an isentropic evaporation process due to depressurization, in which the final pressure is equal to the atmospheric pressure. The flash-off vapor fraction (flash fraction) is the corresponding quality in the final state of the process. The term partial flashing refers to the same process when the final pressure is higher than the atmospheric pressure. 

We assume that the fluid is in a saturated state initially and finally and that the fluid is initially pure liquid (j o = 0). Then the flash fraction (x J) can be computed numerically from Eq. (27.7). The final temperature of the fluid is the boiling point temperature of the... 

The significance of flash fractions is that they give the maximum possible vapor fraction formed in an isentropic process for discharges into the atmosphere. This statement could be derived mathematically starting from Eq. (27.7) and equations for the temperature dependency of the vapor and liquid entropies in saturated state. For instance, in choked isentropic pipe flow the vapor fraction of the fluid at the aperture is always smaller than the flash fraction (because the exit pressure is higher than the atmospheric pressure). 

FIGURE 27.3 Flash fraction versus temperature of storage for some toxic and flammable substances. (Source Kukkonen, 1990)... 

A standard equation for prediction of the fraction of the liquid that flashes can be derived by assuming that the sensible heat contained within the superheated liquid due to its temperature above its normal boiling point is used to vaporize a fraction of the liquid. This isenthalpic analysis leads to the following equation for the flash fraction (Growl and Louvar, 1990),... 

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