Liquid Flashing

Dukler s analysis can be suitable modified for flashing fiow. The two-phase pressure drop is  

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FIG. 26-68 Ratio of mass flux for horizoutal pipe flow to that for orifice discharge for flashing liquids hy the homogeueoiis eqiiilihriiim model, (Leung and Gmlmes, AIChE J, 33 (3), pp. 524-527, 1987 reproduced by permission of AIChE. copy-right 1987. All rights reseroed.)... 

FIG. 26-69 Ratio of mass flux for inclined pipe flow to that for orifice discharge for flashing liquids by the homogeneous equilibrium model. Leung, J. of Loss Prev. Process Ind. 3 pp. 27-32, with kind peimission of Elsevier Science, Ltd, The Boulevard, Langford Lane, Kidlington, 0X5 IGB U.K., 1990.)... 

One applieation is the expansion of liquids or flashing liquids. Although this does not seem to be a very large power applieation, but all of the power removed from a eryogenie proeess reduees the amount of heat introdueed into the proeess by turbulenee. 

When these liquid or flashing liquid turboexpanders are installed, the operators are often surprised at the improved proeess effieieney. 

For a binary, let s denote as V the fractional molar split of the feed into overhead product and as L the fractional split into bottom product. Calculate compositions of the flash separation of feed into vapor v and liquid 1 to give v/1 = V/L. The resulting vapor can be regarded as being composed of a portion d of the overhead product composition and a portion r of the flash liquid composition. 

It is then easy to calculate what the proportions must be so that d x overhead composition -l- r x flash liquid composition = flash vapor composition. Thus ... 

The reason for this simple relationship is that the concept of minimum reflux implies an infinite number of stages and thus no change in composition from stage to stage for an infinite number of stages each way from the pinch point (the point where the McCabe-Thiele operating lines intersect at the vapor curve for a well-behaved system, this is the feed zone). The liquid refluxed to the feed tray from the tray above is thus the same composition as the flash liquid. 

We shall first consider the case of non-flashing liquids. In this situation, there is no critical flow pressure limiting the flow of liquid through a PR valve orifice, as opposed to the case of vapor flow. The discharge rate is a function of the pressure drop across the valve and can be estimated by the following expression ... 

Sizing for Flashing Liquids - PR valves handling fluids which are liquid at inlet conditions but which flash wholly or partially to vapor at discharge conditions can be sized using the following procedure ... 

Sizing for Mixed-Phase Vapor and Liquid Service - PR valve sizing for a mixture of hquid and vapor at inlet conditions may be calculated using the sum of the orifice areas required for each phase individually, in the same way as described above for flashing liquid service. 

M = Rate of vaporization and burning of liquid, kg/hr (selected as equal to the rate of flashed liquid entering the pit)... 

Critieal flow—this is usually the ease sinee eritieal pressure ratios of flashing liquid approaeh the value of 1... 

Spring-loaded ball valves should be used for drain valves. They have to be held open, and they close automatically if released. The size of drain valves should be kept as small as practicable. With liquefied flammable gases and other flashing liquids, % in. should be the maximum allowed. 

If water has to be drained regularly from liquefied flammable gases or other flashing liquids, and if a spring-loaded valve cannot be used, then a remotely operated emergency isolation valve (see Section 7.2.1) should be installed in the drain line. 

On many occasions combustible gas detectors have detected a leak soon after it started, and action to control it has been taken promptly. Installation of these detectors is strongly recommended whenever liquefied flammable gases or other flashing liquids are handled or when experience shows there is a significant chance of a leak [3]. Detectors are also... 

Any flammable liquid under pressure above its normal boiling point will behave like LFG. Liquefied flammable gases are merely the most common example of a flashing liquid. Most unconfined vapor cloud explosions, including the one at Flixborough (Section 2.4), have been due to leaks of such flashing liquids [2],... 

Another theory of liquid-liquid explosion comes from Board et al. (1975). They noticed that when an initial disturbance, for example, at the vapor-liquid interface, causes a shock wave, some of the liquid is atomized, thus enhancing rapid heat transfer to the droplets. This action produces further expansion and atomization. When the droplets are heated to a temperature equal to the superheat temperature limit, rapid evaporation (flashing liquid) may cause an explosion. In fact, this theory resembles the theory of Reid (1979), except that only droplets, and not bulk liquid, have to be at the superheat temperature limit of atmospheric pressure (McDevitt et al. 1987). 

This section addresses the effects of BLEVE blasts and pressure vessel bursts. Actually, the blast effect of a BLEVE results not only from rapid evaporation (flashing) of liquid, but also from the expansion of vapor in the vessel s vapor (head) space. In many accidents, head-space vapor expansion probably produces most of the blast effects. Rapid expansion of vapor produces a blast identical to that of other pressure vessel ruptures, and so does flashing liquid. Therefore, it is necessary to calculate blast from pressure vessel mpture in order to calculate a BLEVE blast effect. 

Method for Explosively Flashing Liquids and Pressure Vessel Bursts with Vapor or Nonideal Gas... 

In many cases, both liquid and vapor are present in a vessel. Experiments indicate that the blast wave from expanding vapor is often separate from that generated by flashing liquid. However, it is conservative to assume that the blast waves from each phase present are combined. This method is given in Figure 6.29. 

Rgure 6.29. Calculation of energy of flashing liquids and pressure vessel bursts filled with vapor or nonideal gas. 

Hasegawa and Sato (1977) showed that when the calculated amount of flashing evaporation of the liquid equals 36% or more, all of the contained fuel contributes to the BLEVE and eventually to the fireball. For lower flash-off values, part of the fiiel forms the BLEVE and part of it forms a pool. It is assumed that if the flashing evaporation is lower than 36%, three times the quantity of the flashing liquid contributes to the BLEVE. 

For prediction purposes, the amount of gas in a BLEVE can be taken as three times the amount of flashing liquid up to a maximum of 100% of available fuel. 

Use of Figure 9.2 requires that the temperature of the liquid be compared to its boiling point and its superheat-limit temperature. Table 6.1 provides these temperatures T), = 231 K, and 7, = 326 K. It is obvious that the liquid s temperature can easily rise above the superheat limit temperature when the vessel is exposed to a lire. Therefore, the explosively flashing-liquid method must be selected. This method is described schematically in Figure 9.5 (equal to Figure 6.29), and described in Section 6.3.3.3. 

When the vessel is only 10% filled, the energy of the explosion is different. Therefore, calculations must be repeated starting from Step 6 of the flashing liquid method. 

Assuming that the blasts from vapor expansion and flashing liquid are simultaneous, the total energy of the surface explosion is ... 

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