Temperature superheat-limit

Propylene is a colorless gas under normal conditions, has anesthetic properties at high concentrations, and can cause asphyxiation. It does not irritate the eyes and its odor is characteristic of olefins. Propjiene is a flammable gas under normal atmospheric conditions. Vapor-cloud formation from Hquid or vapor leaks is the main ha2ard that can lead to explosion. The autoignition temperature is 731 K in air and 696 K in oxygen (80). Evaporation of Hquid propylene can cause skin bums. Propylene also reacts vigorously with oxidising materials. Under unusual conditions, eg, 96.8 MPa (995 atm) and 600 K, it explodes. It reacts violentiy with NO2, N2O4, and N2O (81). Explosions have been reported when Hquid propylene contacts water at 315—348 K (82). Table 8 shows the ratio TJTp where is the initial water temperature, and T is the superheat limit temperature of the hydrocarbon. 

Reid (1976) used the equation-of-state of Redlich-Kwong, which predicts a superheat limit temperature of ... 

As described in Section 6.2.1., British Gas performed full-scale tests with LPG BLEVEs similar to those conducted by BASF. The experimenters measured very low overpressures firom the evaporating liquid, followed by a shock that was probably the so-called second shock, and by the pressure wave from the vapor cloud explosion (see Figure 6.6). The pressure wave firom the vapor cloud explosion probably resulted from experimental procedures involving ignition of the release. The liquid was below the superheat limit temperature at time of burst. 

Thus, the BLEVE theory predicts that, when the temperature of a superheated liquid is below T, liquid flashing cannot give rise to a blast wave. This theory is based on the solid foundations of kinetic gas theory and experimental observations of homogeneous nucleation boiling. It is also supported by the experiments of BASF and British Gas. However, because no systematic study has been conducted, there is no proof that the process described actually governs the type of flashing that causes strong blast waves. Furthermore, rapid vaporization of a superheated liquid below its superheat limit temperature can also produce a blast wave, albeit a weak... 

Temperature determines whether or not the liquid in a vessel will boil when depressurized. The liquid will not boil if its temperature is below the boiling point at ambient pressure. If the liquid s temperature is above the superheat-limit temperature Tj] (Tsi = 0.897 ), it will boil explosively (BLEVE) when depressurized. Between these temperatures, the liquid will boil violently, but probably not rapidly enough to generate significant blast waves. However, this is not certain, so it is conservative to t sume that explosive boiling will occur (see Section 6.3.2). 

Figures 6.30 and 6.31 present the same information for saturated hydrocarbons. In Figure 6.30, the saturated liquid state is on the lower part of the curve and in Figure 6.31 it is on the upper part of the curve. Below T y, the line width changes, indicating that the liquid probably does not flash below that level. Note that a line has been drawn only to show the relationship between the points a curve reflecting an actual event would be smooth. Note that a liquid has much more energy per unit of volume than a vapor, especially carbon dioxide. Note It is likely that carbon dioxide can flash explosively at a temperature below the superheat limit temperature. This may result from the fact that carbon dioxide crystallizes at ambient pressure and thus provides the required number of nucleation sites to permit explosive vaporization.

The above methods assume that all superheated liquids can flash explosively, yet this may perhaps be the case only for liquids above their superheat-limit temperatures or for pre-nucleated fluids. Furthermore, the energies of evaporating liquid and expanding vapor ate taken together, while in practice, they may produce separate blasts. Finally, in practice, there are usually structures in the vicinity of an explosion which will reflect blast or provide wind shelter, thereby influencing the blast parameters. 

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. 

The explosion of a vessel full of liquid above the superheat limit temperature has much more energy, and therefore, causes a much more severe blast than a gas- or vapor-filled vessel. 

The speciflc work done by the fluid in expansion can be read from Figures 6.30 or 6.31 if its temperature is unknown. Saturated propane at a pressure of 1.9 MPa (19 bar) has a temperature of 328 K, almost the superheat-limit temperature. Note that it is assumed that temperature is uniform, which is not necessarily the case. From Figure 6.30, the expansion work per unit mass for saturated liquid propane is... 

The calculation method can be selected by application of the decision tree in Figure 9.2. The liquid temperature is believed to be about 339 K, which is the temperature equivalent to the relief valve set pressure. The superheat limit temperatures of propane and butane, the constituents of LPG, can be found in Table 6.1. For propane, T, = 326 K, and for butane, T i = 377 K. The figure specifies that, if the liquid is above its critical superheat limit temperature, the explosively flashing liquid method must be chosen. However, because the temperature of the LPG is below the superheat limit temperature (T i) for butane and above it for propane, it is uncertain whether the liquid will flash. Therefore, the calculation will first be performed with the inclusion of vapor energy only, then with the combined energy of vapor and liquid. 

In the first case, internal temperature rises slowly, so the liquid propane is also heated. At failure, the liquid temperature will be above superheat limit temperature, and it will flash on release. 

In the second case, temperature rises very rapidly, so the liquid is not heated to a temperature above the superheat limit temperature at failure, and no liquid flashing occurs. To demonstrate the influence of fill ratio, cases of 80% and 10% fill ratio are considered. 

Superheat limit temperature The temperature of a liquid above which flash vaporization can proceed explosively. 

Vessel mptures can also occur when a higher-temperature liquid or solid is combined with a cooler low boiling liquid, transferring sufficient heat from the hotter material to the colder material such that the colder material rapidly vaporizes. No chemical reactions are involved instead, the explosion occurs because the cooler liquid expands as it is converted to vapor, creating high pressures. These are called physical explosions. A common example is a steam explosion, which occurs when liquid water is accidentally introduced into a process vessel operating at an elevated temperature. If the hotter material is above the superheat limit temperature of the evaporating liquid, initial confinement by a vessel is not required to create an explosion pressure wave. 

Extensive laboratory-scale studies have been conducted to investigate the triggering mechanism for RPTs when LNG, liquefied petroleum gas (LPG), and liquid refrigerants contact a hot liquid (usually water). These studies are covered in Sections III and VII. The evidence seems overwhelming that RPTs in these cases result from superheating of the cold, volatile liquid to its superheat-limit temperature where prompt homogeneous nucleation occurs in a time period of a few microseconds. (The properties of a superheated liquid and the concept of homogeneous nucleation are reviewed briefly in Section IX.)... 

The laboratory-scale test results lead naturally to the conclusion that methane-rich LNG, as transported in commerce today, would not undergo an RPT if spiUs were to occur in a marine accident. Normal seawater temperatures are much higher than the superheat-limit temperature of this LNG. However, recent LNG spill tests involving up to 40 m of methane-rich liquids have, on occasion, produced strong RPTs (see Section III,J). In some of these incidents, the RPTs may have occurred after preferential evaporation of the methane to leave an LNG heel much... 

For an external pressure of one atmosphere, is about 0.89-0.90 times the critical temperature of a pure hydrocarbon. For hydrocarbon mixtures, the superheat-limit temperature may be closely approximated by a mole fraction average of the homogeneous nucleation temperature of the pure components. 

As the superheat-limit temperature of light hydrocarbons can be closely approximated as 0.89 times the critical temperature, an equivalent statement would be... 

A number of tests were also carried out with liquefied ethane containing various amounts of heavier hydrocarbons. The compositions where RPTs were reported are given in Table VI. Also shown is the ratio TJT which, from previous tests, would be expected to be slightly greater than unity for an RPT to occur. The agreement is excellent. Note also that, for pure ethane, TJT i 1.09, yet no RPT results if this liquefied gas is simply poured into water. The value TJT 1.09 is close to the upper cutoff of the ethane binaries in Table VI. While theory would still indicate that RPTs are possible, they do not often occur when the water temperature is much above the superheat limit temperature of the liquefied gas. 

For the smelt-water case. Nelson suggested the water in contact with the very hot smelt was, initially, separated by a thin vapor film. Either because the smelt cooled—or because of some outside disturbance— there was a collapse of the vapor film to allow direct liquid-liquid contact. The water was then heated to the superheat-limit temperature and underwent homogeneous nucleation with an explosive formation of vapor. The localized shocks either led to other superheat-limit explosions elsewhere in the smelt-water mass or caused intense local mixing of the smelt and water to allow steam formation by normal heat transfer modes. 

While the mechanism proposed by Nelson explained many of the characteristics of a smelt-water explosion, it had one very serious drawback, i.e., the smelt temperature was significantly higher than the expected superheat-limit temperature of water (1100-1200 K compared to 577 K). For LNG-water, it was shown earlier in Section III that if the water temperature were much higher than the superheat-limit temperature of the LNG, explosions were then rarely noted. For such cases, the filmboiling mode was too stable and collapse of this vapor film was unlikely. 

Based on data of this nature, Shick concluded that little, if any, Na2C03 would move to the high-temperature water phase and, even if this were to occur, the volatility (and superheat-limit temperature) of water would be unaffected. 

Returning to Shick s argument, developed further by Shick and Grace (1982), if the water film accumulates salt, the pressure is depressed but, even more important, they suggest that the 1-bar superheat limit temperature may be significantly increased over the value for pure water (—577 K). The rationale for this assertion stems from considering the P-V iso-... 

Finally, it has often been stated that the maximum pressure which could exist at the source of a superheat explosion is that equivalent to the vapor pressure of the cold liquid at its superheat-limit temperature. For most organic liquids this value would be 30 bar. For pure water, it rises to 90 bar. For concentrated salt solutions, much higher values are possible. 

A modified superheat theory was proposed by Shick to explain molten salt (smelt)-water thermal explosions in the paper industry (see Section IV). (Smelt temperatures are also above the critical point of water.) In Shick s concept, at the interface, salt difiuses into water and water into the salt to form a continuous concentration gradient between the salt and water phases. In addition, it was hypothesized that the salt solution on the water side had a significantly higher superheat-limit temperature and pressure than pure water. Thicker, hotter saltwater films could then be formed before the layer underwent homogeneous nucleation to form vapor. 

Several investigators studied R-12. Holt and Muenker (1972) and Rausch and Levine (1973) made simple spills of this cryogen into water. The highest water temperature used by both teams was —342 K and weak explosions were noted. From Table XVI, it can be seen that this water temperature was barely within the range of the superheat-limit temperature, so no or only minor explosions might have been expected. Henry et al. (1974) spilled R-12 on top of a hot mineral oil. For oil temperatures less than about 409 K, there was little interaction except rapid boiling. Above 409 K, explosions resulted. Henry et al. state that this oil temperature would lead to an interface temperature [see Eq. (1)] close to the expected homogeneous nucleation temperature (—345 K) so that the explosions were to be expected. 

The results of Enger and Hartmann, Rausch and Levine, and Holt and Muenker are in surprisingly good agreement, and their lower water temperature threshold values are close to the expected superheat-limit temperature of R-22 (—327 K). 

As will be described later in this section, for several types of small-scale tests where RFTs would be expected, an increase in the absolute system pressure had a profound effect in suppressing such incidents. As often noted in previous sections, one current theory to explain RPTs invokes the concept of the colder liquid attaining its superheat-limit temperature and nucleating spontaneously. In an attempt to explain the pressure effect on the superheating model, a brief analysis is presented on the dynamics of bubble growth and how this process is affected by pressure. The analysis is due largely to the work of Henry and Fauske, as attested to by the literature citations. 

As will be seen later, there is a maximum superheat and at this point the liquid attains the superheat-limit temperature or homogeneous nucleation temperature and nucleation is prompt. Denoting this limit as T i, we note it is a function of pressure and material tested. 

Temperature in bulk liquid Temperature in bubble Normal boiling point Temperature of water Superheat-limit temperature Critical temperature Specific volume Distance... 

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