Vapor burst

Unstable vapor formation (bumping, geysering, vapor burst) (relaxation instabilities)... 

Check the static instabilities by steady-state correlations, to avoid or alleviate the primary phenomenon of a potential static instability, namely, boiling crisis, vapor burst, flow pattern transition, and the physical conditions that extend the static instability into repetitive oscillations. 

The frequency of the vapor bursts is very high. For an over-all AT of 133°F., each inch of the photographed side of the tube exhibited 84 bursts per sec. A typical burst has a vigorous life of about 0.003 sec. A burst in profile shows that the surrounding liquid is shoved back about 0.16 in. in this time, or the average velocity of the interface is about 4.5 ft./sec. 

Vapor burst instability occurs due to sudden vaporization of the liquid phase with rapid decrease in mixture density. For example, a very clean and smooth heated surface may require high wall superheat for nucleation. The fluid adjacent to the surface is highly superheated and vapor generation is rapid when nucleation starts. This in turn ejects liquid from the heated channel. Rapid vaporization cools the surface, and cooler liquid keeps the vaporization suppressed until wall temperature reaches required nucleation superheat and the process repeats. Vapor burst instabilities are observed during reflood phase of the reemergence core cooling of reactor. 

Interfacing TMqio cavities with packed GC columns is more diflScult since the plasma is extinguished by the vapor burst from the solvent and may also be disrupted by large sample peaks. Packed columns are of value, however, particularly for trace determinations when resolution... 

Liquids examined by FAB are introduced into the mass spectrometer on the end of a probe inserted through a vacuum lock in such a way that the liquid lies in the target area of the fast atom or ion beam. There is a high vacuum in this region, and there would be little point in attempting to examine a solution of a sample in one of the commoner volatile solvents such as water or dichloromethane because it would evaporate extremely quickly, probably as a burst of vapor when introduced into the vacuum. Therefore it is necessary to use a high-boiling solvent as the matrix material, such as one of those listed in Table 13.1. 

At higher vapor loads, the kinetic energy of the vapor rather than the bubble burst supphes the thrust for jets and sheets of hquid that are thrown up as well as the energy from breakup into spray. This yields much higher levels of entrainment. In distillation trays it is the most common limit to capacity. 

The term mist generally refers to liquid droplets from submicron size to about 10 /xm. If the diameter exceeds 10 /xm, the aerosol is usually referred to as a spray or simply as droplets. Mists tend to be spherical because of their surface tension and are usually formed by nucleation and the condensation of vapors (6). Larger droplets are formed by bursting of bubbles, by entrainment from surfaces, by spray nozzles, or by splash-type liquid distributors. The large droplets tend to be elongated relative to their direchon of mohon because of the action of drag forces on the drops. 

An old 100-m pressure vessel, a vertical cylinder, designed for a gauge pressure of 5 psi (0.3 bar), was being used to store, at atmospheric pressure, a liquid of flash point 40°C. The fire heated the vessel to above 40°C and ignited the vapor coming out of the vent the fire flashed back into the tank, where an explosion occurred. The vessel burst at the bottom seam, and the entire vessel, except for the base, and contents went into orbit like a rocket [4]. 

At first it was thought that the spheres burst because their relief valves were too small. But later it was realized that the metal in the upper portions of tlie spheres was softened by the heat and lost its strength. Below the liquid level, the boiling liquid kept the metal cool. Incidents such as this one in which a vessel bursts because the metal gets too hot are known as Boiling Liquid Expanding Vapor Explosions or BLEVEs. 

This worked satisfactorily for a time until some water collected in the dead-end and gradually warmed up as the oil was heated. When the temperature reached 100°C, the water vaporized with explosive violence and burst the equipment. The escaping oil caught fire, five men were killed, and the tank ended up in the plant next door. 

Naphtha vapor from a relief valve on a town gas plant in the UK was ignited by a flare stack. The flame impinged on the napththa line, which burst, starting a secondary fire [7]. 

Fires have often occurred when air is compressed. Above 140°C, lubricating oil oxidizes and forms a carbonaceous deposit on the walls of air compressor delivery lines. If the deposit is thin, it is kept cool by conduction through the pipework. But when deposits get too thick, they can catch fire. Sometimes the delivery pipe has gotten so hot that it has burst or the aftercooler has been damaged. In one case the fire vaporized some of the water in the aftercooler and set up a shock wave, which caused serious damage to the cooling-water lines. 

The hazards of water hammer are described in Section 9,1,5 and the hazards of ice formation in Section 9,1,1, This section describes some accidents that have occurred as the result of the sudden vaporization of water, incidents known as boilovers, slopovers, foamovers, frothovers, or puking, Boilover is used if the tank is on fire and hot residues from the burning travel down to the water layer, Slopover is often used if water from fire hoses vaporizes as it enters a burning tank. Sections 9,1.1 and 12.4.5 describe incidents in which vessels burst because water that had... 

When the reboiler was brought back on line, the water was swept into the heat transfer oil lines and immediately vaporized. This set up a liquid hammer, which burst the surge tank. It was estimated that this required a gauge pressure of 450 psi (30 bar). The top of the vessel was blown off in one piece, and the rest of the vessel was split into 20 pieces. The hot oil formed a cloud of fine mist, which ignited immediately, forming a fireball 35 m in diameter. (Mists can explode at temperatures below the flash point of the bulk liquid see Section 19.5.)... 

The most famous case of a runaway reaction caused by contamination is Bhopal (see Section 21.1). In this case the reaction occurred in a storage vessel. It did not burst but was distorted, and the discharge of vapor was larger than the scrubbing and flare systems could have handled, even if they had been in operation. 

Giesbrecht, H., K. Hess, W. Leuckel, and B. Maurer, 1981. Analysis of explosion hazards on spontaneous release of inflammable gases into the atmosphere. Part 1 Propagation and deflagration of vapor clouds on the basis of bursting tests on model vessels. Ger. Chem. Eng. 4 305-314. 

Giesbrecht et al. (1981) Ignition of vapor clouds after vessel burst (0.226-1000) C3H6 45 0.05... 

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. 

A vessel filled with a pressurized, superheated liquid can produce blasts upon bursting in three ways. First, the vapor that is usually present above the liquid can generate a blast, as from a gas-filled vessel. Second, the liquid will boil upon depressurization, and, if rapid boiling occurs, a blast wiU result. Third, if the fluid is combustible and the BLEVE is not fire induced, a vapor cloud explosion may occur (see Section 4.3.3.). In this subsection, only the first and second types of blast wiU be investigated. 

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. 

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

In the preceding subsections, bursting vessels were assumed to be filled with ideal gases. In fact, most pressure vessels are filled with fluids whose behavior cannot be described, or even approximated, by the ideal-gas law. Furthermore, many vessels are filled with superheated liquids which may vaporize rapidly, or even explosively, when depressurized. 

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

Note that the recommended value for p is not always conservative. In some cases, heat input may be so high that the safety valve cannot vent all the generated vapor. In such cases, the internal pressure will rise until the bursting overpressure is reached, which may be much higher than the vessel s design pressure. For example, Droste and Schoen (1988) describe an experiment in which an LPG tank failed at 39 bar, or 2.5 times the opening pressure of its safety valve. Note also that this method assumes that the fluid is in thermodynamic equilibrium yet, in practice, stratification of liquid and vapor will occur (Moodie et al. 1988). 

In practice, vapor release will not be spherical, as is assumed in the method. A release from a cylinder burst may produce overpressures along the vessel s axis, which are 50% lower than pressures along a line normal to its axis. If a vessel ruptures from ductile, rather than brittle, fracture, a highly directional shock wave is produced. Overpressure in the other direction may be one-fourth as great. The influences of release direction are not noticeable at great distances. Uncertainties for a BLEVE ate even higher because of the fact that its overpressure is limited by initial peak-shock overpressure is not taken into account. 

It is not clear which measure of explosion energy is most suitable. Note that, in the method presented in Section 6.3, the energy of gas-filled pressure vessel bursts is calculated by use of Brode s formula, and for vessels filled with vapor, by use of the formula for work done in expansion. 

Esparza, E. D., and W. E. Baker. 1977b. Measurement of Blast Waves from Bursting Frangible Spheres Pressurized with Flash-evaporating Vapor or Liquid. NASA CR-2811. Washington NASA Scientific and Technical Information Office. 

Maurer, B., K. Hess, H. Giesbrecht, and W. Leuckel. 1977. Modeling vapor cloud dispersion and deflagration after bursting of tanks filled with liquefied gas. Second Int. Symp. on Loss Prevention and Safety Promotion in the Process Irui., pp. 305-321. Heidelberg. 

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