Instantaneous release

Once the scenario has been identified, a source model is used to determine the quantitative effect of an accident. This includes either the release rate of material, if it is a continuous release, or the total amount of material released, if it is an instantaneous release. Eor instance, if the scenario is the mpture of a 10-cm pipe, the source model would describe the rate of flow of material from the broken pipe. 

The dispersion model is typically used to determine the downwind concentrations of released materials and the total area affected. Two models are available the plume and the puff. The plume describes continuous releases the puff describes instantaneous releases. 

Assume a continuous release of pressurized, hquefied cyclohexane with a vapor emission rate of 130 g moLs, 3.18 mVs at 25°C (86,644 Ib/h). (See Discharge Rates from Punctured Lines and Vessels in this sec tion for release rates of vapor.) The LFL of cyclohexane is 1.3 percent by vol., and so the maximum distance to the LFL for a wind speed of 1 iti/s (2.2 mi/h) is 260 m (853 ft), from Fig. 26-31. Thus, from Eq. (26-48), Vj 529 m 1817 kg. The volume of fuel from the LFL up to 100 percent at the moment of ignition for a continuous emission is not equal to the total quantity of vapor released that Vr volume stays the same even if the emission lasts for an extended period with the same values of meteorological variables, e.g., wind speed. For instance, in this case 9825 kg (21,661 lb) will havebeen emitted during a 15-min period, which is considerablv more than the 1817 kg (4005 lb) of cyclohexane in the vapor cloud above LFL. (A different approach is required for an instantaneous release, i.e., when a vapor cloud is explosively dispersed.) The equivalent weight of TNT may be estimated by... 

Initial plume volume flux for dense gas dispersion, voliime/time Continuous release rate of material, mass/time Instantaneous release of material, mass Release duration, time T Absolute temperature, K... 

Sutton Micrometeorology, McGraw-Hill, 1953, p, 286) developed a solution to the above difficulty by defining dispersion coefficients, O, Gy, and O, defined as the standard deviation of the concentrations in the downwind, crosswind, and vertical x, y, z) directions, respectively, The dispersion coefficients are a function of atmospheric conditions and the distance downwind from the release. The atmospheric conditions are classified into six stability classes (A through F) for continuous releases and three stability classes (unstable, neutral, and stable) for instantaneous releases. The stability classes depend on wind speed and the amount of sunlight, as shown in Table 26-28,... 

The puff model describes near-instantaneous releases of material. The solution depends on the total quantity of material released, the atmospheric conditions, the height of the release above ground, and the distance from the release. The equation for the average concentration for this case is (Growl and Louvar, 1990, p, 143) ... 

Explosive Chemical causing a sudden, almost instantaneous release of... 

BLEVE, BOILING LIQUID EXPANDING VAPOUR EXPLOSION Instantaneous release and ignition of flammable vapour upon rupture of a vessel eontaining flammable liquid above its atmospherie boiling point. 

EMGRESP is overly conservative for passive gas dispersion applications. No time-varying releases may be modeled. Dense gas dispersion may be computed for only "instantaneous" releases conditions. 

A massive amount of propane is instantaneously released in an open field. The cloud assumes a flat, circular shape as it spreads. When the internal fuel concentration in the cloud is about 10% by volume, the cloud s dimensions are approximately 1 m deep and 100 m in diameter. Then the cloud reaches an ignition source at its edge. Because turbulence-inducing effects are absent in this situation, blast effects are not anticipated. Therefore, thermal radiation and direct flame contact are the only hazardous effects encountered. Wind speed is 2 m/s. Relative humidity is 50%. Compute the incident heat flux as a function of time through a vertical surface at 100 m distance from the center of the cloud. 

Cases of instantaneous release, as from an explosion, or short-term releases on the order of seconds, arc also and often of practical concern. 

Instantaneous release Escape of a quantity of material over a short time span (typically a few seconds). 

There are also several possibilities for the temporal distribution of releases. Although some releases, such as those stemming from accidents, are best described as instantaneous release of a total amount of material (kg per event), most releases are described as rates kg/sec (point source), kg/sec-m (line source), kg/sec-m (area source). (Note here that a little dimensional analysis will often indicate whether a factor or constant in a fate model has been inadvertently omitted.) The patterns of rates over time can be quite diverse (see Figure 3). Many releases are more or less continuous and more or less uniform, such as stack emissions from a base-load power plant. Others are intermittent but fairly regular, or at least predictable, as when a coke oven is opened or a chemical vat... 

Dining chlorination of hydrocarbons with Lewis acid catalysis, the catalyst must be premixed with the hydrocarbon before admission of chlorine. Addition of catalyst to the chlorine-hydrocarbon mixture is very hazardous, causing instantaneous release of large volumes of hydrogen chloride. 

The worst-case releases are those that assume almost catastrophic failure of the process, resulting in near instantaneous release of the entire process inventory or release over a short period of time. 

Figure 5-2 Puff formed by near instantaneous release of material.

Consider the instantaneous release of a fixed mass of material, Q, into an infinite expanse of air (a ground surface will be added later). The coordinate system is fixed at the source. Assuming no reaction or molecular diffusion, the concentration C of material resulting from this release is given by the advection equation... 

Figure 5-8 Puff with wind. After the initial instantaneous release, the puff moves with the wind.

For instantaneous releases the characteristic source dimension is defined as... 

A is the characteristic source dimension for instantaneous releases of dense gases (length) and... 

A storage tank containing acrolein (ERPG-1 = 0.1 ppm) is located 1500 m from a residential area. Estimate the amount of acrolein that must be instantaneously released at ground level to produce a concentration at the boundary of the residential area equal to the ERPG-1. 

The emergency coordinator has decided that the appropriate emergency response to the immediate release of a toxic material is to alert people to stay in their homes, with doors and windows closed, until the cloud has passed. The coordinator has also indicated that homes 4000 m downwind must not be exposed to concentrations exceeding 0.10 mg/m3 of this material for any longer than 2 min. Estimate the maximum instantaneous release of material (in kg) allowed for these specifications. Be sure to clearly state any assumptions about weather conditions, wind speed, etc. 

A butane tank is located 500 ft from a residential area. Estimate the minimum instantaneous release of butane required to produce a vapor concentration in the residential area equal to the LFL for butane. What continuous release rate is required Assume that the release occurs at ground level. Will the minimum amount increase, decrease, or stay the same if the release occurs above ground level ... 

Release momentum. For jet releases, the amount of air entrained in an unobstructed jet is proportional to the jet velocity. Depending on the orientation of the jet relative to nearby obstructions, the momentum of a jet can be dissipated without significant air entrainment. The degree of initial air entrainment can be an important determinant of the hazard extent, particularly for flammable hazards. It would be (possibly overly) conservative to assume the source momentum is dissipated without air dilution. Explosive releases are high-momentum, instantaneous releases. For explosive releases, a rough first approximation is to assume that the mass of contaminant in the explosion is mixed with 10 times that mass of air. 

Recently, the pesticide company experienced two accidents. The first accident was a small fire of a mixture of liquid pressurised gas and liquid pesticide, which resulted in some material damage. The second accident was an instantaneous release of pesticide powder, which resulted in a number of operators requiring treatment for respiratory problems in a local hospital. Management recognised that safety problems had to be addressed. Additionally, a major concern for the company was the high financial penalties which resulted from late deliveries. Both the safety and the reliability of the operational process had to be improved. The management thought that operator errors were the cause of almost all the problems in the process. 

A chemical that causes a sudden, almost instantaneous release of pressure, gas, and heat when subjected to sudden shock, pressure, or high temperature. 

Solutions were obtained in Section III for the mean concentration resulting from an instantaneous release of a quantity Q of material at the origin in an infinite fluid with stationary, homogeneous turbulence and a mean velocity in the x direction. We now wish to consider the case of a continuously emitting source under the same conditions. The source strength is specified as q (g sec )-... 

Based on the manner of derivation of the Gaussian equations in Section III, we see that the dispersion parameters a-y and are originally defined for an instantaneous release and are functions of travel time from release. Since the puff equations depend on the travel time of individual puffs or releases, the dispersion coefficients depend on this time, i.e., these coefficients describe the growth of each puff about its own center. This is basically a Lagrangian formulation. 

In determining how the dispersion coefficients depend on travel time one may employ atmospheric diffusion theory or the results of experiments. Because of the difficulty of performing puff experiments, however, the coefficients are usually inferred not from instantaneous releases but from continuous releases. Thus, the dispersion coefficients derived from such experiments are essentially a measure of the size of the plume envelope formed by sampling a real meandering plume emitted from a... 

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