Atmospheric stability class

TABLE 26-28 Atmospheric Stability Classes for Use with the Pasquill-Gifford Dispersion Model... 

The atmospheric stability class The lapse rate SfYJdz)... 

Plume rise observations based on single-stack operation were regressed into the above expression and empirically fitted to the following expression, which incorporates atmospheric stability classes into the coefficients ... 

Time, Hours Wind Speed, m/s Atmospheric Stability Class... 

Critical GLC s can usually be calculated based on a unstable atmosphere, thus enabling the designer to determine a worst case scenario. For any given day, typical atmospheric stabihty data can usually be obtained from a local weather bureau, or may be estimated from the so-called Pasquill chart for the appropriate Atmospheric Stability Class (refer to Table 1). 

From Figures 2 and 3, the Pasquill-Gifford dispersion coefficients are obtained for a downwind distance of 2000 meters and for atmospheric stability Class B. 

TABLE 23-24 Typical Atmospheric Stability Classes in Terms of Wind Speed, Insolation, and State of the Sky... 

Figure 6 Cloud footprint to an atmospheric concentration of 20 ppm resulting from the rupture of a 50-mm-diameter chlorine pipe containing either chlorine liquid or chlorine vapor. Release conditions Complete rupture of pipe without shutoff, pipe elevation is 5 m above grade, wind speed is 5 m/sec, atmospheric stability class D, 20 ppm is the Emergency Response Planning Guideline-3 (ERPG-3) concentration for chlorine, the concentration at which life-threatening effects might result from exposure for 1 hour.

Measures of plume and puff spread are based on experimental studies, are independent of height, and are a function of atmospheric stability class. 

One commonly used suite of models that is based on Gaussian plume modeling is the Industrial Source Complex (ISC) Dispersion Models (US EPA, 1995). This suite includes both a short-term model (ISCST), which calculates the hourly air pollutant concentrations in an area surrounding a source, as well as a long-term model (ISCLT), which calculates the average air pollutant concentrations over a year or longer. ISCLT uses meteorological data summarized by frequency for 16 radial sectors (22.5° each) this data format is referred to as a stability array (STAR). Within each sector of STAR, joint frequencies of wind direction, wind speed, and atmospheric stability class are provided. 

Pasquill-Gifford atmospheric stability classes are calculated using the standard deviations of the horizontal wind direction. The stability stratification for the TA-V tower is given using the three-year database. Table 1.4-2 identifies the frequency of stability and wind speed conditions that could be expected in general at TA-V. 

Ps = conditional probabihty of an accident rise with toxic gas release in risk sources grid cell S [km ] Phj = probability of atmospheric stability class [-] P = probabihty of assigned wind breathe direction attached with weather class M[-] ... 

A case study is performed assuming an instantaneous release of the toxic liquid acrylonitrile from a rail tankwagon. After the release of the toxic liquid a pool of 600m is formed from which evaporation occurs, leading to a vapour cloud. This vapour cloud travels with the wind and disperses. The degree of dispersion is determined by the wind speed, the stability of the atmosphere and the surface roughness. The stability of the atmosphere is indicated by the pasquill-stabUity class. By day, the most common atmospheric stability class is class D and a wind speed of 5 m/s is assumed, this weather condition is abbreviated with D5. At night class F is the most common atmospheric stability, associated with a wind speed of 1.5 m/s this weather condition is abbreviated as FI.5. The evaporation and dispersion calculations are performed with EFFECTS 7.6. A neutral gas model is used for the dispersion calculations. 

Figure 1. Example of ammonia maximal concentration in atmosphere ground level (instantaneous outflow 1000 kg NH3, atmosphere stability class E)...

The cloud behavior is affected mostly by the wind speed, the vapor evolution rates from the pool, the atmospheric stability class, and the atmospheric relative humidity (Kapias, 1999 Kapias and Griffiths, 1999a). In the majority of the cases, the cloud will initially be denser... 

The effects of wind speed and atmospheric stability class on the hazardous species centreline concentration in the cloud are depicted in Figs. 37.8 and 37.9. It should be noted that... 

FIGURE 37.9 Atmospheric stability class and wind speed effect on the hazardous species centerline concentration for spills of 16 kg s of SO3 for 600 s. 

The third and often confusing term used in describing incidents is the incident outcome case. As indicated by its definition in Table 1.1, the incident outcome case specifies values for all of the parameters needed to uniquely distinguish one incident outcome from all others. For example, since certain incident outcomes are dependent on weather conditions (wind direction, speed, and atmospheric stability class), more than one incident outcome case could be developed to describe the dispersion of a dense gas. 

Incident outcome case The quantitative definition of a single result of an incident outcome through specification of sufficient parameters to allow distinction of this case fircwn all others for the same incident outcomes. For example,a release of 10 Ib/s of ammonia with D atmospheric stability class and 1.4 mph wind speed gives a particular downwind concentiation profile, resulting, for example, in a 3000 ppm concentration at a distance of2000 feet. 

The EPA ride specifies defauit values of wind speed, atmospheric stability class, and other parameters for the development of the offsite consequence analysis of worst-case scenarios. It also specifies the end point for the consequence analysis, based on the calculated concentration of toxic materials, the overpressure (1 psi) from vapor cloud explosions, and the radiant heat exposure for flammable releases (5 kW/m for 40 seconds). 

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