Assessing The Explosion Risk Of The Equipment In The Refrigeration System By Study And Apply The Pasquill-gifford Puff Model - 2007

ASSESSING THE EXPLOSION RISK OF THE EQUIPMENT IN THE REFRIGERATION SYSTEM BY STUDYING AND APPLYING THE PASQUILL-GIFFORD’S PUFF MODEL Ly Ngoc Minh – Director of Industrial Safety and Health Center Faculty of Chemical and Environmetal Engineering Hochiminh City University of industry (HUI) [email protected] ABSTRACT The study introduces the method to calculate the dispersion of the refrigerant in the air for assessing the environmental risk caused by the explosion of the equipment in the refrigeration system by studying and using the instantaneous source model (puff model); and ability to apply them in order to calculate the dispersion of the hazard gas in the air to predict the impact of the environmental risk in using the refrigeration and air conditioning in Vietnam. Result of our study can use for assessing the environmental risk or predicting the consequence of the environmental risk in using the refrigerant or another hazardous chemicals. I.

INTRODUCTION

Using the refrigerant or another hazardous chemicals can caused some environmental risks, such as: leaked from the tanks, rupture the tanks ... So we have to calculate the dispersion of the hazard gas in the air for assessing that environmental risk. One of the ways is using the mathematical models to do this work. This is one of the models - the instantaneous source model (puff model) as to be shown in Fig.1, Fig. 2 and Fig.3 Mexico City, 1984 LPG Bleve, VCF 650 mŕtvych, 6400 zranených strata 31 300 000 $ Terminál

Fig. 1: Image of the ruptuered risk of the LPG tanks in Mexico City in 1984

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Fig. 2: Image of the domino effect of the eplosion risk of the LPG tanks in Mexico City in 1984

Fig.3: Image of the fire ball upon the explosion risk of the LPG tank II. BASIC THEORY A puff is formed as a result of an instantaneous spill (see Fig. 4). Puff scenarios include catastrophic of bottles, drums, or vessels containing liquids above their normal boiling points. Upon rupture, a fraction of the liquids vaporizes. The flashing vapor is a result of the excess sensible heat vaporizing a fraction of the liquid. This flashing vapor may also entrain some of the remaining liquid.

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Figure 4: Instantaneous source puff model The resulting puff of vapor and the entrained liquid are dispersed in and carried with the surrounding wind. Downwind concentration (vapor only) is computed using Eq. 1  1 y −  exp C ( x, y , z , t ) = 3  (2π ) 2 σ xσ yσ z  2  σ y Q m*

   

2

    exp − 1  z − H r    2  σ z 

  

2

 1 z + H  r  + exp −   2  σ z 

  

2

    

(Eq.1)

Where: • C(x, y, z, t) is the concentration of the gas as a function of x, y, z at time t (mass/length3), • x, y, z are distances from the source (length), • Q*m is the instantaneous source (mass), • σ y and σ z are dispersion coefficients (length), given in Fig. 2 and Fig.3; The stability classes for the puff model are given in Table 1. • u is the wind velocity (length/ time), and • Hr is the height of the release (length). The computed concentrations are for positions projected from the center of the puff as it drifts downwind at the speed of the surrounding wind. Since the puff moves at the same speed as the surrounding wind, the center of the puff is followed using Eq. 2 x = ut (Eq. 2) Where: • x is the center of the puff and downwind from the source (length), • u is wind speed (length/ time), and • t is time after the source is released (time). Set z = 0 to estimate downwind ground – level concentration. The center concentration for a ground level release is estimated by setting y = z = Hr = 0:

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C ( x,0,0, t ) = C (ut ,0,0, t ) =

Qm*

(2π )

3 2

(Eq. 3)

σ xσ y σ z

Figure 5: Horizontal dispersion coefficients for Pasquill – Gifford puff model

Figure 6: Vertical dispersion coefficients for Pasquill-Gifford puff model

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The dispersion coefficients are function of the atmospheric stability classes and the downwind distance from the source. These stability classes are shown in Table 1. Table 1: Atmospheric stability classes for the Pasquill-Gifford dispersion model Wind speed (m/s) <2 2-3 3-5 5-6 >6

Day radiation intensity Strong Medium Slight A A-B B C C

A-B B B-C C-D D

B C C D D

Night cloud cover Calm and Cloudy clear E F D E D D D D

Note: Stability classes for plume model (A, B, C, D, E, F …) compared to stability for puff model (unstable, neutral, and stable): A, B (unstable), C, D (neutral), E, F (stable). III.

CONCLUTION

Result of the study can use for assessing the environmental risk or predicting the consequence of the environmental risk in using the refrigerant or another hazardous chemicals. REFERENCES 1. Joseph F. and B. Diane Louvar; Health and environmental risk analysis; Prentice Hall, Inc., 1998. 2. Daniel A. Crowl/ Joseph F. Louvar; Chemical Process Safety: Fundamentals with Applications; Prentice Hall, Inc., 2001.

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