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CHEMICAL ENGINEERINGTRANSACTIONS 
 

VOL. 48, 2016 

A publication of 

 
The Italian Association 

of Chemical Engineering 
Online at www.aidic.it/cet 

Guest Editors:Eddy de Rademaeker, Peter Schmelzer
Copyright © 2016, AIDIC Servizi S.r.l., 
ISBN978-88-95608-39-6; ISSN 2283-9216 

A Novel Index Based Framework for Assessing Hazards of  
Toxic and Flammable Gaseous Releases in Process Plants 

Emilio Palazzi, Fabio Currò, Erika Lunghi, Bruno Fabiano* 

DICCA – Civil, Chemical and Environmental Engineering Department – Polytechnic School University of Genoa, Via Opera 
Pia, 15 – 16145 Genoa, Italy 
brown@unige.it 

Generally speaking, chemicals are the main source of fire, explosion and toxicity hazards. Notwithstanding 
technological development, enforcement of ATEX Directives and safety management system application, 
hazardous releases and following toxic dispersion or explosions in the process sector still claim lives and 
severe economic losses. Additionally, rather moderate releases of hazardous gases under semi-confined 
geometry are known to present a serious risk so that there is a need in the assessment of the maximum 
admissible gas build-up, in connection with adverse effects. For purpose of providing comprehensive warning 
of the hazardous nature of the considered gas and obtaining a simplified tool, we present a simple unified 
approach which, starting from the chemical-physical and hazardous properties of the released gas allows 
estimating both asphyxiation, fire/explosion and toxic exposure hazards.  

1. Introduction 

The inherent safety approach, which  can be applied also to consolidated processes aims at eliminating or 
reducing hazards, or exposure to them, or the chance of occurrence, by applying well known principles, e.g., 
“substitution” or ”intensification”. Indeed, inherently safer design and technical topics related to hazardous 
phenomena/properties of substances are recognized as prioritized research issues for the 21st century (De 
Rademaeker et al., 2014). However, raw materials in petrochemical plants (e.g. flammable and toxic 
hydrocarbons) are often impossible to be replaced by inherently safer materials, while the application of 
“intensification” in the downstream oil industry, by inventory reduction connected to changes in equipment and 
process design, is still limited as evidenced by accident statistics (Fabiano and Currò, 2012). Additionally, as 
new plants capacities are increasing and often by simple linear extrapolations of existing designs, the hazard 
size is increased, either in proportion to the necessary inventory increases, or over this threshold (e.g. piping 
inventory increases with capacity more than linearly and extended risk assessment approaches seem 
advisable (Milazzo and Aven, 2012). This implies that measures need to be adopted in order to adequately 
quantify release hazard and thereby to mitigate the risks. For example (Windhorst and Koen, 2001), 
considering ethylene plant, the higher value of individual risk increases to the power 1.33 of capacity and the 
risk is proportional to the square of fixed capital. Reasons are mainly connected to larger equipment and 
nozzle sizes resulting in larger release rates and mass released. Chemical releases are the main source of 
fire, explosion and toxicity related events. As reported in Mannan & Lees (2005), about two thirds of impacts 
were mainly initiated by explosion compared to fire, while toxicity exerts a determining role on the number of 
affected people, compared to fire and explosion. Many accidents have occurred in the past as a result of 
inadequate understanding of the post-release evolution or not-correct design of technical protection measures. 
Referring to these last issues, the year 2014 and 2016 mark respectively the 30

th and 40th anniversary of 
Bhopal and Seveso disasters: two notable chemical accidents connected with toxic release into the 
atmosphere, which caused severe and unparalleled damage to both the country and its neighbouring (Palazzi 
et al., 2015). For purpose of providing comprehensive warning of the hazardous nature of a broad range of 
gas and obtaining a simplified tool useful for risk assessment specific for chemical industry installations, we 
present a short-cut unified approach. Starting from the triangle approach, the paper further discusses simple 
hazard indices that can be obtained in analytical form under simplifying but conservative hypotheses. 

                               
 
 

 

 
   

                                                  
DOI: 10.3303/CET1648021

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Please cite this article as: Palazzi E., Curro' F., Lunghi E., Fabiano B., 2016, A novel index based framework for assessing hazards  of  toxic 
and flammable gaseous releases in the process industry., Chemical Engineering Transactions, 48, 121-126  DOI:10.3303/CET1648021  

121



2. Unified approach to release hazards  

In the event of a release, the knowledge of safety properties of the material is essential for estimating 
hazardous areas and set-up proper emergency and evasive actions. All humans exposed within the 
“consequence zone”, defined as areas where the gas concentration exceeds threshold values, are at risk of 
experiencing the adverse effects associated with the exposure to the material originally released in the 
environment. The purpose of this work is to consider release events and connected risk, possibly 
simultaneous, related to the different incident outcomes after containment has failed, namely asphyxiation, 
self-ignition, exposure to toxic substance, radiating heat exposure etc. A rapid approximated method of 
estimating asphyxiation, or toxicity, or flammability characteristics of ternary mixtures is developed starting 
from the hypothesis of homogeneous gas mixtures. The method can consider both nearly instantaneous and 
continuous release mode from a single component or binary mixture point source. Based on the hazardous 
concentration level, it is possible to attain a cautious and accurate knowledge of a particular compound’s 
hazardous region, by means of a ternary diagram depicting in a coordinate fashion of immediate readability: 
• the concentration region, Rp, where the release is inherently hazardous; 
• the concentration region, Rd, where the release, originally at non-hazardous conditions, may fall within the 

region Rp as a consequence of dilution and air entrainment; 
• the concentration range, R = Rp ∪ Rd, where the release is always potentially at risk; 

• the critical dilution, *dy , needed to attain the critical compositions, M
*( *** ,, 321 yyy ), at the boundaries of the 

inherently hazardous region, Rp. Any further dilution of the release P determines a reduction of the hazard, 
down to a reasonably acceptable level. 

2.1 Release characterization 

Scenarios connected to hazardous release clearly consider the logical chain: initiating event-loss of 
containment-hazmat release-effects-damage on targets. The short-cut method here discussed allows 
considering following release type/duration at the point source conditions and connected hazards: 
Single gas: O2 over oxygenated atmosphere formation 
 N2, CO2, etc. under oxygenated atmosphere formation 
 Cl2, NH3, CO2 toxic cloud formation  
 NH3, CH4, C2H6, C2H4, C3H8, etc. flammable cloud formation 
Binary gas mixture O2, with different gas e.g. NH3, CH4, C2H6, C2H4, C3H8 flammable cloud formation 
Duration Nearly instantaneous point source of a total given mass/volume;  
 Continuous point source at a continuous mass flow rate.  
The graphical representation considers that in a three dimensional perspective form, we can depict the 
hazardous limits of a ternary system representing the actual characteristics of the release into the atmosphere 
at constant pressure and temperature, as shown in Figure 1. Under these conditions and the working 
hypothesis of homogeneous mixture and release in air, since the sum of the three molar fraction terms 
(oxygen O, nitrogen N and gas G) equals 1, one can draw the different conditions by the projection of the 
three-dimensional plane, as described in the following chapters, so as to attain the composition evolution from 
the starting conditions taking into account the intrinsic chemical property of the material and the relevant 
hazards. 
 

 

Figure 1: Three dimensional composition of a ternary 
system. 

 

Figure 2: Composition of a ternary release P and 
evolving ideal dilution with air.

2.2 Air dilution of the release 

As amply known, hazard is not solely an inherent property of the chemical involved but it depends also on the 
conditions under which the release P evolves.  

122



The analytical formulae presented in the following are obtained considering the most general application of a 
ternary mixture release-air. Eqs (1)-(3) summarize the composition of a ternary mixture M as a function of air 
dilution, from the starting point source release composition, under the ideal assumption.  

y1 =  y1p yd + 0.21 (1-yd) (1) 

y2 =  y2p yd + 0.79 (1-yd) (2) 

y3 =  y3p yd (3) 

Being: 
yd = release dilution (v/v)  0<yd≤1;      yip= molar fraction of i-th component in the release P;                      
yia= molar fraction of i-th component in air A (e.g.. y1a= 0.21 ; y2a= 0.79). 
We can depict the release dilution in a ternary graph, obtained by simple projection of the three dimensional 
plane previously presented, as shown in Figure 2, identifying all three constituents of interest simultaneously, 
where P(y1p; y2p; y3p), A(y1a; y2a; 0),M(y1; y2; y3). It follows that the upon dilution the resulting gas mixture M 
∈ to the line PA (see Figure 2), or, in other words, the composition modifies from the point P(y1p; y2p; y3p) to 
the point A (0.21; 0.79; 0). 

3. Hazardous releases and critical dilution estimation  

The risk levels connected to a generic gaseous release, possibly characterized by different hazards 
(asphyxiation, inhalation, deflagration, etc.), clearly depends upon the interaction with the environment and the 
consequent dispersion, or build-up in semi-confined regions. As previously remarked the approach here 
outlined is based on the ideal assumption of perfect mixing and is based on following steps: identification of 
the “critical reference concentrations” for the different scenarios (health effect, fire/explosion); analytical 
calculation of the dilution required to attain the reference concentrations; graphical representation of contour 
map describing the concentration region, Rp, where the release is inherently hazardous; the concentration 
region, Rd, where the release, originally at non-hazardous conditions, may fall within the region at risk Rp as a 
consequence of dilution and air entrainment; the concentration range, R = Rp∪Rd, where the release is always 
potentially at risk. 

3.1 Single gas release 

First of all we discuss the rather simple event of a single gas release characterized by different potential health 
effect, different from combustion hazard. For the sake of simplicity, Table 1 summarizes the results obtained 
according to this scenario, by applying Eqs (1)-(3). From Table 1, it can be argued that the hazard connected 
to over-oxygenation extends until by dilution (the critical dilution is easily obtained by Eq(1)), it is attained the 

critical concentration 2501 .
* =y corresponding to health effect for man (K), so that in the ternary diagram the 

risk region is simply R = RK as shown in Figure 3. The boundaries of his region can be expressed as:  









≤≤
≤≤
=≥

≡
7500

7500

250

3

2

11

.

.
.*

y

y

yy

RR Kp  (4) 

Additionally, it must be pointed out that oxygen clearly plays a role determining maximum flame temperature 
and thermal power in given scenarios (e.g. pool fire), especially in enclosures (Vianello et al., 2012). The risk 
levels connected to an oxygen release, possibly in presence of other hazardous releases depends upon the 
interaction with the environment, including increased flammability by oxygen enrichment, and the consequent 
dispersion, or build-up in semi-confined regions (Palazzi et al., 2010), as it will be discussed in the following. 
Analogously, considering a potential suffocating release, hazard region is attained assuming as critical 

concentration 1701 .
* =y , corresponding to under-oxygenation risk (S) that can lead to asphyxiation. In this way 

we obtain the ternary diagram depicted in Figure 4, where it is noteworthy noting that the critical dilution value 

obtained by means of Eq(1), 190.* =dy  holds its validity for any release composition consisting of different non-
toxic compounds, in the absence of any oxygen content. In this case, the boundaries of the hazard region are 
obtained according to the conditions summarized in Eq. (5). 

123



Table 1: Single gas release P: hazardous scenario, calculated critical parameters to attain the corresponding 
boundaries of the hazard region R in a triangular diagram (G= gas; O= oxygen; N = nitrogen). 

Scenario Release P characterization Critical concentration
*y  Critical dilution *dy

Overoxigenated atmosphere Pure oxygen: P ≡ O (1; 0; 0) == Kyy 11
* 0.25 == dKd yy

* 0.051 

Asphyxiating atmosphere 
Pure nitrogen:P ≡ G (0; 1; 0) == Syy 11

* 0.17 == dSd yy
* 0.19 

Pure carbon dioxide: P≡ G(0; 0; 1) == Syy 11
* 0.17 == dSd yy

* 0.19 

Toxic release 

Pure carbon dioxide: P ≡ G (0; 0; 1) == Tyy 33
* 4·10-2 == dTd yy

* 4·10-2 

Pure ammonia: P ≡ G (0; 0; 1) == Tyy 33
* 3·10-4 == dTd yy

* 3·10-4 

Pure chlorine: P ≡ G (0; 0; 1) == Tyy 33
* 10-5 10

-5 
  









≤≤
≤≤

=≤≤
≡

10

10

1700

3

2

11

y

y

yy

RR Sp

.*

 (5) 

The case of toxic compound release the critical concentration is defined by a threshold value for the physical 
effect (T) corresponding for example to IDLH (Mannan and Lees, 2005) expressed as yT. In this case, the 

critical dilution value may be derived from Eq (3) by imposing Tyy 33 =
* . The corresponding ternary diagram is 

depicted in Figure 5, and the resulting boundaries of the hazard region are: 









≥
−≤≤
−≤≤

≡
*

*

*

33

32

31

10

10

yy

yy

yy

RR Tp  (6) 

3.2 Flammable gas 

Flammable gases and liquids are prevalent in today’s process and petrochemical sectors and their 
flammability characteristics are usually represented by index based on flammability limits data.  

 

Figure 3: Risk region belonging to overoxigenated 
atmosphere scenario (K). 

 

Figure 4: Risk region belonging to asphyxiating 
atmosphere scenario (S). 

 
Figure 5: Risk region belonging to toxic atmosphere 
scenario (T). 

Figure 6: Risk region belonging to flammable gas 
scenario (F). 

124



Theoretical and experimental research on flammability limits is still an up-to-date research topic, and a new 
method for estimating the flammability range of fuel-oxidizer mixtures, with or without inert gas was recently 
presented (Giurcan et al., 2015). Referring to the triangular diagram reproduced in Figure 6, the two different 
hazardous regions connected to flammability can be described by simple geometry:  

• Inherent hazardous release  Rp≡ triangle  HoLI; 

• Hazardous release upon air dilution  Rd≡ quadrangular  GHoIJ; 

• Resulting flammable mixture hazardous region in the ternary system: R ≡ Rp ∪ Rd. 
In order to obtain the formal definition of the release hazard, the knowledge of the compound flammability 
region as a function of fuel, oxygen and nitrogen concentrations is the starting point. 
We consider as a relevant example, the flammable limits summarized in Table 2 (Lewis and von Elbe, 1987), 
where, to the purpose of this simplified but conservative approach, we assume for each flammable compound: 

• a constant value of the lower flammable limit LFL corresponding to the minimum value:y3La ≅y3Lo = y3L; 
• a variable upper flammable limit UFL by linear dependence on the composition.  
Making reference to the already mentioned Figure 6, the analytical approach to a release characterized by 
fire/deflagration hazard requires calculating: 

• the equation of the line HoHa; 

• the intersection point, I, between the line HoHa and the line described by the equation y3 = yL; 

• the equation of the line AI. 
An account of these calculation follows.  
The line where the original release is diluted down to HFL, described by the equation HoHa, is given by: 

( ) ( ) ( )[ ] ( ) ( ) HoHaHoHaHoHaHo yyyyyyyyyy 1311323333 12101790 −−−=−=−− ..  (7) 
so that the hazardous region describing the inherent flammability conditions of the release can be expressed:  

( ) ( )[ ]






−−−≤
≥
≥

≡

HaHaHoHo

L

Fp

yyyyyy

y

yy

RR

333233

2

1

1790

0

.
 (8) 

Analogously to the previous approach, we can assume the “critical reference concentration” starting from the 
conservative values of LFL and UFL available in literature by a stepwise calculation. The dilution requirement 

can be determined in terms of “critical dilutions” *dy L and 
*
dy U, as follows. Starting from Eq (3) one can write:   

pd yyy 33=  (9) 

so that *dy L  is easily calculated, by utilizing the values of y3p and of y3 = y3L shown as an example in Table 3.

*
dy U can be obtained by substitution into Eq (7) of the corresponding values of y2 and y3 respectively given by 

Eq (2) and Eq (3). From the resulting Eq (10), it follows that if  y3p = 1 →  ydH = y3Ha; and  if  y3p = y3Ho→  ydH = 1  

( ) ( )[ ]HoHaHapHoHad yyyyyyy 333333 11 −+−−=  (10) 
At last, by substitution into Eq (9) of the values of ydH obtained by Eq (10), the resulting y3H is easily calculated. 

4. Discussion  

By applying the proposed method, in Table 3 we consider some selected flammable gases and some 
pertinent examples of different release composition P, namely (see Figure 6 for symbol explanation): 
P ≡ G pure flammable gas; P ≡ P1  corresponding to the midpoint of the line GH0; 
P ≡ H0  flammable gas at UFL; P ≡ P2  corresponding to the midpoint of the line H0L. 
Starting from the discussed theoretical approach and the triangle representation, rather simple hazard indices 
can obtained in analytical form under simplifying but conservative hypotheses.  
The quantitative indices of immediate applicability can include: critical distance; critical area; flammable 
mixture volume; man exposure and hazardous dose (Palazzi et al., 2014). In particular, the model output 
parameter considered of primary importance is the distance from the release point to the predicted critical 
concentration in the given hazardous scenarios (dilution concentration), defined as “effect distance”. An 
approximated approach can assume a neutrally buoyant dispersion, considering a neutral gas and the 
absence of considerable auto-refrigeration during release. The isopleths ground concentration (v/v) of a 
nearly-instantaneous release of given volume, of a continuous release of given rate can be obtained by as a 
first approach by the Gaussian model and corresponding dispersion coefficients by BNL (Pasquill and Smith, 
1983).  

125



Table 2: Lower and upper flammable limits of selected hydrocarbons in air and oxygen.  

Pure compound Air mixture Oxygen mixture 
 HFLy3Ha LFLy3La HFLy3Ho LFLy3Lo 
NH3 0.28 0.15 0.79 0.15 
CH4 0.15 0.053 0.61 0.051 
C2H4 0.32 0.031 0.80 0.03 

Table 3: Calculation of critical concentration and critical dilution for selected flammable releases. 

Release P characterization  Critical concentration Critical dilution 
y3c = y3L y3c = y3H ydc= ydL ydc= ydH 

G (0; 0; 1)        (pure NH3)    
P1 (0.40; 0; 0.60)   (NH3 and O2)   
Ho (0.79; 0; 0.21) (NH3 and O2)   
P2 (0.47;0;0.53) (NH3 and O2)  

0.15 
0.15 
0.15 
0.15 

0.150 
0.210 
0.610 
 

0.150 
0.168 
0.190 
0.319 

0.150 
0.261 
1.000 
 

G    (0; 0; 1) (pure CH4)  
P1     (0.31; 0; 0.69) (CH4 and O2)  
Ho    (0.61; 0; 0.39) (CH4 and O2)  
P2      (0.33; 0; 0.67) (CH4 and O2)  

0.051 
0.051 
0.051 
0.051 

0.280 
0.392 
0.790 
 

0.051 
0.063 
0.084 
0.154 

0.280 
0.438 
1.000 
 

G   (0; 0; 1) (pure C2H4)  
P1    (0.40; 0; 0.60) (C2H4 and O2) 
Ho   (0.80; 0; 0.20)             (C2H4 and O2)  
P2      (0.42; 0; 0.58) (C2H4 and O2)  

0.031 
0.031 
0.031 
0.031 

0.320 
0.436 
0.800 
 

0.030 
0.033 
0.038 
0.072 

0.320 
0.485 
1.000 
 

5. Conclusions 

For purpose of identifying release hazards and assessing over/under-oxygenation, toxic and fire/deflagration 
risk we present a simple approach and a graphical representation useful as a tool relative risk ranking and an 
indication of whether further study is warranted in a given context. Starting from the relevant information on 
the chemical physical properties of the gas, the method provides comprehensive warnings of the hazardous 
nature of a release. A short-cut dispersion modelling for continuous and instantaneous releases allows 
developing contour map describing the hazardous area following different gas releases. The framework here 
outlined may be used as a simple analytical tool to perform an approximate conservative evaluation and a 
preliminary screening for selecting cases where an in-depth safety analysis may be needed. 

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