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Journal of Loss Prevention in the Process Industries 26 (2013) 839e850

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Journal of Loss Prevention in the Process Industries

journal homepage: www.elsevier .com/locate/ j lp

Comparative study of the methodologies based on Standard UNE60079/10/1 and computational fluid dynamics (CFD) to determinezonal reach of gas-generated Atex explosive atmospheres

J. Telmo Miranda a,*, E. Muñoz Camacho b, J.A. Fraguela Formoso c,J. de D. Rodríguez García d

aDepartamento de Ingeniería Agroforestal, EPS, Universidad de Santiago de Compostela, 27002 Lugo, SpainbDepartamento de Ingeniería Industrial II, EPS Ferrol, Universidad de A Coruña, Ferrol, SpaincDepartamento de Ingeniería Naval y Oceánica, EPS Ferrol, Universidad de A Coruña, Ferrol, SpaindDepartamento de Ingeniería Industrial, EUP Ferrol, Universidad de A Coruña, Ferrol, Spain

a r t i c l e i n f o

Article history:Received 26 September 2012Received in revised form30 January 2013Accepted 24 February 2013

Keywords:Classification and extent of hazardous areasStandard UNE 60079/10/1CFD modellingATEX explosive atmosphere

* Corresponding author. Tel.: þ34 982824161; fax:E-mail addresses: javier.telmo@usc.es, jtemo1@gm

eumucam@udc.es (E. Muñoz Camacho), jafraguela@ujdedios@cdf.udc.es (J.deD. Rodríguez García).

0950-4230/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.jlp.2013.02.015

a b s t r a c t

This study presents two methodologies which can be used to determine the classification and extendedarea of hazardous zones caused by gas, vapours and mists. The first is based on UNE 60079/10/1:Electrical apparatus for explosive gas atmospheres e Part 10: Classification of hazardous areas, whilst thesecond is developed on the basis of Computational Fluid Dynamics (CFD) using the FLUENT softwareapplication. Both methodologies were applied in the same case study of differing leakage componentsfrom a dairy farm steam boiler room fuelled by liquid natural gas (LNG).

The results obtained show that CFD methodology is a powerful tool with a significant capacity fordetermining the size of an explosive atmosphere for a broad range of exhaust sources. This methodologyoffers more conservative results than those obtained from the analytical methodology recommended inStandard UNE 60079/10/1. Results obtained using CFD are more useful in enabling the study of turbu-lence phenomena, boundaries, and diverse initial and contour conditions.

In contrast, the Standard UNE 60079/10/1 methodology is less conservative and aims at determiningthe hypothetical volume Vz of the explosive atmosphere. This volume is a measurement of the ventilationefficiency which is in turn proportional to a massive gas release through an exhaust source divided by thenumber of air changes in the enclosed area.

From the results obtained, it can be confirmed that Standard UNE 60079/10/1 should be revised.� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Flammable substances are manipulated or stored during verydifferent production processes that take place in various sectors.Agriculture is included in these, as well as the textile, chemical,recycling, biomass energy and petrochemical industries. Facilitiesare designed, operated and maintained to prevent flammable ma-terials from leaking and to limit the dimensions of these hazardouszones. This objective should be carried out for all normal andabnormal operating conditions. Therefore, it is important toexamine the components used in such processes and also the

þ34 982224904.ail.com (J. Telmo Miranda),

dc.es (J.A. Fraguela Formoso),

All rights reserved.

systems from which a flammable substance may leak. Conse-quently, diverse modifications can be made in the design of thesesystems in order to reduce leakage.

Directive ATEX 1999/92/EC (1999) establishes the basic pro-visions needed to provide greater protection and safety for theworkers exposed to the risks of explosive atmospheres. Its coun-terpart in Spanish Law is Royal Decree 681/2003 (2003), which hasthe same aim as the European Union directive. In this legislation,hazardous zones and environments where gas-related explosionsmay occur are analyzed and classified. Some of the factors takeninto account include the flammability characteristics of the gas orvapour, the ignition energy and the temperature. The zone typedepends primarily on the extended area of exhaust and ventilation.The zones are classified, according to Appendix I of Royal Decree681/2003 (2003), as: zone 0 (explosive atmosphere presentcontinuously, frequently or for a long period of time), zone 1 (an

J. Telmo Miranda et al. / Journal of Loss Prevention in the Process Industries 26 (2013) 839e850840

occasional formation of an explosive atmosphere) and zone 2(formation of an explosive atmosphere in normal operation notlikely and if it does occur it is short-lived).

A relevant process in controlling explosive atmospheres is thecorrect classification of the zones and the adequate determinationof their size. This depends on the distance calculated before theexplosive atmosphere has dispersed into the air below the lowerexplosive limit (LEL). In order to achieve this goal, two methodol-ogies are described below: one based on UNE 60079/10/1 (2009),and the other on computational fluid dynamics (CFD).

The first steps in establishing a model for an explosive atmo-sphere were carried out thanks to the work done by Hjertager(1982, 1984, 1986, pp. 1303e1350, 1993). Models were proposed intwo and three dimensions to define flow behaviour, combustionand the effects of turbulence influenced gas explosions.

Commercially available from 1996, FLACS (FLame ACcelerationSimulator) is a software package which is based on computationalfluid dynamics. The background of this method as well as someexamples and applications have been published in the Gas Explo-sion Handbook (Bjerketvedt, Bakke, & vanWingerden, 1997). Threeyears later, a work on several cases involving different situationsand geometries validated with FLACS was presented at the ThirdEuropean Meeting on Chemistry, Industry and the Environment(Hansen, Storvik & Wingerden, 1999). This software was developedand improvedwith the additions of various authors as well as thoseof Hjertager, Arntzen (1998) and Van Wingerden, Hansen, andLemousy (1998).

More recently, Ledin (2002) conducted an overview of the latestand most noteworthy models used for gas explosions. An analysisof the whole range of methods based on computational fluid dy-namics e from the most simple to the most complex e has beensummarized. In 2008, an interesting study was completed at theHealth and Safety Laboratory, Harpur Hill, England (Ivings et al.,2008) in which a description of how the classification of ATEXzones on the basis of both Directive 1999/92/CE and Standard UNEEN 60079/10 (2004) was carried out. In order to minimize sec-ondary leaks, ventilation methods are checked along with aspectssuch as the calculation of natural ventilation in confined spaces. Thestudy then revised different experiments which were carried out tovalidate the analysis of leaks through computational fluid dynamics(CFD) methodology. This methodology is also used to classify ATEXzones related to secondary gas leaks in confined spaces and openenvironments.

The purpose of the work at hand is to present and compare twomethodologies for classifying ATEX zones for gases: quantitativemethodology from Standard EN/60079/10/1 (2009) and CFD

Table 1Classifying Atex zones according to ventilation and exhaust level.

Exhaust level Ventilation

Level

High

Availability

Very good Good Mediocre

Continuous (ZONE 0 ED)Non-hazardousa

(ZONE 0 ED)Zone 2a

(ZONE 0 ED)Zone 1a

Primary (ZONE 1 ED)Non-hazardousa

(ZONE 1 ED)Zone 2a

(ZONE 1 ED)Zone 2a

Secondaryb (ZONE 2 ED)Non-hazardousa

(ZONE 2 ED)Non-hazardousa

Zone 2

Note “þ” means surrounded by.a Zone 0 ED, 1 ED or 2 ED indicates a theoretical zone of imperceptible dimensions unb Zone 2, created by a secondary leak, may be exceeded in zones that have contact withc It will be zone 0 if ventilation is weak and the leak is such that the explosive gas atm

absence of ventilation).

methodology. These two methodologies are successfully applied ina case study on a dairy boiler room in a dairy industry.

2. Material and methods

The two methodologies used to determine the size of theexplosive atmospheresdUNE 60079/10/1 and CFDdare brieflydescribed.

2.1. Methodology based on Standard UNE 60079/10/1

UNE 60079/10/1 explains the essential criteria that are neededto assess the risk of explosion and provides guidance for the designand operating parameters needed to reduce these risks. Hence, thisstandard allows us to classify hazardous areas (according to therequirements of R.D. 681/2003) where risks exist because of thepresence of flammable gas or vapour mixed with air, under normalatmospheric conditions. The calculation of the risks involved willbe important in order to select and properly install the equipmentthat will be used in these locations. In some cases, the standardallows us to determine the type of a hazardous area easily,depending on the degree of the emission source, the degree ofventilation and the availability as indicated in Table 2. The standardalso provides examples that describe the general philosophy usedto classify hazardous areas and that aim to expose typical resultsthat can be obtained when using the guidance and standard pro-cedures in different situations.

The objective of UNE 60079/10/1 is to determine the theoreticalvolume Vz representing the volume in which the average concen-tration of flammable gas or vapour will be between 0.25 and 0.5times the LEL. Vz stands for the volume of the flammable shroudfrom the exhaust source, although it is not normally equivalent tothe volume of the hazardous location. The geometry of this theo-retical volume is not usually defined. However, it tends to take aspherical form with respect to the exhaust zone. Moreover, it willbe influenced by ventilation conditions. The methodology pre-sented here entails the following steps:

1. Determination of the exhaust rate: G.2. Calculation of the minimum ventilation flow rate needed to

prevent the formation of an explosive atmosphere: QVmin.3. Determination of the number of air changes: C.4. Calculation of volume Vz (m3).5. Determination of zone type (zone 0, zone 1 or zone 2) using

Table 1.

Medium Low

Very good Good Mediocre Very good, good or mediocre

Zone 0 Zone 0þ

Zone 2Zone 0þ

Zone 1Zone 0

Zone 1 Zone 1þ

Zone 2Zone 1þ

Zone 2Zone 1 or Zone 0c

Zone 2 Zone 2 Zone 2 Zone 1 equalZone 0c

der normal conditions.leaks classified as continuous or primary. In this case the greater size should be taken.osphere is present in a virtually permanent fashion (a situation very similar to the

Table 3Relation between exhaust rate and zone and frequency of ATEX explosive atmo-sphere formation.

Exhaust rate ATEXzone

Likelihoodof explosive atmospherein 365 days

Time span (hours/year)

Continuous 0 P > 10�1 >1000Primary 1 10�1�P > 10�3 Between 10 and 1000Secondary 2 10�3�P > 10�5 Between 0.1 and 10

J. Telmo Miranda et al. / Journal of Loss Prevention in the Process Industries 26 (2013) 839e850 841

Classifying zones according to ventilation, with variables that arefixed as the degree of defined exhaust, as well as the ventilationlevel and availability (Tables 2e4).

6. Calculation of zone size based on Vz.

An overview of this quantitative methodology is presented inFig. 1

From the theoretical volume Vz it is possible to determine thefictitious sphere radius of the extent of the zone to be classified(Fig. 2).

2.2. CFD methodology

The methodology proposed in this section is based on compu-tational fluid dynamics (CFD). The model has been developed usingFLUENT (FLUENT 6.3, 2007), a commercial software applicationfrom Fluent Incorporated. FLUENT is a worldwide software leaderin terms of CFD applications to solve problems involving fluid flowiterations and energy balances. NaviereStokes equations for massand momentum conservation are used as generalized balances.Additionally, heat transfer, mixed species, reaction or speciesconservation equations are solved and turbulence modelsequations are provided.

2.2.1. NaviereStokes equation: mass conservationThe general equation for mass conservation (also known as the

continuity equation) is defined as:

vr

vtþ vðruiÞ

vxi¼ Sm (1)

The term Sm contains additional mass from gas changes.For 2D symmetrical geometries, the continuity equation is given

by:

vr

vtþ vðruxÞ

vxþ vðrurÞ

vrþ rur

r¼ Sm (2)

Whereby x is the axial co-ordinate, r is the radial co-ordinate, ux isaxial speed and ur is radial speed.

Table 2Ventilation level and availability for inside spaces.

Inside spaces

Ventilation availabilityVery good Only applied when there is a back-up artificial

ventilation system or shut-down process when ventilation fails.Good Generally used for building interiors with natural ventilation

or an extraction/ventilation system without back-up.Bad Rarely given. There have to be many obstacles and the ventilation

must be very poor to be classified as “bad”.

Ventilation levelHigh Parameter reserved for ventilation/extraction systems

guaranteeing a determined number of air changesto prevent an explosive atmosphere occurring.

Medium Commonly applied and often in respect of inside spaceswith natural ventilation and air change number/hour over onechange/hour (great majority of rooms with closed windowsand doors).

Low Only when ventilation does not reach 300 changes/hour,as is the case inside drains or sumps and other zoneswith numerous obstacles.

2.2.2. NaviereStokes equation: momentum conservationThe equation for conservationmomentum, in direction i and in a

non-accelerated reference is given by:

vðruiÞvt

þ v�ruiuj

�vxi

¼ �vpvxi

þ vsijvxj

þ rgi þ Fi (3)

In balance (5) r is static pressure, sij is tensile stress, gi is theforce of gravity and Fi is a component of external forces: these couldinclude phase interaction, centrifugal and Coriolis forces. The stresstensor for a Newtonian fluid is defined as:

sij ¼"m

vuivxj

þ vujvxi

!#� 23mvu1vx1

dij (4)

Here m is the molecular viscosity; the second term on the right isdue to the volume dilatation. For symmetrical 2D geometries, thecontinuity equation is given by:

vðruxÞvt

þ1rvðrrruxuxÞ

vxþ1rvðrrruruxÞ

vr

¼ �vpvx

þ1rv

vx

�rm�2vuxvx

�23ðV$uÞ

��þ1rv

vr

�rm�vuxvr

þvurvx

��þFx

and

vðrurÞvt

þ1rvðrrruxuxÞ

vxþ1rvðrrruruxÞ

vr¼ �vp

vrþ1rv

vx

�rm�vurvx

þvuxvr

��

þ1rv

vr

�rm�2vuxvx

�23ðV$uÞ

���2m

urr2

þ23m

rðV$uÞþr

u2zrþ Fr

(5)

where:

V$u ¼ vuxvr

þ vurvx

þ urr

(6)

2.2.3. Standard k � 3 turbulence modelThis model (the simplest complete turbulence model) was

proposed by Launder and Spalding (1974) and further developed byArntzen (1998). It is a semi-empirical model inwhich its equation isderived from Reynolds-Averaged NaviereStokes (RANS) Equations.The major assumptions in this model are that it deals with a wholly

Table 4Classifying ATEX zones.

Gases, vapours or clouds Explosive atmosphere

Zone 0 Constantly present, for prolonged periodsor on an frequent basis

Zone 1 Likely to form on an occasional basisZone 2 Unlikely in normal working conditions.

When occurring of short duration only

Fig. 1. Process for obtaining theoretical volume Vz.

J. Telmo Miranda et al. / Journal of Loss Prevention in the Process Industries 26 (2013) 839e850842

turbulent flow and negligible viscosity effect. Moreover it can onlybe used for wholly turbulent flows.

Using RANS, the variables for solving the NaviereStokes equa-tions are decomposed in component ui and fluctuations u0j. Theresult of this process when applied to the NaviereStokes equationsis:

vðruiÞvt

þv�ruiuj

�vxi

¼�vpvxi

þ v

vxj

"m

vuivxj

þvujvxi

!��23mvu1vx1

�#

þv��ru0iu

0j

vxj

(7)

Speeds and other solutions are now represented with thecalculation time value. Turbulence effects are represented by

Fig. 2. Theoretical volume of atmosp

“Reynolds Stress” ð�ru0iu0jÞmodelled by the Bosussinesq hypothesis

(1868):

�ru0iu0j ¼ mt

vuivxj

þ vujvxi

!��23mvu1vx1

�� dij (8)

Turbulent viscosity, mt, is defined with k, the energy from tur-bulent kinetic energy and 3, the dissipation factor, and comes from:

mt ¼ rCmk2

3(9)

This equation has been adapted from the following trans-portation equations:

here around the exhaust source.

Fig. 3. Gas pipe.

J. Telmo Miranda et al. / Journal of Loss Prevention in the Process Industries 26 (2013) 839e850 843

vðrkÞvt

þ vðruikÞvxi

¼ v

vxi

��mþ mt

sk

�vkvxi

�þ Gk þ Gb � r3 (10)

vðr3Þvt

þ vðrui3Þvxi

¼ v

vxi

��mþ mt

sk

�v3

vxi

�þ G13

3

kfGkþ ð1� C33ÞGbg

� C23r32

k(11)

where Gb is defined as the generation of turbulent kinetic energy.

2.2.4. Energy equationThe energy equation in FLUENT is solved with static tempera-

ture transportation. Temperature equation is obtained from theenthalpy, and it is only solved in special cases, when temperature isa dependant variable. The enthalpy equation is defined as:

v rhð Þvt

þ v ruihð Þvxi

¼ v

vxikþ ktð Þ vT

vxi�vPjhjJj

vxiþ vp

vtþ sikð Þeff

vuivxk

þ Sh

(12)

In this equation, Sh, the heat from chemical reaction, is includedand Kt is the heat exchange kt, defined as the conductivity from theturbulent transport:

kt ¼ CpmtPrt

(13)

Enthalpy is defined as:

h ¼Xj

mjhj (14)

with

hj ¼ZTTref

Cp;j dT (15)

where mj is the mass of the species j.

Fig. 4. Manual cut-off valve.

2.3. Description of the plant studied: the boiler room of a dairyplant

In this work, the boiler room of a dairy plant has been studied.The boiler room consists of a rectangular enclosure with a surfacearea on the floor plan of 99 m2 (9 m � 11 m) and a volume of352m3. It holds two steam boilers fuelled by natural gas and using aforced-air burner. They produce 6000 kg/h of steam to serve thedairy plant’s energy requirements. The site has two entrance/exits.The one at the main entrance leading outside has an area of 2.64 m2

(1.20 m � 2.20 m). There is also another doorwaydwith the samedimensionsdin the boiler area that opens into a room adjacent tothe compressors.

The burner circuit lines are symmetrical for both boilers. Themain pipe (Fig. 3), leaves the liquid natural gas LNG tank and passesinto the boiler room supplying the burners for these boilers. Theyhave amanual cut-off valve (Fig. 4) throughwhich the burner circuitline passes (Fig. 5). After being pre-heated, the gas travels through aseries circuit with a manual cut-off valve, a gas pressure reductionvalve, a filter, a gas pressure control valve and an automatic cut-offvalve. From this valve, two lines branch off: the top one is for thepilot light and the lower one goes through fuel inlet of the burner. Ascheme of the described boiler room is presented in Fig. 6.

In order to compare the two proposed methodologies (CFD andStandard UNE 60079/10/1) a quintuple gas leak with five possiblepores has been considered. One is a 1mm2 pore located in themainvalve inlet in the boiler room, and the other 4 pores are 2.5 mm2

each, and are distributed between the two lines of the boilerburner. The pore sizes were obtained from the practical guideClassifying for Explosive Atmosphere Zones (García & Frances, 2005).

3. Results and discussion

In this section the results of applying the two proposed meth-odologies to the boiler room described in Section 2.3 are presented.

Fig. 5. Burner lines.

Table 5Characteristic parameters of methane gas (CH4).

Methane CH4

LEL 4.4% 0.0350 kg/m3

ESL 17% 0.2048 kg/m3

Density relative to air 0.554Molar mass (kg/kmol) 16.04Gamma co-efficient 1.31Volume mass of liquid (kg/m3) 415Specific heat at ambient temperature Ta, Csl (kJ/(kg K)) 2.226Latent heat from steam at Tb, Clv (kJ/kg) 510.00Gas diffusion co-efficient Cd (m2/h) 0.074Boiling temperature Tb (�C) �161.4Steam pressure (Pv) at 20 �C (kPa) 25456.8Temperature group and class IIA T1CAS number 74-82-8

J. Telmo Miranda et al. / Journal of Loss Prevention in the Process Industries 26 (2013) 839e850844

3.1. Application of the methodology based on Standard UNE 60079/10/1 to the boiler room case study

3.1.1. Identification of flammable substancesThe first step is to identify the flammable substances. As already

mentioned, the fuel used is liquid natural gas composed of 92% ofmethane; the remaining 8% is composed of propane, butane and anodour additive. To simplify calculations, all the gas is considered tobe/as if they were methane. The characteristics of this gas aresummarized in Table 5.

3.1.2. Identification of leak sourcesPossible leakage sources are the different elements along the

fuel line leading to the burner: flanges, flanged valves, instrumen-tation threads, and filters with flanges at both ends.

3.1.3. Degree of leakageThe primary sources for existing leaks provide the degree of

leakage: the inlet valve for the outside gas and burner lines (themanual cut-off valve, gas pressure adjustment valves, the filter, theautomatic gas pressure control valve and the automatic cut-offvalve). None of the existing leaks may be deemed secondary orcontinuous.

3.1.4. Influence of all leaksAs there are six elements with primary leaks, for further cal-

culations, the fresh air flow QVmin has been calculated by adding the

Fig. 6. Distribution of the lines to the pilot light and burner on one boiler.

three greatest values (the manual valve, the pressure control valveand the filter).

3.1.5. Calculation of leakage rateThe values of maximum losses for safety valves and for the other

elements in the boiler room were obtained from the Standard API527:07-1991 and UNE 202007 IN (2006). Thus for a working tem-peratureof40 �C (oneof the least favourable cases), the leak ratesare:

- Manual cut-off valve (gas inlet line): 2.08 � 10�8 kg/s.- Manual cut-off valve (burner line): 2.08 � 10�8 kg/s.- Pressure control panel: 2.08 � 10�8 kg/s.- Pressure control valve: 5.20 � 10�8 kg/s.- Gas filter: 2.08 � 10�8 kg/s.- Automatic pressure cut-off valve: 1.04 � 10�8 kg/s.

Another possibility is to consider the manual cut-off gas inletvalve and obtain the global value for the other five elementscomprising the devices used along the burner line:

- Manual cut-off valve (gas line inlet): 2.08 � 10�8 kg/s.- Burner: 3.64 � 10�8 kg/s.

3.1.6. Definition of degree and availability

- Ventilation (type): natural.- Degree: medium.- Availability: high.- Ventilation inefficiency fv: 4 (fv is the efficiency of ventilation inthe dilution of explosive gas atmospheres with a value rangingfrom f: 1 (ideal situation) to fv: 5 (air flow with difficulties dueto obstacles).

3.1.7. Hazardous zone typeAccording to Table 1, the primary leaks with a medium degree of

ventilation and high availability establish the classification of zone1 for hazardous areas.

3.1.8. Hazardous zone sizeStarting with the QVmin, value obtained by adding the highest

values, Eq. (16) is applied. A ventilation factor fv equal to 4 is takeninto consideration. The explosive atmosphere volume Vz is calcu-lated, in this way and corresponds to a sphere with a radius as seenin Eq. (17):

Vz ¼ fv$QVminC

(16)

Table 6Results obtained for the boiler room considering burner line elements.

Leak Leakage rate G (kg/s) Leakage degree Safety factor k QVmin (m3/s) Variations Vz (m3) Sphere radius in m

Manual valve (inlet) 2.08 � 10�8 Primary 0.25 2.9 � 10�6 1.1 � 10�4 0.11 0.30Manual valve burner line 2.08 � 10�8 Primary 0.25 2.9 � 10�6 1.1 � 10�4 0.11 0.30Pressure control valve 5.20 � 10�9 Primary 0.25 7 � 10�7 1.1 � 10�4 0.026 0.19Pressure control setup 2.08 � 10�8 Primary 0.25 2.9 � 10�6 1.1 � 10�4 0.11 0.30Filter 2.08 � 10�8 Primary 0.25 2.9 � 10�6 1.1 � 10�4 0.11 0.30Automatic valve 1.04 � 10�8 Primary 0.25 1.01 � 10�6 1.1 � 10�4 0.036 0.20Total* Primary 0.25 8.7 � 10�6 1.1 � 10�4 0.33 0.43

* To determine Qvmin and Vz are taken into account the three major leaks of G.

J. Telmo Miranda et al. / Journal of Loss Prevention in the Process Industries 26 (2013) 839e850 845

r ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi�

Vz�

3

s; r ¼ radius of sphere (17)

ð4=3$pÞ

The values obtained for the manual inlet can be found in Table 6.Since the burner circuit is symmetrical, the same values areconsidered to be true for the other burner line. As mentionedabove, it is also possible to calculate the leak for the whole burnerline rather than for each of the five independent elements formingit. The global leakage rate appears in Table 7 along with the othercalculations. In this table, one can observe that the value obtainedfor the radius of the sphere when the whole burner line isconsidered is lower than the one obtained when each element isdealt with individually. For safety reasons, the value that should beused from Table 5 is 0.43 m. This value can be estimated to be0.50 m.

Since methane gas is lighter than air, for vertical distances, thevalue considered was twice than that used for other directions(1 m). Therefore, the resulting zone classification is:

� Zone 1: 0.5 m around the cut-off valve, filters and controlssetup, although 1 m should be applied to vertical distancesabove these elements.

� Zone 2: 0.5 m around zone 1, but also 1 m for vertical distances.

In Figs. 7 and 8 a representation of the ATEX classification zonesfor the elements is shown.

As an additional preventative measure, it is advisable to definethe entire volume surrounding the boiler as zone 2. This decision isalso essential when considering possible ignition sources that maybe found in the zones under study.

3.2. Application of the CFD methodology to the boiler room casestudy

The boiler room is enclosed area and measures 9.9 m � 11 m; ithas a maximum height of 7 m while the minimum height is 4 m.The installation has two grilles for natural ventilation: one locatedat the entrance and the other at the door to the storeroom con-taining the cold storage facility for the plant. The ventilation surfacearea is 0.45m2 and the ventilation flow for a speedmeasuredwith a1 m/s Kestrel k3000 Thermo Hygrometer is 0.45 m3/s.

As mentioned earlier, one pore is the main entry valve locatedin the enclosed area (see Fig. 4). As for the other four, two aresituated on each burner line for both boiler 1 and 2. Thus, the leakon the main valve is 1 mm2, while in each burner line there are

Table 7Boiler room values when burner considered as single element.

Leak Leakage rate G (kg/s) Degree of leakage Safety factor k

Burner 2.43 � 10�8 Primary 0.25

two 2.5 mm2 leaks: one from the pressure control valve and theother from the automatic valve. For simplicity, the geometry hasbeen studied in two dimensions and the horizontal scope of themethane gas leak was assessed using the elements referred toearlier. With this approach, the comparison with the methodologyproposed in UNE 60079/10/1 is possible. In Fig. 9, the fine meshmodel of the boiler room produced by the GAMBIT (GAMBIT 2.3,2007) application with the imposed contour conditions has beenpresented.

The next step is the determination of the speed at which the leakis produced and at which the gas escapes into the area beingexamined. For this task, it is necessary to calculate the emission rateof the leak in kg/s. The leakage rate is obtained using the followingformula:

G ¼ 3:95$a$�M

�P � 105

�T

�0:5(18)

whereby:

� G ¼ mass flow (kg/s).� A ¼ transversal area (m2).� M ¼ molecular mass (g/mol).� P ¼ pressure originating from the leak (Pa).� T ¼ temperature (K).

For the 2.5 mm2 pore leak, the following leakage rate isobtained:

G ¼ 3:95$ð0:0025Þ2$�16

ð183;000� 10;000Þ293

�0:5¼ 1:7$10�3 kg=s (19)

For the 1 mm2 pore emission size, the result is:

G ¼ 3:95$ð0:001Þ2$�16

ð183;000� 10;000Þ293

�0;5¼ 2:6$10�4 kg=s (20)

The next task is the characterization of the leak source byworking with the following equations.

The Choquet or non-Choquet Condition if the following equa-tion is met:

QVmin (m3/s) Variation Vz (m3) Sphere radius in m

3 � 10�6 1.1 � 10�4 0.11 0.30

Fig. 7. ATEX classification zones for the manual inlet valve.

J. Telmo Miranda et al. / Journal of Loss Prevention in the Process Industries 26 (2013) 839e850846

�PsourcePamb

�<

�kþ 12

�KðK�1Þ�1

(21)

ChoquetEnvironment0G¼ C$A$

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffik$r$P$

h�2

kþ1

�ðKþ1ÞðK�1Þ�1ivuut

(22)

Non Choquet Environment0G ¼ C$A$

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2$r$P$

�k

k� 1

�h�PaP

�2,K�1

��PaP

�ðKþ1ÞK�1ivuut (23)

with:

� G ¼ mass flow (kg/s).� C ¼ non-dimensional discharge co-efficient ¼ 0.72.� A ¼ discharge hole area (m2).� k ¼ gas Cp/Cv.� Cp ¼ specific heat of gas at a constant pressure.� r ¼ real density of gas at P and T (kg/m3).� P ¼ absolute upstream pressure at (Pa).� Pa ¼ absolute environmental pressure (Pa).� R ¼ universal gas law constant ¼ 8.314 kJ/mol K.� T ¼ temperature (K).

For ideal gases Cp ¼ Cv þ R with Cp ¼ 35.552 kJ/kmol K.

Thus; k ¼ CpCv

¼ 35:55227:238

¼ 1:305 y r ¼ P$MR$T

¼ 1:83$105$16$10�3

8:314$298¼ 1:182 kg=m3

Reconsidering (21):

�P1

�<

�1:305þ 1

2

��1:3050:305

�0Pðchoquet� non choquetÞ

¼ 183;000 Pa

(24)

Depending on whether a subsonic or sonic regime is present,either Eq. (22) or Eq. (23) has been used. Using Eq. (24), with Machspeed equal to one, the pressure obtained is 183.000 Pa orapproximately 1.83 absolute bar, and this value is close that of thegas inlet pressure in the boiler roomwhich is 2 bar. Density values,heat capacity at constant pressure and other data may also befound. As it is already established in the Choquet environment, Eq.(22) is used. For the 2.5 mm leak, the escape rate is:

Fig. 8. ATEX classification zones for a boiler burner line.

J. Telmo Miranda et al. / Journal of Loss Prevention in the Process Industries 26 (2013) 839e850 847

G ¼ C$ð0:0025Þ2$

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1:305$1:182$183;000$

h�2

1:305þ 1

�ð7:55Þivuut¼ 1:59$10�3 kg=s

(25)

If this result is compared with the one obtained in Eq. (19)(1:7$10�3 kg=s) it can be seen that both values are close and theyhave the same order of magnitude.

Next, the speed of the leak as sonic or subsonic is determined:

1:59$10�3 kg=s0:554 kg=m3 ¼ 2:88$10�3 m3=s ��!speed 2:88$10�3 m3=s

ð0:0025Þ2m2

¼ 461 m=s

As can be seen in the following expression, the gas speed as itleaks from the pore is sonic velocity:

Fig. 9. Fine mesh model of the bo

c ¼ffiffiffiffiffiffiffiffiffiffiffiffiffig$R$TM

r¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1:305$8313$298

16:04

r¼ 449:55 m=s (26)

M ¼ Vc

¼ 461449:55

¼ 1:025 Mach (27)

Since the Choquet environment conditions are met, this is asonic fluid. For the 1 mm2 leak under consideration in the maininlet valve in the boiler room the leak rate is:

Q ¼ C$ð0:001Þ2$

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1:305$1:182$183;000$

h�2

1:305þ 1

�ð7:55Þivuut¼ 2:78$10�4 kg=s

(28)

This is a leakage value very close to the one obtained in (20)ð2:6$10�4 kg=sÞ and which carries a gas emission speed of:

iler room and leakage points.

J. Telmo Miranda et al. / Journal of Loss Prevention in the Process Industries 26 (2013) 839e850848

2:78$10�4 kg=s3 ¼ 4:88$10�4 m3=s ��!speed 4:88$10�4 m3=s

2 2

Fig. 11. Mass fraction contours for the methane (CH4) and classification of zones 1 and2 according to Standard EN 600079-10.

0:554 kg=m ð0:001Þ m

¼ 488 m=s ðsonic speedÞLogically speaking this is sonic.

c ¼ffiffiffiffiffiffiffiffiffiffiffiffiffig$R$TM

r¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1:305$8313$298

16:04

r¼ 449:55 m=s (29)

M ¼ Vc

¼ 488449:55

¼ 1:08 Mach (30)

It is essential to determine gas leak values as they will becomevelocity-inlet contour conditions and as such will be applied to theleak sources in the geometry under examination, when it is vali-datedwith the FLUENTsolver. Figs.10e13 show the results obtained.

Fig. 10 shows the mass fraction contours for the methane fromthe five leak sources throughout the entire boiler room once thesimulation process has reached convergence (3.451 iterations). Asexpected, the greatest concentrations of methane occurred in theareas closest to the leak source, and the methane concentrationdiminished in farther away areas.

In Fig. 11 the gas leak contours for the enclosed area are repre-sented. In addition the circumferences of the 0.5 m and 1 m radiiobtained using UNE 60079/10/1 are also presented here. Zone 1 isrepresented as a broken-line circumference and zone 2 as a contin-uous line. As can also be observed in this Figure, there are numerous“uncovered” zones with values above 25% and 50% for methane LELoutside the zones defined using the European Standard.

The zones of study can be extended and the data from thisextension are given in Figs. 12 and 13. Fig. 12 shows the massfriction contours for the methane produced by the leak in the mainvalve as well as by the leaks in the first burner line of boiler 1. In thisFigure, the circumferences of the of 0.5 and 1 m radii, whichcorrespond with zones 1 and 2 as established in Standard UNE60079, are presented respectively.

It can be seen that these exclusions zones do not cover all therisk zones caused by the leaks (as previously pointed out). TheFLUENT software makes it possible to position the mouse-probeoption over the fine mesh elements and show the level at whichthey are found. Thus, different nodes can be superimposed on thefigure; these clearly represent zones 1 and 2 and their relation to

Fig. 10. Mass contour fraction for the methane gas leaks (CH4) in the boiler room.

50% and 25% of LEL for the methane gas respectively. Using themethodology provided by the European Standard, the calculation ofthese parameters is unavailable. One should cite some of the pointsrepresented in Fig. 12: point 3 which corresponds to a zone 1 at adistance of 1.85 m from leak source and point 4 which is located2 m from the same leak and corresponds with a zone 2.

The simulation in Fig. 13 shows the gas mass fraction contoursfor the leaks located in the burner line of the second boiler. In thisfigure, the two zones considered 1 and 2 according to the EuropeanStandard have been marked. There are nodes such as 1, 4, 5 and 6with values equal to or higher than 50% of the LEL which are foundto be outside the sphere with a radius of 0.5 m as contemplated bythe European Standard. Nodes such as 8, 10 and 11 located at 4, 3.8and 3.5 m of the leak sources respectively (corresponding to a zone2), achieved a clearer picture of the gas cloud extension.

3.3. Comparison between the results obtained with themethodology from UNE 60079/10/1 and the one from CFD

Discrepancies between CFD and UNE 60079/10/1 depend on thenature of the twomethods themselves as well as on the complexityof the environment (e.g. confinement, available ventilation, openand closed spaces, obstacles, etc.). So, in this case, and taking intoaccount the comparison of the results obtained from the two

Fig. 12. Mass fraction contours for methane (CH4), classification of zones 1 and 2 ac-cording to Standard EN 600079-10 and location of the points for zones 1 and 2 outsidethe European standard for the leak in the main valve.

Fig. 13. Mass fraction contours for methane (CH4), zones 1 and 2 classification ac-cording to Standard EN 600079-10 and location of points related to zones 1 and 2outside the European Standard for the double leak on the burner line of boiler 2.

J. Telmo Miranda et al. / Journal of Loss Prevention in the Process Industries 26 (2013) 839e850 849

methodologies proposed for the case studies, it can be seen that themethodology based on UNE 60079/10/1 (2009) provides lessrestrictive values, since it gives horizontal distances of 0.5 m and1 m (zones 1 and 2 respectively) around each one of the leak ele-ments. Overlapping exists in sphere geometries (with the 2D cir-cumferences) at the leak points. Because methane gas is lighterthan air, vertical distances that double the horizontal ones areconsidered; in this case they are up to 1 m.

On the other hand, the methodology based on CFD with theFLUENTsoftware application providesmore conservative results (ascan be seen in Figs. 12 and 13).

For the leak in the main valve in the boiler room (Fig. 12), thehorizontal distances obtained correspond as follows to zone 1 (0.5LEL) at 2.2 m (node 1); 1.85 m (node 3) and 3 m (node 2).

These values are clearly higher than the 0.5 m radius of thecircumference established by the European Standard. As for thenodes found in zone 2 (0.25% LEL), there are, among others: n� 4located 2 m from the leak source; node 5 at a distance of 2.1 m andn� 6 at a distance of 2.7 m. As can be appreciated in the simulation,the contribution made by the leaks allows the gas clouds to extendthroughout virtually the whole left-side surface of the enclosedarea of the boiler room.

The study of Fig. 13 provides similar results. The figure showsthe gas contours for the leaks from the burner line for boiler 2. Themethodology from the European Standard establishes the twospheres (2D circumferences) with 0.5 and 1 m radii respectively,centred on the leak points for zones 1 and 2. Using the CFD facilityto show and evaluate nodes in different positions in the two clas-sification zones, one can affirm that nodes belonging to zone 1 arelocated at a distance of: 0.8, 1, 1.1 and 1.4 m (points 1, 4, 2 and 3)from the leak sources. These are values greater than the 0.5 mestablished in themethodology set out by the European Standard. Asimilar conclusion has been reached for zone 2; the distances fornodes 7 and 12 were in the 1.2 and 1.3 m range (a value similar tothe sphere with a 1 m radius). Nodes 8, 9 and 10 were located atdistances of 4, 3.5 and 3.65 m.

These results are valid for scenarios similar to those providedwith this example: same equipment, same leak sources, flammablesubstances, degree of leakage, leakage rate, degree and availabilityof the ventilation, obstacles, geometry, etc.

4. Conclusions

In the present work, two methodologies for determining zoneclassification and size for explosive atmospheres resulting from

gases, vapours or clouds have been compared. The first of these isan analytical methodology based on the recommendations ofStandard UNE 60079/10/1 (2009) for the determination of thevolume of explosive atmospheres surrounding an emission sourceeven in complex environments. The second methodology based onCFD applied NavierStokes equations to a flow solution for fluidsusing FLUENT software.

The discrepancy between the results obtained is mostly due tothe characteristics of the two methods themselves in addition tothe complexity of the environment. The first analytical methodol-ogy is approximate and it is conditioned by many factors, includingleak rates. In addition the necessity of available literature for leakdata as well as for the characteristics of the equipment involved andthe multiple calculations to be made when numerous leak sourcesmust be evaluated, considerably limit the applicability of thisapproach. Therefore, this methodology may be used for cases inwhich a perfect dilution up to LEL occurs.

CFD methodology proved a more complete and realistic way toassess different case studies since it takes into account differentphenomena such as turbulence, obstacles, and currents. Moreover,the initial and contour conditions might be selected on the basis ofthe characteristics of each case. CFD has provenmost useful in thesecases and it has been demonstrated that predicting risk areas is notpossible with the UNE 60079/10/1.

From the results obtained, it can be concluded that the validity ofstandard UNE 60079/10/1 should be revised because its validity islimited to cases in which the explosive gas dilution up to LEL wasperfect.Moreover, in this revision onemust: consider the sources andextentof theescape (e.g. the rateof emission fromthemanual and theautomatic valves, gasfilters, etc.), thedegreeof ventilation in functionof the dangerous volume parameters (Vz), and the time (t) factor;improve the definition of good or poor availability; and introduce anew section on how to determine the extent of the hazardous areasfor the most common cases in both open and closed spaces, i.e.emission source in subsonic regime (typical cases of vents), sonicemission source (e.g. gas leaks by flanges) and source emission pro-duced by a flammable liquid pool (e.g. accidental spillage).

Acknowledgements

The authors would like to show their gratitude to the staff ofEUROSERUM IBÉRICA, S.L. and in particular to Joaquín GarridoMartín (Manager) and Jesús Garrido Pedráz (plant manager).

References

API 527-07. (1991). Commercial seat tightness of safety relief valves with metal e Tometal seats. American Petroleum Institute.

Arntzen, B. J. (1998). Modelling of turbulence and combustion for simulation of gasexplosions in complex geometries. PhD thesis, Division of Applied Mechanics,Thermodynamics and Fluid Dynamics, The Norwegian University of Science andTechnology, NTNU, Trondheim, Norway.

ATEX 1999/92/EC. (1999). Directive 1999/92/EC (Atex 137) on minimum requirementsfor improving the safety and health protection of workers potentially at risk fromexplosive atmospheres.

Bjerketvedt, D., Bakke, J. R., & van Wingerden, K. (1997). Gas explosion handbook.Journal of Hazardous Materials, 52, 1e150.

Boussinesq, J. (1868). Mémoire sur l’influence des frottements dans les mouvementsréguliers des fluids. Journal des Mathématiques Pures et Appliquées, 13, 377.

FLUENT 6.3. (2007). Tutorial notes. ANSYS.GAMBIT 2.3. (2007). Tutorial notes. ANSYS.García, J., & Francesc, E. (2005). Practical guide “Clasificación de zonas en atmósferas

explosivas”. Barcelona: COITI.Hansen, O. R., Storvik, I., & vanWingerden, K. (1999). Validation of CFD-models for gas

explosions, where FLACS is used as example; model description and experiencesand recommendations for model evaluation. In Proceedings of the Europeanmeeting on chemical industry and environment III (pp. 365e382). Polstka.

Hjertager, B. H. (1982). Simulation of transient comprenssible turbulent reactiveflows. Combustion Science and Technology, 27, 159e170.

J. Telmo Miranda et al. / Journal of Loss Prevention in the Process Industries 26 (2013) 839e850850

Hjertager, B. H. (1984). Influence of turbulence on gas explosions. Journal of Haz-ardous Materials, 9, 315e346.

Hjertager, B. H. (1986). Three-dimensional modeling of flow, heat transfer and com-bustion. Handbook of heat and mass transfer. Houston: Gulf Publishing Company.

Hjertager, B. H. (1993). Computer modelling of turbulent gas explosions in complex2D and 3D geometries. Journal of Hazardous Materials, 34, 173e197.

Ivings, M. J., Clarke, S., Gant, S. E., Fletcher, B., Heather, A., Pocock, D. J., et al. (2008).Area classification for secondary releases from low pressure natural gas systems.Harpur Hill, Buxton: Health & Safety Laboratory.

Launder, H. S., & Spalding, D. P. (1974). The numerical computation of turbulentflows. Computer Methods in Applied Mechanics and Engineering, 3, 269e288.

Ledin,H. S. (2002).A reviewof the state-of-the-art in gas explosionmodelling. ReportHSL/2002/02.HarpurHill, Buxton: FireandExplosionGroup.Health&SafetyLaboratory.

Royal Decree 681/2003. (2003). Protecting the health and safety of workers from risksderived from explosive atmospheres at work.

UNE EN 202707 IN. (2006). Guía de aplicación de la norma une 60079/10. Materialeléctrico para atmósferas de gas explosivas. Clasificación de emplazamientos pel-igrosos. Madrid: AENOR.

UNE EN 60079/10. (2004). Material eléctrico para atmósferas de gas explosivas. Cla-sificación de emplazamientos peligrosos. Madrid: AENOR.

UNE EN 60079/10-1. (2009). Clasificación de emplazamientos. Atmósferas explosivasgaseosas. Madrid: AENOR.

Van Wingerden, K., Hansen, O. R., & Lemousy, T. (1998). Effect of deluge on ex-plosions, FLACS simulations compared to full scale experiments. In 7th Annualconference on offshore installations. ERA-report no 98-0958. ISBN 0-7008-0679-2. London.

Glossary

Vz: theoretical volume for explosive atmosphere in m3

fv: ventilation efficiency index in diluting the gas explosive atmosphere with valuesfrom f ¼ 1 (ideal situation) to f ¼ 5(difficult air circulation due to obstacles)

C: number of air changes per time unit (s�1)Qvo: measurement of total flow of fresh air through space under considerationVo: volume affected by real ventilation close to exhaustG: maximum exhaust rate for the source (mass per time unit kg/s)LEL: lower explosive limit (mass per unit of volume, kg/m3)K: safety factor applied to LEL, normally: K ¼ 0.25 and ¼0.5T: ambient temperature (in degrees Kelvin, K)