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Explosive atmospheresrisk assessment:a case study

Ing. MURE' Salvinasalvina.mure@aria. to.it


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The case study is related to the risk assessmentperformed to fullfill ATEX regulations in a food industry.The plant produces food stabilizers, ingredients, starchesand gums.The raw materials arrive to the plant as powders,contained in bags. The powders pass through hoppers tobe homogenized and sieves and then they are mixedtogether and packed.In each step of the manufacturing the movement of thepowders can generate potential explosive atmosphere ...

HOW TO EVALUATE THE EXPLOSION HAZARDSAND REDUCE IT?


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The risk assessment was carried out in accordance withItalian law no. 81 of Apr. 9th, 2008 "Attuazione dell'articolo 1della legge 3 agosto 2007, n. 123, in materia di tutela dellasalute e della sicurezza nei luoghi di lavord' Title XI"Protection against explosive atmosphere" hereunder shownas D.Lgs. n. 81/08.In accordance with what it is stated in attachment XLIX ofD.Lgs. n. 81/08, for the classification of areas and workingplaces, we refer to the following technical rules: CEI EN 60079-10-1 (CEI 31-87) "Explosive atmosphere -

Part 10 - 1: Hazardous Area Classification and Contro! ofIgnition Sources for Flammable Gases and Vapours" andCEI guide 31-35 and modifications; CEI EN 61241-10 (CEI 31-66) Electrical apparatus for usein the presence of combustible dust Part 10:Classification of areas where combustible dusts are or maybe present and CEI guide 31-36 and modifications.


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For hazard analysis and risk assessment together with preventionand protection measures, we refer to the following rules: EN 1127-1:2011 Explosive atmospheres. Explosion prevention

and protection. Basic concepts and methodology - specifiesgenerai design and construction methods to help designers andmanufacturers in achieving explosion safety in the design ofequipment, protective systems and components;

CEI CLC/TR 50404:2003 "Electrostatics - Code of practice forthe avoidance of hazards due to static electricity"; UNI EN 13463-1:2009 "Non-electrical equipment for use inpotentially explosive atmospheres. Basic method andrequirements"

The italian laws above mentioned derive from EUdirectives and uptake international standards


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In order to proceed with the explosion risk assessment, it's necessary tocollect data about the facilities of the plant, the productive process, thepresence of suction systems, and above ali the characteristics of the rawmaterials that produce the explosive atmosphere.For the purposes of this document, the main terms and definition areillustrated below: Lower Explosive Limit (LEL): the lowest concentration (percentage)

of a gas or a vapor in air capable of producing a flash of fire inpresence of an ignition source (are, flame, heat). The term isconsidered to be the same as the lower flammable limit (LFL). At aconcentration in air lower than the LEL are gas mixtures are "toolean" t o burn. Methane gas has a LEL of S0/o. If the atmosphere hasless than S0/o methane, an explosion cannot occur even if a source ofignition is present.

Minimum ignition temperature of an explosive atmosphere: thelowest temperature of a hot surface that will cause a dust cloud, ratherthan a dust layer, to ignite and propagate flame.


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Explosion Index (Kst): explosive property measured in the laboratoryto quantify the severity of a dust explosion. The Kst value is calculatedas the equivalent pressure in a 1 m 3 sphere from the cube law (Kstvalue =cube root of volume x explosion pressure rise). The ST classis based on the Kst value as follows: ST class O - Kst value =O

ST class 1 - Kst value less than 200 bar m / sec ST class 2 - Kst value between 200 and 300 bar m/ sec ST class 3 - Kst value greater than 300 bar m / sec;

Minimum Ignition Energy (MIE): minimum amount of energyrequired to ignite a combustible vapor, gas or dust cloud.


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In the following table,characteristics.

N ame of substances

AGARCARRAGEENANCELLULOSE l CMCCELLULOSE l HPMCCELLULOSE l MCCELLULOSE l MCEMULSIFIER l DATEMEMULSIFIER l LACTEMEMULSIFIER l MDG 40EMULSIFIER l MDG 90 (kerry)

MALTODEXTRINNOVATION 4600PECTIN l HMPOLIDEXTROSEPROTEIN l ANI MAL l GELATINE APROTEIN l ANI MAL l MILK l WHEYPROTEIN l ANI MAL l MILK l WHEY 60SALTS l CITRATESTARCH l MODIFIED l HOT SWELLINGl WAXY MAIZE E-1422STARCH l NATIVE l COLO SWELLING lWAXYRICESUGAR l FRUCTOSESUGAR l GLUCOSESUGAR l SUCROSESWEETENERS l SUCRALOSEXANTHAN

l 25l

1 144< 300

1 80-1601 116

1 711 1521 711 721 1602 2021 0-2001 150

1 391 211 150

1 1141 1501 631 172

1 84

a list of the

> 1000

>99925-50

29-11020

15-3010

20-10060

> 30040

>1000

>500100

>10010

35010-25400

>1000

Minimum lgnitiontemperarture

c380430

360 autoignition 170370

380 autoignition 170>175280

300280380

380240

> 250 (layer)420

> 500

440

420

>470360

>420490 (layer)

390

substances

Glowingtemperature

c

330-450

320

44oc self ing te m p

.1S shown withLower Maximum Bulk

Explosive explosion DensityLimi pressurem3 bar,, Il

125 70060 400-600

40-50 7,630 20030 6-10

7,83 520950

30 6,7 900


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With reference to the used raw materials, it is required the evaluationof the explodibility characteristics of each typology of powder:Name: Novation 4600LEL (g /m3): 30Average size (pm): 10Absolute density (kg/m3): 500Layer ignition temperature T Smm ("C): 240Cloud ignition temperature Tcl ("C): 320Humidity (%): 6,2Mass humidity content (%): 6,2Explosion overpressure (bar): 10Kst (bar x m/s): 202Explodibility class: St 2MIE(mf):60Conductivity: No


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The explosion risk assessment requires 5 different steps:1. Identification of explosion hazard and estimation of

probability of occurrence of a hazardous explosiveatmosphere;

2. Identification of ignition hazards and determination of thelikelihood of occurrence of potential ignition sources;

3. Estimation of the possible effects of an explosion in caseof ignition;

4. Evaluation of the risk and whether the intended level ofprotection has been achieved;

5. Consideration of the measures to reduce the risks.


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The explosion risk assessment is evaluated by using amethodology with semi-quantitative indicators.

The analytical evaluation of risk can be determined ingenerai accordance with the following formula:R=P*C*D

w h ere: R is the risk, P is the hazard factor, that is the probability ofexistence of the hazard, C is the contact factor, that is the probability that thehazard can cause damage, D is the damage factor, that is the entity ofany damage that might occur.


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STEP 1. Identification of explosion hazardous and determinationof occurrence of a hazardous explosive atmosphereThe probability of occurrence of a hazardous explosive atmosphereis evaluated through Area Classification methods, according to EN60079-10-1 and 61241-10. The correlation between hazard factor Pand area classification is represented in the following table:

Hazardous Area

Zona 0/20

Zona 1/21

Zona 2/22

Zona NE

O,1,2 for gas, vapour20,21,22 for dust

Description

An area in which an explosive gas/dustatmosphere in air is continuously presentor present for long periods or frequentlyAn area in which an explosive gas/dustatmosphere in air is likely to occur innormal operationAn area in which an explosive gas/dustatmosphere in air is not likely to occur innormal operation and if it occurs it willexist only for a short timeAn area in which an explosive gas/dustatmospher e in air never occurs

P Index

3

2

1

o


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STEP 2. Identification of ignition hazards and determination ofthe likelihood of occurrence of potential ignition sourcesThen for each hazardous zone, it shali be determined whichtypes of ignition sources are possible. The significance of aliignition sources that could come into contact with theexplosive atmosphere shali be assessed.The ignition effectiveness of ali the ignition sources shali thebe compared with the ignition properties of flammablesubstances: Minimum Ignition Energy (MIE), Minimum ignition temperature of an explosiveatmosphere, Minimum ignition temperature of a dust layer.This step shali result in a complete list of the effectiveignition sources for each hazardous zone. Mterward thelikelihood of occurrence of the potential ignition sourcesshali be assessed, taking also into account those that can beintroduced e.g. by maintenance and cleaning activities.


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The possible ignition sources to be taken into account are:- Hot surfaces- Flames and hot gases- Mechanically generated sparks- Electrical apparatus- Stray electric currents, cathodic corrosion protection- Static electricity- Lightning- Radio frequency (RF) electromagnetic waves from 104 Hz to 3 *1011Hz- Electromagnetic waves from 3*1011 Hz to 3 * 1015 Hz- lonizing radiation- Ultrasonics- Adiabatic compression and shock waves- Exothermic reactions, including self - ignition of dusts.


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The correlation between the factor C and likelihood of occurrenceof effective ignition sources is represented in the following table:

Presence

Always

Sometimes

Rarely

Never

Description

Sources of ignition which can occurcontinuosly or frequentlySources of ignition which can occur inrare situationsSources of ignition which can occur invery rare situationsSources of ignition which can occur never

C Index

3

2

1

o


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STEP 3. Estimation of the possible effects of an explosionTo estimate the possible effects of an explosion the following shall beconsidered, e.g.:- Flames and hot gases;- Thermal radiation;- Pressure waves;- Flying debris;- Hazardous releases of materials.

The consequences of the above are related to the:- Chemical and physical properties of the flammable substances;- Quantity and confinement of the explosive atmosphere;- Geometry of the surroundings taking into account obstacles;- Strength of enclosure and supporting structures;- Protective equipment worn by the endangered personnel;- Physical properties of the endangered objects.


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The correlation between factor D and the parameters used toevaluate the possible effects of explosion is represented in thefollowing tables:

Hazardous AreaZona 0/20

Zona 1/21

Zona 2/22

Zona NE

0,1,2 for gas, vapour20,21,22 for dust

DescriptionAn area in which an explosivegas/dust atmosphere in air iscontinuously present or present forlong periods or frequentlyAn area in which an explosivegas/dust atmosphere in air is likely tooccur in normal operationAn area in which an explosivegas/dust atmosphere in air is no tlikely to occur in normal operationand if it occurs it will exist only for ashort timeAn area in which an an explosivegas/dust atmosphere in air neveroccurs

D Index3

2

1

o


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D Index will be added to the indexes in the table below (D'):

l

ll

l

lndexParameters

0,00 0,25

Workers presence {W ) ever Occas ionai

Dust explosion index (KST) s 200 bar*m/s 200 < KsTS 300 bar*m/sGas explosion index (Kd s 500 bar*m/s 500 < s 1.000 bar*m /sCloud volume (Vz) < 10 dm 3 10 s Vz < 100 dm 3

Dust layer depth ('-d) ssmm 5 < . . . c t s 50 mm

Cloud Confinement (Cc) ot confined Partly confined

D'= D+ WP + KsT + Vz + Ld +Cc (dust)D'= D+ WP + ~ + Vz +Cc (gas)

0,50

Continuous

> 300 bar*m/s> 1.000 bar*m/s2100 dm3

250mm

Completelyconfined


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STEP 4. Evaluation of the risk and whether the intended level ofprotection has been achievedAnother table, similar to the previous one, allows to determine theindexes of elements of the risk to be added to the product P * C *D '.

l

lndexParameters

o 1 2

Workers presence (W ) ever Occasionai Continuous

Dust explosion index (KsT) s 200 ba r*m/s 200 < KsTS 300 bar*m/s > 300 bar*m/sGas explosion index s 500 bar*m/s 500 < s 1.000 bar*m/s > 1.000 bar*m/sCloud volume (Vz) < 10 dm3 10 s V z< 100 dm! 100 dm3

Dust layer depth (4 ) SSmm 5


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The classification of risk levels is determined by risk R calculatedwith the formula above.In the table below are the hazard classes and their descriptions.

Risk Value R Rischio DescriptionThe probability of an explosive atmospheres is high, there areeffective ignition sources and the exposure level is high, so withconsiderable damage to persons and property.The probability of propagation of the explosion is to be consideredsignificantThe probability of the presence of explosive atmospheres is limitedand there may be effective ignition sources. In case of explosion, theexposure level is moderate, then with moderate damage to personsand property.The probability of propagation of the explosion is to be consideredlimited .The probability of presence of an explosive atmosphere is extremelylimited, as well as the presence of effective ignition sources. Theexposure level is low, so with limited damage to persons andproperty.he probability of propagation of the explosion is to be consideredextremely limited.The presence of an explosive atmosphere is almost impossible orthere are no ignition sources effectively. The level of exposure isnegligible, so there are no damages to persons or property.The probability of propagation of the explosion is considerednegligible.

Measuresldentify and implementimmediate and urgent measuresto prevent and contrai exposureto risk.The assessment will be repeatedlater.The exposure is significant, it isnecessary to bring improvementsto the protection and decreasethe risk.The maintenance of compliancerets with the employer andperson in charge.Will be considered for riskreduction measures on the basisof an assessment ALARP (As LowAs Reasonably Practicable)

No further action is necessary


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STEP 5. Consideration of the measures to reduce the risks

The necessity of having an explosive atmosphere together with on effectiveignition source, lead immediately to the basic principles for the reduction ofrisks:

a) Prevention:avoid or reduce explosive atmospheres; this objective can mainly beachieved by modifying either the concentration of the flammablesubstance to a value outside the explosion range or the concentrationof oxygen to a value below the limiting oxygen concentration (LOC);avoid any possible effective ignition source;

h) Protection:halting the explosion and/or limiting the range to a sufficient level byprotection methods, e.g. isolation, venting, suppression andcontainment; in contrast to the two measures described above, here theoccurrence of an explosion is accepted.


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Area classificationSources of release in

Production unit (dust):- A,BeC

Natural gas line (gas)Charging trucks unit (gas)

Production unitA


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The basic elements for establishing the hazardous zone types are:1. the source of release - a point or location !rom which a

gas/liquid/dust may be released into the atmosphere so that anexplosive atmosphere could be formed;

2. the grade of release - there three basic grades of release, as listedbelow in order of decreasing frequency and likelihood of theexplosive atmosphere being present: continuous, primary andsecondary grade.

I t is necessary, first of ali, t o determine the grade of release inaccordance with the definitions, by establishing the likely frequencyand duration of the release.Having established the grade of the release, it is necessary todetermine the release rate and other factors, which may influence thetype and extent of the zone.In the following table the area classification results for line A areshown:

Product line A


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Source of release Grade of releaseHopperTCl Primary

Filter GV l Secondary

Sieve SVl Secondary

Storage tank Sl Secondary

Connection Sl-SAl Secondary

Filter GV2 Secondary

Mixer SAl Secondary

Connection SA1-S2 Secondary

Filter GVS Secondary

Storage tank S2 Secondary

Storage tank S3 SecondaryConnection S3-SV2 Secondary

Filter GV3 Secondary

Sieve SV2 Secondary

Storage tank S4 Secondary

Packaging Secondary

ZoneInternai: 20External: 22 , extension area 1,05 mInternai, dirty side: 20Internai , de a n side: 22External: 2NE (Non hazardous area)Internai : 20External: 2NE (Non hazardous area)Internai : 20External: 2NE (Non hazardous area)Internai : 20External: 2NE (Non hazardous area)Internai, dirty side: 20Internai , de a n side: 22External: 2NE (Non hazardous area)Internai : 20External: 2NE (Non hazardous area)Internai : 20External: 2NE (Non hazardous area)Internai, dirty side: 20Internai , de a n side: 22External: 2NE (Non hazardous area)Internai : 20External: 2NE (Non hazardous area)Internai : 20External: 2NE (Non hazardous area)Internai : 20External: 2NE (Non hazardous area)Internai, dirty side: 20Internai, clean side: 22External: 2NE (Non hazardous area)Internai : 20External: 2NE (Non hazardous area)Internai : 20External: 2NE (Non hazardous area)Internai : 20External: 2NE (Non hazardous area)

NoteLoading with suction system always activated

The only source of release could be the door that is always closed . During cleaningoperations, the sieve is electrically disconnected and suction system is activated.The only source of release could be a drap door that is always closed

The connection is continuous.The filter has no sources of release . When the maintenance is performed, ad hocoperating instruction has adopted in order to avoid the formation of cloudspotential ly explosive.The only source of release could be the door, but when it is opened the mixer iselectrically disconnected and suction system is activated.The connection is continuous.The filter has no sources of release. When the maintenance is performed, ad hocoperating instruction has adopted in order to avoid the formation of cloudspotential ly explosive.The only source of release could be a drap door that is opened only during cleaningand maintenance operations. In these cases, the storage tank is electricallydisconnected and suction system is activated.The only source of release could be a drap door that is always closedThe connection is continuous.The filter has no sources of release. When the maintenance is performed, ad hocoperating instruction has adopted in order to avoid the formation of cloudspotential ly explosive.The only source of release could be the door that is always closed . During cleaningoperations, the sieve is electrically disconnected and suction system is activated.The only source of release could be a drap door that is opened only during cleaningand maintenance operations. In these cases, the storage tank is electricallydisconnected and suction system is activated.Closed room.


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Internai: 20 /

Hopper and typicalequipment: suction system,

shelf for bags, ecc.

Area classification for hopper

,.-

External: 22, extension area, a= 1,05 m /~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Air


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i l l

Dust/Air

Area classification for fter

This a scheme of cartridgefilter. In Hopper fter GV1,the dust collected is sent to

the production line


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Ignition sources analysisIn order to analyze the potential ignition sources, it is necessary to refer to the technicaldocumentation of equipments, containing also the risk analysis and, in particular, theignition sources analysis, that the manufacturer must do in normal condition (NC), raremalfunction (RM) and very rare malfunction (RMF), according to CE Declaration ofConformity (Machinery Directive 2006).The following table shown part of ignition sources analysis for hopper:

gnition sources(E N 1127-1)

lames and hoases

Ho erTClPresence(Yes/No) Reasons

No t present. The temperature conclitions inside and outside of

Effictive Reasons(Yes/No)

o the hopper are of an order of magnitude lower than the ignitionNevertemperature of the dust treated.

No

o

However, one can not exclude the presence of high temperatureNeversurfaces for the moving parts due to friction in case of failure.

The possible presence of flames and hot gases could be due toho work maintenance operations.NeverHowever, it excludes this occurrence beacuse "Hot work permit"

during

procedure are adopted.Possible mechanical sparks may be generateci outside due to theuse of unsuitable tools. In this regard, we adopt suitable toolsNeverwill be drawn up and operating procedure for this purpose.

The hopper is subject to perioclic maintenancequarterly.The source of initiation occurs in case of failure (egseizure of a bearing), but with the adequatemaintenance such event is not relevant, which isdeemed no effective for the source of ignition.

In normal condition, it is excluded the frictionCan no t be excluded that the friction of the moving parts and Sometimes between metal parts. In case of failure, can no t bespark generating heat, as the speed of the same is greater than 1 excluded that event, and having regard to them / s (paragraph 6.4.3 of the UN I EN 13463-1:2009). speed, the source of ignition may be effective.


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Estimation of the possible effects of an explosionTaking into account the parameters for Cloud volume Vz andCloud confinament Cc, the Indices of damage D was calculate.The main results are shown in the following table:

Vz CcSource of release Zone D D'Cloud volume [dm 3] Cloud confinament358 PartlyConfinedInternai: 20 3 5(0,5) (0,25)

>100 Not ConfinedExternal: 22 l 2(0,5) (O)N.P. Completely ConfinedSieve SVl Internai: 20 3 5(0,5) (0,5)46 Completely ConfinedConnection 51-SAl Internai : 20 3 5(0,25) (0,5)

1200 Completely ConfinedMixerSAl Internai: 20 3 5(0,5) (0,5)

131 Completely ConfinedConnection SAl-52 Internai : 20 3 5(0,5) (0,5)1349 Completely ConfinedStorage tank 52 Internai : 20 3 5(0,5) (0,5)39 Completely ConfinedConnection S3-SV2 Internai: 20 3 5(0,25) (0,5)N.P. Completely ConfinedSieve SV2 Internai : 20 3 5

l (0,5) (0,5)

Evaluation of the risk


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The results of risks for eachfollowing table:

equipment areProduct line A

Equipment

HopperTC1

Sieve SV1

Connection S1-SA1

Mixer SA1

Connection SA1-S2

Storage tank S2

Connection S3-SV2

Sieve SV2

Zone

Internal: 20External: 22Internal: 20External: 2N EInternal: 20External: 2N EInternal: 20External: 2N EInternal: 20External: 2N EInternal: 20External: 2N EInternal: 20External: 2N EInternal: 20External: 2N E

p c3 21 23 2

3 3

3 2

3 3

3 2

3 3

3 2

VzCc

D' Clou dCloud volume confmamen5 2 12 2 o5 2 2

5 1 2

5 2 2

5 2 2

5 2 2

5 1 2

5 2 2

shown tn th

Risk

High (35)Basso (8)High (36)NegligibleHigh (50)NegligibleHigh (36)NegligibleHigh (51)NegligibleHigh (36)NegligibleHigh (50)NegligibleHigh (36)Negligible

Identification of prevention and protection measures


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If ignition hazards are possible and the specific measures cannot beimplemented, the equipment, and the protective systems andcomponents shall be designed and constructed in such a way as to limitthe effects of an explosion to a safe level.Such measures could be: Explosion resistant design, see EN 14460; Explosion venting, see EN 14797; Explosion suppression, see EN 14373; Explosion isolation, see EN 15089, EN ISO 16852.These measures generally refer to the m1t1gation of hazardous effectsfrom explosions inside equipment, protective systems and components.

Identification of prevention and protection measures


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In the following table, the main prevention and protection measureside n tified are shown:

Equipment

HopperTCl

Connection S1-SA1

Preventive measures

Inverter to reduce thespeed of moving partsunder the 1 m/ s.Periodic inspection ofmoving parts in order todetect signs of wear anddeformation. In order toavoid cracks anddelamination of metalparts.Replacement theconnection with one mconductive material.

Protection measures

lnstallation of suitablesystems for suppressionand explosion venting ofcombustion gases

Residua! risk Panels explosion ventingNegligible

Low

Negligible

Conclusion


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The risk assessment approach described taken into account the mainrequirement steps indicated by EN 1127:1 standard and by law no. 81 ofApr. 9th, 2008; moreover, this approach allows to quantify the effectivenessof prevention and protection measures in order to reduce the risk.The main difficulty of this analysis is to have the technical documentation,especially when the equipment are installed before the MachineryDirective. In this case, on-site visits with the technical staff should bemade.The results of this analysis should be included in the Document ofExplosion Protection, mandatory for Italian and European law.A similar approach can also be used for workplaces where there may beflammable gases, vapors and flammable liquid.

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