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Using Purge and Pressurization to Prevent Explosions

By Sean Clarke, Epsilon Technical Services Limited

http://www.empiremagnetics.com/articles/purge_and_pressure_explosion.htm

Introduction

We have mentioned in previous articles that there are many types ofprotection concept. One of the simplest concepts to understand and applyto almost any type of apparatus is purging.

Purge, pressurization and Ex p or EEx p are all terms used to refer to thisconcept. The concept works on the principle of keeping the flammalesustance away from the source of ignition and ensuring the surfacetemperature of the purged enclosure is non!incendive.

OVERVIEW

The techni"ue of pressurizing and purging enclosures of electricalapparatus is to prevent the ingress of a flammale atmosphere. Purging isa widely accepted protection concept for explosion protection. #t isaccepted world!wide $using European %tandards, &'P( or #E) %tandards*and is relatively straightforward to comprehend. Explosion protection isachieved y keeping the potentially explosive atmosphere away from anysource of ignition $thermal or electrical*. The potentially ignition capaleapparatus is mounted inside an enclosure, the enclosure is thenpressurized to a positive pressure relative to the atmospheric pressure $a

positive pressure of +.mar is all that is re"uired*.

(s long as this positive pressure is maintained, no gas $or even dust* wille ale to enter the enclosure, hence the internal e"uipment can not eexposed to a potentially explosive gas. There is however a chance that anexplosive gas mixture may have entered the enclosure prior to the positivepressure eing achieved. To ensure that the enclosure is pressurized witha non!explosive gas $i.e. (ir or &itrogen* the enclosure is -purged- to flushout the existing contents and ensure that all areas of the enclosure contain

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only the purging gas $purging of internal dusts have not yet eenconsidered*. #t normally takes etween and + volume changes toensure that the enclosure is -purged-. $#n Europe the first edition purgestandard defined five air changes as a minimum, in &orth (merica theminimum is defined as + air changes*

#t is a condition of certification for /one e"uipment that power can note applied to the e"uipment until the -purging- $a specified flow of purginggas for a specified time* has een completed. To ensure continuity in theeffect of the purging, the maximum leakage rate for the enclosure is alsospecified. When the purging has een completed, power can still not eapplied until the specified positive pressure $at least +. mar* has eenachieved.

#n the event of a failure to complete the purging cycle $drop in flow orincomplete duration* or if the enclosure pressure drops elow the specifiedpositive pressure, power to the e"uipment shall e removed $for /one *or an alarm indication shall e given $/one 0*.

#n the event either of these conditions, the entire purging cycle startsagain with the full purge time duration. The control of the automaticpurging and pressurization is normally y a -Purge )ontrol 1nit- $P)1*. TheP)1 is re"uired to measure flow and pressure, and must fail!safe in allconditions.

The enclosure that houses the e"uipment to e purged must havesufficient physical integrity to withstand impacts and overpressures. Theenclosure should also e designed to facilitate the free flow of air. (senclosure integrity is re"uired to a level of #P2+ $no holes greater thanmm*, any non!metallic material must e tested for duraility andlongevity $against effects of heat and light etc.*. External considerations,such as the surface temperature of the e"uipment or static from plasticparts, must e considered. To ensure incandescent particles can not e

vented from the e"uipment, a spark arrestor must e fitted $or the ventedgas must e ducted to a safe area*. This techni"ue is virtually unlimitedparticularly in physical size or power rating of the apparatus eingprotected.

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Purge control Units

Purge control units must e ale to measure and act on the followinginformation3

Pressure $pressurization*

'low $purging*

Time $Purging*

The operations performed y the P)1 must e -failsafe- y virtue of testand assessment with one fault $i.e. a valve failing*, this is even more

prevalent with the advent of the (TE4 5irective. 'ailure modes ofcomponents must e considered6 even relays can fail open or short!circuit$normally open relays can -arc!weld- in to a closed position*. Purgingtimers must always re!set to zero if they are interrupted during thepurging cycle or after a purge failure. There are two asic types of control3

)onstant 'low $)'*! The air flow for the purging and pressurization stagesare the same. The flow is left as a constant after it is set, and power isapplied after a set period of time.

7eakage )ompensation $7)* (fter purging, the air flow is reduced to afigure 8ust aove the leakage level to maintain the pressurization. The P)1is re"uired to switch from an initial high flow rate $often referred to as fastpurge* to a much lower flow rate on completion of the purge time. )'systems are simpler to design, ut are more expensive $in air or nitrogen*to run. There are other examples of hyrid systems $)'97)* ut in realitythey are 8ust variations on the two asic types.

P)1-s are normally either pneumatic or electrical. #f the P)1 is mounted inthe safe area only the operation will re"uire verifying $unless it containsintrinsically safe outputs*. P)1-s mounted in the potentially explosiveatmosphere will re"uire certifying oth as safety systems and as potentialignition sources $although sources of ignition from pneumatic systemswere not considered until the (TE4 5irective*.

Each %T(TE of the system is defined in response to the inputs of themonitoring devices. The states are re"uired to e uni"ue. The logical

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conditions for the occupation of each state are re"uired to e uni"uelydefined y :OO7E(& logical expressions. (ll possile cominations of inputconditions are re"uired to e shown in a tale. 'or maintenance purposesit is necessary to work on the apparatus within the enclosure while thepurge is off and the enclosure is open. &aturally this is done under a gas

clearance or hot work permit for safety ut there are other safetyimplications which have to e taken into account. Once the enclosure haseen opened the pressure switch will, via the control unit, cause the powerto e isolated from the enclosure. &aturally power must e on so somemeans are re"uired to y pass this function of the control system. Oneway of achieving this is to wire a key!operated switch in parallel with thepressure switch. #t must e ensured that the purge cycle reinitiates eforepower is applied again.

tandards and !erti"ication

The methodology for designing and testing a purged and pressurizedenclosure has een defined in many %tandards and specific industry codesof practice. The two most commonly used %tandards are3

#.$ !E%E&E! Purge tandard

The standard in Europe for the last 0+ years has een the )E&E7E)standard E& ++;3

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%tatic pressurization

Purge control re"uirements $including flow measurement, truth tales

etc.*

%ource or release covered

Purge testing is mandatory and the test is defined

( minimum of volume changes is no longer mandatory

7eakage tests are re"uired

This -%econd Edition- %tandard is currently eing used for European)ertification, although the vast ma8ority of certified purged e"uipment is

only certified against the re"uirements of the -'irst Edition-. :oth types ofcertification will exist in parallel until @une Ast 0++A, when the (TE45irective will ecome mandatory.

,oning Re-uirements

The European pressurization standard does not indicate the zone of use ofthe purged apparatus, this is defined in the -)ode of Practice-.Purge can eused in /one or /one 0.

/one 3 Power must e automatically removed if pressurization fails. /one03 (n alarm must e raised $as a minimum* if pressurization fails.

?ost commercially availale purge control units have options for -alarm-and -alarm and trip- so that the user can select the appropriate measures.

%orth merica Re-uirements

The &'P( $&ational 'ire Protection (ssociation* 2

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0pe ,purging was intended for 5ivision 0 locations to change interior tonon!hazardous.

Type 4 purging re"uires that if the enclosure pressure is lost, the supply isautomatically disconnected on loss of purge pressure and a re!purge is

re"uired efore the supply is restored. $/one )E&E7E)*

Type B purging does not re"uire supply disconnection on loss of pressureut the e"uipment in the purged enclosure had to e suitale for division0. $This is now possile under the (TE4 5irective for /one )E&E7E)*

Type / purging, ecause of the lower level of risk in division 0, re"uiredonly an indication of loss of purged pressure. $/one 0 )E&E7E)*

Practical implementation

Enclosures

( standard #P2 enclosure may not e suitale for use as a pressurizedenclosure ecause the sealing is in the wrong direction. The standardenclosure is designed to prevent the external environment entering theenclosure which means that they are generally unsuitale for retaininginternal pressure.

'or this reason enclosures from suppliers of pressurized systems are notgenerally the same as general!purpose enclosures. (lso, the enclosuremust e ale to maintain the pressure, on large enclosures the unit can eseen to deform even with relatively low pressures. (dditional hinges andcover olts may e re"uired when the pressure is acting on a large surfacearea. Plastic parts $e.g. switches* should not penetrate the housing walls.Plastic parts may e used externally if, when the plastic is removed, metalparts remain that provide an ingress protection rating of #P2+ $no o8ectsgreater than mm can penetrate the enclosure*. &o live $or potentially

live* parts should e exposed outside of the purged area. (s switches arenormally sealed devices that contain sparking contacts, it is preferale toeither use certified switches or mount the switches inside the purgedenclosure. Plastic ducting should not e used if the plastic part failing doesnot create a fail!safe condition, e.g. on the secondary purge system.

uppl0 and Return o" Purging 3edium

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%pecial precautions are needed where the pressurization method uses afan or lower. #t is generally undesirale to put the fan in a hazardous areaecause this means that the ducting, which extends to a non!hazardousarea, will e elow atmospheric pressure and, unless it is completely leaktight, may draw in flammale gases. The other advantages of putting the

fan in the non! hazardous area is that it does not need to e suitale foruse in hazardous area itself and the ducting is under positive pressurewhich prevents ingress of flammale gas.

The use of compressed air is the normal method of supplying purge air, itmust e noted that several purged enclosure on one supply line may dropthe operational pressure to elow working levels for the pneumatic logic, ifsuch a system is used.

The exhausted air from a purged enclosure may contain small particlesthat have een heated y the internal sources of the enclosure. To preventthese particles eing vented into the potentially explosive atmosphere thefollowing methods are used3

%park arrestors on the air outlet.

5ucting $to a safe area* on the air outlet.

Pressuri4ation

The minimum pressure re"uired is +. mar $+ Pa or +.0C w.g.* and thisshould e achieved with the lowest possile flow of pressurizing gas. Thepressure measurement has to at least raise an alarm if the pressure fallingelow this level so that the working pressure will e aove this. Theenclosure has e tested to prove it will withstand . times its normalworking pressure $minimum 0++ Pa* for 0 minutes without distortion, afigure of + mar is not uncommon as a working pressure.

Purging

Effective purging of the enclosure and its contents has to e provided. (sa guideline, volume changes are generally sufficient if the enclosure haseen designed to a few asic guidelines3

a. (void air traps $pockets*

. (void -channelling- of the purging air

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c. )reate turulence

d. (void sealed volumes

emperature !lassi"ication

%ince the flammale gas is prevented from entering the enclosure theexterior of the enclosure determines the temperature classification. #t is toe noted, however, that internal hot surfaces will remain hot even afterthe power has een removed. ( full assessment of the thermal propertiesof hot parts of a purged enclosure must e conducted.

Internal Energ0 torage

'or things such as capacitors in power supplies this often means eitherwaiting until the charge has leaked away efore the enclosure is opened orfitting leed resistors to ensure it happens. :atteries cannot e dealt within this way and invarialy have to e protected y one of the othermethods, a draft standard for atteries in purged e"uipment outlines themethodology3

Test the attery to get the -worst case- output parameters.

Ensure the output is non!incendive.

(ssess the capacitance and inductance to which the attery is

connected.

Pressuri4ed and Purged Enclosure with Internal ource o" Release

The primary o8ective here is to prevent a flammale atmosphere fromoccurring within the enclosure. This can e achieved either y continuous

dilution or y the use of inert gas. #f the "uantity of the internal source ofrelease can e defined and controlled, dilution can e used. #f the "uantityof release can not e defined, inert gas must e used. #n either case aninitial purge is still re"uired6 where air is the purging medium the intent isto dilute any flammales elow the lower flammale limit. #n the case ofinert gas the intent is to render the interior of the enclosure non!flammale y the removal of oxygen. 'or ovious reasons, apparatus

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capale of igniting a flammale gas must not e located in the dilutionzone of a possile release.

!erti"ication and esting

Pressurized e"uipment can e -self certified- for /one 0. #f the pressurizede"uipment is designed to e used in a /one atmosphere then it must ecertified through a European &otified :ody.

The certification process involves an assessment of the sample against theprovided drawings $to ensure that the constructional re"uirements haveeen met and that the sample is a representative test sample* and aseries of tests. The exact nature and type of testing conducted will varyfrom product to product, ut will typically e as indicated elow.

The level of -impact- for the impact test can vary depending on the

material and the risk of impact. Plastic materials re"uire pre!conditioningefore impacting, and the impact test is conducted with the plastic parts athigh and low temperatures. Plastic parts that penetrate the enclosure mayalso re"uire resistance to light testing.

Purge Veri"ication ests

The test is normally conducted y filling the enclosure with representativetest gasses $(rgon and Delium are used to represent heavy and light

#mpact test on enclosure E&+ +2#mpact test on glass $xA* E&+ +2Thermal shock on glass E&+ +2

Purge test $(rgon and Delium* E&+ +;Overpressure test E&+ +;7eakage test E&+ +;7ow pressure test E&+ +2Temperature rise test E&+ +2Thermal decay test E&+ +2%econdary purge test E&+ +;5ilution test E&+ +;

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explosives gasses* and physically measuring the removal from theenclosure when the inert purge gas is applied. The actual gas that will epresent where the e"uipment will e used can e used, ut (rgon andDelium are normally used as they cover all possile flammale gasatmospheres.

The test is carried out y positioning small ore tues in the purge cainetat positions likely to -pocket- and measuring the actual content of test gas.#nitially the cainet must e at least =+ full of the test sample to eremoved. The test sample is then removed y air purging until acceptalelevels $ased on the 7E7* have een reached.

The time taken to remove the test gasses from the enclosure is referred toas the -purge time-, and will e marked on the certification lael andconducted prior to pressurizing the enclosure and applying power.

The purge time will e dependent on the rate of flow of purging gas andinternal geometry of the enclosure to e purged. :y ventilation and aconsidered approach to the internal configuration, purge times can egreatly reduced $reducing the downtime of e"uipment efore power can eapplied*. There is not a linear relationship etween purge times and purgeflow rate, i.e. douling the air flow will not necessarily half the purge time.

Other t0pes o" pressuri4ation

tatic Pressuri4ation:

%tatic pressurization relies on the enclosure eing pressurized with aninert gas and having a sealed enclosure to maintain the pressurization.

The protective gas shall e inert.

#nternal sources of release of flammale sustances are not

permitted.

The pressurized enclosure shall e filled with inert gas in a non!

hazardous area using the procedure specified y the manufacturer

Two automatic safety devices shall e provided to operate when the

overpressure falls elow the minimum value specified y themanufacturer.

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The automatic safety devices shall only e capale of eing reset y

the use of a tool or a key.

The purpose for which the automatic safety devices are used $i.e. to

disconnect power or to sound an alarm or otherwise ensure safety of

the installation* is the responsiility of the user.

Pressuri4ed and Purged Enclosure with People Wor5ing Inside6Purged rooms7

&aturally inert gas cannot e used and compressed air is not generallyrecommended. #n addition, emergency facilities for the personnel arere"uired. 7ighting and means of escape are of prime importance. Thelighting is re"uired under all circumstances and hence must e protectedy some other suitale means such as flameproof. Fick!out panels or crashars on doors usually provide for escape.

Sean Clarke is a specialist ATEX Compliance Engineer with Epsilon Limited, a company

which designs, test assesses and certifies equipment for the CE marking and

Potentially Eplosi!e Atmosphere certification including the ATEX "irecti!e# Epsilon alsooffers training for $oth users and manufacturers of potentially eplosi!e atmospheres

equipment# %eaders may contact &r# Clarke at the address $elow for furtherinformation or to arrange a free technical meeting#

Epsilon Limited Tel' ()*++ +)) -a' ()*++ +./// 0%L' http'11www#epsilon2

ltd#com

http://www.epsilon-ltd.com/http://www.epsilon-ltd.com/http://www.epsilon-ltd.com/http://www.epsilon-ltd.com/