Drager Fixed Gas Detector - Explosion Protection Brochure

Explosion Protection

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GAS DETECTIONSYSTEMS

9046262_Ex_Schutz_Gaswarnsysteme_engl_nCI.qxd:9046262_Ex_Schutz_Gaswarnsysteme_engl_nCI 15.12.2009 10:36 Uhr Seite 1

Dan Macey

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Explosion hazards mostly arise fromflammable gases and vapours

Instead of avoiding their ignition by explosion protection measures it maybe preferable to detect them before they become ignitable.

DANGER OF EXPLOSION IS LURKING EVERYWHERE

Wherever hazardous situations exist due to thepresence of combustible gases and vapours e.g. inoil & gas exploration and storage, transportationand storage of flammable liquids and gases, inprocesses involving the use of solvents, or in theplastics processing industry, we will alwaysencounter explosion protection measures, mostlyregulated by law, to keep the personnel and plantssafe.

Depending on the application different measuringprinciples for the detection of gases and vapourscan be used: Catalytic bead sensors, point oropen-path infrared sensors. When detectors areused in combination with a central controller suchas Dräger Polytron or Dräger REGARD it is possibleto detect flammable gases and vapours at an earlystage, when concentrations are so low that adangerous condition – a risk of explosion – can bereliably averted.

METHODOLOGY OF EXPLOSION PROTECTION

Flammable gases and vapours can only be ignitedby an ignition source with sufficient high energy orsufficient high temperature if – under atmosphericconditions – they exist in a mixture with atmosphericoxygen in sufficiently high concentrations.This mixture’s concentration is called LEL:Lower Explosion Limit.

To have an ignition three conditions have to be met:1. Concentration of flammable gas or vapour above

the LEL2. Sufficiently high oxygen concentration3. Sufficiently high temperature or sufficient energy

of the ignition source

Vice versa this rule reads: If one of the threeconditions is not met it is reliably ensured that noignition or explosion can take place.

So, measures of explosion protection can be thefollowing:1. Concentration limiting2. Inertisation3. Use of explosion protected apparatus


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The safest way of concentration limiting is theremoval of flammable gases and vapours from theprocess – this is not normally practical. Whereflammable gases and vapours are used, it is normalfor gas detection systems to be used to limitconcentrations. When the process is closed andthe level of flammable gases or vapours is allowedto exceed the LEL level, this is acceptable as longas the oxygen concentration is kept low enough tocontrol the risk of explosion (inertisation).

If these measures however are not sufficient, all ofthe involved electrical devices have to be designedaccording to certain explosion protection standardsso they will not act as a source of ignition ifflammable gases or vapours are released.

Further advice concerning the methodology can befound in the harmonized standard EN 1127-1.

Explosion protection means reliably excluding atleast one of the three prerequisites for ignition.

Gas/vapourin sufficiently high concentration(above LEL).

Air/oxygenin sufficiently high concentration.

Ignition sourcee.g. ignition spark with sufficient energyor sufficiently high temperatures.

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Activation ofcompulsory measures(e.g. shut-down)

Activation of counter measures(e.g. ventilation)

Alarm range 2

Alarm range 1

Safe range

40 %LEL

20 %LEL

0 %LEL

Alarm thresholdsIf the gas concentration rises acounter measure is activated whenreaching the alarm range 1. If thecounter measure is effective the gasconcentration will decrease (bluecurve). If, however, the countermeasure is not effective the concen-tration will keep on rising (redcurve). When reaching the alarmrange 2 compulsory measures areactivated. Properly designed gasdetection systems will rarely or neverreach the alarm range 2.S

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HazardousAtmospheres

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Preventing potentially explosiveatmospheres –primary explosion protection.

BELOW THE LEL NO DANGER OF EXPLOSION

Concentration limiting (1) and inertisation (2) arealso called primary measures because the formationof an ignitable concentration is averted. On the otherhand when using explosion protected instruments (3)this is a secondary measure because not the for-mation of ignitable concentrations is averted, butonly its ignition.

Concentration limiting means active dilution, e.g. byautomatically ventilating fresh air into the hazardousarea if concentrations have risen above the 20 %LELthreshold. If concentrations continue to risebecause the counter measure is ineffective then itis necessary to automatically activate shut-downactions at 40 %LEL e.g. by switching off any non-explosion proof instrument or equipment. Gasdetection systems used for this purpose must betype approved by a Notified Body in respect to theircompliance with the European Standards (formerlyacc. to EN 50054ff, now acc. to EN 61779 orEN 60079-29-1). This is true for the sensor and thetransmitter as well as for the central controller unit.

As inertisation is also a preventive explosionprotection measure, oxygen measuring instrumentscontrolling the inertisation process at least inEurope also have to be type-approved for thispurpose and shall comply with the relevantharmonised standards (e.g. EN 50 104).

LEL ScaleThe lower the LEL the more danger-ous is the substance, as ignitableconcentrations can form more easily.

Ammonia

Carbon monoxide

Formic acid

1.2-Dichloro ethylene

Methyl bromide

1.1.1-Trichloro ethane

Methyl chloride

Acetyl chloride

Formaldehyde

1.1-Dichloro ethylene

1.2-Dichloro ethane

Methanol

1.1-Dichloro ethaneHydrogen cyanide

Methyl amine

Hydrazine

Methane

Hydrogen

Vinylchloride

Ethyl amine

EthanolAcetonitrile

Acrylonitrile

Dimethyl ether

Ethylene

Dimethyl formamide

i-Propanol

Propane

i-Butanen-Butane

n-Butyl acetate

n-Hexane

n-Octanen-Nonanen-Decane

15.5 % v/v

15.0 % v/v

11.0 % v/v

10.5 % v/v

10.0 % v/v

9.5 % v/v

9.0 % v/v

8.5 % v/v

8.0 % v/v

7.5 % v/v

7.0 % v/v

6.5 % v/v

6.0% v/v

5.5 % v/v

5.0 % v/v

4.5 % v/v

4.0 % v/v

3.5% v/v

3.0 % v/v

2.5 % v/v

2.0 % v/v

1.5 % v/v

1.0 % v/v

0.5 % v/v

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Safety relevant data of flammable gasesand vapours.

THE LOWER EXPLOSION LIMIT – LEL

For flammable substances there is a limit concen-tration necessary for ignition. Below this limit amixture of the substance in air cannot be ignitedbecause there is a deficiency of fuel. This limit iscalled the Lower Explosion Limit or LEL.The LEL cannot be calculated but is an empiricalcharacteristic figure which is established bystandardized methods. With some exceptions theLEL lies between 0.5 and 15 % by volume.

GASES AND LEL

Matter above its boiling point commonly is calleda gas. Thus the pressure of a pure liquefied gasis always higher than the atmospheric pressureso that released gases can very quickly formconcentrations above the LEL causing dangerousignitable gas-/air-mixtures.

VAPOURS OF FLAMMABLE LIQUIDS AND

FLASHPOINT

Matter below its boiling point is not only gaseousbut exists in an equilibrium with its liquid (and alsosolid) state which depends on the temperature.

The gaseous component of this matter is calledvapour. A vapour’s pressure is always lower thanthe atmospheric pressure, and, depending on theliquid’s temperature, only certain maximum vapourconcentrations can form. Especially the maximumvapour pressure of a flammable liquid can be so lowthat the LEL concentration can only be exceeded

at a certain temperature. Only above this certaintemperature a flammable liquid’s vapour becomesignitable. This empirical temperature, establishedby standardized methods, is called the flashpointwhich is a very important safety-relevant figure toassess the hazardous nature of flammable liquids.For example the flashpoint of pure Ethanol is 12 °C(so Ethanol is flammable at 20 °C), but for n-Butanolthe flashpoint is 35 °C, so vapours of n-Butanolcannot be ignited at ambient temperatures of 20 °C,but above 35 °C they can.

And indeed: As long as the temperature of aflammable liquid is kept reliably some degreesbelow the flashpoint this is a primary explosionprotection measure!

IGNITION TEMPERATURE AND MINIMUM IGNITION

ENERGY

Sparks and arcs produced electrically (ormechanically) and hot surfaces are the bestknown ones of 13 different sources of ignition.To ignite mixtures of flammable gases or vapoursin air the source of ignition must either have atemperature higher than the empirical ignitiontemperature or sparks must have an energy higherthan the empirical minimum ignition energy.Both the ignition temperature and ignition energycharacteristics are established by standardizedmethods and are relevant when designing orselecting explosion protected instruments for acertain application.

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Nitrobenzene

Methyl pyrrolidone

Tetrahydro naphthalene

Dimethyl acetamide

Cyclohexanol

Dimethyl formamide

Trimethyl benzene

Ethylglycol

n-Butanol

Nonane

Chlorobenzene

Ethyl benzene

i-Butyl acetate

Ethanol

Methanol

Toluene

Acetonitrile

Ethyl acetate

Methylethyl ketone

Cyclohexane

Hexane

Allyl amine

80 °C

70 °C

60 °C

50 °C

40 °C

30 °C

20 °C

10 °C

0 °C

– 10 °C

– 20 °C

– 30 °C

Flashpoint ScaleThe lower the flashpoint the more dangerousand easier to inflame is the liquid.

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LIST OF SAFETY-RELEVANT FIGURES FOR A SELECTIONOF FLAMMABLE GASES AND VAPOURS

Vapour IgnitionGas / Vapour LEL % v/v LEL g/m3 Flashpoint* pressure* temperature

Acetone 2.5 60.5 < – 20 °C 246 mbar 535 °C

Acetylene 2.3 24.9 Gas Gas 305 °C

Acrylonitrile 2.8 61.9 – 5 °C 117 mbar 480 °C

Ammonia 15.4 109.1 Gas Gas 630 °C

Benzene 1.2 39.1 – 11 °C 100 mbar 555 °C

1.3-Butadiene 1.4 31.6 Gas Gas 415 °C

i-Butane 1.5 36.3 Gas Gas 460 °C

n-Butane 1.4 33.9 Gas Gas 365 °C

n-Butanol 1.7 52.5 35 °C 7 mbar 325 °C

n-Butene 1.2 28.1 Gas Gas 360 °C

n-Butyl acetate 1.2 58.1 27 °C 11 mbar 390 °C

Chlorobenzene 1.3 61.0 28 °C 12 mbar 590 °C

Cyclohexane 1.0 35.1 – 18 °C 104 mbar 260 °C

Cyclopentane 1.4 40.9 – 51 °C 346 mbar 320 °C

Diethylether 1.7 52.5 – 40 °C 586 mbar 175 °C

Dimethylether 2.7 51.9 Gas Gas 240 °C

1.4-Dioxane 1.9 69.7 11 °C 38 mbar 375 °C

Epichlorohydrin 2.3 88.6 28 °C 16 mbar 385 °C

Ethanol 3.1 59.5 12 °C 58 mbar 400 °C

Ethylene 2.4 28.1 Gas Gas 440 °C

Ethyl acetate 2.0 73.4 – 4 °C 98 mbar 470 °C

Ethyl benzene 1.0 44.3 23 °C 10 mbar 430 °C

Ethylene oxide 2.6 47.8 Gas Gas 435 °C

n-Hexane 1.0 35.9 – 22 °C 160 mbar 240 °C

Hydrogen 4.0 3.3 Gas Gas 560 °C

Methane 4.4 29.3 Gas Gas 595 °C

Methanol 6.0 80.0 9 °C 129 mbar 440 °C

Methyl chloride 7.6 159.9 Gas Gas 625 °C

Methylethyl ketone 1.5 45.1 – 10 °C 105 mbar 475 °C

Methyl methacrylate 1.7 70.9 10 °C 40 mbar 430 °C

n-Nonane 0.7 37.4 31 °C 5 mbar 205 °C

n-Octane 0.8 38.1 12 °C 14 mbar 205 °C

n-Pentane 1.4 42.1 – 40 °C 562 mbar 260 °C

Propane 1.7 31.2 Gas Gas 470 °C

i-Propanol 2.0 50.1 12 °C 43 mbar 425 °C

Propylene 1.8 31.6 Gas Gas 485 °C

Styrene 1.0 43.4 32 °C 7 mbar 490 °C

Toluene 1.1 42.2 6 °C 29 mbar 535 °C

* Flashpoint is defined for liquids only, vapour pressure at 20 °C reasonable for liquids only


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Avoiding effective ignition sources –secondary explosion protection

Without source of ignition no danger of explosion

If the formation of an ignitable atmosphere cannotsafely be averted or cannot effectively be prevented(e.g. by using gas detection systems), electricalinstruments being used in this area shall not act asa source of ignition: They need to be designedsuch that they cannot inflame these flammableatmospheres.

TYPES OF PROTECTION

Four of seven standardized types of protection areapplied in the gas detection technology: Flame-proof (d), intrinsic safety (i), encapsulation (m),and increased safety (e). By encapsulation hotsurfaces and sparks are prevented mechanicallywhereas in intrinsically safe instruments this ispracticed by electrical power limiting. Generallyflameproof accepts internal explosions but isdesigned such that it withstands the internalexplosion pressure and reliably avoids a flashback.Increased safety is limited to passive devices suchas junction boxes, terminals and cable glands.These are designed such that the risk of forminghot surfaces or sparks is highly reduced. Explosionprotected instruments have to be type approvedand certified by a Notified Body.

EXPLOSION PROTECTION IS A LAW

In Europe explosion protection has become law byconverting the EU-directives 94/9/EC and99/92/EC, also known as ATEX 95 and ATEX 137,

into national ordinances. Manufacturers of explosionprotected instruments have to mark these devicesunitarily by a device category showing the permissi-ble application range, whereas the user of theseinstruments is pledged to classify the potentiallyexplosive atmosphere area into zones dependingon the probability of occurrence of flammableatmospheres and the nature of flammable matter:gases or vapours (G) or dust (D).

For example II 2 GD is a typical device category forinstruments which may be used in zone 1 and zone2 as well as in zone 21 and 22, while instrumentsbeing used in zone 2 must have a marking ofII 3 G at least.

In the USA explosion protection is regulated bythe NEC 505, the relevant marking also impliesapplication hints by terms like Class and Division.

In the USA instruments using either flameproof orintrinsically safe methods of protection are pre-ferred. In most countries the European or Americanexplosion protection standards are accepted.Recently there has been a drive towards the IEC-Exprotection standards, which is based on world-wideIEC explosion protection standards. Dräger gasdetection instruments meet the explosion protectionrequirements of CENELEC (ATEX, Europe),UL (USA), CSA (Canada) and IEC-Ex (world-wide).

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ASSIGNMENT OF TEMPERATURE CLASSES AND EXPLOSION GROUPS AND TYPICAL GASES AND VAPOURS

Temperature class Explosion groupand max. permissible IIA IIB IICsurface temperature ignition energy greater 0.18 mJ ignition energy 0.06 to 0.18 mJ ignition energy smaller 0.06 mJ

T1 450 °C Acetone, Ammonia, Benzene, Hydrogen cyanide, Town gas HydrogenEthyl acetate, Methane, Methanol,Propane, Toluene

T2 300 °C i-Amyl acetate, n-Butane, n-Butanol, 1.3-Butadiene, 1.4-Dioxane, Acetylene1-Butene, Propyl acetate, i-Propanol, Ethylene, Ethylene oxideVinyl chloride

T3 200 °C Amyl alcohol, Gasolines, Dimethylether, Ethyl glycol,Diesel fuel, Fuel oil, n-Hexane Hydrogen sulfide

T4 135 °C Acetaldehyde Diethylether

T5 100 °C

T6 85 °C Carbon disulfide

Example: If a potentially explosive atmosphere is caused by Carbon disulfide, an electrical apparatus is only suitable to be operated in this atmosphere if it is marked byIIC and T6, whereas for n-Hexane atmospheres electrical devices with the marking IIA T3 are sufficiently protected.

II 2 GDType-approved acc. to 94/9/EC

Device category

II (2) GType-approved acc. to 94/9/EC

Device category

Explosion protected

Ex dem IIC T4Type of protection

Explosion group

Temperature class

Explosion protected

Ex ib IIC T4Type of protection

Explosion group

Temperature class

Typical marking of a gas detectiontransmitter acc. to 94/9/EC:Apparatus for zone 1, 2, 21 and 22.

Typical marking of a safety barrieror a performance approved centralcontroller with electrical connectionsinto the hazardous area (zone 1 or 2),but not to be operated in the hazardousarea.

Typical explosion protection markingof an electrical apparatus (e.g. gasdetection transmitter).

Marking of an apparatus acc. toIEC-Ex. Devices marked like thisare only allowed to be operated incountries not belonging to theEuropean Community.


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Warning sign mandatoryWarning sign mandatory for placeswhere explosive atmospheres mayoccur (zones). Organizational meas-ures have to be regarded.S

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According to the directive 99/92/EC (ATEX 137) theoperator has to conduct a risk assessment of thehazardous area and to classify the area into zonesdepending on the probability of the occurrence ofpotentially explosive atmospheres. Also, he has toarrange organizational safety measures and provideevidence by means of an explosion protectiondocument accordingly. Only suitable instrumentsmay be used in certain zones.

Employing the use of gas detection systems whichreliably prevent the occurrence of an ignitableatmosphere, by default reduces the probability thata flammable atmosphere can occur at all; it is notlikely to occur in normal operation – and this isdefined as zone 2. In other words: By means of a

suitable gas detection system a zone 1 areaconverts to a zone 2 area where less complexdesigned and mostly less expensive instrumentsmay be used (e.g. lamps, machines, mobiles, forklifters etc.)

An important requirement however is that thecounter measures activated by the gas detectionsystem are adequately preventing the formation offlammable concentrations. This might not be thecase in the direct vicinity of a gas leak if the gas ismore quickly released than removed or diluted bythe ventilation. But although the proximal areaaround a leak keeps being zone 1, the zone 1 areashrinks drastically by using a gas detection system– a great advantage for the customer.

The hazardous areas shrink: Zone 1 becomes zone 2.

By means of gas detection systems the probabilityof a formation of explosive atmospheres is reduced

DEFINITION OF ZONES ACC. TO DIRECTIVE 99/92/EC

Hazardous places are classified in terms of zones on the basis of the Minimum requirementZone frequency and duration of the occurrence of an explosive atmosphere for device category

Gas 0 A place in which an explosive atmosphere consisting of a mixture with air of II 1Gflammable substances in the form of gas, vapour or mist is present continuously,or for long periods or frequently

1 A place in which an explosive atmosphere consisting of a mixture with air or II 2Gflammable substances in the form of gas, vapour or mist is likely to occur innormal operation occasionally

2 A place in which an explosive atmosphere consisting of a mixture with air of II 3Gflammable substances in the form of gas, vapour or mist is not likely to occur innormal operation but, if it does occur, will persist for a short period only

Dust 20 A place in which an explosive atmosphere in the form of a cloud of combustible II 1Ddust in air is present continuously, or for long periods or frequently

21 A place in which an explosive atmosphere in the form of a cloud of combustible II 2Ddust in air is likely to occur in normal operation occasionally

22 A place in which an explosive atmosphere in the form of a cloud of combustible II 3Ddust in air is not likely to occur in normal operation but, if it does occur,will persist for a short period only

Example: If an apparatus shall be operated in zone 21, the relevant marking must at least be device category II 2D

HazardousAreas


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Proven in gas detection since decades:Pellistor sensors and Infrared sensors.

PELLISTOR SENSORS

The pellistor sensor (or catalytic bead sensor) isa relatively inexpensive flameproof sensor themeasuring principle of which is based on a chemicalreaction with oxygen and thus needs at least 12 % v/v.Without oxygen the pellistor sensor cannot give areading, but also – because of oxygen deficiency –there is no danger of explosion. The pellistor sensormeasures multiple gases and vapours, but withdifferent sensitivity. If the sensitivity for a substanceis too low the pellistor sensor may not be the correctmeasuring principle for a reliable gas detectionsystem. Infrared sensors may be more applicable.

Catalytic bead principleThe heat-of-reaction measuring principle is basedon the fact that flammable gases and vapours evenbelow their LEL-concentration can undergo aflameless oxidation reaction with atmosphericoxygen, if only a suitable hot catalyst is present.The additional released heat of reaction is ameasure for the gas concentration.

The pellistor sensor houses two small measuringbeads called pellistors (artificial term coming frompellet and resistor), because they are used as pre-cisely measuring temperature-dependent resistors.Both the pellistors are made of very porous ceramicmaterial embedding a small platinum wire coil.The active pellistor additionally contains catalyticmaterial. By means of an electrical current ofapprox. 270 mA on the one hand the platinum coilheats up the ceramic bead to ca. 450 °C, on theother hand the platinum coil acts as a measuring

Active pellistor(shown in half cut, schematic)The penetrating methane molecule reactsby means of the heated catalytic beadmaterial with atmospheric oxygen toform water vapour and carbon dioxide.The released heat of reaction causes ameasurable resistance increase of theembedded platinum coil.

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Using

Gas Detection Systems


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INFRARED SENSORS

In contrary to the catalytic bead sensors theinfrared sensors, which are based on a purephysical measuring principle, are not prone to bepoisoned and do not depend on oxygen content formeasurement. By means of tight optical windowsinfrared sensors are separated from the gas to bedetected. Infrared sensors, however, may haveextremely different sensitivities to different gasesand vapours, and some flammable substancessuch as vinyl chloride or acetonitrile are, againstone’s expectations, not detectable at all. TheDräger application laboratory is very experiencedand has worked out sensitivity data for more than300 gases and vapours for different IR transmitters.

The IR measuring principle is based on the factthat molecules of flammable gases and vapoursexcept a few (e.g. H2, NH3, CO, CS2, HCN, H2S,and hydrides) are mostly hydrocarbons, theCH-bonding of which can be excited to vibrationby certain wave-lengths (frequencies) of theIR-spectrum and thus absorb energy. If IR isradiated into an optical system filled with anIR-absorbing gas, an increase of IR-absorptionmight be detectable in a certain wavelength range,because normal air does not absorb IR.

The generation of a stable spectrum of nearinfrared radiation is easily practiced by glow lampsoperated by under-voltage, whereas the design of awavelength-specific IR measuring detector is waymore complex: Pyro-electric crystals, encapsulatedbehind an optical interference filter produce very

resistor dependent on the bead’s temperature.When molecules of a flammable gas penetrate intoa catalytic bead they react with the activated atmo-spheric oxygen which is absorbed in the porousceramic and resultant heat of reaction increases thepellistor’s temperature e.g. by about 2 °C for 10 %LELoctane. The resulting increase of the pellistor’sresistance is in the magnitude of some milli-Ohmsand is proportional to the gas concentration.

Environmental conditionsThe increase of temperature, dependent of the gasconcentration, however, can only be used as ameasurement signal if changes of ambient temper-ature, which might be much greater, are compen-sated. This is realized by a second pellistor, whichin opposite to the described one does not containcatalytic material and thus is measuring only theambient temperature. As part of a Wheatstonebridge this passive pellistor compensates forenvironmental influences, especially for ambienttemperature. For optimized behaviour both thepellistors must fit best concerning their measuringparameters, and they are accordingly matched topairs during manufacturing.

Poison resistanceFor many decades the pellistors manufactured byDräger are of type PR, which means poison resist-ant. Based on their special construction the sensorshave a longer lifetime compared to conventionalsensors when being exposed to industrialatmospheres containing catalyst poisons such assulphur-, phosphor-, lead- or silicone-compounds.

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small intensity depending voltage changes whenbeing exposed to pulsating radiation. Thesevoltage changes are amplified, linearized andfinally converted to a 4 to 20-mA-signal which isproportional to the gas concentration.

But not only gas reduces the measurable IRintensity, but also a contamination of the opticalsystem by dust or mud causes intensity attenuation.This effect is compensated by a second IRmeasuring detector (reference detector), whichgets the optical information by a beam splitter anddetects the IR intensity in a wavelength rangewhere flammable gases do not absorb IR. If boththe measuring detector and the reference detectorindicate an IR-absorption this is not caused by agas but e.g. by dusty reflectors or else. By this wayof compensation the sensor signal becomescontamination resistant, and at a certain degree ofcontamination additionally a beam block signal ormaintenance signal can be generated. With theDräger Polytron IR type 334 or 340 also possiblealterations of the IR-detectors are compensatedby a second IR source (“4-beam-compensation-method”).

The greater the optical system, the more gasmolecules are involved, and the higher is theIR-absorption. And by using greater optical systemslower measuring ranges such as e.g. 10 % of theLEL can be realized to detect leakages at a veryearly stage.

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IR-transmitterThe fact that under normal conditions (no IR-absorbing gas) there is a high measuring signal,makes it possible to implement self diagnosticprocesses: The IR-transmitter is able to detectif e.g. the IR source fails or the optical systemis blocked. The transmitter is fail-safe in thecontext of the IEC/EN 61508 standard and thus issuitable to be used in SIL2-rated safety chains.

Dräger Polytron IR ExSchematic design (2-beam compensation method).

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Only the green part of thelight radiation is filtered outand its intensity measured.

If a gas absorbs the greenpart of the light radiation,its intensity is measurablyreduced.

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Dräger Polytron IR type 334Schematic design (4-beam compensation method).

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Only by proper calibration and sensor positioninga gas detection system’s reliability is ensured

CALIBRATION

Only by calibration with the target gas (“gas to bedetected”) the gas detection transmitter is enabledto give an individual gas concentration reading. Ifseveral gases or vapours are to be detected thetransmitter has to be calibrated for the substancewhich it is least sensitive to. The properly performedcalibration procedure is essential for the reliabilityof a gas detection system.

SENSOR POSITIONING

Please refer also to the IEC/EN 60079-29-2,chapter 8 – Criteria for the placement of sensorsand sampling points.

There are three different sensor placementstrategies:

1. Spot monitoring: The potential sources of leak(e.g. valves, filling nozzles, flanges, bellows) arewell-known and locatable. So sensors can beplaced such that gas leaks can be detected veryearly and reliably.

2. Area monitoring: The potential sources of leakare spread across a large area and are notlocatable (e.g. in hazardous goods stores). Thusthe sensors should be positioned more or lessequidistantly across the entire area.

3. Fence monitoring: The potential sources of leakare not locatable and there are no ignitionsources in the area. Thus sensors are positionedat the outer limit to monitor for hazardous gasconcentrations crossing into neighbouring areas.

Besides the operational experience of the localplant engineers concerning sensor positioningalso the IEC/EN 60079-29-2 “Guide for selection,installation, use and maintenance of apparatus forthe detection and measurement of combustiblegases or oxygen” contains a lot of advices to placegas sensors properly.

It seems to be trivial, but one thing is essential toknow when designing a gas detection system:A gas detector can only detect a gas which is inthe direct vicinity and which can enter the sensor’selements. So it has to be considered that vapoursof flammable liquids are always heavier than air andspread across the floor rather than rising up, andthey even might condensate in other locations if thetemperature decreases. Also, vapours of flammableliquids cannot form combustible concentrationsif the ambient temperature is lower than the flash-point.

Gas detection transmitters for the detection offlammable vapours and heavy flammable gases(especially propane and butane belong to thisgroup) should be placed where these substancesmight accumulate, as low as possible above theground and/or below the leak. On the other handthree gases are well known which are very muchlighter than air: Hydrogen, methane, and ammonia.Commonly these gases will rise under normalconditions, and sensors should be placed abovethe leak.

Met

hane

n-Hexane

n-Nonane

Eth

ylen

e

Ethyl acetate

Hyd

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n

n-Octane

Toluene

n-P

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AcetonePropane

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90

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70

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50

40

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00 10 20 30 40 50 60 70 80 90 100

%LEL

%LEL

Met

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Ethoxypropanol

Ethyl acetate

n-Butyl acetate

Cumene

Methane

Die

thyl

ethe

r

Acetone

Propane

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%LEL

%LELPellistor sensorDifferent sensitivities of aPropane-calibrated pellis-tor sensor (schematic):50 %LEL n-Nonanecause a measuring valueof only 23 %LEL Propanewhile 50 %LEL Ethylenecause a measuring valueof 62 %LEL.

Infrared sensorDifferent sensitivities of aPropane-calibrated infraredsensor (Dräger Polytron IR Ex,schematic): 50 %LEL Ethylacetate cause a measuringvalue of only 22 %LELPropane while 50 %LELMethanol cause a measuringvalue of 76 %LEL. A Propane-calibrated IR Ex is relativeinsensitive to Methane.Hydrogen cannot be detectedat all by IR-technique.

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Gas detection systems

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Central Controllers – Centers for your Safety

Generally gas detection systems consist of explosionprotected remote transmitters (gas detectiontransmitters), which are installed in the hazardousarea, a central controller unit installed in the safearea to process the sensor signals and to triggeralarms and counter measures, and optical andaudible alarm devices which also might be installedin the hazardous area.

CENTRAL CONTROLLER UNITS

Central controller units are installed in the non-hazardous area, and on the one hand they supplythe connected gas detection transmitters with thenecessary voltage, on the other hand they receivetheir measuring signals and status-information toprocess and indicate them. If pre-adjusted alarmthresholds are exceeded central controllers are totrigger alarms reliably. To have a high availabilitycentral controllers often are equipped also with anemergency power supply.

Central controllers might be small single-channel-units for the connection of only one transmitteras well as complete cabinets with built-in deviceracks for many plug-in modules (channel modules)connected to many transmitters, and withcustomized wired alarm circuits.

The channel modules mostly provide several relayswith voltage-free contacts. Depending on whetherthe relays are energized in normal operation or incase of alarm, the voltage-free contacts can beconfigured as NO-contacts (normally open =closed in case of alarm) or NC-contacts (normallyclosed - open in case of alarm).

For primary protection systems at least one relayfor device alarm must be energized in normaloperation (fail-safe principle) so that a mains powerbreakdown can be detected. Furthermore, forrelays having a safety function it is recommendedthat these relays are also fail-safe (de-energized incase of alarm or power-off).

The relay contacts – if necessary upgraded byadditional relays to have redundant contacts – canbe used to activate counter measures (ventilationon/off, ventilation flap open/close, gas supplyon/off, shutdown activation, etc.) and for opticalalarms, to keep the operator informed about thealarm condition even after having switched off theaudible alarm. Switch-off means acknowledgement,and in parallel to the automatic measures thecustomer has to conduct organizational measuresif necessary.

Furthermore a gas detection system often provides4 to 20-mA-outputs (“signal repeaters”), e.g. toprint-out or visualize the current gas concentrationsfor documentation purposes.

Especially the modular concept of the central con-troller Dräger REGARD offers numerous possibili-ties for customized design: Various combinations ofDräger REGARD Ex and 4 to 20-mA-modules,Master card for the acknowledgment of specialalarm configurations and connections to PLCs,HART card as well as a ModBus-Gateway card forBUS-connection to process control systems andPCs with 32-bit operating systems for measurementdisplay and data-logging.

Hazardous area (e.g. zone 1) Safe area

Gas detection transmitters detect a high vapour concentration

The cloud of vapourconcentration is diluted andremoved by fresh air

Activated by the gas detection system the ventilator injects fresh air

Leak evaporates

As a counter measure theventilation is switched on

Central controller realizeshigh vapour concentrationand triggers alarms

Visible alarm

Audible alarm

II (2) GII 2G II 2G II 2G

ST-

1594

-200

7


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Gas detectors

Based on catalytic bead sensor technology as wellas infrared sensor technology Dräger offers a widerange of different gas detectors for the detection offlammable gases and vapours.

ST-

5003

-200

4

ST-

6056

-200

4

GAS DETECTORS

ST-

1389

-200

6

ST-

5675

-200

4

Dräger PEX 30004 to 20-mA-transmitter withpellistor sensor, with built-inexplosion protected 4 to20-mA-converter, 7-segment-display and 2-key-operation(for calibration) to beconnected to central controllerswith 4 to 20-mA-inputs.

ST-

5669

-200

4

Dräger Polytron SE ExSensing head without electro-nics with pellistor sensor andmV-measuring signal to beconnected to special centralcontrollers, measuring range0 to 100 or 0 to 10 %LEL, alsoas high temperature version forup to 150 °C.

Dräger Polytron FX andDräger Polytron 2 XP Ex4 to 20-mA-transmitter withpellistor sensor or IR-sensor(optional), flameproof housing,with display and magnetic wandoperation, ATEX- or UL-approved,to be connected to central con-trollers with 4 to 20-mA-inputs.

ST-

3833

-200

3

ST-

8821

-200

5

DrägerSensor IRExplosion protected transmitterwith IR-sensor in flameproofhousing, similar to Dräger PIR3000, but with pellistor-sensor-emulation for replacement ofpellistor sensors, with mV-outputto be connected to specialcentral controllers.

ST-

1165

9-20

07

Dräger PIR 7000Explosion protected robustIR-transmitter with IR-sensorin flameproof stainless steelhousing for central controllerswith 4 to 20-mA-input, suitable forsafety related systems acc. to SIL2,certified following IEC 61508,ATEX,- IECEx- and UL-approved.

Dräger Polytron ExExplosion protected transmitterfor pellistor sensor, withintrinsically safe current circuits(LC-display, sensor connection,operating facilities) to beconnected to central controllerswith 4 to 20-mA-inputs.

ST-

5643

-200

4

ST-

5651

-200

4

Dräger Polytron IR type 334and type 340Explosion protected robust IR-transmitter with IR-sensor in flame-proof stainless steel housing forcentral controllers with 4 to 20-mA-input, suitable for safety relatedsystems acc. to SIL2, with HARTand RS 485, ATEX- or UL-approved.

ST-

8822

-200

5

Dräger PIR 3000Explosion protected transmitterwith IR-sensor in flameproofhousing, to be connected tocentral controllers with 4 to20-mA-input, ATEX-, IECEx- andUL-approved.

Dräger Polytron IR ExExplosion protected transmitterwith IR-sensor, with intrinsicallysafe current circuits (LC-display,sensor connection, operatingfacilities) to be connected tocentral controllers with 4 to 20-mA-inputs, with open or closedoptical system, different versions.

ST-

3932

-200

5

Dräger Polytron PulsarExplosion protected Open Pathgas detector for the detection ofa gas concentration along a lineof sight of up to 200 m.


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Central controllers

Based on different concepts, on the one hand devicerack with channels modules or DIN-rail mounted con-trollers both to be installed in wall housings or cabinets,on the other hand stand-alone controller units in IP65-

protected ABS-enclosures, Dräger offers controllerswith different properties, which – in combination withsuitable gas detectors – are dedicated for the reliablegas detection in the field of explosion protection.

ST-

6056

-200

4

ST-

7448

-200

6

ST-

478-

2004

CENTRAL CONTROLLERS

4865

8

Dräger Polytron ControllerDevice racks for max. 2,5, or 12 differentchannel modules and oneacknowledgement module,to be installed in cabinets,panels or wall housings.Channel modules with bargraph display and LED statusinformation, three configurablerelays per module.

ST-

335-

2004

Dräger REGARD 1Stand-alone single channelcontroller with ABS-enclosure,with LC display and statusLEDs, configurable for SE Exsensing heads as well as4 to 20-mA-transmitters,five relays, three alarmthresholds and batteryback-up.

ST-

5647

-200

6

ST-

340-

2004

ST-

272-

2004

ST-

5738

-200

6

Dräger REGARD 2410Central controller to beclipped on a DIN-rail forthe connection of fourtransmitters max.

Dräger REGARD 2400Stand-alone centralcontroller for wall mountingto be operated with fourtransmitters max.

Dräger REGARD SystemDevice rack to takeup 16 differentmodules, to beinstalled in cabinets,panels, or wallhousings. Channelmodules with 4 digitmatrix displays andoperation push buttonsfor configuration.

Dräger REGARD 3900Stand-alone centralcontroller with ABS-enclosure for 1 to 16channels, with LC displayand four status LEDs foreach channel, fullyconfigurable via laptopor PC to operate with4 to 20-mA-transmitters.


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Explosion Protection byDräger Gas Detection Technology

EXPLOSION PROTECTION WITH DRÄGER

Dräger gas detection technology means sensors,transmitters, central controllers and competentsupport – our system engineers will design aspecific gas detection system for your application.

The product portfolio of Dräger gas detectiontransmitters encompass numerous different types:

– catalytic bead or infrared sensor technology– point- or open-path-detection– increased safety, intrinsic safety, or flameproof– analogue 4-20-mA-signal, mV signal

or digital HART or RS 485 signal– with or without display at site– with or without onboard relays– low budget or high end equipment

Select between different central controllers foryour special explosion protection application:

– for mV-signal or 4 to 20-mA-signal– single-channel or multi-channel controllers– device racks for cabinet installation or stand-

alone type with ABS-housing– with or without digital communication– manually or PC-supported configuration

Dräger REGARD 3900Stand-alone central controllerwith ABS-enclosure for up to16 channels.

ST-

272-

2004

Dräger REGARDA representative customized gasdetection system.

ST-

523-

98


Dan Macey

1

Drager Fixed Gas Detector - Explosion Protection Brochure

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