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GAS DETECTION TECHNOLOGY

There are areas at many workplaces whichhave such restricted dimensions that per-sonnel who enter them or have to work inside them are hindered in their activitiesor even at risk. Such areas are generally referred to as "confined spaces" if their volume is less than 100 m3, if they lack anynatural or manmade system of ventilationand/or air extraction, or if they are belowtwo metres in either length, width, height or diameter. Confined spaces are to be found not only in industrial settings (tanks,boilers, chemical apparatus, storage con-tainers), but also in many other work areas. For example, windowless cellar rooms, welland sewage shafts, pipelines, mineshafts,tunnels, pits, ditches, canals, double-bottomtanks in ships, bridge and crane supports,and hollow sections in machines, are alsoregarded as confined spaces. It is often difficult to escape from confined spaces because entry and exit is only possiblethrough a manhole. Because of the lack ofspace, the presence or possible formationof harmful substances in gaseous form is ofparticular relevance. Before personnel enterconfined spaces, it must be ensured that nohazardous gas concentrations are present.What is more, continuous monitoring of airquality is necessary the entire time work isbeing carried out in confined spaces. Thisarticle cites a number of practical examplesto illustrate the use of portable measure-ment systems for air monitoring in confined spaces.

The risk posed by different gasesIn principle, toxic and/or combustible gasescan occur or be enriched in all confinedspaces, and all confined spaces may beprone to a lack or surplus of oxygen. In almost all cases, consideration needs to be given to the following four gases:

– Methane: produced by the fermentation of organic material (shafts containing leaves and water, or sewers) or released through leaks in gas pipes (cellar rooms, trenches);

– Hydrogen sulphide: produced by rotting organic material (animal corpses, faeces)and to be found as a constituent of crudeoil and oil products (chemical industry, oil industry);

– Carbon monoxide: produced by incom-plete combustion, e.g. smouldering fires in cable ducts, or released through leaks in exhaust gas pipes or as exhaust fumesfrom heating systems or motor vehicles

– Lack of oxygen due to the consumption of oxygen during the decomposition of organic material, during smouldering firesor when oxygen is displaced by gas leaks(e.g. methane from gas pipes). Danger of suffocation at oxygen concentrations below 19.5 % by volume.

– Oxygen surplus (caused by leaks in areas where personnel work with oxygen)means that substances – including protective clothing of low flammability in normal atmospheres – burn more easily, more quickly, and at higher temperatures.

Explosive fires can be caused. These hazardscan certainly be expected at oxygen con-centrations in excess of 23.5 % by volume.

Depending on the local circumstances, andthe work being carried out, other gasesmay occur, e.g.:– ammonia in refrigeration systems or in

agriculture;

The use of portable gas detectors in confined space applications

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– nitrogen oxides during welding work or as a constituent of diesel exhaust fumes;

– sulphur dioxide during burning of fossil materials;

– chlorine resulting from leaks in water dis- infection applications (waste water, cool- ing water, drinking water, swimming pools);

– hydrocyanic acid in the production of

noble metals and in galvanization;– mercaptanes as odorants in natural gas.

The gases listed above are either toxic andcan affect the lungs, cause illness or poison-ing – depending on their mode of actionand concentration – or are combustible,meaning that there is a risk of explosion incombination with the oxygen contained in

X-am 2000 being used forpersonal confined spacemeasurement

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THE USE OF PORTABLE GAS DETECTORS IN CONFINED SPACE APPLICATIONS GAS DETECTION TECHNOLOGY

the air. In addition, low but nonetheless toxic concentrations of volatile organic com-pounds (VOCs) such as fuels, oils, solvents,pesticides and herbicides may be present.

Safety measuresIf at all possible, confined spaces shouldnot be entered. Often, however, confinedspaces have to be entered in order to carryout necessary maintenance, repair or clea-ning work; in such cases it is essential thata check be carried out prior to entry to de-termine whether toxic or explosive sub-stances are present in the confined space,and whether the oxygen content is too highor too low. Because gases can have diffe-rent densities, it is important to ensure thatmeasurements are performed at floor level,at mid height and at ceiling level of theconfined space. If harmful substances aredetected, it needs to be checked whetherthe substances in question could be remo-ved by ventilation or any other means, orwhether personnel need to don appropriateprotective equipment before entering theconfined space. In the majority of cases, itis also necessary to continue measuring forharmful substances while work is ongoinginside the confined space.

Use of portable measurement systemsFirst, we can distinguish between short-term and continuous measurement systems.A classic example of a short-term measure-ment system is the Dräger Tube. The ad-vantage of this method of measurement liesin its high level of flexibility. Dräger Tubesrepresent an extremely low-cost option, andcan also be used to measure exotic sub-stances. However, the period of measure-ment lasts only a few minutes; to ensureproper monitoring of the atmosphere in aconfined space, many individual measure-ments would have to be carried out. In thiscontext, continuous measurement systemsare the obvious choice, particularly for per-sonal measurements.

As mentioned above, the presence of com-bustible substances (methane), carbon mo-noxide, hydrogen sulphide or a lack or surp-lus of oxygen cannot be ruled out with anydegree of certainty in most cases. A four-gas detector would therefore be the measu-rement system to choose here. Continuousportable measurement systems normallywork with sensors. Combustible substancesare measured using a catalytic Ex sensor,for example, while toxic substances andoxygen are measured by substance-specificelectrochemical sensors. These four sen-sors are used in multigas detectors such asthe devices of the Dräger X-am family, allo-wing the aforementioned substances to bemeasured all at once on a continuous basis.The user is alerted by a visual, audible andvibration alarm the moment one or more li-mit values of the substances to be measu-red is reached or exceeded.

Confined spaces: what are the key characteristics of the measurement systems used?

Entering confined spaces An all-clear measurement has to be perfor-med before personnel enter a confined spa-ce to make sure that no combustible or to-xic substances are present, and to ensurethat there are acceptable concentrations ofoxygen available. For the purposes of the

all-clear measurement, the multigas detec-tor is fitted with a pump and an extensionhose; the extension hose is then fed intothe confined space – through the entranceopening, for example. Ideally, the multigasdetector will feature an integrated pump –this is the case with the Dräger X-am 3000(four-gas detector) and the X-am 7000 (five-gas detector). An extension hose upto 20 metres in length can be used withthe X-am 3000 – in the case of the X-am7000 the hose can be as long as 45 metres. In both devices the internal pump isactivated automatically when the pump ad-apter is fitted. If the extension hose beco-mes blocked or obstructed for any reason(e.g. if it is kinked), the user will be alertedby the devices. The gas concentrations aredisplayed on the device's screen in a clearand easily comprehensible manner so thatpersonnel can decide on the basis of themeasurement result whether and whichmeasures need to be taken before enteringthe confined space.

Working in confined spacesWhen working in confined spaces, it is im-portant to remember that concentrations ofharmful substances can also change whilework is being carried out. This may be dueto inflowing contaminants (e.g. leaks in gaspipes), or indeed the work process itself(e.g. organic substances being released du-ring cleaning processes). Given that this isthe case, personal measurements should beperformed during work in confined spaces.The X-am 2000 is a small and lightweightinstrument which allows measurement ofcombustible substances, oxygen, carbonmonoxide and hydrogen sulphide. The X-am 2000 is the first multigas detector tofeature miniaturized Dräger XXS sensorsand, besides its compact dimensions andlow weight, also offers extremely quick response times and a high level of watertightness. In the event of hazardous gasconcentrations, the X-am 2000 also alertsthe user with a visual, audible and vibrationalarm, allowing the area to be exited or appropriate protective clothing to be donned in case of alarm.

Entering confinedspaces with multi gas detector and self contained, closed circuit escape set

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GAS DETECTION TECHNOLOGY THE USE OF PORTABLE GAS DETECTORS IN CONFINED SPACE APPLICATIONS

Measuring combustible substances: Cat Ex, IR or PID sensor?Different types of sensor can be used tomeasure combustible substances. In thepercentage by volume or LEL (lower explo-sive limit) range, catalytic Ex sensors (CatEx) or infrared sensors (IR) can be used. Inthe LEL concentration range, the Cat Exsensor works on the principle of catalytic

oxidation on a pellistor element of the com-bustible gas to be measured. This type ofmeasurement requires at least 10 % oxygento be present. In the case of the aforemen-tioned Dräger detectors, a Cat Ex sensorerror is given at oxygen concentrations below 10 %, thereby clarifying the situationfor the user. At even higher concentrationsof combustible gases (percentage by volumerange) and therefore even lower oxygenconcentrations, the sensor switches to athermal conductivity measurement and themeasurement result is displayed in percentby volume. The IR sensor does not requireany oxygen in order to measure organiccombustible substances because the sen-sor functions according to the principle ofIR light absorption. In other words, if orga-nic combustible substances need to bemeasured at very low oxygen concentrati-ons, the use of an IR sensor is recommen-ded. What is more, the IR sensor cannot beinfluenced by hydrogen sulphide or other(Cat Ex) sensor poisons. It needs to be re-membered, however, that the IR sensor, unlike the Cat Ex sensor, cannot measurehydrogen, so a combination of Cat Ex andIR Ex sensor may make sense, dependingon the particular application to be carriedout. The X-am 7000 allows these two sensortypes to be used in combination. Both sen-sors can be replaced by the user and storethe necessary data, such as limit values andcalibration data, in the sensor's EPROM(plug and play). This means that no repeatcalibration needs to be performed after asensor has been replaced.

If organic substances need to be detectedin the ppm range rather than the explosivelimit range, the use of a PID (photo ioni-zation detector) is recommended. Measu-ring these substances in the lower concen-tration range makes sense to the extentthat different organic substances have toxic relevance in the ppm range. PIDs are available both as stand-alone devices (e.g.Multi PID 2) and as sensors integrated inmultigas detectors (PID sensor in the X-am7000). When a PID sensor is integrated in the X-am 7000 multigas detector, an Ex and PID sensor can be used in combi-nation.

Flexibility by combining electrochemicalsensorsWhile certain multigas detectors (X-am2000, X-am 3000) are equipped with defined combinations of sensors, other multigas detectors like the MiniWarn and X-am 7000 allow the use of other electro-chemical sensors. This means that theaforementioned sensors needed for specialapplications (e.g. for ammonia, nitrous gases, sulphur dioxide, chlorine, hydro-cyanic acid, mercaptanes) can be used in these devices. These sensors also require no previous calibration. Calibrationdata and alarm limits are transmitted to themeasuring instrument when the sensor ischanged.

Dipl.-Ing. Oliver SchirkDräger Safety AG & Co. KGaAoliver.schirk@draeger.com

ConclusionMultigas detectors, thanks to the features they offer such as alarms and flexibility, are pre-destined for use in confined space applications. The user should focus on aspects such asease of operation, sturdiness and modularity. Recent developments in sensor technologyhave paved the way for miniaturized measuring instruments and, therefore, straightforwardpersonal measurements. As for all-clear measurements, devices with an integrated pumpare the best choice so as to make actual use of the detector as simple as possible and to largely rule out potential measurement errors. The measuring instruments described inthis article can be used to store and download the obtained measured data; together witha user-oriented analysis software (such as GasVision) it is possible then to evaluate andstore the measurement results in graph and tabular form.

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