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There are areas at many workplaces which
have such restricted dimensions that personnel
who enter them or have to work
inside them are hindered in their activities
or even at risk. Such areas are generally
referred to as "confined spaces" if their
volume is less than 100 m3, if they lack any
natural or manmade system of ventilation
and/or air extraction, or if they are below
two metres in either length, width, height
or diameter. Confined spaces are to be
found not only in industrial settings (tanks,
boilers, chemical apparatus, storage containers),
but also in many other work
areas. |
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For example, windowless cellar rooms, well
and sewage shafts, pipelines, mineshafts,
tunnels, pits, ditches, canals, double-bottom
tanks in ships, bridge and crane supports,
and hollow sections in machines, are also
regarded as confined spaces. It is often
difficult to escape from confined spaces
because entry and exit is only possible
through a manhole. Because of the lack of
space, the presence or possible formation
of harmful substances in gaseous form is of
particular relevance. Before personnel enter
confined spaces, it must be ensured that no
hazardous gas concentrations are present.
What is more, continuous monitoring of air
quality is necessary the entire time work is
being carried out in confined spaces. This
article cites a number of practical examples
to illustrate the use of portable measurement
systems for air monitoring in confined
spaces. |
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The risk posed by different gases
Wenn möglich, sollte der Einstieg in enge
Räume vermieden werden. Allerdings ist
das Betreten enger Räume für Wartungs-,
Reparatur und Reinigungsarbeiten häufig
unvermeidbar, und in diesen Fällen ist es
unabdingbar, zuvor festzustellen, ob sich
toxische oder explosive Stoffe in dem
engen Raum befinden und ob der Sauerstoffgehalt
weder zu hoch noch zu gering
ist. Hierbei ist aufgrund der unterschiedlichen
Dichte von Gasen darauf zu achten,
dass Messungen am Boden, in der Mitte
und im oberen Bereich des engen Raumes
durchzuführen sind. Werden Schadstoffe
festgestellt, so ist zu prüfen, ob die Schadstoffe
durch Lüftung oder andere Maßnahmen
entfernt werden können, oder ob ein
Einstieg mit entsprechender Schutzausrüstung
erforderlich ist. In den meisten Fällen
ist die Messung von Schadstoffen auch
während der Arbeit im Confined-Space-
Bereich erforderlich.
Einsatz tragbarer Meßsysteme
Zunächst kann zwischen Kurzzeit- und
kontinuierlichen Messsystemen differenziert
werden. Ein geradezu klassisches Beispiel
für ein Kurzzeitmesssystem sind die Dräger-
Röhrchen. Der Vorteil dieser Messmethode
liegt in ihrer hohen Flexibilität. Bei den Röhrchen handelt es sich um eine sehr
kostengünstige Methode, und auch exotische
Substanzen können mit den Dräger-
Röhrchen gemessen werden. Der Messzeitraum
erstreckt sich allerdings nur über
wenige Minuten und um eine echte Überwachung
der Atmosphäre im engen Raum
zu gewährleisten, müssten viele Einzelmessungen
durchgeführt werden. Insofern
empfiehlt sich insbesondere für personenbezogene
Messungen der Einsatz kontinuierlicher
Messsysteme.
Wie oben erwähnt, ist in fast allen Fällen
das Auftreten brennbarer Substanzen
(Methan), Kohlenmonoxid, Schwefelwasserstoff
und Sauerstoffmangel, bzw. Sauerstoffüberschuss
nicht sicher auszuschließen.
Insofern bietet sich ein Viergas-Messsystem
an. Kontinuierlich messende, tragbare
Messsysteme arbeiten üblicherweise mit
Sensoren. So wird die Messung von brennbaren
Substanzen mit einem katalytischen
Ex-Sensor durchgeführt, toxische Substanzen
und Sauerstoff werden mit stoffspezifischen
elektrochemischen Sensoren gemessen.
Diese vier Sensoren werden in
Mehrgasgeräten, wie den Geräten der
Dräger X-am-Familie eingesetzt, so dass
die genannten Substanzen parallel kontinuierlich
gemessen werden. Der Anwender
erhält über optischen, akustischen und
Vibrationsalarm eine Warnung, sobald ein
oder mehrere Grenzwerte der zu messenden
Substanzen erreicht oder überschritten
worden sind. |
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Confined Spaces
In principle, toxic and/or combustible gases
can occur or be enriched in all confined
spaces, and all confined spaces may be
prone to a lack or surplus of oxygen. In
almost all cases, consideration needs to
be given to the following four gases:
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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); |
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Hydrogen sulphide: produced by rotting
organic material (animal corpses, faeces)
and to be found as a constituent of crude
oil and oil products (chemical industry,
oil industry); |
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Carbon monoxide: produced by incomplete
combustion, e.g. smouldering fires
in cable ducts, or released through leaks
in exhaust gas pipes or as exhaust fumes
from heating systems or motor vehicles |
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Lack of oxygen due to the consumption
of oxygen during the decomposition of
organic material, during smouldering fires
or when oxygen is displaced by gas leaks
(e.g. methane from gas pipes). Danger
of suffocation at oxygen concentrations
below 19.5 % by volume. |
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Oxygen surplus (caused by leaks in
areas where personnel work with oxygen)
means that substances – including pro
tective clothing of low flammability in
normal atmospheres – burn more easily,
more quickly, and at higher temperatures. |
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Explosive fires can be caused. These hazards
can certainly be expected at oxygen concentrations
in excess of 23.5 % by volume. |
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Depending on the local circumstances, and
the work being carried out, other gases
may occur, e.g.: |
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ammonia in refrigeration systems or in
agriculture; |
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nitrogen oxides during welding work
or as a constituent of diesel exhaust
fumes; |
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sulphur dioxide during burning of fossil
materials; |
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chlorine resulting from leaks in water disinfection
applications (waste water, cooling
water, drinking water, swimming pools); |
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hydrocyanic acid in the production of
noble metals and in galvanization; |
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mercaptanes as odorants in natural gas. |
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The gases listed above are either toxic and
can affect the lungs, cause illness or poisoning
– depending on their mode of action
and concentration – or are combustible,
meaning that there is a risk of explosion in
combination with the oxygen contained in the air. In addition, low but nonetheless
toxic concentrations of volatile organic compounds
(VOCs) such as fuels, oils, solvents,
pesticides and herbicides may be present. |
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Safety measures
If at all possible, confined spaces should
not be entered. Often, however, confined
spaces have to be entered in order to carry
out necessary maintenance, repair or cleaning
work; in such cases it is essential that
a check be carried out prior to entry to determine
whether toxic or explosive substances
are present in the confined space,
and whether the oxygen content is too high
or too low. Because gases can have different
densities, it is important to ensure that
measurements are performed at floor level,
at mid height and at ceiling level of the
confined space. If harmful substances are
detected, it needs to be checked whether
the substances in question could be removed
by ventilation or any other means, or
whether personnel need to don appropriate
protective equipment before entering the
confined space. In the majority of cases, it
is also necessary to continue measuring for
harmful substances while work is ongoing
inside the confined space. |
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Use of portable measurement systems
First, we can distinguish between shortterm
and continuous measurement systems.
A classic example of a short-term measurement
system is the Dräger Tube. The advantage
of this method of measurement lies
in its high level of flexibility. Dräger Tubes
represent an extremely low-cost option, and
can also be used to measure exotic substances.
However, the period of measurement
lasts only a few minutes; to ensure
proper monitoring of the atmosphere in a
confined space, many individual measurements
would have to be carried out. In this
context, continuous measurement systems
are the obvious choice, particularly for personal
measurements. |
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As mentioned above, the presence of combustible
substances (methane), carbon monoxide,
hydrogen sulphide or a lack or surplus
of oxygen cannot be ruled out with any
degree of certainty in most cases. A fourgas
detector would therefore be the measurement
system to choose here. Continuous
portable measurement systems normally
work with sensors. Combustible substances
are measured using a catalytic Ex sensor,
for example, while toxic substances and
oxygen are measured by substance-specific
electrochemical sensors. These four sensors
are used in multigas detectors such as
the devices of the Dräger X-am family, allowing
the aforementioned substances to be
measured all at once on a continuous basis.
The user is alerted by a visual, audible and
vibration alarm the moment one or more limit
values of the substances to be measured
is reached or exceeded. |
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Confined spaces: what are the key
characteristics of the measurement
systems used?
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Entering confined spaces
An all-clear measurement has to be performed
before personnel enter a confined space
to make sure that no combustible or toxic
substances are present, and to ensure
that there are acceptable concentrations of
oxygen available. For the purposes of the all-clear measurement, the multigas detector
is fitted with a pump and an extension
hose; the extension hose is then fed into
the confined space – through the entrance
opening, for example. Ideally, the multigas
detector 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 up
to 20 metres in length can be used with
the X-am 3000 – in the case of the X-am
7000 the hose can be as long as 45
metres. In both devices the internal pump is
activated automatically when the pump adapter
is fitted. If the extension hose becomes
blocked or obstructed for any reason
(e.g. if it is kinked), the user will be alerted
by the devices. The gas concentrations are
displayed on the device's screen in a clear
and easily comprehensible manner so that
personnel can decide on the basis of the
measurement result whether and which
measures need to be taken before entering
the confined space. |
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Working in confined spaces
When working in confined spaces, it is important
to remember that concentrations of
harmful substances can also change while
work is being carried out. This may be due
to inflowing contaminants (e.g. leaks in gas
pipes), or indeed the work process itself
(e.g. organic substances being released during
cleaning processes). Given that this is
the case, personal measurements should be
performed during work in confined spaces.
The X-am 2000 is a small and lightweight
instrument which allows measurement of
combustible substances, oxygen, carbon
monoxide and hydrogen sulphide. The
X-am 2000 is the first multigas detector to
feature miniaturized Dräger XXS sensors
and, besides its compact dimensions and
low weight, also offers extremely quick
response times and a high level of water
tightness. In the event of hazardous gas
concentrations, the X-am 2000 also alerts
the user with a visual, audible and vibration
alarm, allowing the area to be exited or
appropriate protective clothing to be
donned in case of alarm. |
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Measuring combustible substances:
Cat Ex, IR or PID sensor?
Different types of sensor can be used to
measure combustible substances. In the
percentage by volume or LEL (lower explosive
limit) range, catalytic Ex sensors (Cat
Ex) or infrared sensors (IR) can be used. In
the LEL concentration range, the Cat Ex
sensor works on the principle of catalytic oxidation on a pellistor element of the combustible
gas to be measured. This type of
measurement requires at least 10 % oxygen
to be present. In the case of the aforementioned
Dräger detectors, a Cat Ex sensor
error is given at oxygen concentrations
below 10 %, thereby clarifying the situation
for the user. At even higher concentrations
of combustible gases (percentage by volume
range) and therefore even lower oxygen
concentrations, the sensor switches to a
thermal conductivity measurement and the
measurement result is displayed in percent
by volume. The IR sensor does not require
any oxygen in order to measure organic
combustible substances because the sensor
functions according to the principle of
IR light absorption. In other words, if organic
combustible substances need to be
measured at very low oxygen concentrations,
the use of an IR sensor is recommended.
What is more, the IR sensor cannot be
influenced by hydrogen sulphide or other
(Cat Ex) sensor poisons. It needs to be remembered,
however, that the IR sensor,
unlike the Cat Ex sensor, cannot measure
hydrogen, so a combination of Cat Ex and
IR Ex sensor may make sense, depending
on the particular application to be carried
out. The X-am 7000 allows these two sensor
types to be used in combination. Both sensors
can be replaced by the user and store
the necessary data, such as limit values and
calibration data, in the sensor's EPROM
(plug and play). This means that no repeat
calibration needs to be performed after a
sensor has been replaced. |
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If organic substances need to be detected
in the ppm range rather than the explosive
limit range, the use of a PID (photo ionization
detector) is recommended. Measuring
these substances in the lower concentration
range makes sense to the extent
that 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 in
multigas detectors (PID sensor in the X-am
7000). When a PID sensor is integrated
in the X-am 7000 multigas detector, an
Ex and PID sensor can be used in combination. |
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Flexibility by combining electrochemical
sensors
While certain multigas detectors (X-am
2000, 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 electrochemical
sensors. This means that the
aforementioned sensors needed for special
applications (e.g. for ammonia, nitrous
gases, sulphur dioxide, chlorine, hydrocyanic
acid, mercaptanes) can be used
in these devices. These sensors also
require no previous calibration. Calibration
data and alarm limits are transmitted to the
measuring instrument when the sensor is
changed. |
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Conclusion
Multigas detectors, thanks to the features they offer such as alarms and flexibility, are predestined
for use in confined space applications. The user should focus on aspects such as
ease of operation, sturdiness and modularity. Recent developments in sensor technology
have paved the way for miniaturized measuring instruments and, therefore, straightforward
personal measurements. As for all-clear measurements, devices with an integrated pump
are 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 in
this article can be used to store and download the obtained measured data; together with
a user-oriented analysis software (such as GasVision) it is possible then to evaluate and
store the measurement results in graph and tabular form. |
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Dipl.-Ing. Oliver Schirk
Dräger Safety AG & Co. KGaA |
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Dräger Safety AG & Co. KGaA |
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Revalstrasse 1 |
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23560 Luebeck, Germany |
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Tel +49 451 882 0
Fax +49 451 882 2080
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