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Introduction
The detection of airborne pollutants plays
an important role in all kinds of different
settings. In areas where there is a chance
that hazardous gas concentrations may be
present, measurements need to be conducted
to ensure that they comply with the
respective limit values. A basic distinction is
made in this context between personal and
area monitoring. Personal monitoring gives
a person information about the concentration
of harmful gaseous substances present
in the air surrounding them, and is therefore
the method of choice in many cases. It
is particularly useful when the person to be
monitored is moving from site to site within
a generally hazardous area. Area monitoring,
on the other hand, makes sense when
a clearly defined area needs to be checked
and the person in question is situated within
this area.
Confined spaces are one typical application
for this type of area monitoring. However,
some stationary tasks like welding can also
prove suitable for area monitoring. Once the
question of whether to use area or personal
monitoring has been clarified, all that needs
to be decided is whether to employ a single
or multigas system – this is the topic which
this article will be dealing with in more
detail.
Single- or multigas: flexibility can be an
important criterion.
It is of course clear from the name what the
difference is between a single and a multigas
detector: either one or several hazardous
substances can be measured at the
same time. However, both single- and multigas
instruments can be further differentiated.
One key aspect, besides the features
offered by the detector, is its flexibility with
regard to the interchangeability of sensors.
For instance, in some detectors the sensors
to be used are clearly defined, while others
allow the use of different types of sensors.
The cost of purchasing one of the less
flexible systems, of course, is lower. However,
if it is likely that the detector will be
used to monitor different substances rather
than just one substance or, in the case
of multigas detectors, a handful of specific
substances, the decision should be in
favour of an instrument which is as flexible
as possible. In such detectors the sensors
can be exchanged to suit the application in
hand. A single-gas detector used to check
oxygen levels, for example, can be used to
monitor carbon monoxide concentrations
following replacement (on a plug and play
basis) of the sensor. If in addition smart
sensors are used, where the calibration
data are stored in the sensor's EPROM, the
detector does not even require recalibration.
At the single-gas level, examples of
instruments designed to detect a single
specific gas include the microPac family
and the new Pac 1000, 3000, 5000. In the
case of the Dräger Pac III, it is possible to
switch between different gas types using
one and the same instrument. As far as
multigas instruments are concerned, the
sensors are clearly defined for the Dräger
X-am 3000, while the X-am 7000 allows
users not only to replace the sensors themselves,
but also switch between different
sensor types.
Single- or multigas – it all depends on
the application
It goes without saying that the application
will determine whether a single- or a multigas
detector system is required. Let us
take a look now at various applications and
the different criteria used as the basis for
decision-making.
Refineries
In the refinery industry, especially during
turnarounds and shutdowns, a large number
of people, often including the staff of other
companies, all need to be equipped with
gas detection instruments at the same
time. The primary gas hazard at a refinery
is posed by hydrogen sulphide. Normally
workers are additionally outfitted with a
filtering escape mask to allow them to evacuate
to a safe area if a warning of high
hydrogen sulphide concentrations is given.
Dräger microPac plus and Pac 1000, 3000,
5000 are single-gas detection systems
which could be used for this purpose. In the
case of the microPac plus, the user can
choose to have either an OK mode or the
concentration itself displayed, while Pac
1000 and 3000 display the concentration
once the set limit value has been reached
or violated, and Pac 5000 displays the
hydrogen sulphide concentration in ppm
throughout the measurement process. All
these instruments give visual, audible and
vibration alarms. |
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When entering confined spaces during
plant shutdowns, other gas hazards besides
hydrogen sulphide are likely to occur.
Depending on the type of confined space,
combustible substances or carbon monoxide
may be present, while low oxygen concentrations
also pose a possible danger. In
such cases where an all-clear measurement
for confined spaces needs to be conducted,
the obvious choice is a multigas detector
equipped with a pump to allow air from
lower-lying confined spaces to be pumped
out through an extension hose and analysed.
When cleaning, welding and soldering
work needs to be carried out it may make
sense to continue measurements while
work is ongoing or to use personal multigas
monitoring.
Steel production
During the steel production process, coke
is used to generate carbon monoxide so
as to reduce the iron oxide (iron ore). This
means that high concentrations of toxic
carbon monoxide are always likely to be
present here, making the steel production
industry a classic example of an application
for personal single-gas detectors. What is
more, CO leak detection also plays a part in
this sector – a single-gas detector specially
equipped with a pump and probe can also
be used for this purpose.
Refrigerated warehouses
In large-scale refrigeration plants in particular,
e.g. cold storage houses, cold stores
and in food and drink production, ammonia
is used as a cooling agent. Because ammonia
gas has a suffocating and corrosive
effect, personal monitoring with a single-gas
detector is the best option here. The singlegas
detector can also be used in such
areas for leak detection so long as it can
be fitted with a pump, extension hose and
leak probe. There are significant differences
as regards the quality of ammonia sensors,
however: the electrochemical Dräger XS
Ammonia sensor, for example, features a
special electrode-electrolyte combination
which achieves a sensor lifetime of two
years even if the sensor is exposed to high
levels of ammonia – during leak detection
operations, for instance.
Gold mining
To extract gold from rock containing very
little of the precious metal, a solution of
potassium or sodium cyanide is used during
a process known as cyanide leaching to
mine even microscopic quantities of gold
in rock (1g of gold per tonne of rock).
Although this is a simple, efficient and
low-cost method, it also releases relevant
concentrations of highly toxic hydrocyanic
acid. Here, too, personal single-gas monitoring
is a sensible option. The microPac
plus is a maintenance-free, easy to operate
and relatively low-cost single-gas instrument
which allows personal monitoring to be conducted
even in less prosperous countries.
Fumigation
Magnesium or aluminium phosphide, which
releases phosphine, is used during fumigation
of goods or timber to kill off pests.
Because phosphine is extremely toxic, personal
monitoring of phosphine concentrations
is needed when container checks are
carried out and indeed during the fumigation
process itself. For this application too,
an easy to use, maintenance-free single-gas
detector is available (microPac plus PH3),
which is specially designed for personal
monitoring and alerts the user by means
of visual, audible and vibration alarms if
the preset limit value concentrations are
reached or exceeded.
Sewers (pipes, confined spaces)
Unlike in the examples of applications
given above, the danger when entering a
sewer is not associated with one particular
gas – all kinds of different substances may
pose a hazard. When organic material
decomposes, for instance, methane may
be generated, with concentrations occurring
which may reach the explosion limit or even
the percent by volume range. What is more,
methane can seep into sewers from leaks
in municipal gas mains, especially as gas
mains and sewage pipes often run directly
next to one another. Decomposition processes
also use up oxygen, while hydrogen
sulphide can be produced if material containing
protein decomposes. In such cases,
at least three sensors should be used in a
portable detector; one sensor to detect
combustible gases (catalytic Ex sensor or
infrared Ex sensor), an electrochemical
sensor for oxygen measurement, and an
electrochemical sensor for measuring
hydrogen sulphide. Ideally, the instrument
should provide a clearly readable display of
all three concentrations at the same time.
One portable instrument which meets these
requirements is the Dräger X-am 3000.
Another substance which is relevant in
the sewage area is carbon dioxide, which
occurs as the by-product of natural biological
metabolic processes, especially in the
biological stages of waste water treatment
plants. In principle, carbon dioxide can be
detected using electrochemical sensors,
though infrared-optical sensors are better
suited to the job because they respond
more quickly. In other words, the perfect
multigas detector for sewage work would
also allow an infrared-optical sensor to be
installed for carbon dioxide. The X-am 7000
multigas detector not only satisfies all
these requirements, but is also water-tight
and allows an IR-CO2 sensor to be combined
with either a catalytic Ex sensor or
an IR Ex sensor. The user can choose to fit
the relevant sensors to suit the application
in hand, and with the X-am 7000 this also
applies to the Ex and IR CO2 sensors. Even
the integrated pump can be retrofitted or
replaced by the user.
Confined spaces in general
What we talked about with respect to sewers
and pipelines is generally applicable
to other confined spaces such as shafts,
piping and cable ducts too. In all these
confined spaces the addition of organic
material can trigger decay processes which
produce the same gas hazards as in sewers.
In addition, incomplete combustion
processes (e.g. cable fire, smouldering fire)
can produce toxic carbon monoxide. In such
applications a multigas detector should be
used which, ideally, allows the measurement
of five gases (Ex, oxygen, carbon
dioxide, hydrogen sulphide, carbon monoxide).
This requirement is likewise met by
the X-am 7000.
Dipl.-Ing. Oliver Schirk
Dräger Safety AG & Co. KGaA
Conclusion
To sum up, we can say that the choice of portable single- or multigas detectors depends
on the application in question. Where toxic substances only occur individually, a single-gas
detector is sufficient. As soon as a shortage of oxygen is also likely to be present in addition
to the toxic substance, the oxygen concentration should be monitored, so the use of
a multigas detector is to be recommended. When conducting all-clear measurements for
confined spaces, a range of different airborne contaminants can be expected, making the
use of a multigas detector (ideally for five gases with replaceable sensors) advisable. |
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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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