Room to breathe

Louise Early explains the gas risks of working in
confined spaces and what steps can be taken to keep
employees safe

Aconfined space is any space large
enough for someone to enter and
perform assigned work, which
has limited means of entry or exit, and
which is not designed for continuous
worker occupancy. This covers just
about every industry, including utilities,
construction, hydrocarbon exploration
and processing, petrochemicals,
marine, agriculture, food processing
and brewing, as well as the emergency
services.

Employers must assess the risks these
workplaces pose to their employees and
endeavour to prevent them. In most cases,
both the assessment and the safe working
system will require testing of the
atmosphere with gas detection equipment.

The gas risks can be divided into three
broad categories: combustible gas, toxic
gas, and oxygen depletion or enrichment.

Combustible gas risks
For combustion to occur the air must
contain a minimum concentration of
combustible gas or vapour. This quantity
is called the lower explosive limit (LEL).

Different compounds have different LELs
so it’s vital that detectors are capable of
detecting at the correct levels.

Typically, storage vessels which have
contained hydrocarbon fuels and oils
present a danger. Other dangers come
from fuel leaks: burst fuel containers;
pipelines on and off site, gas cylinders and
engine-driven plant. For workers in pits,
sewers and other sub-surface locations,
methane formed by decaying organic
matter is an almost universal danger.

Toxic gases and vapours
Confined-space workers may be exposed
to many toxic compounds, depending on
the nature of the work. A risk assessment
should be made to determine which toxic
substances a worker may be exposed to in
any given work situation.

When looking at toxic gases related to
specific applications, the water industry
for example uses many toxic compounds
for cleaning and processing both waste
and clean water. Hazards such as chlorine,
ozone, sulphur dioxide and chlorine
dioxide then pose additional risks both in
storage and treatment areas.

Oxygen – too high or too low?
The normal concentration of oxygen in
fresh air is 20.9%. An atmosphere is
hazardous if the concentration drops below
19.5% or goes above 23.5%. Without
adequate ventilation, the simple act of
breathing will cause oxygen levels to fall
surprisingly quickly.

Combustion also uses up
oxygen, so engine-driven plant
and naked flames such as
welding torches are potential hazards.

Oxygen can also be displaced.

Nitrogen, for example, when used to
purge hydrocarbon storage vessels prior to
re-use, drives oxygen out of the container
and leaves it highly dangerous until
thoroughly ventilated. High oxygen levels
are also dangerous. As with too little, too
much will impair the victim’s ability to
think clearly and act sensibly. Moreover,
oxygen-enriched atmospheres represent a
severe fire hazard.

Gas detector types
Both portable and fixed gas detectors can
be used for confined space monitoring.

Fixed systems typically comprise one or
more detector ‘heads’ connected to a
separate control panel. If a detector reads a
dangerous gas level, the panel raises the
alarm by triggering external sirens and
beacons. This sort of installation is suited
to larger spaces like plant rooms, which
have sufficient room for the hardware or
remote stations that are usually unmanned.

However, much confined space work
takes place in more restricted areas,
making compact portable units more
suitable. Ease of use, with one button
operation, means minimal training is
required while increased safety is ensured.

Combining one or more sensors with
powerful audible and visual signals to warn
when pre-set gas levels are reached,
portable detectors can be carried or worn
wherever they are needed. In addition, a
compact instrument is easily carried in a
confined space, ensuring that pockets of
high gas concentration are not missed.

Certain features should be expected in
every portable gas detector. Clearly, lifesaving
tools for demanding environments
must be as tough as possible, with reliable
electronics housed in impact-resistant
casings. While the need to leave gas
sensors exposed to the atmosphere means
that no instrument can be fully sealed, a
high degree of protection against dust and
water ingress is essential. Toughness
notwithstanding, a well-designed detector
will also be light and compact enough to
wear for an entire shift.

Finally, because of the difficulties of
working in a cramped space, perhaps
under poor lighting, instruments should
be easy to use. No matter how advanced a
detector’s internal architecture or data
management options, personnel in the
field should be faced with nothing more
daunting than a clear display, simple, onebutton
operation and loud/bright alarms.

Louise Early is portable product manager
at Crowcon