MRS TRAINING & RESCUE
OXYGEN IN A CONFINED SPACE
It is essential that oxygen is present within a confined
space as a constituent of the air within the space. There
are, however, two concerns associated with oxygen in or
around confined spaces. These are:
1.Oxygen enrichment
This can occur when certain processes are being
undertaken in or near confined spaces. Examples can
be the use of oxygen leaking from cylinders used in
welding or its production from chemical processes. It
can also occur when other gases such as nitrogen are
removed from air. (Oxygen has a specific gravity of
1.1 and is therefore heavier than air.) The increase of
oxygen content results in a considerable increase in the
flammability of combustible material. This may also
alter the “flammable range” of flammable gases.
2.Oxygen deficiency
This can occur when a confined space is infrequently
opened up to the atmosphere and rusting has occurred.
This rusting is a slow form of combustion and absorbs
oxygen. Again, other processes such as burning and
welding can use up the oxygen content. There are many
examples of fatalities occurring due to this reduction in
the oxygen content. Unconsciousness, and subsequent
fatalities, can result when the oxygen content drops to
around 16%. in order to provide a reasonable level of
safety, the lowest acceptable level of oxygen content
should not be below 19%.
CHEMICALS
Assessments on the effect of chemicals to be used in a
confined space should be fully noted and understood to
ensure the assessed risk of the chemical, and its impact on
those employed to use it, are controlled to a level that is
acceptable.
An upsetting example: a very unfortunate employee
working with a chemical to clean an aviation fuel tank was
so hot while working inside the fuel tank, he decided to
unplug the light to remove it as a heat source. Unplugging
the light caused a spark which ignited the vapour from the
cleaning chemical. The employee suffered severe burns in
the explosion. His injuries prevent him from ever working
again.
Following an investigation, it was revealed that the
cleaning chemical was not required; warm “soapy” water
did as good a job of cleaning the tank. This should have
been realised sooner with an effective assessment of the
risk of utilising the chemical within the confined space.
DETECTING FOR GAS, VAPOUR OR FUME
Every confined space can, and must, be tested prior
to persons entering it. This can be achieved by the use of
tubing or a probe and drawing a sample to the detection
device. An alternative to this is to place or lower a suitable
detection device into the space and allow it to record the
environmental conditions. These will then be available
to those assessing the risks. There are environmental
monitors available that will ‘Bluetooth’ environmental
readings to a smart phone.
38
HSE INTERNATIONAL
Where the assessment of risk suggests a requirement
to test the environment within a confined space, the
space should be opened, ventilated (mechanical and/
or natural) and tested prior to any person entering the
space and, if required, tested again after a predetermined
period ascertained by the control measure from the
risk assessment. Testing should be carried out on every
occasion the confined space is re-entered to ensure nothing
has changed within the space. Even small changes in
barometric pressure can alter the atmosphere within a
confined space.
When persons are working within the confined
space, the atmosphere should be constantly monitored,
particularly if mechanical ventilation is a requirement of the
risk assessment. Constant monitoring will also be required
if the work being undertaken is particularly hazardous, i.e.
cutting and welding.
DETECTION INSTRUMENTS
Detection instruments can be supplied in various
combinations of sensors. These range from single sensor
devices for a specific gas to multi sensor devices for several
gases. They may also vary depending on the design role of
the software attached to the sensor.
ORDER OF TESTING
Ascertaining the presence of oxygen is a priority due
to the influence that it has on both the flammability and
toxicity of other gases. Flammable gases should be tested
next, by toxic gases, fumes and vapours. It should also be
remembered that some gases, e.g. carbon monoxide, are
both toxic and flammable.