Civil Engineering Reference
In-Depth Information
The supply of oxygen, as previously discussed,
can infl uence fi re behaviour and thus recognising how
the supply can be enhanced or reduced is vital when
considering control factors.
Ventilation and air handling systems can provide
an enhanced supply as can the use of oxygen cylinders
(medical use and hot work/cutting work).
Chemical reactions can also cause the release
of oxygen, particularly chemicals such as ammonium
nitrate (fertilisers), sodium chlorate (pesticides), hydrogen
peroxide (water treatment, hair care) and chromate
(variety of industrial processes).
It should also be noted that oxidisers exhibit
highly exothermic reactions when in contact with other
substances, particularly fl ammable substances. Oxidising
agents can also increase the combustibility of substances
which do not normally burn readily in air, or can lower
ignition temperatures to such a point that materials burn
more readily and more violently.
Figure 7.7
Typical fl ammable liquid sign
Gases
Flammable gases are common throughout workplaces
and include natural gas and liquefi ed petroleum gas
(LPG). The most common types of LPG are stored in
pressurised cylinders and include butane and propane
(i.e. cooking, heating, plumbers' torches, etc.); in addi-
tion dissolved acetylene and oxygen mixtures are
utilised in welding operations.
As with the fl ammable vapours given off by fl amma-
ble liquids not only do gases present a signifi cant risk of
fi re they also present an
explosion
risk.
Gases do not always come in containers and can be
produced by chemical reactions such as the degradation
of waste materials within a refuse dump which produces
methane.
Figure 7.9
Typical oxidising agent sign
7.2.6
The chemical process
Conditions
Quite specifi c conditions are needed for the fi re pro-
cess to start and continue. In order for an evaluation of
whether the conditions present the potential for fi re, it is
essential to gather information as to the likelihood that
the conditions will arise.
Flammable materials are provided with information
sheets (Materials Safety Data Sheets - MSDS) which are
produced by the manufacturer for onward transmission
to the suppliers and ultimately the 'end user', as required
by both the Chemical Hazards (Information Packaging
for Supply) Regulations and HSWA, section 6.
The MSDS will include certain information which will
help to analyse the fi re risks.
Certain fi re data is provided (based upon test
conditions):
Figure 7.8
Typical fl ammable gas sign
Dusts
Dusts can be produced from many everyday and work-
place materials such as wood, coal, grain, sugar, syn-
thetic organic chemicals and certain metals. A cloud
of combustible dust in the air can explode violently
if ignited. Explosions and explosive materials are dis-
cussed later in this chapter.
7.2.5 Oxygen
➤
Flashpoint
is the lowest temperature at which a sub-
stance will produce suffi cient vapour to fl ash across
its surface momentarily when a fl ame is applied
The fi nal element of the fi re triangle that should be dis-
cussed is oxygen. Oxygen is contained as an element
within the air that we breathe (19.6%) and therefore is
available in any area that humans can live.
➤
Firepoint
is the lowest temperature at which the
heat from the combustion of the burning vapour is
