The Unseen Danger Working Safely with Flammable Liquids and Powders
Introduction
Of all the hazards which you will encounter at work, the
vapour produced by flammable liquids is amongst the most
dangerous because it is invisible and, if safe working
practices are not followed, potentially deadly.
The burn injuries caused if the vapour is ignited can be
horrific, and sometimes fatal.
In addition, whole factories can be totally destroyed by
the ensuing fire.
Similarly, the build up of electrostatic charge on certain
powders can lead to major explosions that can severely
injure persons and destroy buildings.
Understanding the Risks
Flammable substances should be marked with a fire symbol,
or will have a safety phrase indicating that the substance
is flammable.
The `flash-point' is the temperature at which the vapour
is capable of being ignited.
However, it is safer to treat all flammable substances
with the greatest of care, regardless of their warning
label or flash-point.
The vapours can quickly spread many metres away from the
liquid, and if ignited will tend to `flash' back to the liquid.
Some powders can accumulate a static charge when being
transferred from one container into another, and some
materials are inherently combustible in powder form.
For a flammable vapour or dust to ignite, three things must
be present; fuel, heat, and oxygen. Remove any one of these
components and a fire or explosion will be prevented.
The fuel is provided by the flammable liquid vapour. These
vapours are heavier than air and will sink to the floor and
spread rapidly. They can be removed from the fire triangle
by ensuring that containers are carried and stored with
their lid securely on at all times.
Heat is provided by a source of ignition, such as a spark
from a light switch. Because it is the vapour which burns
and not the flammable liquid, the ignition source could be
some distance away from the container.
Oxygen is in the air all around us. In some cases a chemical
may generate oxygen itself, (for example, oxidising agents),
which will feed oxygen to the fire.
Generally speaking, however, because the oxygen
component is difficult to remove, there are just
two ways of preventing a flammable liquid fire;
by careful control of the flammable vapours, and
by the elimination of all possible sources of ignition.
Removing the Danger
The two best ways of preventing a flammable liquid fire are;
by careful control of the flammable vapours, and
the elimination of all possible sources of ignition.
Local exhaust ventilation, including low level vents, are
necessary wherever flammable liquids are used or stored.
Never use an open-topped container when carrying solvents.
Always use a metal container with a close-fitting lid.
When dispensing flammable liquids always use a safety
container, which has a self-closing cap and prevents the
release of vapour when filling or pouring.
In the laboratory, flammable liquids should be stored
correctly in a safety cabinet.
Open topped process vessels should be covered when in use to
prevent the release of vapours.
Cleaning operations using flammable liquids should be done in
a room or cabinet provided for the purpose.
Always dispose of solvent soaked cleaning rags properly in a
metal bin with a self-closing lid.
In bulk storage areas, bund walls should be used to contain
any spillage. Dispensers should be fitted with automatic
shut-off valves to prevent overspill.
Drums should be fitted with self-closing valves, and drip
trays should be installed to catch small amounts spilled.
Where combustible dust is handled, it is essential to maintain
a high standard of housekeeping. If dust deposits are allowed
to accumulate they can provide the fuel for an explosion.
Sources of Ignition
One way of preventing a flammable liquid fire is to eliminate
all possible sources of ignition. The main sources of ignition
to be found in most workplaces are:
static electricity,
sparks from electrical equipment,
fire,
heat generated by mechanical equipment and
sparks from ferrous tools.
The sparks that can be caused by static electricity are deadly
if flammable vapours are present.
Electrical equipment installed in areas where flammable vapours
may be present should be constructed to a suitable explosion-
protected standard (often denoted by an Ex symbol).
Be aware that portable electrical tools can generate sparks and
their use should be strictly controlled.
Light switches often spark, even when being switched off.
A major cause of fire is hot work, such as welding and
cutting, carried out on vessels containing flammable
vapour, with the likelihood of severe burns or death.
Hot work must only be done under a permit-to-work system.
This will include following site rules and instructions,
and operating only when authorised by a competent person.
Smoking is strictly prohibited where flammable liquids are
handled. Smoke only in designated areas and do not carry
lighters or matches in your pocket.
Frictional heating of moving parts of process plant can result
in hot surface temperatures on the equipment, particularly if
it is poorly maintained. This should not be allowed to exceed
the ignition temperature of any flammable liquids in use in the vicinity.
Some non-electrical tools can also be a significant ignition
source if they are allowed to generate a spark. Some examples
are impact sparks generated by tools dropped onto the floor,
metal shovels and scrapers used to clean up a solidified spill,
and truck forks contacting with a metal drum.
Control Measures
Perhaps the most common safeguard against electrostatic
build up is the earthing of all containers and equipment
containing flammable liquids.
Earthing contacts should be regularly maintained and kept clean.
Portable containers should be earthed before use with
bonding clips connected by a wire to a fixed earth point.
The clips must not be removed until after the transfer process.
If for some reason this is not possible the process must be stopped.
Splash filling of process vessels and containers should be avoided.
Always ensure the liquid is poured without splashing by
directing it against the inside edge of the container.
Take particular care when adding dry powders to vessels containing flammable liquids.
Use aluminium scoops, or pour powders from an anti-static bag into an earthed chute.
As an additional precaution, wear anti-static clothing
and footwear, and ensure the floor is clean and not painted with a non-conductive coating.
Your Responsibilities
To prevent static charges building up on your body and
on your clothes, you should wear anti-static clothing and footwear.
This safe flow of electrostatic energy will however be
impeded if the floor is dirty, as this destroys the
conductive properties of the flooring material. So
follow good housekeeping procedures and always clean up spillages, however small.
For the same reason, floors should not be painted
unless it is with special conductive paint.
Don't be tempted to remove your anti-static clothing,
even for a short period. The movement of one material
against another will produce a high static charge and
turn you into a potential source of ignition.
Remember, simply pouring a flammable liquid into another
container can generate a static charge.
Before dispensing any flammable liquid, always attach an
earthing cable to both vessels.
Make sure that the clips are clean and connect with bare metal.
Report any equipment which is damaged to your supervisor and do not use it.
Don't tamper with safety equipment such as flame arrestors.
When not in use, containers of flammable liquids should be
securely closed and returned to storage areas or cabinets.
All sources of ignition should be eliminated; keep matches, lighters etc. away from the work area.