Civil Engineering Reference
In-Depth Information
Control
Critical to the management of explosive atmospheres is
the avoidance or reduction of potentially explosive mater-
ials within an atmosphere.
The substitution of fl ammable substances by inert
materials or limiting the concentrations of the fl amma-
ble substances to avoid their explosive range must be
considered at the top of any explosion management
hierarchy.
Such controls may be the replacement of a fi ne
dusty material by a less dusty granular material or
reducing the fl ammable gas to the absolute minimum.
Limiting the concentration to avoid the explosive range
with mechanical systems linked to ventilation which may
be actuated via gas or fl ow detectors (including alarms)
should be considered. In the case of combustible liquids
the objective should be to reduce the concentration of
any mist formed, below the lower explosion limit, which
in turn will ensure it is suffi ciently below its fl ashpoint to
prevent explosion.
An alternative mechanism may be the use of adding
inert gases, e.g. nitrogen and carbon dioxide, utilising
water vapour or inerting using a powdery substance
such as calcium carbonate. With appropriate dispersal
these materials can prevent the formation of an explosive
atmosphere, which we term inerting.
can be broken down into primary and secondary explo-
sions. The concentrations required for a dust explosion
are generally not seen outside process vessels and thus
the most signifi cant dust explosions start from within a
piece of equipment, e.g. mixers, hoppers and silos.
The fi rst explosion is known as primary causing
a rupture of the vessel releasing the fl ammable gas/
air mixture into the atmosphere. With dust generally
suspended in the air around the process equipment or
dust lying undisturbed within the building a secondary
explosion occurs.
A similar chain of events can be seen in relation
to gas explosions, particularly when a gas explosion
involves pressurised containers. The consequences of
gas explosions range from no damage to total destruc-
tion and can lead to fi res and BLEVEs as indicated in
Figure 7.30.
7.5.5
Principles of explosion management
The principles of explosion management can be broken
down into two discrete areas: those of control and miti-
gation, each will be dealt with separately. The Dangerous
Substances and Explosive Atmospheres Regulations
(DSEAR) require the application of a hierarchy of control
measures to manage the risk of accidental explosion.
Design and construction - equipment, protective
systems and system components
When considering the types of equipment, protective
systems and components that will contain fl ammable
substances, reasonably practicable steps should be
made to keep the substances enclosed at all times and
the materials of construction should be non-combustible.
Where necessary leak detection systems should
be fi tted and particular attention should be given to the
following areas:
DSEAR regulation 6(4)
(a) the reduction of the quantity of danger-
ous substances to a minimum;
(b) the avoidance or minimising of the
release of a dangerous substance;
(c) the control of a release of a dangerous
substance at source;
(d) the prevention of the formation of an
explosive atmosphere, including the appli-
cation of appropriate ventilation;
(e) ensuring that any release of a danger-
ous substance that may give rise to risk
is suitably collected, safely contained,
removed to a safe place, or otherwise
rendered safe, as appropriate;
(f) the avoidance of -
(i) ignition sources including electro-
static discharges; and
(ii) adverse conditions which could
cause dangerous substances to give
rise to harmful physical effects; and
(g) the segregation of incompatible danger-
ous substances.
➤
Joints
➤
Piping
➤
Areas that may be subject to impact
➤
Areas that may be subject to hazardous interactions
with other substances.
The detection systems should provide advanced warning
of any leakage from the equipment, systems or compon-
ents, so that appropriate steps can be taken to prevent
the consequential build-up of any fl ammable atmosphere.
Dilution by ventilation
While quite effective with gas and vapour in relation to
dusts, ventilation is of limited effectiveness and provides
suffi cient protection only when the dust is extracted from
the place of origin and deposits of combustible dust can
be prevented.
