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