Introduction to Fire Safety Management
Introduction to Fire Safety Management
Introduction to Fire Safety Management
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<strong>Introduction</strong> <strong>to</strong> <strong>Fire</strong> <strong>Safety</strong> <strong>Management</strong><br />
Table 7.5 Showing the zones for dusts<br />
Zones for dusts<br />
Zone 20 A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is present continuously, or<br />
for long periods or frequently.<br />
Note: In general these conditions, when they occur, arise inside containers, pipes and vessels, etc.<br />
Zone 21 A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is likely <strong>to</strong> occur in normal<br />
operation occasionally.<br />
Note: This zone can include, among others, places in the immediate vicinity of, e.g., powder fi lling and emptying<br />
points and places where dust layers occur and are likely in normal operation <strong>to</strong> give rise <strong>to</strong> an explosive concentration<br />
of combustible dust in mixture with air.<br />
Zone 22 A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is not likely <strong>to</strong> occur in<br />
normal operation but, if it does occur, will persist for a short period only.<br />
Note: This zone can include, among others, places in the vicinity of equipment, protective systems, and components<br />
containing dust, from which dust can escape from leaks and form dust deposits (e.g. milling rooms, in which dust<br />
escapes from the mills and then settles).<br />
s<strong>to</strong>red, etc. These should be located away from other<br />
buildings and actual parts of the plant should be as<br />
remote from one another as is possible. Ideally buildings<br />
should be of single s<strong>to</strong>rey in nature but kept as low<br />
as possible and the explosion prone part of any pro -<br />
cess should be as high as possible, ideally on the roof <strong>to</strong><br />
minimise the possibility of building collapse.<br />
Where any hazardous part of the plant is located<br />
within a building the area should be reinforced and<br />
protected from the rest of the area by a blast wall. As<br />
discussed previously the area should be vented <strong>to</strong> avoid<br />
damage (structural) from any overpressure.<br />
Escape routes and other emergency response<br />
planning must take in<strong>to</strong> account the explosive nature, as<br />
should any electrical equipment.<br />
Under the Dangerous Substances and Explosive<br />
Atmospheres Regulations 2002 there is a requirement<br />
<strong>to</strong> identify hazardous contents (containers and plant) <strong>to</strong><br />
ensure that the selection of the correct equipment and<br />
systems can take in<strong>to</strong> account the level of and likelihood<br />
of there being an explosive atmosphere.<br />
7.6 Case study<br />
Flixborough disaster 1974<br />
One of the most serious accidents in the his<strong>to</strong>ry of the<br />
chemical industry was the explosion at about 16:53<br />
hours on Saturday 1 June 1974 at the Nypro (UK) site at<br />
Flixborough, which was severely damaged. Twenty-eight<br />
workers were killed and another 36 others were injured<br />
as a direct result of the explosion and subsequent fi re. It<br />
is recognised that the number of casualties would have<br />
been more if the incident had occurred on a weekday, as<br />
the main offi ce block was not occupied.<br />
134<br />
Outside the plant, 53 persons were reported injured<br />
with 1821 houses and 167 shops suffering damage<br />
ranging from major (required rebuilding) <strong>to</strong> broken<br />
glazing from the pressure waves.<br />
The overall cost of the damage at the plant itself<br />
<strong>to</strong>gether with the damage outside was estimated at over<br />
£75 million.<br />
The cause of the Flixborough explosion was a<br />
release of about 50 <strong>to</strong>ns of cyclohexane, due <strong>to</strong> failure of<br />
a temporary pipe. The fl ammable cloud was ignited about<br />
1 minute or so after the release. A very violent explosion<br />
occurred. The blast was equivalent <strong>to</strong> an explosion of<br />
about 16 <strong>to</strong>ns of TNT.<br />
Five days prior <strong>to</strong> the explosion, on 27 March 1974,<br />
it was discovered that a vertical crack in reac<strong>to</strong>r No. 5<br />
was leaking cyclohexane. The plant was subsequently<br />
shut down for an investigation. The investigation that<br />
followed identifi ed a serious problem with the reac<strong>to</strong>r and<br />
the decision was taken <strong>to</strong> remove it and install a bypass<br />
assembly <strong>to</strong> connect reac<strong>to</strong>rs No. 4 and No. 6 so that the<br />
plant could continue production.<br />
During the late afternoon on 1 June 1974 the<br />
temporary 20 inch bypass system ruptured, which is<br />
extremely likely <strong>to</strong> have been caused by a fi re on a nearby<br />
8 inch pipe. This resulted in the escape of a large quantity<br />
of cyclohexane, which formed a fl ammable mixture and<br />
subsequently found a source of ignition.<br />
At approximately 16:53 hours there was a massive<br />
unconfi ned vapour cloud explosion which caused exten -<br />
sive damage and started numerous fi res on the site.<br />
Eighteen fatalities occurred in the control room as a<br />
result of the windows shattering and the collapse of the<br />
roof. No one escaped from the control room. The fi res<br />
burned for several days and after ten days those that still<br />
raged were hampering the rescue work.<br />
The characteristic of the gas explosion at Flixborough<br />
is that the dense fuel (cyclohexane) was able <strong>to</strong> form a