Introduction to Fire Safety Management

Note:
This zone can include, among others, places surrounding zones 0 or 1.
Table 7.4
Showing the zones for gases and vapours

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Introduction to Fire Safety Management
134
stored, etc. These should be located away from other 
buildings and actual parts of the plant should be as 
remote from one another as is possible. Ideally build-
ings should be of single storey in nature but kept as low 
as possible and the explosion prone part of any pro 
-
cess should be as high as possible, ideally on the roof to 
minimise the possibility of building collapse.
Where any hazardous part of the plant is located 
within a building the area should be reinforced and 
protected from the rest of the area by a blast wall. As 
discussed previously the area should be vented to avoid 
damage (structural) from any overpressure.
Escape routes and other emergency response 
planning must take into account the explosive nature, as 
should any electrical equipment.
Under the Dangerous Substances and Explosive 
Atmospheres Regulations 2002 there is a requirement 
to identify hazardous contents (containers and plant) to 
ensure that the selection of the correct equipment and 
systems can take into account the level of and likelihood 
of there being an explosive atmosphere.
7.6
Case study
Flixborough disaster 1974
One of the most serious accidents in the history of the 
chemical industry was the explosion at about 16:53 
hours on Saturday 1 June 1974 at the Nypro (UK) site at 
Flixborough, which was severely damaged. Twenty-eight 
workers were killed and another 36 others were injured 
as a direct result of the explosion and subsequent fi re. It 
is recognised that the number of casualties would have 
been more if the incident had occurred on a weekday, as 
the main offi ce block was not occupied.
Outside the plant, 53 persons were reported injured 
with 1821 houses and 167 shops suffering damage 
ranging from major (required rebuilding) to broken 
glazing from the pressure waves.
The overall cost of the damage at the plant itself 
together with the damage outside was estimated at over 
£75 million.
The cause of the Flixborough explosion was a 
release of about 50 tons of cyclohexane, due to failure of 
a temporary pipe. The fl ammable cloud was ignited about 
1 minute or so after the release. A very violent explosion 
occurred. The blast was equivalent to an explosion of 
about 16 tons of TNT.
Five days prior to the explosion, on 27 March 1974, 
it was discovered that a vertical crack in reactor No. 5 
was leaking cyclohexane. The plant was subsequently 
shut down for an investigation. The investigation that 
followed identifi ed a serious problem with the reactor and 
the decision was taken to remove it and install a bypass 
assembly to connect reactors No. 4 and No. 6 so that the 
plant could continue production.
During the late afternoon on 1 June 1974 the 
temporary 20 inch bypass system ruptured, which is 
extremely likely to have been caused by a fi re on a nearby 
8 inch pipe. This resulted in the escape of a large quantity 
of cyclohexane, which formed a fl ammable mixture and 
subsequently found a source of ignition.
At approximately 16:53 hours there was a massive 
unconfi ned vapour cloud explosion which caused exten -
sive damage and started numerous fi res on the site.
Eighteen fatalities occurred in the control room as a 
result of the windows shattering and the collapse of the 
roof. No one escaped from the control room. The fi res 
burned for several days and after ten days those that still 
raged were hampering the rescue work.
The characteristic of the gas explosion at Flixborough 
is that the dense fuel (cyclohexane) was able to form a