Civil Engineering Reference
In-Depth Information
Table 7.5
Showing the zones for dusts
Zones for dusts
Zone 20
A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is present continuously, or
for long periods or frequently.
Note:
In general these conditions, when they occur, arise inside containers, pipes and vessels, etc.
Zone 21
A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is likely to occur in normal
operation occasionally.
Note:
This zone can include, among others, places in the immediate vicinity of, e.g., powder fi lling and emptying
points and places where dust layers occur and are likely in normal operation to give rise to an explosive concentration
of combustible dust in mixture with air.
Zone 22
A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is not likely to occur in
normal operation but, if it does occur, will persist for a short period only.
Note:
This zone can include, among others, places in the vicinity of equipment, protective systems, and components
containing dust, from which dust can escape from leaks and form dust deposits (e.g. milling rooms, in which dust
escapes from the mills and then settles).
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
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.
