A review of HSE's risk analysis and protection-based analysis ...

Assumption Explanation Application Comment
Releases from highpressure
gas pipelines
are assumed to be
vertical jets.
Rupture of a high-pressure gas
pipeline is a violent event that
results in disruption of the ground
cover and formation of a crater
around the point of failure. It is
assumed that the gas is projected
vertically upwards from this crater.
The probability that a Potential ignition sources for
release of flammable accidental releases are many and
material will be ignited. varied. On-site sources may include
welding activity or equipment
containing flames, such as boilers or
furnaces. Off-site sources may
include vehicles, traffic lights,
smoking and domestic appliances.
Risk analysis studies for The distance to which the cloud
toxic substances like generated by a vessel failure remains
chlorine model failure hazardous depends on the quantity
of the vessel in two released to atmosphere. The larger
ways – release of the the quantity, the greater the distance
entire vessel contents at which the cloud is still hazardous.
upwards into the air;
and release of the entire
contents downwards, so
that half is retained in
the vessel bund. The
vessel is assumed to be
completely full when it
fails.
Risk analysis of
pipelines.
Risk analysis of MHPs
and MHIs handling
flammable substances.
Risk analysis of
installations handling
toxic, pressurised
liquefied gases (such as
chlorine).
There is evidence from accident reports, experiments and computer
modelling work that the release of gas may be in directions other than
vertical. HSE's pipeline risk analysis models are to be covered by the
model evaluation exercise (see Section 2.4.1).
Currently ignition probabilities are established using expert
judgement informed by research on the numbers of potential ignition
sources present in different typical types of land use (residential,
industrial, rural etc.). However, HSE has invested in further research
in this area with a view to putting these values on a more rigorous
basis in the future. It is partly because of the uncertainty associated
with establishing ignition probabilities that many analyses of MHIs
handling flammable materials are currently protection based.
This assumption considers two different ways in which a chlorine
storage vessel could fail catastrophically, representing the two
extremes of a spectrum of possible failures. The assumption of 100%
fill of the vessel is related to the Hazardous Substances Consent that
the site would hold for storing the material. A site would be entitled to
store this much material for as long and as often as they liked. Note
that the kind of vessel failure event considered here is very different
from a BLEVE.