Page 196 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

Receptors 711 73
ments. The thermal and overpressure distances are themselves assumptions underlying such event combinations produce
a function of many factors including release rate, release vol- very conservative (highly unlikely) scenarios that typically
ume, flammability limits, threshold levels ofthermal/overpres- overestimate the actual hazard zone distances. This is done
sure effects, product characteristics, and weather conditions. intentionally in order to ensure that hazard zones encompass
A hazard zone must be defined in terms of specific damage the vast majority of possible pipeline release scenarios. A fur-
thresholds that could occur under defined scenarios. An exam- ther benefit of such conservatism is the increased ability of
ple of a damage threshold is a thermal radiation (heat flux) level such estimations to weather close scrutiny and criticism from
that causes injury or fatality in a certain percentage of humans outside reviewers.
exposed for a specified period of time. Another example is the As an example of a conservative hazard zone estimation, the
overpressure level that causes human injury or specific damage calculations might be based on the distance at which a full
levels to certain kinds of structures. Such damage threshold pipeline rupture, at maximum operating pressure with subse-
criteria are further discussed in Chapter 14. quent ignition, could expose receptors to significant thermal
Receptors falling within the hazard zones are considered to damages, plus the additional distance at which blast (overpres-
be vulnerable to damage from apipeline release. In the case of a sure) injuries could occur in the event of a subsequent vapor
gas release, receptors that lie between the release point and the cloud explosion. The resulting hazard zone would then repre-
lower flammable concentration boundary of the cloud are sent the distances at which damages could occur, but would
considered to be at risk directly from fire. Receptors that lie exceed the actual distances that the vast majority of pipeline
between the release point and the explosive damage boundary release scenarios would impact.
may additionally be at risk from direct overpressure effects. More specifically, the calculations could be based on conser-
Some receptors within the hazard zone would also be at risk vative assumptions generating distances to the LFL boundary.
from thermal radiation effects from ajet fire as well as from any doubling this distance to account for inconsistent mixing, and
secondary fires resulting from the rupture-ignition event. See adding the overpressure distance for a scenario where the igni-
Chapter 14 and Figure 7.8. In the case ofliquid spills, migration tion and epicenter of the blast occur at the farthest point.
of spilled product, thermal radiation from a potential pool fire, However, such conservatism may also be excessive, leading to
and potential contamination would define the hazard zone. inefficient and costly repercussions-in the case of land-use
Because an infinite number of release scenarios-and subse- decisions, for example.
quent hazard zones-are possible, some simplifying assump- An estimation of potential hazard zones from pipeline
tions are required. A very unlikely combination of events is releases is an aspect ofthe previously discussed spill score. The
often chosen to represent maximum hazard zone distances. The same issues that are used to establish relative consequences are

Thermal effects 1

Thermal effects 2
Wind

A / I

.* Overpressure 1

lgnitability range
Figure 7.8 Thermal and overpressure damage zones.