Page 173 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

7/150 Leak Impact Factor
a jet fire to a massive fireball and detonation. On ignition, a with minimum lateral spreading. An HVL cloud will normally
flame propagates through the cloud, entraining surrounding air be negatively buoyant (heavier than air) due mostly to the evapo-
and fuel from the cloud. If the flame propagation speed rative cooling of the material. These dense vapors tend to slump
becomes high enough, a fireball and possibly a detonation can and flow to low points in the immediate topography. Typical
occur. The fireball can radiate damaging heat far beyond the cloud configurations are roughly cigar or pancake shaped.
actual flame boundaries, causing skin and eye damage and sec- A stable atmospheric condition is usually chosen for release
ondary fires. If the cloud is large enough, a “fire storm” can be modeling, in order to generate scenarios closer to worst case
created, generating its own winds and causing far-reaching scenarios. Atmospheric stability classes are discussed in
secondary fires and radiant heat damage. Chapter 14 and shown in Table 14.3 1. This stability class repre-
Ignition probabilities are discussed in Chapter 14. sents some fraction of possible weather type days in any year.
Under very favorable conditions, unignited cloud drift may lead
Overpressure wave to extended hazard zone distances, but such events are seen to be
rare, difficult to estimate, and generally considered within the
In rare cases, a vapor cloud ignition can lead to an explosion. conservative assumptions already included in these estimates.
An explosion involves a detonation and the generation of blast Many variables affect the dispersion of vapor clouds. In
waves, commonly measured as overpressure in psig. Anuncon- general, these include
fined vapor cloud explosion, in which a cloud is ignited and the
flame front travels through the cloud quickly enough to gener- Release rate and duration
ate a shock wave, is a rare phenomenon. Such a phenomenon is 0 Prevailing atmospheric conditions
called an overpressure wave. A confined cloud is more likely to Limiting concentration
explode, but confinement is difficult to accurately model for an Elevation of source
open-terrain release. The intensity of the overpressure event is 0 Surrounding terrain
inversely proportional to the distance from the explosion Source geometry
point-the intensity is less at greater distances. Various over- 0 Initial density ofrelease [5].
pressure levels can be related to various damages. An overpres-
sure level of 10 psi generally results in injuries (eardrum Release duration is not as critical in estimating maximum
damage) among an exposed population. Higher overpressure cloud size since the release rate will diminish almost instantly
levels cause more damages but would only occur closer to the as the pipeline rapidly depressures under the pipeline rupture
explosion point. It is conservatively assumed that an uncon- scenario. Smaller size leaks could create vapor clouds that
fined vapor cloud explosion can originate in any part of the would be more dependent on release duration, especially under
cloud. Therefore, the overpressure distance is conservatively weather conditions that support cloud cohesiveness, but these
added to the LFL distance (the ignition distance) for purposes scenarios are not thought to produce maximum cloud sizes.
of hazard zone estimation. The extreme complexities surrounding a vapor release sce-
The manner in which an ignited vapor cloud potentially nario make the problem only approximately solvable for even a
transforms from a burning event to an exploding event is not relatively closed system. An example of a somewhat closed
well understood. It rarely occurs when the weight of airborne system is a well-defined leak from a fixed location where the
vaporis less than 1OOOpounds [83]. terrain is known and constant and where weather conditions
Should a detonation occur, widespread damage is possible. A can be reasonably estimated from real-time data. A cross-
detonation can generate powerful blast waves reaching far country pipeline, on the other hand, complicates the problem by
beyond the actual cloud boundaries. Most hydrocarbodair adding variables such as soil conditions (moisture content, tem-
mixtures have heats of combustion greater than the heat of perature, heat transfer rates, etc.), topography (elevation pro-
explosion of TNT [8], making them very high energy sub- file, drainage pathways, waterways, etc.), and often constantly
stances. The possibility of vapor cloud explosions is enhanced changing terrain and weather patterns (amount of sunshine,
by closed areas, including partial enclosures created by trees or wind speed and direction, humidity, elevation, etc.).
buildings. Unconfined vapor cloud explosions are rare but Even though it vaporizes quickly, a highly volatile pipeline
nonetheless a real danger. Certain experimental military bombs product can form a liquid pool immediately after release. This
are designed to take advantage of the increased blast potential could be the case with products such as butane or ethylene. The
created by the ignition of an unconfined cloud of hydrocar- pool would then become a secondary source of the vapors.
bodair vapor. Vapor generation would be dictated by the temperature of the
Damages that could result from overpressure (blast) events pool surface, which in turn is controlled by the air temperature,
are discussed in Chapter 14. the wind speed over the pool, the amount of sunshine to reach
the pool, and the heat transfer from the soil (Figure 7.6). The
Vapor cloud size soil heat transfer is in turn governed by soil moisture content,
soil type, and both recent and current weather. Even if all of
The predicted vapor cloud size is a function of variables such as these factors could be accurately measured, the system is still a
the release rate, release duration, product characteristics, nonlinear relationship that cannot be exactly solved.
threshold concentrations of interest, and surrounding environ-
ment (e.g., weather, containment barriers, ignition source prox- Cloud modeling
imity) at the release site.
A cloud of lighter-than-air vapors such as natural gas or A vapor cloud that covers more ground surface area, either due
hydrogen will normally be buoyant-it will tend to rise quickly to its size or its cohesiveness, has a greater area of opportunity