Page 172 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

Dispersion 7/149
The relative density of the gas in the atmosphere will partly area of a trench fire can be greater than for an unobstructed jet
determine its dispersion characteristics. A heavier gas will gen- fire because more of the heat-radiating flame surface may be
erally stay more concentrated and accumulate in low-lying concentrated near the ground surface [83].
areas. A lighter gas should rise due to its buoyancy in the air. Calculating hazard zones from jet fires is discussed later in
Every density of gas will be affected to some extent by air this chapter and in Chapter 14. Those discussions illustrate that
temperature, wind currents, and terrain. pressure, diameter, and energy content of the escaping gas are
A product that stays in liquid form when released from the critical determinants in the thermal effects distances.
pipeline poses different problems. Environmental insult,
including groundwater contamination, and flammability are Vapor clouds (vapor spills)
the most immediate problems, although toxicity can play a role
in both short- and long-term scenarios. Of great interest to risk evaluators are the characteristics of
For purposes of risk assessment, dispersion goes beyond vapor cloud formation and dispersion following a product
the physical movement of leaked product. Thermal and blast release. Vapor can be formed from product that is initially in a
effects can range far beyond the distance that the leaked mole- gaseous state or from aproduct that vaporizes as it escapes or as
cules have traveled. The calculation of a hazard zone expands it accumulates in pools on the ground. The amount of vapor put
the concept of dispersion to include these additional ramifica- into the air and the vapor concentrations at varying distances
tions. Dispersion is normally the determining factor of a hazard from the source are the subject of many modeling efforts.
zone. Dispersion and. hence, hazard zone, are also intuitively At least two potential hazards are created by a vapor cloud.
closely intertwined with spill quantity. This risk analysis One occurs if the product in the cloud is toxic or displaces oxy-
assesses dispersion somewhat separately from spill size in the gen (that is, acts as an asphyxiant). The threat is then to any sus-
interest of risk management-there are risk mitigation oppor- ceptible life forms that come into contact with the cloud. Larger
tunities to reduce spill size or dispersion independently. clouds or low-lying clouds provide a greater area of opportu-
Reductions in dispersion are assumed to reduce the potential nity for this contact to occur and hence carry a greater hazard.
consequences. From a risk standpoint, the degree of dispersion The second hazard occurs if the cloud is flammable. The
impacts the area of opportunity because more wide-ranging threat then is that the cloud will findan ignition source. causing
effects offers greater chances to harm sensitive receptors. fire andor explosion. Larger clouds logically have a greater
Reductions in the amount or range of the spill may occur chance of finding an ignition source and also increase the
through natural processes of evaporation and mixing and damage potential because more flammable material may be
thereby reduce the potential consequences. Similarly, reduc- involved in the fire event.
tions in the harmful properties of the substance reduce the risk. Of course, the vapor cloud can also present both hazards:
This may occur through natural processes such as biodegrada- toxicity and flammability.
tion, photolysis, and hydrolysis. If the by-products of these
reactions are less harmful than the original substance, which Vapor cloud ignition
they often are, the hazard is proportionally reduced. Barriers
that either limit dispersion or protect receptors from hazards When an escaping pipeline product forms a vapor cloud the
also reduce risks. entire range of possible concentrations of the product/air mix-
Several dispersion mechanisms-the underlying processes ture exist. Within a specific fuel-to-air ratio range, the vapor
that create the dispersion or hazard zone area-are examined cloud will be flammable. This is the range between the upper
in this section. The hazard zone for a gas release is established ,flammability limit (UFL) and the lower ~fkurnmabiiit~ limit
through either a jet fire or a vapor cloud. The hazard zone for (LFL), which are the threshold concentration levels of interest
a liquid release arises from either a pool fire or a contamina- (also called explosion limits) representing the concentration of
tion scenario. HVL hazard zones can arise from any of these the vapors in the air that support combustion. Ignition Is only
mechanisms. possible for concentrations of vapors mixed with air that fall
between these limits. Outside these limits, the mixture is either
Jet fire too rich or too lean to ignite and burn. Because mixing is by no
means constant, the LFL distance will vary in any release event.
Kelease of a flammable gas carries the threat of ignition and A flammable gas will therefore be ignitable at this point in the
subsequent fire. Thermal radiation from a sustained jet or torch cloud.
firc, potentially preceded by a fireball, is a primary hazard to Although ignition is not necessarily inevitable, there is often
people and property in the immediate vicinity of a gas pipeline a reasonable probability of ignition due to the large number of
failure. in the event of a line rupture, a vapor cloud will form, possible ignition sources--cigarettes, engines, open flames.
grow in size as a function of release rate, and usually rise due to residential heaters, and sparks to name just a few. It is not
discharge momentum and buoyancy. This cloud will normally uncommon during gaseous product release events for the igni-
disperse rapidly and an ignited gas jet, or unignited plume, will tion source to be created by the release of energy, including
be established. If ignition occurs before the initial cloud static electricity arcing (created from high dry gas velocities),
disperses, the gas may bum as a rising and expanding fireball. contact sparking (e.g., metal to metal, rock to rock, rock to
A trench fire is a special type ofjet fire. It can occur ifa dis- metal), or electric shorts (e.g., movement of overhead power
charging gas jet impinges on the side of the rupture crater or lines).
some other obstacle. This impingement redirects the gas jet, It is conservative to assume, then, that an ignition source will
reducing its momentum and length while increasing its width, come into contact with the proper &el-to-air ratio at some point
and possibly producing a horizontal profile fire. The affected during the release. The consequences of this contact range from