Page 171 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

7/148 Leak Impact Factor
Table 7.6 Liquid spill size analysis options

Variables used in the analysis Application
Flow rate only Assumes high volume; full line rupture at MOP. Higher level screening to assess differences in
consequence potential among different liquid pipeline systems (different locations, diameters, products,
etc.). Generally assumes that things get worse uniformly in proportion to increasing leak size.
Flow rate and draindown potential Adds more resolution to identify potential consequence differences along a single pipeline route since
relative low spots are penalized for greater stabilization release volumes after section isolation.
Add basic terrain considerations Improves evaluations ofpotential consequences in general since site-specific variables are included.
Examples of terrain variables include slope, surface flow resistance, and waterway considerations
obtained from maps or from general field surveys. Water body intersects can be determined and
characterized based on the water body’s flow (slope at intersect point can be proxy for water flow rate).
Hole size and pressure More realistic, but must include probabilities of various hole sizes. Provides for an estimate ofpinhole leak
volumes over several years.
Particle trace Also calledflowpath modeling, this is normally a computer application (GIS) that determines the path ofa
hypothetical leaked drop of liquid. Includes topography and sometimes surface flow resistance. The
computer routine is sometimes called a costingfunction. Accumulation points and water body intersects
are determined. Arbitrary stop points of potential flow paths may need to be set.
Particle trace with release volume Adds aspect of ground penetration (soil permeability) and driving force ofthe volume release in order to
considerations better characterize both the depth and lateral spread distance of the leak. Flow path stop points are
automatically determined. Volumes may be determined based on worst case releases or probabilistic
scenarios.
Add aquifer characteristics Adds hydrogeologic subsurface component to surface flow analyses to model groundwater transport of
portions of a leak that may contact the aquifer over time. More important for toxic and/or environmentally
persistent contamination scenarios.

wind patterns, rate of air entrainment, and temperature changes relative size of that impacted area is the subject of this portion
must be considered. In simple terms, a gaseous cloud of highly ofthe consequence assessment.
volatile liquids (HVLs) will be formed from the initial leak, As modeled by physics and thermodynamics, spilledproduct
which in turn transitions into a combination of secondary will always seek a lower energy state. The laws of entropy tell
sources. The secondary sources include the quantity of imme- us that systems tend to become increasingly disordered. The
diately flashing material, the vapor generation from a liquid product will mix and intersperse itself with its new environ-
pool, and the evaporation of airborne droplets. ment in a nonreversible process. The spill has also introduced
The initial release rate will be the highest release rate of the stress into the system. The system will react to relieve the stress
event. This rate will decrease almost instantly after the rupture. by spreading the new energy throughout the system until a new
As the depressurization wave from a pipeline rupture moves equilibrium is established.
from the rupture site, pressures inside the pipeline quickly drop The characteristics of the spilled product and the spill
to vapor pressure. At vapor pressure, the pipeline contents will site determine the movement of the spill. The possibilities
vaporize (boil), releasing smaller quantities of vapor. are spills into the atmosphere, surface water, soil, groundwa-
Releases of a highly volatile liquid are similar in many ter, and man-made structures (buildings, sewers, etc.). Accu-
respects to the vapor release scenario. Key differences include rately predicting these movements can be an enormously
the following: complex modeling process. For releases into the atmosphere,
product movement is covered in the discussion of vapor
HVLs have multiphase characteristics near the release point. dispersion. Liquid dispersion scenarios cover releases into
Product escapes in liquid, vapor, and aerosol form, increas- other media. Some spill scenarios involve both the spill of a
ing the vapor generation rate in the immediate area of the liquid and vapor generation from the spilled liquid as it
leak. disperses.
Liquid pools might form, also generating vapors. For purposes of many assessments, accurate modeling ofthe
As the pipeline reaches vapor pressure, the remaining liquid dispersion of spilled product will not be necessary. It is the
contents vaporize through flashing and boiling, until the propensity to do harm that is of interest. A substance that causes
release is purely gaseous. great damage even at low concentrations, released into an
Lower vapor pressures (compared to pure gases) generally area that allows rapid and wide-ranging spreading, creates the
lead to heavier vapors (negatively buoyant), more cohesive greatest hazard.
clouds with more concentrated product, and possibly higher If a product escapes from the pipeline, it is released as a
energy potential. gas andor a liquid. As a gas, the product has more degrees of
freedom and will disperse more readily. This may increase or
decrease the hazard, since the product may cover more area,
C. Dispersion but in a less concentrated form. A flammable gas will entrain
oxygen as it disperses, becoming an ignitable mixture. A toxic
A release of pipeline contents can impact a very specific area, gas may quickly be reduced to safe exposure levels as its
determined by a host of pipeline and site characteristics. The concentration decreases.