Page 176 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

Dispersion 71153
Alternatively, a simple geometry expression can be used to rate of release, soil permeability, surface flow resistance,
calculate pool radius, once a pool depth is assumed. In either groundwater movement, surface water intersects, etc., all of
case, assumptions must be made regarding rate of penetration which are very location specific.
into the soil, evaporation, and other considerations. As one possible generalization. the contamination can be
Thermal radiation is related to the emissivity and transrnissiv- modeled as being proportional to the potential pool size. Pool
ity. In accounting for shielding by surrounding layers of smoke, size can be estimated as described previously. Some multiple of
emissivity is related to the normal boiling point of the material. this pool size (2x to lox, perhaps) can be used as a standard rel-
Higher boilingpoint fluids tend to burn with sooty flames. ative measure of contamination distance. This will of course
Emissivity has been correlated to boiling point by means of routinely over-and underestimate the true distances, but. when
the following relationship: used consistently, can help rank the damage potential.
E, = -0.3 I3 xTb+l 17 Spill migration
where E, IS the effective emissive power (kW/m2) and T, While a full topographical, hydrogeological analysis is the best
is the normal boiling point in degrees Fahrenheit [5]. way to estimate contamination potential, some basic concepts
Transmissivity is a measure ofhow much ofthe emitted radi- of migration of hydrocarbons through a medium such as soil
ation is transmitted to a potential receptor. It is mainly a func- and water can be used to model the range of a spill. Depending
tion of the path to the receptor: distance, relative humidity, and on numerous factors, a hydrocarbon spill will spread laterally
flame temperature. Water and carbon dioxide tend to reduce the as well as penetrate the soil. Quantities of lighter-than-water
transmissivity. hydrocarbons that penetrate the soil will often form a pancake
With assumptions like constant transmissivity, the thermal shape at some level below the surface, as gravity and buoyancy
radiation from a pool fire is related to spill size and boiling forces are balanced and the spill spreads laterally.
point. The emissivity value can be used in an inverse square The surface spread and soil penetration depth and movement
relationship to calculate thermal radiation levels at certain dis- through soil are generally related to the product and soil charac-
tances from the fire. teristics captured in a variable termed h-vdruulic conducrivici..
Equations for pool growth and emissivity are shown here An additional consideration that impacts the depth of penetra-
because they offer the opportunity to extract some simplifying tion and the spread is the soil retention capacity (or residual
assumptions, as will be shown later. The calculation scheme saturation capacity, hydrocarbon retained by soil particles)
could be as shown in Table 7.7. Increasing soil retention reduces the
spread of the spill. The product viscosity is the chief product
tstiinate spill size+ calculate pool area- add boiling point+ characteristic to be considered. The soil permeability is well
calculate relative hazard distance correlated with both the hydraulic conductivity and the soil
retention capacity, so it is a valuable variable for the station
risk model. For example, a scoring regime can be set up for
Contunzinution potential which the contamination potential is partly a function of the
soil retention and hydraulic conductivity. In Table 7.7, higher
Most spills of hydrocarbon liquids will present hazards related scores represent higher spread of contaminants.
to both fire and contamination. Potential damages from each The phenomenon of source strength, the intensity with
hazard type tend to overlap. and are interchangeable in some which dissolved chemicals may be released from a spilled
cases and additive in others. Contamination potential some- hydrocarbon into water, is considered in assessing the product
times depends on the thermal radiation potential-if the prod- hazard component ofthis model (where that assessment should
uct burns on release, then the contamination potential is consider the presence of trace amounts of hazardous compo-
diminished or eliminated. nents) and in the use of groundwater depth as a variable.
The environment can be very sensitive to certain substances. Deeper groundwater affords more opportunity for soil reten-
Contaminations in the few parts per billion or even parts per tion and may minimize the lateral spread of the spill. Any
trillion are potentially of concern. If contamination is defined changes to the hydraulic conductivity of the soil due to the
as IO parts per billion, a IO-gallon spill of a solvent can contam- spilled hydrocarbon are beyond the resolution of this model.
inate a billion gallons of groundwater. A 5000-gallon spill from
a pipeline can contaminate 500 billion gallons of groundwater
to 10 ppb. The potential contamination is determined by the
simple formula: Table 7.7 Soil retention and hydraulic conductivities for various
types of soil
V,XC,=Vp*X~,,
where
V, = volumeofspill
Vz,y = volume of' groundwater contaminated Stonekoarse gravel 5 I 100 I0
x
8
Cy = average concentration of contaminant in spilled material Gravel/coarse sand I5 I 0-0- I h
Coarse to medium sand
Cg% = average concentration of contaminant in groundwater.
Medium to fine sand 25 4
Fine sand to silt 40 ] 0-8- 10-2 1
It is very difficult to generalize a contamination area esti- Clay 10 -10. I 0 -4 I
mate. Any estimate is highly dependent on volume released