Page 170 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

Leak Volume 7/147
As one approach to assessing relative release rate impacts, Leak rate is determined with a worst case line break sce-
the leak volume can be approximated by calculating how much nari-a full bore rupture. As with the atmospheric dispersion,
vapor will be released in IO minutes. Our interest in the leak choosing this scenario allows us to incorporate the line size
volume under this approach is not a contradiction to the earlier and pressure into the hazard evaluation. A 36-in. diameter high-
statement of primary dependence on leak rate. The conversion pressure gasoline line poses a greater threat than a 4-in. diame-
ofthe leak rate into a volume is merely a convenience under this ter high-pressure gasoline line, all other factors being equal.
approach that allows a combined vapor-liquid release to be This is because the larger line can potentially create the larger
modeled in a similar fashion (see Appendix B). spill.
The highest leak rate occurs when the pressure is the highest Alternatively, several scenarios of failure hole sizes can be
and the escape orifice is the largest. This leads to the assump- evaluated and then combined, as was noted for the vapor release
tion that, in most cases, the worst case leak rate happens near modeling. The scenarios would represent the distribution of
the instant of pipeline rupture, while the internal pressure is all possible scenarios and would require that the relative proba-
still the highest and after the opening has reached its largest bility of each hole size be estimated. This requires additional
area. As will be discussed later, the highest leak rate generally complexity of analysis. However, because this approach incor-
produces the largest cloud. As the leak rate decreases, the porates the fact that the larger hole size scenarios are usually
cloud shrinks. As an exception for the case of a dense cloud, more rare, it better represents the range of possibilities. It also
vapors may ‘‘slump’’ and collect in low-lying areas or “roll” can evaluate scenarios where small amounts, below detection
downhill and continue to accumulate as the cloud seeks its capabilities, are leaked for very long periods and result in larger
equilibrium size. In modeling the 10-minute release scenario, total volume spills. This requires evaluation of leak detection
we are conservatively assuming that all the vapor stays capabilities and the construction of representative scenarios.
together in one cloud for the full 10-minute release. We are Because the release of a relatively small volume of an incom-
also conservatively neglecting the depressuring effect of 10 pressible liquid can depressure the pipeline quickly, the longer
minutes worth of product leakage. This is done to keep the cal- term driving force to feed the leak may be gravity and siphon-
culation simple. The 10-minute interval is chosen to allow a ing effects or pumping equipment limitations. A leak in a low-
reasonable time for the cloud to reach maximum size, but not lying area may be fed for some time by the draining of the
long enough to be counting an excessive mass of well- rest of the pipeline, so the evaluator should find the worst
dispersed material as part of the cloud. The amount of product case leak location for the section being assessed. The leak
released and the cloud size will almost always be overesti- rate should include product flow from pumping equipment.
mated using the above assumptions. Again, for purposes ofthe Reliability of pump shutdown following a pipeline failure is
relative risk assessment, overestimation is not a problem as considered elsewhere.
long as consistency is ensured. See Appendix B for more dis- Based on the worst case leak rate and leak location for the
cussion of leak rate determinations. section, the relative spill size can be scored according to how
An alternative approach that avoids some of the calculation much product is spilled in a fixed time period of, say, 1 hour.
complexities associated with estimating release quantities is to Leaks can be (and have been) allowed to continue for more than
use pressure and diameter as proxies for the release quantities. 1 hour, but leaks can also be isolated and contained in much
Using a fixed damage threshold (thermal radiation levels; see shorter periods. The 1-hour period is therefore arbitrary, but
page 308), it has been demonstrated that the extent of the threat will serve our purposes for a relative ranking. This approach
from a burning release of gas is proportional to pressure and will distinguish the more hazardous situations such as high-
diameter [83]. Therefore, pressure and diameter are suitable throughput, large-diameter liquid pipelines in low-lying areas.
variables for assessing at least one critical aspect of the poten- In many scenarios, reaction to a liquid spill plays a larger role
tial consequences from a gas release. As in the first approach, in consequence minimization than does reaction to a gas
this can incorporate conservative assumptions regarding cloud release. An adjustment to the spill score can be applied when it
formation and dispersion. can be shown that special capabilities exist that will reliably
Because the immediate hazards from vapor releases are reduce the potential spill size by at least 50%, as is detailed in
mostly influenced by leak rate, leak detection will not normally later sections.
play a large role in risk reduction. One notable exception is a The consequences arising from various liquid spill volumes
scenario where leak detection could minimize vapor accumula- is closely intertwined with the dispersion potential of those
tion in a confined space. volumes. Therefore, evaluating these scenarios is often best
done by simultaneously considering spill volume and disper-
Hazardous liquid spills sion, as is discussed later in this chapter. Some modeling
options are shown inTable 7.6.
Potential liquid spill size is a variable that depends on factors
such as hole size, system hydraulics, and the reliability and Highly volatile liquid releases
reaction times of safety equipment and pipeline personnel.
Safety equipment and operation protocol are covered in other Calculating the quantity of material released under flashing
sections of the assessment, so the system hydraulics alone are conditions is a very complex task, due to the quite complex
used here to rank spill size. phenomena that take place during such a release. The process
Based on the expected potential hazards from a liquid spill, represents a nonlinear, non-equilibrium process, for at least
including pool fire and contamination potential, the spill vol- part of the episode. Beyond the quantity calculation, the vapor
ume is a critical variable. Potential spill volume is estimated cloud generation calculation adds further complications. Many
from potential leak rates and leak times. variables such as weather conditions, heat transfer through soil,