Page 160 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

Product hazard 7/137
The j7ash point is defined as the minimum temperature at Exotherm. ’C N,
which the vapor over a flammable liquid will “flash when >400 0
exposed to a free flame. It tells us what temperature is required 305400 1
to release enough flammable vapors to support a flame. 2 15-305 2
Materials with a low flash point (<100”F) ignite and burn read- 125-2 15 3
ily and are deemed to be flammable. If this material also has a <125 4
boiling point less than lOO”F, it is considered to be in the most
flammable class. This includes methane, propane, ethylene, The immediate threat from the potential energy of a pressur-
and ethane. The next highest class of substances has flash ized pipeline is also considered here. This acute threat includes
points of less than 100°F and boiling points greater than 100°F. debris and pipe fragments that could become projectiles in the
In this class, less product vaporizes and forms flammable event of a catastrophic pipeline failure. Accounting for internal
mixtures with the air. This class includes gasoline. crude petro- pressure in this item quantifies the intuitive belief that a
leum, naphtha, and certain jet fuels. pressurized container poses a threat that is not present in a non-
A material is termed combustible if its flash point is greater pressurized container.
than 100°F and it will still bum. This class includes diesel and The increased hazard due solely to the internal pressure is
kerosene. Examples of non-combustibles are bromine and thought to be rather small because the danger zone is usually
chlorine. very limited for a buried pipeline. When the evaluator sees an
Use the following list or Appendix A to determine the NFPA increased threat, such as an aboveground section in a populated
N, value (FP = flash point; BP =boiling point [26]): area, she may wish to adjust the reactivity rating upward in
point value. In general, a compressed gas will have the greater
Noncombustible Nf=O potential energy and hence the greater chance to do damage.
FP > 200’F N,= 1 This is in comparison to an incompressible fluid.
100°F < FP < 200°F Nf=2 The pressure hazard is directly proportional to the amount of
FP < 100°F and BP < 100°F N,=3 internal pressure in the line. Although the MOP could be used
FP < 73°F and BP < 100°F N,= 4 here, this would not differentiate between the upstream sections
(often higher pressures) and the downstream sections (usually
More will be said about flammability in the discussion of lower pressures). One approach would be to create a hypotheti-
vapor cloud dispersion later in this chapter. cal pressure profile of the entire line and, from this, identify
normal maximum pressures in the section being evaluated.
Reactivity, N, Using these pressures, points can be assessed to reflect the risk
due to pressure.
Occasionally, a pipeline will transport a material that is unstable So, to the N, value determined above, a pressure factor can be
under certain conditions. A reaction with air, water, or with itself added as follows:
could be potentially dangerous. To account for this possible
increase in hazard, a reactivity rating should be included in the Incompressible Fluids (Liquids) Pressure Factor
assessment of the product. The NFPA value N, is used to do this. 0-100 psig internal pressure 0 pts
Although a good beginning point, the N, value should he
modified when the pipeline operator has evidence that the sub- >lo0 psig 1 Pt
stance is more reactive than the rating implies. An example of Compressible Fluids (Gases)
this might be ethylene. A rather common chain of events in &50 psig 0 pts
pipeline operations can initiate a destructive series of detona- 5 1-200 psig 1 Pt
tions inside the line. This is a type ofreactivity that should indi- >200 psig 2 pts
cate to the handler that ethylene is unstable under certain
conditions and presents an increased risk due to that instability. Total point values for N, should not be increased beyond 4
The published N, value of 2 might not adequately cover this points, however, because that would minimize the impact of the
special hazard for ethylene in pipelines. flammability and toxicity factors, N, and N,, whose maximum
Use the following list or Appendix A to determine the N, point scores are 4 points.
value [26]:
Example 7.1: Product hazard scoring
N, = 0 Substance is completely stable. even when heated under
fire conditions A natural gas pipeline is being evaluated. In this particular
N, = 1 Mild reactivity on heating with pressure section, the normal maximum pressure is 500 psig. The evalua-
Nr = 2 Significant reactivity, even without heating tor determines from AppendixA that the N, for methane is 0. To
N, = 3 Detonation possible with confinement this. he adds 2 points to account for the high pressure of this
Nr = 4 Detonation possible without confinement. compressible fluid. Total score for reactivity is therefore 2
points.
Note that reactivity includes self-reactivity (instability) and
reactivity with water. Toxic& Nh
The reactivity value (N,) can be obtained more objectively by
using the peak temperature of the lowest exotherm value as The NFPA rating for a material’s health factor is N,. The N,
follows [26]: value only considers the health hazard in terms of how that