Page 94 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

4/72 Corrosion Index
Sulfates bers. Potential for carryovers due to incorrect operations,
Carbonates improperly sized equipment, or unusual levels of contami-
Conductivity, nants received should be included in the evaluation.
O&Mpructices-an evaluation of the actions taken by the
A detailed assessment of internal corrosion would use the operator to prevent introduction of contaminants. This may
actual measurements of the concentrations, especially where include the degree of human intervention required and the
such measurements are easily available to the evaluator. number of redundancies that can interrupt a sequence of
Weightings can be assigned based on the perceived role of the events that might otherwise result in increased contaminant
contaminant in corrosion. Point scales can then be developed concentrations. See also the discussion under mitigation
based on the weightings and the expected range of measure- measures.
ments, best case to worst case, for each contaminant. Highestflow velociv + highestprofle-an evaluation of the
normal and worst case high flowing velocities and an assess-
Upset potential This aspect of internal corrosion measures ment of this effect on erosion potential and contact time
the potential for increased product corrosivity under abnormal between contaminant and pipe wall. Both the average high
conditions. This might include unintentional introduction of and the peak velocities should be of interest.
contaminants and changes in flow patterns that might aggravate Lowestflow velociv + lowest profile-an evaluation of the
previously insignificant corrosion potential. The introduction normal and worst case low flowing velocities and an assess-
of contaminants is a function of (1) the processing prior to ment of this effect on erosion potential and contact time
delivery into the pipeline, (2) equipment capabilities and fail- between contaminant and pipe wall. Both the average low
ure potential, and (3) operations and maintenance practices of and the lowest velocities should be of interest.
the facility delivering the product into the pipeline.
Changes in flow patterns including stagnant flow conditions, Points can be assigned to these factors based on observed or
can be considered to be “upsets.” Low flow rates can lead to reported conditions and can be combined for a final assessment
increased chances of liquid or solid dropout and accumulation of upset potential.
at low spots, whereas high flow rates can lead to erosion.
Contaminant dropout may lead to increased contact time Simplified scoring ofproduct corrosivity
between pipe wall and product, maybe at higher contaminant
concentrations (at low spot accumulation points, for instance). In many cases, the amount of detail described above may not be
Anything that leads to increased corrosive contaminant contact warranted for scoring internal corrosion potential. This is espe-
with pipe walls will logically increase corrosion potential and cially true if the risk evaluation is primarily used as a high-level
rate. Note, however, that subsequent higher flow rates might screening tool. In this case, the above factors can be considered
sweep accumulations and hence be a mitigation measure as more generally and perhaps outside a formal scoring protocol.
described later. These considerations can then be used to assign point values in
Erosion is the removal of pipe wall material caused by the amore qualitative fashion.
abrasive or scouring effects of substances moving against the A simple schedule can be devised to assign points to the prod-
pipe wall. It is a form of corrosion only in the pure definition of uct corrosivity if a more generalized approach is appropriate:
the word, but is considered here as an internal corrosion poten-
tial. High velocities and abrasive particles in the product stream Strongly corrosive 0 pts
are the normal contributing factors to erosion. Impingement Mildly corrosive 3 pts
points such as elbows and valves are the most susceptible ero- Corrosive only under special conditions I pts
sion points. Gas at high velocities may be carrying entrained Never corrosive 10 pts
particles of sand or other solid residues and, consequently, can
be especially damaging to the pipe components. “Strongly corrosive” suggests that a rapid damaging kind of
Historical evidence of erosion damage is of course a strong corrosion is possible. The product is highly incompatible with
indicator of susceptibility. Other evidence includes high prod- the pipe material. Transportation of brine solutions, water,
uct stream velocities (perhaps indicated by large pressure products with H,S, and many acidic products are examples of
changes in short distances) or abrasive fluids. Combinations of materials that are highly corrosive to steel lines.
these factors are, of course, the strongest evidence. If, for “Mildly corrosive” suggests that damage to the pipe wall is
instance, an evaluator is told that sand is sometimes found in possible but only at a slow rate. Having no knowledge of the
filters or damaged valve seats, and that some valves had to be product corrosivity can also fall into this category. It is conser-
replaced recently with more abrasion-resistant seat materials, vative to assume that any product can do damage, unless we
he may have sufficient reason to penalize the pipe section for have evidence to the contrary.
erosion potential. “Corrosive only under special conditions” means that the
The overall assessment of upset potential, as a contributing product is normally benign, but there exists the chance of intro-
factor to internal corrosion potential, can be accomplished ducing a harmful component into the product. CO, or saltwater
through an evaluation and scoring of the following: excursions in amethanepipeline are a common example. These
natural components of natural gas production are usually
Equipment-an evaluation of the types of equipment used to removed before they can get into the pipeline. However, equip-
remove contaminants or prevent contaminant introduction ment used to remove such impurities is subject to equipment
into the pipeline, and the reliability of such equipment. failures, and subsequent spillage of impurities into the pipeline
Examples include product filters, dehydrators, and scrub- is apossibility.