Page 89 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

Scoring the corrosion potential 4/67
A.
Example 4.1 : Scoring road casings B. Chemical and marine 0 Pt
Chemical and high humidity
0.5 pt
A section of steel pipeline being evaluated has several road C. Marine, swamp, coastal 0.8 pt
crossings in which the carrier pipe is encased in steel. D. High humidity and high temperature 1.2 pts
There are two aboveground valve stations in this section. E. Chemical and low humidity 1.6 pts
One of these stations has approximately 25 ft of pipe supported E Low humidity and low temperature 2 pts
on concrete and steel pedestals. The other one has no supports. G. No exposures 2 pts
The evaluator assesses the section for atmospheric corrosion
“facilities” as follows: A. Chemical and marine Considered to be the most corro-
sive atmosphere, this includes certain offshore production
Casings 1 Pt facilities and refining operations, especially if in splash-zone
Groundair interface 2 pts environments. The pipe components are exposed to airborne
supports 2 pts chemicals and salt spray that promote oxidation, as well as
occasional submersion in water.
Picking the worst case, the point value for this section is 1 pt.
The evaluator feels that the number of casings and number of B. Chemical and high humidiw Also quite a harsh environ-
supports and number of groundair interfaces are roughly ment, this may include chemical or refining operations in
equivalent and chooses not to use the multiple occurrences coastal regions. Airborne chemicals and a high moisture con-
option. If other sections being evaluated have a significantly tent in the air combine to enhance oxidation of the pipe steel.
different number of occurrences, adjustments would be needed
to show the different risk picture. A distinction between a sec- C. Marine. swamp, coastal High levels of salt and moisture
tion with one casing and a section with two casings is needed to combine to form a corrosive atmosphere here.
show the increased risk with two casings.
In many modem assessments, segmentation is done so that D. High humidity and high temperature Similar to the situ-
sections with atmospheric exposures are distinct from those ation above, this case may be seasonal or in some other way not
that have no such exposures. A cased piece of pipe will often be as severe as the marine condition.
an independent section for scoring purposes since it has a dis-
tinct risk situation compared with neighboring sections with no E. Chemical and low humidity While oxidation-promoting
casing. The neighboring sections will often have no atmos- chemicals are in the air, humidity is low, somewhat offsetting
pheric exposures and hence no atmospheric corrosion threat at the effects. Distinctions may be added to account for tempera-
all. This sectioning approach is a more efficient way to perform tures here.
risk assessments as is discussed in Chapter 2.
I? Low humidity The least corrosive atmosphere will have
no airborne chemicals, low humidity, and low temperatures.
A2. Atmospheric type (weighting: 20% of
atmospheric corrosion) G. No exposures There are no atmospheric exposures in the
section being evaluated.
Certain characteristics of the atmosphere can enhance or accel- In applying this point schedule, the evaluator will probably
erate the corrosion of steel. They are thought to promote the need to use judgment. The type of environment being consid-
oxidation process. Oxidation is the primary mechanism evalu- ered will not usually fit specifically into one ofthese categories,
ated in this section. Some of these atmospheric characteristics but will usually be comparable to one of them. Note however,
and some simplifying generalities about them are as follows: that given the low point values suggested here, scoring this vari-
able does not warrant much research and scoring effort.
Chemical composition. Either naturally occurring airborne
chemicals such as salt or CO, or man-made chemicals such
as chlorine and SO,,(which may form H,SO, and H,SO,) Example 4.2: Scoring atmospheric conditions
can accelerate the oxidation of metal.
Humidity. Because moisture can be a primary ingredient of The evaluator is comparing three atmospheric conditions.
the corrosion process, higher air moisture content is usually The first case is a line that runs along a beach on Louisiana’s
more corrosive. Gulf Coast. This most closely resembles condition C. Because
Temperature. Higher temperatures tend to promote corrosion. there are several chemical-producing plants nearby and winds
may occasionally carry chemicals over the line, the evaluator
A schedule should be devised to show not only the effect of a adjusts the C score down by 50%.
characteristic, but also the interaction of one or more character- The second case is a steel line in eastern Colorado. While the
istics. For instance, a cool, dry climate is thought to minimize line is seasonally exposed to higher temperatures and humidity,
atmospheric corrosion. If a local industry produces certain air- it is also frequently in cold, dry air. The evaluator assigns a point
borne chemicals in this cool, dry climate, however, the atmos- value based on an adjusted condition E This is 1.6 pts. equiva-
phere might now be as severe as a tropical seaside location. lent from a risk standpoint to condition E, even though there is
The following is an example schedule with categories for no chemical risk.
several different atmospheric types, ranked from most harsh to The final case is a line in southern Arizona. Experience con-
most benign, from a corrosion standpoint: firms that this environment does indeed experience only minor,