Page 95 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

Scoring the corrosion potential 4/73
“Never corrosive” means that there are no reasonable possi- Inhibitor injection When the corrosion mechanism is fully
bilities that the product transported will ever be incompatible understood, certain chemicals can be injected into the flowing
with the pipe material. product stream to reduce or inhibit the reaction. Because oxy-
The evaluator may also wish to interpolate and assign point gen is a chief corroding agent of steel, an “oxygen-scavenging”
values between the ones shown. chemical can combine with the oxygen in the product to pre-
vent this oxygen from reacting with the pipe wall. A more com-
BZ. Preventions (weighting: 50% ojinternal mon kind of chemical inhibitor forms a protective barrier
corrosion) between the steel and the product-a coating, in effect.
Inhibitor is reapplied periodically or continuously injected to
It is often economically advantageous to transport corrosive replace the inhibitor that is absorbed or displaced by the prod-
substances in pipe that is susceptible to corrosion by the uct stream. In cases where microorganism activity is aproblem.
substance. In these cases, it is prudent to take actions to reduce biocides can be added to the inhibitor. The evaluator should be
the damage potential. confident that the inhibitor injection equipment is well main-
Having assessed the potential for a corrosive product, the tained and injects the proper amount of inhibitor at the proper
evaluator can now examine and evaluate mitigation measures rate. Inhibitor effectiveness is often verified by an internal
being employed against potential internal corrosion. A point monitoring program as described above.
schedule, based on the probable effectiveness of the measures, A pigging program may be necessary to supplement inhibitor
will show how the risk picture is affected. In the following injection. The pigging would be designed to remove free liquids
example schedule. points are added for each preventive action or bacteria colony protective coverings, which might otherwise
that is employed, up to a maximum of 10 points. interfere with inhibitor or biocide performance.
A nticovvosion activities heingperfimzed:
Internal coating Internal coating can take several forms
None 0 pts including spray-on applications of plastics, mortar, or concrete
Internal monitoring 2 pts as well as insertion liners for existing pipelines. New materials
Inhibitor injection 4 pts technology allows for the creation of “lined” pipe. This is usu-
Not needed 10 pts ally a steel outer pipe that is isolated from a potentially damag-
Internal coating 5 pts ing product by a material that is compatible with the product
Operational measures 3 pts being transported. Plastics, rubbers, or ceramics are common
Pigging 3 pts isolating materials. They can be installed during initial pipe
fabrication, during pipeline construction. or sometimes the
lvbnr This. of course, means that no actions are taken to material can be added to an existing pipeline. Such two-
reduce the risk of internal corrosion. material composite systems are also discussed in the design
index (Chapter 5). For purposes of this part of the risk assess-
hternal monifoving Normally, this is done in either of two ment, the evaluator should assure himself that the composite
ways: ( 1 ) by an electronic probe that can continuously transmit system is effective in protecting the pipeline from damage due
measurements that indicate a corrosion potential or (2) by a to internal corrosion. A common concern in such systems is the
coupon that actually corrodes in the presence of the flowing prod- detection and repair of a leak that may occur in the liner.
uct and is removed andmeasuredperiodically. Each ofthese meth- The internal coating can be judged by the same criteria as
ods requires an attachment to the pipeline to allow the probe or coatings for protection from atmospheric corrosion and
coupon to be inserted into and extracted from the flowing product. buried metal corrosion described in this chapter. Note that
Another method involves the use of a spool piece-a test an internal coating that is applied for purposes of reduction
piece of pipe that can be removed and carefully inspected for in flow resistance might be of limited usefulness in corrosion
evidence of internal corrosion. Searching for corrosion prod- control.
ucts in pipeline filters or during pigging operations is yet
another method of inspectiordmonitoring. Operational nieaswes In situations where the product is
To be creditable under this section, an inspection method normally compatible with the pipe material but corrosive impu-
requires a well-defined program of monitoring and interpreta- rities can be introduced, operational measures are often used to
tion of the data at specified intervals. It is further implied that prevent the impurities. Systems used to dehydrate or filter a
appropriate actions are taken, based on the analysis from the product stream fall into this classification. A system that strips
monitoring program. sour gas (sulfur compounds) from a product stream is another
Where a corrosion rate is actually measured, the overall example. Maintaining a certain temperaturc on a system in
internal corrosion score can be somewhat calibrated with this order to inhibit corrosion would also be a valid operational
information. Ideally, the scores will reflect the corrosion poten- measure. These systems or measures are termed operurionul
tial and will correlate well with more direct evidence such as a here because the operation ofthe equipment is often as critical
measured corrosion rate. Caution must be exercised however, as the original design. Procedures and mechanical safeties
when assigning favorable scores based solely on the non- should be in place to prevent corrosive materials from entering
detection of internal corrosion at certain times and at limited the pipeline in case of equipment failure or system overloads.
locations. It is important to note that the potential for corrosion The evaluator should check to see that the conditions for which
might be high and is worth noting. even when no active corro- the equipment was designed are still valid especially if
sion is detected. More is said about corrosion rate later in this the effectiveness of the impurities removal cannot be directly
chapter and in Chapter 14. determined. The evaluator should look for consistency and