surfaceo fapipe or vessel to the next interface that reflects sound waves.
Generally,t his measures wall thickness, but As can can be occasionally
fooled, by midwall pipe flaws. Bscan instruments are much more powerful
because they produce cross-sectional imagess imilar to x-rays. The C-scan
systemsp roduce at hree-dimensional view of as urface usingc omplex and
expensive equipment. C-scan systems can be very useful for large, critical
surfaces sucha sa ircraft skins, but they are much less commonly used in
process plants at this time because of cost, speed of coverage, and very large
quantity of data produced.
11.1.3V isual Inspection
Visual inspection is rarely practicali naworking process plant. However,
whenever an opportunity presents itself, full advantages hould be taken of
it. Only through direct visual inspection can one be absolutelyc ertain as
to the condition of ap rocess unit. Consequently,d uring any turnaround
activities, internal visual inspection should be carried out to verify the results
of on-line monitoring programs, choose new locations for monitoring, etc.
11.1.4D estructiveA nalysis
Destructive analysis is less practicalt han visual inspection but can be
important and should, therefore, be performed as often as possible. The
most common opportunity ariseswhenpiping is replaced for one reason or
another.Sections of the replaced piping can then be examined for evidence
of stress corrosion cracking (SCC),h ydrogen-inducedc orrosion( HIC),
pitting,and any other unusual or unexpected corrosive activity.
11.1.5C hemical Analysis
Chemical analysis is them onitoringo ft he chemical compositiono r
other chemical characteristics of the process fluids. Because corrosion is an
electrochemical phenomenon, chemical analysis can be au seful indicator
of corrosivity of the process. In water systems, for instance, pH and
conductivity are two parametersfrequently monitored to give an indication
of corrosivity.I ncreases in concentrationso fc orrodents sucha sH 2 So r
CO 2 can also be useful in determiningc orrosivity,b ut suchm easurements
requirem ore complex equipmenta nd are usually infrequently made.
Another technique that can be useful involves analyzing process streams
for the presence of corrosionproducts, e.g., the quantity of metal and metallic
compounds. Thist ype of analysis can yield ag ood estimate of the current
metal loss from corrosion. When compared with previous data, it can be used
to establish trendsand approximate corrosion rates. Such data must be used
with caution, however.F or instance, “iron counts” in fluid produced by oil
wells can be agood indicatorofthe amount of corrosiontaking place on the
casing, tubing, and completion equipment. If ironispresent naturally in the
Corrosion Monitoring
535
CAT8247—CHAPTER 11—6/10/2006—12:56—SRIDHAR—XML MODEL B–pp. 533–547
Generally,t his measures wall thickness, but As can can be occasionally
fooled, by midwall pipe flaws. Bscan instruments are much more powerful
because they produce cross-sectional imagess imilar to x-rays. The C-scan
systemsp roduce at hree-dimensional view of as urface usingc omplex and
expensive equipment. C-scan systems can be very useful for large, critical
surfaces sucha sa ircraft skins, but they are much less commonly used in
process plants at this time because of cost, speed of coverage, and very large
quantity of data produced.
11.1.3V isual Inspection
Visual inspection is rarely practicali naworking process plant. However,
whenever an opportunity presents itself, full advantages hould be taken of
it. Only through direct visual inspection can one be absolutelyc ertain as
to the condition of ap rocess unit. Consequently,d uring any turnaround
activities, internal visual inspection should be carried out to verify the results
of on-line monitoring programs, choose new locations for monitoring, etc.
11.1.4D estructiveA nalysis
Destructive analysis is less practicalt han visual inspection but can be
important and should, therefore, be performed as often as possible. The
most common opportunity ariseswhenpiping is replaced for one reason or
another.Sections of the replaced piping can then be examined for evidence
of stress corrosion cracking (SCC),h ydrogen-inducedc orrosion( HIC),
pitting,and any other unusual or unexpected corrosive activity.
11.1.5C hemical Analysis
Chemical analysis is them onitoringo ft he chemical compositiono r
other chemical characteristics of the process fluids. Because corrosion is an
electrochemical phenomenon, chemical analysis can be au seful indicator
of corrosivity of the process. In water systems, for instance, pH and
conductivity are two parametersfrequently monitored to give an indication
of corrosivity.I ncreases in concentrationso fc orrodents sucha sH 2 So r
CO 2 can also be useful in determiningc orrosivity,b ut suchm easurements
requirem ore complex equipmenta nd are usually infrequently made.
Another technique that can be useful involves analyzing process streams
for the presence of corrosionproducts, e.g., the quantity of metal and metallic
compounds. Thist ype of analysis can yield ag ood estimate of the current
metal loss from corrosion. When compared with previous data, it can be used
to establish trendsand approximate corrosion rates. Such data must be used
with caution, however.F or instance, “iron counts” in fluid produced by oil
wells can be agood indicatorofthe amount of corrosiontaking place on the
casing, tubing, and completion equipment. If ironispresent naturally in the
Corrosion Monitoring
535
CAT8247—CHAPTER 11—6/10/2006—12:56—SRIDHAR—XML MODEL B–pp. 533–547