334
Fundamentals of Corrosion
covering the subject is the Corrosion Engineering Handbook by Philip
A. Schweitzer (published by CRC Press).
From the above sources, or from manufacturers’ data sheets, the
corrosion rate for uniform corrosion can be determined. This will
enable the engineer to determine the amount of material that will
be lost over the required life of the equipment, thereby permitting a
corrosion allowance to be established.
8. Economic considerations. Typically at this point, the viable alternatives
can easily be reduced to a handful of materials, often two or three.
Economics is often the basis for making the final selection. However,
making the choice on the basis of minimum material costs is not
always the best overall economic alternative. Other factors that must
be considered are:
a. Fabrication costs, including the cost of making the equipment,
transportation to the job site, and the installation. Some of the
cost differences may be subtle. For example, carbon steel may
not require postweld heat treatment but 1¼ Crl-2 Mo alloy might.
Thus, the cost differences would be greater than the differences
between the cost of the two alloys.
b. Maintenance costs, including a judgment about how frequently
the equipment will need to be inspected for corrosion damage.
c. Fouling can be a problem where there are large surface areas
of materials with an otherwise acceptable corrosion rate. For
example, a large tower packed with steel packing may generate
tons of corrosion product even when the corrosion rate is moderate. This corrosion product can foul the small passages through
the packing and plug downstream heat exchangers and other
equipment.
d. Reliability requirements. Some equipment may require a high level
of reliability. This would be the case for equipment with a long
delivery time and where spare equipment cannot be held in
inventory. Conversely, less conservative material choices can be
made if spare equipment is available and having an equipment
failure does not create a serious safety or environmental event.
e. Repair costs, including judgments about how easily and quickly
the equipment can be repaired. Will major components have to
be replaced rather than repaired?
f. Cost of lost production due to equipment inspection, repair, or
replacement must be considered.
Although it may be difficult to do with high levels of confidence, these judgments can be reduced to dollar estimates. NACE
Standard RP-02-72 Recommended Practice — Direct Calculation
of Economic Appraisals of Corrosion Control Measures provides
Fundamentals of Corrosion
covering the subject is the Corrosion Engineering Handbook by Philip
A. Schweitzer (published by CRC Press).
From the above sources, or from manufacturers’ data sheets, the
corrosion rate for uniform corrosion can be determined. This will
enable the engineer to determine the amount of material that will
be lost over the required life of the equipment, thereby permitting a
corrosion allowance to be established.
8. Economic considerations. Typically at this point, the viable alternatives
can easily be reduced to a handful of materials, often two or three.
Economics is often the basis for making the final selection. However,
making the choice on the basis of minimum material costs is not
always the best overall economic alternative. Other factors that must
be considered are:
a. Fabrication costs, including the cost of making the equipment,
transportation to the job site, and the installation. Some of the
cost differences may be subtle. For example, carbon steel may
not require postweld heat treatment but 1¼ Crl-2 Mo alloy might.
Thus, the cost differences would be greater than the differences
between the cost of the two alloys.
b. Maintenance costs, including a judgment about how frequently
the equipment will need to be inspected for corrosion damage.
c. Fouling can be a problem where there are large surface areas
of materials with an otherwise acceptable corrosion rate. For
example, a large tower packed with steel packing may generate
tons of corrosion product even when the corrosion rate is moderate. This corrosion product can foul the small passages through
the packing and plug downstream heat exchangers and other
equipment.
d. Reliability requirements. Some equipment may require a high level
of reliability. This would be the case for equipment with a long
delivery time and where spare equipment cannot be held in
inventory. Conversely, less conservative material choices can be
made if spare equipment is available and having an equipment
failure does not create a serious safety or environmental event.
e. Repair costs, including judgments about how easily and quickly
the equipment can be repaired. Will major components have to
be replaced rather than repaired?
f. Cost of lost production due to equipment inspection, repair, or
replacement must be considered.
Although it may be difficult to do with high levels of confidence, these judgments can be reduced to dollar estimates. NACE
Standard RP-02-72 Recommended Practice — Direct Calculation
of Economic Appraisals of Corrosion Control Measures provides