New Appendix G of API-653 Provides for Tank Bottom Inspections
By Vincent A. Carucci
Aboveground storage tank bottoms must be periodically inspected and
assessed for their integrity. This will generally set the required interval
for internal tank inspection. During this inspection, the entire bottom
should be visually examined for holes, cracked welds, generally corroded or
pitted areas, and previously repaired areas. Additional inspection methods
will also typically be used to obtain all the data needed to assess the
integrity of the bottom. This can include:
- Magnetic flux leakage and/or eddy current testing of the entire
bottom to establish the base values of remaining plate thickness.
- Vacuum box testing of floor seam welds and the bottom-to-shell
junction weld to identify through-wall defects that were not apparent
from the visual inspection.
- Ultrasonic thickness measurements to quantify the remaining
- Magnetic particle or liquid penetrant testing of the
bottom-to-shell junction weld, and possibly floor plate welds, to
verify that surface cracks or excessive pitting are not present.
API-653, Tank Inspection, Repair, Alteration and Reconstruction,
does not explicitly state the inspection procedures that must be used, nor
their extent. These are left up to the owner/operator to decide. However,
with Addendum 1 issued September 2003, API-653 contains a new Appendix G
that provides guidance for qualifying both tank bottom examination
procedures and the personnel who perform tank bottom examinations.
The owner/operator may apply the Appendix G guidelines as written, or
modify them to suit their particular applications and needs. The
introduction of Appendix G uses the words “guidelines” and “guidance” in
describing the information contained in it. However, it is clear that the
intent is to follow these guidelines, or at most, adapt them for specific
situations while still meeting their intent.
All tank inspections are important in order to evaluate the tank’s
structural integrity. However, bottom integrity evaluation and the
inspection data that are needed to conduct it are clearly the most
important factors to “get right” for obvious reasons.
- Hydrocarbon leaks from the bottom go directly into the foundation
and can potentially proceed much further.
- The bottom may have been leaking for a long time before the
leak is detected.
- Severe bottom leakage could undermine the foundation, which
could result in a more extensive tank failure.
- The underside of the bottom is not visible, and severe
bottom corrosion and pitting is often from the underside.
With the advent of API-653 in 1991, much more attention has been paid
to tank inspection and integrity evaluation in general and the bottom in
particular. There has thus been a large increase in the number of
individuals and companies performing tank inspections. Technology
improvements and inspection techniques have also kept pace with this
increased demand. Because of all these factors, it is important that
qualified inspection tools, procedures, and personnel are used in tank
bottom examinations. This is where Appendix G comes in.
The approach that Appendix G uses to present its guidelines is similar
to that used with welding procedures and welder qualification. Some of the
same terminology is also used. The following highlights the overall
structure and some of the information contained in Appendix G. Refer to
Appendix G for complete details.
- Definitions. Several terms are defined. Among
these are essential variables, non-essential variables, qualification
test, and scanning operator or operator. Note the similarities to
welding terminology. For example, an essential variable in a tank
bottom examination procedure is one that cannot be changed without the
procedure and scanning operators being requalified.
- Tank Bottom Examination Procedures. An authorized
inspection agency that performs bottom inspections must have and use
qualified tank bottom examination procedures (TBP). These procedures
provide direction for those who perform the bottom inspection. Each
TBP must address essential and non-essential variables. Each TBP
specifies limits on the appropriate variables. This general approach
is analogous to a welding procedure.
- Tank Bottom Examiners. Examiners must be
qualified for the work that they actually do (e.g., operate scanning
equipment or perform follow-up bottom thickness measurement). The
purpose of qualifying a tank bottom examiner is to confirm their
ability to satisfactorily use a qualified procedure to determine tank
bottom condition. This individual is analogous to a welder.
- Qualification Testing. A test must be performed
on a sample of the bottom with designed flaws in it in order to
qualify the examination procedure and equipment. Appendix G specifies
the minimum number, type, and location of the flaws to be included in
the sample plate. This is analogous to a welding procedure
- Qualification Test Acceptance Standards.
Acceptance criteria are specified that must be met when qualifying
either an examination procedure or an examiner.
- When qualifying either a procedure or a scanning operator, the
operator must be able to detect a specified percentage of flaws.
For example, if the remaining bottom thickness is less than 0.05
in., 90% - 100% of the flaws must be found.
- When qualifying either a procedure or examiner who “proves up”
indications (i.e., determines remaining plate thickness), the
examiner must be able to determine flaw depth based on the type of
tank bottom (i.e., uncoated, coating < 0.03 in. thick, coating >
0.03 in. thick).
- Qualification Test Variables. Appendix G presents suggestions for
essential and non-essential variables.
- Essential variables are items that may have a significant
effect on the examination quality if they are changed from those
used in the qualification test. Examples include:
- Scanner equipment
- Prove-up equipment
- Prove-up procedure
- Plate thickness
- Coating thickness
- Distance from shell
- Critical equipment settings
- Threshold settings
- Calibration or functional check
- Non-essential variables are items that will have less effect
on the examination quality and may be different for different
types of bottom scanners. Examples include:
- Scanner speed
- Scanning pattern
- Height limitations
- Overlap between scans
- Plate cleanliness
- Non-critical equipment settings
Appendix G is an important and welcome addition to API-653. Its use
will go a long way toward improving the reliability of tank bottom