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Tank Floor / Bottom Inspection
Tank 125: 28' Foot Diameter
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endangering personnel, affecting the environment and interrupting the operator's business.  In a 1988 API worldwide survey,
tank ruptures accounted for 5 % of the 132 releases that occurred worldwide between 1970 and 1988 but accounted for
almost 19% of the released material.  An example of a failure with dramatic results was in January 1988 in Pennsylvania,
where 500,000 gallons of fuel flowed from an above ground tank into the Monogahela River, the major source of water for
many local towns.  The cost of clean up, damage to the environment and adverse publicity associated with this and other
releases spawned present tank regulations and the development of API 653.
hydrocarbon production.  However, relative to pressure equipment, limited information is available for tank integrity
management.  Tanks vary considerably in size, to American Petroleum Institute (API) 620 and 650 tanks where the size may
be tens of millions of liters.  Perhaps the perception that tanks are simple, ambient pressure equipment leads to them receiving
less attention in the technical literature.  Additionally, the generally high reliability and perception of tanks as infrastructure
rather than plant has meant that tank maintenance approaches have tended to be reactive.  Whatever the case, review of tank
design and operating experience shows that tank issues can be complex and responses to leaks have been costly and anything
but simple.
The failure of a tank can have several undesirable effects such as...
Tanks have been around since the beginning of ...
Whether published standards for tank integrity ...
are available or not, it is apparent that well planned preventive, rather than reactive, measures should be taken in tank
maintenance and reliability.  It is interesting to note that in the USA, tank regulations and rules generally focus on mitigative
rather than preventive aspects; for example leaks and spills are mitigated by secondary containment rather than prevented by
design and inspection.  The importance of inspection and condition monitoring in avoiding failures, maintaining safety and
optimizing availability is unquestionable. However, in a competitive business environment, down time for inspection requires
considerable justification.
Facilities with tanks often present additional risks beyond site ...
risks such as potential injury to site personnel, damage to equipment and lost business.  Tanks are often located in areas of
environmental value or, because of the encroachment of suburbia, are close to the community.  Furthermore, incidents may
create unfavorable publicity through media coverage. Consideration of the cost of litigation and fines from accidental releases
alone can warrant setting up an inspection program.  Companies therefore require a consistent approach for assessing tank
integrity and maintaining compliance with industry standards and regulatory, that is, community requirements. Such an
approach must show that tanks are not leaking and will not leak before next inspection reduce the potential for releases
maintain tanks in safe operating condition, and make repairs and determine if and when replacement is necessary.
The standards and recommended practices ...
produced by the American Petroleum Institute (API) are recognized as worldclass.  Tank selection has historically been a
complex process of optimizing an array of requirements such as design, capacity and cost.  Other factors include corrosion
prevention systems and environmental regulations.  In planning to design and construct new tankage, there are ample standards
geared to provide agreement on design and fabrication between the supplier and purchaser.  Such standards ensure that the
tank will not fail when put into service and were not intended to deal with long term maintenance and inspection.  There are a
number of API standards and recommended practices which provide guidelines on design, fabrication, operation, cleaning,
inspection and repair of tanks and which can be used to develop tank integrity programs and procedures.  Selected information
is contained in Appendix 1.  The most important guide on in-service integrity is API 653.
API 653 departs from most inspection specifications ...
in that it requires an engineering analysis of the inspection data. Thickness measurements are evaluated to ensure that the tank
is structurally sound, within allowable stresses for the required design conditions and will not leak before the next inspection.  
Confirming that a tank will not leak goes beyond ensuring that it will not fail catastrophically, since even a small leak is
unacceptable. API 653 emphasizes the need for engineering experience when evaluating a tank's suitability for service.  It
requires that evaluation be conducted by organizations that maintain or have access to engineering and inspection personnel
who are technically trained and experienced in tank issues.
An inspection program should address the four main storage tank components:  ...
the roof, shell, bottom and foundation. There are several subcategories within these main components, including the tank
including fixed fire fighting systems and floating roof drains.  These will not be considered here.
Compliance and API 653 costs time and money.
Although compliance with API 653 is not mandatory, such industry standards have always had the standing of "good industry
practice" in the view of most regulatory authorities.  Compliance with API 653 or a corporate or other equivalent is really an
investment, in that the long term costs are likely to be more than recouped, due to avoided costs of site remediation from
spills, potential fines and lost business.  API 563 may also more directly reduce costs in demonstrating that tanks built to older
design standards continue to be fit for service.
Engineering analysis methods are potential alternatives to repairing a problem tank ...
The decision on which approach to take, repair or analysis should be made on a case-by-case basis on relative costs and
schedule considerations.  If using the API 653 shell-thickness calculations based on minimal data does not cause a severe
fill-height restriction or mandate extensive repairs, then the additional expense and time required for further analysis may not
be justified.  However, if the initial inspection and evaluation results show that there is a significant problem then the additional
inspection and evaluation may be worthwhile.  Thickness "averaging" is possible. With this approach, credit is taken for
reinforcement provided by thicker regions that are next to corroded regions of a tank shell.  Similar credit may be taken by
performing thickness calculations based on specific elevations of corroded regions.  This accounts for actual hydrostatic head
imposed at the corroded region, rather than making its minimum required thickness equal to that required at the bottom of the
particular shell course.
If analysis is required, API 653 provided guidelines for many types of repairs  ...
and alterations, including patch plates, alteration of nozzles, bulge repairs, bottom repairs or replacement, roof repairs, floating
roof seal repairs, hot taps and repair of defective welds.  Tank Inspection and Leak Methods Appendix C of API 653 contains
comprehensive checklists to perform in-service and out-of-service visual inspections. Some checklist items relate to tank
operational factors, such as whether the level control is operational, while other items relate to structural integrity issues. The
philosophy of API 653 is to gather data and to perform a thorough initial inspection in order to establish a baseline for each
tank inspection against which future inspections may be used to determine rates of corrosion or changes that might affect
fitness for service.  The scope of inspection is always subject to interpretation: for instance, a cursory or limited inspection
may miss the one pit in the floor that can lead to a leak.  To inspect for floor top-side corrosion, it is essential that the floor is
cleaned by grit blasting. While expensive (several tens of thousands of dollars for a crude tank), it has proven to be the only
sure way of uncovering defects. It is usually found that tank integrity assurance costs are dominated by cleaning/sludge
removal activities prior to inspection and application of confined space entry precautions, rather than by inspection costs.
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