Api 575 rev.2

API 575
Guidelines and Methods for
Inspection of Existing Atmospheric
and Low-pressure Storage Tanks
CAIRO
23-27 Feb 2008
Hesham Moharram
For the benefit of business and people

2
1 Scope
2&3 References& Definitions
4 Types of Storage Tanks
5 Reasons for Inspection &
Causes of Deterioration.
6 Inspection Frequency &
Scheduling.
7 Methods of Inspection
8 Leak Testing & Hydraulic
Integrity of the Bottom
9 Integrity of Repair &
Alterations
10 Records
11 Appendix A
SUMMARY

4Title of the presentation - DD/MM/YY
Scope1
This document provides useful information and
recommended practices for the maintenance and
inspection of atmospheric and low-pressure
storage tanks.
While some of these guidelines may apply to
other types of tanks, these practices are intended
primarily for existing tanks that were constructed
to API Spec 12A or API Spec 12C, and API Std 620
or API Std 650.

References& Definitions2&3
2.References:
API, ASME, ASNT, ASTM, OSHA
3.Definitions:
3.1 alteration: Any work on a tank involving cutting, burning,
welding, or heating operations that changes the physical
dimensions and/or configuration of a tank.
Examples of alterations include:
a. The addition of a man way or nozzle exceeding 12 in.
NPS (nominal pipe size).
b. An increase or decrease in tank shell height.

2&3
3.2 applicable standard: The original standard of construction,
such as API standards unless the original standard of
construction has been superseded or withdrawn from publication;
in this event, applicable standard means the current edition of the
appropriate standard.
3.3 atmospheric pressure: When referring to (vertical) tanks,
the term “atmospheric pressure” usually means tanks designed to
API Std 650. API Std 650 & 653 provides for rules to design tanks
for “internal pressure” up to 21/2 lbf/in.2.
References& Definitions

2&3
3.4 authorized inspection agency: It can be one of the following:
a. The inspection organization of an insurance company which is
licensed or registered to and does write aboveground storage
tank insurance
b. An owner or operator of one or more aboveground storage
tank (s) who maintains an inspection organization for activities
relating only to his equipment and not for above ground
storage tanks intended for sale or resale.
c. An independent organization or individual under contract to
and under the direction of an owner or operator and
recognized or otherwise not prohibited by the jurisdiction in
which the aboveground storage tank is operated. The owner or
operator’s inspection program shall provide the controls
necessary for use by authorized inspectors contracted to
inspect aboveground storage tanks.

2&3
3.5 authorized inspector: An employee of an authorized
inspection agency that is certified as an aboveground storage
tank inspector per API Std 653, Appendix D.
3.6 bottom-side: The exterior surface of the tank bottom, usually
used when describing corrosion. Other terms with the same
meaning are “under-side” or “soil-side.”
3.7 change-in-service: A change from previous operating
conditions involving different properties of the stored product such
as specific gravity or corrosivity and/or different service conditions
of temperature and/or pressure.

2&3
3.8 examiner: A person who assists the API authorized tank
inspector by performing specific non-destructive examination (NDE)
on the tank but does not evaluate the results of those examinations
in accordance with API Std 653 or this recommended practice,
unless specifically trained and authorized to do so by the owner or
user. The examiner does not need to be certified in accordance with
API Std 653 nor needs to be an employee of the owner or user, but
shall be trained and competent in the applicable procedures in which
the examiner is involved. In some cases, the examiner may be
required to hold other certifications as necessary to satisfy owner or
user requirements. Examples of other certification that may be
required are American Society for Non-Destructive Testing SNT-TC-
1A or CP189, or American Welding Society Welding Inspector
Certification. The examiner’s employer shall maintain certification
records of the examiners employed, including dates and results of
personnel qualifications and shall make them available to the API
Authorized Inspector.

2&3
3.9 inspector: An Authorized Inspector and an employee
of an Authorized Inspection Agency who is qualified and certified
to perform tank inspections under this standard.
3.10 MFL (magnetic flux leakage): An electromagnetic scanning
technology for tank bottoms. Also known as MFE (magnetic flux
exclusion).
3.11 product-side: The interior surface of a tank bottom, usually
used when describing corrosion. Other terms with the same
meaning are “top-side” or “product-side.”
3.12 owner/operator: The legal entity having control of and/or
responsibility for the operation and maintenance of an existing
storage tank.

2&3
3.13 reconstruction: The work necessary to re-assemble a tank that
has been dismantled and relocated to a new site.
3.14 reconstruction organization: The organization having assigned
responsibility by the owner/operator to design and/or reconstruct
a tank.
3.15 repair: Any work necessary to maintain or restore a tank to a
condition suitable for safe operation. Typical examples of repairs
includes:
a. Removal and replacement of material (such as roof, shell, or
bottom material, including weld metal) to maintain tank
integrity.
b. Re-levelling and/or jacking of a tank shell, bottom, or roof.
c. Addition of reinforcing plates to existing shell penetrations.
d. Repair of flaws, such as tears or gouges, by grinding and/or
gouging followed by welding.

2&3
3.16 shell capacity: The capacity that the tank can hold based
on the design liquid level (see API Std 650).
3.17 soil-side: See definition for bottom-side.
3.18 storage tank engineer: One or more persons or
organizations acceptable to the owner or user who are
knowledgeable and experienced in the engineering disciplines
associated with evaluating mechanical and material
characteristics affecting the integrity and reliability of tank
components and systems. The tank engineering, by consulting
with appropriate specialists, should be regarded as a composite
of all entities necessary to properly address technical
requirements and engineering evaluations.

2&3
3.19 tank specialist: Someone experienced in the design and
construction of tanks per API Std 620 and/or API Std 650, and the
inspection and repair of tanks per API Std 653.
3.20 top-side: See definition for product-side.

4
4.1 GENERAL
Important factors such as the volatility of the stored fluid and the
desired storage pressure result in tanks being built of various types,
sizes, and materials of construction. In this document, only
atmospheric and low pressure storage tanks are considered.
Guidelines for inspection of tanks operating at pressures greater than
15 lbf/in.2 (103 kPa) gauge are covered in API RP 572.
Types of Storage TanksGENERAL

4
4.1.1 Storage Tanks with Linings and/or Cathodic
Protection
Tank Bottom Lining (API RP 652)
Cathodic protection (API RP 651)
4.1.2 Storage Tanks with Leak Detection Systems
Design Guidelines for leak detection (API Std 650,Appendix I)
GENERAL Types of Storage Tanks

4
4.1.3 Storage Tanks with Auxiliary Equipment
Most storage tanks are provided with some of the following auxiliary
equipment such as liquid-level gauges, high-and low-level alarms and
other overfill protection systems, pressure- relieving devices, vacuum
venting devices, emergency vents, roof drain systems, flame
arrestors, fire protection systems and mixing devices. Stairways,
ladders, platforms, handrails, piping connections and valves,
manholes, electric grounding connections (as required) and cathodic
protection systems are considered examples of storage tank auxiliary
equipment.
GENERAL Types of Storage Tanks

4
4.2 ATMOSPHERIC STORAGE TANKS
4.2.1Construction Materials and Design Standards
Atmospheric storage tanks are designed to operate with their gas and
vapor spaces at internal pressures approximating atmospheric
pressure.
Such tanks are usually constructed of carbon steel, alloy steel,
aluminium or other metals, depending on service. Additionally, some
tanks are constructed of non-metallic materials such as reinforced
concrete, reinforced thermosetting plastics, and wood. Some wooden
tanks constructed to API Spec 12E are still in service.
Types of Storage TanksATMOSPHERIC
STORAGE TANKS

4
4.2.2 Use of Atmospheric Storage Tanks
AST in the petroleum industry are normally used for fluids having a
true vapor pressure that is less than atmospheric pressure. Vapor
pressure is the pressure on the surface of a confined liquid caused by
the vapors of that liquid. Vapor pressure varies with temperature and
increases as the temperature rises.
Crude oil, heavy oils, gas oils, furnace oils, naphtha, gasoline, and
non-volatile chemicals are usually stored in atmospheric storage
tanks. Many of these tanks are protected by pressure -vacuum vents
that maintain the pressure difference between the tank vapor space
and the outside atmosphere to just a few ounces per square inch.
STORAGE TANKS

4
4.2.3 Types of Atmospheric Storage Tank Roofs
The most common type of
atmospheric storage tank is
the fixed cone roof tank,
up to 300 ft (91.5 m) in diameter &
64 ft (19.5 m) in height (larger
diameter tanks have been built,
mostly outside the U.S.)
These roofs are normally supported by
internal structural but can be fully self-
supporting in smaller diameters
(typically, 60 ft [3 m] diameter or less).
STORAGE TANKS

4
The umbrella roof has radially-arched
segmental plates with integral framing
support members (usually without
internal support columns).
In the steel dome roof tank, the roof
plates are usually formed with curved
segments joined to be self-supporting.
STORAGE TANKS

4
The floating-roof tank is designed to
minimize filling and breathing losses
by eliminating or minimizing the vapor
space above the stored liquid. The
shell and bottom of this type of tank
are similar to those of the fixed roof
tanks, but in this case, the roof is
designed to float on the surface of the
stored liquid. Older styles of floating
roofs include single steel deck details
without annular pontoons. Such roofs
have no reserve buoyancy and
are susceptible to sinking in service.
STORAGE TANKS

4
Annular-pontoon and double-deck Some floating-roof tanks have
fixed aluminium geodesic dome roofs installed on top of the tank shell to
reduce product vapor loss or to eliminate the need to drain rainwater
from the roof. These are considered internal floating roofs is a tank with
a fixed steel cone roof over a floating roof.
STORAGE TANKS

4 Types of Storage TanksATMOSPHERIC
STORAGE TANKS

4
Floating-roof sealing systems are used
to seal the space between the tank wall
and the floating roof, typically with a
mechanical seal. This type of seal
consists of a shoe which is a plate that
is pressed against the tank wall by
springs (or by counter-weights in older
designs) or other tensioning system,
with a flexible vapor membrane
attached between the shoe and the
floating roof outer rim.
STORAGE TANKS

4
4.3 LOW PRESSURE STORAGE TANKS
3.3.1 Description and Design of Low-pressure Storage
Tanks
Low-pressure storage tanks are those designed to operate with
pressures in their gas or vapor spaces exceeding the 2.5 lbf/in.2 (18
kPa) gauge permissible in API Std 650, but not exceeding the 15
lbf/in.2 (103 kPa) gauge maximum limitation of API Std 620. These
tanks are generally constructed of steel or alloy steel and are usually
welded, although riveted tanks in low-pressure service are still found.
Rules for the design and construction of large, welded, low-pressure
storage tanks are included in API Std 620.
Types of Storage TanksLOW PRESSURE
STORAGE TANKS

4
4.3.2 Use of Low Pressure Storage Tanks
Low-pressure storage tanks are used for the storage of the more
volatile fluids having a true vapor pressure exceeding the pressure
limits of API Std 650, but not more than 15 lbf/in.2 (103 kPa) gauge.
Light crude oil, some gasoline blending stock, light naphtha, pentane,
some volatile chemicals, liquid oxygen and liquid nitrogen are
examples of liquids that may be stored in low-pressure storage tanks.
STORAGE TANKS

4
4.3.3 Types of Low Pressure Storage Tank Roofs
Tanks that have cylindrical shells and
cone or dome roofs are typically used
for pressures less than about 5 lbf/in.2
(34.5 kPa) gauge. Tank bottoms may
be flat or have a shape similar to the
roof. Hold-down anchorage of the shell
is generally required.
Vapor Dome Roof
STORAGE TANKS

4
For pressures above about 5 lbf/in.2
Gauge (34.5 kPa), hemispheroid,
spheroid, and noded spheroid tank
types are commonly used. Tanks for
this application are now typically
constructed as spheres. Such tanks
are designed to withstand the vapor
pressure that may be developed
within a tank having no devices or
means to change or relieve the internal
volume. Plain Hemispheroids
STORAGE TANKS

4
Hemispheroid Plain Spheroid
STORAGE TANKS

32
Reasons for
Inspection and
Causes of
Deterioration
5

5
5.1 REASONS FOR INSPECTION
a. Reduce the potential for failure and the release of stored products.
b. Maintain safe operating conditions.
c. Make repairs or determine when repair or replacement of a tank may
be necessary.
d. Determine whether any deterioration has occurred, and if so, prevent
or retard further deterioration.
e. Keep ground water, nearby waterways, and the air free of hydrocarbon
and chemical pollution.
f. Regulatory compliance.
g. Risk management through data gathering and prioritization of
maintenance and capital expenditures.
Reasons for Inspection & Causes of DeteriorationREASONS FOR
INSPECTION

5
5.2 DETERIORATION OF TANKS
External Corrosion
Foundation material used for forming a pad under the bottom may
contain materials that can promote corrosion.
Poor drainage may allow water to remain in contact with the tank bottom.
Accumulation of the fluid under the tank can cause external corrosion of
the tank bottom.
An improperly sealed ring wall, as shown in Figure, may allow moisture
to accumulate between the tank and the support, thereby accelerating
corrosion.
DETERIORATION
OF TANKS
Reasons for Inspection & Causes of Deterioration

5
Foundation Seal
Reasons for Inspection & Causes of DeteriorationDETERIORATION
OF TANKS

5
External Corrosion
The lower tank shell can become severely corroded externally at, or
just above, or below, the grade line, where soil movement has raised
the grade level to cover the lower portion of the shell.
External corrosion also occurs when external insulation absorbs ground
or surface water by wicking action, or when damaged or improperly
sealed openings around nozzles and attachments allow water behind
the insulation.
A sulfurous, acidic or marine atmosphere can damage protective
coatings and increase the rate of corrosion.
OF TANKS

5
External Corrosion
The type of tank and the construction details used can affect the
location and extent of external corrosion. Inspections should look for
areas where tank construction details cause water or sediment to
accumulate.
Riveted tanks offer many niches where concentration cell corrosion can
occur. Leaks at seams of riveted tanks may cause failure of external
coatings, allowing external corrosion to develop.
OF TANKS

5
Internal Corrosion/Deterioration
Riveted tanks offer many niches where concentration cell corrosion can
occur. Leaks at seams of riveted tanks may cause failure of external
coatings, allowing external corrosion to develop.
Internal corrosion in the vapor space above the liquid of these tanks
can be caused by hydrogen sulfide vapor, water vapor, oxygen, or any
combination of these.
OF TANKS

5
Internal Corrosion/Deterioration
In the areas in contact with the stored liquid, corrosion is commonly
caused by acid salts, hydrogen sulfide or other sulfur compounds, or
contaminated water that settles out and mixes with solids on the
bottom of the tank. This layer is typically referred to as bottom
sediment and water (BS&W).
Stress corrosion cracking can be concern where the product is known
to be reactive with welds and heat-affected zones. Areas of high stress
concentration, such as at weld seams and around nozzle necks, are
more susceptible to accelerated corrosion due to the effects of product
contact.
Hydrogen blistering, hydrogen grooving, caustic stress corrosion
cracking, electrolytic corrosion and acid erosion. (API 571)
OF TANKS

5
5.3 DETERIORATION OF OTHER THAN FLAT BOTTOM
AND NON-STEEL TANKS
Concrete tanks can be attacked by the tank contents, can crack
because of settlement or temperature changes, or can spall because of
atmospheric conditions, resulting in exposure of the steel reinforcing
bars to further atmospheric corrosion.
External stress corrosion cracking of stainless steel tanks may be a
concern if chlorides that may be present in insulation get wet or enter
the insulation.
Aluminium can be affected by impurities such as acids or mercury
compounds in process streams or wastewater.
OF TANKS

5
5.4 LEAKS, CRACKS, AND MECHANICAL
DETERIORATION
Leaks can occur at improperly welded or riveted joints, at pipe thread
or gasket connections or cover plates, or at crack-like flaws (including
arc strikes on plates) in welds or in plate material. Three-plate in lap-
welded tank bottoms are particularly prone to defects that can lead to
leaks.
Crack-like flaws can result from a number of causes, including
deficiencies in design, fabrication, and maintenance. The most likely
points for crack-like flaws to occur are at the bottom-to-shell details,
around nozzle connections, at manholes, around rivet holes or around
rivet heads, at welded brackets or supports, and at welded seams. The
lower-shell-to-sketch-plate or shell-to-bottom weld is especially critical
because in relatively large or relatively hot tanks, there is a higher
likelihood this detail can develop a crack-like flaw due to high stresses.
OF TANKS

5
5.5 DETERIORATION AND FAILURE OF AUXILIARY
EQUIPMENT
Examination of tank venting devices should be included in periodic
inspection to ensure that proper operation and protection are
maintained. See API RP 576 for information regarding inspection of
pressure-relieving devices.
OF TANKS

5
5.6 SIMILAR SERVICE METHODOLOGY FOR
ESTABLISHING TANK CORROSION RATES
Similar service uses experience from tanks in similar locations and
services to establish corrosion rates.
The similar service should be examined independently for both the
product-side and the soil-side corrosion since the mechanisms and
corrosion rates are entirely independent of one another.
OF TANKS

44
Inspection Frequency
and Scheduling
6

6 Inspection Frequency and Scheduling
6.1 FREQUENCY OF INSPECTION (API 653)
6.2 CONDITION-BASED INSPECTION SCHEDULING
CONDITION-BASED
INSPECTION SCHEDULING

6
In most cases, the new thickness includes some excess thickness.
This excess thickness may be the result of any one or all of the
following factors:
a. Additional thickness added to the required thickness as a
corrosion allowance.
b. Additional thickness resulting from using the closest, but larger,
nominal plate thickness, rather than the exact value calculated.
c. Additional thickness from deliberately setting the minimum
acceptable thickness of plates for construction purposes.
d. Additional thickness on the upper portions of shell courses not
required for product loading at that level.
e. Additional thickness, available due to a change in tank service
or a reduced operating fill height.
Inspection Frequency and SchedulingCONDITION-BASED

6
For structural members and parts, such as roof supports and
platforms, normal accepted industry practice for structural design
(such as methods provided in the Steel Construction Manual
issued by the American Institute of Steel Construction) can be
used to calculate the allowable loads of members in the new
condition.
For external piping, nozzles, and valves, the methods provided
in API 570 and ASME standards can be used to determine
minimum acceptable thickness.
Inspection Frequency and SchedulingCONDITION-BASED

6
6.3 RISK-BASED INSPECTION
This methodology evaluates both the likelihood associated with a tank
leak or failure and the consequences of such a leak or failure. By
assessing factors associated with the likelihood and the consequence
of failure, and by performing appropriate evaluation, RBI provides
guidance on:
a. When to inspect.
b. The nature of the inspection.
c. Potential NDE technologies to use.
d. What interval will be required for the next inspection.
This evaluation should be performed by personnel experienced with
tank operations, degradation mechanisms, and scenarios relating to
consequences of tank failures. (API RP 580 & API Publ 581).
Inspection Frequency and Scheduling
RISK-BASED INSPECTION

6
6.4 FITNESS-FOR-SERVICE EVALUATION
Aboveground storage tanks can be evaluated to determine fitness for
continued service. API RP 579 provides fitness-for-service (FFS)
evaluation criteria for tanks based on what is known or can be
determined about the tank from various inspections. Different levels of
assessment are provided, depending on the information available for
evaluation and resources (experience and money) available. FFS
deals primarily with the evaluation of defects and flaws such as
corrosion, pitting, crack-like flaws, laminations, and distortions that can
affect remaining service life. Specific criteria for evaluating defects or
flaws are provided for in API RP 579.
Inspection Frequency and SchedulingFITNESS-FOR-SERVICE
EVALUATION

7
7.1 PREPARATION FOR INSPECTIONS
Before entering or re-entering any tank, appropriate safety precautions
are necessary. These precautions are discussed in detail in API Std
2015 and API RP 2016. Generally, such precautions include, but are
not limited to, the following:
a. Removal of hazardous gases.
b. Removal of gas-generating, pyrophoric, or toxic residues.
c. Isolation from any source of toxic or gas-generating fluids by the
use of blinds or by disconnection/isolation.
d. Assurance of an atmosphere that contains sufficient oxygen. Where
applicable, OSHA rules for safe entry into confined spaces should
be followed (Title 29, Code of Federal Regulations, Part
1910.146).
e. Potential for collapse of either fixed or floating roofs.
Methods of InspectionPREPARATION FOR
INSPECTIONS

7.2 EXTERNAL INSPECTION OF AN IN-SERVICE TANK
7.2.1 Ladder and Stairway Inspection
Should be examined carefully for corroded or broken parts. Its condition of
may be checked by visual inspection and by sounding.
7.2.2 Platform and Walkway Inspection
Can be inspected in the same manner as ladders and stairways. The
thickness of walking surfaces used by personnel can be checked at the
edges with calipers and in other areas by tapping with a hammer. Low
spots where water can collect should be checked carefully because
corrosion may be rapid in such areas. Drain holes can be drilled in the
area to prevent future accumulation of water.
EXTERNAL INSPECTION
OF AN IN-SERVICE TANK

7.2.3 Foundation Inspection
The foundations of tanks may be made of sand or other fill pads; crushed
stone pads or stone-filled grade bands; steel and concrete piers, ring
walls, or natural earth pads. Pads should be visually checked for erosion
and for uneven settlement.
7
The opening or joint
between a tank bottom and
the concrete pad or base
ring should be sealed to
prevent water from flowing
under the tank bottom.
Methods of InspectionEXTERNAL INSPECTION

7.2.4 Anchor Bolt Inspection
The condition of anchor bolts can usually
be determined by visual inspection.
Visual inspection can be aided by
removal of the nuts, one by one, or
supplemented by ultrasonic thickness
examination. Anchor bolts nuts should
be checked for a snug fit to the anchor
bolt chair top plate (i.e., there should be
no distance between the location of the
nut on the bolt and the anchor chair top
plate.)
7 Methods of InspectionEXTERNAL INSPECTION

7.2.5 Grounding Connection Inspection
The grounding connections should be visually checked for intact
connection and for corrosion at the point where they enter the earth or
attach to a grounding rod and at the tank ground clip.
If any doubt exists about the condition of the grounding connection, its
resistance can be checked. The total resistance from tank to earth should
not exceed approximately 25 ohms.
7 Methods of InspectionEXTERNAL INSPECTION

7
7.2.6 Protective Coating Inspection
Rust spots, blisters, peeling, cracking and coating due to lack of adequate
bond, are all types of common paint failure. Rust spots and blisters are
easily found by visual inspection. Coating bond failure is not easily seen
unless a blister has formed or has broken. Care should be taken not to
significantly damage protective coatings during inspection. Paint blisters
occur most often on the roof and on the side of the tank receiving the
most sunlight. Coating bond failure commonly occurs below seam leaks.
Other points at which the paint may fail are in crevices or depressions
and at tank seams that are welded, riveted, or bolted. The paint on the
tank roof is especially susceptible to accelerated failure. The paint on
floating roofs should be inspected carefully, especially in areas where
there is standing water or product.

7
7.2.7 Insulation Inspection
Visual examination is normally performed. Detailed inspection should
be conducted around nozzles, around the saddles of horizontal tanks,
and at caulked joints.
Areas of insulation may need to be removed prior to such inspection
especially on the shaded side of the tank, on roofs, below protrusions,
and at areas of obvious water Intrusion.
Insulation support clips, angles, bands, and wires should be spot-
checked for tightness and signs of corrosion and breakage. If access
is available internally, many of these areas can be checked by UT
from the inside.
Corrosion under insulation (CUI) is most aggressive in temperature
ranges between 120°F (49°C) and 200°F (93°C).

7
7.2.8 Tank Shell Inspection
If any foreign material or soil has collected around the bottom of the
shell or if the tank has settled below grade, a close inspection should
be made at and below the grade line.
Tanks with products at elevated temperatures in cold climates may
have water present near the shell and under the bottom because of ice
or snow build up around the tank.
Corrosion products or rust scale can be removed by picking, scraping,
wire brushing, or blasting (with sand, grit or water under high pressure)
so that the depth and extent of the corrosion may be evaluated. The
potential hazards of using such methods should be evaluated
beforehand. For example, hammer testing or removal of heavy scale
should not be done with the tank under pressure or otherwise in
service.

7
7.2.8.1 Thickness Measurements (API 653)
Sun, shade, prevailing winds, and marine environments may affect the
rate of external corrosion significantly. These factors need to be
considered when determining the number and location of thickness
measurements to be taken.
In obtaining shell thickness, special attention should be given to the
upper 24 in. (61 cm) of uncoated shells of floating-roof tanks. These
portions of the shell plates can corrode at a higher rate than the lower
shell plates because of constant exposure to the atmosphere on both
sides.

7
7.2.8.2 Stiffeners and Wind Girders
Can be inspected visually and by hammer testing. Thickness
measurements should be made at points where corrosion is evident.
Any pockets or crevices between the rings or girders and the shell
should receive close attention. If the stiffening members are welded to
the shell, the welds should be visually checked for cracks. The areas
can be checked by the magnetic particle or liquid penetrant
examination method. If the magnetic particle method is used for
detecting cracks while the tank is in service, current flow (prod
techniques) should not be used because of the danger of sparks. For
this type of test, a permanent magnet or electromagnet (magnetic flow)
technique should be used.

7
7.2.8.3 Caustic Cracking
If caustic or amine is stored in a tank, the tank should be checked for
evidence of damage from caustic stress corrosion cracking,
sometimes referred to as caustic embrittlement. The most probable
place for this to occur is around connections for internal heating units
or coils. This type of deterioration is manifested by cracks that start on
the inside of the tank and progress through to the outside. If this
condition exists, the caustic material seeping through the cracks will
deposit readily visible salts (usually white). Thorough cleaning and
checking with indicating solutions is necessary before welded repairs
are conducted on steel that has been affected by caustic stress
corrosion cracking. Cracking may occur during welding repairs in such
areas.

7
7.2.8.4 Hydrogen Blisters (API 571)
They are found most easily by visual examination and by touch.
Visual examination should be aided by use of hand-held lighting of
sufficient candlepower (at least 100 lumens) under low ambient
lighting conditions, holding the flashlight against the shell so the light
beam shines parallel to the shell surface. Many small blisters can be
found by running fingers over the metal surface.

7
7.2.8.5 Leaks, Crack-like Flaws, and Distortion
Leaks are often marked by a discoloration or the absence of paint in
the area below the leaks. The nature of any leaks found should be
carefully determined. If there are any indications that a leak is believed
to be due to a crack, the tank should be removed from service as soon
as possible, and a complete inspection should be made to determine
the repairs required.
Crack-like flaws can be found at the connection of nozzles to the tank,
in welded seams, and in the metal ligament between rivets or bolts,
between a rivet or bolt and the edge of the plate, at the connection of
brackets or other attachments to the tank, and at the connection of the
shell to the bottom of a welded tank.

7
Deformation can be caused by settlement of the tank, wind,
earthquake, internal pressure in the tank due to defective vents or
relief valves, an operating or induced vacuum in the tank, severe
corrosion of the shell, movement of connected piping, improper
welding repair methods, and other mechanical damage.
Settlement or frost heave of the soil beneath a tank bottom can cause
deformation of the shell at the bottom edge.
When a welded tank has significant deformations, weld seams may be
highly stressed and can crack. The joints most susceptible to cracking
are those at connections, at the bottom to shell joint, at floating-roof
deck lap seams, the shell-to-roof joint, and at vertical shell seams.

7
7.2.8.6 Rivet Inspection
If the tank is of riveted or bolted construction, a number of randomly
selected rivets or bolts should be checked for tightness.

7
7.2.9 Tank Roof Inspection
The roof or top head of a tank can be inspected for significant thinning
by ultrasonic thickness examination or even by MFL scanning (if the
roof condition is thought sound enough to withstand the weight of the
equipment). Hammer testing may dislodge scale from the internal plate
surfaces into stored product and is not a recommended method of
establishing roof plate integrity for personnel loading. Suitable fall
protection should be used when working on roofs. In general, the
inspector should always walk on weld seams if they are present,
because of the extra thickness available to support body weight.

7
Excessive gaps between the shell and the seal (s) of a floating roof
can indicate improper seal installation, altered seal condition due to
tank operations or long-term wear and tear or a malfunctioning seal (s)
due to external influences (earthquake, high winds, snow and ice).
Visual inspection may be adequate to determine seal condition, and
corrections may be possible while the tank is in service. If permanent
repairs cannot be made, the defective areas and any temporary
repairs should be noted in the records so that permanent repairs can
be made when the tank is removed from service. Seal damage can
occur if the maximum operating level is exceeded when portions of the
seal are pushed up above the top angle or plate edge.

7
Drainage systems on floating roofs should be inspected frequently for
leakage or blockage. If the drains are blocked, an accumulation of
liquid can cause floating roofs to sink or to be severely damaged.
Proper operation of check valves in drainage sumps should be verified
on a regular schedule, especially for those in fouling or corrosive
service.
Platforms and guardrails on a roof should be checked carefully in the
same manner as described earlier for stairways and ladders.

7
7.2.10 Auxiliary Equipment Inspection
Tank pipe connections and bolting at each first outside flanged joint
should be inspected for external corrosion. Visual inspection combined
with scraping and picking can reveal the extent of this condition.
The soil around the pipe should be dug away for 6 in. - 12 in. (150 mm
- 300 mm) to allow for inspection.
Flame arrestors should be opened at appropriate intervals, and the
screens or grids should be visually inspected for cleanliness and
corrosion. Bees and mud daubers occasionally plug arrestors.
Other auxiliary equipment should be inspected to ensure that it is in an
operable and safe condition.

7.3 EXTERNAL INSPECTION OF OUT-OFSERVICE TANKS
7.3.1 External Tank Bottom Inspection
Tank bottoms that rest on pads or on soil can be reliably inspected for
soil-side corrosion using inspection technology developments such as
magnetic flux leakage (MFL) bottom scanners or robotic inspection
equipment. With these developments, tunnelling under or completely
lifting a tank just for soil-side bottom inspection should normally be
avoided. It should be remembered that inspection of a tank by lifting may
necessitate a hydrostatic test that would be unnecessary with other
methods. As it is difficult to refill a tunnel properly, tunnelling should be
applied only to locations near the edge of the tank. Clean sand or washed
gravel are the best types of fill material.
EXTERNAL INSPECTION OF
OUT-OFSERVICE TANKS

7.3.2 External Pipe Connection Inspection
same as when the tank is in service (see 7.2.5).
7.3.3 External Tank Roof Inspection
All roof plates should be checked for thickness, regardless of the external
appearance. The inside surface of the roof plates may be susceptible to
rapid corrosion because of the presence of corrosive vapors, water vapor,
and oxygen. The interiors of the pontoons or double decks on floating
roofs should be inspected visually. Metal thickness measurements should
also be taken. A bright, portable light (of at least 100 lumens) will be
needed for this work.
Coupons approximately 12 in. X12 in. (300 mm X 300 mm) in size can
also be removed from the roof to check for under-side corrosion and rafter
condition. All coupons should be round or have rounded corners; no
square-cornered coupons should be cut.
7 Methods of InspectionEXTERNAL INSPECTION OF
OUT-OFSERVICE TANKS

7.3.3 External Tank Roof Inspection
All seals should be inspected visually for corroded or broken parts and for
worn or deteriorated vapor barriers. Any exposed mechanical parts-such
as springs, hanger systems and other tensioning devices, and Shoes-are
susceptible to mechanical damage, wear, and atmospheric or vapor
space corrosion.
Most floating-roof tanks are equipped with guides or stabilizers to prevent
rotation. These guides are subject to corrosion, wear, and distortion and
should be inspected visually. If the guides are distorted or the roof is no
longer in alignment with these guides, the roof may have rotated
excessively. The shell should then be inspected for deformations or other
defects as previously outlined in this section.
7 Methods of InspectionEXTERNAL INSPECTION OF
OUT-OFSERVICE TANKS

7
7.3.4 Valve Inspection
All valves on the tank should be inspected when the tank is out of
service. If leakage or significant deterioration is noted, consideration
should be given to valve replacement while the tank is out of service.
Valves can be refurbished if there is sufficient time during the out-of-
service period but this option could affect the return to service schedule
for the tank. Water draw-off valves should be inspected to determine
their condition.
Bonnet and flange bolts should be examined to ensure that they have
not significantly corroded and that they are tight and have proper
engagement length.
Methods of InspectionEXTERNAL INSPECTION OF
OUT-OFSERVICE TANKS

7.4 INTERNAL INSPECTION
7.4.1 Precautions
The tank must be emptied of liquid, freed of gases, and washed or
cleaned out as appropriate for the intended inspection. Appropriate
certification of the tank for personnel entry and inspection work should be
part of the permit process. Many tanks that are cleaned after removal from
service are not completely gas-free or product free unless particular
attention is paid to areas where hydrocarbon build-up can be overlooked
otherwise. Such areas include fixed roof support columns, floating-roof
legs, and guide poles all fabricated from pipe or other closed sections
without drainage holes and bearing pads or striker pads on the bottom
that may have leak paths (exhibited by product weeping). Diffusers and
other internal piping extensions inside the tank open to the product can
retain product in the piping inverts and should also be completely drained
and cleaned, and made otherwise safe prior to inspection work.
INTERNAL INSPECTION7 Methods of Inspection

7
7.4.2 Preliminary Visual Inspection
Visual inspection is important for safety reasons since the condition of
the roof or top head and any internal supports should be established
first. The vapor space, the liquid-level line, and the bottom are areas
where corrosion will most likely be found. If large areas are severely
corroded, it may be best to have them water or abrasive-blasted.
From a personnel safety or equipment operability standpoint, it may
be necessary to remove light coatings of oil or surface rust.
Inspectors should also be alert to accumulation of dry pyrophoric
(self-igniting when exposed to ambient conditions) material that may
ignite during inspection. These accumulations may occur on the tank
bottom, in the seal rim space areas, or on the top of rafters. Such
accumulations that cannot be cleaned out prior to inspection should
be kept moist to reduce the potential for ignition.
Methods of InspectionINTERNAL INSPECTION

7
7.4.3 Types and Location of Corrosion
The vapor space above the stored liquid can be an area of significant
corrosion. This is caused by the presence of corrosive vapors, such
as hydrogen sulfide, mixed with moisture and air.
The vapor-liquid interface is another region that may be subject to
accelerated corrosion, especially when fluids heavier than water are
stored.
When the stored fluid contains acid salts or compounds, they may
settle to the bottom of the tank; and if water is present, a weak
(corrosive) acid will form. Pitting-type corrosion can occur in the top of
tanks directly under holes or openings where water can enter.
The weld heat affected zone (HAZ) has been found to corrode at an
accelerated rate in relation to the surrounding shell plate material.

7
7.4.3 Types and Location of Corrosion
Among other types of deterioration that can occur on the shells of
storage tanks are hydrogen blistering, caustic stress corrosion
cracking, galvanic corrosion between dissimilar metals in close
proximity, and mechanical cracking.
These types of deterioration occur less frequently on the roofs and
bottoms of tanks. Carbon steel that contains slag inclusions and
laminations is more susceptible to hydrogen blistering. Caustic stress
corrosion cracking may occur in tanks storing caustic products. Hot,
strong caustic can also cause accelerated general corrosion.
Areas of residual stresses from welding or areas highly stressed from
product loading are most susceptible to caustic corrosion. Such
corrosion thrives when the temperature rises above 150ºF (65ºC) and
is most likely to occur around heating coil connections at the tank wall
or at piping supports on the bottom.

7
7.4.4 Tank Bottoms
Good lighting is essential for a quality visual inspection. A minimum
100-watt halogen light is usually adequate, but more light is better.
Magnetic flux leakage (MFL) inspection devices can be used to
rapidly scan for metal loss in the tank bottom plates.
Statistical methods are also available for assessing the probable
minimum remaining metal thickness of the tank bottom, and the
methods are based on a sampling of thickness scanning data. The
number of measurements taken for a statistical sampling will depend
on the size of the tank and the degree of soil-side corrosion found.
Typically, 0.2% - 10% of the bottom should be scanned randomly.
The collection of thickness data is required to assess the remaining
bottom thickness. In addition, the outer circumference next to the
shell should be included in the statistical sampling.

7
7.4.4 Tank Bottoms
Seams of riveted tanks can be checked by running a thin-bladed
scraper or knife along the riveted seam.
Representative sections or coupons (minimum size 12 in. [300 mm]
each way) may be taken to confirm the results of magnetic flux
leakage or ultrasonic examinations. The increasing accuracy of
magnetic flux leakage, ultrasonic scanning, and other automated
methods makes coupon removal less useful, especially considering
the time and expense associated with replacing the coupons.
Coupons may be advisable in assessing the root cause of soil-side
corrosion.
If settlement is detected (internally or externally), the magnitude of
the settlement should be measured. (API Std 653, Appendix B,
provides guidelines for evaluation of tank bottom settlement.)

7
7.4.4 Tank Shell
The area of highest stress in flat bottom tanks is commonly at the
shell-to-bottom joint detail and this area can be susceptible to
corrosion. Close visual inspection for this area should be done for
evidence of corrosion or other defects. It should be noted that a
riveted shell to bottom joint using a structural angle detail is
considered a mechanical joint, not a welded joint, and may not be
suitable for certain types of examination.

7
7.4.6 Testing for Leaks
The hydrostatic test will be the best method for detecting shell leaks.
If a hydrostatic test is not to be made, a penetrating oil (such as
diesel or automobile spring oil) can be sprayed or brushed on one
side of the shell plate in suspect areas and the other side can then be
observed for leakage. The liquid penetrant method used for finding
cracks can also be used in much the same manner, with the
penetrant applied to one side of the plate and the developer applied
to the other side. For either method, approximately 24 hours may be
required for leaks to become evident. Tank bottom leak detection
methods are described in Section 8.

7
7.4.7 Linings
Special inspection methods may be needed when the inside surfaces
of a tank are lined with a corrosion resistant material such as steel or
alloy steel cladding, rubber or other synthetic fabric, organic or
inorganic coatings, glass, or concrete (see API RP 652).
To avoid mechanical damage to the linings, considerable care should
be taken when working inside tanks lined with rubber, synthetics,
glass, or organic or inorganic coatings. Glass-lined tanks are
especially susceptible to severe damage that cannot be easily
repaired.
Concrete linings are difficult to inspect adequately, primarily because
the surface is porous. Concrete-lined steel bottoms are impractical to
inspect unless the concrete is removed.

7
7.4.8 Roof and Structural Members
If corrosion is noted on the roof and upper shell, then structural
members may also be thinning, possibly at as much as twice the rate
of the thinning of the roof or shell, since both sides of the structural
members are exposed to the corrosive vapors. ( API Std 653,
Appendix C).
7.4.9 Internal Equipment
Pipe coils, coil supports, swing lines, nozzles, and mixing devices
should be visually inspected.

7.5 TESTING OF TANKS
When storage tanks are built, they are tested in accordance with the
standard to which they were constructed. The same methods can be used
to inspect for leaks and to check the integrity of the tank after repair work.
When major repairs or alterations have been completed, such as the
installation of anew tank bottom or the replacement of large sections of
shell plate, the test requirements are specified in API Std 653, Section12.
Consideration should also be given to the notch toughness of the shell
material at the air and water temperatures existing at the time of the test.
A discussion of notch toughness and brittle fracture can be found in API
RP 571 and in API Std 653, Section 5.
TESTING OF TANKS7 Methods of Inspection

7.5 TESTING OF TANKS
If water is not available and if the roof of the tank is reasonably air tight
or can be made so, a carefully controlled air test using air pressure not
exceeding 2 in. (0.50 kPa) of water pressure may be applied. This type of
test is of very little use as a strength test and is used only in inspection for
leaks.
7.6 INSPECTION CHECKLISTS
API Std 653, Appendix C provides sample checklists of items for
consideration when conducting external and internal inspections.
7 Methods of InspectionTESTING OF TANKS

86
Leak Testing &
Hydraulic Integrity of
Bottom
8

8.1 GENERAL
Tanks that have impermeable foundations (reinforced concrete), under-
tank liners, or tanks constructed with double bottoms, provide an inherent
leak detection system which directs leaks to the perimeter of the tank
where they can be visually detected in accordance with the leak detection
provisions of API Std 650, Appendix I.
It is anticipated that leak test personnel (examiners) have qualifications
consistent with API Std 653.
General8 Leak Testing & Hydraulic Integrity of Bottom

8.2 LEAK INTEGRITY METHODS AVAILABLE DURING OUT-
OF-SERVICE PERIODS
8.2.1 Evaluation by Visual Examination
A Visual Test may be direct type when the surface is readily accessible
to place the eye within 24 in. (61 cm) of the surface at an angle of not less
than 30 degrees. The minimum illumination is 15-foot candles (25 lumens)
for general viewing and 50-foot candles (100 lumens) for viewing of small
anomalies. Visual test may be remote by using mirrors, cameras or other
suitable instruments.
LEAK INTEGRITY METHODS
8 Leak Testing & Hydraulic Integrity of Bottom

8.2.2 Evaluation by Wicking Examination of Shell to Bottom Weld
This test is a practical test because it provides information regarding the
actual hydraulic integrity of the weld with a product less viscous than the
product being stored. A leak could be easily located and repaired. Wicking
test of the shell-to-bottom weld (corner weld) is the process of applying a
highly penetrating oil or dye penetrant to one side of a weld (initial pass or
completed weld as required by the applicable standard of construction or
repair), then letting it stand for at least four hours (12 hours is preferred)
and observing if it penetrates to the other side of the weld.
8.2.3 Evaluation by Bubble Test Examination Pressure
For this method, the inside surface of the bottom is coated with an
indicator solution. Air at not more than 3 in. (0.75 kPa) of water pressure
is injected by a hose under the bottom of the tank through the clay seal or
through a drilled and tapped hole (or holes) in the bottom. The bottom is
then inspected for bubbles, which will indicate any leaks.
8 Leak Testing & Hydraulic Integrity of BottomLEAK INTEGRITY METHODS

8.2.4 Evaluation by Bubble Test Examination Vacuum
The vacuum box method is particularly useful on the flat bottom of a tank
but can also be adapted to the shell and the shell-to-bottom joint. In this
method, the suspect area is first coated with an indicator solution. In cold
weather, it is important that the leak-testing liquid be formulated for use at
the temperature involved. The method requires a minimum vertical
clearance of 6 in. (150 mm) between the bottom and any obstruction for
placement of device and accessibility to viewing the local area being
examined.
8.2.5 Evaluation by Liquid Penetrant
The dyes are either colour contrast (viewable in white light against a
contrasting colour developer) or fluorescent (visible under ultraviolet or
black light).

8.2.6 Evaluation by Magnetic Particle Examination
The magnetic particles are either colour contrast (viewable in white light)
or fluorescent (visible under ultraviolet or a black light) type. The colour
contrast type is either wet or dry type.
8.2.7 Evaluation by Detectable Gas
8.2.7.1 Under-bottom Injection
The injection of inert gas with a tracer under the tank. An advantage
of this method is that welded repairs can be made immediately with
the inert gas under the bottom and a re-check can be made
immediately after repairs.
8.2.7.2 Above-bottom Injection
The typical and preferred approach for implementing this leak test is
to perform it with liquid in the tank as described in 8.3.2. Liquid
loading has two primary advantages: 1) dispersion of the chemical
marker is facilitated by the liquid; and 2) liquid loading will increase
the probability of opening small cracks that might be closed without
the pressure from loading.

8.3 LEAK DETECTION METHODS AVAILABLE DURING IN-
SERVICE PERIODS
8.3.1 Evaluation by Leak Detection Systems Using Volumetric/Mass
Measurement Technology
LEAK DETECTION METHODS
8
8.3.1.1 Evaluation by Leak Detection by Volumetric Level and
Temperature Measurement
Volumetric level and temperature measurement technologies use
sensors to measure the level of a liquid in the tank over time.
8.3.1.2 Leak Detection by Mass Balancing
Mass measurement technologies use sensors to measure the
pressure of a liquid in the tank over time by use of a
differential pressure sensor.
Leak Testing & Hydraulic Integrity of Bottom

8
8.3.2 Evaluation by Detectable Gas Above-bottom in Liquid
Inoculation (Chemical Marker Technology)
Detectable marker chemical (inoculate) has been applied to existing,
replacement, and new tank bottoms. The tank is full or partially full
of product or water prior to testing and may be used on coated
plates, or tank bottom plates prior to coating or lining.
8.3.3 Evaluation by Acoustic Emission Examination
Acoustic emission testing has the ability to localize a detected leak.
The detection method includes the use of sound sensors that can be
triangulated to locate a leak point.
Leak Testing & Hydraulic Integrity of BottomLEAK DETECTION METHODS

94
Integrity of Repairs &
Alterations
9

9.1 GENERAL
Not every defect or non-conformity will require repair. The decision to
repair or not repair should be made by an engineer familiar with storage
tank design, construction and maintenance issues.
9.2 REPAIRS
9.2.1 Repairs to Welded Tanks
Repairs made by welding on the bottom, shell, or roof of a tank should be
conducted and inspected in accordance with API Std 653
9.2.2 Repairs to Riveted or Bolted Tanks
Repairs can be made by riveting or bolting, using the procedures given in
the original standards for riveted or bolted tanks. Repairs to these tanks
may also be made by welding if the weldability of the steel is first
confirmed by physical testing.
9 Integrity of RepairsGeneral & Repairs

9 Bottom Repairs
9.2.3 Bottom Repairs API Std 653
If complete tank bottom plates must be replaced, the replacement plates
can be taken into the tank through a slot that is cut in the bottom shell
course. A perimeter layer of clean sand fill, metal grating, or a concrete pad
should be installed under and at least 3 in. (76 mm) beyond the projection
of the new bottom so that the shell is supported on the foundation through
the new bottom.
Integrity of Repairs

9 Shell Repairs
9.2.4 Shell Repairs API Std 653
9.2.4.1 Since the reinstallation of door sheets can be difficult for even
experienced tank specialists, the following procedure is suggested:
a. Locate the door sheet where the bottom plate is reasonably level for a
distance of at least 5 ft on either side of the door sheet vertical seams.
b. Make the door sheet cuts so that the vertical and horizontal weld joints
meet the weld spacing requirements in API Std 653, Section 7.
Leaving a shell lip by making the bottom door sheet cut above the
shell-to-bottom weld can provide sufficient stiffness if bottom buckling
is a concern.
c. Provide reinforcement
d. After reinstalling the door sheet, radiograph the weld in accordance
with API Std 653, 12.2.
9.2.4.2 Non destructive examination requirements API Std 653

9 Roof Repairs
9.2.5 Roof Repairs API Std 653
Roof plates can usually be replaced in the same manner in which they were
installed when originally constructed.
9.3 Special Repair Methods
When deep pits in tank plates are not closely spaced or extensive and thus do
not affect the strength of the tank, they may be repaired or filled by a number
of methods. Filling with air-hardening adhesive-to-steel epoxies may be
suitable if it will not be affected by the tank’s contents.
Leaks in roofs can be repaired by soft
patches that do not involve cutting,
welding, riveting, or bolting of the steel.
Soft patches can be made from a variety
of materials, including rubber, neoprene,
glass cloth, asphalt, and mastic or epoxy
sealing materials

10 Records & Reports
10.1 GENERAL
Good records form the basis of an effective inspection program and
allow for properly scheduled inspections. Accurate and complete
records help predict when repairs and replacements may be
needed, reducing the potential for safety and environmental
hazards.
10.2 RECORDS AND REPORTS API Std 653
A complete record file should consist of at least three types
of records:
a) design and construction records,
b) repair/alteration records, and
c) inspection records.
RECORDS

10 Form & Organization
10.3 FORM AND ORGANIZATION
The report should clearly breakdown the following categories of
recommendations:
a. Those areas that require immediate repair or change that are
mandatory in order to maintain the continued safety, health and
environmental concerns of the facility and that should not be
delayed.
b. Those areas that should be repaired to extend the tank life
that may fail before the next internal inspection.
c. Those areas that can be deferred until the next internal
inspection without jeopardizing health, environment or safety
and that the owner/operator wants to defer.
d. Those items that are strictly non-threatening areas of concern
such as cosmetic issues, settlement that is within the API Std
653 tolerances.
All recommendations should be backed up with supporting
calculations, photos and the rationale for such recommendations.
RECORDS

9 Ultrasonic Thickness
(UT) Measurement
A.1 Ultrasonic Thickness (UT) Measurement
Dual-element transducers can have the ability to measure thin sections
from 0.050 in.-1.000 in. (1.3 mm Ð 25 mm).
Holes in the material or sections of less than 0.050 in. (1.27 mm)
measured with too low a frequency will provide either no reading or
a false reading.
Epoxy coatings have a velocity approximately half that of the steel, so
that the ultrasonic tool will read the epoxy coating thickness as
twice its
actual thickness (0.015 in. [15 mils] epoxy would read as 0.030 in. [30
mils]). Selection of a single-crystal transducer operating in the so-
called echo-to-echo mode can prevent this coating thickness error.
However, the single-crystal transducer has poor resolution for small
diameter deep pits.
APPENDIX A

9 Integrity of Repairs
A.2 Ultrasonic Corrosion Testing
(ASME) recommends 10% minimum overlap for readings based on
the transducer diameter. Large diameter transducers will not find
small diameter deep corrosion pits.
A.3 Ultrasonic Shear Wave Testing
Shear wave inspection can be used to assist in the discrimination
between laminations and inclusions in material. Automated shear
wave is especially effective for this purpose. The most general
application of shear wave transducers is to detect defects in butt-
welded joints, usually in lieu of radiography.
APPENDIX A

9 Integrity of Repairs
A.4 Magnetic Flux Leakage Bottom Inspection
The user should make sure that the scanner is calibrated properly
and has a validation and/or calibration test plate. A primary
advantage of these tools is the ability to detect product-side pitting,
soil-side corrosion, and holes in the tank bottom in an efficient and
economical manner. All of the systems require some additional
inspection to quantify detected flaws.
A.5 Robotic Inspection
These robotic crawler devices are designed for total immersion in
liquids and have been successful in providing ultrasonic thickness
information on tank bottoms in clear finished product storage such
as gasoline, naphtha and some crude oil. This equipment needs to
be used under carefully controlled circumstances and within API
safety guidelines for work on tanks in service.
APPENDIX A

Api 575 rev.2

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