iFluids Tank Inspection services

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iFluids Tank Inspection Services
Bottom Tank Inspection
Magnetic Flux Leakage Inspection techniques have been widely used in the Oil field
Inspection Industry for over a quarter of a century for the examination of pipe, tubing and
casing both new and used. It is only in the last ten years that this inspection technique has
been applied to Above Ground Storage Tank Floors in an attempt to provide a reliable
indication of the overall floor condition within an economical time frame.
In order to understand how this particular application of Magnetic Flux Leakage works it is
necessary to understand the basic principles of the technique. Everybody is familiar with the
ability of a magnet to “stick” to a carbon steel plate. This is due to the fact that magnetic lines
of force (flux) much prefer to travel in the carbon steel plate than in the surrounding air. In
fact this flux is very reluctant to travel in air unless it is forced to do so by the lack of another
suitable medium.
For the purposes of this particular application a magnetic bridge is used to introduce as near a
saturation of flux as is possible in the inspection material between the poles of the bridge.
Any significant reduction in the thickness of the plate will result in some of the magnetic flux
being forced into the air around the area of reduction.
Sensors which can detect these flux leakages are placed between the poles of the bridge, our
MFL equipment can detect in real time these material losses and help the inspector to identify
the critic zones in the tank floor for a UT inspection.
iFluids inspector were training in flux leakage and the use of the equipment.

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The ultrasonic prove up necessary must be carried out by personnel who are adequately
trained and qualified.
Mark 4 Tank Floor Scanner
Manual & Mapping Tank Floor Bottom Scanner
Overview
The Mark IV takes the precision, reliability, and durability of its predecessors to another level with
upgraded, user-friendly software and a lightweight build. Its slim design boasts an unprecedented
weight of only 65 lb.
The convenience of using the new Mark IV will be outweighed only by the confidence in its
reporting features and ability to discover volume loss in real time with its true real-time display.
SOFTWARE
User-Friendly Software:
The software guides the operator through an intuitive process that begins with quickly
generating a sketch of the tank floor bottom and ends with a comprehensive detailed MFL
map of the tank floor bottom.
Real-Time Display:
The real-time “A-Scan” display features an LED signal response that runs concurrently with
a “C-Scan” mapping preview that clearly illustrates where the defect is relative to the

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magnetic bridge. This allows defects to be located even faster and drastically eliminates the
needs of the scanner to be constantly moving while locating defects.
Tank Floor Bottom:
In less than 10 minutes, the operator can quickly recreate the tank floor plate layout,
including annular plates. Detailed information for each plate is not required until the operator
is ready to begin the MFL scan.
Speed Tracking:
Our new speed tracking feature provides the operator immediate feedback so they are
confident they are scanning within an optimal speed range, ensuring consistent, reliable, and
accurate results.
Speed Compensation Software:
Speed Compensation Software allows operators scanning at different speeds to obtain the
same results. The signal response is normalized when scanning within the speed tracking
range.

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It must be remembered that this is not just “thickness measurement” but rather corrosion
evaluation and the technician must have a full understanding of the technique that should be
applied.
This is not a subject that we can cover within the limits of this paper. It will, however, be the
subject of the next in a series of papers regarding the NDT of Storage Tanks.
iFluids have carried out this MFL Inspection with excellent results in India to many clients,
some of them three tanks belonging to IOCL, HPCL and others.
Dimensional ControlDimensional ControlDimensional ControlDimensional Control
Whether it is a single pipe spool, a complex structure, or an entire facility, if it has to fit first
time, iFluids surveys system of precise dimensional control solutions are right for you.
With the largest private fleet of precision survey equipment in the world, and proprietary
software systems providing guaranteed sub-millimetre accuracies, to better than 0.05mm
where required, no job is too precise.
When failure is not an option, only a company achieving a 99.9% success rate can make the
kind of promise your project needs. Never worry again, as the integrated iFluids approach
improves quality, schedules and project efficiency whilst lowering costs, and saving time.
Storage Tank Inspection
Responsive service and more information at less cost
Take advantage of the proven expertise and total value IFLUIDS Above Ground Storage
Tank Inspections offer. Our highly responsive technicians are certified to API 653, and can
provide you with immediate on-site evaluation and reporting and are available 24/7.
IFLUIDS's exceptional combination of certified inspectors, in-depth experience, and state-of-
the art equipment, deliver the overall cost savings and added value you're looking for.
Our DX-20 Magnetically- Attached Ultrasonic / Remote Visual Testing Crawler has
complete ultrasonic inspection and API-653 regulatory compliance capabilities for tanks and
vessels of all sizes. It does not require scaffolding, ladders, or sky climbers, and features
manless entry into confined or hazardous spaces. The DX-20 is also able to negotiate 1" high
obstacles, rivets, lap joints, and welds.
IFLUIDS also employs the MFE 2412 to inspect and detect bottom side & topside corrosion,
allowing our technicians to accurately examine tank floors at rates of 60 or more feet per
minute.

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Above Ground Storage Tank Evaluation Program
• Our Above Ground Tank Inspection services include:
• Nondestructive testing services
• Ultrasonic thickness measurement using our DX-20 remote crawler
• MFE 2412 floor scanner
• DX-20 Magnetically- Attached Ultrasonic / Remote Visual Testing Crawler
• Our Tank Inspection Qualifications include:
• API 653 certified inspectors
Mechanical Integrity Program capabilities:
• Thickness Inspections
• Engineering Evaluations
• Corrosion Rate and Remaining Life Determination
• Fitness For Service Evaluations (FFS)
• Turnkey Tank Management Program
Our targeted tank inspections look to find and quantify such damage and deterioration phenomenon
as:
• Corrosion Rate
• Remaining Life
• Under Floor Corrosion
• Corrosion Under Insulation (CUI)
• Tide Line and Corrosion Zones
• Settlement and Planar Tilt
Above Ground Storage Tanks within our inspection expertise include:
• SP-001 Tanks
• UL-58
Our inspections are tailored to your process equipment and risk mediation programs such as:
• Mechanical Integrity Program (MIP)
• Process Safety Management (PSM)
• Spill Prevention Control and Countermeasure (SPCC)
• Resource Conservation and Recovery Act (RCRA)
• Chlorine and Chlorine Dioxide Service
• Flammable or Hazardous Chemicals
• Proprietary Process Chemicals

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We utilize the following tests in our inspections to gain an understanding of the condition of your
equipment.
• Visual Inspections (VT)
• Ultrasonic Thickness Testing (UTT)
• Magnetic Particle Testing (MT)
• Penetrant Testing (PT)
• Radiographic Testing (RT)
• Magnetic Flux Leak Detection (MFL)
• Vacuum Box Testing (VBT)
Peripheral Considerations:
• Secondary Containment Sizing
Our API inspectors have been properly trained and certified and commmonly perform API
653 tank inspections. Many industries call upon iFluids to perform other popular services
according to the API 570 and API 510 standards.
Applications include:
• Tank shells, roofs and floors
• Confined Space inspections
• Hazardous area inspection
• Benefits include:
• Immediate on-site evaluation and reporting
• More information at less cost
• Automatic data acquisition analysis and API-653 calculations
Overview of API 653 - Tank Inspection, Repair, Alteration, and Reconstruction
API 653, Tank Inspection, Repair, Alteration, and Reconstruction, Fifth Edition, is a standard
developed and published by the American Petroleum Institute (API) and covers the

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inspection, repair, alteration, and reconstruction of steel aboveground storage tanks used in
the petroleum and chemical industries. The first edition of API 653 was published in January
of 1991. The fifth and most recent edition of the standard was published in November of
2014.
The requirements set forth in API 653 are meant to cover those steel storage tanks that were
constructed under the standards of API 650 and its predecessor API 12C. If there are any
conflicts found between this standard and those, API 653 is to take precedence. While it was
specifically written to apply to those tanks constructed under API 650 and API 12C, API 653
can, at the owner’s discretion, be applied to tanks constructed under other standards as well.
This standard provides the minimum requirements for maintaining the integrity of welded or
riveted, non-refrigerated and refrigerated, atmospheric pressure, aboveground storage tanks
after they have been placed into service. It only applies to maintaining the integrity of the
foundation, bottom, shell, structure, roof, attached appurtenances, and nozzles to the face of
the first flange, first threaded joint, or first welding-end connection of the tank.
What is MFL?
Magnetic Flux Leakage (MFL) inspection is a method of non-destructive testing (NDT) used
to detect and assess corrosion, pitting and wall loss in lined and unlined metallic storage tanks
and pipelines. A powerful magnet is used to magnetize the steel. In areas where there is
corrosion or missing metal, the magnetic field “leaks” from the steel. MFL tools use sensors
placed between the poles of the magnet to pinpoint the leakage field.
How it Works
In tank floor inspection, the floor of the tank is swept with the MFL tool. The area is flooded
with magnetic flux and rare earth magnets are used to temporarily magnetize the steel while
the magnetic field changes are recorded and analyzed. If the magnetic field is uniform, there
are no flaws in the tank floor. If the magnetic field is distorted, internal or external flaws are
present, such as pitting or corrosion and this distortion or “leakage” can be measured by the
sensors.
Technicians proceed to mark areas that need to be verified by visual and ultrasonic
inspections. The results obtained from the MFL inspection can be reported and used to
establish an existing baseline for the equipment or to determine remaining wall and fit-for-
service corrosion calculations in accordance with API, EEMUA and other applicable
standards.

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Magnetic Flux Leakage - Magnetic Bridge
Magnetic Flux Leakage - Corrosion Pit
Why Use MFL?
• Rather than replacing plant assets, condition assessment using MFL and spot repair can
often extend the life of a storage tank or pipeline.
• MFL inspection technology allows asset managers to proactively manage a tank or
pipeline by repairing problematic locations.
• MFL Inspection reduces the risk of tank or pipeline failure.
THE TRUTH ABOUT MAGNETIC FLUX LEAKAGE AS APPLIED TO TANK FLOOR INSPECTIONS
Magnetic Flux Leakage (MFL) Inspection techniques have been widely used in the NDT oil
field inspection industry for over a quarter of a century for the examination of pipe, tubing
and casing, both new and used. It is only in the last fifteen years that MFL inspection
techniques have been applied to above ground storage tank floors in an attempt to provide a

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reliable indication of the overall floor condition within an economical time frame. In most
cases, these MFL inspections are being carried out by industrial inspection NDT companies
who do not have the depth of experience in the technique that most of the oil field tubular
inspection companies have.
At the same time this relatively new application of Magnetic Flux Leakage brings with it some
additional problems not evident in the inspection of tubulars where certain parameters can be
quite closely controlled. Probably the greatest of these is that tank floors are never flat, whereas
tubulars are generally always round. The ability to obtain any reasonably consistent
quantitative information is seriously impacted by this general unevenness of most tank floors.
The application of rigid accept/reject criteria based on signal amplitude thresholds has proved
to be absolutely unreliable as regards truly quantitative information. A more realistic approach
is required in the application of this NDT inspection technique and in the design of the MFL
inspection equipment to ensure that there are fewer incidences of significant defects being
missed.
The following information outlines some of the major considerations that need to be addressed
in order to achieve reliable, fast and economical inspections of above ground storage tank
floors.
Magnetic Flux Leakage (MFL)
In order to understand some of the problems associated with this particular application of
Magnetic Flux Leakage (MFL), it is necessary to understand the basic principles of the
technique. Most people are familiar with a magnet’s ability to “stick” to a carbon steel plate.
This happens because the magnetic lines of force (flux) prefer to travel in the carbon steel plate
rather than in the surrounding air. In fact, this flux is very reluctant to travel in air unless it is
forced to do so by the lack of another suitable medium. For the purposes of this particular
application, a magnetic bridge is used to introduce as near a saturation of flux as is possible in
the inspection material between the poles of the bridge. Any significant reduction in the
thickness of the plate will result in some of the magnetic flux being forced into the air around
the area of reduction. Sensors which can detect these flux leakages are placed between the poles
of the bridge. Figure 1 graphically illustrates this phenomenon.
MFL Inspection- Magnetic Bridge Corrosion Pit

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Figure “1”
The MFL Inspection Environment
In order to optimize the effectiveness of the MFL inspection, it is necessary to consider the
environment and address the physical restrictions imposed by the actual conditions found when
examining the majority of tank floors.
Climatic Conditions
Invariably, the range of temperature and humidity conditions will vary enormously worldwide.
The effect on both operator and equipment must be taken into consideration. Human beings do
not function well in extremes of temperature. Use of the MFL inspection equipment should not
place too great a burden on them from either a physical or mental point of view. In other words,
the simpler, more reliable and easy to use the MFL inspection equipment is made, the more
reliable the inspection results.
Tank Floor Cleanliness
By their very nature, the majority of above ground storage tanks are dirty and sometimes dusty
places to work. The conditions in this regard vary widely and are dependent upon how much
effort the tank owner/operator is willing to expend in cleaning the floors in preparation for
Magnetic Flux Leakage scanning and inspection. As an absolute minimum, a good water blast
is necessary and all loose debris and scale should be removed from the inspection surface. The
surface does not necessarily have to be dry but puddles of standing water need to be removed.
The cleaner the floor, the better the inspection.
Storage Tank Surface Condition
Significant top surface corrosion and/or buckling of the tank floor plates represents a serious
limitation to both the achievable coverage in the areas concerned and also the achievable
sensitivity during the MFL inspection. While it is understood that very little can be done to
improve this situation prior to inspection, it must be considered in the design of the MFL
inspection equipment and its effect on the sensitivity of the inspection appreciated by both the
owner/operator of the tank as well as the person conducting the examination. Any physical
disturbance of the MFL scanning system as it traverses the tank floor will result in the
generation of noise. The rougher the surface, the greater the noise and, therefore, a reduction
in achievable sensitivity.
MFL Equipment Design Considerations
It is vital that Magnetic Flux Leakage NDT equipment used for storage tank floor inspection is
designed to handle the environmental and practical field conditions that are consistently
present. A piece of MFL inspection equipment designed in a laboratory and tested in ideal
conditions will invariably have significant short comings in real world applications.

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Electromagnets/Permanent Magnets
Powerful rare earth magnets are ideally suited for MFL inspections applications. They are more
than capable of introducing the required flux levels into the material under test. Electromagnets
by comparison are bulky and heavy. They do have an advantage in that the magnetic flux levels
can be easily adjusted and “turned off” if necessary for cleaning purposes. Permanent magnet
heights can be adjusted to alter flux levels, but the bridge requires regular cleaning to remove
ferritic debris. The buildup of debris can have a significant impact on system sensitivity.
Sensor Types
MFL inspection tools typically use one of two types of sensors: Coils and Hall Effect Sensors.
They are both capable of detecting the magnetic flux leakage fields caused by corrosion on
tank floors. There is a fundamental difference, however, in the way that they respond to leakage
fields.
Coils
Coils are passive devices and follow Faraday’s Law in the presence of a magnetic field. As a
coil is passed through a magnetic field, a voltage is generated in the coil and the level of this
voltage is dependent on the number of turns in the coil and the rate of change of the flux
leakage. From this, it is clear that speed will have some influence on the signals obtained from
this type of sensor.
Hall Effect Sensors
Hall Effect Sensors are solid state devices which form part of an electrical circuit and, when
passed through a magnetic field, the value of the voltage in the circuit varies dependent on the
absolute value of the flux density. It is necessary to carry out some cross referencing and
canceling with this type of sensor in order to separate true signals from other causes of large
variations in voltage levels generated by the MFL inspection process.
There is disagreement within the industry as to which is the best type of sensor to use for this
application. Hall Effect Sensors are undeniably more sensitive than coils. However, in this
application, coils are more than adequately sensitive and are more stable and reliable. Hall
Effect sensors prove to be too sensitive when surface conditions are less than perfect, which
results in an unreliable inspection and the generation of significant false calls.
MFL Technique Application Considerations
Coverage Limitations
It is virtually impossible to achieve 100% coverage using this technique due to the physical
access limitations. The MFL inspection equipment should be designed so that it can scan as
close as possible to the lap joint and shell. There are obviously compromises to be made as the
wheel base of the scanner is an important consideration on tank floors that are not perfectly
flat. Smaller scanning heads can be used in confined spaces to increase coverage.

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Topside/Bottom Side Differentiation
Magnetic Flux Leakage cannot differentiate between the response from topside and bottom
side indications. Some attempt has been made to use the eddy current signals from topside
defects for the purposes of differentiation based on frequency discrimination. This is unreliable
on real tank floors due to the uneven nature and lack of cleanliness of the inspection surface.
In most cases, visual techniques are perfectly adequate for this purpose. Contrary to what is
expected, the MFL response from a topside indication is significantly lower in amplitude than
that from an equivalent bottom side indication. This means that, to some degree, the influence
of the top side indications can be “tuned out” to allow a reliable assessment of the under floor
condition.
Quantitative Assessment of Indications
Magnetic Flux Leakage is a qualitative, not quantitative inspection tool and is a reliable
detector of corrosion on tank floors. Due to the environmental and physical restrictions
encountered during real inspections, no reliable quantification of indications are possible.
Amplitude alone is an unreliable indication of remaining wall thickness as it is more dependent
on actual volume loss. Defects exhibiting various combinations of volume loss and through
wall dimension can give the same amplitude signal. Couple to this the continually changing
spatial relationship of magnets, sensor and inspection surface and it is absolutely clear that an
accurate assessment of remaining wall thickness is virtually impossible. Truly quantitative
results can only be obtained using a combination of Ultrasonic testing and Magnetic Flux
Leakage.
The Single Level Threshold
Commercial expediency has brought about the implementation of accept/reject criteria using a
single level threshold approach. MFE Enterprises, as a manufacturer of MFL inspection
equipment, does not support this approach. As previously stated, the amplitude of signals alone
is not a reliable indicator of remaining wall thickness. Significant indications can be completely
missed especially in cases where the equipment does not incorporate some form of real time
on line display. In order to carry out a reliable MFL inspection, the operator must have as much
information as possible available to him in the form of an easy-to-interpret real time display.
The use of a blind single threshold is absolutely indefensible in this application.
MFL Operator Training and Qualification Requirements
Currently, there is limited training available to users of the MFL equipment in regard to this
application. MFE Enterprises Inc. recognizes this fact and offers initial basic training in
magnetic flux leakage and the use of MFL inspection equipment on delivery of the scanner.
This is obviously geared to our equipment and is quite specific. The ultrasonic prove up
necessary must be carried out by personnel who are adequately trained and qualified. This is
not just a “thickness measurement,” but rather a corrosion evaluation and the technician must
have a full understanding of the technique that should be applied.

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ULTRASONIC PROVE UP IN SUPPORT OF MAGNETIC FLUX LEAKAGE TANK FLOOR INSPECTIONS
It is generally recognized that Magnet Flux Leakage (MFL) is only a truly qualitative
technique and that indications detected using MFL must be assessed using a more
quantitative method. Invariably this task falls to Ultrasonic testing for the accurate
assessment of indication severity.
Over the last several years, there have been many ultrasonic methods and techniques used for
the evaluation of corrosion in support of MFL Tank Floor Inspections. A very large
percentage of the methods were severely flawed and it was apparent after questioning the
technicians involved that they did not have a real understanding of what was required. There
is a very big difference between “Thickness Measurement” and “Corrosion Assessment”.
Localized pitting corrosion is sometimes very difficult to find and measure using
compression wave Ultrasonic techniques due to the very shape of the defect. Many excellent
MFL inspections have been compromised by the inability of a bad Ultrasonic method to both
find and accurately assess the severity of indications. In most cases it is not the technicians
fault as there is very little published information in this regard and even less training
available.
The following is a description of the equipment and methods used that have proved to give
the best results possible in the assessment of MFL indications using the compression wave
Ultrasonic Technique.
ULTRASONIC INSTRUMENTS
Digital thickness gauges are not ideally suited for this application especially when evaluating
localized pitting type corrosion. Most of these gauges use peak or leading edge data from the
first and second back wall response to determine thickness. It is almost impossible to obtain
two reliable back wall responses from an irregular shaped indication. In the case of very small
pits the normal back wall response will obviously interfere with the accurate measurement of
the pit depth. The trigger gates are often set relatively high with this type of equipment to
prevent false measurements from noise. Indication responses can be very low amplitude and
therefore rejected using this method. Scanning to locate small indications is impossible with
digital thickness gauges. The technician is limited to a move and measure technique (pecking)
that has a limited chance of actually finding the indication at all. This type of equipment is
seriously limited in this application and is not recommended.
There are digital thickness gauges that have a rudimentary A-scan display. These are normally
small, low resolution displays. Even if the processing speed is up to processing the data real
time it is often the case that the screen update speed is too slow to allow any scanning technique
to be used. This type of equipment is also seriously limited in this application and is not
recommended.
B-scan equipment uses the ultrasonic data to provide a profile view of wall thickness and can
be useful as a reporting function but in most cases does not allow the technician access to the
A-scan information necessary to carry out a reliable evaluation of the Ultrasonic data. The data

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for the display is obtained in much the same way as in Digital Thickness Gauges and is
therefore seriously limited in this application and is not recommended.
Ultrasonic apparatus that should be used for the assessment of corrosion indications is one that
has a high resolution real time A-scan display that will allow scanning speeds of at least 6
inches per second. Most of the current commercially available instruments are more than
capable of meeting these requirements. The very early digital instruments had serious
limitations regarding processor and display update speeds. A good analogue display is still
superior to most digital displays for this application. Having said that, the latest digital display
instruments are more than fast enough to give acceptable results. The A-scan display allows
the technician to maximize the response and identify the first and closest facet of the indication
for the accurate measurement of remaining wall thickness.
There are differences between instruments from different manufacturers and the combination
of transducer and instrument is extremely important. Similarly configuration transducers from
different manufacturers do not always perform the same and should be carefully matched to
the ultrasonic instrument being used. A combination of transducer and instrument from the
same manufacturer is recommended as a starting point. If the instrument allows adjustment of
pulse amplitude and width it is possible to experiment with other transducers to obtain the best
results possible.
ULTRASONIC TRANSDUCERS
Where nominal plate thickness is in the range of 0.25" – 0.50" it has been determined that, in
most instances, the best results can be achieved by using a 5 MHz, dual element transducer
0.375" in diameter. In some cases it is necessary to resort to using smaller higher frequency
transducers (surface condition) or larger lower frequency transducers (thicker plates). The
implications as regards to beam spread and resolution must be considered when using these
alternative configurations. Slight focusing of the elements is required to minimize inaccuracy
on thinner materials. The chosen transducer should exhibit minimal cross talk even when worn
down to its limits. Different transducers vary widely in this regard. A fairly hard ware face is
important unless you want to spend a small fortune on replacements. Tank floor plates can be
extremely abrasive. Good quality cables are a must.
ULTRASONIC COUPLANTS
There are many proprietary couplants on the market that perform very well in this application.
As a general rule the higher the viscosity the better. In most cases, water and plenty of it is
more than capable of achieving the required results and is a lot cheaper.
MARKING MATERIALS
The choice of marking material can have a significant adverse effect on the ultrasonic prove-
up. Wax crayons are the worst followed by paint markers. It is sometimes impossible to couple
sound through these materials. This is obviously of concern as the mark should be exactly
where the indication is located. A good quality chalk is recommended for indication location

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marking. After Ultrasonic prove up, the indications can be marked more permanently with a
wax crayon or a paint marker.
METHOD
The method and technique described below is geared towards the assessment of localized
pitting corrosion rather than lake type corrosion. By its very nature, lake type corrosion is much
easier to locate and assess with a high degree of reliability unlike pitting corrosion, which can
be extremely difficult to locate and measure unless the correct method and techniques are used.
INSTRUMENT CALIBRATION
Using a step wedge and the transducer of choice, adjust the range and delay controls to display
a calibrated range so that the first two back wall responses can be displayed on the screen. In
the case of 0.25" nominal plate thickness, this would mean two responses at 0.25" and 0.50"
respectively. Adjust the gain to raise the first back wall response to full screen height. All
responses should be adjusted to the time base. All gates should be disabled at this time. If the
instrument has a reject control, make sure it is turned off or at zero. If provided, adjustment of
any pulse amplitude or pulse width controls can be used to optimize the shape of the signal
envelope.
Reference to any digital read outs of thickness can lead to significant errors in measurement in
this application due to the variations in signal envelope generated by different corrosion pits.
It is therefore recommended that no reference be made to gates based on signal amplitude and
the digital read outs associated with them. All responses should be referenced to the point at
which the signal breaks the time base.
TECHNIQUE
Lock all controls other than gain to prevent any inadvertent adjustment to the base line setup.
Evaluation of the located pit can now begin. Liberally spread the couplant of choice over the
area of plate concerned. Place the transducer adjacent to the location of the pit and obtain a
back wall response from the nominal plate thickness. The gain should be adjusted, at this time,
to bring the first back wall response to full screen height. Note where the first facet (left hand
edge) of the first back wall response breaks the time base. This is your nominal plate thickness.
Add a further 6db for scanning and note this setting.
Slowly scan the marked location of the pit. Even at this gain setting there may be no discreet
signal from the corrosion pit at this time. The only indication of anything amiss may be a dip
in the amplitude of the back wall response. Home in on the dip in back wall response and
increase the gain until the response from the corrosion pit is evident. Maximize the earliest
response from the corrosion pit by small scanning movements of the transducer. Continue
increasing the gain until the response from the pit reaches full screen height and note where the
first facet of the indication (left hand edge) breaks the time base. This will be your remaining
wall thickness measurement to be recorded. If the gain is so high at this point that electronic
noise from the instrument is evident, raise the signal response only as high as necessary to
discriminate between signal facets and the electronic noise.

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Do not forget to adjust the gain to the scan setting established earlier before moving to the next
indication. Calibration should be checked at regular intervals as any wear in the face of the
transducer will affect the accuracy of the measurements taken. At higher gains electronic noise
and cross talk may be evident on the time base but it is relatively easy to differentiate between
the noise and a true signal response. It is sometimes necessary to extrapolate exactly where the
signal breaks the time base based on the slope of the first facet. (left hand edge)
Using this method, it is possible to obtain the most accurate measurement of remaining wall
thickness from the Ultrasonic data presented.
RECORDING OF DATA
It is quite common these days to see thickness data presented to three decimal places. In the
above described method, it is possible to make a judgment as to indications that break the base
line between graticules. In this case you can split the reading rather than go to the third decimal.
ie. If a signal breaks the baseline between 0.16 and 0.17 it will be recorded as 0.165.
For each indication the following information should be recorded at a minimum:
1) Plate number
2) Indication number
3) Nominal plate thickness
4) Minimum Remaining wall thickness (at location of pit)
5) Measurement datum point (eg. South West corner of plate)
6) X – Y location measurement and direction from datum (eg. 25" North 16" West)
This will allow the technician or tank owner to accurately locate any indication at a later date
should any marking be erased.
GENERAL INFORMATION
Many signals generated during an MFL Inspection may not be due to corrosion pitting. Weld
scars, arc strikes and other magnetic anomalies, even lamination inclusions can trigger
responses. Whatever the cause, responses must be identified. If there is definitely no Ultrasonic
evidence of corrosion these other causes must be considered and investigated.
Accurate measurement of pitting corrosion is problematical when coatings are present. The
thicker the coating the more difficult it becomes. Even if it is possible to transmit sound through
the coating material and evaluate the carbon steel plate underneath, accurate pit depth
measurement is unreliable because the velocity in the coating is generally unknown and as
mentioned earlier it is very difficult to obtain repeat responses from small corrosion pits to
allow a peak or edge measurement technique between responses. Invariably the residue of

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energy being returned from the nominal thickness around the pit will interfere with this
technique. Any high amplitude localized signals will require the removal of the coating to
properly evaluate the accurate depth of the indication. Low frequency eddy current techniques
have been developed to overcome this perceived weakness of MFL and to avoid the necessity
of removing coating. The accuracy of the information from such devices is questionable and is
dependent on the coating having an even thickness. This is rarely the case on fiberglass coated
floors as the coating thickness can vary significantly throughout the floor area.
The ultrasonic energy reflected from a clean back wall will have a relatively tight signal
envelope as it is being reflected back from a flat surface. This is not true of the energy reflected
from corrosion. It’s irregular profile means that energy is being reflected from many different
surfaces and therefore the signal envelope will be much wider on the time base and of a much
lower amplitude. This is why it is so important to make significant gain increases to properly
evaluate the significance of the indication. Another point worth bearing in mind is that with
conical shaped reflectors that have a peak there will be very little, and sometimes no energy
reflected from the tip of the indication. In this case it will be seen that the ultrasonic data will
underestimate the severity of the indication.
Methodologies for Magnetic Flux Leakage (MFL) and Above Ground Storage Tank (AST) Inspection
The history of MFE/MFL technology and tank floor inspection
Initially, tanks were visually inspected and only obvious defects were noted. At some point,
the responsible parties brought pick hammers into the tank and hit the floor to check the solidity
of the floor. Some hammer swingers noted that they could tell the condition of the floor based
on the sound coming from the floor when struck by the hammer. This was some sort of pre-
acoustic technique which led to the cutting of coupons and the first evidence of a statistical
analysis approach to tank floor inspection. Eventually, ultrasonic NDT was applied to this
application. Ultrasonic coupons were born and yet tanks continued to leak at an alarming rate.
Meanwhile, pipeline inspection companies were utilizing MFE/ MFL technology to perform
pipeline integrity inspections. These companies were asked to help resolve the tank floor
inspection problem the way they had helped resolve pipeline inspection applications. The tank
floor scanner was invented based on the same MFE/ MFL principle technology applied to
pipeline scanners. The main point here is that nothing is 100% as regards to this application,
but each step is an improvement in moving closer and closer to the ultimate solution.
Manual tank floor scanning with real time display
Manual tank floor scanners with a real time display are scanners that are pushed across the tank
floor and do not include a drive motor. The real time display provides the technician with a
visual display of a signal response from flux leakage. If the response is only used as a location
device, and is not used to implicate pit depth, then the speed the scanner is pushed has a minimal
impact on the inspection outcome. With the manual scanner, an alarm is included which will
flash if the real time display indicates a signal. The alarm must be reset to stop the flashing
alarm. Some companies strongly support manual scanning because this process is the least

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dependent on tying signal amplitude to pit depth. The argument is that signal strength is more
relative to the overall volume loss of the pit than it is to the pit depth. If the unit uses a coil
sensor, it would need a fixed speed motor to provide a uniform inspection speed if the signal
response is going to be evaluated in terms of some linear response to pit depth.
Stop-On-Defect tank floor scanning without real time display
This type of MFE/ MFL scanner would use a hall sensor and would have a drive motor but no
real time display. This technique has been dubbed a “blind single level threshold”. This unit
would be calibrated trying to match pit depth to signal amplitude. The unit would use a
calibration plate with a known depth of defect. The unit would have a gain switch that could
be adjusted to amplify or de-amplify the signal, such that the signal from the calibration plate
would trigger the threshold alarm, and thereby shut down the drive motor. The unit would
have lights across the width of the scanner to represent the approximate location of the pit in
the X or side to side direction. Any light that illuminated would be marked for prove up by
ultrasonic inspection. One issue with this approach, is that there are some who believe that
since the signal strength from flux leakage is affected by the total volume loss, the stop on
defect without real time display could pass over a pit that has the required depth but not the
required total volume loss to equal or exceed the threshold volume loss. The defect could,
therefore, be missed. If this were to occur, the technician would not know that the anomaly
was missed, because there is no real time display to show the pit response that failed to reach
the required amplitude threshold. Another issue with this approach, is that general corrosion
on part of the tank floor could cause the technician to desensitize the unit as compared to
areas where there was isolated pitting. If the volumetric assumption is correct, then it would
explain why pits and even holes have been missed with this approach. On the other hand,
there are supporters who believe that the lack of a real time display allows the technician to
find signals higher than the general noise level, but ignore the signals within the noise level
range.
Stop-On-Defect tank floor scanning with real time display
The addition of a real time display to the stop-on-defect approach allows the technician to see
a signal even if the signal does not reach the preset threshold. This advantage allows the
operator to mark and check any signal displaying on the real time display. Then, the technician
can decide if the signal that did not reach the threshold has sufficient pit depth to be included
in the anomalies that are of interest. The technician can then adjust the threshold to include this
anomaly or the technician can adjust the sensitivity of the unit to cause this anomaly to trigger
the threshold. Supporters of stop-on-defect units agree that the main advantage of this approach
is that it can be used to discern signals that are just slightly higher than the general noise level.
This could be difficult to do with a manual unit where the technician relied on a real time
display and had to find all the signals that were just one or two bars above the general noise
level.
Mapping tank floor scanning using a Multiple color map presentation
Mapping systems are motor driven and have the added sophistication of computers, software,
and some form of encoder that measures wheel travel and tells the software program where the

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sensor is on the floor at all times. The signal responses are recorded and saved to a file. At
some point, the files are transferred to the tank floor drawing. At the end of the inspection, the
technician can provide an automated report that provides color images of the tank floor plates
with pitting that is relative in volume loss or pit depth to the color on the tank floor map. The
map of the tank floor makes it easy for the tank owner to know immediately what type of repair
strategy will be needed. Some argue that the map is more representative of the volume loss
than of the pit depth. It is the pit depth that determines repair strategy and remaining floor life.
However, supporters of the mapping approach argue that the map provides valuable repair
information and that if the owner is reasonable as regards to the known limitations of the
technology, there is considerable value to this approach.
So, which method is correct?
I believe the application of manual, stop-on-defect, and mapping is absolutely tied to the
condition of the tank floor. Tanks that have low density of general corrosion or low density of
isolated pitting are most suitable for manual scanning. This approach will find all the pitting
and will do so very economically.
If the tank floor has a mid to high density of general corrosion pitting, then the stop-on-defect
unit is preferable. The ,stop-on-defect can be set up to stop only when signals come out of the
noise and exceed the general noise level. This screening would be difficult to do simply by
looking at signals and trying to make the determination on the fly. This method would
successfully find the pitting that is visible above the general noise level and this is the best that
can be inspected in this situation. It is also the most economical approach for this particular top
floor condition.
If the scanner is placed on the tank floor and the screen lights up with multiple levels of color,
the technician can become overwhelmed in regards to signal interpretation. This condition
favors a mapping tool, where all these signals can be mapped. Some of the signals can be
checked with ultrasonic equipment to determine an expectation of pit depth in general regions
in comparison to map color. These map colors are more representative of bands of pit depth
and not necessarily relative to absolute pit depth. This approach would allow for a fairly rapid
inspection and is relative information when planning a comprehensive repair.
The bottom line is that there are applications for all of the approaches. Sometimes the decision
of which unit may be made on price or prior experience of tank floor conditions. It is my
opinion that the decision of which application should not be made until the technician is in the
tank and has performed some scanning to determine density and depth of defects.
Factors affecting the reliability of Magnetic Flux Leakage Surveys Of Above Ground
Storage Tank Floors
Over the past 15 years the preferred method for the inspection of Above Ground Storage Tank
Floors has proven to be by the Magnetic Flux Leakage method. Unfortunately, service
companies have been less than forthcoming with some of the limitations of this particular

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application of MFL. This has led to some confusion within the industry as to what this
inspection method is capable of achieving. There are too many stories in circulation about
significant defects being underestimated or missed completely. The following information
addresses some of the limiting factors of Magnetic Flux Leakage surveys and what to expect
from this inspection technique.
Limiting Factors
Adjustment of Equipment
It is vital that the equipment is adjusted in accordance with the manufacturers
recommendations. These adjustments will vary in relation to both thickness of the plate to be
inspected and the thickness of any coating. In order to create leakage fields from corrosion it
is necessary to achieve near saturation of the magnetic flux in the material. If near saturation
does not exist then no leakage field will be generated and therefore there will be nothing for
the sensors to detect.
Surface Condition
The top surface condition of the plate to be inspected absolutely dictates the achievable level
of sensitivity of the inspection. The rougher and dirtier the surface, the more noise that will be
generated from the inspection process and the lower the achievable sensitivity. Real signals
may be masked by the level of noise generated.
Plate Curvature
Tank floors are seldom flat. The degree of curvature of the plates may have a significant impact
on the reliability of the inspection. When the curvature of the plate causes an increase in lift off
by either the magnetic bridge or sensor the sensitivity may suffer dramatically. This can often
be overcome by scanning in more than one direction. Curvature of plates close to lap joints
and/or the shell can make it virtually impossible to scan these areas reliably. In some cases
smaller scanning heads can be used to overcome this problem.
Material Properties
The material itself can also have a significant impact on the inspection. Most floor plates are
manufactured using low carbon steel, which lends itself very well to this technique. Higher
grade materials require a much higher magnetizing force to achieve the same levels of
saturation. This also applies to materials whose properties have been degraded by the service.
i.e. Sour Crude Storage. In this case the material suffers from hydrogen attack and it may not
be possible to achieve the necessary flux saturation of the material to achieve the required
sensitivity. This can also be a major contributor to noise levels. The inspection direction can
also have an impact on the amplitude of signals from the same sized corrosion pit. It has also
been noted that noise levels can vary even on clean smooth surfaces due to either variations in
the material properties or method of manufacture.
Accessibility

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It has often been claimed that MFL can achieve 100% coverage of the floor area. In reality this
is virtually impossible to achieve due to the overlapping nature of the plates and the number of
obstructions in most floors. It is also difficult to inspect adjacent to the shell because of the
curvature due to edge settlement. There are often many areas of the floor that are inaccessible
to the regular scanners. Smaller scanning heads can be used to alleviate some of this problem
but it is rarely possible to cover the entire floor using MFL alone.
Product Side & Underside Discrimination
Magnetic Flux Leakage is incapable of reliably discriminating between product side and
underside corrosion. In most cases this is not a problem as it is possible to discriminate signals
visually. In the case where there is a known topside problem that has been documented and
passivated (by coating) after an earlier inspection and removal of the coating is not necessary
then it is helpful to the inspection agency to have access to the earlier inspection information.
This can speed the inspection process significantly. The very nature of MFL means that in
some cases product side corrosion cannot be detected by the floor scanner. As a general rule
the response from a product side pit with the same volume loss as an equivalent underside pit
will be significantly less. This is due in part to the higher flux leakage levels being retained
within the depth of the pit itself whereas on the other side (underside) of the ligament the
leakage field will be above the surface and therefore in the direct path of the sensors.
Coated Floors
In addition to the above limitation and in the case of thicker coatings, there is another factor
that affects achievable sensitivity. While it is often possible to overcome the additional lift off
of the magnetic bridge by lowering it within the carriage, the same cannot be done with the
sensor array to place it as close to the surface as possible because the coating is in the way.
This obviously means that it will no longer be possible to achieve the same levels of sensitivity
that could be demonstrated on the uncoated plate of the same thickness. In all such cases, it is
necessary to run a simulation prior to the inspection to see what can be achieved.
Thresholding
Some manufacturers employ a blind thresholding approach to this application in order to semi-
automate the inspection. It is deemed a blind threshold if the MFE/MFL equipment does not
provide a real time display that displays a viewable signal as the scanner passes across the tank
floor. With blind thresholding tools there is no display and signals generated that may be
significant will not come to the attention of the operator as the operator will not know the signal
took place. In other words, the floor could have a defect that caused a signal that for whatever
reason fell just short of the required threshold level and therefore the unit would not stop. With
no display to see the signal, the unit would continue to roll across the floor and the operator
would never know the signal occurred. The manufacturer’s claim is that the unit can be set up
so that only defects depths of concern to the tank owner can be found and all signals for defects
that are not deep enough to warrant further investigation can be ignored. This approach claims
to save time and money.

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IFluids does not support this blind threshold approach and feel that this is a major reason why
significant defects have been missed. The amplitude of signals generated from corrosion pitting
is more a function of the overall volume loss than the remaining wall thickness. This indicates
that three dimensions (length, width and depth) affect the signal response and not just pit
depth. Defects exhibiting largely different amplitudes can have the same through wall
dimension. Couple this with the need to control and monitor all the inspection parameters
(virtually impossible in this application) and it means that this approach is seriously flawed
both theoretically and in practical terms. IFluids provides a real time display that must be
monitored by the operator. If the same above described situation occurred with an MFE stop
on defect unit, the operator would see the signal even if it did not exceed the threshold
level. The operator could investigate as many of these signals as deemed necessary to gain
confidence in the current therehold or could make changes in the threshold as necessary to
reflect the proper threshold based on the ultrasonic readings obtained when investigating
signals.
Corrosion Mapping of Flux Leakage Signals
Systems that use a multiple threshold and supposedly accurately map the results in the form of
a color coded output related to wall thickness are available on the market. It is fashionable to
computerize everything these days. Unfortunately this is one application that could suffer by
this approach. First of all, Magnetic Flux Leakage is incapable of accurate quantification of
remaining wall in this application owing to the volumetric nature of the defects as well as other
factors such as lift off of sensor and/or magnetic bridge.
Originally IFluids. did not offer mapping scanners due to the false perceived advantage of being
able to determine pit depth with MFE/MFL as opposed to using ultrasonic tools to obtain
quantitative information. Competitors made (in our opinion) false claims. Through input from
customers, it was later revealed that they found out or already understood the problems as
regards to false claims. However, customers liked the automated reports and color map
presentations of the tank floor overall condition.
They learned they could not use the equipment with ultrasonic data as regards large numbers
of small repair patches but they could make repair decisions as regards the overall condition of
the tank floor as “in general” larger signals were “usually” indicative of deeper pits. These
factors led to a determination by MFE to build a mapping scanner but of offer it without the
false claims of MFE/MFL providing pit depth.
Operator
As with any equipment the operator is responsible for its correct use and the validity of any
inspection is absolutely dependent upon the operators integrity. It is probably better to have a
good operator with the worst equipment rather than a bad operator with the best equipment. Of
course, the best situation is to have a certified operator, equipment, and procedure. All
operators should, as a minimum, be able to show proof of training in the use of the equipment
they are operating along with a reasonable understanding of the technique. IFluids provides 8
hours training free of charge other than for the reimbursement of expenses when the equipment
is purchased from MFE. MFE also provides training according to API 653 Appendix G. The

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options exist for test plate training and/or 40 hours of training based on API 653. NO TWO
PIECES OF EQUIPMENT ARE THE SAME. It is imperative that the end user goes for the
best equipment and a good level of confidence in the person using it if they want to get the best
inspection possible. This is not the type of equipment or technique that can be made to be
independent of the operator. Some manufacturer’s claim equipment is “technology driven not
operator dependent”. All MFE/MFL equipment and most NDT equipment operation are to
some degree “operator dependent”.
Commercial pressures
No two tank floor inspections are the same and can range from smooth, clean, flat, defect
free, rough, dirty, buckled and corroded throughout. It is obvious that a good inspection of
the latter is going to take significantly longer and require more effort than the former. In spite
of this situation many of these inspections are bid on a lump sum basis? It is impossible to
foresee the amount of time and effort involved. A tight bid on a bad floor with extensive
corrosion is bound to put additional pressure on the contractor. It may induce pressure to
make a financial decision to limit the time spent on that particular project. What normally
happens in a case like this is that some of the corrosion detected goes un-reported as the
contractor tries to make the best of a bad job. As already discussed, if the decision is made to
select spots to mark on the floor on the basis of amplitude of signal alone, it stands to reason
that significant defects will be overlooked. This process may lead to missed defects and
leaking floors. This should not happen if the inspection is based on time and material with the
end user taking some responsibility for progression of the work. Invariably the best
inspections are carried out by this method.
Note to end users and tank owners:
On a daily basis hundreds of tank floors are being successfully inspected by competent well
trained operators using reliable equipment who have a full understanding of the above and are
not afraid to mention the limitations of the technique. There is nothing else out there that can
compete with the reliability and speed of Magnetic Flux Leakage provided that the technique
is properly applied and that all the limitations are fully understood.
Ask questions!
Make sure the operators are trained and certified on the particular unit they will be using to
inspect your tanks!
Make sure the inspection company has a certified procedure in place for inspecting tank floors!
Make sure the operator can provide a “function test” to prove the equipment works on the
thickness of floor and coating that will be inspected!
For more information regarding iFluids Engineering Services & Past Project Track Record
please visit here

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