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THIS DOCUMENT IS NOT A SPECIFIC PROCEDURE
IT IS INTENDED AS A GUIDE TO THE PREPARATION OF USER SPECIFIC PROCEDURES
Table of Contents
Section Title
1.0
Purpose
2.0
Scope
3.0
References
4.0
Personnel Qualification
5.0
Equipment
6.0
Floor Condition Considerations
7.0
Flux Leakage Limitations
8.0
Function Test
9.0
Electronic Gain Setting
10.0
Inspection Method
11.0
Ultrasonic Prove Up
12.0
Reporting
13.0
Safety Considerations
1.0 Purpose
The Purpose of this procedure is to establish a method for the Magnetic
Flux Leakage Inspection of Tank Floors to ensure the maximum achievable coverage and best
possible sensitivity to under floor corrosion.
2.0 Scope
This procedure shall be used when utilizing any of the MFE 2412/1212
series of scanning systems for the inspection of Tank Floors looking for under floor
corrosion.
3.0 References
3.1 MFE2412/1212 Operations
and Maintenance Manual
3.2 The truth about Magnetic Flux
Leakage as applied to Tank Floor Inspections. Paper presented by D.M. Amos, MFE
Enterprises Incorporated, at the International Chemical and Petroleum Industry Technology
IV A.S.N.T. Topical Conference, Houston, Texas, June 22nd 1995
4.0 Personnel Qualification
4.1 Personnel operating this
equipment shall have demonstrated to the employer a full understanding of this application
of Magnetic Flux Leakage and an ability to use and maintain the equipment in line with
this procedure and the relevant Operation and Maintenance manuals.
4.2 Personnel
conducting the Ultrasonic Prove Up of indications detected by the Flux Leakage equipment
shall be trained, qualified and certified in accordance with a nationally recognized
training and certification scheme. They shall be capable of demonstrating a full
understanding of corrosion analysis using ultrasonics as an accurate quantification tool
for remaining wall thickness.
5.0 Equipment
5.1 The equipment utilized for the
Flux Leakage Inspection of Tank Floors using this procedure shall be any of the MFE
2412/1212 Scanning systems currently available.
5.2 This equipment uses very
powerful rare earth magnets in order to near saturate the plate in the area of the
scanning head. Coil sensors placed between the poles of the magnetic bridge detect the
flux leakage fields and provide a low voltage output signal to the electronic module for
processing and real time display.
5.3 The electronic
processing and display module is common to both scanning systems.
5.4 The effective scan width
of the MFE 2412 scanner is 12 inches and the MFE 1212 scanner, 6 inches. The real time LED
display has twelve channels, one for each inch of scan width in the case of the MFE 2412
and for each half inch in the case of the MFE 1212.
5.5 Precise location of
indications is readily achievable using this equipment.
5.6 Proper use of this
equipment does not require the setting of calibration thresholds as the amplitude of the
signal is an unreliable indicator of remaining wall thickness.
6.0 Floor Condition Considerations
The achievable sensitivity of any Flux Leakage Inspection on any given
floor is directly dependent on the overall top surface condition. In ideal conditions it
is possible to find very small corrosion areas on the underside of the plate. As the top
surface deteriorates in regard to cleanliness, topside corrosion, and /or floor plate
curvature the achievable inspection will become relatively less sensitive and the results
more and more unreliable. Every effort must be made to ensure that the top surface is as
clean as possible and free from any product residue, rust or loose scale. The effect of
any undulations or buckling of the floor plates must be taken into account in the
assessment of indications and in the scanning method.
7.0 Flux Leakage Limitations
7.1 Flux Leakage is a Qualitative
not Quantitative inspection tool. It is very good at detecting magnetic anomalies and
covering the very large floor areas of tanks very quickly. It is not, however capable of
the accurate quantification of the signals as regards the remaining wall thickness. The
amplitude of the signals is a function of the volume loss of material rather than a
measure of the thickness loss. This is the main reason that MFE Enterprises Incorporated
does not support either the single level threshold or quantified mapping approach to this
particular application. Accept/ reject criteria based on the above two methods can lead to
significant defects going undetected when only the amplitude of the signal is taken into
consideration.
7.2 There are reasons for
the generation of Flux Leakage Signals other than from plate corrosion. Localized changes
in material properties, weld scars and arc strikes can also cause significant indications.
This must be born in mind during the inspection process and the cause of signals properly
evaluated.
8.0 Function Test
As previously stated the achievable sensitivity is very much dependent on
the material under test and the top surface condition. Prior to starting an inspection it
is necessary to check that the equipment is functioning as intended and is set up to
obtain the best possible sensitivity. The physical set up of the equipment as regards
magnet and sensor heights dictates the level of sensitivity. For any given thickness of
plate and for any thickness of coating it is recommended that a system function test plate
as described in the Operation and Maintenance Manual be constructed to prove the
capability of the system.
9.0 Electronic Gain Setting
As previously stated the mechanical arrangement of magnet and sensors
dictates the achievable sensitivity. If there are no leakage signals due to a poor
mechanical set up then it will not matter how much electronic gain is incorporated there
will be nothing to see. Fine tuning of the electronic gain and mechanical set up are
necessary in order to achieve the best possible signal to noise ratio and defect
detection. These parameters should always be arrived at using the function test plate
prior to inspections.
10.0 Inspection method
10.1 Each plate within the tank
must be scanned in a logical manner to ensure the maximum coverage possible. The use of
the smaller scanning head may be required adjacent to welds, obstructions and the floor to
shell areas which are difficult to get to using the large scanning head.
10.2 Scanning across the weld
areas is not possible owing to scanner clearance limitations. The entire floor is scanned
on a plate by plate basis, the scanning head being moved clear of the weld area whilst
transitioning from plate to plate.
10.3 Full coverage of the
floor area using these systems is virtually impossible due to the many and varied access
limitations. If full coverage is required it is possible to manually scan the omitted
areas using ultrasonics.
10.4 The systems are optimized for
a medium walking pace. Once properly set up variations in speed will effect the amplitude
of the signal but not the detection capability of the machine. Faster speeds will result
in much higher amplitude signals. This is also true of the background noise levels. An
optimum speed giving the best signal to noise ratio is preferred. In a very high noise
environment it is sometimes necessary and acceptable to reduce scanning speeds and
increase the electronic gain to optimize the signal to noise ratio. This optimum set up
may vary from floor to floor and even from plate to plate.
10.5 The object of the exercise is
to:-
10.5.1 Identify any
indication that clearly exceeds the
background noise levels.
10.5.2 Determine the type of
indication i.e. isolated pitting or lake type.
10.5.3 Confirm the
indication by scanning from different directions.
10.5.4 Locate and mark the
exact position of pitting corrosion and outline any area of lake type corrosion for
further investigation to determine the worst case occurrences within the area marked.
10.6 Accurate location of
individual leakage signals is achieved by rocking the system backwards and forwards over
the defect until the operator is satisfied that he is stopped over the top of the
indication. By noting the highest affected channel number and marking both the center of
the array and the corresponding rule mark on the front of the scanning head good location
can be achieved.
10.7 In the evaluation of
indications it is necessary to determine the reason for the response. In the absence of of
an obvious topside source or geometric anomaly it must be assumed that the indication is
from an underside source.
10.8 Magnetic Flux Leakage
cannot differentiate between top side and under side corrosion.With these systems the
response to topside indications is often less than those obtained from an equivalent
bottomside indication. This must be born in mind when topside corrosion is present. When
high amplitude signals are obtained from the same area it will be necessary to investigate
further and to determine whether bottomside indications also exist at that location.
10.9 All indications determined to
be significant shall be indelibly marked for subsequent ultrasonic prove up.
11.0 Ultrasonic Prove Up
The MFE 2412 and 1212 scanning systems are ideally set up for the
detection of isolated pitting corrosion. Isolated pitting with a small base area and large
through wall dimension are often difficult to find and measure Ultrasonically. The reason
for this is that sufficient sound energy must be returned from the area of the greatest
through wall penetration point to give a readable response. If the right equipment and
technique is not applied in the ultrasonic prove up of flux leakage signals significant
errors can occur.
11.1 Digital read out thickness
meters are not recommended.
11.2 Digital displays with very
slow screen rewrite frequency are difficult to use as they preclude any real time scanning
capability.
11.3 In order to carry out a good
assessment of corrosion it is necessary to use a real time A scan display. Analog displays
are far superior for this application than digital. Analog sets, however, are becoming
extremely hard to get hold of as manufacturers turn to the cheaper and more reliable
digital technology. Data processing and display speeds are the important consideration
when using digital ultrasonic equipment.
11.4 A good quality twin crystal
compression wave contact transducer in the order of 0.375" in diameter, with a
frequency of approximately 5 Mhz. will generally suffice for the ultrasonic prove up. Some
focusing of the sound beam is an advantage for this application.
11.5 It may be necessary to change
to a different transducer when examining through some coatings or on particularly rough
surfaces.
11.6 Ultrasonic coupling can be
maintained using any of the proprietary brands of couplant. Water works surprisingly well
for this application.
11.7 The instrument must be
properly calibrated and the ultrasonic prove up carried out in accordance with a specific
procedure.
11.8 The location and minimum
remaining wall thickness must be indelibly marked on the floor plate.
12. Reporting
The final report shall include sufficient information, organized in such a
manner so that the client can quickly relocate and identify indications found during the
inspection for repair or replacement consideration. Marking the floor, alone, is not
sufficient. It will be necessary to produce a log of all recordable indications along with
a sketch showing the floor plate layout.
12.1 The following information must be
included as a minimum for every recordable indication:-
1. Plate number
2. Indication number
3. Indication type
4. Datum Point
5. X and Y direction and distance from datum.
6. Minimum Ultrasonic reading of indication.
7. Adjacent nominal thickness
8. Percentage wall loss
12.2 Approximate location of
indications can be indicated on a floor plate layout sketch depending on the number of
indications identified. If necessary individual plate sketches can be added for reporting
clarity purposes.
12.3 Should the client require an
assessment of the topside condition a similar log will be necessary except with the
severity determined with the aid of a pit gauge .
13. Safety Considerations
13.1 The MFE range of equipment
employs extremely powerful rare earth magnets. When handling or lifting the equipment it is
vital that precautions are taken to prevent either injury to persons or damage to the
equipment.
13.2 When transporting the systems
over long distances it is recommended that the custom transportation box provided is used.
This includes a keep plate to reduce the effect of the powerful magnetic field produced by
the magnetic bridge.
13.3 Close proximity of the
magnetic bridge to magnetic storage media, heart pacemakers, credit cards can have a
detrimental affect and must be avoided at all times.
13.4 It is recommended that all
maintenance, cleaning and adjustments are carried out on a wooden bench or platform to
minimise the risk of personel or equipment damage. All tools should be kept well away from
the magnetic bridge until needed.
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