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.
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 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.
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.
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.
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 marking. After Ultrasonic prove up, the indications can be marked more permanently with a wax crayon or a paint marker.
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.
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.
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.
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.
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 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.
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