Magnetic Flux Leakage

The equipment utilized for the Flux Leakage Inspection of Tank Floors By Nova Data Testing is the   MFE 2412/1212 Scanning System or the Silverwing MLF 2000 Tank Floor Scanner.

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. The electronic processing and display module is common to both of the MFE Scanning Systems.

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. Since MFL is a quantitative inspection procedure, no calibration can be made to meet all configurations of defects possible. The functioning plate shall have the ability to determine that all sensors are functioning. The functioning plate must be able to have the ability to represent both through holes and soil side corrosion. Since the plate is used only for instrument functioning, the functioning plate used for a coated tank bottom may be a bare plate or a nonferrous material may be placed on top of the plate to simulate coating.

This equipment makes use of very powerful rare earth magnets. When disassembling the unit or moving the magnetic carriage great care should be exercised to prevent damage to either the equipment or injury to the operator. All tools should be kept well away from the magnetic bridge until required. Maintenance work should always be carried out on a wooden work bench.

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.

Areas covered with scale and dirt will cause vibration in he sensor array. The vibration will give signals at the lower end of the screen on the MFE 2412 which is commonly referred to as noise or grass. Rough surfaces will require changes in the scanning operation. 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 will be masked by the level of noise generated. Scanning speed can be lowered with increase in machine gain control. This method should only be used whenever the surface can not be cleared of the vibration causing condition. Where scale and debris are encountered, the scale and debris must be removed. Scraping of the surface will enhance the ability to do the magnetic flux leakage examination but will greatly increase the likelihood of surface pitting being hidden by filling them with debris. Uneven plates will cause signals that may or may not be true indications of plate thickness reductions. Whenever signals are created in wavy or uneven plates, the signal producing area must be immediately scanned in numerous directions to determine if the indication is real or false. If absolute results cannot be determined, the indication must be considered real.

Tank floors are never flat. The degree of curvature of the plates can 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 suffers. This can be overcome by scanning in more than one direction. Curvature of plates close to lap joints and/or the shell can make it difficult to scan these areas reliably.

The properties of the material itself can 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 service. 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 direction of rolling 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.

Magnetic Flux Inspections cannot achieve 100% coverage of the floor area. This is not possible 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. Corners of plates cannot be accessed by the scanner. Areas not accessible to the scanner such as the corners of plates must be checked by ultrasound.

Magnetic Flux Inspections are 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 repaired (by coating) after an earlier inspection and removal of the coating is not necessary, then it is helpful to the inspection company to have access to the earlier inspection data. In many cases product side corrosion cannot be detected by the floor scanner. The response from a product size pit with the same volume loss as an n 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 underside of the floor the leakage field will be above the surface and therefore in the direct path of the sensors.

Coated Floors will affect the sensitivity when the coatings are determined to be of a thicker layer than 6 mils. It may be 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. Therefore it is no longer possible to achieve the same levels of sensitivity that could be demonstrated on the uncoated plate of the same thickness.

The operator is responsible the correct use and validity of any inspection. Since there is no national standard for qualifications of the operator, the operator must have had training in both the use and maintenance of the MFL equipment. The operator must also have a good understanding of the theory and techniques of MFL inspections as well as knowledge of the limitations of the inspection. Since the inspection is used in conjunction with an ultrasonic follow-up, the operator should also be familiar with this technique and theory.

The operator will be familiar with all the maintenance and adjustments required when using any MFL system. The operator shall have a copy of the equipment manufacturer’s recommendation for calibration, set-up and inspection. The operator shall be required to follow all the manufacturers’ guideline for the inspection.

MFL scanning systems are ideally suited 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. This is because 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 the flux leakage signals significant errors can occur.

In order to carry out a good assessment of corrosion it is necessary to use a real time A scan display. Using the A scan display it is much easier to verify the location and detection of very small localized indications. A good quality 5 Megahertz twin crystal focused contact transducer with either a .375" or .500" diameter element is sufficient for the ultrasonic prove up. A suitable Couplant should be used to maintain good surface contact with the area to be inspected. The amplitude of the signal from the corroded back wall must be raised to at least the same screen height as the calibration reflector. The instrument must be properly calibrated and the ultrasonic prove up conducted according to a specific ultrasonic procedure. It is important to identify the nearest facet of the reflection and the true remaining wall thickness read from the time base.

The following information must be included as a minimum for every recordable indication: