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Magnetic Flux and SLOFEC Inspection of Thick Walled ComponentsA. Bönisch
KontrollTechnik, Schwarmstedt, Germany
F.H. Dijkstra , J.A. de Raad
Röntgen Technische Dienst bv, Rotterdam, The Netherlands
Magnetic Flux Leakage (MFL) inspection of low-alloy carbon steel components is attractive while, contrary to
ultrasonic inspection, no acoustic coupling is needed between the sensor system and the object. Furthermore MFL is a fast and reliable method to detect local corrosion. The well-known and widely used traditional MFL method however is, despite efforts to improve, limited to a thickness of up to no more than 15 mm.
This paper describes an improved highly sensitive MFL method with an upper thickness limit of at least 30 mm. The extended thickness capability of the new MFL tool makes the method suitable for a much wider range of applications, not only for inspection of thick components but also for thinner walls covered with thick non-metallic protection layers such as glass fibre reinforced epoxy coatings on floors of (oil)storage tanks. Moreover this improved MFL method is able to differentiate surface from back wall defects, which is a unique and very useful feature.
The new MFL method, known as "SLOFEC" in the meantime has successfully been applied in the field on a variety of components. Background and applications of this new intriguing MFL tool for the NDT industry are described in this paper.
As a consequence one can observe that some constructions are inspected over their full surface with NDT screening tools, , because all places are considered of equal risk, e.g. the floor of an (oil) storage tank. On other components, e.g. on a pressure vessel, NDT can be limited to certain critical areas. The increasing
knowledge of risk, failure and fracture mechanics has influenced the need to improve or adapt the capabilities of some NDT methods.
The MFL method to inspect steel components fits very well in a full surface coverage and low cost inspection approach. As such it has been the prevailing method to inspect long distance pipelines for decades. Over the past decade, despite its limited quantitative capabilities, MFL became the most common method to inspect tank floors . Unfortunately this "traditional" MFL method is limited to a thickness of 10 or at best 15 mm under favourable field conditions. The demand for MFL tools with a larger thickness range is known for decades, but considerable efforts to increase the range of traditional MFL tools so far were hardly successful. Only marginal improvements could be achieved at a high cost and weight penalty.
The need for a fast and simple NDT technique which does not require acoustic coupling liquid as required in traditional ultrasonic inspection , was a major incentive to develop tools based on the MFL principle. Moreover an MFL system is rather tolerant to surface condition, removal of loose and excessive debris prior to inspection is sufficient. Because of this and other merits MFL has become the premier method to inspect long distance pipelines from the inside. This is done on stream with so called "intelligent pigs". The full pipe surface is inspected to reveal local metal loss either inside or outside. Over a number of decades these tools have been
optimised and reached capabilities near perfection .
In the eighties the method was selected to inspect floors of (oil)storage tanks. Such tools now are a commodity. Figure 1 shows a system in use to inspect a tank floor. In more recent years some derivative MFL tools to inspect pipes from the outside were introduced.
|Fig 1: MFL inspection of a tank floor||Fig 2: Adjustable MFL pipe scanner|
For inspection of bare or painted pipe from the outside, instead of single diameter scanners sometimes adjustable yoke and scanner constructions are used to make them suitable for a range of diameters. With such adjustable scanners, of which one is shown in Figure 2, inspection cost per meter of pipe can be reduced, a factor of paramount importance in the maintenance inspection world.
The MFL method is very suitable to detect local corrosion and is qualitative rather than quantitative. Gradual thinning can not be detected. Once one needs quantitative data complementary methods e.g. ultrasonic inspection has to be applied.
Another considerable drawback of the MFL technique is that rather heavy and bulky scanners are needed, adapted to the geometry of the component. This limits the application to large constructions of uniform geometry such as storage tanks, pipe lines and long lengths of plant piping.
Despite some of the described limitations MFL has obtained a good reputation in industry also due to its high reliability of defect detection.
The MFL method can only be applied on low alloy carbon steels which have a high magnetic permeability. The well known principle is illustrated in Figure 3.
|Fig 3: Principle of MFL to detect metal loss|
A magnet within a yoke construction is used to establish a uniform magnetic flux in the material to be inspected.
The magnetisation should be up to a high level close to magnetic saturation. Usually strong permanent magnets
are used to generate the magnetic field, but sometimes electromagnets are used if sufficient power is available,
even combinations of both to achieve superimposed magnetisation. In a defect free plate the magnetic flux is
uniform. In contrast a metal loss type defect, such as local corrosion or erosion, not only distorts the uniformity
of the flux but a small portion of the magnetic flux is forced to "leak" out of the plate. Sensors placed between
the poles of the magnet or yoke construction can detect this small local "leakage".
The amount of distortion and leakage is dependent on depth, orientation, type and position (topside/back wall) of the defect. Defects are often of erratic form. Various combinations of volume loss can result in the same flux leakage level although not having the same depth. This causes that the method is and remains rather qualitative and not quantitative, despite efforts to apply signal analysis and adaptive learning software programs to improve depth sizing. Very often this mainly qualitative character is acceptable for industry in return for its high speed , full surface coverage and in particular its high probability of defect detection.
Most of the MFL inspection tools make use of "passive" Hall effect sensors to detect flux leakage as indication of metal loss. The systems using Hall elements we call "traditional" MFL tools.
Due to physical limits of the size of magnets and total weight of the necessary scanner there is an optimum in performance of traditional MFL tools. As a consequence thickness range is limited to 10 or at best 15 mm under favourable circumstances.
Sensitivity drops dramatically with increasing thickness. Thus the challenge to design a tool for a much greater wall thickness remained.
Trying to increase the range of MFL tools, another sensor type in combination with a few other essential equipment modifications ultimately solved the problem. Instead of the "passive" Hall sensors, as illustrated in figure 3, "active" eddy current sensors are used to detect flux leakage, even better, these sensors can detect changes in flux density inside the plate. The sensing is virtually "in the plate" and this explains its higher sensitivity for variations of the magnetic flux than a passive sensor "at the plate surface".
The principle has been known and systems existed already for a considerable time . It is applied for steel
(boiler) tube inspection not exceeding say 5 mm wall thickness, thus not for extreme thicknesses up to 30 mm
being the subject of this paper.
Eddy currents in steel have a small penetration depth due to the high relative magnetic permeability, say 500 or more. This limits penetration of the eddy currents to the outer surface. This so called "skin effect" is strongly reduced by magnetic saturation of the wall, causing a low relative permeability, say close to 1. This allows the eddy currents to penetrate much deeper, up to the full wall thickness.
Magnetic saturation not only creates a low permeability and uniform flux, it also suppresses the usual local permeability variations in the material. This eliminates an enormous source of noise, which can hardly be filtered out, and otherwise would prohibit proper functioning of flux sensing systems.
Figure 4 shows the relative sensitivity curves for traditional and improved MFL.
|Fig 4: Relative sensitivity comparison Between MFL and SLOFEC|
Using phase information the type of defect can automatically be sorted out including a reasonable level of defect
severity. In addition, although there is still room for improvement, the new system provides some information on
general wall thickness reduction.
Despite all these merits it can not replace ultrasonic inspection in terms of absolute accuracy.
Experiments in the laboratory and field trials proved that with the improved MFL system a thickness of up to 30 mm and probably more can be inspected with a much higher overall sensitivity than with traditional MFL, this applies certainly for thickness range beyond 5 mm.
The technical thickness limit of this new system is determined by the combination of sufficient magnetic saturation ( bias field) of the full wall thickness of the component and a low enough eddy current frequency to penetrate the full wall without sacrificing on inspection speed. Most probably the weight of the magnetic yoke and scanner dictate the real physical upper limit. The limits have not fully been explored yet. The now existing system, suitable for approximately 30 mm wall thickness, seems to be an optimum.
The "new" MFL technology is called "SLOFEC". The acronym SLOFEC stands for Saturation LOw Frequency Eddy Current.
At first SLOFEC systems were built for customer specific "one-off "solutions. In fact a specific inspection
problem which could not economically be solved with regular NDT provided the challenge. The customer
needed a system to inspect thick large diameter buried bullet tanks from the inside to detect corrosion under the
external tar coating. SLOFEC offered an affordable solution to inspect the tanks which otherwise had to be
lifted, cleaned and inspected ; a not attractive expensive procedure.
Figure 5 shows the partly buried tanks with a diameter of 5 metres. Figure 6 shows the typical "one-off" scanner, with adjustable diameter, built for this job.
|Fig 5: Inside inspection of buried tanks with adjustable SLOFEC scanner||Fig 6: Adjustable SLOFEC scanner for large cylindrical components|
Over the past 5 years several one-off scanners have been built and applications successfully been completed. Gradually the capabilities and market potential of SLOFEC were better understood. This resulted in the development of more versatile systems, intended for thick flat or slightly concave or convex surfaces, suitable for a multiple of applications for which market demand had been established.
|Fig 7: Prototype SLOFEC scanner on trial plates|
Traditional MFL inspection tools are able to cope with non-metallic coatings of up to 2 mm on wall thicknesses of approximately 7 mm. Once thicker coatings are applied SLOFEC can be used. Experiments showed that the new system still works very well with a coating thickness of up to10 mm.
A prototype of a SLOFEC tool to inspect flat plates, as present in storage tanks, is shown in figure 7 during trials in the RTD laboratories.
Because of the many merits of SLOFEC an increasing number of other applications emerged. This is a learning process with cross-fertilisation of ideas.
Some inspection demands are hard to meet. Although the technique can be suitable for the inspection job the need for special scanners sometimes requires long lead times and the cost of one-off scanners prohibits an economic solution.
Weight and size can also be a problem. For tank floor inspection this is solved by a modular construction with a reasonable weight of each part.
For inspection of towers one can create special provisions for access and handling, the selection of the method then depends on a cost-benefit balance.
It is obvious from the above that the development of SLOFEC was and still is "market driven".
These defects were local, of different size and geometry, partly and through full thickness, individual and superimposed on one or two sides. In addition different types of thinning were machined. Figure 8 shows the SLOFEC prototype system on the test plates during these qualification tests. These extensive tests provided the information to quantify and qualify SLOFEC performance. This same procedure has been repeated with the professional scanner as shown in figure 9.
|Fig 8: Prototype SLOFEC scanner during qualification tests in Japan||Fig 9: Professional SLOFEC scanner during qualification tests in Japan|
Most customers require an additional validation and preferably a (long) track record for "something new". NDT service providers understand that and appreciate that some clients in return are prepared to be the "launching customer". Usually validation is done on a real object. Sometimes blind tests on test coupons with real corrosion are included in such validation trials.
|Fig 10: SLOFEC scanner during validation in a Japanese oil storage tank|
It should also be mentioned that results these days should be presented in a way which eases interpretation, e.g. mapping of results of tank floor inspection. Preferably, according to a new trend, formatted in a way that it fits in a customer specific data storage facility. If possible, the results of recurrent inspections should be so reproducible that progress of corrosion can be automatically) established by dedicated software. Altogether product development and market introduction is not only time consuming but very costly despite the essential co-operation of customers.
In addition to the applications described above, some other applications are described in this chapter to illustrate the capabilities of SLOFEC:
The SLOFEC system is often used on pipes. Individual scanners are required to match specific pipe diameters. Figure 11 shows such a single diameter scanner. These one-off scanners require long lead times for scanner production or preparation, often making the inspection too costly. To better cope with varying diameters a modular and adjustable scanner as shown in figure 12 was built. Its first successful application was on an offshore platform, to detect random local corrosion in crude oil lines with a diameter of 200 mm and a wall thickness of 10 mm. A wrapped, non-uniform coating was present, with a nominal thickness of 5 mm.
|Fig 11: Single diameter SLOFEC scanner for pipes||Fig 12: Multi diameter SLOFEC pipe scanner||Fig 13: SLOFEC scanner to inspect boiler tube fire walls|
SLOFEC can also be applied on components with rough surfaces, e.g. on boiler tube walls, for which a special scanner as shown in figure 13 was built. The system has also been used on zinc or aluminium coated components such as flare lines.
In general, SLOFEC is applied as a highly sensitive and fast screening method to detect randomly located, single-spot or cluster corrosion. Local corrosion often has a semi-spherical shape and is very hard to detect with any other NDT method. Radiography can be an alternative, but is often not selected because of its high cost and radiation hazards particularly in case of offshore applications. Moreover, results are not instantly available. Although SLOFEC is often able to detect metal loss exceeding approximately 25 % of the wall thickness, even in thick and/or coated parts, the method is not sufficiently quantitative to establish remaining life time or integrity of a component. In almost all cases ultrasonic gauging is used complementary to measure remaining wall thickness.
Because ultrasonic measurements cannot cope with thick coatings these have sometimes to be removed, but now only locally at the locations previously selected by SLOFEC.
SLOFEC is a fast screening method to detect random local metal loss on coated and non-coated components, the method is suitable to thicknesses of at least 30 mm.
SLOFEC has a higher defect detection sensitivity than MFL, moreover it is able to differentiate near surface from far surface defects.
SLOFEC has been developed mainly based on market demand. It can be considered as a new NDT tool after its qualification and validation.
Although not fully matured yet the method enables components that are currently not inspectable to be inspected, especially where relatively thick or coated components are concerned.
There are an increasing number of successful applications building a growing track record. SLOFEC has the potential to become the future method for next generation tank floor inspection systems.
Appropriate mapping and presentation of results will support its further market penetration.
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