·Table of Contents
·Methods and Instrumentation
New Applications of Electromagnetic Techniques in NDT InspectionsJavier Guerra
Avda. Montes de Oca, 1
28709 San Sebastián de los Reyes (Madrid)
There are currently two ways to increase the reliability and field of application of inspections: one consists of using various traditional techniques in a complementary manner, and the other of using new techniques.
At this respect, the appearance of new inspection technologies based on electromagnetic principles will allow the inspections currently performed to be complemented and improved. These electromagnetic techniques involve two aspects of interest: on the one hand, the movement of the sensor across the area of interest is simplified since it is not necessary to use any coupling medium between the two, and on the other higher scanning speeds are achieved.
This paper will deal with the incorporation of complementary techniques in NDT inspections and with new electromagnetic techniques that might be used for new applications in industrial plants
Javier Guerra is engineer by the Politechnic University of Madrid (Spain). He has been working in NDE related activities since 86, mainly in Steam Generator ET tube inspections and other ET applications as Control Rods, RPV Head penetrations, Fuel, Condensers..... He is working at Tecnatom as responsible of the Eddy Current Division.
Jesus Vazquez is engineer by the Politechnic University of Madrid (Spain). He has been working in NDE related activities since 83, mainly in Steam Generator ET tube inspections and other ET applications as Control Rods, RPV Head penetrations, Fuel, Condensers..... He is working at Tecnatom as responsible of the Eddy Current development. group
In the past, the inspection codes and standards required tracking of the status of certain discrete components that were considered to be the most critical from the point of view of safety and reliability. The inspection of these components was performed using specific techniques that had no associated permanent records of basic information, owing to which the results were not questioned.
At present, higher levels of reliability are required, this requiring validation of the techniques and the use of other complementary approaches. Consequently, there is a need for the results to be repeatable and permanent, this requiring the introduction of automatic inspection systems based on mechanical positioning equipment and data acquisition and processing systems.
The future evolution of the requirements will affect not only the techniques used but also the components to be inspected. A demonstration of this is the RBI (Risk Based Inspection) concept, which considers that both the components affecting the safety of the plant and those having an impact on plant availability should be inspected. The techniques may require lower degrees of absolute accuracy overall, but will also require greater efficiency as regards determination of the degree to which the components have deteriorated.
At this respect, the emergence of new inspection technologies based on electromagnetic principles will make it possible to complement and enhance the inspections currently performed. These electromagnetic techniques involve two aspects of interest: on the one hand, the movement of the sensor across the area of interest is simplified since it is not necessary to use any coupling medium between the two, and on the other higher scanning speeds are achieved.
There are certain components which, due to their geometric configuration and type of material, make inspections based on current techniques complex, limited or particularly time consuming. Consequently, there is a need to improve the conventional techniques and to use new approaches complementing the traditional ones or facilitating the inspection of other components.
A good example of the above is the inspection of reactor vessels of nuclear power plants. The scope of such inspections is included in Western codes, which require the base material to be inspected without directly contemplating the cladding. Reactor vessel inspections have traditionally been performed using ultrasonic techniques, but there is an increasing requirement for inspection of the cladding, which is difficult with ultrasonics but simpler using electromagnetic techniques.
Another example is erosion-corrosion in piping, where the new electromagnetic techniques make continuous measurement of the pipe wall thickness possible without the need to remove heat insulating lagging.
Electromagnetic techniques also allow automatic inspections to be carried out on ferromagnetic piping with thicknesses of between 3 and 12 mm, which by their design are difficult to access, and also make it possible to inspect the inner surface of rotors, which are traditionally inspected by magnetic particles, etc.
One of the electromagnetic techniques most widely used in the industry is that based on Eddy Currents. This technique, which is habitually used for the inspection of small diameter tubes (heat exchangers, condensers, etc.), is being incorporated as a complementary technique to ultrasonic inspections, and in some cases is replacing other techniques that are slower or provide less information.
The eddy current technique is based on coils that generate a magnetic field that is introduced into the material being inspected. If the material is free from defects, this magnetic field remains fixed; if, however, any anomaly is detected in the material, the magnetic field changes and gives rise to a variation in the original signal, and this may be measured. This technique is useful for conducting, generally non-ferromagnetic materials, and is highly sensitive to surface defects, which may be located and dimensioned with great accuracy.
Examples of the above are the inspections of vessel closure head and bottom penetrations in PWR and BWR nuclear plants, shroud inspections, reactor vessel inspections, etc. Other applications that are increasingly requested are the eddy current inspection of bolts, threaded zones, turbine rotor bores, alternator retaining rings, etc.
As indicated above, eddy current-based techniques may be applied to many components, as well as to small diameter tubes, providing accurate information on the position and size of surface defects.
New electromagnetic techniques
In general, these techniques are based on physical principles that have been known for some time but that to date have not had any particularly practical application in NDT.
Although the aim is not to provide an exhaustive description of all these techniques, some of the most significant will be presented because of their potential for application in the industry.
Remote Field (RFEC)
This electromagnetic method is based on the use of a sensor with two coils separated by a constant distance. One of these (the emitting coil) emits an alternating signal that is divided into two parts: one direct and the other that passes through the wall of the component being inspected. The other coil (receiver) receives the signal issued by the remote field, but not the one taking the direct route, since this will have been attenuated by the distance between the two coils. On passing through the wall of the component being inspected, the remote field will be altered if there is any defect present.
In order to obtain good discrimination of the defects, techniques incorporating one emitting coil and an array (several units) of receiving coils are generally used.
These techniques are generally used for the inspection of ferromagnetic materials over a wide range of diameters. Typical components are heat exchangers with ferromagnetic tubes and piping in general.
Magnetic Flux Leakage (MFL)
These techniques are based on the use of continuous magnetic fields that are introduced by means of magnets or electromagnets into the material to be inspected. A sensor, normally located inside this field and close to the surface to be scanned, detects variations in the field when it passes through areas of the material containing defects.
The sensors normally used as detectors are coils, Hall sensors or magnetorresistive sensors, among others. Each has its particular field of application, and the latter type is highly sensitive and is allowing the spectrum of application to be considerably widened.
These techniques have traditionally been used for inspection of the ferromagnetic piping used in gas lines, and have recently been applied also to the inspection of all types of ferromagnetic tubes, for measurement of wall thickness and, with certain limitations, for the detection of defects.
The recent appearance of highly sensitive sensors has made it possible to use this method for a large number of applications, such as the detection of defects in ferromagnetic materials with a poor surface finish, or located beneath insulating material or other types of conducting materials (as is the case of vessel walls with cladding). Furthermore, because of its power to penetrate materials (up to some 30 mm) and sensitivity, the technique may be used with ferromagnetic materials or materials of no great thickness for the detection of defects.
Pulsed Eddy Currents
Pulsed Eddy Current techniques are based on the use of coils excited by short-lived, high amplitude current pulses that generate a large number of low frequencies, this allowing the eddy currents to be generated at great distances from the sensor.
This technique makes it possible to achieve great depths of penetration into the material and also allows the thickness of piping to be measured without the need to remove heat lagging, as long as this measures no more than 75 mm in thickness.
Advantages of electromagnetic techniques
Techniques based on the electromagnetic effect have certain advantages over others, such as ultrasonic or magnetic particle techniques, making them more suitable for many applications.
Among these characteristics is the fact that no coupling medium is required and that they do not even need to be in contact with the surface scanned, along with the possibility of using high scan speeds.
The techniques allow defects to be located and dimensioned with high levels of repeatability, and make it possible to produce records that are long-lasting and open to analysis over time.
The use of computerized systems for data acquisition and analysis makes interpretation of the results relatively simple.
The field of application of traditional NDT techniques in industrial inspections presents a number of limitations that are being overcome as complementary techniques are incorporated or as the new techniques being developed are adapted.
Facilitating and increasing the reliability and accuracy of inspection results, and widening the field of application of NDT techniques, is an on-going objective that is being achieved as a result of the contribution made by new electromagnetic techniques.
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