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Probes for Remote Field Testing of Heat
Exchangers: Configurations and Capabilitiesby David Mackintosh* and Brian Beresford+
Principle of Operation
Remote field testing is a technique commonly used for NDT of small bore steel tubes such as those found in heat exchangers. The term remote field derives from the relatively large spacing of two to three tube diameters between the transmitting and receiving coils. The large spacing ensures that there is no direct coupling between the coils. Instead, the magnetic field travels from the transmitting (or exciter) coil outwards through the tube wall, axially along the outside of the tube and back through the tube wall to the receiving (detector) coil (Figure 1). Remote field testing is therefore known as a through transmission technique. When the probe moves into an area with wall thinning, the shielding effect of the tube wall is reduced and the field at the detector coil changes in two ways: its strength increases and its time of flight decreases. Remote field testing has approximately equal sensitivity to metal loss on the inside and outside of the tube wall. Metal loss increases the amplitude; it does not matter whether that increase occurs on the inside or outside wall. The overall strengths and limitations of remote field testing are described in detail in ASTM E 2096: Standard Practice for In Situ Examination of Ferromagnetic Heat-exchanger Tubes Using Remote Field Testing (ASTM International, 2000).Figure 1 - A basic remote field testing probe. The arrows show the flow of magnetic field from the exciter coil to the detector coil. The direction of flow is normal to the actual lines of magnetic field.
Detector and Exciter Response
For small bore tubes, the exciter coil is usually made larger than the detector (Figure 1) to produce a higher magnetic field and therefore higher sensitivity to pits. The detector coil size is based on a trade off between two values: the number of turns (the more turns, the greater the voltage output) and the dimensions (the slimmer the coil, the higher the resolution and sensitivity). At a small area of metal loss, the basic probe in Figure 1 actually produces two indications, one for each coil. An area of metal loss long enough to cover both coils doubles the response. The remote field testing inspector must be aware of these effects to obtain accurate results.
Remote field testing has approximately equal sensitivity to metal loss on the inside and outside of the tube wall.
Detector Configurations
The single bobbin coil is known as an absolute coil because its voltage output is directly proportional to the local value of the magnetic flux density (Figure 2). Absolute coils are excellent for sizing large volume discontinuities such as general wall loss and erosion. A differential detector is formed by wiring two adjacent absolute coils with opposing outputs. The differential detector subtracts out slow variations in magnetic flux density and produces a larger response to sharp changes in the field, such as those at pits. Differential coils can also be used to size large volume metal loss, although with less accuracy than the absolute coil.Figure 2 - Common detector configurations. The arrows indicate how the absolute and differential bobbin coils can be split into arrays of smaller coils for greater sensitivity
Both absolute and differential bobbin coils can be split into a set of smaller coils, making what is known as an array detector (Figure 2). Each coil in the array is sensitive to the immediately adjacent segment of the tube circumference. Array probes tend to be more sensitive to small discontinuities and they also produce data that intuitively indicates the circumferential extent of metal loss from the response of each coil. The use of array detectors is becoming more common and likely represents the next step up in accuracy and sensitivity for remote field testing. One of the great promises of array detectors is improved performance at tube support plates. The main disadvantage of arrays is that they produce more data for the analyst to interpret, a problem which can be offset by software with improved capabilities for displaying and manipulating multichannel data.
The configurations discussed in this section are summed up in Table 1. The five point scale is used for clarity, but is based on personal experience and is not meant to imply a quantitative comparison.
Basic Probe at Tube Support Plates
Discontinuities in heat exchangers often occur near tube support plates and baffle plates. (Baffle plates serve a dual function of supporting the tubes and directing the flow of fluid within the heat exchanger. The discussion of tube support plates in this paper applies also to baffle plates.) Near tube support plate areas, remote field testing has limited sensitivity and accuracy due to the interruption by the tube support plate of the field traveling on the outside of the tube. Figure 3 shows that on one side of the tube support plate, the basic remote field testing probe creates what could be called a shadow, which is a zone scanned by the exciter but not the detector. Mainly because of its size, the exciter coil is less sensitive to pits than the detector, so the shadow is an area of limited sensitivity to pits. One way to remove the shadow is to use a probe with more coils, as explained in the next section.Figure 3 - Near a tube support plate, the basic two coil remote field testing probe has a shadow (gray area) that is not examined by the detector coil: (a) probe approaches tube support plate; (b) detector has scanned to edge of tube support plate; (c) due to tube support plate, no field is reaching detector (gray zone will not be scanned by detector); (d) exciter detector coupling is resumed (gray zone will be scanned by exciter only).
Multiple Coil Probes
The need to detect and size disconti-nuities near tube support plates has led to the use of probes that have more than just one exciter and one detector (Figure 4). Adding an extra exciter to a probe (Figure 4b) improves performance near tube support plates by removing the shadow zone and gives excellent sensitivity to pits. The disadvantage of this double exciter probe is the complexity of its indications. For example, one small area of metal loss may produce three indications, one for each coil. The double exciter also creates interference between long metal loss indications that are sensed by the three coils simultaneously, which reduces the accuracy of sizing large volume metal loss or pits near or inside large volume metal loss. The double detector probe (Figure 4c) creates only two indications in each detector channel for a small area of metal loss (similar to the basic probe in Figure 1), with correspondingly less interference between different areas of metal loss. The disadvantage of the double detector configuration is that there is double the data to display and analyze, and the indications from the two detectors are not aligned in the strip charts.The configurations discussed in this section are summed up in Table 2. Again, the five point scale is used for clarity, but is based on personal experience and is not meant to imply a quantitative comparison.
Figure 4 - Probes: (a) with the basic remote field testing probe, a tube support plate interrupts the field traveling from exciter to detector; (b) the double exciter probe always has at least one exciter transmitter to the detector coil; (c) the double detector probe always has at least one detector receiving the exciter signal. The double exciter and double detector probes do not have a shadow next to the tube plate like the probe in Figure 3.
Conclusion
There is no single remote field testing probe or detector that can detect and size all types of metal loss in all regions of a heat exchanger with maximum effectiveness. The strengths and limitations of each probe configuration must be understood to improve data analysis and to choose the best probe for the application, tube discontinuities and priorities of the plant owner. Plant owners often choose the remote field test vendor who demonstrates the best sensitivity to pits, but there is usually a trade off between performance for pits and for large volume metal loss.
References
ASTM International, ASTM E 2096: Standard Practice for In Situ Examination of Ferromagnetic Heat-exchanger Tubes Using Remote Field Testing, West Conshohocken, Pennsylvania, ASTM International, 2000.
* Canspec Group, Inc., 7450 - 18 St. NW, Edmonton AB T6P 1N8, Canada; (780) 440-2131; fax (780) 490-2426; <dmackintosh@canspec.com>.
+ Canspec Group, Inc., 7450 - 18 St. NW, Edmonton AB T6P 1N8, Canada.
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