5.2.1 Tube testing using internal probes
Eddy current testing of tube using internal probes is one of the major applications of eddy current testing, and is used for testing tubular heat exchangers, for instance in condensers in power generation plants, and large air conditioning units. The testing is carried out to detect corrosion and cracking at the inside and outside surfaces, and estimation of the depth of any flaws is an important aspect of the testing so that action can be taken before tubes start leaking.
Tube testing probes and their sensitivity
The standard absolute internal probe for tube testing, commonly called a bobbin probe, consists of a short cylinder with a coil wound in a groove around its circumference (see FIG.
5.29a). Differential bobbin probes have two coils wound side by side (FIG. 5.29b). The tubes normally tested by this method generally have an outside diameter in the range approximately 12 mm to 30 mm, with wall thicknesses in the range approximately 0.7 mm to 3 mm. The probe diameter is matched to the internal diameter of the tube being tested, with a clearance of between approximately 0.8 mm to 1.5 mm on diameter, the larger clearance being for large diameter or damaged tubes.
FIG. 5.29. Sections through a standard absolute bobbin probe (a) and a standard differential bobbin probe (b). Centring discs are not always present.
The coil(s) in bobbin probes produce magnetic fields similar to those produced by surface testing probes, but because the coil is concentric with material of the tube, the eddy currents flow circumferentially around the tube wall. Since flaws can be detected only if they distort the flow of eddy currents, this means that a lamination or other separation of the material parallel to the surface of the tube cannot be detected. Fortunately, this is not a common flaw in tubes, and is usually not very harmful to the service of the tube. However, it also means that narrow circumferential or transverse flaws cannot be detected, as shown in FIG. 5.30.
Such flaws include fatigue cracks and stress corrosion cracks. Stress corrosion cracks are often branched rather than a single material separation, and, if so, may be detected. Fatigue cracks, however, are not branched, so the likelihood of detection is very low. Flaws which progress along the tube, like seams, and flaws which are relatively wide, like fretting grooves and corrosion pits, can be readily detected if they are of a significant size.
FIG. 5.30. Sketch of a tube being tested with a bobbin probe showing that a transverse crack does not distort the eddy currents, and so is not detected.
Other probe designs are available which are capable of detecting narrow transverse cracks as well as other flaws. These include zigzag probes, and differential probes with the two coils mounted with their axis normal to the tube axis and at right angles to each other (FIG. 5.31).
One of these types of probe should be used when fatigue or stress corrosion cracking is a known problem or is suspected.
FIG. 5.31. (a) multi-coil pancake style probe and (b) zigzag style probe.
To obtain adequate sensitivity to flaws at the outside surface of the tube, the depth of penetration of the eddy currents must be adequate to obtain a relatively strong eddy current intensity at the outer surface. The depth of penetration can be increased by reducing the test frequency but, just as with surface testing, the coil dimensions have a major effect on the size of the coil's magnetic field and consequently on the depth of penetration of the eddy currents.
For surface testing, the coil diameter can be increased to increase the penetration, but with tube testing, the coil diameter is controlled by the tube inner diameter, and it is the depth and particularly the axial length of the coil windings which are increased to increase the penetration. However, increasing either the depth or length of the coil reduces sensitivity to small discontinuities. For the best compromise between penetration and sensitivity, the coil
length and thickness (depth of the windings) should be approximately equal to the tube wall thickness. Note that, since probes with wide coils are designed for penetration, they are also designed to operate at low frequencies.Absolute probes have only one coil sensing the test material. The balance load may be within the probe, or elsewhere in the electric circuit.
Differential probes have two coils sensing the test material, with the coils connected on adjacent arms of the bridge circuit. Because of this, differential probes give a signal only when the two coils sense different conditions. Therefore, identical conditions and gradually varying conditions, like gradual wall thinning, cannot be detected by differential probes. Long flaws, such as a seam in a tube, will give a signal only at their beginning and end. In addition, because they give more complex signals (discussed later), interpretation of signals can be much more difficult. Despite these disadvantages, they are more commonly used than absolute probes. This is largely because probe wobble noise is much less, and temperature drift and wandering of the spot because of changes in the conductivity of the tube are almost absent.
As for surface testing, the test sensitivity also depends on the degree of magnetic coupling.
When testing with internal coils, the magnetic coupling is measured as the fill factor, given the symbol η (the Greek letter ‘eta’), and defined as the ratio of the average area of the coil to the inside area of the tube.
The fill factor can also be expressed as a%. For maximum sensitivity, the fill factor should be as high as possible compatible with easy movement of the probe in the tube. Note that the fill factor can never exceed 1 (100%).
Selection of test frequency
The impedance diagram for testing tubes with internal probes is of the same general shape as the impedance diagram for surface testing of materials. Since detection of flaws at both the inner and outer surfaces is required, the test frequency is chosen so that the tube wall thickness is less than the effective depth of penetration. That is, the operating point is located on a thickness curve rather than on the conductivity curve. The location where each of the thickness curves meets the conductivity curve occurs when the tube wall thickness equals the effective depth of penetration. The location of this point on the conductivity curve depends on the test frequency, the conductivity of the curve and the tube diameter - as any of these increases, this point moves further down the conductivity curve. Note that this is similar to surface testing except that, for tubes, the tube diameter is the relevant parameter, not the coil diameter as for surface testing. The reason for this is that it is actually the diameter of the eddy currents that is the significant parameter. For surface testing with standard probes, the strongest eddy currents flow immediately beneath the coil, so the diameter of the eddy currents is essentially the same as the coil diameter. For tubes, the eddy currents flow in the tube and so their diameter is essentially that of the tube.
The basis for the selection of test frequency is the same as that for the selection of frequency for surface testing for subsurface flaws or thickness measurement. The frequency should be low enough so that the eddy current intensity at the outside surface is relatively high, but low enough to give a relatively high degree of phase separation between signals from different thicknesses. Therefore, the normal operating frequency should be the frequency which gives a f90 separation between the signals from a shallow inside surface flaw and a shallow outside surface flaw. However, because of the difference in configuration between surface testing and tube testing, this frequency is that at which the tube wall thickness equals approximately 1.1
f90 =530/(t2σ) (5.4) where
f90 = the operating frequency (kHz), t = the tube wall thickness (mm) , and
σ = the conductivity of the test material (% IACS)
An impedance diagram showing the signals from a shallow inside surface flaw and a shallow outside surface flaw is shown in FIG. 5.32. It can be seen that, as the frequency increases, the phase separation between the inside surface flaw and the outside surface flaw increases, but the amplitude of the outside surface flaw decreases relative to the inside surface flaw.
FIG. 5.32. Impedance diagram showing the signals from a shallow inside surface flaw and a shallow outside surface flaw at three different frequencies. The increase in the phase separation and the decrease in the amplitude of the outside surface flaw relative to that of the inside surface flaw can be seen.
Flaw signals from absolute probes
At f90, the signals obtained using an absolute probe from a tube with a shallow inside surface flaw, a shallow outside surface flaw, and a through hole appear as shown in FIG. 5.33 when the display is rotated so that the orientation is similar to that obtained from the same conditions during surface testing. Note that a signal from decreasing fill factor is not normally obtainable. Instead, for tube testing, the phase control is adjusted so that the signal from a shallow inside surface flaw is approximately horizontal. Note that the illustration of absolute probe signals in the ASME code is rotated 1800 and is therefore upside down with respect to FIG. 5.33. However the Code states that signals may be rotated to the upper quadrants at the convenience of the operator. Since it is standard practice internationally when performing surface testing to adjust the phase to give the signal orientation shown in FIG. 5.33, it is strongly recommended that the same orientation be used during tube testing to avoid possible confusion.
FIG. 5.33. The instrument display using an absolute probe for shallow inside and outside surface flaws and a through hole at f90. The fill factor signal is not normally obtainable so is shown dashed.
At f90, all flaw signals appear in the quadrant between the signals for a shallow inside surface flaw and those for a shallow outside surface flaw. Flaw signals which appear between those for a shallow inside surface flaw and a through hole indicate a flaw at the inside surface, and flaw signals which appear between those for a shallow outside surface flaw and a through hole indicate a flaw at the outside surface.
Absolute probe signals from other conditions
During testing of in-service tubes, signals from a number of conditions other then flaws are displayed on the screen. Some of these signals can appear at similar phase angles to those of flaws. It is therefore important to be familiar with these signals and how to distinguish them from flaw signals. These conditions include the following.
(a) Probe wobble
Probe wobble appears as a variation in fill factor and so gives an approximately horizontal signal either side of the operating point for all test frequencies, as shown in FIG. 5.34. This signal is readily distinguishable from flaw signals, but can add noise to other signals and so is undesirable. High frequencies increase the relative intensity of the inside surface eddy currents and so cause greater probe wobble signals. The signals can therefore be reduced by decreasing the test frequency. This may not be acceptable because of decreased sensitivity to the outside surface conditions, so if probe wobble noise is a problem, a slightly larger diameter probe should be used. Alternatively, the probe being used could be wrapped with adhesive tape to reduce wobble.
FIG. 5.34. The instrument display using an absolute probe for various flaws, probe wobble and a dent at f90.
(b) Dents
Dents can be present in tubes in heat exchangers because of the buildup of corrosion products between the tube and a baffle plate or tube support sheet, and from other causes. The stresses associated with them can lead to stress corrosion cracking or fatigue cracking. A dent causes a reduction in inside and outside diameters without any significant thinning or the tube wall, and therefore appears as an increase in fill factor at all test frequencies, as shown in FIG. 5.34.
This signal is readily distinguishable from flaw signals.
(c) Ferromagnetic conditions at the inside surface
A ferromagnetic inclusion at or near the inside surface of a nonferrous tube or an accumulation of iron oxide corrosion product give a similar signal. In both cases the ferromagnetic material increases the amount of flux which in turn increases the inductive reactance of the coil. The signal produced is therefore in the upwards direction on the impedance diagram, whatever the test frequency. However, although the direction on the impedance diagram varies little with frequency, the direction relative to the fill factor direction or the direction of flaw signals varies considerable. This can be seen in FIG. 5.35. At f90 (and at higher frequencies) the signal appears between the shallow inside surface flaw and shallow outside surface flaw signals, and so could be mistaken for a flaw signal (see the two lower operating points in FIG. 5.34).
A signal from a ferromagnetic condition at the inside surface can be distinguished from a flaw signal by retesting at a lower frequency, for example, 1/4 f90 or lower. As the frequency is reduced, the angle between the shallow inside surface flaw and the shallow outside surface flaw signals decreases, as shown in the upper operating point in FIG. 5.34. That is, the outside surface flaw signal rotates anticlockwise towards the inside surface flaw signal. However, a signal from a ferromagnetic condition at the inside surface will rotate slightly clockwise.
(d) Ferromagnetic conditions at the outside surface
A ferromagnetic condition at the outside surface gives the same signal as a ferromagnetic condition at the inside surface except that, because the eddy currents which are affected by it have a phase delay with respect to the inside surface eddy currents, the signal shows a phase shift. In addition, the eddy currents at the outside surface have a lower intensity than those at the inside surface, so the signal amplitude is less for a ferromagnetic condition at the outside surface. That is, the difference is the same as the difference between the signals from flaws at the outside and inside surfaces. At a test frequency of f90, outside surface signals are rotated 900 clockwise with respect to inside signals. At lower frequencies, the phase rotation is less, but the signal amplitude is greater, whereas at higher frequencies, the phase rotation is greater, but the signal amplitude is less.
FIG. 5.36 shows the signal from a ferromagnetic condition at the outside surface. It could be confused with a signal from a dent, but the two can readily be distinguished if required by retesting at a different test frequency. The signal from a ferromagnetic condition at the outside surface will show phase rotation with respect to the signal from an inside surface flaw, as stated above, whereas a dent signal will remain approximately 1800 from the inside surface flaw signal.