Table of contents


1. INTRODUCTION
2. THE ARRAY
3. CHARACTERIZING PERFORMANCE USING MACHINED NOTCHES
4. SUMMARY

INTRODUCTION

Periodic inspection of industrial heat exchangers is an important consideration in the efficient operation of a power generating facility. Since the operating life of the heat exchangers is finite, failures will occur. Flaws may develop in small areas due to local conditions that can cause relatively rapid deterioration of the tubes while overall performance of the unit may appear normal. Inspection of the unit at the convenience of the operator allows the physical condition to be established, and repairs to be made avoiding costly unexpected failures. But the cost of the inspection process must also be considered. The expense of taking a system out of service for evaluation is considerable and hence, in some instances, only a partial inspection of the system can be conducted during the time allowed. Ultrasonic inspection of heat exchanger tubes is usually conducted from the tube bore since access to the outside surface is generally not possible. Conventional inspection relies on either a rotating radial transducer or an axially mounted transducer with a rotating mirror. Both of these techniques have the same fundamental limitations--they are time consuming. To inspect the entire tube wall, the probe must rotate continuously while incrementing the axial position for each rotation. Additionally, since the polar size of a detected flaw is sometimes used in evaluating its criticality, the rotational drive mechanism must be capable of reasonably precise indexing. It has also been difficult to develop an effective bore probe for small I.D. tubes. Obviously any method of increasing the speed of testing is desirable. This presentation reviews a technique utilizing array transducers in boreside applications for greatly increased inspection speeds. The results of use of the design in 13 mm and 19 mm diameter tubes is also presented.

THE ARRAY

Figure 1 - Desired Sound Field Characteristics of the Array

The important aspect of this approach is that the single rotating transducer of the conventional designs is replaced by an ultrasonic array. The elements of the array are positioned so that the ultrasonic fields emanate radially outward as shown in Fig.1.

The space available in smaller tubes prohibits individual conventional transducers being mounted to accomplish this so the array is assembled as a single unit. A piece of acoustically absorptive backing is first machined as a cylinder with eight flat areas where the piezoelectric elements are mounted. Fig.1 shows the sound field configuration of the array. The array elements are angled toward the axis of the tube to achieve the oblique incidence for shear wave inspection as discussed earlier and as shown in the figure. It can also be seen that although the sound fields do not overlap in the polar plane, critical flaws are typically large enough that they will be detected by at least one array element.

CHARACTERIZING PERFORMANCE USING MACHINED NOTCHES

An evaluation was conducted to determine the relative sensitivity of the boreside array probe to notches of various depths as well as the acoustic profile of the individual sound fields in both axial and polar directions. Characterization of the ultrasonic performance of this probe was initiated using artificial "flaws" due to the limited availability of documented flaw samples. Three "flaw" geometries were used all of which emanate from the outside tube surface. These include circumferential notches, partial notches and finally notches at various angles to the tube axis. All notches were machined in 13 mm (0.5 inch) outside diameter stainless steel tube samples having a 1.25 mm (0.05 inch) wall thickness.

The notches were made using a 0.8 mm (0.032 inch) blade. This does not accurately simulate the closed cracks likely to be encountered in actual heat exchanger tubing but it does represent an easily manufactured standard for initial testing. The presence of two corners at the bottom of the notch rather than one in a true crack may account for some slight inflections we noticed in the acoustic profiles but we don't believe the effect is significant in this evaluation.

Nine circumferential notch samples were fabricated with depths ranging from 0.12 mm (.005 inch) to 1.14 mm (0.045 inch) in 0.12 mm (.005 inch) increments. The probe was moved axially through the sample until the maximum signal response was encountered. This maximum signal level was then plotted as a function of notch depth in Fig.3. Notches with depths greater than 0.8 mm (0.032 inch) yield the same peak response. This is probably due to the corner geometry being larger than the lateral sound field dimensions. At smaller notch depths, however, the peak response decreases proportionally. This decay probably becomes very nonlinear as the notch depth approaches zero but the decay appears to be nearly linear over the range plotted. Further testing is necessary to characterize the nature of this response.

Figure 2 - Notch Echo Amplitude as a Function of Circumferential Notch Depth	Figure 3 - Notch Echo Amplitude as a Function of Partial Notch Depth

The partial notch geometry is shown in Fig.4. Again nine samples were fabricated with depths incremented by 0.12 mm (0.005 inch). In these samples, as the notch depth increases, the polar dimension of the notch also increases and this more closely represents crack geometry encountered in tube walls. Fig.4 also shows the maximum axial response as a function of notch depth. Again we see that there exists a notch size above which no further increase in response echo occurs. It can also be seen that notches only .12 mm (.005 inch) deep and having a circumferential length of 2.5 mm (.1 inch) yield signal levels only 4 db below the response of a near through-wall notch. This is an encouraging indication of the sensitivity of this inspection method.

Figure 4 - Crossover of Echo Levels

Figure 5 - Crossover Level as a Function of Notch Depth

Since the circumferential size of these notches does vary, we also characterized the "crossover level" of the probe as a function of notch size. As a probe is rotated relative to a reflector in the tube wall, the response from one array element will decrease while the response from the next adjacent element will increase as shown in Fig.5.

At some point in this rotation, the responses from the two elements will be of equal amplitude. If the alarm threshold in the flaw detector is adjusted to this level or lower, then a notch of this size or larger cannot be missed by passing between two adjacent sound fields. We define this signal level as the "crossover level" for the particular notch size--the level at which the response signals from two adjacent array elements crossover each other. Naturally this "crossover level" will be a function of flaw detector setup and calibration. For our evaluation, we adjusted the instrumentation so that a 1.1 mm deep notch response yielded a signal 80% of the total screen height. We then measured the "crossover level" in terms of screen height as a function of notch depth. Those results are presented in Fig.6 and indicate that an alarm threshold set at 40% screen height will guarantee that all flaws of this geometry and having a depth greater than 0.6 mm (0.024 inch) will be detected.

The angle notch geometry is shown in Fig.7. Nine samples were fabricated all similar to the 0.64 mm deep partial notch except that the angle of the notch relative to the tube axis was varied from 0 degrees to 16 degrees in 2 degree increments. As expected, the peak signal response decreases with increasing notch angle as shown in the figure. Crossover level as a function of notch angle is shown in Fig.8 and indicates that a 20% alarm threshold will guarantee flaws of this geometry oriented less than 5.5 degrees from the tube axis will be detected.

Figure 6 - Notch Echo Amplitude as a Function of Notch Angle

Figure 7 - Crossover Level as a Function of Notch Angle

Finally the axial echo-dynamic profiles of several circumferential and partial notches were plotted. The axial echo-dynamic profile is the amplitude response of the notch as a function of axial displacement of the probe assembly. As we'll demonstrate later, this profile is useful in the determination of maximum acceptable probe speeds during the inspection. The profiles for the circumferential notches are shown in Fig.9. The inflection at the peak of the 0.12 mm (.005 inch) profile as well as other less noticeable inflections in the other profiles are probably due to interfering signals from the multiple notch corners as discussed earlier.

Figure 8 - Longitudinal Profile with Several Half-Node Reflections

Fig.9 shows echo-dynamic profiles for partial notches of various depths.

If we assume that the 0.62 mm (0.025 inch) deep partial notch is representative of a critical flaw and if the flaw detector alarm threshold is set at -12 db on the RELATIVE AMPLITUDE axis, then the notch will be detected for a l.5 mm (0.06 inch) probe displacement. Knowing this, the maximum probe speed can be estimated. Although the alarm circuitry can be latched on a single threshold violation, a more conservative approach limits probe speed such that the notch would be detected on no fewer than five consecutive pulse cycles. Using a pulse repetition frequency of 1250 Hz for each channel, the maximum probe speed is computed to be greater than 0.45 m/sec. (18 in/sec.). Even using the 0.12 mm deep partial notch, the maximum probe speed is 0.25 m/sec. (10 in/sec.). Naturally these probe speeds should be evaluated on real flaws since the echo-dynamic profile will probably vary.

Author

Thomas J. Carodiskey
Krautkramer Branson P.O. Box 350 Lewistown, Pa. 17044
Homepage: NDTnet Exhibition

The Paper was presented on the UTonline Application Workshop in May '97