UT imaging can be used to obtain two- and three-dimensional pictures of cracks, or other types of flaws, in a pressure vessel component from an automated ultrasonic examination. An ultrasonic transducer (either longitudinal or shear wave) is moved over the vessel’s surface with a scanning fixture. The X-Y coordinates of the transducer locations on the vessel and the UT signals obtained by the transducer are recorded on a magnetic disk as the transducer is moved over the vessel’s surface. A computer subsequently constructs an image of the crack (or other type of flaw) based on the UT signal amplitude or distance data correlated with the location of the transducer.
Both plan view and cross-sectional UT images can be obtained for permanent records to very accurately determine the location, orientation, and size of a crack.
These images are especially useful to a fitness-for-service analysis, that is critically dependent upon an accurate knowledge of the crack’s dimensions with respect to the maximum principal membrane stress (see Section 750).
UT imaging can also be used to “map” internal corrosion from the I.D. surface of a vessel, using a longitudinal wave UT transducer. The image constructed by the computer will depict the I.D. surface contour of the vessel, which represents the internal corrosion loss. This can be most useful when it is desired to determine the size of locally corroded areas with a remaining wall thickness that is possibly less than the minimum required, to evaluate if repairs are necessary when it is preferred not to open the vessel for an internal inspection.
It should be noted that UT imaging does not improve upon the accuracy of the UT data itself, and, therefore, it is not a substitute for using the optimum UT procedures for flaw detection and sizing discussed above for manual UT examinations. However, the UT image can be very beneficial for analyzing the UT data and giving a permanent record of flaw location, orientation, and size. Characteristics of a flaw can become readily apparent in a UT image. The image can be very important for evaluating the significance of a flaw that could be missed with a manual UT examination.
Time based sizing generally provides greater accuracy for determining depth, especially when the crack is relatively small with respect to the diameter of the transducer. Therefore, time based sizing should be used whenever a fitness-for-service analysis is made to evaluate the integrity and reliability of a vessel.
Several variations of time based sizing have been developed, but only the general concept is discussed here. Different vendors may use different techniques, but they are all significantly more complicated to apply than amplitude based sizing. All time based sizing techniques are highly dependent on the skill and expertise of the technicians performing the examination, and only the most qualified vendors who can appropriately demonstrate the accuracy of their procedures should be used.
Figure 700-21 illustrates one shear wave UT technique that can be used to determine the depth of a crack. The ultrasonic wave propagates through the material as a wave front having a width at the surface equal to the diameter of the transducer. The width of the wave front spreads somewhat as it propagates, but this can be minimized by the use of high frequencies and large diameter transducers. Two peaks, referred to as a doublet, can be observed in the oscilloscope display for the position of the transducer.
A peak with a relatively high amplitude (Peak R) will be developed by the corner reflection of the ultrasonic wave from the base of the crack at the I.D. surface. This reflected peak will normally have maximum amplitude at the distance in the oscilloscope display corresponding to the thickness of the vessel shell.
The portion of the ultrasonic wave that passes over the tip of the crack is diffracted, which results in the formation of a second peak (Peak D) with an appreciably lower amplitude (referred to as a tip-diffracted satellite pulse). The amplitude of the tipdiffracted peak can be maximized for better observation by moving the transducer, which moves the reflected peak from the distance in the oscilloscope display that corresponds to the thickness of the shell and decreases its amplitude. Nevertheless, the sensitivity control on the ultrasonic instrument usually has to be adjusted quite high to be able to see the tip-diffracted pulse, and, therefore, it can be difficult to distinguish from the background noise. One aid to distinguishing the tip-diffracted peak from the noise is that the tip-diffracted peak will move in unison with the reflected peak (i.e., as a doublet) across the oscilloscope display as the transducer is moved.
The separation between the reflected and diffracted peaks is constant whenever they are observed together, regardless of the position of the transducer, and is the result of the difference in the “time-of-flight” of the ultrasonic wave from the base and tip of the crack. Therefore, the distance in the oscilloscope display of the separation of the peaks indicates the depth of the crack (D´ in Figure 700-21).
Other techniques for time base sizing can be used, and, in fact, may be superior for some types of cracks. The actual technique that is used is based to a large extent on the vendor’s knowledge and experience. The vendor should be required to demonstrate the accuracy of the technique by examining a test block containing cracks similar to those in the pressure vessel that is being evaluated. Consult Company Inspection Groups, or a specialist for recommendations.
Figure 700-20 illustrates how the depth of a crack is determined using amplitude based sizing techniques. The crack was detected by shear wave UT as shown in Figure 700-19. The occurrence of a relatively high amplitude peak in the oscilloscope display at a distance corresponding to the thickness of the shell confirms that the crack has initiated at the I.D. surface.
The peak will normally decrease in amplitude as the transducer is moved closer to the weld, as shown for the position of Transducer B in Figure 700-20, when the “corner” reflection attributable to the base of the crack at the I.D. surface is lost. The amplitude of the peak reflected from the crack can be quite low, and it should be increased to approximately 80 percent of screen height (12 dB) by adjusting the sensitivity control on the ultrasonic instrument.
Amplitude of the peak will remain relatively constant as the transducer is moved closer to the weld. Only a portion of the ultrasonic wave will be reflected by the crack as the transducer is moved further towards the weld, and some of the wave passes over the tip of the crack. It is generally assumed that the tip of the crack is located by a decrease in peak amplitude to 50% of the maximum after adjustment of the sensitivity control (i.e., 6 dB drop), which is shown for the position of Transducer C. The depth of the crack is determined by the distance that the transducer is moved along the surface from the indication of the base of the crack (position A) to the indication of the tip of the crack (position C).
Amplitude base sizing is reasonably satisfactory for crack depths that exceed the diameter of the transducer. However, other characteristics of the crack (such as changes in orientation, roughness, and width) can affect the amplitude of the peak, which can lead to errors in sizing.
Amplitude based sizing is generally not satisfactory for cracks shallower than the diameter of the transducer. This is the situation with most vessel inspections. A 1/8-inch deep crack may not significantly affect the integrity and Reliability of a vessel, whereas a ¾-inch crack would require repair before the vessel is returned to service. Amplitude based sizing tends to oversize small cracks and undersize large ones, and, therefore, may not accurately discriminate between the shallow superficial crack and the deeper one that could jeopardize safe operation.
The ability to determine the depth of a crack through the thickness of a vessel shell is a very important attribute of shear wave UT. However, the accuracy of the depth measurements made with UT can vary considerably, depending on the technique used and the skill of the technician.
UT crack sizing techniques are classified as either “amplitude based” or “time based.” Amplitude based techniques were the first to be developed, and are still the most frequently used. They are relatively simple to use, but it has been learned that they do not always give accurate results. Time based techniques have been more recently developed and are capable of providing significantly more accurate depth measurement. However, they are more difficult to use than amplitude based techniques, and, therefore, appreciably more skill and expertise is necessary to obtain accurate results.
Shear wave UT is very useful for detecting cracks that have developed during service. Figure 700-19 illustrates how shear wave UT, calibrated according to Figure 700-18, can be used to detect a crack in the heat affected zone of a weld joint. This crack has started at the back surface of the workpiece (I.D. of the vessel), and has propagated towards the front (O.D.) surface.
Shear wave UT is performed by moving the transducer along the O.D. surface of the vessel towards the weld joint. No reflection will be observed in the oscilloscope display with the position of Transducer A in Figure 700-19, because the ultrasonic shear wave is reflected by the I.D. surface away from the transducer.
A relatively strong reflection from the base of the crack will be observed in the oscilloscope display for the position of Transducer C, because the base of the crack at the I.D. surface of the vessel acts like a notch in the back surface of a test block. The amplitude of this peak can exceed DAC, and it will appear at a distance in the oscilloscope display corresponding to the thickness of the vessel shell at this location.
When the transducer is moved closer to the weld, as depicted by the position of Transducer D, the amplitude of the peak will usually decrease significantly, and it will appear in the oscilloscope display at a distance corresponding to a depth less than the thickness of the vessel shell. The crack does not always provide a good surface for reflecting the ultrasonic shear wave back to the transducer for detection, because it is usually not oriented perpendicular to the wave. However, cracks normally have a rough (or faceted) texture that will reflect at least a small portion of the ultrasonic wave. The amplitude of this reflection can be very low and difficult to distinguish from the “noise” that results from the relatively high gain setting that is required to obtain satisfactory peak heights from the drilled holes in the test block during calibration. ASME Code, Section V, Article 4, now requires recording all reflections with amplitudes of 20% DAC and greater as flaws. This is a significant increase in the sensitivity of crack detection from the previously used 50% DAC recording level.
Considerable skill and experience is required for a technician to properly interpret reflections with amplitudes this low as an indication of a crack. Cracks that give very low amplitude reflections with ultrasonic shear waves at 45 degrees can give stronger reflections at 60 degrees or 70 degrees. Ultrasonic waves at the greater angles will be more nearly perpendicular to a crack propagating through the vessel shell. Therefore, it is advisable to perform shear wave UT at two or more angles, to maximize the probability of detecting the cracks.
When the transducer is moved still closer to the weld, the ultrasonic shear wave will eventually pass above the tip of the crack, as depicted for Transducer E. The wave will once again be reflected by the I.D. surface of the vessel away from the transducer, and the peak will disappear. Disappearance of the peak can give a rough indication of the depth of a crack, but it should never be used by itself to evaluate the integrity and reliability of a vessel. Other shear wave UT techniques are available that provide much more accurate crack depth and size data.
It is possible for the ultrasonic shear wave reflected by the I.D. surface of the vessel to be reflected subsequently by the crack, as depicted for the position of Transducer B. The peak for this type of reflection from the crack will appear in the oscilloscope display at a distance that is greater than the thickness of the vessel shell. It will then be necessary for the technician to use geometry to determine the actual depth at which the wave is reflected by the crack. The actual I.D. surface of a vessel will not always be a good enough reflector to produce reflections of this type from cracks, because this surface can be roughened considerably by corrosion.
Very fine cracks, such as those resulting from stress corrosion, and cracks that are filled with a corrosion scale may permit a portion of the ultrasonic wave to propagate through. This wave propagation will further reduce the amplitudes of the peaks in the oscilloscope display attributable to these types of cracks, and, therefore, they can be difficult to detect. Proper calibration of shear wave UT with test blocks containing side drilled holes will not guarantee that cracks will always be detected. A special test block containing cracks of the type that may have developed in the pressure vessel should be used to demonstrate that the shear wave UT procedure being used is capable of detecting the cracks, and that the technician has the expertise to properly interpret the data. This concept is gradually being adopted by ASNT and ASME. Contact CRTC Engineering analysis for information and assistance.
Using shear wave UT to detect cracks in nozzle welds or other vessel components with complex geometries can be more difficult than for the previous example. The technician performing the examination will have to know the actual geometry and dimensions of the component, to determine the angles for the ultrasonic shear waves that are required to probe the locations where cracks are most likely to develop. In addition, more complex component geometries can have surfaces that reflect the waves back to the transducer. The technician will have to be able to distinguish these reflections from the surfaces of the vessel component from reflections coming from cracks.
Shear wave UT is calibrated using a test block manufactured from a material similar to the workpiece (i.e., with the same velocity of sound), that has side-drilled holes and a notch on the back surface as shown in Figure 700-18. The test block should have a thickness within 1 inch of the thickness of the workpiece. The side-drilled holes should be 3/16-inch diameter for test blocks up to 4 inches thick, and increase 1/16-inch in diameter for each 2-inch increase in thickness of the test block greater than 4 inches.
The transducer is first moved along the surface until the reflection from the ¼-thickness drill hole attains maximum peak amplitude at the position of Transducer A in Figure 700-18, and the delay control on the ultrasonic instrument is adjusted to shift the peak to a distance on the oscilloscope display equal to the depth of the hole from the surface.
Next, the transducer is moved along the surface until the reflection from the ¾-thickness drill hole attains maximum peak amplitude at the position of Transducer C, and the material velocity control on the ultrasonic instrument is adjusted until the peak coincides with the distance on the oscilloscope display corresponding to the depth of this hole from the surface. Moving the transducer along the surface until a reflection of maximum peak amplitude is obtained from the ½-thickness hole at the position of Transducer B should produce this peak at a distance in the oscilloscope display corresponding to the depth of this hole below the surface.
Then, the transducer is moved until a reflection of maximum peak amplitude is obtained from the ¾-thickness hole when the ultrasonic shear wave is reflected by the back surface of the test block at the position of Transducer E. This peak should appear in the oscilloscope display at a distance corresponding to 5/4 the thickness of the test block. The maximum peak amplitudes determined in this manner will establish a distance amplitude curve (DAC) for shear wave UT. The sensitivity control on the ultrasonic instrument should be adjusted until the amplitude of the highest peak is at least 80% of the oscilloscope screen height.
A very strong reflection is normally obtained from the notch on the back surface with an ultrasonic shear wave at 45 degrees, which can exceed the DAC. However, ultrasonic shear waves at 60 degrees and 70 degrees will usually produce reflections from the notch that are below the DAC.
Shear wave UT is used primarily to detect and determine the size of cracks that have developed during service. Forms of deterioration that can result in cracking include mechanical and thermal fatigue, creep, stress-corrosion, and hydrogen attack, among others (see Section 730). Although shear wave UT can provide very good data for the evaluation of the integrity and reliability of a vessel, quality of the data is highly dependent on the skill and expertise of the technician performing the examination. Only highly qualified technicians should be used who have demonstrated their ability to correctly detect and size the types of flaws that might have developed in the vessel.
The use of longitudinal wave UT for detecting, locating, and determining the size of hydrogen blisters is illustrated in Figure 700-17. The blisters are internal flaws that have a reflecting surface at a depth from the front surface that is less than the distance to the back surface (thickness) of the shell component. Therefore, the blisters will cause reflected peaks to appear in the oscilloscope display at a distance less than the thickness of the shell.
The peaks attributable to the blisters will usually be lower than the DAC, because the surface of the blisters will not normally be as good a reflector as the back surface (similar to the effect of a corroded surface). Furthermore, very thin blisters may have sufficient contact between their opposite surfaces to permit propagation of some of the ultrasonic wave through the blister. This situation is depicted by the position of Transducer B, in which case a small reflection from the back surface will appear in the oscilloscope display. However, no portion of the ultrasonic wave can propagate through blisters with more widely separated opposite surfaces, and no reflection from the back surface will be observed, as depicted by the position of Transducer C.
The distance of the peaks on the oscilloscope display resulting from the blisters indicates their depths below the surface, and the size of each blister can be estimated by moving the transducer along the surface until the reflection attributable to the blister disappears.
Figure 700-16 illustrates the use of longitudinal UT for determining the remaining thickness of a corroded shell. If the transducer is placed on a location of the shell component that is not corroded, as depicted by the position of Transducer A in Figure 700-16, a back reflection will be observed at a distance equal to the original thickness of the shell. On the other hand, if the transducer is placed on a location where internal corrosion has occurred, as depicted by the position of Transducer B in Figure 700-16, a back reflection will be observed that is less than the original thickness. The back reflection from a corroded surface of a vessel shell component can have an amplitude below the DAC, because irregularities in the corroded surface can scatter some of the reflected longitudinal wave.
The distance of the back reflection on the oscilloscope display is the remaining thickness of the shell component, and the size of the corroded area can be determined by moving the transducer along the surface until the back reflection returns to the original thickness.
Calibration of longitudinal wave UT consists of developing a “distance amplitude curve (DAC)” for the instrument and transducer, using test blocks manufactured from a material similar to the workpiece (i.e., with the same velocity of sound). Calibration for determining the remaining thickness of a corroded component can be accomplished by placing the transducer sequentially on test blocks (or on different locations on a single “step block”) with different known thicknesses, as illustrated in Figure 700-15.
The amplitude of the reflection (echo) received by the transducer from the back surface decreases as the thickness of the test block increases, due to attenuation of the ultrasonic wave propagating through the block. An initial pulse is obtained from the front surface of the test block, which is positioned on the zero thickness line on the oscilloscope, by adjusting the “time delay” (or “sweep”) control on the ultrasonic instrument. The back reflections from the other known thicknesses are then shifted as necessary to coincide with the proper thickness lines on the oscilloscope display, by adjusting the material velocity control on the ultrasonic instrument.