Sonic impulses are emitted by a flaw in a ductile material subjected to a gradually increasing applied stress, due to the release of strain energy in the intensified stress field associated with the flaw. The release of strain energy creates stress waves that propagate through the material at the speed of sound. These stress waves behave like sonic impulses, and are, therefore, commonly referred to as acoustic emissions. However, there may be no significant release of strain energy in a brittle material to cause acoustic emissions prior to the occurrence of fracture.
Acoustic emissions attributable to flaws in metals are usually inaudible, but they can be thought of as being similar to the familiar sounds emitted by a piece of wood as it is gradually bent to the breaking point. The sounds not only reveal that the wood is cracking, but an individual hearing the sound can also sense the direction that it is coming from to help locate the piece of wood. It is useful to draw another analogy for brittle materials, such as a glass rod that is bent to the breaking point. No warning sounds will be heard as the glass rod is bent before it breaks.
The nominal applied stress in a component of a pressure vessel manufactured from a ductile metal can be well below the yield strength of the material, but the local stress concentrations developed at flaws can be high enough to cause highly localized plastic deformation or enlargement of the flaw by microcrack propagation. Both plastic deformation and crack propagation produce acoustic emissions that can be detected by acoustic sensors to disclose the existence of the flaw.
A single sensor can usually detect acoustic emissions originating anywhere in a relatively large vessel, because attenuation of the sonic impulses in a metal shell is very low. However, several sensors are usually employed to obtain confirming indications, and, more importantly, to locate the origin of the emissions in the shell by triangulation.
AE was developed to assess the integrity of pressure-containing equipment in the nuclear power industry. Acoustic sensors are attached to the O.D. surface of the equipment to detect and locate flaws in the material during a pressure test. AE can be a very useful NDE method to use for the in-service inspection of pressure vessels, especially because one test will cover all components of a vessel and weld joints in the shell.
It is sometimes thought that RT is the best NDE method that can be used for detecting flaws, because it is mandated by the ASME Code for certifying the quality of newly constructed pressure vessels. This is not true. RT has attained its preeminence in the ASME Code by virtue of the evolution of weld joint efficiencies and permitted design details around the types of fabrication defects that can be readily detected by RT.
The reliance of the ASME Code upon RT should not be construed to imply that it is the optimum NDE method to use for the in-service inspection of vessels. In fact, most of the flaws that can develop as a consequence of the deterioration of a vessel during service can be better detected by other NDE methods.
One circumstance where RT can be used to considerable advantage is when a direct comparison is desired between the present condition of a vessel and its condition when new, and other NDE methods were not used during construction to provide baseline data.
A significant limitation upon the use of RT for in-service inspection is that it will only detect cracks that are essentially parallel to the direction of the incident radiation, and have a sufficient width to be visible in the radiograph as limited by the grain of the film. The detectability of cracks diminishes greatly as they deviate further from this orientation. Taking several radiographs with different angles of incident radiation can overcome this shortcoming, but this would considerably increase the time and cost of the inspection.
RT does not give a reliable indication of the depth of a flaw through the shell of a pressure vessel. Therefore, the depth of a crack detected by RT must be measured by another NDE method, such as UT, to evaluate the effect of the crack upon the integrity and reliability of the vessel.
RT is also severely limited for the in-service inspection of nozzle openings and welds, which tend to be locations of relatively high stress where deterioration during service is likely to occur. Nozzles are usually fabricated from plate and forging or pipe materials with different thicknesses. Satisfactory radiographs of nozzle openings can rarely be obtained, because the workpiece must have an essentially uniform thickness for the variation in density of the radiograph to be within an interpretable range.
RT is the most time-consuming and expensive of all NDE procedures. The set-up time for the equipment is usually much greater than the time required for the exposure, and this must be followed by development of the film in a darkroom and interpretation of the resulting radiograph. Several man-hours can be required for each exposure. Additional time and cost penalties are incurred indirectly by restricting and delaying other work in the area due to the serious radiation hazard associated with RT. Therefore, the suitability of other NDE methods for detecting the forms of deterioration that might have occurred during service should be investigated before RT is employed.
Despite all of the disadvantages associated with the use of RT for the in-service inspection of pressure vessels, there can be no dispute that a radiograph can provide very valuable data concerning the integrity and reliability of a vessel. The radiographs provide permanent records that can be compared to the results of future inspections or reinterpreted in the light of new information concerning the deterioration that can occur during service.
The acceptability of a radiograph for the detection of flaws is determined with a device, referred to as a “penetrameter,” that is placed upon the surface of the workpiece when the exposure is made. Penetrameters are relatively thin pieces of material with radiation attenuation characteristics similar to the workpiece that contain holes with diameters of 1, 2, and 4 times the thickness, as illustrated in Figure 700-24.
The quality level required for a radiograph is designated by a two-part expression X-YT, where X is the maximum thickness permitted for the penetrameter as a percentage of the thickness of the workpiece, Y is the diameter of the hole as a multiple of the thickness, and T is the thickness. A quality level of 2 to 2T is adequate for most applications of RT for the in-service inspection of pressure vessels, and is consistent with the requirements of ASME Code, Section V.
A radiograph is considered to be acceptable for the quality specified if the entire outline of the penetrameter is visible, the density of the penetrameter is within the required range of 1.8 to 4.0, and the hole is discernable. However, the acceptability of the radiograph for flaw detection is limited to areas that have densities within 15% under and 30% over the density of the hole in the penetrameter. These limits may not encompass the entire range of densities in a radiograph, and interpretation of areas with densities outside these limits is of questionable validity. It is important to recognize that the workpiece must have an essentially uniform thickness for the background density of the radiograph to be within these limits.
Note that the penetrameter functions only to determine that a radiograph has acceptable quality. The penetrameter does not serve as a calibration standard. Therefore, it should not be used to estimate flaw sizes, and should not be used to establish acceptance limits for flaws based upon relative densities in the radiograph.
The exposure of the photographic film by radiation passing through the workpiece is determined by the intensity of the radiation multiplied by the time of the exposure. The optical density of the developed negative increases with increasing exposure. ASME Code, Section V, requires a radiograph to have a density between 1.8 and 4.0 for proper visual interpretation. The exposure must be adjusted if the density of the radiograph is not between these limits, by changing either the time of the exposure or the intensity of the incident radiation.
Flaws that reduce the thickness of the workpiece through which the radiation passes will increase the exposure, and, therefore, the density of the radiograph. However, the density of the flaw image compared to that of the surrounding workpiece cannot be relied upon to give an accurate indication of the depth of the flaw. The threedimensional shape and orientation of the flaw can significantly affect the density of the image and are not always revealed by the two-dimensional silhouette in the radiograph.
The radiation which passes through the workpiece is recorded by photographic film. The radiographs are usually interpreted visually with the aid of a high intensity light source (light box), but optical densitometers or image analyzers are occasionally used.
Two primary characteristics of the film can affect the sensitivity of RT for detecting flaws: gradient and grain. Gradient is the difference in optical density of the negative resulting from exposure by different intensities of radiation. A film with high gradient exhibits relatively large differences in density when exposed by radiation of relatively small differences in intensity. In other words, a high gradient results in a relatively high contrast in the negative, which makes small differences in the intensity of radiation passing through the workpiece visible. Therefore, films that have a high gradient will provide the greatest sensitivity for detecting small flaws that cause only a small attenuation of the radiation.
Grain results directly from photo-sensitized crystals in the film. The darkened crystals impart a visually apparent “graininess” to the transparent negative that limits the detail that can be resolved by viewing the negative, regardless of the difference in density of adjacent areas of the film. Therefore, very fine flaws can be obscured by the grain of the film can obscure very fine flaws, so the flaws and may not be visible in a radiograph, although they significantly reduce the effective thickness of the workpiece. Fine grain films usually also have a high gradient, and are preferred for RT to obtain the greatest sensitivity for the detection of flaws.
Another characteristic of film that can affect the sensitivity of RT for detecting flaws is speed, which is a measure of the sensitivity of the film to the radiation passing through the workpiece. A high speed film requires less exposure to radiation to produce the same optical density in the developed negative than a low speed film. Therefore, less exposure time is required for high speed films to produce satisfactory radiographs than with low speed films for the same intensity of incident radiation. The reduction of exposure time can be significant, especially for relatively thick workpieces, when low intensity radiation sources are used for in-service inspection. However, high speed films provide less gradient and have coarser grain than low speed films. It is usually inadvisable to sacrifice the greater sensitivity of fine grain films with a high gradient for detecting flaws in order to obtain the advantage of shorter exposure times.
The film is loaded into flexible cassettes in a darkroom, and the cassettes are usually held against the surface of the workpiece with magnets. The cassettes incorporate a “radiographic screen” that improves the image recorded on the film, by both intensifying the exposure of the film by the radiation passing through the workpiece, and by filtering out scattered radiation to reduce fogging. Lead foil is commonly used for the radiation screen. Radiation penetrating the workpiece interacts with the lead atoms in the foil, which causes them to “fluoresce,” intensifying the exposure of the film. The lead foil will also absorb most of the low energy scattered radiation, while having little effect upon the intensity of the higher energy radiation passing through the workpiece. In this manner, fogging of the film by the scattered radiation is greatly reduced, but exposure of the film by the transmitted radiation is not significantly affected.
Both x-rays and gamma rays can be used as the incident radiation. Energy and intensity are the most important characteristics of the incident radiation. The energy of the incident radiation determines its ability to penetrate the workpiece. Higher energy radiation is required to penetrate thicker workpieces but it reduces the scatter of radiation passing through the workpiece. The intensity of the radiation reaching the photographic film after it has passed through the workpiece controls the length of time required to properly expose the film. Higher intensity radiation is required for thicker workpieces to obtain reasonable exposure times, because attenuation of the radiation passing through the workpiece increases with material thickness.
X-rays are usually produced by x-ray tubes, which are electronic devices that convert electrical energy into x-rays. The voltage of the x-ray tube determines the energy of the x-rays produced, and the current controls the intensity of the radiation. Consequently, higher voltages and currents are required for thicker workpieces, which necessitate larger x-ray tubes and electrical power sources. The bulk of the equipment required to produce high energy-high intensity radiation with x-ray tubes severely restricts portability. Therefore, RT systems suitable for pressure vessels are generally not satisfactory for shell thicknesses greater than 2 inches.
Linear accelerators (LINAC) can also be used to produce x-rays. They are much more practical than x-ray tubes for producing the high energy-high intensity radiation required for thick workpieces, and some systems are sufficiently portable. However, the equipment is still quite cumbersome to handle.
Gamma-rays are created by the radioactive decay of unstable isotopes of naturally occurring or artificially produced elements. Cobalt-60 and iridium-192 are the two isotopes most commonly used for RT. The radiation has a relatively high energy for penetrating thick workpieces. However, the intensity of the radiation is generally lower than that produced by x-ray tubes or LINACs, and it diminishes with time as the radioactive isotope decays. Therefore, the time required to properly expose the film can be quite long. Nevertheless, the equipment required for RT with gamma rays from a radioactive isotope is much less cumbersome than that required to produce x-rays, and it is therefore more suitable.
A “point” source of radiation would provide the sharpest images of flaws on the photographic film exposed to the radiation passing through the workpiece. However, actual sources of radiation used for RT are provided with an aperture of approximately 1/10 to 1/4 inch to obtain sufficient intensity of radiation to expose the film in a reasonable length of time. Consequently, some “geometric unsharpness” results in the image of the flaw, because the source of radiation is not a true point source. The unsharpness is reduced by large source-to-film distances relative to the thickness of the workpiece. The source-to-film distance should be at least five times the thickness of the workpiece to give satisfactory clarity.
It is extremely important to recognize that the radiation sources used for RT have much higher energies and intensities than those used for medical x-rays and therefore can very severely damage animal tissues. Severe disability or death can result from exposure to the radiation. Therefore, it is essential to provide proper shielding of the radiation source and to prevent unauthorized access to the area when RT is being performed. Guidance for shielding and restriction of access should be obtained from knowledgeable safety and health specialists before performing the RT.
X-rays and gamma rays penetrate steel, but the intensity of the incident radiation will be attenuated as it passes through the material. The degree of attenuation depends on the thickness and density of the material.
Flaws can have the effect of reducing the thickness of material through which the radiation must pass by interposing cavities or impurities of lower density in the workpiece. Therefore, there is less attenuation of radiation passing through the flaw than through the surrounding material, as illustrated in Figure 700-22.
Photographic film placed opposite the source of radiation will be exposed by the radiation that has passed through the workpiece (transmitted radiation) and, consequently, the flaw will appear as a dark image on the developed negative (referred to as a radiograph). It is important to recognize that the radiation passing through the workpiece does not directly interact with the flaw. The flaw is detectable only because it alters the thickness of material through which the radiation passes. The image of the flaw on the radiograph is actually the silhouette of the three-dimensional flaw projected onto the two-dimensional surface of the film, as illustrated in Figure 700-23.
Flaws will not always significantly reduce the thickness of material through which the radiation must pass. The flaw in Figure 700-22b is identical to the flaw in Figure 700-22a, except that it is rotated 90 degrees. With this orientation, the flaw will not significantly reduce the thickness of material. Therefore, the attenuation of radiation passing through this flaw will be essentially the same as that for the radiation passing through the surrounding material, and there will be no indication of the flaw.
RT is very useful for detecting both surface and internal flaws, and it is the primary NDE procedure required by the ASME Code to verify the quality of welds during construction. However, the associated radiation hazard makes it difficult to use for inspection of pressure vessels during shutdowns, when other personnel are working on the same vessel or close to it. The investment in equipment (radiation sources and darkroom facilities for processing film) can also be quite high, but this can be offset by the use of qualified contractors.
Article 2 of ASME Code, Section V, gives the minimum requirements for an RT procedure for pressure vessels.
UT is a very efficient NDE method. A large amount of data for evaluating the integrity and reliability of a vessel can be obtained in a relatively short period of time, without requiring extensive preparation of the vessel or interfering with other work in the area.
Longitudinal wave UT is applicable for determining the remaining wall thickness of a corroded pressure vessel under almost any circumstances, and most UT technicians have the skill and experience to perform these examinations. Longitudinal wave UT will also detect and locate hydrogen blisters, or similar internal flaws. Shear wave UT is very useful for detecting cracks and provides essentially the only method for determining the size (depth) of cracks with sufficient accuracy for making a fitness-for-service analysis to evaluate the integrity and reliability of a vessel.
The greatest limitation on the use of UT for in-service inspections is that the accuracy of the data obtained is highly dependent on the skill and expertise of the technicians performing the examinations. This is especially true for detecting and sizing cracks by shear wave UT. Qualification of procedures and certification of technicians is not sufficient to guarantee acceptable results. Only those vendors should be used that can demonstrate that they have the required skill and expertise to provide accurate data. Samples of various types of cracks that have developed in pressure vessels during service should be saved for future use as “standards” for the qualification of UT procedures and technicians for in-service inspections.
Another limitation on the use of UT for in-service inspection is that it was not used during the construction of most of the vessels that are now operating. Consequently, many indications of flaws detected by UT are very difficult to classify as either innocuous fabrication flaws or more serious indications of deterioration occurring during service. UT procedures are available that can usually distinguish between fabrication flaws and those that have developed during service, but they require highly skilled UT examiners to properly apply. The increasing use of UT during the construction of vessels, as provided for in PVM-MS-4750 and PVM-MS-4749, included in this manual, may eventually alleviate this difficulty.