Examination, this chapter like I promise in the last post, Piping and Fabrication will gave to you, I think the description and explanation about The Examination on Piping & Fabrication is detail enough, enjoy your self.
Types of Examinations. When used in the various codes, examination refers to the verification work performed by employees of the fabricator, much of which falls into the category of NDE. NDEs most often referenced by code and applied to the fabrication and installation of piping components and systems are:
• Visual
• Radiographic
• Ultrasonic
• Liquid penetrant
• Magnetic particle
Eddy current examination is often used to evaluate the quality of straight lengths of pipe as they are manufactured but is not often used in fabrication activities. Although not referenced by most codes, bubble testing, halogen diode probe testing, or helium pass spectrometer leak testing may be invoked by contract when, in the opinion of the designer, they will contribute to the integrity of the system. While these methods are referred to as leak tests, their methodology is outlined in Article 10 of ASME Section V Nondestructive Examination.
Accept-reject criteria and the extent to which the various NDEs are to be applied are in the applicable code. The following are brief descriptions of NDEs as they apply to piping. For much more detailed information the reader is referred to various publications of the American Society for Nondestructive Testing (ASNT),36 particularly the Nondestructive Testing Handbooks.
1. Visual examination: Visual examination is probably the oldest and most widely used of all examinations. It is used to ascertain alignment of surfaces, dimensions, surface condition, weld profiles, markings, and evidence of leaks, to name a few. In most instances the manner of conducting a visual examination is left to the discretion of the examiner or inspector, but more recently, written procedures
outlining such things as access, lighting, angle of vision, use of direct or remote equipment, and checklists defining the observations required are being used. Visual examination takes place throughout the fabrication cycle along with QA and QC checks. At setup, this would consist of verifying materials, weld procedures, welder qualifications, filler metal, and weld alignment, and on completion of fabrication, such things as terminal dimensions, weld profile, surface condition, and cleanliness.
2. Radiographic examination: When the need for greater integrity in welding must be demonstrated, the most frequently specified examination is radiography. Since the internal condition of the weld can be evaluated, it is referred to as a volumetric examination.
Radiographic sources used for examination of piping are usually X-rays or gamma rays from radioactive isotopes. While X-ray equipment is often used, it has limitations in that it often requires multiple exposures for a single joint, and special equipment, such as linear accelerators, are needed for heavier thicknesses. Although X-ray machines produce films with better clarity, they are not as practical in the field because of space limitations and portability. In the field, radioactive isotopes are used almost exclusively because of their portability and case of access. For wall thicknesses up to about 21⁄₂ in (63.5 mm) of steel, the most commonly used isotope is iridium 192. Beyond this cobalt 60 is used for wall thickness up to about 7 in (179 mm).
Radioactive sources normally used in piping work range in intensity from a few curies up to about 100 curies. Each source decays in intensity in accordance with its particular half-life. As the intensity decays, longer exposure times are required. Iridium 192 has a half-life of 75 days, while cobalt 60 has a 5.3-year half-life. Radioactive sources have finite dimensions and as a result produce a shadow effect on the film. This is referred to as geometric unsharpness, and it is directly proportional to the source size and inversely proportional to the distance between the source and the film. ASME Section V has established limits for geometric unsharpness.
Ideally for pipe, the source is placed inside the pipe and at the center of the weld being examined, with film on the outside surface of the weld, thus permitting one panoramic exposure. Where geometric unsharpness precludes this practice, the source may be placed on the inside on the opposite wall and a portion of the weld is shot. Several exposures will be needed. The source may also be placed outside the pipe and the exposure made through two walls. Again this requires multiple exposures and longer exposure times. A radiograph is considered acceptable if the required essential hole or wire size information on this subject.
3. Ultrasonic examination: Ultrasonic examination is used in piping for the detection of defects in welds and materials as well as for determining material thickness. A short burst of acoustic energy is transmitted into the piece being examined and echoes reflect from the various boundaries. An analysis of the time and amplitude of the echo provides the examination results.
A clock in the equipment acts to initiate and synchronize the other elements. It actuates a pulsar to send a short-duration electrical signal to a transducer, usually at a frequency of 2.5 MHz. The transducer converts the electrical signal to mechanical vibration. The vibration as ultrasound passes through a couplant (such as glycerine) and through the part at a velocity which is a function of the material. As the
sound reflects from various boundaries, it returns to the initiating transducer or sometimes to a second one where it is converted back to an electrical signal which is passed to a receiver amplifier for display on a cathode-ray tube. The horizontal axis of the display relates to time and the vertical axis relates to amplitude. The indication on the extreme left will show the time and amplitude of the signal transmitted from the transducer. Indications to the right will show the time and degree of reflection from various boundaries or internal discontinuities. The ability of an ultrasonic examination to detect discontinuities depends a great deal on the part geometry and defect orientation. If the plane of the defect is normal to the sound beam, it will act as a reflecting surface. If it is parallel to the sound beam, it may not present a reflecting surface and accordingly may not show on the oscilloscope. Therefore, the search technique must be carefully chosen to assure that it will cover all possible defect orientations.
The most serious defect in a pipe butt weld is that which is oriented in the radial direction. The most commonly used technique for detecting such defects is the shear wave search. In this procedure, the transducer is located to one side of the weld at an angle to the pipe surface. The angle is maintained by a lucite block which transmits the sound from the transducer into the pipe. The sound will travel at an angle through the pipe and weld. Being at an angle, it will reflect from the pipe surfaces until it is attenuated. Any surface which is normal to the beam, however, will reflect a portion of the sound back to the transducer and show as an indication on the oscilloscope. If the beam angle and the material thickness are known, the reflecting surface can be located and evaluated.
Prior to and periodically during each search, the equipment is calibrated against artificial defects of known size and orientation in a calibration block. The block must be representative of the material being searched (i.e., an acoustically similar material, with appropriate thickness, outside contour, surface finish, and heattreated condition). A variation of ultrasonic examination can be used to measure material thickness. If the speed of sound within the material is known, the time it takes for the signal to traverse the thickness and return can be converted to a thickness measurement.
Types of Examinations. When used in the various codes, examination refers to the verification work performed by employees of the fabricator, much of which falls into the category of NDE. NDEs most often referenced by code and applied to the fabrication and installation of piping components and systems are:
• Visual
• Radiographic
• Ultrasonic
• Liquid penetrant
• Magnetic particle
Eddy current examination is often used to evaluate the quality of straight lengths of pipe as they are manufactured but is not often used in fabrication activities. Although not referenced by most codes, bubble testing, halogen diode probe testing, or helium pass spectrometer leak testing may be invoked by contract when, in the opinion of the designer, they will contribute to the integrity of the system. While these methods are referred to as leak tests, their methodology is outlined in Article 10 of ASME Section V Nondestructive Examination.
Accept-reject criteria and the extent to which the various NDEs are to be applied are in the applicable code. The following are brief descriptions of NDEs as they apply to piping. For much more detailed information the reader is referred to various publications of the American Society for Nondestructive Testing (ASNT),36 particularly the Nondestructive Testing Handbooks.
1. Visual examination: Visual examination is probably the oldest and most widely used of all examinations. It is used to ascertain alignment of surfaces, dimensions, surface condition, weld profiles, markings, and evidence of leaks, to name a few. In most instances the manner of conducting a visual examination is left to the discretion of the examiner or inspector, but more recently, written procedures
outlining such things as access, lighting, angle of vision, use of direct or remote equipment, and checklists defining the observations required are being used. Visual examination takes place throughout the fabrication cycle along with QA and QC checks. At setup, this would consist of verifying materials, weld procedures, welder qualifications, filler metal, and weld alignment, and on completion of fabrication, such things as terminal dimensions, weld profile, surface condition, and cleanliness.
2. Radiographic examination: When the need for greater integrity in welding must be demonstrated, the most frequently specified examination is radiography. Since the internal condition of the weld can be evaluated, it is referred to as a volumetric examination.
Radiographic sources used for examination of piping are usually X-rays or gamma rays from radioactive isotopes. While X-ray equipment is often used, it has limitations in that it often requires multiple exposures for a single joint, and special equipment, such as linear accelerators, are needed for heavier thicknesses. Although X-ray machines produce films with better clarity, they are not as practical in the field because of space limitations and portability. In the field, radioactive isotopes are used almost exclusively because of their portability and case of access. For wall thicknesses up to about 21⁄₂ in (63.5 mm) of steel, the most commonly used isotope is iridium 192. Beyond this cobalt 60 is used for wall thickness up to about 7 in (179 mm).
Radioactive sources normally used in piping work range in intensity from a few curies up to about 100 curies. Each source decays in intensity in accordance with its particular half-life. As the intensity decays, longer exposure times are required. Iridium 192 has a half-life of 75 days, while cobalt 60 has a 5.3-year half-life. Radioactive sources have finite dimensions and as a result produce a shadow effect on the film. This is referred to as geometric unsharpness, and it is directly proportional to the source size and inversely proportional to the distance between the source and the film. ASME Section V has established limits for geometric unsharpness.
Ideally for pipe, the source is placed inside the pipe and at the center of the weld being examined, with film on the outside surface of the weld, thus permitting one panoramic exposure. Where geometric unsharpness precludes this practice, the source may be placed on the inside on the opposite wall and a portion of the weld is shot. Several exposures will be needed. The source may also be placed outside the pipe and the exposure made through two walls. Again this requires multiple exposures and longer exposure times. A radiograph is considered acceptable if the required essential hole or wire size information on this subject.
3. Ultrasonic examination: Ultrasonic examination is used in piping for the detection of defects in welds and materials as well as for determining material thickness. A short burst of acoustic energy is transmitted into the piece being examined and echoes reflect from the various boundaries. An analysis of the time and amplitude of the echo provides the examination results.
A clock in the equipment acts to initiate and synchronize the other elements. It actuates a pulsar to send a short-duration electrical signal to a transducer, usually at a frequency of 2.5 MHz. The transducer converts the electrical signal to mechanical vibration. The vibration as ultrasound passes through a couplant (such as glycerine) and through the part at a velocity which is a function of the material. As the
sound reflects from various boundaries, it returns to the initiating transducer or sometimes to a second one where it is converted back to an electrical signal which is passed to a receiver amplifier for display on a cathode-ray tube. The horizontal axis of the display relates to time and the vertical axis relates to amplitude. The indication on the extreme left will show the time and amplitude of the signal transmitted from the transducer. Indications to the right will show the time and degree of reflection from various boundaries or internal discontinuities. The ability of an ultrasonic examination to detect discontinuities depends a great deal on the part geometry and defect orientation. If the plane of the defect is normal to the sound beam, it will act as a reflecting surface. If it is parallel to the sound beam, it may not present a reflecting surface and accordingly may not show on the oscilloscope. Therefore, the search technique must be carefully chosen to assure that it will cover all possible defect orientations.
The most serious defect in a pipe butt weld is that which is oriented in the radial direction. The most commonly used technique for detecting such defects is the shear wave search. In this procedure, the transducer is located to one side of the weld at an angle to the pipe surface. The angle is maintained by a lucite block which transmits the sound from the transducer into the pipe. The sound will travel at an angle through the pipe and weld. Being at an angle, it will reflect from the pipe surfaces until it is attenuated. Any surface which is normal to the beam, however, will reflect a portion of the sound back to the transducer and show as an indication on the oscilloscope. If the beam angle and the material thickness are known, the reflecting surface can be located and evaluated.
Prior to and periodically during each search, the equipment is calibrated against artificial defects of known size and orientation in a calibration block. The block must be representative of the material being searched (i.e., an acoustically similar material, with appropriate thickness, outside contour, surface finish, and heattreated condition). A variation of ultrasonic examination can be used to measure material thickness. If the speed of sound within the material is known, the time it takes for the signal to traverse the thickness and return can be converted to a thickness measurement.
4. Liquid penetrant examination: Penetrant-type examinations are suitable for surface examinations only but are very sensitive. They require a fairly smooth surface, since surface irregularities such as grinding mark indications can be confused with defect indications. The surface to be examined is thoroughly cleaned with a solvent and then coated with a penetrating-type fluid. Sufficient time is allowed to permit the fluid to penetrate into surface discontinuities. The excess penetrant is removed by wiping with cloths until all evidence of the penetrant is removed. A developer which acts somewhat like a blotter is then applied to the surface. This an indication. Obviously, the success of the examination depends on the visibility of the indication. To enhance this, the penetrant contains colored dyes which can be seen under normal light, or fluorescent dyes which are viewed under ultraviolet light. The most common case is a red dye penetrant with a white developer.
FIGURE A6.26 Ultrasonic shear wave search. (a) Search arrangement; (b) oscilloscope
5. Magnetic particle examination: Magnetic particle examination is essentially a surface-type examination, although some imperfections just below the surface are detectable. This type of examination is limited to materials which can be magnetized (paramagnetic materials), since it relies on the lines of force within a magnetic field.
The item to be examined is subjected to a current which will produce magnetic lines of force within the item. The surface is then sprayed with a fine iron powder. The powder will align itself with the lines of force. Any discontinuity normal to the lines of force will produce a leakage field around it and a consequent buildup of powder which will pinpoint the defect. The examination must be repeated at 90° to detect discontinuities which were parallel to the original field. There are a great many variations of magnetic particle examination depending on the manner in which the field is applied and whether the particles are wet or dry and fluorescent or colored.
5. Magnetic particle examination: Magnetic particle examination is essentially a surface-type examination, although some imperfections just below the surface are detectable. This type of examination is limited to materials which can be magnetized (paramagnetic materials), since it relies on the lines of force within a magnetic field.
The item to be examined is subjected to a current which will produce magnetic lines of force within the item. The surface is then sprayed with a fine iron powder. The powder will align itself with the lines of force. Any discontinuity normal to the lines of force will produce a leakage field around it and a consequent buildup of powder which will pinpoint the defect. The examination must be repeated at 90° to detect discontinuities which were parallel to the original field. There are a great many variations of magnetic particle examination depending on the manner in which the field is applied and whether the particles are wet or dry and fluorescent or colored.
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