DURING THE FABRICATION of a boiler, pressure vessel, and such related components as boiling water reactor piping or steam generator tubes, various types of nondestructive inspection (NDI) are performed at several stages of processing, mainly for the purpose of controlling the quality of fabrication.
In-service inspection is used to detect the growth of existing flaws or the formation of new flaws. This can be done while the vessel is in operation or down for servicing. The inspection methods used include visual, radiographic, ultrasonic, liquid penetrant, magnetic particle, eddy current, and acoustic emission inspection, as well as replication microscopy and leak testing. The assurance of component quality depends largely on the adequacy of NDI equipment and procedures and on the qualification of personnel conducting the inspection.
In many cases, nondestructive inspection, both prior to and during fabrication, must be done to sensitivities more stringent than those required by specifications. The use of timely inspection and rigid construction standards results in the reduction of both the costs and delays due to rework. Quality planning starts during the design stage. For inspections to be meaningful, consideration must be given to the condition of the material, the location and shape of welded joints, and the stages of production at which the inspection is to be conducted. During fabrication, quality plans must be integrated with the manufacturing sequence to ensure that the inspections are performed at the proper time and to the requirements of the applicable standard.
In the newest nuclear plants, quality design planning includes:
- Avoidance of complex weld geometries to facilitate attachment of ultrasonic transducers to the surface at the best positions
- The increased use of ring forgings for pressure vessel components; this means that there are no
- longitudinal welds that have to be inspected in service. The result is a reduction in the amount of inservice inspection and man-rem exposures
- Incorporating large numbers of access points for introducing mechanized inspection equipment, which can be operated remotely, thus avoiding exposures to operators and enabling more accurate processing than is possible with handheld inspection equipment
- The elimination of welds between cast austenitic components; inspection of welds through cast welds is difficult because they are opaque to ultrasonic inspection to a large degree.
Nondestructive Inspection of Boilers and Pressure Vessels
Boiler and Pressure Vessel Code and Inspection Methods
Pressure vessels--both fossil fuel and nuclear--are manufactured in accordance with the rules of the applicable American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code. For nuclear vessels, section XI of the ASME code establishes rules for continued nondestructive inspections at periodic intervals during the life of the vessel. One feature of the rules in section XI is the mandatory requirement that the vessel be designed so as to allow for adequate inspection of material and welds in difficult-to-reach areas. Section III of the code describes the material permitted and gives rules for design of the vessel, allowable stresses, fabrication procedure, inspection procedure, and acceptance standards for the inspections. Pressure vessels are constructed in various sizes and shapes, and some of the largest are those manufactured for the nuclear power industry. Some pressure vessels are more than 6 m (20 ft) in diameter and 20 m (70 ft) in length and weigh almost 900 Mg (1000 tons). Thickness of the steel in the walls of these vessels ranges from about 150 mm (6 in.) to more than 400 mm (15 in.), although many pressure vessels and components are fabricated from much thinner material. Joining of the many vessel sections is accomplished by welding. Welders of pressure vessels are qualified according to section IX of the ASME Boiler and Pressure Vessel Code, and welding is done in accordance with qualified welding procedures.
Nondestructive inspection of welds is only a part of the inspection requirements; the materials themselves must be inspected prior to welding. For pressure vessels that are not constructed according to the ASME code, it is a matter of agreement between the manufacturer and the user as to whether NDI methods are to be employed and which method or methods are to be used.
Nondestructive Inspection Methods. An appendix to each section of the ASME code establishes the methods for performing nondestructive inspection to detect surface and internal discontinuities in materials. Four inspection methods are acceptable: radiographic, magnetic particle, liquid penetrant, and ultrasonic inspection. All these methods are mandatory for nuclear vessels, for section III, and for division 2 of section VIII of the code. Ultrasonic inspection is listed in division 1 of section VIII of the code as nonmandatory. Leak testing, eddy current inspection, acoustic emission inspection, and visual inspection are included in section V. Details as to which method is to be used and the required acceptance standards are specified in the appropriate articles on materials and fabrication. All NDI personnel must be qualified and certified to SNT-TC-1A procedures.
Radiographic Inspection. Methods of radiographic inspection are extensively detailed in the ASME codes; radiography using either x-rays or radioisotopes as the radiation source is permitted. Radiography is the oldest inspection method detailed in the codes and is probably the most understood and the most widely accepted. A principal reason for its wide use is that radiography provides a permanent record of the results of the inspection. This record is important because the inspector can review the radiographs at any time to ensure that federal, state, or insurance requirements have been met. Acceptance standards were developed according to the limits of radiography (what can or cannot be detected by the method) and by the quality level obtainable by the manufacturing practices used to produce the vessels. Essentially, the acceptance standards do not permit the existence of indications of the following types of flaws: cracks, incomplete fusion, incomplete penetration, slag inclusions exceeding a given size that is not related to the thickness of the part, and porosity that exceeds that presented in illustrated charts provided in the codes. These standards result from the ability to distinguish among porosity, slag, and incomplete fusion in the radiograph; more important, they also mean that no indications of cracks or of incomplete fusion are permitted.
Magnetic Particle Inspection. The procedures for magnetic particle inspection reference ASTM E 709 or section V of the ASME code for the method. Acceptance standards permit no cracks, but rounded indications of discontinuities are permitted provided they do not exceed a certain size or number in a specified area. Magnetic particle inspection is universally used on ferromagnetic parts, on weld preparation edges of ferromagnetic materials, and on the final welds after the vessel has been subjected to the hydrostatic test. A magnetic particle inspection must be conducted twice on each area, with the lines of magnetic flux during the second application at approximately 90° to the lines of magnetic flux in the first application. Depending on the shape of the part and its location at the time of inspection, magnetization can be done by passing a current through the part or by an encircling coil and sometimes by a magnetic yoke. The acceptance level is judged by a qualified operator and is subject to review by an authorized code inspector.
Liquid penetrant inspection is usually employed on nonferromagnetic alloys, such as some stainless steels and highnickel alloys. The acceptance standards are the same as for magnetic particle inspection and are also judged by an operator, subject to review by a code inspector. The methods are specified to those contained in ASTM E 165 or section V of the ASME code. Water-washable, postemulsifiable, or solvent-removable penetrants can be used. A waterwashable color-contrast penetrant is usually employed because it is easy to handle, requires no special ventilation, and is nontoxic. Sometimes, special requirements dictate the use of either a solvent-removable color-contrast penetrant or a fluorescent penetrant.
Ultrasonic inspection is used to inspect piping, pressure vessels, turbine rotors, and reactor coolant pump shafts. Straight-beam ultrasonic inspection is specified to detect laminations in plates and to detect discontinuities in welds and forgings. This technique is described in general and specific terms in section XI of the ASME code, in the United States Nuclear Regulatory Commission Regulatory Guide 1.150 (Ultrasonic Testing of Reactor Vessel Welds During Preservice and Inservice Examinations), and in companion reports written by utility ad hoc committees. Angle-beam inspection is specified for welds, and a more detailed procedure is presented, including reporting requirements, It is mandatory, however, that ultrasonic inspection, either by straight beam or angle beam, be conducted to a detailed written procedure. These procedures are usually developed by the manufacturer. Specifications and standards for steel pressure vessels are given in ASTM A 577, A 578, and A 435. Acceptance standards for the inspection of welds by ultrasonics closely parallel the acceptance standards for radiography. Cracks, incomplete fusion, and incomplete penetration are not permitted. The size permitted for other linear indications is the same for the slag permitted by radiography. However, ultrasonic inspection can detect cracks better than radiography, but it is sometimes difficult to separate cracks from other linear indications by ultrasonics. Furthermore, ultrasonic inspection procedures refer to the amplitude of the signal obtained from a calibration notch, hole, or reflector placed in a standard reference block, but not all slag inclusions or cracks in an actual workpiece present a similar response to that obtained from the artificial calibrator. Advanced ultrasonic systems (see the section "In-Service Quantitative Evaluation" in this article) and the improvements in codes and regulations have combined to make ultrasonic inspection one of the most commonly used nondestructive methods in the power industry. Advanced ultrasonic methods are intended to ensure that the vessel remains fit for continued service by detecting and sizing defects that could degrade structural integrity.
Acoustic emission (AE) inspection has been used for the following applications:
- Inspection of chemical and petrochemical vessels
- Monitoring nuclear plant components or systems during hydrotests, plant operation, or preservice pressure testing of the primary system
- Monitoring during pressure testing of intentionally flawed vessels
- Monitoring fiber-reinforced plastic tanks, with the major problems being associated with poor
- manufacturing techniques that allow dust and other foreign objects to be mixed in with the resin
- Monitoring liquefied petroleum gas storage tanks. The main problems associated with this type of test program include the different propagation paths and attenuation coefficients caused by the geometry of the vessel and the correct transducer locations and spacing
Rupture tests on experimental vessels with wall thicknesses up to 150 mm (6 in.) have been monitored, and such tests help define the acoustic emission response patterns that can be used to recognize incipient vessel failures. However, many of the vessel rupture tests monitored by acoustic emission have been conducted mainly to provide fracture mechanics data, and the acoustic emission monitoring was an add-on feature.
Acoustic emission tests are often conducted during preshut down operations in an effort to identify areas requiring special maintenance or during special tests while under slightly varied operational conditions. Data on wave propagation and failure mechanisms have been recorded and used to develop reliable acoustic emission evaluation techniques. The advancement of AE inspection techniques includes the introduction of an AE methodology into Section V, Article 12, of the ASME Boiler and Pressure Vessel Code as a December 1988 Addendum. Other AE methodologies in the inspection of fiber-glass and metal pressure vessels are described in the article "Acoustic Emission Inspection" in this Volume.
Eddy Current Inspection. One of the rapidly increasing applications for this method is the inspection of thin-wall (0.9 to 1.5 mm, or 0.035 to 0.060 in.) Inconel alloy steam generator tubing and heat exchanger tubing. The focus of steam generator tube inspection by eddy current has shifted from concentrating solely on the detection and characterization of tube wall denting and wastage to more complex tube wall degradation mechanisms. The newer degradation mechanisms consist of intergranular attack (IGA), stress-corrosion cracking (SCC), mechanical wear, and pitting in the presence of copper.. In response to the complexity of these newer problems, eddy current instrumentation has also evolved from a single-frequency to a multiplefrequency configuration. The analog instrumentation has been replaced by digital multiple-frequency instrumentation, offering more consistent data acquisition. The wider dynamic range offered by the digital instrumentation allows analysis of eddy current data obtained from traditionally difficult areas, such as dented tube support/tube sheet interfaces and roll transition/roll expansion areas within the tube sheet. Additional information, including typical data produced by multiplefrequency instrumentation, can be found in the article "Eddy Current Inspection" in this Volume.Another advancement is that of remote-field eddy current testing, which has been used to examine nuclear fuel rods and other tubular products. The article "Remote-Field Eddy Current Inspection" in this Volume can be consulted for details and results of analyses.
Replication microscopy, or field metallography, is an effective and economical means of obtaining an image of a component surface, permitting both on-site and laboratory examination and evaluation of the metallurgical condition of the material (see the article "Replication Microscopy Techniques for NDE" in this Volume). Material sample removal from critical components is a costly and time-consuming process that often necessitates part replacement, weld repair, and stress relief. The replication examination is completed without having to cut out a portion of the component, and the metallurgical data can be determined in fine detail. Therefore, material evaluation can be performed with a nondestructive and cost-effective method that does not require the removal of material samples.
Specimen (Surface) Preparation. Critical surface locations for inspection are identified, and external insulation is removed. The area to be replicated is polished with progressively finer grits and compounds. The final compound is a 0.25 m diamond polishing paste, which ensures a surface finish of 1 m or better. The surface is then etched to reveal the microstructural detail. A plastic film is applied, cured, and carefully removed. The surface of the film in contact with the polished area retains a precise, reverse image of the etched component surface.
The principal application of the replication technique is in revealing metallurgical anomalies and incipient damage (cracking). In the absence of fatigue and/or preexisting defects, creep cracks are theorized to initiate by the formation,growth, and linkup of voids into microcracks, which in turn consolidate to form macrocracks. The propagation of the macrocracks can then lead to final failure. In actuality, a creep-fatigue interaction better describes the type of damage mechanism most often encountered. It is theorized that the creep mechanism described previously is accomplished by a fatigue mechanism. In a creep-fatigue interaction, one may observe a mixture of intergranular and transgranular crack growth surfaces with the absence of voiding. The presence of preexisting flaws affords an initiation site for cracking and subsequent material degradation.