Nondestructive Inspection of Boilers and Pressure Vessels
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.
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 (Ref 2). 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 (Ref 3).
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 (Ref 4) 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 (Ref 5) 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 (Ref, 1). 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 (Ref 6), A 578 (Ref 7), and A 435 (Ref 8). 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.