Kuppan T. Heat Exchanger Design Handbook
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914
Chapter
14
9.7 General Procedure in Radiography
The steps of performing a radiographic examination are as follows:
1.
Surface preparation
2.
Selection of radiation source, depending upon the thickness of the part and location of
the
component/assembl
y
3.
Selection of film and exposure parameters for desired quality (i.e., sensitivity, density),
and selection of intensifying screens
4.
Processing of exposed films
5.
Interpretation of radiographs
6.
Record keeping and perservation of radiographs
9.8 Reference Documents
Radiography is one of the most widely established methods for the detection of internal
as
well as surface defects, and has been written into many national codes and standards.
ASTM
standards E94, E142, E155, E272, E390. E592,
E586,
and E747 cover radiographic examina-
tion.
ASME
Code Section
V
and VIII, Div. 1, Appendix 4, covers RT.
9.9 Safety
One of the most important considerations in x-ray or gamma-ray work is the exercise
of
ade-
quate safeguards for personnel.
9.10 Identification Marks
The identification marks such as pressure vessel number, job number, seam number, segment
number, or spot number, date, and manufacturer’s symbol should appear
in
each film as radio-
graphic images.
9.11 Location Markers
Location markers should appear as radiographic images and should be placed
on
the part, and
their locations shall be hard stamped or engraved on the surface of the part being radiographed.
Lead marker accessories containing sets of alphabets and numbers including arrow and oblique
are available
in
the market.
9.12 Processing of X-Ray Films
All the radiographs should be free from fogging and processing defects. While processing,
radiograph details should not be lost due to scratches, finger marks, etc. The Welding Engineer
Data Sheet provides guidance for troubleshooting radiographs [47].
9.1
3
Surface Preparation
Weld surfaces should be ground free of ripples
so
that the resulting radiographic image cannot
mask
or
be confused with the image of discontinuities. Prior dressing of the weld surface
ensures interpretation of films with
a
minimum amount of referring back to the welds them-
selves,
In
the case
of
submerged arc welding, no surface preparation is required provided the
weld bead is smooth and free from valleys or ridges.
QA,
QC,
Inspection,
and
NDT
91
5
9.1
4
Radiographic Techniques for Weldments
of Pressure Vessels
The common techniques used for the radiography of welds in cylindrical shapes are
(1)
single-
wall, single-image (unidirectional) (Fig. 12a), (2) single-wall, single-image (panoramic) (Fig.
12b),
(3)
double-wall, single-image for pipe welds greater than
88.9
mm (Fig. 12c), and
(4)
double-wall, double-image for pipe welds with
OD
less than or equal to
88.9
mm (Fig.
12d).
Whenever possible, single-wall technique is preferred. This technique is employed for plates,
shell welds, and castings of larger sizes. Common techniques for various fillet welds are shown
in Fig. 13.
A
diagrammatic representation of the film and source is shown in Fig. 14.
Panoramic Radiography with Isotopes
One of the important advantages using isotopes is that by a single exposure, the entire circum-
ference
of
a weld can be radiographed, provided there is accessibility
to
keep the source at the
center
of
the pipe. Panoramic radiography (Fig.
15)
shows the radiation source inserted
in
the
center through an adjacent hole
[48].
Gamma-Ray Technique
A
gamma-ray technique for branch welds is shown in Fig.
16.
SOURCE
OUERLAPPING
f
ILHS
Source
I/
d
Figure
12
Radiographic techniques for cylindrical shape testing. (a) Single wall, single image (unidi-
rectional);
(b)
single wall-single image (panoramic); (c) double wall, single image; and (d) double wall,
double image (From Ref.
33.)
91
6
Chapter
I4
Film
I
Source
Figure
13
Common
RT
techniques for corner fillet welds.
0
cl
0
0
0 0
0
Figure
14
Diagrammatic representation of the film and source.
9.1
5
Full
Radiography
ASME Code Section
VIII,
Div. 1, UW-11 lists the conditions for which the
full
RT
for the
welded joint is mandatory. Some
of
these conditions are:
1.
All
butt welds
in
the shell and heads of vessels used to contain lethal substances
2.
All butt welds in vessels
in
which the least nominal thickness at the welded joint exceeds
1.5
in
(38.1
mm).
3.
All butt welds in the shell and heads
of
unfired steam boilers having design pressures
exceeding
50
psi.
7-
Telef
lex
cable
Figure
15
Panoramic radiography.
QA,
QC,
Inspection, and
NDT
91
7
Figure
16
Gamma rays technique for branch welds. (From ref.
48.)
4.
All butt welds in nozzles, communicating chambers, etc., attached to vessel sections or
heads that are required to be fully radiographed under parts 1 or
3.
Spot Radiography
Spot radiography should be in accordance with
ASME
Code Sec.
VIII,
Div. 1, UW-52. Spot
radiography means that one spot is examined on each vessel for each 50-ft increment of the
weld or part thereof for which a joint efficiency from column (b) of Table UW-12 is selected.
For
vessels with less than
50
ft of weld, 50-ft increments of weld may be represented by one
spot examination. Length of each radiography shall be at least
6
in
(152.4
mm). Further, all
T
joints must be radiographed and at least one shot must be taken on each longitudinal and
circumferential seam (Fig.
17).
9.1
6
Radiographic
Quality
The quality of the radiographs that decides the efficiency of flaw detection is determined by
two factors: sensitivity and density.
Radiographic Sensitivity
The effectiveness of a radiographic technique to bring out the smallest details in a given
thickness of an object is referred to as
radiographic sensitivity.
The more the exactness, the
better is the sensitivity. Radiographic sensitivity is affected by two factors:
(
1)
contrast-the
magnitude of change in density produced on the film because the defect and sound material
absorb different amounts of radiation-and
(2)
definition-the amount of blurring of the pro-
jected shape of the defect produced
on
the film. The terms and definitions that best represent
Figure
17
Spot radiography.
91
8
Chapter
I4
(1) and (2) are thickness sensitivity and unsharpness, respectively [49]. In quantitative terms,
sensitivity is expressed as a percentage ratio of the smallest size of an artificially produced
defect shown on the radiograph to the thickness of the object under examination. The smaller
the numerical value of sensitivity, the better is the radiographic sensitivity. The artificial defect
in
the form of a hole, step, or wire of various dimensions is called as image quality indicator
(IQI)
or
penetrameter.
Density of Radiographs
The
density
of a radiograph refers to its darkness. Areas exposed to relatively large amounts
of radiation are known as areas of high density and areas exposed to less radiation are known
as areas of light density. The density of the radiograph should be minimum of
1.8
for x-rays
and 2.0 for gamma rays. When the density of the radiation varies by more than -15% or
+30%
from the density through the body of the penetrameter, additional penetrameters should be
placed in the exceptional areas.
Image Quality Indicators (IQI)
An
image quality indicator (IQI), also called a penetrameter (or penny), is a simple geometric
form used to judge the quality of a radiograph. It is made of the same material as or a similar
material to the component being examined. The dimensions of the
IQ1
bear some numerical
relation to the thickness of the part being examined. They are placed on the test piece during
setup and are radiographed at the same time as the test piece.
IQIs
are preferably located
in
regions of maximum test piece thickness
and
greatest test piece to film distance, and near the
outer edge of the central beam of radiation. The degree to which the image of the
IQ1
is visible
in the developed image is a measure of the quality of that image.
ZQZ
Designs.
A number of IQ1 designs are used by different authorities. There are American
standards, British standards, and French and German standards. Three basic types of IQ1 are
available. They are the wire type, step-wedge type, and strip-hole type.
The most widely used
wire-type
IQ1
is of the
DIN
type (German), conforming to
DIN
54109 Fe, Cu, and Al. It specifies a set of three parameters, each containing seven equidistant
wires. The diameter of the wires varies in geometric progression. Each diameter is represented
by a whole number. The three sets are 1-7, 6-12, and 10-16. The ASTM standard for wire-
type
IQ1
is E747 Fe, Cu, and Al. Wire-type IQ1 is shown in Fig. 18a.
The
step-wedge type
is the simplest
IQI.
This type of penetrameter
is
commonly used in
UK-BS
3971, and
in
France-AFNOR Code. It consists of small steps, whose thickness
(0.5
to
5%
of specimen thickness) increase in geometric progression. Step-wedge type
IQ1
is shown
in
Fig. 18b. Step-hole type is step-wedge type with holes.
For the
strip-hole type
IQI,
the most common and widely used version is the ASTM or
ASME penny. Strip-hole
IQ1
is
also known as plaque-type IQI, whose thickness is about
2%
of the specimen thickness. This consists of a small rectangular piece of metal with appropriate
material with three holes. The diameters of the holes are multiples of thickness: IT,
2T,
and
4T, where T is the thickness of IQI. Plaque type
IQ1
are described in ASTM
E
142/ASME
SE
142 Fe, Cu, Al, Monel, and Ti. Strip-hole type
IQ1
is shown in Fig. 18c.
Number
of
IQIs
One
IQ1
should be used for each radiograph, generally. In case of radiography using a pan-
oramic exposure, three IQIs placed at 120" apart is accepted.
How to Calculate
IQ1
Sensitivity
An IQ1 is used to indicate the quality of radiographic technique and not to measure the size
of
a defect that is shown.
--
QA,
QC,
Inspection,
and
NDT
91
9
6
Wires
equaly
spaced
ASTM
Minimun
25
Ws(1
inch)
Minimum
5
CMs(2
inches)
for
setsCSLD
.1
A
I-
a
I
--
U.S.
Bureau
of
ships
desiq
Step type
----
-
Step-Hole
type
B.W.R.A.
Designs
A.
F.N.O.R.
Desians
b
C
Figure
18
IQ1
types.
(a)
Wire;
(b)
step wedge; and
(c)
strip hole.
920
Chapter
14
Sensitivity for wire-type
IQ1
is based on the smallest size of wire diameter visible, and
for strip-hole type
IQ1
it is based on diameter
of
the smallest hole:
smallest size
of
penny visible
Percent of sensitivity
=
x
100
thickness
of
the object
For the strip-hole type or plaque-type
IQIs,
the
IQ1
image and the specified hole are the
essential indications of sensitivity. The thickness of the
IQ1
and the hole diameter to be seen
are generally specified in the code. The following are the different sensitivity levels of inspec-
tion with strip-hole type pennies:
1-1T 2-1T &IT
1-2T 2-2T &2T
1-4T 2-4T 4-4T
Normally the image of 2-2T hole should be visible in the radiograph. Critical components
require a level of 1-2T or 1-lT, and less critical components may need only a quality level
of
2-4T or 4-4T.
Examination of Radiographs
The examination of radiographs of welds should be made on the original films using a viewing
device of suitable illuminating power. The standard of acceptance shall be made with respect
to the reference chart of x-rays given
in
ASME Code Section
VIII,
Div.
1,
Appendix 4.
X-Ray Clues to Welding Discontinuities
When interpreting radiographs of weldments, one must
look
for different types of discontinu-
ities (undercut, porosity, lack
of
fusion, etc.). All discontinuities, however, must be considered
before deciding to accept or reject and to correct welding practices. Some
of
the common weld
defects and their typical radiographic appearances are given in Table 10.
Acceptance Criteria
For porosity, radiographs are often compared with charts showing arbitrary arrays of pores.
The charts are intended to limit the porosity to a certain percentage
of
the area on the radio-
graph. Slag inclusions are usually accepted on the basis of length (the only dimension readily
Table
10
Weld Defects and Radiographic Appearances
[6,
501
Defect Image
Crack
Appears as a fine, dark, zig-zag irregular line, may be branched.
Lack of fusion
Very narrow, straight dark line, parallel to the weld image.
Lack of penetration
Continuous or intermittent dark straight line at the center of the weld
seam.
Tungsten inclusions
Since tungsten has a higher atomic number than the parent metal,
namely iron, the radiographic indications are white distinct marks.
Void
Looks rounded and black, and readily recognized with well defined
outlines.
Slag inclusions
Look less distinct than voids. Dark indications and irregular shapes.
May appear singly or be linearly distributed or scattered
throughout the weldment.
Porosity
or
gas inclusion
Comes in clusters, or in random fashion. The image will be normally
or
blow hole
round dark spots with smooth edges.
921
QA,
QC,
Inspection,
and
NDT
measured by radiographs). Planar defects (cracks, lack of fusion etc.) are normally not permit-
ted [49]. An interpretation is complete and a better conclusion is arrived at only after a thor-
ough visual examination of the part.
For acceptance criteria regarding image density, relevant indications, maximum size
of
rounded indications, aligned rounded indications, spacing, clustered indications, etc
.,
refer to
ASME Code Section VIII, Div.
1,
Appendix 4, and Section
V,
or the
IIW
collection of refer-
ence radiographs.
Documentation
When reporting and documenting the results of radiographic film interpretation, complete and
accurate information must accompany the radiographs. This is required for subsequent cus-
tomer review, and possibly regulatory agency review. As per ASME Code Section
V,
the films
shall be retained up to
5
years, by the manufacturers.
9.1
7
Recent Developments in Radiography
Over the last few years, several new radiographic techniques have been developed. Important
developments in the field of radiography are discussed in Ref. 27 and Hellier [51]. Some of
the developments follow.
Tomography
Computer tomography in radiography is a three-dimensional (3D) inspection technique that
can be applied for various kinds of objects and materials. This method was adapted from the
medical computerized axial tomography (CAT) scanner. The method involves the repeated
scanning of a specimen with a narrow beam of radiation in a series of steps around the speci-
men. The radiation impinges on a linear array of detectors whose output is digitized and
provides a reconstruction of an image or slice, which allows a view of the specimen perpendic-
ular to the direction of the radiation [52].
Neutron Radiography
This refers to radiographic inspection using neutrons rather than electromagnetic radiation.
Neutrons bring out information that is not revealed by x-rays or gamma rays, because the
absorption mechanism of neutrons is different from that of x-rays and gamma rays.
Radioscopy
Radioscopy is similar to radiography; the only difference is that an image receptor replaces
the radiographic film. The receptor is an image intensifier coupled to a low-light-level camera.
The image shows up
on
a high-resolution monitor. An operator views the image when it
appears on the screen; no film processing is involved. This feature provides on-line examina-
tion of longitudinal and circumferential pipe and shell welds. Radioscopy is nearly 10 times
as fast as film radiography, considering time to develop film and interpret radiographs, and the
image can be recorded on videotape [53]. Radioscopy is included in ASME Code Section
V.
Fluoroscopy
X-ray images can be made visible instantaneously by use of a fluorescent screen instead of a
photographic film. Thus, this provides a process of real time image viewing. For permanent
records, fluoroscopy images can be photographed (fluorography
).
Gammascopy
Gammascopy systems with image processing facility are now commercially available for in-
spection of thick structural parts with iridium- 192 or cobalt-60 sources. The sensitivity and
resolution obtainable in the gammascopy are inferior to those of film radiography.
922
Chapter I4
Digitization of Radiographs and Laser Scanner System
Although radiographic film remains the main image capture medium in many applications, it
has disadvantages in long-term storage of images because of the possibility of image quality
loss
over time, difficulty of access and retrieval, and strict storage environment requirements
[51].
NDT Scan, a recent development of DuPont NDT Systems, converts high-quality film
radiographs from analog to digitized form, which offers the prospects of image storage in
a
digital format on optical media, analysis, recall, and simultaneously providing solutions to the
problems associated with film.
Imaging Plate
This is a high-sensitivity imaging plate that essentially replaces conventional radiographic
film
while utilizing standard radiographic equipment. It
is
estimated that the imaging plate can
be
reused about
40,000
times
[51].
High-Energy Radiography
Radiation sources such as synchrotrons and betatrons are available.
Gamma Radiography
The development of modern gamma cameras permits safe and reliable remote operations.
Apart
from the commonly used isotopes, a number
of
other sources as europium, ytterbium, etc. are
finding increasing applications
[27].
Microfocus X-Rays Source
The microfocus x-rays source can produce extremely sharp radiographs at
exceedingly
short
film-to-focus distances. An outstanding application is the radiography of
bore-side tube-to-
tube-sheet weld of steam generators as shown in Fig.
19
[27].
0
Q/FILM
CASSEftE
0
ROD
ANODE
Figure
19
Microfocus
X-rays
source for bore
side
tube-to-tubesheet
welding.
(From
Ref.
27.)
QA,
QC,
Inspection, and
NDT
923
10
ULTRASONIC TESTING (UT)
Ultrasonic testing (UT) is one of the most widely used methods of nondestructive inspection.
This method uses sound waves having frequencies in the megahertz range. In UT technique,
high-frequency sound waves are transmitted through the material being inspected. The sound
waves travel through the material with some loss of energy (attenuation) and are reflected at
interfaces or discontinuities such as cracks, flaws, inclusions, and seams, as illustrated in Fig.
20
[29,54].
The reflected sound beam from the flaws or interface is detected and analyzed to
define the presence and location
of
discontinuities or the thickness
of
the material. It is also
especially suited for determining characteristics
of
engineering materials such as elastic moduli,
study of metallurgical structure, grain size, and density variation.
10.1
Test Method
The primary ultrasonic testing techniques and their applications are
[55]
:
1.
Pulse echo: most effective for cracks, inclusions and thickness measurements.
2.
Through transmission: most effective for porosity, grain size and density measurements.
3.
Resonance: most effective for detection of thickness and laminar discontinuities.
There are three modes of ultrasonic vibrations that are frequently used in ultrasonic testing
of materials. These are (1) longitudinal waves (Fig. 21a),
(2)
transverse waves (Fig. 21b), and
(3) surface or Rayleigh waves (Fig. 21c).
Most ultrasonic weld testing is done manually by an operator manipulating an ultrasonic
probe over the test object and visually monitoring the screen of an oscilloscope. The pulse-
echo method with A-scan data presentation is most commonly used for inspecting welds. This
system utilizes a cathode ray tube (CRT) screen to display the test information. The schematics
of A-scan instrument is shown in Fig. 20c.
10.2
Application
of
Ultrasonic Technique in
Pressure Vessel Industry
This technique’s primary application is the detection and characterization of internal disconti-
nuities [28]. It is also used to determine effectiveness of weld overlay bondingkladding, and
to measure thickness [33]. The information possible to obtain from the UT data includes defect
location, orientation, size, and type. Only ultrasonics and radiography can substantially reveal
subsurface flaws.
Ultrasonic Plate
Tester.
Ultrasonic plate testers (Fig. 22) are available to perform edge-to-
edge testing before cutting or welding operations in order to check quickly the presence of
laminations, gross internal discontinuities such
as
pipes, ruptures, or slag inclusions. Shell plate
areas on which the branches are to be welded are checked ultrasonically to ensure that the
steel is free from cluster inclusions. This is to avoid any likelihood of lamellar tearing due to
stress introduced by welding [43].
Ultrasonic Examination
of
Nozzle Welds.
Ultrasonics is of prime importance for examining
the attachment welds of nozzles and branches where the wall thickness involved is in the order
of 25 mm
or
greater
[43].
Various
automated and semiautomated equipment has been devel-
oped for double-fillet type through nozzles. The branch weld is normally subject to
100%
examination before and after weld heat treatment. The equipment almost invariably employs a
longitudinal wave probe operated from the bore of the nozzle. Figure 23 shows the UT exami-
nation of a defect in double-fillet nozzle welding.