Kuppan T. Heat Exchanger Design Handbook
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904
Chapter
I4
1. White light illumination for color contrast processes.
2.
Ultraviolet illumination for fluorescent processes.
3.
Ambient visible light in darkened inspection rooms.
4.
Visible light from ultraviolet sources.
The fluorescent light is more sensitive, due to the fact that the human eye can more easily
discern a fluorescent indication.
7.1
5
Evaluation
of
Indications
Prior to the evaluation of test results, it is necessary to interpret the indications. Identifying
indications requires much practice. Indications can be classified as:
(1)
false indications,
(2)
nonrelevant indications, and
(3)
true or relevant indications. False indications are due to im-
proper cleaning, rough surfaces, contaminations due to penetrants on the hands of operator,
etc. Nonrelevant indications are caused by surface irregularities. All nonrelevant indications
shall be regarded as a defect until the indication either is eliminated by surface conditioning
or
is evaluated by other NDT methods and proved to be nonrelevant. True indications are
those caused by discontinuities in the specimen. Only those indications are to be studied while
evaluating the results of the examination.
7.1
6
Acceptance Standards
For welds and materials as per ASME Section VIII Div.
1,
Appendix
8,
all surfaces to be
examined shall be free of (1) relevant linear indications,
(2)
relevant rounded indications
greater than
%6
in
(4.8
mm), and
(3)
four or more relevant rounded indications in a line
separated by
I/ih
in (1.6 mm)
or
less edge to edge.
7.17 Postcleaning
Penetrants are difficult to remove completely from discontinuities, and if they are corrosive
to
the material, or otherwise not compatible with the product application, they should be avoided.
7.18 Recent Developments in PT
PT
accessories are available from Magnaflux, a Division of ITW, Inc., USA. They include
Ni-Cr test panels (Japanese) and TAM panels. Ni-Cr test panels are used for evaluating
penetrant system performance and sensitivity; they are available in four types of flaw depths,
10,
20,
30,
or
50
pm. A TAM panel
is
a fast and reliable means of monitoring the proper
functioning of a liquid penetrant system. A stainless-steel panel with five star-shaped flaws of
decreasing size is on one half, with a rough sand-blasted surface on other half, for checking
sensitivity and removability
of
penetrants.
8
MAGNETIC PARTICLE INSPECTION (MT)
Magnetic particle inspection is a nondestructive testing technique to locate surface and subsur-
face discontinuities such as cracks, inclusions, pores, shrinkages, laps, folds, seams, etc., aris-
ing out of manufacturing operations and service constraints in ferromagnetic materials. Ferro-
magnetic materials include most of the iron, nickel, and cobalt alloys. Nonmagnetic metals
such as aluminum alloys, copper and copper alloys, titanium and its alloys, and austenitic
stainless steels and cladding of these metals cannot
be
inspected by this method.
QA,
QC,
Inspection,
and
NDT
905
8.1
Principle
Magnetic particle inspection is based on the principle that if a ferromagnetic object is magne-
tized, the surface or subsurface discontinuities lying at an angle to the magnetic lines of force
cause an abrupt change in the path of magnetic flux flowing through the object. This results
in local flux leakage at the surface over the discontinuity (Fig. 9). When finely divided ferro-
magnetic particles are applied over the surface, some of the particles will be attracted to the
regions of leakage field and will pile up and bridge the discontinuity. The piling up of magnetic
particles along the discontinuity indicates its location, shape, and extent. While using this
method, it should be seen that the magnetic field must be in a direction that will intercept the
principal plane of the discontinuity.
8.2
Applications
Magnetic particle inspection
is
used for testing of weld preparations, weld cutbacks on double-
sided welds, roots of welds made from one side only, stub, branch, and nozzle welds and
attachment welds to shell, load-carrying nonpressure parts like supports and lifting lugs, and
areas where temporary attachments have been removed. The latter areas are particularly vulner-
able to cracking [43].
8.3
Reference Documents
ASTM
Specifications
E
109-Dry powder magnetic particle inspection
E
138-Wet powder magnetic particle inspection
E
125-MT inspection of ferrous castings
E
269-Definitions of terms related to MT
E
275-MT inspection of steel forgings
E
340-Definition of terms, symbols, and conversion factors relating to MT
Figure
9
Magnetic particle testing principle-local
flux
leakage at the surface over discontinuity.
906
Chapter
14
ASME Section V, Article
7,
and ASME Section VIII, Div.
1,
Appendix
6
and 7 (for steel
castings) are also used.
8.4
Test Procedure
There are several basic steps in applying magnetic particle testing:
1.
Surface preparation
2.
Magnetization of the test object under evaluation
3.
Application of magnetic particles
4.
Interpretations of the patterns formed
5.
If required, demagnetization of the items being tested.
8.5 Factors Affecting the Formation and Appearance
of
the
Magnetic Particles Pattern
There are many factors that can affect the formation and appearance of the magnetic particles
on the surface of the component being tested. The most common include [41,44]:
(1)
disconti-
nuity size, shape, distance below the surface, and its orientation,
(2)
direction and strength
of
the magnetic field,
(3)
method
of
magnetization employed, (4) intensity of current used,
(5)
size, shape and mobility
of
the magnetic particle and the method
of
application, and
(6)
shape
of the component.
8.6 Merits
of
Magnetic Particle Inspection
For detection of surface defects, liquid penetrant testing and magnetic particle examination are
being very widely used. In the case of ferromagnetic materials, magnetic particle technique
will also detect subsurface flaws that are not open to surface. Important advantages of the
MT
method are [41]:
(1)
it is rapid and simple to operate,
(2)
indications are produced directly on
the surface of the component, and
(3)
a skilled operator can make a reasonable estimate of
crack depth with suitable magnetic powders and proper technique.
8.7
Limitations of the Method
Magnetic particle testing has certain limitations, like
(1)
the method is not applicable for non-
magnetic metal,
(2)
the sensitivity to detect subsurface flaw decreases exponentially with dis-
tance below the surface,
(3)
exceedingly heavy currents are required at times to examine very
large units, and (4) current imparted by electrodes to induce magnetic fields can cause arc
burns, which must be avoided because they can be a source of corrosion attack.
8.8
Written Procedure
The written procedure should contain as minimum requirement the items given in Table
8.
Magnetic Particle Examination Procedure Deficiencies
Deficiencies may include (1) incorrect method; e.g., coil method specified for specimens hav-
ing
L/D
ratios less than
2, (2)
incorrect magnetizing currents,
(3)
failure to perform examina-
tion at more than one current on complex sections, and (4) incorrect sequence of examination
or failure to demagnetize when the second operation produces a lower
flux
density than the
first [2].
QA,
QC,
Inspection,
and
NDT
907
Table
8
Written Procedure for MT
1.0
Scope: materials, shapes, and sizes to be
7.0
Magnetization current (type and amperage)
examined
7.1
Magnetizing field adequacy
2.0
Reference document
8.0
Application of examination medium
3.0
Magnetizing technique
9.0
Evaluation of indication
4.0
Examination equipment
10.0
Acceptance criteria
5.0
Surface preparation
11
.O
Demagnetization
6.0
Examination medium: type
of
ferromagnetic
12.0
Reports
particles to be used, manufacturer, color, wet
13.0
Records
or dry powder, etc.
8.9
Magnetizing Current
For the purpose of magnetizing parts for magnetic particle inspection, either alternating current
(which has relatively little penetrating power and hence is good for surface flaw detection) or
direct current (good for the detection of subsurface discontinuities) or half-wave rectified alter-
nating current, which has the merits of
DC
and AC, is widely used. Since
it
is difficult to
correlate the results obtained with AC with the result obtained using DC,
it
is important to
specify
in
the specifications or drawings whether AC or DC is to be used. In the event that
this is not specified, DC shall be used.
8.1
0
Equipment
for
Magnetic Particle Inspection
The magnetic particle test equipment essentially consists of
(1)
a means to magnetize the
component,
(2)
a device for application of magnetic particles, and
(3)
demagnetization of
components (if required) after inspection.
8.1
1
Magnetizing Technique
There are numerous methods of magnetization. Some of the methods include (1) coil magneti-
zation
(Fig.
lOa),
(2)
prod magnetization (Fig. lob), and
(3)
yoke magnetization (Fig. 10c).
Prod Magnetization
Prod magnetization produces a circular magnetic field. This will detect defects that are length-
wise in the test piece. Prods are portable current-carrying copper conductors, which are used
to magnetize localized areas as shown in Fig. lob. Magnetization is accomplished by pressing
the prods against the surface area to be examined. When current flows through the prods, a
circular magnetic field is created in the test part. The magnetic flux is perpendicular to the line
joining the two prods, and the flux density is greatest along this line. During prod magnetiza-
tion the prods are spaced
150
to
200
mm apart and in line with the suspected discontinuities.
AC, DC, or half-wave DC (HWDC) can be used to suit the job requirement. Mostly HWDC
magnetization is used for prod magnetization. Normally
100
to
120
A
current per inch of prod
spacing would be required. Care must be taken to avoid burning of the part under the contact
points.
To
avoid arcing, a remote control switch
is
built into the prod handles to permit the
current to be turned on after the prods have been properly positioned.
Direction of Magnetization
At least two separate examinations shall be performed on each area. During the second exami-
nation, the lines of magnetic
flux
shall be approximately perpendicular to those used during
the first examination. Examination shall be done by the continuous method.
908
Chapter
I4
aocks
detected
Defect
\
Magnetic
/fitld
Cable
ines
of
magnetic
forces
racks
parallel
to
lmes
of
fom
will
show
lines
of force
will
not
show
b
art
iclr
Magnetic
lines
at
fme
Figure
10
Magnetising techniques. (a) Coil magnetization;
(b)
prod magnetization; and
(c)
yoke
magnetization. (Courtesy American Welding Society.)
909
QA,
QC,
Inspection,
and
NDT
Prod Spacing
Prod spacing should not exceed
8
in (203.2 mm). Shorter spacing may
be
used to accommodate
the geometric limitations of the area being examined or to increase the sensitivity, but not less
than
3
in
(76.2
mm).
Yoke Magnetization
Yoke magnetization introduces a longitudinal field to show up defects perpendicular to the
flaw, or across the surface of the part. Both permanent and electromagnetic yokes are used.
Permanent magnetic yokes have magnetic strength limitations but are most useful for spot
inspection
of
welds. They can be used when electricity is not available. Electromagnetic yokes
are U-shaped with a coil to cany current as illustrated in Fig. 10c. Longitudinal magnetic fields
are set up
in
the test specimen when the coil is energized either by AC or DC current. Alternat-
ing current electromagnetic yokes should have a lifting power of at least 10 lb at the maximum
pole spacing. Directions of magnetization should be same as for the prod technique. Pole
spacing should be limited to
2
to
8
in
(50.4
mm to 203.2 mm).
Examination Coverage
All the examinations should be conducted with sufficient overlap to ensure 100% coverage at
the required sensitivity.
8.1
2
Inspection Medium (Magnetic Particles)
The inspection medium shall consist of finely divided ferromagnetic particles. They must have
good mobility, very fine size and shape, high permeability, and low retentivity. The particles
may be in dry powder form (for the
dry
method) or suspended
in
a suitable liquid medium
such as water
or
kerosene (for the wet method). The size of the dry particles is about
60
pm
and a blend
of
elongated and spherical particles is used to attain good sensitivity and mobility.
The wet particle size is about
40
to
60
km.
Application of Examination Medium
The magnetic particles should be applied in such a manner that a
thin
uniform dust-like coating
settles upon the surface
of
the test part while the part is being magnetized. Specially designed
powder blowers are used for this purpose.
8.1
3
Inspection Method
According to the condition
of
the magnetic particles used for inspection, the inspection tech-
niques are known as the dry method and the wet method.
Dry Method
In the dry method, dry magnetic particles are used for examination. The dry powder method
is preferred for surface and subsurface defects
[44].
The powders are available in colors that
will contrast with the background of the part being examined. For example, black or red
powders are for use with light-colored surfaces, grey on dark, and yellow on either. The mag-
netic particles are sprinkled over the surface by using a powder blower operated by a low-
pressure air supply. The particles line up along the discontinuities as they reach the surface.
When using dry particles, test sensitivity depends to
a
large extent on how the excess particles
are removed. In the field, a small syringe or powder blower is used to push excess parti-
cles off the test area under a light burst of air without disturbing the lightly held powder
particles along the discontinuities. Inspection by the dry method is carried out with portable
magnetizing equipment.
910
Chapter
14
Wet Method
In the wet method, the magnetic particles are suspended in a liquid base of oil or water. The
magnetic particles used in this method are finer than those used for the dry method, which
makes the wet method more sensitive to detection of fine surface defects. Two types
of
wet
magnetic particles are used. They are (1) visible magnetic particles, either reddish or black,
observed under normal white light, and
(2)
fluorescent magnetic particles, which fluoresce
when exposed to near-ultraviolet (black) light. The inspection bath is mostly flowed, sprayed,
or brushed over the specimen surface. Most magnetic particle inspection by the wet method is
done with stationary-type equipment, although portable equipment may be used.
Comparison of
Dry
Method and Wet Method
The dry powder method is more sensitive than the wet powder method for the detection
of
near-surface discontinuities but not as sensitive as wet method in detecting fine discontinuities.
The dry method is convenient to use in conjunction with portable equipment for the inspection
of large components or for field inspection, whereas the wet method
is
used with stationary-
type equipment.
8.14 Surface Preparation
Surfaces to be inspected shall be clear and free from oil, grease, sand, loose rust, scale, ripples,
welding spatters, or any other surface contaminants that could interfere with the interpretation
of the magnetic particles indications. Surface preparation by grinding or machining may
be
necessary where surface irregularities could mask indications due to discontinuities. Magnetic
particle test shall not be performed if the surface temperature of the part exceeds
600°F.
8.1
5
Evaluation
of
Indications
The types of indications obtained by magnetic particle inspection due to “piling up”
of
the
magnetic particles at the lines of flaws are as follows:
1.
Relevant indications are those that result from mechanical discontinuities.
2.
Linear indications are those indications in which the length is greater than three times the
width.
3.
Rounded indications are circular or elliptical with the length equal to or less than three
times the width.
4.
Nonrelevant indications may arise due to metallurgical discontinuities and magnetic per-
meability variations. These nonrelevant indications shall be reexamined by other suitable
NDT methods such as
FT
and ET.
8.1
6
Demagnetization
When the residual magnetism in the part could interfere with subsequent processing or usage,
the part shall be demagnetized after completion
of
the test.
8.17
Record
of
Test Data
The information such as material, method used (dry or wet), type
of
magnetization, type
of
current, amount of current, and nature of defects should be recorded at the time of each test
for future reference.
QA,
QC,
Inspection, and
NDT
91
I
8.18 Interpretation
1.
Surface defects appear sharp and distinct.
2.
Subsurface flaws appear rough and fuzzy.
3.
Width of surface flaw indication varies with depth.
8.19 Acceptance Standards
For pressure vessels, heat exchangers, fittings, and components manufactured to ASME Section
VIII Div.
1,
as per Appendix
6,
all surfaces to be examined shall be free of
(1)
relevant linear
indications,
(2)
relevant rounded indications greater than
%h
in
(4.8
mm), and
(3)
four or more
relevant rounded indications in a line separated by
!46
inch
(1.6
mm) or less edge to edge.
8.20 MT Accessories
Various MT accessories are marketed by
M/s
Magnaflux, a Division of ITW, Inc.,
USA,
and
by others. They include residual field indicators, ASME field indicators, quantitative quality
indicators, centrifuge tube and stand for ascertaining bath concentration, easy-to-use puffer
bulb for applying dry method powders, copper braided pads to prevent burning by ensuring
good electrical contact between test part and contact plates, test bar with artificial surface
and subsurface discontinuities for testing with yoke method, and test block with subsurface
discontinuities for testing with the prod method and coil method. For testing the magnetic
particles, a Betz ring is used. The Betz ring has holes drilled at different depths. As a magnetic
field is induced in the Betz ring, magnetic particles are sprinkled to the surface. The number
of visible line indications determines the sensitivity of the particles. Typically, specifications
require visual detection of the fifth hole, which produces a broad indication
[39].
9
RADIOGRAPHIC TESTING (RT)
The term
radiography
usually implies a radiographic process that produces a permanent image
on a film. Radiographic testing (RT) technique using x-rays and gamma rays is one of the
important methods of NDT, and has developed, as
it
is applied today, into a very reliable
technique. The technique utilizes electromagnetic radiation to penetrate
an
object to reveal
information about its internal conditions, particularly internal flaws and surface flaws/disconti-
nuities.
9.1 Principle
of
Radiography
When a radiation beam passes through a specimen, some of the radiation energy is absorbed
and the intensity of the beam is reduced. To make a radiographic examination of the object,
the intensities of the radiation that pass through the specimen must be recorded and compared.
Variations in beam intensity are seen as difference in shading that are typical of the types and
sizes of the flaw present. The recording is being achieved either by letting the rays strike a
photographic film (Fig.
11)
or by letting the rays fall on a fluorescent screen that absorbs some
of the radiation and converts it to visible light, thus producing a visible image known as
fluoroscopy.
9.2 Application
Surface flaws that are detectable by radiography include undercuts, concavity at the weld root,
incomplete filling
of
the grooves, excessive reinforcements, overlaps and electrode spatter,
91
2
Chapter
I4
SOURCE
RADIATION
FILM
/
Figure
1 1
Radiographic testing principle.
and internal flaws such as blow holes, gas inclusions, porosity, piping, slag inclusion, cracks,
incomplete penetration, incomplete fusion; and tungsten inclusions. More information
on
flaw-
detecting capabilities is discussed later in this section.
9.3
Radiation
Sources
(X-Rays
and Gamma
Rays)
Two types of radiation sources are commonly used in weld inspection. They are x-ray
ma-
chines and radioactive isotopes. X-rays are produced by machines that range from portable,
low-energy units in the range of
50-400
kV
used for inspection of relatively thin objects, say,
metal thickness up to
7.5
cm steel equivalents, to betatrons and linear accelerators up to
30
meV, capable of radiography of thick objects up to
20
in
(508
mm) of steel.
Gamma-ray radiation is emitted by radioisotopes, the two most common being cobalt-60
and iridium-192. Cobalt-60 will effectively penetrate a thickness up to approximately
8
in (203
mm) of steel, whereas iridium-192 is effectively limited to a steel thickness of about
3
in
(76.2
mm). The minimum thickness
of
steel on which x-rays may be normally used is up to and
including
19.05
mm, and gamma-ray sources may normally be used for thicknesses greater
than 19.05 mm.
Comparison of X-Ray and Gamma-Ray Radiography
Most industrial radiography is performed with x-rays. It is highly effective and versatile, and
with modern techniques, it is very accurate.
The
principal advantage
of
gamma-ray radiography
is lower cost, the source can
be
very small
so
that transportability is easy, and it is economical
for low-volume test pieces. However, gamma rays are not as sensitive to small defects as are
x-rays in thickness less than 1 in
(25.4
mm), cracks are the most difficult to detect and gener-
ally require a longer exposure time than with x-rays, and they do not provide as sharp a
definition
[43].
9.4
Merits
and
Limitations
Merits.
Radiography has been the standard for volumetric inspection of weldments for dec-
ades. In spite of high cost, hazards, and the time-consuming nature of the process, radiography
QA,
QC,
Inspection,
and
NDT
91
3
stands out as the
NDT
method that gives a permanent record of test results. Among other
advantages, it has surface and subsurface inspection capability, and radiographic images aid in
characterizing (identifying type) discontinuities [28].
Limitations.
A significant limitation of radiography is that discontinuities must be favorably
aligned with the radiation beam. This is usually not a problem for discontinuities round in
cross section, such as slag inclusions and porosity [28,33]. On the other hand, cracks and
planar defects that diverge considerably from the direction of the radiation beam would nearly
miss detection, and therefore ultrasonics is sometimes employed in addition to radiography
when very critical examination of a weld is required [33]. Angular notch-type discontinuities
such as lack of fusion, incomplete penetration, and cracks must be oriented to the radiation
beam so as to show up on the x-rays film
[45].
Coarse grain structure, especially in large-pass
welding, can cause problem due to diffraction mottling effects 1461.
9.5
Radiographic Test Written Procedure
The radiographic test written procedure is given in Table
9.
Radiographic Examination Procedures Deficiencies
These may include incorrect exposure geometry, film placement, source selection, penetrameter
selection, incomplete film identification, and failure to submit a technique sheet with sketch
PI
*
9.6
Requirements
of
Radiography
Essential requirements for producing a radiograph are [28]:
1.
A source of radiation
2.
X-ray film
3.
A trained person for operating the equipment
4.
A
means of processing the exposed film
5.
A
trained person capable of interpreting the radiographic images
Table
9
Radiographic Test Written Procedure
1
.O
Scope
1
1
.O
Density
2.0 Techniques
12.0
Sensitivity
3.0
Surface preparation
13.0
Processing
of
films
4.0
Identification system
14.0
Processing technique
5.0
Location markers
15.0
Personnel certification
6.0 Radiation sources
16.0 Evaluation
of
radiographs
7.0 Film
17
.O
Reporting
8.0
Screens
18.0 Retention
of
radiographs
9.0
Focus to film
or
source to
film distance.
10.0 Penetrameter
10.2 Number of penetrameters
10.3Placement of penetrameter