ASTM E690-98
(Practice)Standard Practice for In Situ Electromagnetic (Eddy-Current) Examination of Nonmagnetic Heat Exchanger Tubes
Standard Practice for In Situ Electromagnetic (Eddy-Current) Examination of Nonmagnetic Heat Exchanger Tubes
SCOPE
1.1 This practice describes procedures to be followed during eddy-current examination (using an internal, probe-type, coil assembly) of nonmagnetic tubing that has been installed in a heat exchanger. The procedure recognizes both the unique problems of implementing an eddy-current examination of installed tubing, and the indigenous forms of tube-wall deterioration which may occur during this type of service. The document primarily addresses scheduled maintenance inspection of heat exchangers, but can also be used by manufacturers of heat exchangers, either to examine the condition of the tubes after installation, or to establish baseline data for evaluating subsequent performance of the product after exposure to various environmental conditions. The ultimate purpose is the detection and evaluation of particular types of tube integrity degradation which could result in in-service tube failures.
1.2 This practice does not establish acceptance criteria; they must be specified by the using parties.
1.3 This standard does not purport to address the safety problems associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: E 690 – 98
Standard Practice for
In Situ Electromagnetic (Eddy-Current) Examination of
Nonmagnetic Heat Exchanger Tubes
This standard is issued under the fixed designation E 690; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope cation and Certification
NAS-410 NAS Certification and Qualification of Nonde-
1.1 This practice describes procedures to be followed during
structive Personnel (Quality Assurance Committee)
eddy-current examination (using an internal, probe-type, coil
assembly) of nonmagnetic tubing that has been installed in a
3. Terminology
heat exchanger. The procedure recognizes both the unique
3.1 Standard terminology relating to electromagnetic ex-
problems of implementing an eddy-current examination of
amination may be found in Terminology E 1316, Section C,
installed tubing, and the indigenous forms of tube-wall dete-
Electromagnetic Testing.
rioration which may occur during this type of service. The
document primarily addresses scheduled maintenance inspec-
4. Summary of Practice
tion of heat exchangers, but can also be used by manufacturers
4.1 The examination is performed by passing an eddy-
of heat exchangers, either to examine the condition of the tubes
current probe through each tube. These probes are energized
after installation, or to establish baseline data for evaluating
with alternating currents at one or more frequencies. The
subsequent performance of the product after exposure to
electrical impedance of the probe is modified by the proximity
various environmental conditions. The ultimate purpose is the
of the tube, the tube dimensions, electrical conductivity,
detection and evaluation of particular types of tube integrity
magnetic permeability, and metallurgical or mechanical dis-
degradation which could result in in-service tube failures.
continuities in the tube. During passage through the tube,
1.2 This practice does not establish acceptance criteria; they
changes in electromagnetic response caused by these variables
must be specified by the using parties.
in the tube produce electrical signals which are processed so as
1.3 This standard does not purport to address all of the
to produce an appropriate combination of visual displays,
safety concerns, if any, associated with its use. It is the
alarms, or temporary or permanent records, or combination
responsibility of the user of this standard to establish appro-
thereof, for subsequent analysis.
priate safety and health practices and determine the applica-
bility of regulatory limitations prior to use.
NOTE 1—The agency performing the testing or examination shall meet
the requirements of Practice E 543.
2. Referenced Documents
5. Significance and Use
2.1 ASTM Standards:
5.1 Eddy-current testing is a nondestructive method of
E 543 Practice for Evaluating Agencies that Perform Non-
locating discontinuities in tubing made of materials that
destructive Testing
conduct electricity. Signals can be produced by discontinuities
E 1316 Terminology for Nondestructive Examinations
located either on the inner or outer surfaces of the tube, or by
2.2 Other Documents:
discontinuities totally contained within the tube wall. When
SNT-TC-1A Recommended Practice for Personnel Qualifi-
using an internal probe, the density of eddy currents in the tube
cation and Certification in Nondestructive Testing
wall decreases very rapidly as the distance from the internal
ANSI/ASNT-CP-189 ASNT Standard for Qualification and
surface increases; thus the amplitude of the response to outer
Certification of Nondestructive Testing Personnel
surface discontinuities decreases correspondingly.
MIL-STD-410E Nondestructive Testing Personnel Qualifi-
5.2 Some indications obtained by this method may not be
relevant to product quality. For example, an irrelevant signal
This practice is under the jurisdiction of ASTM Committee E-7 on Nonde- may be caused by metallurgical or mechanical variations that
structive Testing and is the direct responsibility of Subcommittee E07.07 on
Electromagnetic Methods.
Current edition approved Dec. 10, 1998. Published February 1999. Originally
published as E 690 – 79. Last previous edition E 690 – 91. Available from Standardization Documents Order Desk, Bldg. 4, Section D,
Annual Book of ASTM Standards, Vol 03.03. 700 Robbins Ave. Philadelphia, PA 19111–5904, Attn: NPODS.
3 5
Available from Smerican Society for Nondestructive Testing, 1711 Arlingate Available from Aerospace Industries Association of America, Inc., 1250 Eye
Plaza, P.O. Box 28518, Columbus, OH 43228–0518. Street, N.W., Washington, DC 20005.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E690–98
are generated during manufacture but that are not detrimental 6.1.15 If specified in the contractual agreement, NDT agen-
to the end use of the product. Irrelevant indications can mask cies shall be qualified and evaluated in accordance with
unacceptable discontinuities occurring in the same area. Rel- Practice E 543. The applicable edition of Practice E 543 shall
evant indications are those that result from nonacceptable be specified in the contractual agreement.
discontinuities. Any indication above the reject level, which is
7. Apparatus
believed to be irrelevant, shall be regarded as unacceptable
7.1 Electronic Apparatus:
until it is proven to be irrelevant. For tubing installed in heat
7.1.1 The electronic apparatus shall be capable of energiz-
exchangers, predictable sources of irrelevant indications are
ing the probe coils with alternating currents of suitable
lands (short unfinned sections in finned tubing), dents,
frequencies, and shall be capable of sensing changes in the
scratches, tool chatter marks, or variations in cold work.
electromagnetic response of the probes. It is important to note
Rolling tubes into the supports may also cause irrelevant
that a differential coil probe system tends to maximize the
indications, as may the tube supports themselves. Eddy-current
response from abrupt changes along the tube length, while a
testing systems are generally not able to separate the indication
single coil probe system usually responds to all changes.
generated by the end of the tube from indications of disconti-
7.1.2 Since many gradual changes are irrelevant, a differen-
nuities adjacent to the ends of the tube (end effect). Therefore,
tial coil system may permit higher gain than an absolute coil
this examination may not be valid at the boundaries of the tube
system, which enhances the response to small, short defects.
sheets.
Electrical signals produced in this manner may be processed so
as to actuate an audio or visual readout, or both. When
6. Basis of Application
necessary, these signals may also be further processed to
6.1 The following criteria may be specified in the purchase
produce a permanent record. The apparatus should have some
specification, contractual agreement, or elsewhere, and may
means of providing relative quantitative information based
require agreement between the purchaser and the supplier.
upon the amplitude or phase of the electrical signal, or both.
6.1.1 Type of eddy-current system, and probe (coil assem-
This may take many forms, including calibrated sensitivity or
bly) configuration,
attenuation controls, multiple alarm thresholds, or analog or
6.1.2 Location of heat exchanger, if applicable,
digital readouts, or combination thereof.
6.1.3 Size, material, and configuration of tubes to be exam-
7.2 Readout Devices, which require operator monitoring,
ined,
such as an oscilloscope or oscillograph presentation, may be
6.1.4 Extent of examination, that is, length, tube sheet areas,
used when necessary to augment the alarm circuits. This may
straight length only, minimum radius of bends, etc.,
be necessary, for example, to find small holes, indications of
6.1.5 Time of examination, that is, the date and location of
which tend to be nearly in phase with the response from lands
the intended examination, and the expected environmental
in skip-fin tubing. Since the lands cause very large signals to
conditions,
occur, phase discrimination may not prevent irrelevant alarms
6.1.6 The source and type of material to be used for
from tube support, if the alarm is set to reject the hole. By
fabricating the calibration standard,
observing an oscilloscope or oscillograph, however, the ability
6.1.7 The type(s), method of manufacture, location, dimen-
to detect this type of defect may be improved, especially in
sions, and number of artificial discontinuities to be placed on
areas between the tube supports.
the calibration standard,
7.3 Test Coils—Test coils shall be capable of inducing
6.1.8 Allowable tolerances for artificial discontinuities, and
current in the tube and sensing changes in the electrical
methods for verifying compliance,
characteristics of the tube. The test coil diameter shall be
6.1.9 Methods for determining the extent of end effect,
selected to yield the largest practical fill-factor. The configu-
6.1.10 Maximum time interval between equipment calibra-
ration of the test coils may permit sensing both small, localized
tion checks,
conditions, which change rapidly along the tube length, such as
6.1.11 Criteria to be used in interpreting and classifying
pitting or stress corrosion cracks, and those which may change
observed indications,
slowly along the tube length or from tube to tube, such as
6.1.12 Disposition of examination records and calibration
steam cutting, mechanical erosion, or metallurgical changes.
standard,
The choice of coil diameter should be based upon requirements
6.1.13 Contents of examination report, and
judged to be necessary for the particular test situation.
6.1.14 If specified in the contractual agreement, personnel
7.4 Single-Coil or Differential-Coil Probe Systems:
performing examinations to this practice shall be qualified in
7.4.1 Single-Coil Probe Systems—In a single-coil probe
accordance with a nationally recognized NDT personnel quali-
system, the signal obtained from the interaction between the
fication practice or standard such as ANSI/ASNT-CP-189,
test coil, and the portion of the test specimen within its
SNT-TC-1A, MIL-STD-410E, NAS-410, ASNT-ACCP, or a
influence is often balanced against an off-line reference coil in
similar document and certified by the certifying agency, as
a similar specimen, often with the aid of electrical compensa-
applicable. The practice or standard used and its applicable
tion. In some systems, electrical balancing of the test coil is
revision shall be identified in the contractual agreement be-
accomplished entirely by the use of an electrical balance
tween the using parties.
reference.
7.4.2 Differential-Coil Probe Systems—In a differential-coil
NOTE 2—MIL-STD-410 is canceled and has been replaced with NAS-
410, however, it may be used with agreement between contracting parties. probe system, the reference coil is identical to (again, often
E690–98
with the aid of electrical compensation), and on the same 8.2 The tube used when adjusting the sensitivity and phase
longitudinal axis as the test coil. In this type of configuration, settings of the apparatus shall be of the same material,
both coils function simultaneously as test and reference coils,
dimensions, and configuration as the tubes installed in the heat
and the instrument responds only to unbalance voltages (that is, exchanger.
differential voltages) between the two coils.
8.3 It is important to note that artificial discontinuities may
7.4.3 In either the single or differential coil system, some
not be representative of natural discontinuities and may not
form of original balance is attained and it is the disruption of
provide a direct relationship between instrument response and
this balance which provides the response signals that indicate
discontinuity severity. They are intended only for establishing
deviations in the tube wall as compared to the original sample.
an approximation of sensitivity levels to various types of
7.5 Speed-Sensitive Equipment—Eddy-current equipment
conditions. The relationship between instrument response and
that produces a variation in discontinuity signal response with
discontinuity size, shape, and location is important and should
variations in the test-scan speed. This is characteristic of
be established separately, particularly as a function of test
equipment that employs filter networks to attenuate the de-
frequency.
tected signal at frequencies below or above, or both, an
8.4 Since there may be a need to compare the results of this
adjustable or fixed frequency. Speed insensitive d-c coupled
test procedure, as applied to a particular tube or heat ex-
equipment provides a constant discontinuity signal response
changer, with the results of prior or subsequent examinations,
with changing test speeds.
it is important that a record be kept of each examination. The
7.6 Driving Mechanism—A means of mechanically travers-
reference standard should be maintained to provide a basis for
ing the probe coil through the tube may be used. Whether the
comparing results from successive examinations. In this situ-
probe is traversed through the tube manually or mechanically,
ation it is recommended that the reference standards should
care should be taken to maintain as uniform a probe speed as
provide at least three levels of readout, so that test data may be
possible to produce repeatable indications of discontinuities
referred against a standardization curve. Tubes with indications
when using speed sensitive equipment.
in excess of a predetermined level should be recorded to
7.7 Phase-Selective System—An instrumentation system
identify the affected tube, its location, and, when necessary, to
that includes built-in circuitry to indicate phase differences in
describe the level of response.
the response signal relative to the excitation signal. This ability
8.5 Any combination of notch size and shape might be
aids in discriminating between abnormal conditions in the tube
wall (cracks, pitting, wear from the tube supports) and normal incorporated in a standard tube. This determination must be
change (lands in skip-fin tubing, the tube support itself, made by the party responsible for the examination, within the
contaminants in or about the tube such as sludge, etc.). Phase context of the overall considerations mentioned in 9.1, and the
may also provide information on defect position relative to the functions a standard
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