ASTM E690-98(2004)e1
(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
SIGNIFICANCE AND USE
Eddy-current examination is a nondestructive method of locating discontinuities in tubing made of materials that conduct electricity. Signals can be produced by discontinuities located either on the inner or outer surfaces of the tube, or by discontinuities totally contained within the tube wall. When using an internal probe, the density of eddy currents in the tube wall decreases very rapidly as the distance from the internal surface increases; thus the amplitude of the response to outer surface discontinuities decreases correspondingly.
Some indications obtained by this method may not be relevant to product quality. For example, an irrelevant signal may be caused by metallurgical or mechanical variations that are generated during manufacture but that are not detrimental to the end use of the product. Irrelevant indications can mask unacceptable discontinuities occurring in the same area. Relevant indications are those that result from nonacceptable discontinuities. Any indication above the reject level, which is believed to be irrelevant, shall be regarded as unacceptable until it is proven to be irrelevant. For tubing installed in heat exchangers, predictable sources of irrelevant indications are lands (short unfinned sections in finned tubing), dents, scratches, tool chatter marks, or variations in cold work. Rolling tubes into the supports may also cause irrelevant indications, as may the tube supports themselves. Eddy-current examination systems are generally not able to separate the indication generated by the end of the tube from indications of discontinuities adjacent to the ends of the tube (end effect). Therefore, this examination may not be valid at the boundaries of the tube sheets.
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.
This standard does not purport to address all of the safety concerns, if any, 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:E690–98 (Reapproved 2004)
Standard Practice for
In Situ Electromagnetic (Eddy-Current) Examination of
Nonmagnetic Heat Exchanger Tubes
This standard is issued under the fixed designation E690; 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 (´) indicates an editorial change since the last revision or reapproval.
´ NOTE—Editorial changes made throughout in January 2004.
1. Scope cation and Certification in Nondestructive Testing
ANSI/ASNT-CP-189 ASNT Standard for Qualification and
1.1 Thispracticedescribesprocedurestobefollowedduring
Certification of Nondestructive Testing Personnel
eddy-current examination (using an internal, probe-type, coil
NAS-410 NAS Certification and Qualification of Nonde-
assembly) of nonmagnetic tubing that has been installed in a
structive Personnel (Quality Assurance Committee)
heat exchanger. The procedure recognizes both the unique
problems of implementing an eddy-current examination of
3. Terminology
installed tubing, and the indigenous forms of tube-wall dete-
3.1 Standard terminology relating to electromagnetic ex-
rioration which may occur during this type of service. The
amination may be found in Terminology E1316, Section C,
document primarily addresses scheduled maintenance inspec-
Electromagnetic Testing.
tion of heat exchangers, but can also be used by manufacturers
of heat exchangers, either to examine the condition of the tubes
4. Summary of Practice
after installation, or to establish baseline data for evaluating
4.1 The examination is performed by passing an eddy-
subsequent performance of the product after exposure to
current probe through each tube. These probes are energized
various environmental conditions. The ultimate purpose is the
with alternating currents at one or more frequencies. The
detection and evaluation of particular types of tube integrity
electrical impedance of the probe is modified by the proximity
degradation which could result in in-service tube failures.
of the tube, the tube dimensions, electrical conductivity,
1.2 This practice does not establish acceptance criteria; they
magnetic permeability, and metallurgical or mechanical dis-
must be specified by the using parties.
continuities in the tube. During passage through the tube,
1.3 This standard does not purport to address all of the
changes in electromagnetic response caused by these variables
safety concerns, if any, associated with its use. It is the
in the tube produce electrical signals which are processed so as
responsibility of the user of this standard to establish appro-
to produce an appropriate combination of visual displays,
priate safety and health practices and determine the applica-
alarms, or temporary or permanent records, or combination
bility of regulatory limitations prior to use.
thereof, for subsequent analysis.
2. Referenced Documents
NOTE 1—The agency performing the testing or examination shall meet
2.1 ASTM Standards: the requirements of Practice E543.
E543 Specification for Agencies Performing Nondestruc-
5. Significance and Use
tive Testing
5.1 Eddy-current examination is a nondestructive method of
E1316 Terminology for Nondestructive Examinations
locating discontinuities in tubing made of materials that
2.2 Other Documents:
conduct electricity. Signals can be produced by discontinuities
SNT-TC-1A Recommended Practice for Personnel Qualifi-
located either on the inner or outer surfaces of the tube, or by
discontinuities totally contained within the tube wall. When
using an internal probe, the density of eddy currents in the tube
This practice is under the jurisdiction of ASTM Committee E07 on Nonde-
wall decreases very rapidly as the distance from the internal
structive Testing and is the direct responsibility of Subcommittee E07.07 on
surface increases; thus the amplitude of the response to outer
Electromagnetic Methods.
Current edition approved January 1, 2004. Published February 2004. Originally
surface discontinuities decreases correspondingly.
approved in 1979. Last previous edition approved in 1998 as E690 – 98. DOI:
10.1520/E0690-98R04E01.
2 3
For referenced ASTM standards, visit the ASTM website, www.astm.org, or Available fromTheAmerican Society for NondestructiveTesting (ASNT), P.O.
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Box 28518, 1711 Arlingate Ln., Columbus, OH 43228-0518.
Standards volume information, refer to the standard’s Document Summary page on Available from Aerospace Industries Association of America, Inc., 1250 Eye
the ASTM website. Street, N.W., Washington, DC 20005.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
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E690–98 (2004)
NOTE 2—MIL-STD-410 is canceled and has been replaced with NAS-
5.2 Some indications obtained by this method may not be
410, however, it may be used with agreement between contracting parties.
relevant to product quality. For example, an irrelevant signal
may be caused by metallurgical or mechanical variations that
6.1.15 If specified in the contractual agreement, NDT agen-
are generated during manufacture but that are not detrimental
cies shall be qualified and evaluated in accordance with
to the end use of the product. Irrelevant indications can mask
Practice E543.The applicable edition of Practice E543 shall be
unacceptable discontinuities occurring in the same area. Rel-
specified in the contractual agreement.
evant indications are those that result from nonacceptable
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
exchangers, predictable sources of irrelevant indications are
7.1.1 The electronic apparatus shall be capable of energiz-
lands (short unfinned sections in finned tubing), dents,
ing the probe coils with alternating currents of suitable
scratches, tool chatter marks, or variations in cold work.
frequencies, and shall be capable of sensing changes in the
Rolling tubes into the supports may also cause irrelevant
electromagnetic response of the probes. It is important to note
indications, as may the tube supports themselves. Eddy-current
that a differential coil probe system tends to maximize the
examination systems are generally not able to separate the
response from abrupt changes along the tube length, while a
indication generated by the end of the tube from indications of
single coil probe system usually responds to all changes.
discontinuities adjacent to the ends of the tube (end effect).
7.1.2 Since many gradual changes are irrelevant, a differen-
Therefore, this examination may not be valid at the boundaries
tial coil system may permit higher gain than an absolute coil
of the tube sheets.
system, which enhances the response to small, short defects.
Electrical signals produced in this manner may be processed so
6. Basis of Application
as to actuate an audio or visual readout, or both. When
6.1 The following criteria may be specified in the purchase
necessary, these signals may also be further processed to
specification, contractual agreement, or elsewhere, and may
produce a permanent record. The apparatus should have some
require agreement between the purchaser and the supplier.
means of providing relative quantitative information based
6.1.1 Type of eddy-current system, and probe (coil assem-
upon the amplitude or phase of the electrical signal, or both.
bly) configuration,
This may take many forms, including calibrated sensitivity or
6.1.2 Location of heat exchanger, if applicable,
attenuation controls, multiple alarm thresholds, or analog or
6.1.3 Size, material, and configuration of tubes to be exam-
digital readouts, or combination thereof.
ined,
6.1.4 Extent of examination, that is, length, tube sheet areas, 7.2 Readout Devices, which require operator monitoring,
straight length only, minimum radius of bends, etc.,
such as an oscilloscope or oscillograph presentation, may be
6.1.5 Time of examination, that is, the date and location of used when necessary to augment the alarm circuits. This may
the intended examination, and the expected environmental
be necessary, for example, to find small holes, indications of
conditions,
which tend to be nearly in phase with the response from lands
6.1.6 The source and type of material to be used for
in skip-fin tubing. Since the lands cause very large signals to
fabricating the reference standard,
occur, phase discrimination may not prevent irrelevant alarms
6.1.7 The type(s), method of manufacture, location, dimen-
from tube support, if the alarm is set to reject the hole. By
sions, and number of artificial discontinuities to be placed on
observing an oscilloscope or oscillograph, however, the ability
the reference standard,
to detect this type of defect may be improved, especially in
6.1.8 Allowable tolerances for artificial discontinuities, and
areas between the tube supports.
methods for verifying compliance,
7.3 Examination Coils—Examination coils shall be capable
6.1.9 Methods for determining the extent of end effect,
of inducing current in the tube and sensing changes in the
6.1.10 Maximum time interval between equipment refer-
electrical characteristics of the tube. The examination coil
ence checks,
diameter shall be selected to yield the largest practical fill-
6.1.11 Criteria to be used in interpreting and classifying
factor. The configuration of the examination coils may permit
observed indications,
sensing both small, localized conditions, which change rapidly
6.1.12 Disposition of examination records and reference
along the tube length, such as pitting or stress corrosion cracks,
standard,
and those which may change slowly along the tube length or
6.1.13 Contents of examination report, and
from tube to tube, such as steam cutting, mechanical erosion,
6.1.14 If specified in the contractual agreement, personnel
ormetallurgicalchanges.Thechoiceofcoildiametershouldbe
performing examinations to this practice shall be qualified in
based upon requirements judged to be necessary for the
accordance with a nationally recognized NDT personnel quali-
particular examination situation.
fication practice or standard such as ANSI/ASNT-CP-189,
7.4 Single-Coil or Differential-Coil Probe Systems:
SNT-TC-1A, MIL-STD-410E, NAS-410, ASNT-ACCP, or a
7.4.1 Single-Coil Probe Systems—In a single-coil probe
similar document and certified by the certifying agency, as
applicable. The practice or standard used and its applicable system, the signal obtained from the interaction between the
examinationcoil,andtheportionofthetestspecimenwithinits
revision shall be identified in the contractual agreement be-
tween the using parties. influence is often balanced against an off-line reference coil in
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E690–98 (2004)
a similar specimen, often with the aid of electrical compensa- A special consideration in this type of examination is the high
tion. In some systems, electrical balancing of the examination probability of certain types of defects occurring in the area of
coil is accomplished entirely by the use of an electrical balance the tube supports. It is recommended that a way of simulating
reference. the tube support (such as a simple outside diameter ring of a
7.4.2 Differential-Coil Probe Systems—In a differential-coil material similar to the tube support) be supplied, so that the
probe system, the reference coil is identical to (again, often influenceofthetubesupportonthediscontinuitysignalmaybe
with the aid of electrical compensation), and on the same observed.
longitudinal axis as the examination coil. In this type of
8.2 The tube used when adjusting the sensitivity and phase
configuration, both coils function simultaneously as examina-
settings of the apparatus shall be of the same material,
tion and reference coils, and the instrument responds only to
dimensions, and configuration as the tubes installed in the heat
unbalance voltages (that is, differential voltages) between the
exchanger.
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 examination-scan speed. This is characteristic
be established separately, particularly as a function of exami-
of equipment that employs filter networks to attenuate the
nation frequency.
detected 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
examination procedure, as applied to a particular tube or heat
equipment provides a constant discontinuity signal response
exchanger, with the results of prior or subsequent examina-
with changing examination speeds.
tions, it is important that a record be kept of each examination.
7.6 Driving Mechanism—Ameans of mechanically travers-
The reference standard should be maintained to provide a basis
ing the probe coil through the tube may be used. Whether the
for comparing results from successive examinations. In this
probe is traversed through the tube manually or mechanically,
situation 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 examination
possible to produce repeatable indications of discontinuities
data may be referred against a standardization curve. Tubes
when using speed sensitive equipment.
with indications in excess of a predetermined level should be
7.7 Phase-Selective System—An instrumentation system
recorded to identify the affected tube, its location, and, when
that includes built-in circuitry to indicate phase differences in
necessary, to describe the level of response.
the response signal relative to the excitation signal.This ability
8.5 Any
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