METHODE ET DISPOSITIF DE DETECTION DE DEPOTS MAGNETIQUES DANS DES ECHANGEURS DE CHALEUR ET DES APPAREILS ANALOGUES

METHOD AND MEANS FOR DETECTING MAGNETIC DEPOSITS IN HEAT EXCHANGER AND THE LIKE

Abrégé anglais

METHOD AND MEANS FOR DETECTING MAGNETIC DEPOSITS IN HEAT
EXCHANGER AND THE LIKE
Abstract of the Disclosure
Deposits of magnetite on coils in a heat exchanger of a
steam generator are detected by measuring the magnetic
reluctance within the coils. A probe for measuring the
reluctance includes a magnetic core, an excitation coil
wound on the core, and a magnetic flux detector such as
a Hall generator. Changes in flux density established by
the excitation coil are detected by the Hall generator,
thus indicating the presence of magnetite deposits.

Revendications

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A probe for detecting deposits of magnetite in steam
generator tubes and the like comprising a magnetic core, an
excitation coil wound on said core for establishing a magnetic
field, a variable current source interconnected for energizing
said coil, a magnetic flux density detector for detecting magnetic
flux density, and a Gauss meter interconnected with said detector
for measuring magnetic flux changes.
2. A probe as defined by claim 1 wherein said detector
comprises a Hall generator.
3. A probe as defined by claim 2 and further including
means for pushing said probe through a tube.
4. A probe as defined by claim 3 wherein said means for
pushing comprises a non-magnetic tube.
5. A probe as defined by claim 3 and further including
a current source interconnected for energizing said coil and a
Gauss meter interconnected with said Hall generator for measuring
magnetic flux changes.
6. A probe as defined by claim 3 and further including
non-magnetic guide rings for supporting said magnetic core.

Description

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MET}IOD AND MEANS E'OR DETECTING MP.GNETIC DEPOSITS IN
HEAT EXCHANGER AND THE L:[:KE
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This invention relates generally to inspection equipment
for tubular products such as coils in heat exchangers,
and more particularly the invention relates to an improved
m~thod and means for detecting deposits of magnetite in
; S heat exchangers.
Heat eXchan~ers such as used in connection with steam
driven electric power generators are subjected to very
high temperatures and pressures. Further, deposits of
;~ 10 impurities from fluids used in the heat exchangers tend
to build up on the tubes and support structure in the
heat exchanger which can lead to tube thinning and crack-
ing. Thus, the tubes must be periodically inspected to
insure safe operation.
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In particular, steam generator units associated with
nuclear reactor plants can experience tube denting in
the support plate region due to a buildup of deposits
containing magnetite. Heretofore, no single technique
has been developed which is capahle of predictin~ the
presence of magnetite buildup in the crevice gap region
of steam generators, although attempts have been made
to detect the presence of magnetite by eddy current,
acoustic, and radiographic techniques. Oth~r attempts
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at detecting the presence of magnetite have been through the use o~
induced mechanical vibrations in the -tubes of the heat exchanger.
An object o~ the invention is a probe for magnetically
detecting the presence o~ magnetite deposits in a heat exchanger.
Briefly, in accordance with the invention, there is
provided a probe for detecting deposits o~ magnetite in steam
generator tubes and the like comprising a magnetic core, an
excitation coil wound on said core ~or establishing a magnetic
` field, a variable current source interconnected for energizing
said coil, a magnetic ~lux density detector for detecting magnetic
flux density, and a Gauss meter interconnected with said detector
for measuring magnetic flux changes.
B~ energizing the coil and moving the probe through a
tubular product such as a heat exchanger, deposits of magnetite
can be detected and crevice gap clearance can be measured by
detecting the changes in magnetic flux density.
The invention and objects and features thereof will be
more readily apparent from the following detailed description
and appended claims when taken with the drawings.
In the drawings, Figure 1 is a section view of a heat
exchanger such as used in a nuclear reactor steam system.
Figure 2 is an enlarged view of a portion of a tube and
support plate in the heat exchanger of Figure 1.
Figure 3 is a functional block diagram of equipment
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useful in the method of detecting magnetic deposits in
accordance with the present invention.
Figure 4 is a side view in section of a tube and support
plate and a probe for detecting deposits of magnetite in
accordance with the invention.
Referriny now to the drawings, Figure 1 is a section
view of a nuclear reactor heat exchan~er in which pri-
mary fluid from the reactor flows into inlet 10, through
tubes 12, and exits through outlet 14. Feed water for
steam enters through inlet 16 and flows downwardly into
contact with tubes~12, and the exchange o heat from
tubes 12 to the water creates steam which exits through
outlet 18 to drive a tur~ine. The heat exchanger also
conventionally includes a moisture separator 20 and
swirl-vane moisture separators 22.
The tubes 12 which interconnect the primary fluid from
the inlet 10 to the outlet 14 are supported within the
exchanger by a support structure 24 including plate 26
which supports the tube~ 12. As above indicated, de-
posits of magnetite form in the heat exchanger, and the
buildup of ma~netite in the crevice gap region where
the tubes are supported by plates 26 can create denting
of the tubes which can lead to thinning and cracking of
the tube walls,
Figure 2 is an enlarged view of the crevice gap region
within circle 30 of Figure 1 and shows the tube 32 pass~
ing through support plate 34 with the buildup of magne-
tite 36 in the crevice between tube 32 and plate 34. In
an acute case, the buildup of magnetite fills the crevice
gap and causes denting of the tube which eventuallY can
cause cracking and thinning of the tube walls. Hereto-
fore, no adequate inspection technique has been known
-, which will predict the presence of magnetite buildup

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in the crevice gap region as shown in Figure 2,
Figure 3 is a functional block diagram of equipment for
detecting ma~netite deposits in acccrdance with the
present invention. The equipment includes a probe com-
prising a bobbin 50 which supports a magnetic core 52
with an excitation coil 54 wound on the core. Mounted
on the bobbin 50 is a flux detector 56 which preferably
comprises a Hall generator.
A current source 58 is interconnected to energize the
coil 54, and a Gauss meter 60 is interconnected with the
Hall generator 56 for detecting magnetic flux densities.
The output of the Gauss meter may be interconnected with
a digital oscilloscope 62 and an X-Y plotter 64.
The probe is mounted on a suitable carrier such as a
Nylon tube, and the carrier and probe are pushed through
the tube of the heat exchanger by means of a suitable
motor drive 66. Thus, the energized coil sets up a mag-
netic flux pattern, and as the probe is pushed through
the tube changes in the flux pattern due to the presence
of magnetite in close proximity to the tube causes an
increase in flux density which is detected by the Hall
; 25 generator and measured by the Gauss meter 60. Accord-
ingly, the presence and location of deposits of magne-
tite can be established~
Figure 4 is a cross section of support plate 80 and tube
` 30 82 extending therethrough with a probe shown generally
at 84 and probe carrier 86 positioned within tube 82.
Typically, the support plate 80 comprises carbon steel
and tub~ 82 comprises Inconel. Probe 84 comprises a
soft iron core 88 which is mounted between two non-
magnetic guide rings 90 with the coil 92 wound on core
88. The two guide rings are held together by a bolt 94
with nut 96 fastened on cne end of bolt g4 and the
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other end of the bolt threadably engaging the tube 86.
The Hall generator is mounted on one of the rings in
close proximity to the tube wall. Carrier 86 typically
, comprises a non magnetic tube such as Nylon~ and the
wire interconnections to the coil and to the Hall
generator are placed within tube 86.
I~hile the invention has been described with reference to
a specific embodiment, the description is illustrative
of the invention and is not to be construed as limiting
the invention. Various modiiications and applications
may occur to those skilled in the ar~ without departing
from the true spirit and scope of the invention as de-
fined by the appended claims.
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