GB2055473A

GB2055473A - Method and means for detecting magnetic deposits in tubular plant

Info

Publication number: GB2055473A
Application number: GB8019687A
Authority: GB
Original assignee: Electric Power Research Institute Inc
Current assignee: Electric Power Research Institute Inc
Priority date: 1979-06-18
Filing date: 1980-06-17
Publication date: 1981-03-04
Also published as: CA1162987A; SE8004469L; FR2459490A1; BE883851A; GB2055473B; DE3022060A1; JPS566178A

Abstract

Deposits of magnetite on tubes 82 in a heat exchanger, e.g. a steam generator, are detected by measuring the magnetic reluctance within the tubes. A probe 84 for measuring the reluctance includes a permanent magnet (or a magnetic core and an excitation coil would on the core 88, 92) and a magnetic flux detector such as a Hall generator mounted for example on one of the non-magnetic rings 90. Changes in flux density as the probe is pushed through the tubes are detected by the Hall generator, thus indicating the presence of magnetite deposits. The probe includes a non-magnetic tube 86 for pushing it through the heat exchanger tubes. <IMAGE>

Description

SPECIFICATION Method and means for detecting magnetic deposits in tubular plant This invention relates generally to inspection of tubular plant such as coils in heat exchangers, and more particularly the invention relates to an improved method and means for detecting magnetic deposits in such plant.

Heat exchangers such as used in connection with steam driven electric power generators are subjected to very high temperatures and pressures. Further, deposits of 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 cracking. Thus, the tubes must be periodically inspected to ensure safe operation.

In particular, stream 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 capable of predicting the presence of magnetite build-up 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. Other attempts at detecting the presence of magnetite have been through the use of induced mechanical vibrations in the tubes the heat exchanger.

The present invention provides a method of detecting magnetic deposits on support means for tubing (e.g. in heat exchangers, especially steam generators) comprising the steps of inserting a magnetic flux generator and detector into the tubing, moving the generator and detector through the tubing, and measuring the density of detected flux, changes of flux density being indicative of magnetic deposits on the support means in close proximity to the tubing.

It also provides a probe for detecting magnetic deposits in heat exchangers, the probe comprising a magnetic core, an excitation coil wound on the core for establishing a magnetic field, and a magnetic flux density detector for detecting changes in magnetic flux density; and apparatus for detecting magnetic deposits in heat exchangers, the apparatus comprising (a) a probe having means for establishing a magnetic field and a magnetic flux detector for detecting changes in magnetic flux density, and (b) means for pushing the probe through a tube of a heat exchanger.

A probe in accordance with the invention can have means for establishing a magnetic field, such as a permanent magnet or a magnetic core and excitation coil; a magnetic flux detector in the probe detects changes in flux density and magnetic reluctance due to the presence of magnetite deposits.

Means can be provided for moving the probe through a tubular body, and of establishing the magnetic field and moving the probe through tubing such as that of a heat exchanger, deposits of magnetite can be detected and crevice gap clearance can be measured by detecting and measuring the changes in magnetic flux density.

The invention is illustrated by way of example only in the following detailed description of a preferred embodiment to be taken with the accompanying drawings, in which: Figure 1 is a sectional eievation 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 useful in the method of detecting magnetic deposits in accordance with the present invention; and 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.

Referring now to the drawings, Figure 1 is a section view of a nuclear reactor heat exchanger in which primary fluid from the reactor flows into inlet 10, through tubes 12, and exits through outlet 14. Feed water for steam enters through inlet 1 6 and flows downwardly into contact with tubes 12, and the exchange of heat from tubes 12 to the water creates steam which exits through outlet 1 8 to drive a turbine. The heat exchange also conventionally includes a moisture separator 20 and swirl-vane moisture separators 22.

The tubes 12 which channel the primary fluid from the inlet 10 to the outlet 14 are supported within the exchanger by a support structure 24 including plates 26. As above indicated, deposits of magnetite form in the heat exchanger, and the build-up of magnetite in the crevice gap region 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 30 of Figure 1 and shows a tube 32 passing through a support plate 34 with the buildup of magnetite 36 in the crevice between tube 32 and plate 34. In an acute case, the build-up of magnetite fills the crevice gap and causes denting of the tube which eventually can cause cracking and thinning of the tube wall. Heretofore, no adequate inspection technique has been known which will predict the presence of magnetite build-up in the crevice gap region as shown in Figure 2.

Figure 3 is a functional block diagram of equipment for detecting magnetite deposits in accordance with the present invention. The equipment includes a probe comprising 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 connected to energize the coil 54, and a Gauss meter 60 is connected to the Hall generator 56 for detecting magnetic flux densities. The output of the Gauss meter may be connected to 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 a tube of the heat exchanger by means of a suitable motor drive 66. The energized coil sets up a magnetic flux pattern, and, as the probe is pushed through the tube, change 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 generator and measured by the Gauss meter 60.

Accordingly, the presence and location of deposits of magnetite can be established.

Figure 4 is a cross section of support plate 80 and tube 82 extending therethrough with a probe shown generally at 84 and probe carrier 86 positioned within tube 82. Typically, the support plate 80 is of carbon steel and tube 82 is of Inconel (Trade Mark). Probe 84 comprises a soft iron core 88 mounted between two non-magnetic guide rings 90 with a coil 92 wound on core 88.

The two guide rings are held together by a bolt 94 with nut 96 fastened on one end of bolt 94 and the 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 is typically a tube of non-magnetic material such as nylon, and the wire connections to the coil and to the Hall generator are placed within tube 86.

Claims

1. A method of detecting magnetic deposits on support means for tubing comprising the steps of inserting a magnetic flux generator and detector into the tubing, moving the generator and detector through the tubing, and measuring the density of detected flux, changes of flux density being indicative of magnetic deposits on support means in close proximity to the tubing.

2. A method according to claim 1 wherein the tubing is that of a heat exchanger.

3. A method according to claim 2 wherein the tubing is that of a steam generator

4. A probe for detecting magnetic deposits in heat exchangers, the probe comprising a magnetic core, an excitation coil wound on the core for establishing a magnetic field, and a magnetic flux density detector for detecting changes in magnetic flux density.

5. A probe according to claim 4 including nonmagnetic guide rings supporting the magnetic core.

6. A probe according to claim 4 or 5 wherein the detector comprises a Hall generator.

7. A probe according to any of claims 4 to 6 having means for pushing it through a tube of a heat exchanger.

8. A probe according to claim 7 wherein the pushing means comprises a non-magnetic tube qn which the core is mounted.

9. A probe according to any of claims 4 to 8 having a current source connected for energizing the coil and a Gauss meter connected to the detector for measuring magnetic flux changes.

1 0. Apparatus for detecting magnetic deposits in heat exchangers, the apparatus comprising (a) a probe having means for establishing a magnetic field and a magnetic flux detector for detecting changes in magnetic flux density, and (b) means for pushing the probe through a tube of a heat exchanger.

11. Apparatus according to claim 10 wherein the means for establishing a magnetic field comprises a permanent magnet.

12. Apparatus according to claim 10 wherein the means for establishing a magnetic field comprises a magnetic core having an excitation coil wound thereon.

13. A method of detecting magnetic deposits in a heat exchanger, the method being substantially as hereinbefore described with reference to the accompanying drawings.

14. Apparatus for detecting magnetic deposits in a heat exchanger, the apparatus being substantially as hereinbefore described with reference to the accompanying drawings.