·Table of Contents
·Methods and Instrumentation
Development and Application of Sound Pulse and Multi-frequency Eddy Current Equipment
Dong Zhenjun,Tao Yuchun
Guangdong Nuclear Power Joint Venture Co. Ltd.
Eddysun (Xiamen) Electronic Co., Ltd.
This paper introduces the configuration, the principium and the
application of the sound pulse and multi-frequency eddy current tester. By the
feedback experience on site of Daya bay nuclear power station, it proves to be
well complementary each other between two inspection methods. It also properly
meet needs of electric power, petrochemical and nuclear industries, especially
for the high speed NDT inspection of the tubes of heat exchangers in service.
Keywords: sonic pulse, inspection tester, in service.
In electric power, petrochemical and nuclear industries, a variety of
pipes are in use, for instance, high and lower pressure pipes, heat exchangers
tubes, condenser tubes and heat pipes of a boiler etc. Under higher
temperature and pressure and strong erotic fluid, it is easy to cause crack
and through wall holes like defect in these pipes. In order to let these pipes
in safe operation and reduce the economical losses, it is very important to
carry out the non-destructive testing (NDT) and evaluation for these pipes in
time and effectively. For time on the site NDT is short, a fast, accurate, and
reliable detection equipment is necessary. A sound pulse inspection has been
developed to meet these requirements complemented by the eddy current
inspection. Sound pulse inspection with very high inspection rates which is
applicable for a large amount of heat exchangers tubes and any material tubes.
It can detect holes and through wall cracks, but the sound pulse has very low
sensitivity to the cracks, which do not penetrate tube wall. The eddy current
inspection rate is lower than that of the sound pulse inspection, but it is
reliable, high sensitive and very useful for some important parts and tubes
requiring high inspection accuracy. Two methods can be combined to reach high
inspection speed, accuracy and reliability.
2 Detection Principle
2.1 The principle of sound Detection
When a sound wave
with a certain velocity propagates in a pipe, an echo will be caused by a hole
or a through wall crack in the pipe wall because of the sound leakage from the
hole and the crack. According to this an emitter of the sound wave and a
receiver of its echo are put at one side, a basic sound pulse detection
equipment can be formed. Shown in Fig. 1, if a sound pulse P is emitted at
moment t, after time interval T its echo E induced by leaking hole or crack is
received, and the position of the hole or the through wall crack from the
sound source can be determined as follows:
Where, V is sound wave propagation velocity in air.
Fig 1 |
2.2 Principle and advantage of multi-frequency eddy current detection
In order to eliminate effectively a variety of disturbances in the
eddy current testing, the testing equipment has to be able to discriminate and
analyze various kinds of eddy current signals simultaneously. In the
multi-frequency eddy current detection several actuating frequencies are
simultaneously applied to the same probe and more eddy current signals than
that of the single frequency are acquired. The signal process module of the
multi-frequencies eddy current equipment is important portion. It uses
potentiometer combination,coordinate rotations etc hardware and software
techniques, comprehensively analyzes the extracted eddy current signals,
effectively eliminates various disturbance and increases the ratio of signal
to noise. Through these steps, the purpose of increasing sensitivity,
reliability and accuracy of the eddy current on-site testing can be achieved.
The details are as follows:
- Detection of different depth defects
In accordance with the theory of electromagnetic field, different frequency electromagnetic waves
have different penetration depths in the material being tested. The higher
the testing frequency is, the smaller its penetration depth is.
Simultaneously adopting two or several detection frequencies with different
parameter settings (field strengthen, gain and phase), defects and their
shapes in different depth layers can be detected.
- Elimination of various disturbances
In various elimitable
disturbances they are the signals caused by supporting plates of the heat
exchange tubes,tube sheets, dints and sediments and rusty spots on the
tubes,cold machining of the tubes,erosive pits of the rotor bores and the
blade surfaces of the steam turbine,noises of oxidized surface layers and
probe lift off in the tubes. Through theory and practice it has been proved
that the above mentioned disturbances and eddy current signals of the
defects induced by different actuating frequencies on the specimen being
tested are different on their impedance planes. Using this principle, two or
several frequency sine signals will be applied to the probe simultaneously
and the acquired eddy current signals respectively are rectified, amplified
and rotated through multiple channels. Then by means of several mixing
calculation units disturbance signals can be eliminated and the defect
signals can be clearly and actually obtained.
3 Equipment principle
The principle block circuit of EEC-ET/XB Multi-frequency Eddy Current
Sound Pulse Detection Equipment is shown in Fig. 2. They are integrated in
the compact equipment. The peripherals include keyboard, monitor, alarm and
probe etc. The software of the equipment includes operating system, defect
acquisition and analysis, recording and evaluation etc. The design of the
software is based on WINDOWS 95/98 and its operations are very simple.
The application of the equipment does not limit to the inspection of tubes
and it can be applied to inspections of ferromagnetic/non-ferromagnetic and
non-metal materials etc. It can inspect straight flexing,wound tubes without
shape limitation. In 1998/1999 it had been used in tube inspections of high
and low pressure heaters, condensers and boilers during unit refueling
outages. The equipment used to inspect 20% of the tubes of the condensers by
eddy current method and 80% tubes by the sound pulse method. However, for in
the tubes of high and low pressure heaters there are many bowing sections,
they were inspected by the sound pulse method only. The results are good and
there was no leakage from these inspected tubes after almost a year continuos
EEC-ET/XB Sound pulse,Multi-frequency Eddy Current equipment combines the
advantages of the eddy current and the sound pulse inspections and it has been
integrated in a portable compact equipment and also can be applied to field
installation inspections in electric power and petrochemical industries.
- Kaihui Chen etc, "Eddy Current Test of Steam Turbine Condenser Titanium
Tubes During Unit 2 Secondary Refueling Outage of Guangdong Nuclear Power
Station", Symposium on New Techniques Conference 1996
- Junming Lin, Kaihui Chen etc, "The Application of A Phase Amplification
Technique in the Remote field Eddy Current Testing", Proceedings of
9th Asia Pacific NDT Conference, 1998