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UltrasonicInspectionforWallThicknessMeasurementatThermalPowerStationsARTICLEJANUARY2011
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International Journal of Engineering Research and Technology.
ISSN 0974-3154 Volume 4, Number 1 (2011), pp. 89-107 International
Research Publication House http://www.irphouse.com
Ultrasonic Inspection for Wall Thickness Measurement at Thermal
Power Stations
S. Bhowmick
Department of Mechanical Engineering, Indian Institute of
Technology, Kharagpur, India
Abstract
This paper is mainly concentrated upon the various scientific
technologies involved in the ultrasonic inspections frequently
conducted at the thermal power stations. Not only in day to day
industrial maintenance operations, ultrasonic inspection which is
considered as one of the most popular Non Destructive Testing
methods has immense contribution to the field of Research &
Development operations also, a branch which has its unit at
condition monitoring cell at thermal power stations & also at
the most renowned R&D organisations around the world. Now days
particularly at the Thermal Power Stations every moment new
challenges are faced in the power generation technology which
necessitates the development of more advanced Condition Monitoring
Techniques. Although Condition Monitoring at Thermal Power Stations
occupy a vast region starting from Vibration Based Condition
Monitoring to Non Destructive Examinations. The Non Destructive
Testing not only includes the involvement of skilled technicians,
engineers but also highlights a vast, endless research domain with
state of the art technology in modern day industries. Further
analysis of the technologies results in the academic achievements
also. This paper deals with the prospects, scope &
contributions of the state of the NDT Ultrasonic Techniques for day
to day maintenance at the Thermal Power Stations.
Keywords: Ultrasonic Inspection, Non Destructive Testing, Power
Station, Maintenance.
Introduction Nowadays, Condition Monitoring operations at the
Thermal Power Stations & various other industries has
popularized & supported the development of Non Destructive
Testing Techniques not only in India but round the globe. Although,
Condition
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90 S. Bhowmick
Monitoring operations include a vast domain, starting right from
the maintenance activities by the plant engineers & technicians
to computerised failure analysis & reliability engineering
under proactive maintenance at the well known Research &
Development organisations & institutes. Among the chiefly
practised Maintenance operations Non Destructive Testing deserves
special mention. The various types of Non Destructive Examinations
include Ultrasonic Inspection, Remote Visual Inspection, Eddy
Current Testing, Radiographic Examination, Vibration Analysis &
various other methods whose concepts are still under research
studies. All these methods are equally vital both from the
Industrial as well as the research point of view. Its industrial
field includes oil & gas industries, aircraft industries, iron
& steel industries, power generation industries & various
other small scale & large scale industries. However, in this
paper we are just highlighting upon Ultrasonic Inspection &
more specifically upon its basic principles, technology behind wall
thickness measurement & scope. It is known that any engineering
operation is conducted under certain sections viz., Machinery or
Technical accessories, Technology behind operation, Results &
Scope of operation. We will now categorically discuss the above
mentioned facts. Technical accessories for Ultrasonic Inspection
Every Ultrasonic Inspection consists of certain common technical
accessories whose specification differs with their makers. However,
same technical process exists behind such accessories. Hence,
technically the components can broadly summarize under the
following categories: Transducers/Probes: The Transducers &
probes mainly act as a converter i.e. it performs the conversion
from one form of energy to another. In case of Ultrasonic
Inspection, it plays the role of interfacing between the mechanical
& the electrical energy (pulse). Depending upon the industrial
needs, ultrasonic transducers of various technical specifications
& make are utilized for thickness measurements & crack
detections. Types of Transducers based upon Modes of Operation
Ultrasonic transducers can be used in the time, attenuation,
frequency, and image domains. Time domain transducers measure the
time of flight and the velocity of longitudinal, shear, and surface
waves. Time domain transducers measure density and thickness,
detect and locate defects, and measure elastic and mechanical
properties of materials. These transducers are also used for
interface and dimensional analysis, proximity detection, remote
sensing, and robotics. Attenuation domain transducers measure
fluctuations of transmitted and reflected signals At a given
frequency and beam size these transducers are used for defect
characterization and determining surface and internal
microstructures. They also can be used for interface analysis.
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Ultrasonic Inspection for Wall Thickness Measurement 91
Frequency domain transducers measure the frequency dependence of
ultrasonic attenuation, thereby providing ultrasonic spectroscopy.
These transducers are especially used for microstructure analysis,
grain boundary studies, determining porosity and surface
characterization, and phase analysis. Image domain transducers
measure the time of flight and are used for attenuation mapping as
function of discrete point analysis by raster C-scanning or
synthetic aperture techniques. These transducers can provide
surface and internal imaging of defects, microstructure, density,
velocity, or mechanical properties. True 2D or 3D imaging can be
provided. Again depending upon the geometry, make & method of
contact with the test sample, probes can be broadly divided into
contact & non contact ones
Figure 1.1: Types of Transducers Based Upon Nature of Contact.
Now we shall discuss mainly the technical set up of those
Transducers frequently utilized for operations at the Thermal Power
Stations. Contact Type Transducers Ultrasonic testing (UT) is
widely used by industry for quality controls an equipment integrity
studies. Major uses include flaw detection and wall thickness
measurements. Using ultrasonic techniques it is also possible to
measure the thickness of process pipes and vessels with ultrasonic
transducers. Wall thickness measurements are especially important
in corrosion studies where corrosion can cause a uniform reduction
in wall thickness over a period of time. When a piezoelectric
crystal is driven by high-voltage electrical pulses, the crystal
rings at its resonant frequency and produces short bursts of high
frequency vibrations. These sound wave trains generated by the
ultrasonic transducer or search unit are transmitted into the
material being tested. When the search unit is in direct contact
with the test material, the technique is known as contact testing.
Ultrasonic pulses are also reflected from the back surface of the
material and this signal represents the total distance travelled.
The pulse received from the back surface can also represent the
width, length, or thickness of the material depending on its
Transducers
ImmersionTypeProbes
NonContactProbes
ContactTypeProbes
AngleBeamNormalBeamDualProbe
SingleProbe
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92 S. Bhowmick
orientation. Ultrasonic thickness testing measures the wall
thicknesses of pipes and vessels by measuring the total distance
travelled by the ultrasonic pulses, which is represented by the
distance from the initial pulse or front surface to the back
reflection from the back surface. Ultrasonic flaw and thickness
indications are frequently displayed on an instrument or computer
display screen. In ultrasonic testing, a search unit may be thought
of as an ultrasonic probe or transducer containing one or more
piezoelectric crystals. The search unit is driven for 1 to 3ms,
producing a short burst of ultrasonic waves. The ultrasonic waves
are transmitted through the material where it is reflected by the
back surface. After this initial burst of pulses is transmitted,
the transducer acts as a receiver, waiting to receive the reflected
wave train or echo pulse. This transmitting receiving cycle is
repeated 60 to 1000 times or more based on transducer design and
application requirements. To avoid confusion, sufficient time must
be allowed to elapse between transmitted pulses to permit return of
the echo pulse and provide for the decay of the initial pulse.
Figure 1.2: A Block diagram of an Ultrasonic Testing by a
Transducer. However depending upon the transmission & receiving
of the ultrasonic beams, contact type transducers can be again
categorized into dual element transducer, single element transducer
& angle beam transducers (mainly used for flaw detection).
However in dual element transducers, two separate crystals are
present for transmission & receiving separated by acoustic
barrier while in angle beam probes one or many crystals formed in
array may be utilized. The simplest in arrangement is the single
crystal probe in which only one crystal is present for the entire
function. However, details regarding the angle beam probe are
beyond the scope of this paper.
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Ultrasonic Inspection for Wall Thickness Measurement 93
Figure 1.3: Various Types of Transducers based upon Geometry
& Crystal Construction.
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94 S. Bhowmick
Piezoelectric Crystals & Piezoelectric Effects Along with
the brief description of contact type transducers, we should also
highlight the piezoelectric crystals & their working principles
since their role is vital for understanding transducer operation
& construction. A piezoelectric substance is one that produces
an electric charge when a mechanical stress is applied (the
substance is squeezed or stretched). Conversely, a mechanical
deformation (the substance shrinks or expands) is produced when an
electric field is applied. When a piezoelectric crystal is driven
by high-voltage electrical pulses, the crystal rings at its
resonant frequency and produces short bursts of high frequency
vibrations. These sound wave trains generated by the ultrasonic
transducer or search unit are transmitted into the material being
tested. When the search unit is in direct contact with the test
material, the technique is known as contact testing. The
piezoelectric crystal in the search unit converts the reflected
sound wave or echo back into electric pulses. Ultrasonic pulses are
also reflected from the back surface of the material and this
signal represents the total distance travelled. The pulse received
from the back surface can also represent the width, length, or
thickness of the material depending on its orientation. Ultrasonic
thickness testing measures the wall thicknesses of pipes and
vessels by measuring the total distance travelled by the ultrasonic
pulses, which is represented by the distance from the initial pulse
or front surface to the back reflection from the back surface.
Ultrasonic transmitters and receivers are mainly made from small
plates cut from certain crystals. If no external forces act upon
such a small plates electric charges are arranged in certain
symmetry and thus compensate each other. Due to external pressure
the thickness of the small plate is changed and thus the symmetry
of the charge. An electric field develops and at the silver-coated
faces of the crystal voltage can be tapped off. This effect is
called Direct Piezoelectric Effect. Pressure fluctuations and thus
also sound waves are directly converted into electric voltage
variations by this effect; the small plate serves as receiver. The
direct piezoelectric effect is reversible is reversible (reciprocal
piezoelectric effect). If voltage is applied to the contact face of
the crystal the thickness of the small plate changes, according to
the polarity of the voltage the plate becomes thicker or thinner.
Due to an applied high frequency a.c. voltage the crystal
oscillates at the frequency of the a.c. voltage. A short voltage
pulse of less than 1/1000000 seconds and a voltage of 300-1000 v
excites the crystal into oscillations at its natural frequency
(resonance), which depends on the thickness and the material of
small plate. The thinner the crystal, the higher its resonance
frequency. Therefore it is possible to generate an ultrasonic
signal with a definite primary frequency. The thickness of the
crystal is calculated from the required resonance frequency f0-
according to the following formula: T=V/2F Where V= velocity of the
crystal material; f= resonance frequency of the crystal;
T=thickness of the crystal.
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Ultrasonic Inspection for Wall Thickness Measurement 95
Figure 1.4: Piezoelectric Effects caused due to various Circuit
Design & Charging Processes.
Figure 1.5: Piezoelectric effect causing energy conversions
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96 S. Bhowmick
Non Contact Ultrasonic Transducer One of the mention worthy
development in the NDT techniques is the introduction of Non
Contact Ultrasonic transducers with perfect air/gas impedance (Z)
matching. Non contact was made possible by the development of
high-transduction piezoelectric transducers in 1997(U.S. and
international patents) and the creation of a dedicated non
contacting ultrasonic analyzer in 1998 to 2003. Although for few
years the concept of Non Contacting Ultrasound remained a dream
because of mismatch of acoustic impedance but the development of
dry coupling for longitudinal and shears wave transducers operating
at frequencies up to 25MHz was the NCU transducer precursor. Since
1983, these transducers have been used to characterize thickness,
velocity, elastic, and mechanical properties of green, porous, and
dense materials. This research was followed by the development of
planar and focused air/gas propagation transducers, which utilized
a less than 1 Mrayl acoustic impedance matching layer of a
nonrubber material on the piezoelectric material. These 250kHz to
5MHz air-coupled (AC) transducers with polymer acoustic impedance
matched layers depended on high-energy or tone burst excitation,
and high signal amplification, and were somewhat application and
range limited. In 1997, Mahesh C. Bhardwaj* produced and evaluated
transducers with compressed fiber as the final acoustic impedance
matching layer. These transducers produced unprecedented and
phenomenal transduction in air. This work was instrumental in the
development of current noncontacting transducers with perfect air
acoustic impedance matching. As a result, current noncontacting
transducers, covering the range of a 50 kHz to >5MHz, can now be
propagated though practically any medium including
very-high-acoustic impedance materials such as steel, cermets, and
dense ceramics.
(a) Operation (b) EMAT Transducer Construction
Figure 1.6: General Operation & construction block diagrams
of a typical NCU unit.
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Ultrasonic Inspection for Wall Thickness Measurement 97
Working Principal NCUTM transducer signal-to-noise ratio (SNR)
is determined by SNR = 20 logVx Vn [dB] where Vx is the received
signal in volts Vn is noise voltage The SNR is determined without
signal processing and includes the noise associated with measuring
instruments, cables, etc. NCU transducer sensitivity (S) is
determined by S = 20 logVx V0 [dB] where Vx is the received signal
in volts; V0 is the excitation voltage. NCU transducers generate
immense acoustic pressure in air over their frequency range. Again,
in some cases, magnetic field & electric fields are used to
generate the Ultrasonic Wave & thereby strengthening the
acoustic pressure. Some Industrial Advantages
1. No requirement of couplant, since NCU Transducers overcomes
the drawback of the conventional UT methods to attenuate in air
medium in absence of water, grease, glycerine & other
couplants.
2. Can be used both in contact & also at distance from the
surface. This feature is very much vital particularly in cases of
boiler tubes where there is surface deposition & those areas
where surface contact is not available, NCU unit is enable to
provide Wall Thickness Measurement.
3. No surface preparations prior to thickness measurement
operations results in the saving of both Time & Labour.
Couplants Air is a poor conductor of Ultrasonic Waves at the
available Transducer frequencies. Impedance mismatch will occur if
even a thin film of air is present between the transducer & the
test piece. It will directly obstruct the transmission of sound
waves between the robe and the test piece. Hence it is essential to
eliminate air, air bubbles between the transducer & the test
piece in order to achieve measurement accuracy. Even some
ultrasonic thickness measuring devices which are highly sensitive
shows LOSS OF SIGNAL to indicate the presence of air. Hence the
ultimate solution to this technical complexity is the utilisation
of a compound known as couplant. It fills up the space between the
contacts of the transducer & test piece and thereby eliminating
the presence of air bubbles in the path of wave transmission.
However, the couplants popular for industrial as well as R&D
use are grease, oil, water, glycerine and chemical pastes. However,
certain parameters vital for couplant selections are: Surface
nature & topography of the test piece. Surface temperature of
the test piece. Chemical nature of the surface and the couplant to
prevent any type of chemical
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98 S. Bhowmick
reactions, corrosion etc. Whether any pre surface preparation
like cleaning etc. is required or not.
Theory of Operations A typical ultrasonic instrument consists of
the following components:
i. An electronic signal generator for producing bursts of
alternating voltage. ii. A transducer for transmission and
reception of the ultrasonic waves to and
from the test piece. iii. A couplant to act as a medium of wave
propagation between the transducer
and the test piece. iv. An electronic device to amplify or
demodulate or modify the signal from
the transducer. v. A display screen or indicator to demonstrate
the thickness readings or
ultrasonic waves received from the various layers of the test
piece. vi. An electronic clock or timer to control the sequence of
actions and thereby
acting as a reference point. Ultrasonic Inspection
Figure 2.1: Types of Ultrasonic Inspection based on operation.
Since the Pulse Echo Technique is popular at Thermal Power
Stations, hence this paper is highlighting this technique only. But
the major distinction between the two methods is that the Thorough
gives the measurement of the signal attenuation while Pulse Echo
Technique measures Pulse Echo Technique The techniques causes
detection of echoes produced when an ultrasonic pulse is reflected
at an interface of test piece. In this process short bursts of
ultrasonic energy are introduced at regular intervals into the test
specimen. If the pulses encounter a reflecting surface, some or all
of the Ultrasound energy is reflected back. Both the
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Ultrasonic Inspection for
reflected energy and timereception of ay response fr The Pulse
Echo Techdetection.
Figure 2.2:
Ultrasonic Wave PropagIn air sound travels by cdirections of
travel. Howedirections, so a number oLongitudinal waves and Sin the
industrial sectors, waves travel through soliattenuated, or die out
in constant through a given hmaterial and these differelasticity of
each material.
Figure 2.3: A T
Wall Thickness Measurement
e delay between the transmission of initialfrom the test piece
are measured. hnique is utilised both in thickness measurem
A Typical Scale Thickness Measuring Gauge
gation and Wave Characteristics compression and rarefactions of
the air mever, in solids, the molecules can support vibof different
types of sound waves can be ghear waves are mostly preferred for
thicknessso their properties are particularly highlighids and
liquids at relatively high speeds, bugases. The velocity of a
specific ultrasonic homogenous material. The velocities differ
frences are largely due to the differences .
hickness result of a Typical Ultrasonic Instru
99
l pulse and the
ment and crack
e.
molecules in the brations in other enerated. Since s
measurements hted. Ultrasonic ut more readily
wave ode is a from material to in density and
ument.
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100 S. Bhowmick
Basically the Ultrasonic waves can be broadly categorized into
the following types Longitudinal Waves The longitudinal waves, also
known as compressional wave mode, consist of alternate compression
and rarefaction zones along the direction of propagation. The
propagation of this sound is caused by elastic bond between the
particles, wherein particle as it moves from its equilibrium
position, pushes or pulls the adjacent particles, which in turn
transmit energy to next particle and so on. Almost all the energy
originates as sound and may be converted into other wave modes upon
interference. This mode can propagate in all the three medium i.e.
Solids, Liquids and Gases and also has the highest velocity
compared to the three modes. This mode includes a large section of
straight beam probes ranging in frequency from.5 MHz to 25 MHz and
can thus measure large test specimens.
Figure 2.4: A diagrammatic representation of longitudinal wave
propagation. Shear Waves This wave mode, also known as Transverse
Mode, is next in importance in terms of industrial practice as
compared to Longitudinal Waves. Transverse Waves are visualised
readily in terms of vibrations of a rope that is shaken
rhythmically, in which each particle vibrates up and down, rather
than parallel to the direction of wave motion. Transverse waves
cannot be supported by the elastic collision of the adjacent
molecules. For the propagation of transverse waves it is necessary
that each particle exhibit strong force of attraction to its
neighbour, so that as the particle moves back and forth, it pulls
the neighbours with it. This makes the sound to move through the
material with a velocity of 50% of Longitudinal for the same
concerned material. Transmission of these waves is again not
supported by air and water.
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Ultrasonic Inspection for Wall Thickness Measurement 101
Direction of Wave
Particlemovement
Figure 2.5: A diagrammatic representation of shear wave
propagation. Lamb Waves Lamb waves also called Plate Waves consist
of complex vibration that occurs throughout the thickness of the
material and hence they are utilized to detect discontinuities in
thin sheets, only a few wavelengths thick. The wave propagation is
affected by material density, elastic properties and the structure
of the material as well as thickness of the test piece and the
frequency. The two basic types of Lamb waves are Symmetrical and
Asymmetrical. Surface Waves These waves are known as Raleigh Wave
mode and travel along smooth, rough and curved surface of
relatively thick solid parts. The surface waves suffer high
attenuation and travels at velocity of 90% of that of transverse
waves. Factors Affecting Wave Propagation During transmission
through the test specimen, the sound waves encounter losses in
energy due to various factors. These losses in case of Ultrasonic
waves are collectively known as Attenuation. Factors contributing
to energy losses can broadly categorized as follows: Transmission
Losses. Interference Losses. Beam spread Losses.
Transmission losses occur during the transmission of Ultrasonic
waves due to scattering, absorption and acoustic impedance effects.
Interference losses occur when the sound beam produced by on
oscillating particle interferes with the sound wave produced by
another vibrating particle. This could be the result of Phase
shifts, wave fringes, diffraction etc. Losses due to scattering
occur mainly because of certain irregularities in the lattice
structure. There may be sections even in a flawless test specimen
where the space
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102 S. Bhowmick
lattice structure of the material is defective or foreign
matter, which accumulates very small quantities at the grain
boundaries. The propagation of sound waves is very strongly
affected by this inhomogeneity. Grain boundaries and foreign matter
reflect very small sound portions into all directions. This effect
is called Scattering. Near Field Effects The crystal produces a
system of waves ina limited area, which leads to a sound field
shaped in a very complecated manner. In order to explain these
procedures, two point shaped sources P1 and P2 are taken which can
transmit spherical wave. It is also assumed that both the sources
produce maxima and minima of the same amplitude. In the space
sarrounding these points P and P there are certain where the path
difference between the two points is just i.e. at these points a
minimum of he one wave overlaps a maximum of the other wave. At
these points the waves compensate to each other. In order to make
the sound radiation from the crystal surface simplified, the
surface is subdivided into many small points. Each point of the
crystal is considered to be the starting point of a spherical
wave(Huygens principle). Due to the interference of all these waves
a complicated system of maxima and minima occurs in the sound
field. Behind the crystal a number of interference maxima and
minima can be seen. On the central beam there is the last
maximum(the main maximum) and from this point o noother maxima and
minima can exist. The area of maxima and minima upto the main
maximum is called near field. The distance between crystal and main
maximum is the near field length N. The near field length dependson
the face of the crystal i.e. the square of its diameter, the
frequency of the sound waves and the sound velocity in the material
in which the waves propagate. To a circular crystal the following
formula applies:
Where N= length of the near field; D=Probe diameter; =
Ultrasound wavelength. If the wavelength is less than the
Ultrasound transducer diameter, then the near field is calculated
as follows:
Near field effect may mainly cause sensitivities inconsistencies
while searching for discontinuities smaller than the probe
diameter. Hence, it is not recommended to inspect in near field.
The length of the near field can be adjusted by varying delay line
thickness, probe frequency and diameter.
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Ultrasonic Inspection for Wall Thickness Measurement 103
Leading Edge of Pulse (Extent of Dead Zone) Transducer Near
Zone
FarZone
Figure 2.6: Near Field and Far Field Zones in a specimen. Far
Field Effects & Other Losses As from a particular distance of
near field lengths the sound pressure on the central beam is
directly reduced proportionally to the distance of the crystal.
This area of the sound beam is called Far field. The area between
near field and far field is called Transition Area. It can be
stated that the shape of the sound beam depends on the sound
velocity; the frequency velocity means that the material of the
test object influences the shape of the sound beam. If a certain
material is concerned and thus the sound velocity known, a larger
near field and the small divergence angle are obtained by
increasing the testing frequency. The same effect can be reached by
increasing the crystal diameter. The geometry of the crystal and
the wave characteristics are the reason for the interference
effects. Again, wave propagation always leads to loss of energy. A
small portion of the oscillation energy of a mass particle involved
in a wave motion is lost during the conversion into heat; This
influence is called absorption and the resulting losses are known
as absorption losses. Calibration of Instruments Accuracy and
precision are the ultimate tools for any industrial operations. It
also appears the acceptance criteria of any technical inspection
result. Ultrasonic Inspection is no way exception to this policy. A
Calibration process can briefly defined as the checking of
instrumental control and its parameters at regular intervals in
order to ensure the accuracy of its results. In order to have a
comparative idea, reference test specimens having nationally or
industrially certified technical specifications, are utilised.
Since mainly normal beam probes are utilised at Thermal Power
Stations for Ultrasonic Inspection purposes, hence the techniques
comprising the calibration procedures are highlighted in the
following categories: Linearity of Time Base Check: In order to
check horizontal linearity or sweep linearity, it is necessary to
adjust multiple echoes obtained from longitudinal wave
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104 S. Bhowmick
transducer placed on a flat surface of standardised test block.
It is should be made clear that the distance between the initial
pulse and the first back surface reflection is always greater than
the successive echoes, and hence alignment should always commence
with first echo signal and not with the initial pulse. The echoes
should be so arranged that the leading or the left hand side of
each echo should always coincide with the divisions on the
horizontal scale on the CRT screen. The other types of calibrating
procedures like Linearity of amplification, Sensitivity check,
Resolution check and estimation of Dead zone etc. are considered to
be vital for flaw detection measuring units. However, for
Ultrasonic Scale measuring instruments we mainly rely upon the
relation of Sound velocity, time taken for transmission and
reception and the distance travelled. Among these, the velocity of
sound varies from material to material. Hence, the only objective
is to detect the exact sound velocity through a particular type of
reference material block for accurate measurement of specimens of
same material but of varying dimensions. Need and Contributions of
Ultrasonic Inspection to R&D and Thermal Power Stations in
India Shortage of power supply is an issue of perennial worry in
India. In place of increasing the National capacity of power
generation, some power stations are even obliged to decommission
their installed unit because of unavoidable technical faults. Among
the other technical reasons, forced outage deserves special. As per
the statistical records, the generating capacity was 76,700 MW in
1997 which was expected to be doubled in 10-15 years requiring
7,000 MW per annum which was not possible by the National
Organisations alone without the investment of private sectors and
foreign investments. Again, globally depleting coal resources has
worsened the catastrophy. The depletion of coal resources also
results in the degradation of quality of coal. Due to presence of
impurities like quartz, stones etc. much of the supplied coal is
rejected in Coal Handling Plants at the Thermal Power Stations,
thereby causing a huge loss to the National Economy. Even poor
quality of coal adversely affects the plant machinery also and
reduces their service life. To some extent this equipment damage is
accompanied by river water also. Even under these circumstances,
Thermal Power Station authorities under public sectors are bound to
purchase low grade quality coal at a relatively high price. Boiler
Tube Leakage, which has proved to be one of the major concerns of
Thermal Power Stations, can be considered as one of the negative
effects of coal and water quality. In recent years, boiler tube
failure and the subsequent capital overhauling has caused severe
forced outage at the major Thermal Power Stations & thereby
causing decommissioning of certain dependable units at the Power
Plants. Boiler Shut downs for months on virtue of Condition
Monitoring & other maintenance operations have become quiet
frequent, thereby affecting the state electricity revenue system
& causing shortage of power supply. Various factors chiefly
contributing to the boiler tube failure are combustion of low grade
coal, utilisation of river water containing impurities like silt,
poor tube
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Ultrasonic Inspection for Wall Thickness Measurement 105
material, prolonged deposition of scales inside the tubes,
overheating resulted from control of parameters at the
instrumentation room and various other reasons which still attract
highly intellectual long term research initiatives. Condition
monitoring operations can be considered as a fast hand remedial
measure to all these long term problems. Many reputed industrial
and R&D organisations perform such operations as outsourcing
agencies at the Thermal Power Stations. However, Wall Thickness
Measurement operations have proved to be a vital tool both for
condition monitoring operations as well as R&D activities. A
boiler tube with scale deposition greater than 32 microns can be
declared as failed tube, but acid cleaning can be conducted for
saving the tube before this deposition occurs. Wall Thickness
Measurement and related Scale Thickness Measurements are the two
effective methods which can act as guidelines to these acid
cleaning operations. An accurate measurement not only indicates the
tube which is likely to fail but it also helps in saving the tube
by causing an effective cleaning. Generally the scales found inside
the tubes are mainly composed of Fe2O3, Fe3O4, CaCO3 etc. Every
year Thermal Power Stations under public sectors spend several lacs
of rupees after the condition monitoring during periods of capital
overhauling, whereas a sincere initiative in the Wall Thickness
Measurement and Scale Detection operation with an R&D backup
will help to reduce boiler tube failure, thereby preventing capital
overhauling and forced outage not only in a single Thermal Power
Station but at all the Power houses throughout the country. Only
requirement for this success is a team of dedicated scientists well
coordinated with field engineers at the Thermal Power Stations.
Further Research and Development activities are carried on to
increase the operational efficiency and mobility to minimize
capital overhauling period. State Electricity Boards generate and
distribute powers, set tariffs and collect revenues. However they
suffer from chronic financial problems because of rising generating
cost accompanied by eroded revenues due to pilferage, bad debts and
supply of power for agricultural sector at subsidised rates. They
operate without a guideline on how to price power. However, at the
Thermal Power Stations various boiler accessories are subjected to
scale thickness measurements. These include Platen Superheater,
Pendant Reheater, Economiser and various other type of tubes found
within the boiler units during capital overhauling.
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106 S. Bhowmick
Figure 3.1: Areas of Wall Thickness Measurement inside a Boiler
Unit During Capital Overhauling. Acknowledgement I convey my deep
respect, honour and gratitude to the following intellectual
personalities, industrial and R&D organisations for their whole
hearted and precious contributions to the field of Condition
Monitoring and Non Destructive Testing. This paper is dedicated to
the devoted activities of NDT professionals, field engineers,
scientists, research and industrial personalities for upgradation
of Power Generation Systems in India. Prof. Mihir Sarangi,
Department of Mechanical Engineering, Indian Institute of
Technology, Kharagpur, Dr. Hassan Shaaban, Professor of Metallurgy,
AEA, all the Thermal Power Stations in India for their patronage
and supports to Condition Monitoring Operations, American Society
of Non Destructive Testing, Central Mechanical Engineering Research
Institute, A Constituent Establishment of CSIR, Central Power
Research Institute, An Autonomous Society under Ministry of Power,
Krautkamer NDT Ultrasonic Systems, Clarkson University, Potsdam,
New York 13699 U.S.A., SIS Institute of Non Destructive Testing,
Chennai, Donald N. Bugden, Vice President-Marketing, Magnetic
Analysis Corporation, Olympus NDT & GE Inspection Technologies.
References
[1] Berke, Michael, Nondestructive Material Testing with
UltrasonicsIntroduction to the Basic Principles, Krautkramer GmbH,
19901992.
[2] Betti, F., Guidi, A., Raffarta, B. et al., TOFDThe Emerging
Ultrasonic Computerized Technique for Heavy Wall Pressure Vessel
Welds Examination, 2002.
[3] Bhardwaj, M.C., High Efficiency Non-Contact Transducers and
a Very High Coupling Piezoelectric Composite,WCNDT, 2004.
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Ultrasonic Inspection for Wall Thickness Measurement 107
[4] Bhardwaj, M. C., Evolution of Piezoelectric Transducers to
Full Scale Non-Contact Ultrasonic Analysis Mode,WCNDT, 2004.
[5] Bhardwaj, Mahesh C., Non-Contact Ultrasound: The Final
Frontier in Nondestructive Analysis, Publication #SW302, March
2002.
[6] Frielinghaus, Rainer, Examples of Ultrasonic Application
Examples of Ultrasonic Application for Nondestructive Testing of
Plastics, Krautkramer GmbH, 1990.
[7] Hoover, Kelli et al., Destruction of Bacterial Spore by
Phenomenally High Efficiency Noncontact Ultrasonic Transducers,
November 2002.
[8] Krautkramer, The Krautkramer Ultrasonic Book, Krautkramer
GmbH & Co., 1998.
[9] Kobe Steel, Ltd., New Non-destructive Examination (TOFD
System), date and author unknown.
[10] NDT.net, Nondestructive Testing EncyclopediaUltrasonic
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[12] Sommer, Jrg,The Possibilities of Mobil Hardness Testing,
Krautkramer, GmbH date unknown.
[13] Splitt, G., Ultrasonic Probes for Special TasksThe Optimum
Probe for Each Application, Krautkramer GmbH, date unknown.
[14] Splitt, G., Piezocomposite TransducersA Milestone for
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[15] Paul E. Mix, INTRODUCTION TO NONDESTRUCTIVE TESTING, A
Training Guide, Second Edition.
Biography Saswata Bhowmick is a Mechanical Engineer with
professional specialisation in Machine Operations. He has obtained
a BTech Degree in mechanical engineering from West Bengal
University of Technology but has started his career by joining as
Junior Supervisor in a Chain Industry under private concern even
before pursuing engineering degree. During, his engineering career,
he received acknowledgement certificates for remaining associated
with academic as well as research projects in various Reputed
Industrial and R&D Organisations and is attached as executive
delegate with reputed industrial associations. His area of research
is R&D operations i.e. Operation & Utilization of any
machines related to mechanical engineering in R&D and modern
industrial sectors. He joined Indian Institute of Technology for
Technical Assistance in Power Plant based project & is
presently working as a Project Scientist, NDT and Tribology under
the department of mechanical engineering in the same project.