Ultrasonic Inspection for Wall Thickness

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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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.

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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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.


Figure 1.3: Various Types of Transducers based upon Geometry & Crystal Construction.

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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.


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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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.


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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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

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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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.


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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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.


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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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


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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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.


[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 TestingTOFD Principles,date unknown.

[11] Rao, B. P. C., Jayakumar, T., Kalyansundaram, P., et al., TOFD for Sizing of Defects Using Shear Horizontal Waves Generated by Electromagnetic Acoustic Transducers (EMATs), Proceedings of NDE-98, Trivendrum, 1998.

[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 Ultrasonic Testing, Krautkramer GmbH, date unknown.

[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.

Ultrasonic Inspection for Wall Thickness

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