ect signal analysis

GUIDELINES FOR EDDY CURRENT ANALYSIS

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CONTENTS PAGE

EXECUTIVE SUMMARY 2

INTRODUCTION 4

OBJECTIVES 5

EDDY CURRENT FUNDAMENTALS 6

EDDY CURRENT INSPECTION PRINCIPLES 19

SIGNAL ANALYSIS 26

DISCUSSION 35

RECOMMENDATIONS 36

REFERENCES 40

DISTRIBUTION LIST 91

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EXECUTIVE SUMMARY

EDDY CURRENT SIGNAL ANALYSIS - GUIDELINES

OVERVIEW

Eddy Current (EC) testing is an electromagnetic non-destructive inspection technique generally used to inspect tubular or thin sheet non-ferrous products. Due to the limited penetration depth, it is generally used to detect surface breaking or near surface indications.

BACKGROUND

The ASME code has been adopted, by the eddy current fraternity in South Africa, as the basic requirements for system calibration. This technique has however numerous limitations. This document provides guidelines for the analysis of typical indications found in Eskom Heat Exchangers. OBJECTIVES

The main objective of this document is to provide the reader with a basic knowledge of the eddy current inspection technique and to stipulate a set of guidelines to be used during defect characterisation.

APPROACH

The inspection process, consisting of the calibration procedure, data acquisition and signal analysis has been investigated to identify any factors that could lead to errors in the measurement accuracy. Guidelines are provided in this document to eliminate or suppress the error factors and thereby increasing the accuracy of eddy current measurements.

RESULTS

The following issues have been identified as limiting factors in the accurate classification of defect signals.

Analyst’s qualification and experience. Calibration procedure and reference samples. Detection limitations introduced by using a low fill factor probe (below 80%). Difference in signal response as a function of defect orientation. Indication geometry viz. The influence of both volume and depth on the phase angle. Analysis graphs relating only a single parameter to depth viz. either phase angle or

amplitude. Both of these has to be considered when determining the depth of an indication

CONCLUSIONS

The application of a specific calibration procedure to all types of damage is not recommended and inaccuracies in the depth measurement are inevitable.

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RECOMMENDATIONS

Procedures have to be established , based in these guidelines, to increase the accuracy of the eddy current signal analysis

INDUSTRY PERSPECTIVE

The project was performed in collaboration with an inspection company. The guidelines estalished in this document can be used to benefit not only the power generation industry, but also the petrochemical, mining and tube manufacturing sectors

KEYWORDS

Eddy Current Inspection, Heat Exchanger, Damage mechanisms, Analysis guidelines

FUTURE REVIEW

This document is a working document implying that new defect signals and calibration procedures would be added as the need arise. The process of establishing a calibration curve, stipulated in this document, has to be applied to all types of tubes and defect mechanisms.

RETURN ON INVESTMENT The return on investment can be calculated based on the number of unplanned outages required to identify leaking tubes, as well as in the delay of a component replacement.

The detection and identification of unexpected leaks has a direct cost implication on the power generation. Normally a half load is sufficient to identify a leaking tube. The time interval is normally 12 hours. At a rate of R20/MWh the total cost for such an intervention is estimated at R43200. Related costs, such as manpower, are not included in this estimate.

Accurate eddy current measurement can faclitate the delay in component replacement by reducing the total number of plugged tubes. An estimated amount of R6.6m was saved, on interest alone, when the Matla Unit 2 Main condenser retube was delayed for 3 years.

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1. INTRODUCTION

Non-destructive testing is technology by which the detection and sizing of indications and the material properties can be performed without damaging or destroying the sample being inspected. The detection of defects is made possible due to changes in the material properties such as density, velocity of sound, thermal conductivity, magnetic permeability and electrical conductivity. The characterisation of defects with regard to depth, length and position, whether surface or volumetric (within a sample) could be accomplished using different techniques viz. visual (surface), radiographic (volumetric), ultrasonic (volume and surface), acoustic emission (volume and surface), liquid penetrants (surface), magnetic stray flux (surface or subsurface indications in ferromagnetic materials), infrared thermography (surface) and electromagnetic (surface and subsurface).

Eddy current inspection is an electromagnetic inspection technique whereby surface or subsurface indication can be detected in electrical conductive samples. It is commonly used for inspecting tubular products, either internally or externally, inspecting for surface or subsurface defects when a non-conductive coating is present or access is limited. Apart from defect detection it can also be applied for material sorting based on the electrical conductivity and magnetic permeability of the sample and sample geometry measurements based on the probe lift off and electrical conductivity of the sample.

Different types of eddy current inspections are possible based on the material being tested. Normal eddy current technique, by which the same probe is used to generate and detect the change in material properties (near field), is ideal for inspecting nonmagnetic conductive materials such as aluminium, aluminium brass, copper (alloy 141), copper alloys (viz. admiralty brass alloy 443, 444 and 445), titanium, Cu Ni –alloys (70-30 alloy 715, 80-20 alloy 710, 90-10 alloy 706), stainless steel (viz. types 304 304N and 316).

Slightly ferromagnetic materials such as Monel (Alloy 400) require a saturation eddy current technique, whereby electromagnetic coils or permanent magnets are used, to align magnetic domains in the direction of the saturating magnetic flux lines, near the eddy current test coils. Thus, the material appears as a no domain nonmagnetic material to the eddy current flow and defects can be identified.

Highly magnetic material such as ferritic stainless steels (viz. type 439) and carbon steels cannot be completely saturated. Partial saturation is however possible provided that the magnet is properly sized and selected for the specific material, tube size and wall thickness. The partial saturation technique relies on variations in magnetic permeability decreasing proportionally with flaw depth.

Remote field eddy current testing (RFT) enables a volumetric inspection of ferromagnetic tubing. The RFT probe consists of two coils (generally greater than two tube diameters apart). The one generates the eddy current field (exciter) and the other senses the field (receiver). A magnetic field is projected through the material and a defect is characterised according to the signal size and phase lag between the receiver and exciter coils. This method is not as definitive as normal eddy current techniques and knowledge of failure mechanisms and the position of indications is crucial, since the areas (distance of coil separation) in front of and behind support plates are “blind” to defect detection.

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Flux leakage is the most popular method for inspecting ferromagnetic tubing. Typically a pair of induction coils are used to detect tube defects, one for both internal (ID) and external (OD) defects and the other for only internal defects. One differentially connected induction coil is placed between the poles of a powerful permanent magnet and senses the changes in flux leakage, which result form both the ID, and OD defects. The second differential coil is located outside the magnetic poles and is operated at a lower gain so that it only senses the changes in the flux leakage resulting from ID flaws. Since differential coils are insensitive to gradual wall loss, Hall effect element probes are installed around the one end of the magnetic pole and facilitate gradual wall loss detection.

2. OBJECTIVES

The eddy current inspection technique is an indirect means of establishing the test material condition. Defect detection is based on the changes caused by the defect on a secondary parameter, in this case the probe’s impedance. The accuracy is limited by the calibration and reference samples used and the calibration procedure applied. Data acquisition has to be monitored closely and a proper analysis procedure established for the different kinds of indications. The additional limitation linked to human errors is also introduced into the error equation. All of these factors have lead to inaccurate eddy current measurements and defect characterisation at Eskom Power Station.

The objective of this project was to identify the origin of the measurement errors and establish a guideline for eddy current tube inspection and defect analysis that will eliminate or decrease these inaccuracies. The following aspects were addressed for five main types of indications found in Eskom Heat exchangers viz. internal pitting, external pitting, dezincification, ammoniac attack and steam erosion:

Calibration standards and procedure Data acquisition Data analysis and measurement techniques

Although only five types of defects were addressed, a guideline has been established whereby the inspection, characterization and classification of indications can be qualified and validated. Aspects such a destructive examination of indications, indication verification, root cause analysis of actual defects found in Eskom Heat Exchangers have been addressed.

The objective of this report is to present the reader with a basic knowledge regarding the theory of eddy current, the usage of eddy currents within Eskom, typical indications found and a set of guidelines for eddy current inspection, analysis and reporting. By no means has an attempt been made to discuss the complex electromagnetic theory of the eddy current inspection technique or to address the intricacies of signal generation and probe design. These factors have purposely been omitted so that the basics of the eddy current technique would not be obscured with complex formulas or computer simulation.

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3. EDDY CURRENT FUNDAMENTALS

3.1. Magnetic fieldOersted discovered that a magnetic field is always associated with a current flowing through a conductor or coil. The direction of the homogeneous magnetic field inside the coil can be determined by the right hand rule which states that if the solenoid is gripped with the right hand, with the fingers pointing in the direction of the conventional current flow, then the thumb outstretched parallel to the axis of the solenoid points in the direction of the magnetic field. The strength of this magnetic field is directly proportional to the current and the number of coil turns and indirectly proportional to the length of the coil. An inhomogeneous magnetic field exits outside the coil.

3.2. Magnetic Flux Associated with the magnetic field is the magnetic field density B (Wb/m2), which can be defined as the number of flux lines that cross a surface area perpendicular to the flux lines. It has the same direction as the magnetic field and the magnitude depends on the distance from the coil and the current through the coil.

The total magnetic flux p (Wb) contained within the loop is the product of the magnetic field density and the area of the coil.

3.3. Magnetic permeabilityMagnetic permeability () can be defined as the ability of a material to conduct magnetic flux lines.

3.4. Equation governing eddy current generationThe eddy current inspection technique is based on an electromagnetic interaction between a current carrying coil (Primary circuit) and the test sample (Secondary circuit).

The current flowing through any electrical circuit is governed by Ohm’s law which states that the current is equal to the driving voltage (Primary circuit) divided by the primary circuit impedance.

Ip = Vp / Zp - Eq 1

The eddy current coil forms part of the primary circuit. The primary impedance consist of a resistance and a inductive reactance component of an alternating current is used to excite the coil.

The current passing through the coil normally varies sinusoidally, thus:

Ip = I0 sin (t) - Eq 2I0 is the peak current value is the frequency of excitation in radians per second and is defined as 2F (if F is in Hertz)t is the time

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Figure 1. The induction of eddy current is based on the electromagnetic interaction between a primary circuit that is excited using an alternating current and the resultant eddy currents induced in the test sample material

A magnetic flux (p) exists around the coil (Oersted) and is proportional to the number of turns in the coil (Np) and the current (Ip)

p Np Ip

The magnetic (p) flux is in fact directly proportional to the magnitude of the applied current, the rate of change (frequency) of the current and the coil parameters such as the inductance, coil diameter and length, wire thickness and number of turns as well as the core material.

Faraday’s law of induction states a voltage Vs is created in a region of space when there is a changing magnetic field. When we apply this to our coil:

Vs = -Np (dp / dt) - Eq 3

Where dp / dt is the rate of change in magnetic flux (p) caused by the primary circuit excitation.

Since the coil current varies sinusoidally with time, total magnetic flux in the coil also varies sinusoidally

p = 0 sin (t)0 is the magnetic flux corresponding to excitation current through the coil (I0)

The induced voltage as described by equation 3 results in

Vs = -Np 0 cos (t) - Eq 4

Equation 4 indicates that the induced voltage also varies periodically with time. If we bring the coil (primary circuit) close to a conductive test sample, Ohm’s law states that

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if there is a driving voltage (Vs) , within a conductive medium, and the samples impedance is finite, current will flow. The current is governed by the following equation:

Is = Vs / Zs - Eq 5Is is current through the sampleVs is induced voltageZs is impedance or total opposition to flow of current in the sample. The impedance is a total of both the resistance and the inductive reactance of the coil

These induced currents are known as eddy currents because of their circular paths. They in turn generate their own magnetic field, which, according to Lenz’s law, opposes the primary field

s Is

and

E = p - s - Eq 6E is the equilibrium magnetic flux surrounding the coil in the presence of a test sample.

The influence of permeability, conductivity and sample geometry on the equilibrium flux is the crux on which eddy current inspection technique is based.

If the primary circuit is placed in air, then the equilibrium flux is equal to the primary magnetic flux (p).

However, the closer the primary circuit is brought to the test sample, the larger the secondary magnetic flux and the lower the total value of the equilibrium magnetic. The flow of eddy currents, in the test sample, results in resistive (Ohmic) losses if a defect is detected, and an increase in the equilibrium magnetic flux. The Ohmic losses can be attributed to the increased distance that the eddy current must travel to traverse the defect indication. This is reflected as a change in probe impedance. This change in the total impedance of the eddy current probe is detected by various means. In the equation form:

Z E - Eq 7

And

V = Z Ip - Eq 8

Equation 7 indicates that a coil’s impedance is a function of the magnetic field surrounding it and in turn the magnetic field is governed by the induced current in the specimen.

To summarize, flux is set up by passing an alternating current through the test coil. When this coil is brought close to a conductive sample, eddy currents are induced. In addition, the magnetic flux associated with the eddy currents opposes the coil’s

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magnetic flux, thereby decreasing the net flux. This results in a change in coil impedance and voltage drop. It is the opposition between the primary (coil) and the secondary (eddy current) fields that provides the basis for extracting information. This process is called eddy current testing (ET).

It should be noted that if a sample is ferromagnetic, then equation 6 still applies but the magnetic flux is strengthened despite the opposing eddy current effects.

3.5. Nature of eddy current flow

Eddy currents are closed loops of induced current circulating in planes perpendicular to the magnetic flux. They normally travel parallel to the coil’s winding and parallel to the surface (depending on the orientation of the coil relative to the surface). Eddy current flow is limited to the area of the inducing magnetic field and varies sinusoidally with time.

Figure 2. The induction of eddy currents as a function of the depth into the test material

The test frequency determines depth of penetration into the specimen; as frequency is increased, the penetration depth decreases and the eddy current distribution becomes denser near the specimen’s surface (nearest to the test coil). Test frequency also affects the sensitivity to changes in material properties and defects.

Figure 2 shows the variation in the algebraic relationship and the oscilloscope display of eddy current and magnetic field distribution as the depth into the specimen is increased. Both the eddy currents and magnetic flux gets weaker with depth because of “skin effect”. In addition to this attenuation, the eddy currents lag in phase with depth. The two parameters that are linked to the decrease in distribution and phase lag viz. the eddy current signal amplitude and the phase angle is the two major parameters used for indication characterization.

3.5.1.Skin Effect

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Eddy currents, induced by a changing magnetic field, concentrate near the surface adjacent to the excitation coil. The depth of penetration decreases as the test frequency, conductivity or permeability is increased. This phenomenon is known as the skin effect and is analogous to the situation in terrestrial heat conduction where daily surface temperature fluctuations are not appreciable below the earth’s surface.

Skin effect arises as follows: The eddy currents flowing in the test object at any depth produce magnetic fields, which oppose the primary field, thus reducing net magnetic flux and causing a decrease in current flow as depth increases. Alternatively, eddy currents near the surface can be viewed as shielding the coil’s magnetic field thereby weakening the magnetic field at greater depths and reducing induced currents.

Figure 3 The reduction of the eddy current density with an increase in depth as seen through the reduction of signal amplitude

The equation for flow of induced currents is

2 J = (J / t)J is current density is conductivity is magnetic permeability 2 is a differential operator of second order.

For a semi-infinite (thick) conductor the solution to the above equation is

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(Jx / J0) = e - sin(t - ) - Eq 9

Jx / J0 is the ratio of eddy current density Jx at depth x to the surface density J0

e = 2.718 is the base of natural logarithms is given by (x / ) where = (f) -½ the standard depth of penetration

Equation 9 can be separated into two components:

(Jx / J0) e -

which describes the exponential decrease in eddy current density (signal amplitude)with depth and

(Jx / J0) sin(t - )

denoting the increasing time or phase lag (signal phase angle) of the sinusoidal signal with depth into the material.

3.5.2.Penetration Depth

Figure 4 illustrates the change in eddy current density in a semi-infinite conductor. Eddy current density decreases exponentially with depth.

Figure 4 Eddy current density as a function of the depth into the test sample

The depth at which eddy current density has decreased to 1/e or 36.8% of the surface density is called the standard depth of penetration. The standard depth of penetration is calculated by solving equation 9 and is given as

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= 50.3 (/r F) (mm) is the resistivity in .cm or (172.4/) is the conductivity expressed as the %IACS (International Annealed Copper Standard)F is the test frequency in hertzr is the relative permeability (dimensionless)The constant of 50.3 is introduced into this formula to provide a standard depth penetration in millimeters while the other units are not based on the SI system.

The skin depth equation is strictly true only for infinitely thick material and planar magnetic fields. Using the standard depth, , calculated from the above equation, makes it a material/test parameter rather than a true measure of penetration.

Figure 5 indicates the typical penetration depths for selected tube materials typically used in Eskom heat exchangers as a function of the frequency.

The depth of penetration is influenced by both the permeability and the conductivity of the test material. Higher permeability values facilitate the flow of magnetic flux lines through the test material. Thereby increasing the eddy current density on the surface, thus increasing the “shielding “ effect of the primary magnetic field and reducing the depth of penetration. Higher conductivity values have a similar effect on the penetration depth.

In the case of surface probes, it is not the coil cross-section but the coil diameter D0

that largely controls penetration. The magnetic field, in a thick material under a surface probe, penetrates to a depth of (D0/4). One can decrease the penetration to less than (D0/4) by increasing the test frequency, but decreasing the frequency will not increase the penetration depth appreciably. Increasing the coil diameter however decreases the sensitivity towards small indications and a compromise needs to be established based on the inspection requirements and the type of discontinuity one wishes to detect.

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Figure 5. Standard depth of penetration calculated for materials typically used in Eskom heat exchangers

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3.5.3.Phase lag

The signal produced by a flaw depends on both amplitude and phase of the currents being obstructed. A small surface defect and large internal defect can have a similar effect on the magnitude of test coil impedance. However, because of the increasing phase lag with depth, there will be a characteristic difference in the test coil impedance vector. This effect allows location and extent of a defect to be determined.

Phase lag is derived from equation 9 for infinitely thick material. It represents a phase angle lag in radians between the sinusoidal eddy currents at the surface and those below the surface. It is denoted by the symbol (beta) and is given by:

= x/ = x/(50 (/fr) ), radians

where x is distance below the surface in the mm

Figure 6 Eddy Current Phase Lag Variation With Depth in Thick Samples.

The phase lag is 57° or one radian, when x is equal to one standard depth of penetration. This means that the eddy currents flowing below the surface, at one standard depth of penetration, lag the surface currents by 57°. At two standard depths of penetration, they lag the surface currents by 114°.

Phase lag is the parameter that makes it possible to determine the depth of a defect. It also allows discrimination between defect signals and false indications. It is the main parameter in eddy current testing.


3.6. Probe impedance and impedance plane

The test coil can be characterised by two electrical quantities viz. the Ohmic resistance and the inductive reactance

The Ohmic resistance R, which is an opposition to the flow of electrical current, is given by Ohm’s law as

V = IR

and is constant for both direct and alternating current.

The inductance is the property of an electric circuit in which a varying current induces an electromotive force in the circuit (self) or in a neighbouring circuit (mutual).

L = N p /I

N is the number of coil turnsp is the magnetic flux of the primary circuit (Weber)I is the current (Ampere)

The inductive reactance is the opposition to changes in the alternating current flow through a coil and is also measured in ohms.

XL = L = 2fL is the angular frequencyL is the inductanceF is the test frequency

The impedance of the coil is the total opposition to an alternating current and is determined by both the resistance R and the inductive reactance.

The impedance Z is a complex vector quantity and can be written as:

Z = R + iXL

(Assuming that the coil capacitance is negligible)

It is common practice to represent the coil impedance on a two-dimensional impedance graph display. The so-called complex impedance plane. The inductive reactance is plotted on the y-axis and the resistance on the x-axis. The coil impedance is represented by a point P (operating point) situated at a distance.

Z = (R2 + XL2)

From the origin at an angle given as

= Arctan (XL/R)

The position of the operating point is determined by the following:

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type of probe, the test sample geometry, the excitation frequency, sample conductivity the distance between the probe and the surface of the test sample.

If the resistance and inductance of the coil in free space are taken as R0 and L0

normalised impedance plane diagrams can be produced by plotting inductive reactance (L/L0) against resistance (R/L0) where is the angular frequency. The impedance locus is that of a semi – circle with centre at XL/X0 = 0.5 and RL/X0 = 0 and radius 0.5.

Figure 7 Impedance graph display for a solid cylindrical bar and thin tube wall.

The eddy current operating point (Balance Point) can be found on this locus. Various factors influence the exact position of the operating point:

Resistivity: The impedance point moves up the curve with increasing resistivity. The typical response from a defect that increases the resistivity will therefore be on this direction.

Thickness: As test material becomes thinner, causing increased resistance to eddy currents, the impedance point moves up the curve.

Frequency: As frequency is increased, eddy currents are sampling a thinner layer close to the surface and the impedance point moves down the curve. When the

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L/L0

RL/L0

frequency is decreased eddy currents penetrate deeper into the material and the impedance point moves up the curve.

Probe diameter: An increasing coil diameter moves the impedance point down the curve.

Figure 8 Factors influencing the position of the operating point on the impedance curve display

The ideal operating point for eddy current inspection is near the “knee” of the curve. The angle variation between lift off and resistivity at the top of the curve is small. Small lift off variations at the bottom of the curve cause large impedance changes. Impedance is mainly composed of resistance at the bottom of the curve. Resistance depends very much on temperature and a temperature change can lead to false interpretations in this region.

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Figure 9 A simplified artistic presentation of the impedance plane graph indicating the operating point of the eddy current system. The signal response for the ASME calibration tube as detected on the 25kHz channel is displayed to show the typical amplitude and phase variation around the actual operating point. The largest signal amplitude indicates the internal groove indication.

The ideal operating point for eddy current inspection is near the “knee” of the curve. The angle variation between lift off and resistivity at the top of the curve is small. Small lift off variations at the bottom of the curve cause large impedance changes. Impedance is mainly composed of resistance at the bottom of the curve. Resistance depends very much on temperature and a temperature change can lead to false interpretations in this region.

The curve shown in the impedance graph display can be regarded as frequency locus for a selected probe diameter and test sample (Conductivity, material thickness and fill factor are constant). A qualitative explanation of the shape of the curve can be given as follows.

Eddy current generation is dependent on a changing magnetic field. If the excitation frequency is zero, no change in the magnetic field exists and no eddy current are generated. Since no current is flowing in the test sample, no resistance to the flow of current is possible and R is zero. The normalised inductive reactance is one since the probe’s inductive reactance does not change (L = L0 and L / L0 = 1).

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Signal response because of probe wobble

10% Internal groove signal

10% External groove signal

Eddy currents are generated in the sample as soon as the frequency is no longer 0. If eddy currents are flowing in the test sample then resistance to the flow of current exist. With an increase in the frequency, the eddy current penetration depth is reduced. The density of eddy currents on the surface of the test sample increase, causing a higher resistance to the flow of currents. The increasing eddy current density at the surface also leads to an increase in the secondary magnetic field strength, reducing the equilibrium impedance of the probe and hence the inductive reactance.

A further increase in the frequency results in an even higher concentration of eddy currents on the surface. The larger secondary field reduces the equilibrium impedance of the probe even further and the inductive reactance is reduced to zero. With the majority of eddy currents being concentrated on the surface, the resistance to eddy current flow is reduced, since the volume of eddy current flow in the sample is reduced.

The only two parameters that can be changed, by the operator, in order to move up or down the impedance locus are the fill factor and the frequency. Hence the selection of frequency plays an important role.

4. EDDY CURRENT INSPECTION PRINCIPLES

4.1. Frequency

4.1.1.SelectionSelection of inspection frequency is normally a compromise. For instance, penetration should be sufficient to reach any subsurface flaws that must be detected eg. external pits using an internal bobbin probe. Unfortunately, as the frequency is lowered, the sensitivity to flaws decreases somewhat and the speed of inspection may be influenced. The highest possible frequency that provides the required penetration depth is selected. Frequency selection for internal indications is easy, since the internal surface condition are the only limiting factor. Frequencies ranging from a few kilohertz to one or two megahertz are usually selected, depending on the type of material. External defect detection requires the selection of a low frequency and a slight sacrifice in sensitivity has to be made.

The selection of suitable test frequencies for inspecting tubes are based on the following two principles viz. adequate phase discrimination between defect signals and other indications and a good phase separation between internal and external defect signals.

The sensitivity to various discontinuities and damage is related to the depth of penetration and is characterized by the signal to noise ratio as indicated by Figure 10.

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Figure 10. Variation of eddy current parameters as a function of the frequency.

Figure 11. Amplitude variation, as a function of frequency, of the external damage (OD) relative to various extraneous eddy current signals.

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The basic frequency selection is based on the type of material, material thickness and the type and position of defects that has to be detected. The basic frequency is selected to be sensitive towards all types of damage, both internal and external, and should be capable of accurately characterizing all indications. The basic frequency should be sensitive to all relevant extraneous discontinuities and be capable of discriminating between these indications and defect indications.

The frequency referred to as f90 has proven to be the most successful selection for a basic frequency. This frequency yields a 90 phase separation between the 10% internal groove indication and the 10% external indication. The frequency can be empirically derived by using a ratio of 1:1 to one between the material thickness (or required penetration depth) and the skin depth. The frequency is then given as:

f90 = 3 / t2 Kilohertz is the resistivity in micro ohm centimeters t is the tube wall thickness in millimeters

4.1.2.Multi-frequencyDetection and evaluation of specific discontinuities is based on a multi-frequency analysis technique whereby auxiliary frequencies can be used to either confirm the type of damage or be used to measure the extent of the damage. For instance, although inlet end erosion can be detected by the basic frequency in the differential mode, analysis and characterization is performed in the absolute channel at a lower frequency. Multi-frequency analysis technique is also used to differentiate between internal pitting and magnetic inclusions, by reducing the test frequency the phase angle between the fill factor and the inclusion signal is increased. Verification of defect signals is based on the correlation between the amplitude, phase angle, shape and depth estimation between the various frequencies.

Thus by incorporating additional high and low frequencies into the inspection procedure facilitates the interpretation of complex signals. The auxiliary frequencies should be significantly different from the basic frequency. Normally auxiliary frequencies applied are selected to be twice and half the basic frequency. Harmonics between the selected frequencies are eliminated through interaction between the soft- and hardware.

4.1.3.Mixing The extraneous discontinuities, either internal such as internal surface roughness, or external, such as the support structures, perturb the eddy current signal and result in a complex signal usually masking possible damage. By using multi-frequency mixing techniques, the extraneous discontinuities can be suppressed and damage can be identified and characterised.

A mixing frequency should be very sensitive to the discontinuity that requires suppression, but insensitive to the possible damage. Normally the suppression of an internal discontinuity requires a mixing frequency higher than the basic frequency and for an external discontinuity a lower frequency. The shape of the unwanted signal is normally the limiting factor. Although the phase angle and the amplitude prior to mixing can be the same, it is normally a large difference in shape that results in poor suppression of the discontinuity.

4.1.4.Influence

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The frequency selection has an influence on all of the signal parameters except the signal duration that is determined by the axial length of the indications. The effects of frequency on the signal amplitude and phase angle are shown in appendix 1. The eddy current response for selected indications is shown to indicate the change in amplitude, phase angle and shape as the frequency is increased. Note the percentage depth correlation for the various indications as measured on different frequencies.

Figure 12. Variation in amplitude and shape for the internal 10% groove indication. This indication is used to set the phase angle and thus no change is noted.

Figure 13. Variation in phase angle and shape for the internal 1.5mm hole indication. This defect is used to set the amplitude and thus no change is noted.

Figure 14. Variation in amplitude, phase angle and shape for the internal 80% external flat bottom hole indication.

Figures 12 to 15 indicate the change of signal parameters, as the frequency is increased. The amplitude for internal indications increases as the frequency is

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increased. The phase angle for internal indications shows a small increase as the frequency is increased. The shape of the lissajous signal tends to close up as the frequency is increased.

The amplitude for external indications decreases with an increase in frequency, since the penetration depth is reduced. The phase angle shows a significant increase.

The signal to noise ratio is dependant on the external or internal nature of either the indication or the noise. If both the defect and the noise origin are internal, then the ratio increase slightly as the frequency is increased. (Figure 14)

The ratio of external defects to internal noise would decrease as the frequency is increased since the higher frequency is more sensitive to the internal noise. On the other hand, if the noise signal is due to external circumstances and the defect is internal, then the ratio increases drastically as the frequency is increased. Both external noise and defect indications result in a slight decrease of the signal to noise ratio.

Figure 15. The signal to noise ratio for internal indications increases as the frequency is increased.

4.2. Inspection modes

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4.2.1.Absolute modeThe absolute inspection mode is based on a single coil configuration with the other leg of the Wheatstone bridge being balanced either electronically or using a second coil inside a reference sample. The impedance of the single coil is changed as the coil moves over an indication and remains unbalanced until the indication is traversed. The absolute inspection mode is sensitive to long gradual indications and provides the profile of these indications. The interpretation of the signal is made easy by the absence of the signal shape parameter.

The absolute mode is however not sensitive to small volume defects such as pitting. The absolute inspection mode is sensitive to temperature variation and probe wobble since the external or electronically balance cannot cater for the localized inspection coil conditions.

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Figure 16. The time base and lissajous signal display for the absolute inspection mode.

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4.2.2.Differential Mode

The differential inspection mode is based on a double coil configuration with either coils forming the two balancing legs of the wheatstone bridge. The close proximity of the coils (normally separated by 1 to 3 mm) reduces the sensitivity towards localized coil condition variation such as temperature variations and probe wobble. The differential mode is sensitive to small volume indications such as pitting. Long and gradual indications are difficult to detect and can only be identified as two edge signals when sharp edges are present.

Figure 17. The different probe configurations used to conduct the various inspection modes.

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Figure 18. The time base and lissajous signal display for the differential inspection mode.

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5. SIGNAL ANALYSIS

5.1. Signal Parameters

5.1.1.Eddy Current Signal Amplitude

The analysis of eddy current signals and the resultant defect characterisation is based on the following parameters viz. amplitude, phase angle, signal shape, duration and signal to noise ratio. The eddy current signals can be presented either in an x-y display (impedance) or a time base display. The duration as well as the signal to noise ratio can easily be measured on the time base display while the signal shape, amplitude and phase angle can be identified using the x-y display.

The signal amplitude can be related to the volume of the indication. The amplitude is normally measured from peak to peak, but both the x-component and the y-component can be measured separately to provide a more accurate defect assessment. The amplitude varies with other discontinuity characteristics such as volume and concentration of conductive deposits, dent size, the support plate gap, etc. The amplitude of a constant volume external indication increases as the depth increases and forms the basis of amplitude analysis. The orientation of the defect influences the signal amplitude and the maximum value is measured when the defect orientation is perpendicular to the eddy current path.

The amplitude of internal indications increases as the frequency is increased. This can be attributed to the skin effect with a larger concentration of eddy currents on the internal surface of the tube. The amplitude is also influenced by the fill factor and the orientation of the defects with respect to the probe. Experiments indicated that defects at the top of the tube could be missed if the fill factor is less than 80%. This is because the distance between the probe and the defect is increased and the eddy current density near the defect is reduced.

Figure 19. Peak to peak amplitude measurement for a differential (left) and an absolute (right) signal.

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Figure 20. Horizontal maximum (left) and Vertical Maximum (right) is two techniques that can also be used to measure the signal amplitude.

5.1.2.Phase Angle

The phase angle is measured either clock or anti-clockwise relative to a reference point. According to the ASME calibration procedure, the phase angle is measured clockwise from the negative x-axis (Cartesian system). The phase is normally measured between the negative horizontal axis and the line drawn from peak to peak. Numerous other measurements techniques, to quantify the phase angle, can also be used, such as the maximum rate of change and the gausian angle for double amplitude signal (symmetric or unsymmetric) or a customised phase angle measurement for multiple amplitude signals.

The signal phase angle, combined with the directional nature of the signal, is used to characterise the type of indication viz. to differentiate between an internal pit indication and a dent indication. The phase angle can be related to the indication depth for both internal and external indications. An increase in the frequency increases the phase angle of a through hole and thus the gradient for internal indication depth measurement. The accuracy and the sensitivity (increase in amplitude) towards internal indications are increased by increasing the frequency. The difference in phase angle between the 10% internal and 10% external indications determines the accuracy of the measurement. The larger the delta phase value, the better the accuracy in measurement.

Figure 21. Peak to peak phase angle measurement for a differential (left) and an absolute (right) signal.

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5.1.3.Signal Duration

The signal duration can be measured on the time base display and is an indication of the axial length of the indication. It is important not to confuse the phase lag with the signal duration. The phase lag is a determined by the eddy current induction into the material and the signal duration is an indication of the time that a specific indication influence the coil impedance. The duration of the detection is normally longer than the actual length of the discontinuity because of the influence of primary magnetic field surpassing the coil width. The primary field, around the test coil, is a function of the test frequency and the probe design.

Figure 22 Measurement of the signal duration for a differential (left) and an absolute (right) signal

5.1.4.Signal to Noise ratio

The signal to noise ratio is the ratio of the amplitude of the defect signal to the amplitude of an indication containing no useful information. This ratio is a qualitative means for determining the examination sensitivity. The accuracy in depth estimation is determined by this ratio.

The signal to noise ratio increases as the fill factor increases and varies with the test frequency and the mixing frequency for specific discontinuities. Accurate depth measurement is possible if the ratio is at least 3:1. Signal parameters can still be measured at a lower ratio but the measurements are questionable due to the signal disturbance.

Figure 23. Signal to noise ratio of a typical internal pit indication

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5.1.5.Signal Shape

The shape of the eddy current signal can be interpreted on the x-y base display. The shape refers to the symmetry of the lissajous signal and the size and openness of each lobe. The shape of the signal is used to determine the type of indication. The frequency, probe design and the type of indication have an influence on the shape of the signal. The lissajous signal tends to close up as the frequency is increased or as the coil width and coil spacing is reduced.

Figure 24. Note the difference in signal shape between a dent indication and a 60% external indication.

5.2. Calibration

As mentioned previously, the eddy current inspection method is an indirect measurement technique i.e. the indication detection is based on the influence of the electrical conductivity, permeability, geometry and the test frequency on the probe impedance. Calibration and reference samples, consisting of artificially manufactured indications, are used to calibrate and establish calibration curves, used to determine the defect characteristics. It is very seldom that all the signal parameters for an artificial defect correspond to that of a real defect but the phase and amplitude values provide a means of determining the depth of indications.

The accuracy of measurement depends on the correlation between the artificial and real defect geometries. This project has indicated that for a one sided internal wall loss indication an error of 50% is possible if the geometry of the indications is not taken into account.

The accuracy and the type of reference indications normally used, are dependent on the manufacturing technique. Spark eroded notches and slots are used to simulate cracks, while internal and external machined grooves and drilled holes are adequate to simulate manufactured or service indicated tube defects. The limitation in measurement accuracy can be attributed to poor calibration and analysis procedures. Since only two calibration tubes, viz. the differential (to simulate short localized wastage) and the absolute (to simulate long gradual wastage) tubes are used to establish amplitude vs. depth and a phase angle vs. depth curves, it is important to identify the limitations of these curves.

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The differential calibration tube complies to the ASME code which requires the following defects to be present:

Indication Type Diameter1 / Width(Depth is given as % of Tube Wall Thickness)

10 % Internal Groove 1mm100% Through Hole 1.5mm100% Through Hole 2mm80% External FBH2 2mm60% External FBH 3mm40% External FBH 5mm

4 x 20% External FBH 5mm10 External Groove 1mm

These indications are used to establish the required phase vs. depth calibration curves for the various channels.

The hole (1.5mm) is one of the reference indications (10% internal groove is used to adjust the phase angle), and is used to set the amplitude value. The phase angle is adjusted by either setting the hole to 45 or the internal groove indication to 0 depending on the availability of a suitable calibration tube or on the inspection requirements.

The depth of an internal indication is measured against an assumed linear relationship between the 10% internal groove and the hole indications. This relationship is assumed since it is difficult to manufacture and verify internal indications. Internal indications can be differentiated from external indications by noting that a phase angle of less than 30 (based on figure 25) depicts an internal indication. Indication phase angles exceeding 30 are regarded as external indications.

Figure 25 The phase angle vs. depth curve for the 25kHz differential channel.

Various depth external indications are used to determine the phase vs. depth relationship for external indications. Flat bottom machined holes are normally used.

1 Valid for a 23 x 1mm tube2 Flat Bottom Hole

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The diameter of these holes is adjusted to ensure a constant indication volume. The amplitude of the eddy current signals should therefore be constant and assist in the calibration process. Determining the volume of a flat bottom hole is easier than for a round bottom hole and is therefore used. The phase angle variation between the 10% internal and external indication is normally between 40 and 120, depending on the frequency.

The absolute channel is calibrated using the following artificial indications:

Indication Type Width(Depth is given as % of Tube Wall Thickness)

10% External Circumferential Wastage 40mm20% External Circumferential Wastage 40mm30% External Circumferential Wastage 40mm40% External Circumferential Wastage 40mm50% External Circumferential Wastage 40mm50% Internal Circumferential Wastage 40mm40% Internal Circumferential Wastage 40mm30% Internal Circumferential Wastage 40mm20% Internal Circumferential Wastage 40mm10% Internal Circumferential Wastage 40mm

Since the absolute inspection mode is insensitive to small indications, a calibration tube consisting of large volume indications is used to calibrate the absolute channels. The phase angle provides little information as to the depth of an indication when an absolute channel is used. An amplitude vs. depth curve is established using above-mentioned indications and used to determine the extent of gradual wall loss.

Figure 26 The amplitude vs. depth graph for the 9kHz absolute channel.

The 20% external wall loss area is used to adjust the signal response. The phase angle is normally set to 90 and the amplitude to 0.5volts. These values can differ based on the calibration and analysis procedure and the type of defect expected.

5.3. Limitations and errors inherent to above mentioned calibration technique

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The aim of this project was to identify the limitations in the eddy current inspection technique, determine the origin of these errors and if possible, to eliminate or reduce the error by modifying the calibration procedure, data

acquisitioning and signal analysis technique. And to document these findings as guidelines (see appendix 4)

By applying the above mentioned calibration technique the following limitations are introduced into the inspection:

5.3.1.The procedure does not provide a means for evaluating the probe centralization. Probe movement and defect orientation is thus introduced into the inspection equation. These factors have proven to be main contributing factors for inaccurate measurement. The error factor increases drastically as the fill factor is reduced. Errors of up to 60% wall loss can be made.

5.3.2.The circumferential extent of the defect plays an important role in the depth estimation based on the amplitude vs. depth graph. Since a partial circumferential extent implies a reduction in volume, the amplitude values for these types of indications need to be modified. Errors of up to 50% in the wall loss can be made.

5.3.3.The above-mentioned calibration method assumes that the phase angle is a measurement of the indication depth and the amplitude can be related to the indication volume. The phase angle and amplitude are also related and both combined should be used to determine the extent of the damage. Large diameter indications tend to be underestimated and small diameter indications overestimated. Errors of up to 20% in the wall loss can be made.

5.3.4.The linear assumption for phase angle values of small volume internal indications has been proven inaccurate. Indications exceeding the diameter of the reference defect (1.5mm) tend to be overestimated and indications below the reference diameter underestimated. Errors of up to 20% in wall loss are possible.

5.4. Indication analysis guidelinesIt is clear that significant errors are inherently part of the calibration and analysis procedure currently used for eddy current inspections. A guideline has been compiled for signal analysis and defect characterization based on the results of this project and can be found in appendix 4. These guidelines should be used to establish a calibration and analysis procedure for each type of indication identified.

6. DISCUSSION

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Eddy current signal analysis requires human intervention, due to limitations in analysis software. Automatic analysis is usually performed on a single frequency and only two signal parameters viz. the amplitude and the phase angle are taken into consideration during defect characterisation.

Semi-automatic or manual (human intervention) is not without problems. Signal interpretation is based on the analyst’s experience and knowledge. Without proper analysis procedures and related software settings, errors can be introduced into the eddy current results that are not due to the limitations of the eddy current technique but can be attributed to poor inspection management.

Proper training and procedures are essential for accurate eddy current signal analysis. These can only be established if destructive examination is conducted on pulled tubes. After the root cause analysis has been performed a set of reference tubes has to be manufactured simulating the defect found. These reference tubes should be designed to address the relevant signal parameters such as amplitude, phase angle, signal shape and duration. The resultant calibration curves should then be incorporated into an analysis procedure for the specific type of defect and relevant software settings made to facilitate signal analysis.

This project has highlighted a few issues that can contribute to inaccurate depth measurement. The guidelines stipulated in appendix 4 have to be incorporated into the analysis procedure of the inspection company. Applicable procedures and personnel can be qualified prior to an inspection, by incorporating tubes from the tube library into the mock-up heat exchanger, and verifying the results found.

7. RECOMMENDATIONS

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The following recommendations are made to in order to eliminate or reduce errors intricately part of the current eddy current inspection technique.

7.1. Calibration tube

7.1.1.Differential Calibration tube

7.1.1.1. The calibration tube should at least consist of the following artificially manufactured reference defects.

Indication Type Diameter / Width(Depth is given as % of Tube Wall

Thickness)10 % Internal Groove 1mm100% Through Hole 1.5mm80% External FBH3 1.5mm60% External FBH 1.5mm40% External FBH 1.5mm

4 x 20% External FBH 1.5mm10 External Groove 1mm

7.1.1.2. In addition to these indications, three 1.5mm holes positioned at 120 circumferential orientation and 30mm apart. These indications would be used to assess the probe centralization.

7.1.1.3. Internal Flat bottom holes (1.5mm diameter) have to be spark eroded on the internal surface of the calibration tube. The depth of these indications should be verified with an independent inspection technique such as an IRIS (Ultrasonic based internal inspection system) or a surface eddy current technique designed to inspect from the outside surface of the tube. The depth of these indications should range between 20% to 80% wall loss with 20% intervals.

7.1.1.4. The quality of the calibration tube and the accuracy of the reference defects should be verified.

7.1.1.5. The calibration tube has to be placed into a protective sleeving, reducing any damage that may occur during inspections.

7.1.1.6. The calibration tubes should become part of the tube sheet library, in addition to the reference samples and provided to the inspection company prior to an inspection.

7.1.1.7. The condition of the calibration tube should be verified upon return and if any damage is identified then the inspecting company would be held liable for replacing the calibration tube.

7.1.2.Absolute calibration tube

3 Flat Bottom Hole

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7.1.2.1. The calibration tube should at least consist of the following artificially manufactured reference defects.

Indication Type Width(Depth is given as % of Tube Wall Thickness)

10% External Circumferential Wastage 30mm20% External Circumferential Wastage 30mm30% External Circumferential Wastage 30mm40% External Circumferential Wastage 30mm50% External Circumferential Wastage 30mm50% Internal Circumferential Wastage 30mm40% Internal Circumferential Wastage 30mm30% Internal Circumferential Wastage 30mm20% Internal Circumferential Wastage 30mm10% Internal Circumferential Wastage 30mm

7.1.2.2. Each defect should be separated by a 20mm spacing so that measurement from any direction is possible.

7.1.2.3. A 1.5mm hole, introduced in all the calibration and reference tubes, would facilitate eddy current signal comparison between samples.

7.2. Reference tube

7.2.1.A set of reference tubes should be manufactured, for each type of defect identified in Eskom Heat Exchangers. These samples should simulate typical defect geometry and orientation.

7.2.2.The reference samples should be designed to provide sufficient information regarding the signal parameters viz. amplitude variation can be simulated using different volume indications.

7.2.3.The reference tubes as well as the sample defects should be entered into the tube library for future reference.

7.2.4.Analysis procedures should be compiled based on the results obtained from the actual defects and the reference tubes.

7.3. Probe designThe probe design criteria is based on the coil width and spacing, the type and thickness of wire used, the coil thickness, the bobbin material and the actual assembly process. This project has only concentrated on the centralisation of the probe and the effect of a reduced fill factor on the inspection results.

7.3.1.The probe centralization method has to be addressed.

7.3.2.Each probe has to be given an unique number and the properties of each probe, such as signal quality, signal t noise ratio, baseline drift, etc should be established prior to an inspection.

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7.4. Inspection quality manual A quality manual should be established that would address the following issues:

7.4.1.Calibration procedure

7.4.1.1. A calibration procedure should be established based on the requirements of the inspection i.e. the inspection level. For instance, a different calibration technique is required if only pluggable indications have to be identified as opposed to detailed analysis of each tube.

7.4.1.2. Calibration procedures should be compiled for the various tube materials and designs in Eskom Plants.

7.4.2.Data acquisition procedure

7.4.2.1. A detailed procedure and technique sheet has to be established for data acquisitioning.

7.4.3.Analysis procedure

7.4.3.1. Analysis procedures and relevant system settings has to be compiled for each type of indication.

7.4.3.2. These procedures should address the various signal parameters and their relevance in the defect characterization.

7.4.3.3. Guidelines provided in this document should be incorporated in the analysis procedures.

7.4.3.4. Analysis criteria for the different levels of inspections have to be identified and software developed to cater for these requirements viz. measure depth and volume on a phase vs. amplitude graph.

7.5. Personnel training

7.5.1.A training program has to be established to provide the analysts with applicable information to analyze eddy current signals.

7.5.2.Forums have to be established whereby analysts can share their experience and information and learn from other people’s experience.

7.5.3.Training and experience records have to be kept and certification issued based on these documents.

7.6. Inspection Quality assessment

7.6.1.An independent third party should perform quality assessment during eddy current inspection to insure proper defect characterization and accurate component assessment.

7.6.2.The quality assessment should include the calibration process, the data acquisitioning technique, monitoring of inspection progress, signal evaluation and reporting.

7.7. Destructive examination and defect verification

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7.7.1.The inspection process should cater for tubes to be pulled.

7.7.2.Destructive examination of these tubes would facilitate root cause analysis and signal verification.

7.7.3.New types of indications should be subjected to a proper destructive examination prior to the reference samples being manufactured.

8. REFERENCES

8.1. Cecco, V.S., Van Drunen, G., Sharp, E.L., “Eddy Current Manual”, Atomic Energy of Canada Limited, September 1983

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8.2. De La Pintiere, l., “Multi-frequency Eddy Current Examination of Heat Exchanger Tubing”, Intercontrole INC. December 1983

8.3. Stegemann, D.H., “Eddy Current Technique Level 3 Course Material”, University of Hannover, July 1990

8.4. Guiblin, B., “Procedure for axial probe eddy current evaluation”, GDL Procedure CC.P/255 Rev 0., Electricite de France, April 1996

8.5. Krzywosz, K., Tombaugh, R., Syverson, L., Lozier, G., “Balance-of-Plant Heat Exchanger Condition Assessment Guidelines”, EPRI TR-10035, July 1992

8.6. Burrows, M.L., “A Theory Of Eddy Current Flaw Detection”, University of Michigan, 1964

8.7. Golis, M.J., “ASNT Levele 3 Eddy Current Study Guide”, ASNT-SG-ET3-83, 19838.8. Van Drunen, G., Cecco, V.S., “Recognizing limitations in eddy current testing), NDT

International, Vol 17 No 1, February 19848.9. Krzywosz, K., ”Eddy Current Pit Sizing: Revisited”, Paper No. Krzywosz-23:1

presented at the Third EPRI Balance-of-Plant Heat Exchanger NDE Workshop, June 1994

8.10. Lozier, G.M., “Field Experiences in ECT Pit Sizing” New York Power Authority 8.11. Baker, R.A., Tombaugh, R.S., “Eddy Current Examination of Heat Exchanger Tubing

with Inside Surface Pitting”, Materials Evaluation, January 19908.12. ASME Section V, Article 8, “Eddy Current Examination Method for Installed Non-

ferromagnetic Heat Exchanger Tubing”, 1992

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Appendix 1Effect of frequency variation on the eddy current signal

response

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EDDY CURRENT SIGNAL ANALYSISFREQUENCY RESPONSE

Report No. RES/RR/00/12983Appendix 1Page 42

Graph 1. Phase angle measurements of 1.5mm diameter external indications at various frequencies

Graph 2. Phase angle measurements of 1.5mm diameter external indications at various frequencies

Remarks:


EDDY CURRENT SIGNAL ANALYSISFREQUENCY RESPONSE

Report No. RES/RR/00/12983Appendix 1Page 43

Graph 3. Amplitude vs. Phase Angle measurements for 1.5mm diameter indications

Remarks:

EDDY CURRENT SIGNAL ANALYSIS Report No. RES/RR/00/12983

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Frequency Response – ASME – Impedance Plane Appendix 1Page 44

Frequency response for typical ASME calibration defects (19mm Probe 0°Orientation)Ref Def 1: 10% Internal Groove

9kHz 15kHz 25kHz 40kHz

Ref. Def 2: 100% 1.5mm Hole9kHz 15kHz 25kHz 40kHz

Ref Def 3: 80% Flat Bottom Hole9kHz 15kHz 25kHz 40kHz

Ref Def 4: 60% Flat Bottom Hole9kHz 15kHz 25kHz 40kHz

EDDY CURRENT SIGNAL ANALYSISFREQUENCY RESPONSE – ASME – IMPEDANCE PLANE

Report No. RES/RR/00/12983Appendix 1

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Frequency response for typical ASME calibration defects (Continued)Ref Def 6: 4x20% Flat Bottom Hole


Ref Def 7: 10% External Groove9kHz 15kHz 25kHz 40kHz

EDDY CURRENT SIGNAL ANALYSISFREQUENCY RESPONSE – DEFECTS – IMPEDANCE PLANE


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Frequency response for typical localized internal defects (19mm Probe 0°Orientation)Small Volume Internal Pitting


Small Volume Internal Pitting9kHz 15kHz 25kHz 40kHz

Large Volume Internal Pitting9kHz 15kHz 25kHz 40kHz

EDDY CURRENT SIGNAL ANALYSISFREQUENCY RESPONSE – DEFECTS – IMPEDANCE


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PLANE

Frequency response for typical localized external defects (19mm Probe 0°Orientation)External pitting


Ammoniac attack9kHz 15kHz 25kHz 40kHz

Baffle wear9kHz 15kHz 25kHz 40kHz

EDDY CURRENT SIGNAL ANALYSISFREQUENCY RESPONSE – DEFECTS – IMPEDANCE PLANE


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Frequency response for typical localized manufacturing or other indications (19mm Probe 0°Orientation)

Dent9kHz 15kHz 25kHz 40kHz

Mechanical damage9kHz 15kHz 25kHz 40kHz

Bulge9kHz 15kHz 25kHz 40kHz

Deposits9kHz 15kHz 25kHz 40kHz

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Appendix 2Effect of fill factor and orientation on the eddy current signal

response

EDDY CURRENT SIGNAL ANALYSISFILL FACTOR AND ORIENTATION


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Measurements of various 1.5mm indications – Different Fill Factor and orientationGraph 1. Phase Angle Range

Graph 2. Amplitude Range (Internal)

Remarks:

EDDY CURRENT SIGNAL ANALYSISFILL FACTOR AND ORIENTATION


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Measurements for various 1.5mm indications – Different Fill Factor and orientationGraph 3. Amplitude range (External)

Graph 4. Variation of Phase Angle measurements as a function of % Fill Factor

Remarks:

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Appendix 3Service induced indications identified in Eskom Heat Exchangers

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EDDY CURRENT SIGNAL ANALYSISSERVICE INDUCED INDICATIONS


Page 53

Typical External Indications found at various Eskom Power Station

Ammoniac Attack – AA BSP/ASP

Steam Erosion – SE (Top of Tube)

Ammoniac Attack – AA BSP/ASP Ammoniac Attack – AA BSP/ASP

External Pitting - EP Dent – DE (Along Tube)

Steam Impingement – SI Baffle Wear – BW (At SP) Tube Fretting – TF (Between SP) Circumferential Cracking - CC


(Top of Tube)

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EDDY CURRENT SIGNAL ANALYSISSERVICE INDUCED INDICATIONS


Page 54

Typical Internal Indications found at various Eskom Power Station

Tube End Erosion - TEE Corro-Tip Erosion - CTE Internal Erosion - ER Block Erosion

Internal Pitting – PIT (CW Water) Plug Type Dezincification - PIT

Dezincification defect – DEZ(Under Scale)

Internal Pitting – PIT (CW Water)

Internal Pitting – PIT (CW Water) Internal Pitting – PIT (Fouling)

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Appendix 4Eddy current signal analysis guidelines


EDDY CURRENT SIGNAL ANALYSISGUIDELINES

Report No. RES/RR/00/12983Appendix 4APage 56

Indication Code DescriptionAA ASP or BSP Circumferential groove, between 1 and 2mm wide, situated adjacent to support plates or tube sheet

Indication Label Characteristics

AMMONIAC ATTACK(Condensate grooving)

Type : Localised External Wall Loss with Circumferential ExtentHEX - Planar Position : Adjacent to Air Extraction ZonesPosition along Tube : At edge of support plate or far tube sheet edgeCirc. Orientation : Deepest area at bottom of tube

Eddy Current Signal Interpretation

Channel Presentation

F25(X) F25(Y) FMIX(Y) FA(Y) Indication Profile Characterisation Channel FMIX

Eddy Current Signal Analysis Impedance Plane DisplayInterpretation Channel/s : FMIX (25 and 12 kHz)

Verification Channel/s : FD25 (Y-comp), FA6 (Y-comp)

Measurement Parameters : Amplitude in Mixing Channel

Measurement Technique : Peak To Peak

Lower Detection Limit(Amplitude)

:Ampl. 300mV

Phase Range : 27 125 (FMIX)

Calibration Defects : Cal Tube: FA9 100% hole @ 2V

FMIX 100% hole @1.5V

Reference Defects :2mm one sided wide circumferential grooves @ 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80% and 90% WL

Defect Classification : Ampl.: 0 – 0.6V Class A: 0 – 20%

Ampl.: 0.6 – 1.8V Class B: 20 – 40%

Ampl.: 1.8 – 4V Class C: 40 – 60%

Ampl.: 4 – 7.2 V Class D: 60 – 80% FMIX Measurement ParameterAmpl.: > 7.2V Class E: 80 – 100% Amplitude (Peak to Peak)

Plugging Criteria : Class D & E

Remarks: The build-up of corrosion products and iron oxides near the support plates or tubesheet can influence the depth measurement

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Graph 1. Amplitude measurement of reference tube and actual defects identified

Graph 2. Calibration curve for mixing channel indicating real defect validation. (86% FF, 0° Orientation)

Remarks:The difference between the real defect curve and the artificial indication curve can be attributed to the fact that the AA indications normally exhibit a random circumferential groove pattern consisting of more than one wall loss area.

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PHOTO 1. Typical ammoniac attack found in the Lethabo Unit 2 Main Condenser.

PHOTO 2. Typical ammoniac attack found in the Grootvlei Unit 2 Main Condenser

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Report No. RES/RR/00/12983Appendix 4BPage 59

Indication Code Description

BWExternal localized wall loss underneath the support plate with indication width corresponding to support plate thickness. Circumferential grooving is found in severe baffle wear samples and can be attributed to the malleable nature of the material.


BAFFLE WEAR

Type : Localized external wall thinning HEX - Planar Position : Top and side periphery tubes near steam inletsPosition along Tube : Underneath support plates Circ. Orientation : Extent of damage varies circumferentially



F25(X) F25(Y) FMIX(Y) FA(Y) Indication Profile Characterisation Channel FA

Eddy Current Signal Analysis Impedance Plane DisplayInterpretation Channel/s : FA (9 kHz)Verification Channel/s : FMIX (Y)Measurement Parameters

: Amplitude in absolute channel

Measurement Technique : Peak To PeakLower Detection Limit(Amplitude)

:Ampl. 300mV

Phase Range : 55 130Calibration Defects : Cal Tube: FA9 20% Ext Groove @ 0.5V & 90°

FMIX 100% hole @1.5V

Reference Defects :20mm wide one sided circumferential grooves @ 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80% and 90% WL

Defect Classification : Ampl.: 0 – 0.2V Class A: 0 – 20%Ampl.: 0.2 – 0.5V Class B: 20 – 40%

Ampl.: 0.5 – 1.2V Class C: 40 – 60%

Ampl.: 1.2 – 2.0 V Class D: 60 – 80% FA Measurement ParameterAmpl.: > 2.0 Class E: 80 – 100% Amplitude (Peak to Peak)

Plugging Criteria : Class D & ERemarks: The build-up of corrosion products and iron oxides near the support plates or tubesheet can influence the depth measurement.

Amplitude measurement can be increased by increasing the calibration reference defect amplitude setting.

EDDY CURRENT SIGNAL ANALYSIS Report No. RES/RR/00/12983

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GUIDELINES Appendix 4BPage 60


BWExternal localised wall loss underneath the support plate with indication width corresponding to support plate thickness. Circumferential grooving is found in severe baffle wear samples and ca be attributed to the malleable nature of the material. Steam Erosion

Graph 1. Amplitude vs. depth analysis curve as measured on the 9kHz absolute channel

Graph 2. Amplitude vs. depth curve for 15kHz absolute channel

Remarks: The circumferential groove caused by the tube vibration should be analysed according to the guidelines stipulated for ammoniac attack.


Report No. RES/RR/00/12983Appendix 4BPage 61

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BWExternal localised wall loss underneath the support plate with indication width corresponding to support plate thickness. Circumferential grooving is found in severe baffle wear samples and ca be attributed to the malleable nature of the material. Steam Erosion

PHOTO 1. Baffle wear identified near a steam outlet at Arnot Power Station.

PHOTO 2. Shallow baffle wear (22% wall loss) identified at Arnot Power Station


Report No. RES/RR/00/2983Appendix 4CPage 62

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CTEInternal localised wall thinning at the end of the plastic insert, pushed into the tube to limit tube inlet end erosion. The depth gradually decreases as the depth into the tube increase.


CORRO TIP EROSION

Type : Localised internal wall thinning

HEX - Planar Position :Random throughout the tubesheet. (Found in high CW velocity areas)

Position along Tube : Tube inlet (End of corro insert)Circ. Orientation : Deepest area at bottom of tube




Eddy Current Signal Analysis Impedance Plane DisplayInterpretation Channel/s : FA (9 kHz)Verification Channel/s : FD25 (Y), FMIX (Y)Measurement Parameters

: Amplitude in Absolute Channel


:Ampl. 200mV

Phase Range : 0 45Calibration Defects : Cal Tube: FA 20% Internal Groove @ 0.5V & 90°

FMIX 100% hole @1.5VReference Defects : Internal one sided wall loss areas (Circ Ext. > 90°)

Depths of 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80% and 90% WL

Defect Classification : Ampl.: 0 – 0.2V Class A: 0 – 20%Ampl.: 0.2 – 1.5V Class B: 20 – 40%Ampl.: 1.5 – 2.9V Class C: 40 – 60%Ampl.: 2.9 – 3.7 V Class D: 60 – 80% FA Measurement ParameterAmpl.: > 3.7V Class E: 80 – 100% Amplitude (Peak to Peak)

Plugging Criteria : Class D & ERemarks: Differential channel response is normally saturated.



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CTEInternal localised wall thinning at the end of the plastic insert, pushed into the tube to limit tube inlet end erosion. The depth gradually decreases as the depth into the tube increase.

PHOTO 1. Internal erosion at the tip of a corro-insert.

PHOTO 2. Sectioned sample of heat exchanger tube to show the circumferential distribution of damage


Report No. RES/RR/00/12983Appendix 4DPage 64

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DENTMechanical damage caused either during manufacture or operation. Localised denting of tube results in a localised area of turbulence inside the tube and could lead to blocked or restricted tubes


DENT

Type : Localised indications

HEX - Planar Position :Top and side periphery tubes – service induced Within tube bundles – Manufacturing indications

Position along Tube : Along the length of tubeCirc. Orientation : Top of tube (Service induced)



F25(X) F25(Y) FMIX(Y) FA(Y) Indication Profile Characterisation Channel F25

Eddy Current Signal Analysis Impedance Plane DisplayInterpretation Channel/s : FD25 Verification Channel/s : FMIX (Y)Measurement Parameters : Amplitude in Basic Differential ChannelMeasurement Technique : Peak To PeakLower Detection Limit(Amplitude)

:Ampl. 1V

Phase Range : 150 190Calibration Defects : Cal Tube: FD25 100% hole @ 2V

FMIX 100% hole @1.5VReference Defects : Random dents introduced into a reference tube

Defect Classification : Ampl.: 1 – 5V DENT AAmpl.: 5 – 10V DENT B

Ampl.: 10 – 15V DENT C

Ampl.: < 20V DENT D FD25 Measurement ParameterPlugging Criteria : DENT D and restricted tube (Less than 70% fill factor) Amplitude (Peak to Peak)Remarks: Dent indications are not detrimental to the use of tube but can lead to restricted or blocked tube,

Abrasion damage and stress corrosion cracking



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DENTMechanical damage caused either during manufacture or operation. Localised denting of tube results in a localised area of turbulence inside the tube and could lead to blocked or restricted tubes

PHOTO 1. Dent indication found at Lethabo Power Station.

PHOTO 2. Internal abrasion of dent indication resulting in severe wall loss. This indications was identified at Lethabo Power Station


Report No. RES/RR/00/12983Appendix 4EPage 66

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Indication Code DescriptionPITX Localised external wall loss indications, normally situated at the top of the tube.


EXTERNAL PITTING

Type : Localised External Wall Loss HEX - Planar Position : Top and side periphery tubes near steam inletsPosition along Tube : Between support plates Circ. Orientation : Deepest area at top of tube



F25(X) F25(Y) FMIX(Y) FA(Y) Indication Profile Characterisation Channel FD25

Eddy Current Signal Analysis Impedance Plane DisplayInterpretation Channel/s : FD25

Verification Channel/s :FD12 (Y), FMIX (Y), FD9 – Depth estimation for all frequencies should be within 20%

Measurement Parameters : Amplitude in Differential Basic ChannelMeasurement Technique : Peak To PeakLower Detection Limit(Amplitude)

:Ampl. 300mV

Phase Range : 27 125Calibration Defects : Cal Tube: FD25 100% hole @ 2V, Ext FBH’s

FMIX 100% hole @1.5V, Ext FBH’s

Reference Defects :Ext. FBH of various depths (20%, 40%, 60%, 100%) and diameters (1mm, 1.5mm, 2mm, 2.5mm, 3mm)

Defect Classification :

Depth classification according to phase angle and amplitude measurements for various depth and diameter indications. Classification according to

Class A: 0 – 20%Class B: 20 – 40%Class C: 40 – 60%Class D: 60 – 80% FD25 Measurement ParameterClass E: 80 – 100% Phase angle and Amplitude

Plugging Criteria : Class D & E and Ampl >0.5mV (Peak to Peak)Remarks: High fill factor bobbin probe and centralisation of probe is of the utmost importance



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Graph 1. Phase angle measurements for various diameter through holes. Indications form 1.5mm diameter ad higher appears to be underestimated and between 1.5 and 0.4mm diameter overestimated.

Graph 2. Amplitude values for various diameter through hole indications

Remarks:



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PITX Localised external wall loss indications, normally situated at the top of the tube.

Graph 3. Phase angle values for various depth and diameter external indications

Graph 4. Amplitude values for various depth and diameter external indications

Remarks:Large amplitude (<1V) external indications are underestimated by up to 30% if only phase angle analysis is used. Small amplitude (>1V) external indications are overestimated by up to 20% if only phase angle analysis is used




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Graph 5. Calibration curve relating the amplitude, phase angle and indications depth. Results of inspection conducted on external pits found at Lethabo compared favourably with actual measurements if these curves are used for analysis.

Remarks:Due to current software limitations, it is difficult to analyze according to graph 5 and therefore manual depth Estimation is required which has an influence on the analysis time period

Graphs 1 to 5 is based on results obtained from an 82% fill factor probe with 0° defect orientation. Since external pitting indications have been identified at the top of the tubes it is important to use the maximum fill factor allowed, as well as to centralize the probe. Effects of fill factor and defect orientation on the amplitude and phase angle has to be eliminated prior to depth estimation based on graph 5.




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PHOTO 1. External pitting identified at Lethabo Power Station. The indications were distributed in a 5 degree range on top of the tube

PHOTO 2. Profile image of a typical external pit found.




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PHOTO 3. Steam impingement can also be classified as localised external wall loss and the same analysis criteria applies.


Report No. RES/RR/00/12983Appendix 4FPage 72

Indication Code DescriptionGIWL Generalised internal wall loss along the length of the tube. Tube profile indicates ovalisation of internal surface

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due to abrasion


INTERNAL WALL LOSS(OVALISATION)

Type : Generalised Internal Wall Loss HEX - Planar Position : Random through out water boxPosition along Tube : The length of the tube Circ. Orientation : 2 x <90° wall loss areas situated at 180° from one another




Eddy Current Signal Analysis Impedance Plane DisplayInterpretation Channel/s : FA Verification Channel/s : FD25 (Y), FD12 (Y) Baseline variationMeasurement Parameters

: Amplitude in Absolute Channel

Measurement Technique : Measured from tube baselineLower Detection Limit(Amplitude)

:Ampl. 200mV

Phase Range : 20 45 F9Calibration Defects : Cal Tube: FA9 20% Ext. Groove @ 0.5V & 90°

Reference Defects :Single and Double sided 30 mm wide internal wall loss areas @ 10%, 20%, 30%, 40%, 50% WL

Defect Classification Single Double Classification: Ampl.: 0–0.2 0-0.3 Class A: 0 – 20%

Ampl.: 0.2-0.4 0.3-0.6 Class B: 20 – 40%

Ampl.: 0.4-0.8 0.6–1.2 Class C: 40 – 60%

Ampl.: 0.8-1.2 1.2-1.8 Class D: 60 – 80% FA Measurement ParameterAmpl.: > 1.2V >1.8V Class E: 80 – 100% Amplitude from tube baseline

Plugging Criteria : Class D & ERemarks: Single or double-sided nature can be verified by destructive analysis. Phase angle analysis to

determine the circumferential extent has been performed with limited success.




GIWLGeneralised internal wall loss along the length of the tube. Tube profile indicates ovalisation of internal surface due to abrasion

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Tube baseline

Graph 1. Phase angle variation for various depth of indications exhibiting different internal circumferential extent

Graph 2. Amplitude measurement indicating the reduction of amplitude as the circumferential extent of the internal indications decrease

Remarks:




GIWLGeneralised internal wall loss along the length of the tube. Tube profile indicates ovalisation of internal surface due to abrasion

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Graph 3. Amplitude vs. depth curve for three different circumferential extent internal indications

Remarks:


Report No. RES/RR/00/12983Appendix 4GPage 75

Indication Code DescriptionINT ERO

Localised internal wall thinning generally found at the bottom of the tube. The longitudinal extent of the indications exceeds the coil width and resembles two edge signals

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INTERNAL EROSION

Type : Localised internal wall thinning HEX - Planar Position : Random across tube sheetPosition along Tube : Intermittent along the tube length Circ. Orientation : Deepest area is generally found at the bottom of the tube.




Eddy Current Signal Analysis Impedance Plane DisplayInterpretation Channel/s : FA Verification Channel/s : FD25 (Y) – Two edge signalsMeasurement Parameters : Amplitude in Absolute ChannelMeasurement Technique : Peak To Peak – (Double absolute depth estimation)Lower Detection Limit(Amplitude)

:Ampl. 300mV

Phase Range : 0 50

Calibration Defects :Cal Tube: FA9 20% Ext Groove @ 0.5V & 90°

FD25 100% hole @1.5VReference Defects : Internal one sided wall loss areas (Circ Ext. > 90°)

Depths of 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80% and 90% WL

Defect Classification : Ampl.: 0 – 0.2V Class A: 0 – 20%Ampl.: 0.2 – 1.5V Class B: 20 – 40%Ampl.: 1.5 – 2.9V Class C: 40 – 60%Ampl.: 2.9 – 3.7 V Class D: 60 – 80% FA Measurement ParameterAmpl.: > 3.7V Class E: 80 – 100% Amplitude (Peak to Peak)

Plugging Criteria : Class D & ERemarks:





Graph 1. Amplitude analysis of absolute channel provides accurate depth estimation for internal

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erosion indications

Graph 2. Variation of the amplitude measurement as a function of the circumferential extent of the indications

Remarks:





PHOTO 1. Internal Erosion identified at Camden Power Station.

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PHOTO 2. Internal erosion identified at Koeberg Power Station


Report No. RES/RR/00/12983Appendix 4HPage 78


SEGeneralised external wall thinning of tube due to erosion of high impact steam particles. Generally situated at the top of the tube and between support plates.

Indication Label CharacteristicsSTEAM EROSION Type : Generalised External Wall Loss

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HEX - Planar Position : Top and side periphery tubes near steam inletsPosition along Tube : Between support plates Circ. Orientation : Deepest area at top of tube




Eddy Current Signal Analysis Impedance Plane DisplayInterpretation Channel/s : FA Verification Channel/s : Additional absolute channelMeasurement Parameters : Amplitude in Absolute ChannelMeasurement Technique : Peak To PeakLower Detection Limit(Amplitude)

:Ampl. 200mV

Phase Range : 27 130Calibration Defects : Cal Tube: FA 20% Ext Groove @ 0.5V & 90°

Reference Defects : 20mm wide one sided external wall loss areas @ 10%,20%, 30%, 40%, 50%, 60%, 70%, 80% and 90% WL

Defect Classification : Depth analysis determined from the amplitude vs.Depth graph established using the absolute calibration

Tube.

Wall thickness measurement should be doubled due to FA Measurement Parameterthe reduction in circumferential extent Amplitude (Peak to Peak)

Plugging Criteria : Class D & ERemarks: Similar analysis guidelines are applicable for tube fretting





Graph 1. Amplitude vs. depth analysis of external steam erosion indications. (9kHz)

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Graph 2. Amplitude vs. depth analysis of external steam erosion indications. (15kHz)

Remarks:





PHOTO 1. Steam erosion identified in the upper periphery tubes at Hendrina Power Station.

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PHOTO 2. Typical surface condition of steam erosion indication.




TFGeneralised external wall loss caused by the tube to tube wear between two adjacent tubes. Fretting can be attributed to tube vibration

Indication Label CharacteristicsTUBE TO TUBE WEAR Type : Generalised External Wall Loss

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HEX - Planar Position : Near adjacent tubesPosition along Tube : Between support plates Circ. Orientation : Deepest area adjacent to neighbouring tubes

Indication Profile PHOTO 3 Typical tube to tube wear identified at Arnot PS

PHOTO 4. Tube to tube wear surface reveals directional wear marks caused by vibration

Remarks: Similar analysis guidelines are applicable for tube fretting


Report No. RES/RR/00/12983Appendix 4IPage 82


PITLocalised internal pitting. Intermittent pits are indicated by the code PITS and continuous pitting, usually associated with dezincification, by the code PITTING.

Indication Label CharacteristicsINTERNAL PIT Type : Localised internal wall loss

HEX - Planar Position : Random throughout the tubesheet

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Position along Tube : Along the length of the tube

Circ. Orientation :Random around the tube with the bottom of the tube being the preferential orientation



F25(X) F25(Y) FMIX(Y) FA(Y) Indication Profile Characterisation Channel FD25

Eddy Current Signal Analysis Impedance Plane DisplayInterpretation Channel/s : FD25 Verification Channel/s : FD12 (Y), FD6, FMIXMeasurement Parameters

: Phase Angle in FD25


:Ampl. 200mV

Phase Range : 0 30Calibration Defects : Cal Tube: FD25 100% hole @ 2V

10% Int Groove @ 0

Reference Defects :Various diameter and depth internal FB Spark eroded holes @ 10%, 20%, 30%,40%, 50%, 60%, 70%, 80% and 90% WL

Defect Classification :According to modified Phase vs. depth graph the following classifications are made:Class A: 0 – 20% Class B: 20 – 40%Class C: 40 – 60% Class D: 60 – 80%Class E: 80 – 100%If generalised wall loss are present then the depth should FD25 Measurement ParameterBe adjusted accordingly Phase Angle (Peak to Peak)

Plugging Criteria : Class D & ERemarks:





Graph 1. Phase angle vs. depth curve for various diameter internal spark eroded indications

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Graph 2. Normalised phase vs. depth curve for different diameter internal indications.

Remarks:.



Indication Code DescriptionPIT

Localised internal pitting. Intermittent pits are indicated by the code PITS and continuous pitting, usually associated with dezincification, by the code PITTING.

Graph 3. Amplitude measurement of various diameter and depth internal spark eroded indications

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Remarks:.





PHOTO 1. Internal pitting found at Matla Power Station resulting from plug type dezincification.

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PHOTO 2. Internal pitting identified at Camden Power Station





PHOTO 3. Internal pits found at Komati Power Station.

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PHOTO 4. Internal pits at Camden caused by high concentration of chlorides in CW water





PHOTO 5. Symmetrical pit identified at Camden Power Station.

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PHOTO 6. Plug type dezincification identified as pit indication at Lethabo PS





PHOTO 7. Pitting underneath a scale layer (Hendrina Power Station)

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PHOTO 8. Pitting caused by large concentration of chlorides in the CW water (Tutuka PS)





PHOTO 9. Internal pitting filled with a scale deposit (Hendrina PS).

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PHOTO 10. Severe dezincification at Komati Power Station resulting in internal pit indications.

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