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New and Emerging InspectionTechnologies for Flow AcceleratedCorrosion in Fossil Power Plants

TP-114349

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New and Emerging Technologies for Flow

Accelerated Corrosion in Fossil Power Plants

TP-114349

November 1999

EPRI Project Manager

Pedro F. Lara

EPRI 3412 Hillview Avenue, Palo Alto, California 94304 PO Box 10412, Palo Alto, California 94303 USA800.313.3774 650.855.2121 askepri@epri.com www.epri.com

EPRI NDE Center 1300 WT Harris Blvd., Charlotte, North Carolina 28262 PO Box 217097,Charlotte, North Carolina 28221 USA 704.547.6100

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DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES

THIS DOCUMENT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ACCOUNT OFWORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCH INSTITUTE, INC. (EPRI).NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THE ORGANIZATION(S) BELOW, NOR ANYPERSON ACTING ON BEHALF OF ANY OF THEM:

(A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I) WITHRESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEMDISCLOSED IN THIS DOCUMENT, INCLUDING MERCHANTABILITY AND FITNESS FOR A PARTICULARPURPOSE, OR (II) THAT SUCH USE DOES NOT INFRINGE ON OR INTERFERE WITH PRIVATELY OWNEDRIGHTS, INCLUDING ANY PARTY'S INTELLECTUAL PROPERTY, OR (III) THAT THIS DOCUMENT ISSUITABLE TO ANY PARTICULAR USER'S CIRCUMSTANCE; OR

(B) ASSUMES RESPONSIBILITY FOR ANY DAMAGES OR OTHER LIABILITY WHATSOEVER (INCLUDINGANY CONSEQUENTIAL DAMAGES, EVEN IF EPRI OR ANY EPRI REPRESENTATIVE HAS BEEN ADVISEDOF THE POSSIBILITY OF SUCH DAMAGES) RESULTING FROM YOUR SELECTION OR USE OF THISDOCUMENT OR ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED INTHIS DOCUMENT.

ORGANIZATION(S) THAT PREPARED THIS DOCUMENT

EPRI NDE Center

ORDERING INFORMATION

Requests for copies of this report should be directed to the EPRI Distribution Center, 207 Coggins Drive, P.O. Box23205, Pleasant Hill, CA 94523, (800) 313-3774.

Electric Power Research Institute and EPRI are registered service marks of the Electric Power Research Institute, Inc.EPRI. POWERING PROGRESS is a service mark of the Electric Power Research Institute, Inc.

Copyright 1999 Electric Power Research Institute, Inc. All rights reserved.

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iii

CITATIONS

This document was prepared by

EPRI NDE Center

1300 WT Harris Blvd.

Charlotte, NC 28262

Principal Investigator

I. Lara, Pedro

This document describes research sponsored by EPRI.

The publication is a corporate document that should be cited in the literature in the followingmanner:

New and Emerging Inspection Technologies for Flow Accelerated Corrosion in Fossil Power

Plants: 1999. TP-114349.

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v

ABSTRACT

This document describes the Nondestructive Evaluation (NDE) technologies that arecurrently available or under development to help quantify the integrity or the rate of

degradation of piping affected by flow accelerated corrosion in fossil power plants. The

report classifies the technologies into those that require the pipe to be bare (lightlycoated) and those that can measure through insulation or liners. Also, it presents thetechnologies that are best suited to perform a local metal loss assessment or provide a

more complete or global view of the system.

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1

NEW AND EMERGING INSPECTIONTECHNOLOGIES FOR FLOW ACCELERATED

CORROSION IN FOSSIL POWER PLANTS

Executive Summary

Flow accelerated corrosion (FAC) causes leaks and ruptures in carbon steel piping in

power plants. Because these ruptures have included worker fatalities and significant

property damage in some situations, the issue has been the focus of considerable attention

in the past few years. In response to these events, EPRI has published several reports that

address various aspects of the corrosion process including mitigation 1, component

susceptibility analysis1,2,3

, component degradation prediction methodology3,4,5

, and

recommended nondestructive evaluation (NDE) practices 6,7.

This report revisits the NDE practices while incorporating the recent technology

advances. These new NDE developments provide the utility owner with increased

thickness measurement accuracy and better tools to assess wall thickness throughinsulation. In addition, some of these new technologies are aimed at expanding the

inspection area for a very low incremental cost; that is, at providing a more global view

of the system for the same dollars invested in inspection.

In this report the technologies are divided into contact and non-contact; that is, those that

require the piping to be bare (thinly coated), and those that can survey through insulation.

Furthermore, the technologies are classified according to their suitability to assess the

corrosion rate. EPRIs degradation prediction methodologies include corrosion rate

assessments of the components. The corrosion rate estimation is then used as part of the

planning process for the next inspection or for replacing the component.

This evaluation found that,

Monitoring the rate of FAC in bare piping has been made more cost effective with theadvent of AutoGrid, a new technology.

FAC detection through insulation is feasible, with some limitations, and theavailability of new technologies (real-time radiography and pulsed eddy current) havemade the inspection operations more cost effective.

Measuring the rate of FAC through insulation is feasible and the improvements in thepulsed eddy current and Compton backscattering techniques may make the in-serviceassessments more plausible.

Methods for detection of FAC globally are being developed but are not yet

established.

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2

Background

Flow-accelerated corrosion (FAC) has been the cause of many carbon steel piping and

vessel failures that carry water or steam. Because these ruptures have included worker

fatalities and significant property damage in some situations, the issue has been the focus

of considerable attention in the past few years. In response to these events, EPRI has

published several reports that address various aspects of the corrosion process including

mitigation 1, component susceptibility analysis 1,2,3, component degradation prediction

methodology 3,4,5, and recommended nondestructive evaluation (NDE) practices 6,7.

The mechanism that causes FAC is a combination of metal oxidation from iron to

magnetite, and the dissolution or removal of the magnetite layer by the fluid flow. At

low fluid velocities, the dissolution of the magnetite is slow enough and a protective layer

is formed on the metal surface. Under these conditions the rate of corrosion is relatively

slow and controlled by the mass transfer of ions through the magnetite layer. As the fluid

velocity increases above the so called breakaway value, the protective oxide film is

removed by the surface shear stress and the corrosion accelerates to significantly higher

rates, so the term flow-accelerated corrosion 1.

The mechanism suggests that impurities contributing to the dissolution of the protectivelayer enhance the corrosion process. In particular, although the presence of oxygen is a

contributor to the corrosion process, the total removal of oxygen from the system is

detrimental since no protective layer can be formed. Traces of oxygen in the range of

100 parts per billion are typically recommended1.

FAC occurs in single and 2-phase flow carbon steel systems, and in both small and large

bore piping. Components that promote the formation of vortices, secondary flows, or

turbulence are more prone to FAC1. These include:

Elbows,

Bends,

Tees, Reducers,

Pipe entries,

Downstream of valves and flow control orifices.

For fossil power plants the following systems have been determined to most likely

experience FAC5:

Piping around boiler feed pump,

Tubesheets and tubes in the high and low pressure heaters,

Heater drain lines,

Economizer inlet tubing, Piping to economizer header,

Deaereator shell,

Heat recovery steam generator tubing.

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3

This report revisits the NDE practices applicable to carbon steel piping while

incorporating the recent technology advances.

In this report the technologies are classified according to the level of technical maturity

into 3 categories: established, new, or emerging. Established techniques exhibit well-

accepted deployment procedures and accuracy levels. New techniques have proven

performance under some scenarios but have not yet gained wide acceptance. Emergingtechnologies have not yet demonstrated field ability to size damage with any accuracy.

Established technologies provide the user with an accurate assessment of the local pipe

conditions. The user then estimates the overall piping integrity by inferring wall

condition based on these localized measurements (statistical inference). In order to help

reduce the risk of not identifying a deteriorated area while performing these limited

surveys, EPRI has developed CHECWORKS, 8

, a comprehensive predictive model that

helps the utility operator target for inspection those pipe locations that are likely to suffer

the highest corrosion rates.

Some of the new and emerging NDE technologies discussed in this report are aimed at

further minimizing the integrity uncertainty by expanding the area of inspection for a

very low incremental cost. The objective is to provide the utility operator with a moreglobal view of the system for the same dollars invested in inspection.

In this report the technologies are divided into contact and non-contact, that is, those that

require the piping to be bare (thinly coated), and those that can survey through insulation.

Furthermore, the technologies are classified according to their suitability to assess the

corrosion rate. EPRIs degradation prediction methodologies include corrosion rate

assessments of the components. The corrosion rate estimation is then used as part of the

planning process for the next inspection or for replacing the component 5.

The inspection technologies are listed in Table 1 (page 4) for bare piping, and in Table 2

(page 5) for insulated piping. These technologies are discussed next.

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4

Established New Emerging Accuracy Comments Vendors

Local Metal

Loss

Detection

Radiography +/- 5% for

Ug=0.02 inch

Provides accurate wall thickness measurement and good pit resolution. Expensive when per unit

area of coverage. Use Iridium sources for equivalent steel thickness < 2", Cobalt for equivalent

steel thickness < 6".

various

Ultrasonics +/- 0.005 in. Provides accurate wall thickness measurement of bare or coated piping. Accuracy affected by pipe

temperature and roughness.

various

Wide-scan

Real Time

Radiography

Qualitative

assessment

Suitable for an rapid wide-area scan to identify areas that need further evaluation in straght water

filled piping 8" to 36" in diameter and up to 24" diameter elbows. Can resolve a flaw with a

radiographically projected image of 1/4" in diameter and survey 1" away from obstructions.

Requires 4" and 12" clearance at the detector and source side respectively.

IHI Southwest

Narrow-scan

Real Time

Radiography

Not

established

Suitable for an rapid narrow band scan to identify areas that need further evaluation for pipes up to

18" in diameter including insulation. Can detect a 35% 3/4" diameter FBH in a 6 inch diameter pipe.

Sensitivity will be less for larger diameter piping.

Lixi

Low Frequency

Electromagnetics

Not

established

Suitable for an rapid wide band scan to identify areas that need further evaluation in water filled

piping. Can detect isolated pits through coatings up to 0.2" thick. Performance may be affected by

material property changes.

Testex

Corrosion

Rate

Assessment

Autogrid +

Ultrasonics

+/- 0.005 in. Suitable for corrosion monitoring. Combines the accuracy of ultrasonic measurement with precise

sensor placement.

Southwest

Research

Institute

Global Metal

Loss

Detection

Magnetostrictive

Guided Waves

Not

established

Suitable for qualitative evaluation of large inaccessible areas. Can detect a 1% pipewall cross

section reduction with a 7" spatial resolution. The inspection range best for piping up to 16" in

diameter and depends on carrier fluids and piping geometry.

Southwest

Research

Institute

Piezoelectric

Guided Waves

Not

established

Device still under development for global detection of piping damage. Can detect a 0.46% reduction

in cross-section with a spatial resolution of 1.5". Range of detection dependent on wave-mode.

Rules for wave-mode selection still under development.

Penn State

University;

Plant IntegrityLtd. (UK)

Tangential Guided

Waves

Not

established

Suitable for qualitative evaluation of inaccessible areas. Can detect a 10% volume loss over the

1.5" wide sensor-to-sensor band. Performance may be affected by the presence of internal

deposits.

Magna-Tec

All-Tech

Active Infrared

Thermography

Not

established

May detect 25% FBH with diameters equal to nominal wall thickness. Lesser defect depths may not

be detectable. Performance affected by the coating thickness.

Thermal

Wave

Imaging

Table 1Inspection Technologies for Bare Piping

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5

Established New Emerging Accuracy Application Vendors

Local Metal

Loss

Detection

Radiography +/- 5% for

Ug=0.02 in.

Provides accurate wall thickness measurement and good pit resolution. Expensive per unit area of

coverage. Use Iridium sources for equivalent steel thickness < 2", Cobalt for equivalent steel

thickness < 6". External corrosion products may interfere with the measurement.

various

Wide-scan

Real TimeRadiography

Qualitative

assessment

Suitable for an rapid wide-area scan to identify areas that need further evaluation in straight water

filled piping 8" to 36" in diameter and up to 24" diameter elbows including insulation. Can resolve aflaw with a radiographically projected image of 1/4" in diameter and survey 1" away from

obstructions. Requires 4" and 12" clearance at the detector and source side respectively.

IHI Southwest

Narrow-scan

Real Time

Radiography

Not

established

Suitable for an rapid narrow band scan to identify areas that need further evaluation for pipes up to

18" in diameter including insulation. Can detect a 35% 3/4" diameter FBH in a 6 inch diameter pipe.

Sensitivity less for larger diameter piping.

Lixi

Pulsed Eddy

Current

+/- 0.04" Suitable of general wall loss detection of piping through insulation. Will not detect isolated damage. RTD

APTECH

Compton

Backscattering

Not

established

Device still under development. May be suitable for measurement of localized damage through

insulation.

Nuclear

Measurement

Corp.

Corrosion

Rate

Assessment

Pulsed Eddy

Current

+/- 0.04 in. Exhibits repeatibility of +/- 0.005 inch which is suitable for corrosion monitoring. Grid points can be

painted on the insulation, or tool integrated with Autogrid.

RTD

APTECH

Compton

Backscattering

Not

established

May be suitable for corrosion monitoring once the tool's repeatability is quantified. Grid points can

be painted on the insulation, or tool integrated with Autogrid.

Nuclear

Measurement

Corp.

Global Metal

Loss

Detection

Magnetostrictive

Guided Waves

Not

established

Suitable for qualitative evaluation of large inaccessible areas. Can detect a 1% pipewall cross

section reduction with a 7" spatial resolution. The inspection range best for piping up to 16" in

diameter and depends on carrier fluids and piping geometry.

Southwest

Research

Inst.

Piezoelectric

Guided Waves

Not

established

Device still under development for evaluation of inaccessible areas. Can detect a 0.46% reduction

in cross-section with a spatial resolution of 1.5". Range of detection dependent on wave-mode.

Rules for wave-mode selection still under development.

Penn State

University;

Plant Integrity

Ltd. (UK)

Table 2

Inspection Technologies for Insulated Piping

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6

FLOW ACCELERATED CORROSION INSPECTION OF BARE PIPING

NDE technologies for bare piping include those that are used for localized assessment, those that

are suitable for corrosion rate monitoring, and those that are targeted to provide a global view of

the system.

Local Detection of Metal Loss in Bare Piping

Established technologies for localized metal loss assessment include ultrasonics and radiography.

These techniques have been extensively documented elsewhere including two comprehensive

EPRI reports 6,9. These reports list the essential variables, accuracy, and operating procedures of

both ultrasonics and radiography. Because of the availability of these documents only a brief

description of the techniques will be given here.

Ultrasonics

Ultrasonic thickness measurement can be performed on bare ambient temperature piping with an

accuracy of +/- 0.005 inch.

As the piping temperature increases, the accuracy of the ultrasonic measurement decreases andcalibration corrections are warranted. Temperature affects the couplants viscosity, the delay

line velocity, and to a small degree the speed of sound in the steel (the delay line is the

nonmetallic material that is placed in front of the transducer for protection purposes). When

surveying hot piping, ASTM E797 recommends a -1% per 100F correction to the thickness

measurement when the calibration procedure calls for ambient temperature calibration blocks10

.

Ultrasonic thickness measurement accuracy also decreases as the backwall surface roughness

increases because the uneven surface scatters and attenuates the backwall echo. When surveying

corroded surfaces, the use of dual-crystal transducers with a focal distance close to the nominal

thickness is recommended.

Radiography

Radiographic techniques can be used to measure the pipe wall thickness as well as detecting the

aerial extent of localized damage.

FAC measurements with radiography use the tangential technique (Figure 1) 6,9. In this

technique a radiographic image of the pipe wall is obtained by configuring the equipment so that

the source-to-film centerline is tangent to the pipe section to be surveyed. The pipe wall

thickness is obtained from the image by multiplying it times the calculated magnification factor.

Alternatively, a specimen of known dimensions can be placed on the pipe surface at the tangent

point, and the magnification factor estimated from the specimens image. Wall thickness

measurements from tangential radiography have an estimated accuracy of +/- 5%, when the

geometrical unsharpness is less than 0.02 inch.

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Figure 1. Radiographic Techniques9

Radiographic measurements can also be performed using the double wall technique. Thistechnique is useful to identify localized damage on a component and to accurately assess the

aerial extent of the damage. It will not, however, accurately size the depth of wall damage.

Therefore, the double wall technique must be supplemented with other NDE methods when

performing component integrity or life assessments.

New technologies for metal loss detection in bare piping include wide- and narrow-scan real-

time radiography.

Wide-Scan Real-Time Radiography

A new radiographic method, called ThruVu, has been developed under EPRI funding to scan

piping over a wide area and display the acquired image in real time

11

. This real-timeradiographic method permits identification of wall thinning areas, as well as localized damage at

a much lower cost per unit of inspected area when compared to conventional radiography. Once

deteriorated areas are qualitatively identified, subsequent examination with more accurate metal

loss measurement techniques, such as ultrasonics, is warranted (Figure 2).

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Figure 2. Wide Scan Real Time Radiography The ThruVu System 11

ThruVu can be used with various radiographic sources. However, it typically deploys an

Amersham 660 camera with a low intensity Iridium-192 source (30 or more curies). The

tungsten collimator has a narrow V-shaped slit that produces a fan shaped beam of penetrating

radiation. This low radiation intensity level limits the personnel exclusion zone to about 50 feet.

ThruVu deploys an array of solid state detectors that are sensitive to variations in radiation

intensity. The size and number of solid state elements determines the systems spatial resolution.

The linear detector model currently used is 16 inches long and includes 128 1/8-by-1/8

elements. With this configuration, the system is capable of resolving wall loss areas that exhibit

a radiographically projected image of in diameter.

ThruVu is most efficiently utilized when performing double-wall assessments. The currentspatial resolution suggests that its applicability to tangential surveys is limited. The sensor array,

when laid perpendicular to the scanning axis, allows the system to perform double-wall

radiographic images of straight piping ranging from 8 to 36 in diameter, and elbows up to 24

in diameter.

To perform a survey, a temporary track consisting of two guides is secured on the pipe using a

pair of brackets. Cords or straps are then used to secure the tracks in place. A motorized cart

assembly that includes the source and the detector is placed on the pipe and locked to the track,

the latter guiding the cart as it performs the survey. This cart-track design permits the survey of

horizontal, vertical, and elbow configurations as long as 4 and 12 inches of clearance is available

at the detector and source side respectively. The side clearance requirement is only 1 inch, and

the unit can survey within 1 inch of obstructions.

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Narrow-Scan Real-Time Radiography

Alternatively, a real-time radiographic survey can be performed along a narrow band using a

different system design12

. This new technology, the Lixi Profiler, is packaged as a portable

hand held unit, which permits the operator to scan at will any accessible piping area for wall loss

after performing a simple calibration. The device is suited for identification of FAC followed by

further verification with more accurate metal loss evaluation techniques (Figure 3).

Figure 3. Narrow Scan Real Time Radiography The Lixi Profiler12

The Profiler can be operated as a hand held unit because it uses a very low intensity Gadolinium

153 gamma ray source and its weight is low. The exposure to the operator with this source is

typically less than 2 mR, hence, it does not require an exclusion zone, and the device weighs

only 9 pounds. Piping up to 18 inches in diameter can be surveyed with this technology.

The Profiler detector uses a scintillator material and a photomultiplier tube that measures the

light intensity of the scintillator material. Various detector shapes are available including a

commonly used slit 1 long by 1/8 wide. A palm-sized computer records and displays the data

in graph form with equivalent steel thickness in the Y-axis and time in the X-axis.

In order to convert the detected radiation intensity into an equivalent steel thickness, a calibration

procedure must be followed. This procedure must be performed with a pipe of similar

dimensions and filled with fluid at the same level as the actual field pipe. Changes in the water

level will affect the baseline values.

Because the data is stored as thickness versus time, it requires that the operator record the

position of the probe independently in order to relate the archived values to piping locations.

One method to establish the probe position is to survey the segment at a constant pace and record

the time lapsed with a stopwatch.

Low-frequency Electromagnetic Method

Emerging technologies for the assessment of FAC of bare piping include the low frequencyelectromagnetic method

13. This recently developed device allows the operator to scan a wide

band of accessible and hard to reach piping by running a lightweight, hand held sensor unit and

acquiring the data on a laptop computer (Figure 4).

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Figure 4. Low Frequency Electromagnetic Method Testex PS 2000 13

Because the device transmits and receives electromagnetic signals, it does not require liquid

couplant or the removal of the coating, and needs minimal surface preparation.

The device inspects the pipe along a wide band by deploying an array of sensors (up to 64). The

amplitude and phase information of the electromagnetic signal is displayed graphically in real

time by the laptop computer. These anomalies are then correlated to flaw depth via calibration.

When operating the unit, it is recommended that the scanner be moved at constant speed over a

piping segment. This allows the identification of flaw locations from the recorded data, as the

unit does not currently have a wheel encoder to establish the flaw positions on the computer

screen.

Once the flaws are identified and ranked, verification with a more accurate wall loss

measurement technique is recommended.

Corrosion Rate Assessment in Bare Piping

Corrosion rate estimations are an important part of the FAC management strategy since the

corrosion rates affect the operational acceptance of the component, and influence the system

repair plans by determining the component life estimates.

A component is judged to be suitable for continued service if the predicted wall thickness at the

time of the next planned inspection is greater than the minimum accepted value 4, in accordance

with the formula,

Wall predicted > = wall (Corrosion Rate x Time-next-inspection x Safety Factor)

Likewise, a component is considered to have a given remaining useful life in accordance with theformula,

Life = (Wall Minimum acceptable thickness) / (Corrosion Rate x Safety Factor)

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11

EPRI recommends that the corrosion rate of a component be determined by establishing an

inspection grid on the component, comparing the thickness measurement at each grid point with

that obtained in the previous inspection, and selecting as the corrosion rate the maximum value

obtained from all the points in accordance with the formula,

Corrosion Rate = Max [(Wall Wall previous inspection) / (time between inspections)]

The suggested grid patterns for components of various diameters are given in Table 3.

Errors in the corrosion rate estimation can be introduced either by

Changes in the equipment calibration between inspections, or

Variations in the probe placement between surveys.

In order to reduce the errors in the equipment calibration, plants typically prefer to inspect the

components when the equipment is shut down and the components are at room temperature.This procedure avoids the errors associated with surveying hot surfaces with ultrasonics, as

discussed above.

To minimize the errors introduced by variations in probe placement, EPRI funded the

development of the AutoGrid system. This technology will be discussed next.

AutoGrid

The AutoGrid system 14 combines the accuracy of ultrasonics with a novel low-cost method of

recording the survey locations. This system, developed by Southwest Research Institute under

funding from EPRI, accurately locates the ultrasonic transducer in three-dimensional space by

triangulation using an array of acoustic microphones. Using this system the operator can

accurately return to a surveyed location and compare the new measurement with one previously

acquired (Figure 5).

Pipe Size Outside Diameter Maximum Grid Size

inch inch inch

2 2.375 1.00

3 3.5 1.00

4 4.5 1.17

6 6.625 1.73

8 8.625 2.25

10 10.75 2.81

12 12.75 3.3314 14 3.67

16 16 4.19

18 18 4.71

20 20 5.23

24 24 6.00

>24 - 6.00

Table 3

Maximum Grid Sizes for Standard Pipe Sizes5

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Figure 5. The AutoGrid System14

AutoGrid locates the position by attaching two acoustic emitters to the transducer. These

airborne sound signals are received by 3 microphones, which compute the transducer location by

triangulation. The system requires that 3 permanent reference locations be established on the

pipe. Then, a grid pattern is constructed for the area of inspection by the computer relative to

these points. The grid points are virtual; that is, their coordinates are stored in a laptop computer

rather than having them painted on the pipe surface. This avoids the difficulty of laying out the

grid pattern and having the marks subsequently wear off. In addition, the system helps the

operator find the marked location by continuously displaying the position of the probe in relation

to the target point.

Thickness measurements obtained with the help of AutoGrid can be organized as part of a

comprehensive data management system such as CHECWORKS, 8

. The data can be archived,annotated, displayed, and exported to CHECWORKS for further analysis.

Global Detection of Metal Loss in Bare Piping

Global external inspection techniques include longitudinal guided waves, tangential guided

waves, and active thermography. These technologies are still under development for application

to FAC and, therefore, have been classified as emerging.

Two types of longitudinal guided wave technologies are currently under development using

magnetostrictive and piezoelectric sensors.

Magnetostrictive Guided Wave

The magnetostrictive guided wave system is currently suited to scan large sections of piping

from one location. The system can detect defects with an equivalent cross-sectional reduction of

at least 1% and place the defect location with an accuracy of 2 inches 15.

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13

The magnetostrictive guided wave system is reported to achieve its largest inspection range in

straight, drained pipe, where it can inspect sections in excess of 100 feet long. The inspection

range is reduced when the piping is filled with water, when the inspected section includes sharp

bends, tees, and elbows, or when the coating is of the bituminous type. The current tool

performs best when surveying piping of 16-inch diameter or less. The inspection range

deteriorates for larger diameter piping.

The technology measures the net reduction in cross-sectional area. Accordingly, defects with

different morphologies, but same net cross sectional reduction, have equivalent signals. That is,

localized deep pits and widespread shallow defects with the same loss in cross-sectional area are

ranked equally in severity. Furthermore, the signals acquired have a spatial resolution of 7

inches. Defects that are within 7 inches are lumped together and reported as one large defect

with a cross-sectional area reduction equal to the sum of the individual contributions.

In addition, areas that are severely corroded will provide a large number of return echoes that

will be superimposed, making it difficult to perform accurate sizing analysis. In these situations

the technology provides a 3 tier grading system namely, good condition, lightly corroded, or

heavily corroded.

Because of these performance characteristics, once defects are detected and located, follow upmeasurements should be performed with more accurate local metal loss evaluation tools.

The magnetostrictive system deploys lamb-wave transmitter and receiver units that consist of

encircling coils and biasing magnets. The lamb wave is generated in the pipewall by disturbing

the bias magnetic field established by the magnets with a current pulse applied to the encircling

transmitter coil (Joule Effect). Conversely, the defect signals are detected by monitoring the

current that is generated in the receiver coil as the lamb wave crosses its bias magnetic field

(Villary Effect) (Figure 6).

Figure 6. Magnetostrictive Guided Wave Method Southwest Research System 15

Piezoelectric Guided Wave

Guided wave inspection technologies that deploy piezoelectric transducers are also under

development 16. One of these technologies uses either fully or partially encircling comb

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14

transducers (Figure 7). This technology has been reported to detect defects with an equivalent

cross-section reduction of 0.5%, and has been capable of inspecting 24-inch diameter pipelines.

Figure 7. Piezoelectric Guided Wave Inspection Pennsylvania State University CombTransducer System16

The comb transducer is reported to allow the control of the Lamb wave modes generated. This

mode control permits the optimization of defect detection as well as minimizes the loss of energy

to the transported fluid or the external coating, hence, maximizing the range of inspection. The

inspection procedures, however, are still under development, and tool operation requires

supervision from an expert in the process.

The comb transducer consists of an array of transducers placed normal to the pipe surface.

Each transducer in the array produces a periodical vibration in phase and at the same frequency

as to generate a guided wave with a wavelength equal to the transducer spacing. This

relationship between the transducer spacing and the wave mode permits the optimization of the

inspection range.

Tangential Guided Waves

Global ultrasonic inspection can also be performed with tangential guided waves. This emerging

technology is suited for evaluation of hard-to-reach locations such as piping laying on saddle

supports, or as a rapid scanning device to qualify piping areas for further evaluation with more

accurate metal loss techniques.

The system allows 360 examination of piping ranging from 2 to 36 in diameter. Surface

preparation is minimal since the system deploys electromagnetic acoustic transducers (EMAT)

that require no couplant, and can perform surveys through coatings up to 0.02-inch thick.

The device uses a saddle-like, hand-held sensor unit that is placed on top of the pipe withtransmitter and receiver electromagnetic acoustic transducers (EMAT) located 90 apart (Figure

8). The transmitted wave reaches the receiver along the short cord (90), by taking the long path

(270), or by performing a full trip around the circumference before reaching the transducer

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(450). The time-of-flight of these signals are recorded and correlated with the volumetric metal

loss along each of the travel paths. In this way, the technology surveys a band 1.5-inch wide that

completely encircles the pipe. The operator then moves the sensor unit to attain 100% coverage17.

Figure 8. Tangential Guided Wave Method Magna Scan System17

The system can detect a 10% volume loss over the surveyed band; however, it provides no

information on the type of corrosion damage. Therefore, the corrosion indications are typically

marked for further evaluation with more accurate metal loss techniques.

Active Infrared Thermography

Active Infrared Thermography is another emerging technology under development for global

inspection of FAC18

. This technology may be suited for detection of defects with diameterequal to the wall thickness and 25% deep. The performance deteriorates in the presence of thick

coating.

Active Infrared Thermography detects thin areas by quickly heating the pipes external surface

with pulsed flash lamps then recording the change in the surface temperature with an infrared

camera. With this procedure, internal pipe defects are identified by contrasting their hotter

appearance from the cooler image of the pipe at large.

The cameras are designed with a small form factor that can be manipulated by a single operator

in tight environments. Furthermore, the system highlights defect indications automatically, for

further comparison with calibration signatures.

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FLOW ACCELERATED CORROSION INSPECTION OF INSULATED PIPING

NDE technologies for insulated piping are classified in the same way as in the bare piping

section; that is, localized assessment, corrosion rate monitoring, and global surveying.

Local Detection of Flow Accelerated Corrosion in Insulated Piping

Local detection techniques include radiography, real time radiography, pulsed eddy current, and

Compton backscattering.

Radiography

Internal wall loss assessment through insulation using radiography is well established. Internal

wall loss assessments with an accuracy of +/- 5% can be obtained using the tangential

configuration as long as the geometrical unsharpness remains within 0.02 inch6,9

.

Since the film and the source (by necessity) are placed outside the insulation, the source-to-

object distance must be increased relative to bare piping procedures to attain the geometrical

unsharpness value of 0.02 inch. Therefore, measurements through insulation require longer

exposure times to obtain the recommended exposure densities of 1.5 to 2. EPRI recommends theuse of fine grain high-resolution film for wall thickness assessments

9. However, if this

technique leads to exposure times that are impractical, then faster speed films can be used.

As was discussed in the bare piping case, double wall exposures can be used to locate the

damage and assess the aerial extent of the affected area, but actual verification of the remaining

wall thickness should be done with other techniques.

Real-time Radiography

FAC can also be detected through insulation with real-time radiography for the commonly

deployed low-density insulation materials such as urethane or polystyrene foams. The detection

capabilities of the wide-band ThruVu system or the narrow-band Lixi Profiler are little

affected by the presence of these types of insulation systems.

The advantage of these newly developed techniques is that the cost per unit area of inspection is

much lower when compared with conventional radiography. However, because these tools use

the double-wall assessment technique, the thickness values provided must be verified with more

accurate metal loss procedures.

Higher density insulation systems such as calcium silicate or asbestos have been reported to

interfere with the ThruVu system. When pieces of these types of insulation are missing, an

apparent wall loss is recorded in the image 19.

Pulsed Eddy Current

FAC can also be detected with pulsed eddy current technology. The wall thickness of pipes with

diameters greater than 2 inches can be measured with an accuracy of +/- 0.04 inch20,21

.

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Pulsed eddy current systems are easy to deploy. Since no surface preparation or coupling media

is required, the operator simply places the probe at the insulated location and, within a few

seconds, obtains a thickness reading (Figure 9). Furthermore, since the technique is not sensitive

to probe alignment, it is very tolerant of the operators skill, which helps attain good

measurement repeatability in the field.

Figure 9. Pulsed Eddy Current Method The Incotest System 21

Pulsed eddy current technology is not suited for the detection of isolated damage. The eddy

current field, when used through 2 inches of insulation, illuminates an area 4 to 8 inches indiameter. Isolated pits may go undetected when applying the technique.

Pulsed eddy current uses a transmitter and a receiver coil. During operation, a train of current

pulses is sent to the transmitter coil, and the voltage decay of the induced eddy current field is

monitored by the receiver. The remaining wall thickness is determined by measuring the decay

time (or decay rate) of the eddy current field and comparing it to calibration standards.

Use of the technology does require some survey planning, particularly in pipe racks or in the

vicinity of carbon steel vessels. The proximity of large carbon steel masses will affect the

measurements and may require the realignment of the coil sensor away from the interfering

mass.

Compton Backscattering

Compton backscattering is another technology currently under development for measurement of

pipe wall thickness through insulation.

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This process has been shown to have an accuracy of 5% for wall thickness less than 0.25 and

10% for thickness greater than 0.25 but less than 0.5. For thickness greater than 0.5 the

performance of the system degrades. These accuracy targets are achieved when the material, the

internal fluids, and the survey geometry are well-characterized 22.

This technology has the advantage of exhibiting a spatial resolution of 0.75 and 1 inch for liftoff

distances of 2 and 4 inches respectively. This resolution enables the technique to detect pits with0.5-inch diameter and quantify their depth when their diameter exceeds 1 inch in 4-inch thick

insulated piping.

The Compton backscattering device includes a high-energy radioactive source with photon

energies greater that 0.1 MeV. The detectors, which can be of the scintillation type, are placed

near the entry point of the source radiation to capture the very weak backscattered radiation

emanating from the tested material. When the location of the source and the detector relative to

the specimen are accurately established, the intensity of the scattered radiation is proportional to

the density and thickness of the material. Since the density of steel is known precisely, the pipe

wall thickness can be accurately determined 23.

The intensity of this backscattered radiation also depends on insulation thickness, and service

fluid. Since insulation height is typically variable and unknown, practical implementation of thisemerging technology will require the development of liftoff corrections to the detected signal.

Corrosion Rate Assessments through Insulation

The NDE procedures to establish corrosion rate through insulation in piping susceptible to FAC

are not yet established. However, the pulsed eddy current and Compton backscattering

techniques can potentially be used for this service.

Pulsed Eddy Current

The pulsed eddy current technology can potentially be used to monitor the rate of FAC online, in

insulated piping. Because the monitoring procedure has not been validated, the application has

been classified as emerging.

The pulsed eddy current technique is suited for corrosion rate monitoring because it exhibits a

repeatability of +/- 0.005-inch, which is equivalent to that exhibited by ultrasonics. Furthermore,

monitoring can be performed online, while the component is hot, without deteriorating the tools

repeatability.

As was mentioned above, the device is very tolerant of variations in the field conditions. No

surface preparation is required, and changes in the probes alignment will not affect the

performance. In addition, because the tools hardware consists of a voltage measurement unit,

an accurate clock, and coil probes, no field adjustment of the device is required. Therefore, the

operators skill level is not critical to achieve the repeatability target.

As in the case of bare piping, a monitoring grid pattern should to be established on the

component. This grid pattern can be painted on the insulation, rather than on the surface of the

component. Also, the pulsed eddy current device, in principle, can be integrated with the

AutoGrid system, although this service is not yet available.

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Because the pulsed eddy current system illuminates a relatively large area, about 4 inches in

diameter for a 2-inch liftoff, less monitoring stations are required when compared to ultrasonics

to attain a similar aerial coverage of the component. Accordingly, the grid pattern can be made

sparser than that suggested in Table 3.

Compton Backscattering

Corrosion rates can also potentially be monitored with the Compton backscattering technique for

piping with wall thickness less than 0.5.

This emerging technique is still under development, as its repeatability has not yet been

established. However, since the technology requires that a calibration curve of radiation-counts-

versus-wall thickness be established before performing the survey 22, its repeatability is likely to

be less robust than that exhibited by the pulsed eddy current technique.

However, because the Compton backscattering technique has a good spatial resolution, and the

errors brought about by the liftoff variations are eliminated once the baseline values at grid

points are established, the technique represents an alternative for online monitoring of FAC.

As with pulsed eddy current devices, the grid pattern can be established on the insulation ratherthan the component itself, either by painting the survey stations or by integrating the device with

the AutoGrid system. Because the spatial resolution is 0.75 inch, the grid spacing suggested in

Table 3 is applicable.

Global External Inspection of Insulated Piping

Global external inspection techniques for insulated piping include longitudinal guided waves

with either magnetostrictive or piezoelectric sensors. As mentioned when referring to bare

piping inspection, these are emerging technologies.

Magnetostrictive Guided Wave

The magnetostrictive guided wave systems can have an inspection range greater than 100 feet aslong as the sensors are placed directly on the pipe surface by removing a short piece of

insulation. When placing the coils and magnets in close contact with the pipe surface, the system

can detect defects with a cross-sectional reduction of 1%, with spatial resolution of 7 inches, and

locate defects with an accuracy of +/- 2 inches. As mentioned before, the tool performs best

when surveying piping of 16-inch diameter or less. The inspection range deteriorates for larger-

diameter piping.

The magnetostrictive guided wave system can be operated with the transmitter and receiver coils

placed outside the insulation. However, this configuration reduces the inspection range and

deteriorates the systems performance. The preferred manner is to remove a narrow band of

insulation all the way around the pipe and place the sensors directly on the pipes external

surface. Guided waves are then produced which travel along the length of the pipe (beneath theinsulation) and detect damage.

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Piezoelectric Guided Wave

Guided wave inspection technologies that deploy piezoelectric transducers can also be used to

inspect insulated piping, but they require removal of the insulation at the location where the

transducers are to be coupled to the pipe. These systems have been reported to inspect insulated

piping with minor range deterioration once the wave-mode is properly selected 13. This

technology requires that the partially encircling comb be coupled to the pipe surface at the

location where lamb waves are to be launched. This guided wave technology has been reported

to detect defects with an equivalent cross-sectional reduction of 0.5%, in piping up to 24 inches

in diameter16.

Conclusions

This evaluation on new and emerging technologies for FAC concluded that:

Monitoring the rate of FAC in bare piping has been made more cost effective with the adventof AutoGrid, a new technology.

FAC detection through insulation is feasible, with some limitations, and the availability ofnew technologies (real-time radiography and pulsed eddy current) has made the inspectionoperations more cost effective.

Measuring the rate of FAC through insulation is feasible and the improvements in the pulsededdy current and Compton backscattering techniques may make the in-service assessmentsmore plausible.

Methods for detection of FAC globally are being developed but are not yet established.

References

1. Chexal, B., et.al., Flow Accelerated Corrosion in Power Plants, EPRI Report No. TR-106611-R1, 1998.

2. Dooley, B., Condition Assessment Guidelines for Fossil Fuel Power Plant Components,

EPRI Report No. GS-6724, 1990.3. Gosselin, S.R., Ammirato F.V., Walker, S.M., Risk-Informed Inservice Inspection

Evaluation Procedure, EPRI Report No. TR-106706, 1996.

4. Chexal, V.K., Recommendations for an Effective Flow Accelerated Corrosion Program,EPRI Report No. NSAC-202L-R1, 1996.

5. Dooley, R.B., et.al., Guidelines for Controlling Flow-Accelerated Corrosion in FossilPlants, EPRI Report No. TR-108859, 1997.

6. Walker, S.M., Ammirato, F.V., Nondestructive Evaluation of Ferritic Piping for Erosion-Corrosion, EPRI Project No. NP-5410, 1987.

7. Nottingham, L.D., Sherlock, T.P., NDE Guidelines for Fossil Power Plants, EPRI ReportTR-108450, 1997.

8. Chexal, B., CHECWORKS Flow Accelerated Corrosion, EPRI Report No. TR-103198-P1, 1998.

9. Becker, F.L., Lube, B.M., Walker, S.M., Guide for the Examination of Service WaterPiping, EPRI Report No. TR-102063, 1994.

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10.ASTM Standard Recommended Practice for Measuring Thickness by Manual UltrasonicPulse-echo Contact Method, Designation E-797-90.

11.Gothard, M., Field Trials and Testing of Prototype ThruVu Real Time RadiographicDevice, EPRI Report GC-110152-SI, 1998.

12.Walker, S.M., Demonstration of the Prototype Lixi Portable Density Profiler, EPRI ReportGC-109055, September 19, 1997.

13.Ramchandran, S, Ramchandran, S., McDougal, L., Non-Destructive Testing of Boiler Tubefrom the Fireside (Outer Diameter), Proceedings: Third International Conference on BoilerTube Failures in Fossil Plants, EPRI Report TR-109938, Pg. 5-33, April 1998.

14.Walker, S.M., Development and Commercialization of Next Generation NDE Devices forFlow-Accelerated Corrosion, EPRI Report MI-107959, 1998.

15.Brophy, J.W., Assessment of Magnetostrictive Sensor Technique: Detecting Flow-Accelerated Corrosion in Feedwater Piping, Revision 1, EPRI Report No. TR-108449-R1,1997.

16.Spanner, J.C., Ultrasonic Guided Wave Inspection of Piping, EPRI Report TR-107419,November 1997.

17.Kenefick, S., Evaluation of the Quality NDT Magna Scan Ultrasonic Examination System,EPRI Piping and Bolting Memorandum, May 1997.

18.Zayicek, P., Shepard, S.M., Summary Report of Advanced IR NDE of Service Water PipingSystems, EPRI Report TR-107463, October 1997.

19.Angell, B., Personal Communication, May 1999.

20.Walker, S.M., Martinez, E.Q., MacDonald, D.E., Evaluation of the TransientElectromagnetic Probing (TEMP) System Through for Detection of Wall Thinning ThroughInsulation, EPRI Report TR-101680, December 1992.

21.Brett, C.R., Assessment of the Pulsed Eddy Current Technique: Detecting Flow-AcceleratedCorrosion in Feedwater Piping, EPRI Report No. TR-109146, December 1997.

22.Klevans, E.H., et.al., An Erosion-Corrosion Monitor Using Gamma Ray Backscatter, EPRI

5

th

Piping and Bolting Inspection Conference, San Antonio, June 1999.23.Bryant, L.E., Nondestructive Testing Handbook Radiography and Radiation Testing,

American Society of Nondestructive Testing, Volume 3, 1985.

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About EPRI

EPRI creates science and technology

solutions for the global energy and energy

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