STANDARDIZATION OF ACOUSTIC EMISSION TESTING … S Tilley1 ASME... · STANDARDIZATION OF ACOUSTIC EMISSION TESTING OF FOSSIL POWER PLANT ... content and method of fabrication

STANDARDIZATION OF ACOUSTIC EMISSION TESTING OF FOSSIL POWER PLANT SEAM-WELDED HIGH ENERGY PIPING

John M. Rodgers

Acoustic Emission Consulting, Inc.

Richard M. Tilley Electric Power Research Institute

ABSTRACT The structural integrity of seamed fossil high energy

piping has become a major safety and O&M issue again

with eight recent failures of seam-welded piping since

1992. These include failure of six superheat link piping

segments, two of them catastrophic, and the failures of two

long-seamed bends in hot reheat lines. Advanced methods

of inspecting piping welds with ultrasonic techniques, such

as Time Of Flight Diffraction and Focused/Phased Arrays,

is pushing back the envelop of detection to earlier stages of

creep damage. But these are still very expensive and

involve considerable logistics planning and downtime to

perform. EPRI has sponsored development activities since

1986 to mature the utilization of a real-time online

evaluation method for seam-welded piping: Acoustic

Emission (AE) Guidelines were published in 1995, and

over 90 full-scale tests have been performed from 1996-

2003 to develop a database and correlate results with other

established evaluation methods.

Tests to date have shown high sensitivity to early

stage creep damage, which is evidenced by development of

cavities (cavitation) around nonmetallic inclusions and

carbides in the grain boundaries of the weld heat affected

zone and fusion zone. Successful double-blind testing with

advanced ultrasonsic inspection methods, and additional

confirmation with advanced cryo-cracking metallography,

have proven both the reliability and sensitivity of the AE

technique. The economics of the method are highly

favorable. Only small areas of insulation need to be

removed every 4.6-6.1 m (15-20 ft) to weld “waveguides”

to the piping surface. These form a linear location array

along the length of piping, providing global coverage of the

piping system. Testing is performed online with normal

peak loading and load cycling. No outage schedule is

required to perform the AE examination. The ASTM

E07.04 Subcommittee on Acoustic Emission is currently

developing a standard based on the EPRI testing database:

Document WK658, “Standard Test Method for Acoustic

Emission Examination of Seam-Welded High Energy

Piping.” Results will be presented showing that the AE

method has become a reliable and economical field

evaluation tool for seamed high energy piping.

Seam-Welded Piping Issues Ever since the catastrophic failures of seam-welded,

hot reheat (HRH) piping at Southern California Edison’s

Mohave plant in 1985, and Detroit Edison’s Monroe plant

in 1986, utility companies have been carefully considering

the need for periodic inspections of critical piping to guard

against creep-induced failures. Figure 1 illustrates the

creep damage mechanisms associated with seam-welded,

high energy piping. A number of serious defects in seamed

piping were removed after inspections in the late 1980’s,

and for a number of years there were no more catastrophic

failures (Foulds et al, 1996).

Beginning in 1992, however, there have been six

known failures of seam-welded superheat (SH) link piping

supplied with CE boilers, as well as two failures in hot

reheat long seamed bends. Two of these have been

catastrophic: Virginia Power’s Mt. Storm Unit 1 in June

1996, and Kansas City Power & Light’s Hawthorne Unit 5

in August 1998 (Figure 2). No loss of life occurred in

either of those two failures, but the cost of repairs and loss

of power generation is of critical concern to utility

companies in this age of growing competition. All failures

of SH link piping have occurred on units with accumulated

service time of 125,000 to 225,000 hrs. Compounding the

problem of inspection is the inaccuracy of supplied

documentation, which may not reflect the true alloy

To be presented at the ASME Pressure Vessel & Piping Conference, San Diego, July 2004 1 of 19

content and method of fabrication. The Hawthorne SH link

piping was not known to be seam-welded. The general

aging of fossil plants will continue to raise concerns about

the safety of operating seamed high energy piping systems.

Even seamless piping systems have had problems,

including creep-related failures of circumferential welds,

and the through-wall creep failure of a seamless SH bend

that had been improperly fabricated. Current strategies for

effectively managing the safety and life of seam-welded

piping are based upon periodic inspection of the weld area

for evidence of inservice damage.

Inspection Issues The normal inspection process is arduous: wait for a

scheduled outage, remove insulation from large areas of

piping, and conduct ultrasonic (UT) inspections of every

inch of seam weld. The documentation issue creates

additional uncertainty about what and where to inspect.

Often the utility must resort to acid etching sections of

piping to identify the seam weld condition and its location

on the pipe, especially if the piping has been finish

machined after fabrication. Certain areas, such as welds

under hanger attachments and at floor penetrations, are not

typically inspected due to the additional costs of access. In

some cases, the inspections have not produced reliable

results. The through-wall failure in a Sabine HRH elbow in

1992 was preceded by UT inspections five and three years

before failure. The Mt. Storm SH link piping failure in

1996 was preceded by UT inspection 14 months prior to

failure. The failure of a long seamed HRH bend at Gaston

in 2001 raised additional concern about projecting the

results of inspection from other line segments. The same

bend on the mirror image HRH line of the two-line system

had been inspected with Time of Flight Diffraction (TOFD)

UT five years earlier, but had not indicated any problems.

The lack of a predictable re-inspection interval to avoid

this type of failure has become problematic on top of the

high inspection costs and plant downtime. In addition

many utilities are going to extended outage schedules,

limiting the time and interval available for periodic off-line

inspections.

More automated UT techniques using TOFD and

Focused Array Transducer System (FATS) have come to

fore in the past two years as reliable methods of pushing

back the envelope of detection of creep-induced defect

conditions; but these are even more expensive to apply as

general survey methods than conventional multi-angle,

pulse-echo UT inspections. Conventional stress and

materials analysis methods are only marginally useful in

predicting location and timing of the development of creep

damage. Changing conditions on the line with time may

alter the effectiveness of hangers, and general relaxation

effects change stresses from the original installed

condition. Peaked weld geometries, welding materials,

methods and post-weld heat treatment add an additional

range of variables to the prediction of when and where. A

physical test that could inexpensively locate areas of

potential damage---while the unit is in operation--- would

offer the best safety and cost solution to the problem of

seam-welded piping management. That technology is

acoustic emission monitoring, and the results to date are

very encouraging.

Acoustic Emission Monitoring Acoustic emission (AE) has been used

extensively in the past thirty years in the petrochemical,

nuclear, and aerospace industries, and has been

incorporated into a variety of society inspection codes and

practices (ASME, ASTM, ASNT). Aerial manlift vehicles

on the transmission and distribution side of electric utility

companies have been routinely inspected with AE since the

early 1980’s (ASTM F914-85). Acoustic emission is an

attractive alternative to conventional inspection. Its

principal features as applied to high energy piping are:

1. It is a passive monitoring technique, which listens for

the high frequency sounds of material failures and defect

growth. The detected energy comes from sound waves

generated by growing flaws, and developing metallurgical

damage mechanisms (eg. yielding, inclusion decohesion,

deformation twinning, etc)

2. Emission sources can be “source-located” within a

few inches on linear lengths of piping by time-of-arrival

techniques, similar to seismology.

3. It is a global inspection. Any defect growing in the

metal path can be detected and located. This includes

seam, closure, and hanger welds, drains, vents, etc.


4. Instrumentation of the line is minimal. Removal of 0.1

m2 (1 ft2) of insulation every 4.6-6.1 m (15-20 ft) along to

length of the line is required to install AE Waveguides.

These are 0.6 cm (¼ inch) diameter stainless steel rods

welded to the pipe surface, with a platform for mounting

the AE sensor at the other end (Figure 3). Welding is

allowed by ASME B31.1 code either offline or during plant

operation. Both manual welding and stud-welding

attachment are practiced.

5. Testing is performed online, while the plant is

operating and load cycling. Feedback is real-time, and

does not require an outage to perform testing.

6. After initial setup and calibration of equipment,

monitoring can be conducted remotely via modem

communication. 1-2 weeks of monitoring is required for a

typical 152 m (500 ft) piping system.

7. Direct inspection costs are only 10-30% of that

required for ultrasonic inspection, and no plant downtime is

required. This allows more frequent testing (3 yrs

average) than would be practicable or affordable with UT,

and increases the safety margin for early detection of

problems.

Equipment and AE Testing Set-up The process of AE monitoring applied to piping

systems starts with installation of piezoelectric transducers

on thermal stand-offs (or waveguides) along the length of

the piping. Spacing intervals for the sensor are typically

4.6-6.1 m (15-20 ft), and installation of the sensors does

not require full removal of piping insulation. Acoustic

emissions generated in the piping structure are detected by

the sensors and relayed to the data acquisition system. A

schematic of the hardware set-up is shown in Figure 4.

There are several unique requirements for successful

monitoring of high energy piping with AE:

1. The use of high frequency sensors (300-400 kHz) and

high frequency filtering (>200 kHz) to mitigate the effects

of the steam flow background noise. This noise is

predominant below 300 kHz, and would obscure detection

and accurate source location if lower frequency, or

broadband, sensors were used.

2. The employment of a “floating” or automatic

threshold that can control the sensitivity of detection by

keeping the voltage threshold of detection above the

average background noise.

3. The use of active linear source location to determine

the position of emitting sources on the line. The accuracy

of location is dependent on the distance between

sensor/waveguide positions, the pipe diameter, and the

position of the emitting source. In the middle of the array

between two sensors, accuracy has been demonstrated at

+/- 2.4 cm (+/- 6 inches). Near the sensors accuracy may

degrade to +/- 60 cm (+/- 24 inches). Still, this yields a

very finite area for follow-up inspection.

4. The use of active AE signal feature filtering to further

refine the data and eliminate obvious noise sources, such as

flow turbulence.

5. Simultaneous recording of piping temperature and

pressure are required to provide correlation between active

AE sources and the likely source mechanism. Data Evaluation and Correlation

During startups, shutdowns, and normal operation, the

piping system is a rich source of acoustic events, which are

detected by the sensor array on the piping. By evaluating

time of arrival differences at two adjacent sensor positions,

the source of the emission can be located. Some of the

detected emissions are associated with processes that are

not of concern with regard to pipe integrity. These sources

include steam flow turbulence (noise) and mechanical

friction. The acquired data is accordingly filtered to focus

on sources associated with in-service damage. Figure 5

provides an illustration of the processing that is used to

correlate AE sources with weld creep damage.

The primary characteristics of seam weld creep-

related sources are behavioral in nature—they respond to

the pressure in the piping (hoop stress) and other

mechanical sources of stress (geometry, hanger supports,

etc.). During online conditions with normal peak load

cycling, creep-related sources reveal themselves by

repeated behavior with each peak load cycle:

1. The sources are sensitive to pressure, and may show a

pronounced effect of emission rate with pressure (Figures

5, 6, 8, 9).


2. During load cycling, emission rates will typically peak

near the start of the peak load period.

3. There is periodic emission activity during steady-state

pressure and temperature conditions.

4. The AE location profile is typically spread out over 1

meter (40 inches) or more of piping length, and shows

intermittent high-density locations of activity (Figure 5).

5. The amplitude range of emission sources broadens to

higher values with higher activity rates (Figure 5).

6. Emission rates are much higher during startup

conditions, even before substantial pressure loading. This

demonstrates that the damaged area is responsive to

stresses even when the piping is not in the creep regime

(>510 C , 950 F).

The amount of emission generated by the creep

mechanism, the repetitive nature with each peak load cycle,

and the extensive dynamic range (45-90 dB amplitude) of

signals, is extraordinarily different from normal ductile

fracture mechanisms--- such as fatigue crack growth in

mild steels. Many thousands of locatable signals are

sometimes accumulated over several feet of weld length

and several cycles (days) of steam line operation at peak

load. The sheer numbers of the sources is inconsistent with

a ductile crack growth mechanism, which produces

infrequent emission of more limited dynamic range with

repeated load cycles. The acknowledged mechanism of

creep in seam welds is the development of cavities

(cavitation) around nonmetallic inclusions and carbides on

the grain boundaries in the fine-grained heat affected zone

(HAZ) or fusion zone of the seam weld (Figure 1). Isolated

cavities soon give way to aligned cavitation along grain

boundaries, then coalescing into scattered microcracking.

Final consolidation and linking into macrocracking along

the seam weld direction occurs in the last stage of growth,

which can be very rapid depending on a host of factors

(wall thickness, annealing state, inclusion densities,

thermal and localized mechanical stresses, etc).

The early stage of this process involves the degradation

of the bonding between particles and the metal matrix.

These are load-carrying interfaces, and their eventual

failure (decohesion) is the most plausible explanation for

the amount and dynamic range of emission detected in the

creep process. From the viewpoint of classifying AE

behavior, this bears similarity to the experience of

monitoring an organic-based composite material that has

incurred extensive matrix damage. This also explains the

emission that has been noted during the thermal excursion

on startups, even without pressure in the system, when the

piping is clearly not operating in the creep range.

Damaged particle-to-matrix interfaces are prone to

disbonding under high strain conditions, and startups are

known to produce an even higher axial strain than at full

load operation. Indeed the results of the extensive EPRI

field testing program to date has yielded detection of

cavitation damaged seam welds that have not developed to

the stage of micro- or macro-cracking.

Lest this raise the concern that cracking would not be

detected were it occurring, a separate test program

conducted in collaboration with a UK utility demonstrated

that creep crack growth in small specimens produced ever

increasing emission with increased crack growth rate.

Again the emission rate was orders of magnitude higher for

the increment of crack growth than would have been

expected at lower temperatures and growth under fatigue

conditions. The decohesion mechanism remains active

throughout the creep regime, regardless of whether induced

by directed stress at the tip of an active crack or in a

volume of weld without visible cracking. This leads to

certain detection of the creep-related failure process from

very early stages, approximated at 50% of remaining life.

Field Testing Program In 1986 EPRI began investigating AE testing to

quickly screen large areas of piping while the plant is

operating, then come back later for a closer look with

conventional UT examination methods. Beginning in 1991

with a joint R&D program with Pacific Gas & Electric,

EPRI charted the course for the development of a reliable

field procedure. That program produced the EPRI

Acoustic Emission Monitoring Guidelines (Morgan and

Foster, 1995), a comprehensive guide to seam-welded

HRH line inspection. It was demonstrated in double-blind

testing in 1994 on PG&E’s Potrero Unit 3 HRH line, where

the results from AE testing and conventional UT were

successfully corroborated.


Over the period of 1995-2000 an extensive field

testing program was undertaken to build a database of

results performed to the EPRI Guidelines criteria. These

were used to verify the methodology, refine evaluation

criteria, and compare the results with other

inspection/evaluation methods. Thirty inspection programs

were conducted on both HRH and SH seam-welded piping

systems for twelve different utility companies. Table 1

provides a summary of the more significant findings, and

follow-on results to date. Some of these have included

Tailored Collaboration projects between EPRI and its

member utilities, including Central & Southwest (Central

Power & Light and West Texas Utilities, now a division of

American Electric Power), American Electric Power,

Kentucky Utilities (now LGE Energy), Illinois Power (now

Dynegy), United Illuminating (now part of PSE&G

Energy), Southwestern Public Service Co (Xcel Energy)

and Sierra Pacific Power. Other companies have started

their own inspection programs, including Sunflower

Electric, Oklahoma Gas & Electric, Grand River Dam

Authority, Northern Indiana Public Service Co, PECO

Energy (Exelon), Minnkota Power, and Entergy.

A typical inspection of 1502 m (500 ft) of HRH

piping with UT will often require extensive scaffolding,

complete insulation removal and reapplication, and 2-3

weeks of plant downtime. This can easily run into the

$300K range for this length of piping. Often time is spent

evaluating material conditions that do not represent active

defect sources (original welding flaws, plate laminations,

e.g.). An AE test program will typically cost <$60K for

the same coverage. AE only detects active (developing)

flaw sites, and therefore focuses the priorities for follow-on

inspection. This can lead to findings that would otherwise

be ignored or not pursued aggressively enough. Several

examples of this occurred during AE testing programs.

In the original proof-of-method AE test on PG&E’s

Potrero Unit 3 HRH line, several defect conditions were

found that would otherwise not have been noted without

the AE testing. These included a cracked saddle weld in a

hanger support, cracks in a cast elbow, and ID-connected

thermal fatigue cracking in the parent material adjacent to a

spool closure weld. None of these defects was a serious

structural threat, but the sensitivity of the AE technique

was established. The following sections summarize some

specific results from the field testing program.

Central Power & Light Joslin Unit 1 HRH An early AE test using the EPRI guideline approach

was the inspection of Central Power & Light’s Joslin Unit

1 HRH line in April 1997. A series of AE sources was

being source-located at a mechanical hanger location on

the bottom of the line (Figure 6). This position showed

unusual behavior for a hanger location—AE was observed

to be sensitive to the pressure gradient, and continued

under steady-state high load conditions. In a normal

inspection scenario, the hanger would not have been

removed. During the following outage, the hanger strap

was removed and the indications verified in the seam weld

by both manual and automated (multi-angle pulse echo and

TOFD) UT inspection. A 25 cm (10 inch) square plate

section was removed from the line and the suspected seam

weld location investigated with both conventional

metallography at 500X, and with more sensitive

“cryocracking” and SEM examination at 5000X (Figures

7a, 7b). A reliable method of early creep detection is a

method known as cryocracking. It is capable of identifying

the condition when damage still exists as isolated

cavitation around carbides and inclusions in the weld HAZ.

The specimen is scored at the desired analysis location,

submerged in liquid nitrogen to supercool the material, and

then impacted to break the material along the preferred

plane. Careful examination under SEM magnifications of

up to 5000X is required to observe the fine structural detail

of the fracture surface. The conventional metallographic

preparation showed no cavitation or cracking to verify the

UT indications. The SEM examination showed that creep

cavitation was present in the lower weld bead. This early

test demonstrated that the online testing methodology with

AE was both very sensitive to early stage creep damage,

and that it could also be verified with the proper UT

inspection methods and a more sensitive metallographic

technique. Using AE monitoring to direct the follow-up

inspection averted a potentially serious future problem, one

that would not likely have been discovered with

conventional inspection approaches.


American Electric Power Tests Two tests were conducted on AEP’s Gavin Units 1 &

2. Gavin Unit 1 was the subject of another double blind

test with UT inspection in the summer of 1996. The lower

152 m (500 ft) of the dual-line, 366 m (1200 ft) HRH

piping system was inspected first with both manual and

TOFD UT during a planned outage. This was followed by

online AE testing for several weeks during peak load

cycling operation. AE detected two areas of activity on

elbow closure welds, and two minor areas of activity on

turbine leads. The closure weld indications correlated with

OD-connected shallow cracks that had been visually

ground out during the outage. The two areas on the turbine

leads also matched UT indications. The pattern of

intermittent indications, and length of line affected,

matched up well between the UT and AE methods. None

of these indications was an immediate structural problem.

A spool piece with a high density of plate laminations in

another turbine lead was flagged as a potential problem by

AEP. But the AE results showed nothing of significance

going on in this structure, and future costs for re-inspecting

this area with UT on a regular basis can be averted.

Testing of the full 366 m (1200 ft) of Gavin Unit 2 in 1997

revealed no significant AE indications anywhere on the

line. The estimated cost savings on this line by using AE

screening in place of conventional UT inspection was

$500K, not counting potential lost power generation.

Kentucky Utilities Tests Two significant inspection programs were conducted

at Kentucky Utilities Ghent Unit 1 and Brown Unit 3 in

1997. Several exit elbows in the HRH link piping at Ghent

were found to have highly active AE sources during startup

and subsequent online AE monitoring (Figure 8). Manual

UT inspection confirmed all five indicated areas of

activity. Weld repairs were conducted to remove most of

the indicated weld material. AE Monitoring was

performed after weld repair. The two elbows that had been

repaired by through-wall removal of weld metal showed a

90% reduction in AE activity to very low levels. The SH

link piping was also monitored on this unit, and showed no

significant activity. Such was not the case at Brown Unit

3.

The seam-welded SH link piping leads (3.7 m [12 ft]

vertical segments) were monitored at KU’s Brown Unit 3

during a unit restart following the Spring outage in 1997.

Both leads were observed to have active AE sources in

areas below the penthouse roof. These areas were similar

to the location of a through-wall failure on a SH link at

Alabama Power’s Gaston plant in 1992. Monitoring was

performed during online operation again in Feb. 1998,

prior to an outage when the links were scheduled for

replacement. Similar AE activity was observed in the

suspect areas (Figure 9). The West lead was removed and

sent to the EPRI NDE Center in Charlotte, NC for further

analysis. A phased-array automated UT inspection was

performed on the seam weld, and significant indications

were observed in a 60 cm (24 inch) long area matching the

AE source location results. A 1.2 cm (3 inch) plug sample

was taken for metallographic analysis. No cracking or

cavitation was evident with normal metallographic

preparation and examination at 500X, so the specimen was

sent to Materials & Mechanical Engineering in Austin, TX

for further analysis. The cryocracking examination

identified evidence of early-stage creep cavitation in the

weld centerline (Figure 10). These sophisticated UT and

metallographic methods confirmed that AE had identified a

developing problem well ahead of the appearance of

microcracking, which later would coalesce into a major

crack. This added perhaps years to the early detection of

the problem, and allowed the utility to remove a future

hazard on a reasonably planned schedule.

Illinois Power Tests Another recent AE monitoring program at Illinois

Power’s Baldwin and Wood River plants identified

potential creep problems in several seam-welded elbows

and bends. Structural Integrity Associates (Silver Spring,

MD) was contracted to perform TOFD and FATS UT

inspection on one of the clamshell elbows on Baldwin Unit

1. They found evidence of early-stage creep cavitation in

the midspan extrados weld of the elbow, in the location the

AE results had predicted (Figure 11). This result matched

well with the expectations of the AE monitoring, which

identified this area as a low-activity source. The utility

now knows where the bellwether indicators are on the line,


References and the approximate level of damage it represents.

Periodic future inspections or metallographic examination

can be used to plan a replacement or repair schedule

consistent with the company’s operating objectives.

Foulds, J. R., Viswanathan, R., Landrum, J.L., 1996,

“Guidelines for the Evaluation of Seam-Welded High

Energy Piping,”Electric Power Research Institute, . TR-

104631.

Morgan, B.C., Foster, C. L., 1995, “Acoustic Emission

Monitoring of High-Energy Piping, Volume 1: Acoustic

Emission Monitoring Guidelines for Hot Reheat Piping,”

Electric Power Research Institute,. TR-105265-V1.

Conclusions

Acoustic emission has proven its worth in these and

other test programs. Approximately 30% of lines tested

have shown no significant findings, and most others have

shown only minor activity at suspect locations. The

majority of seam weld findings have been in elbows and

bends, followed by hanger locations on horizontal line

segments. These are known to be higher stressed areas,

and offer further validation of the AE methodology. The

correlation with follow-on nondestructive inspection has

been very good, but the lesser sensitivity of UT inspection

methods will generally not confirm early stage creep

damage at the isolated cavitation stage. The economics of

inspection and relative certainty of detection at an early

stage of creep damage should be increasingly attractive to

companies attempting to manage their piping systems in a

climate of reduced capital and O&M spending.

Munson, R., Rodgers, J., Tilley, R., 1999, “The

Utilization of Advanced Metallographic Techniques to

Verify In-service Damage in Long-seam Welded, High

Energy Piping,”EPRI Fossil Plant Inspection Conference,

Atlanta.

Foulds, J.R., Carnahan, R.A., 1997, “Examination of

Sabine 2 Hot Reheat Pipe Seam Weld Cracking,” Electric

Power Research Institute, TR-107141.

Munson, R., 1998 “Metallurgical Examination, Hot

Reheat Plug, Central Power & Light Joslin SES,” Report to

EPRI.


Table 1. Summary of AE Tests with Significant Findings on Seam-Welded, High Energy Piping Through 12/98

Station and Unit

AE Indications

Verified by Comments

Potrero 3 HRH 2 East lead elbows Automated UT & RT One elbow inspected. Near-ID linear indications, porosity in elbow. Not sure seam welded. On lower neutral axis. Associated with casting defects (shrinkage porosity)

Hanger attachment on horizontal segment.

Visual Cracked saddle weld. Repaired. No AE after repair.

Closure weld at East lead elbow. Manual UT Circ weld cracking (minor).

Montour 1 HRH Minor indications in turbine lead bend and near horizontal hanger support and at branch connection.

100% UT TOFD No significant findings on first inspection. Turbine lead bend later removed. Creep cracking in branch connection found with TOFD 2 yrs after first inspection.

Gavin 1 HRH, partial.

2 elbow closure welds, shallow OD cracks, one of them 270° around circumference.

MT Lower 152 m (500 ft) of line double blind UT to ERPI specs, both manual and TOFD. Circ. weld cracks ground out prior to AE testing.

2 indications, turbine leads, in seam. Manual, TOFD UT Manual UT confirmation of strongest sources (near ID, discontinuous over 76 cm (30 in) from shallow elbow to hanger--matches AE locations). TOFD confirmation of other source (midwall, discontinuous, past horizontal elbow).

Coal Unit 2 HRH

7 stantions (Hanger attachments). Cooldown only--no online indications.

MT Some manual UT of seam welds around hanger, bend locations. MT findings confirm multiple cracks in fillet welds on stantion welds.

E.S. Joslin HRH

3 seam weld indications under and near hangers.

Manual, automated UT

Manual and automated UT confirmation (Multi-angle shear and TOFD)--midwall indications. One indication removed. Cryo-cracking analysis by M&M Engrg (Austin, TX) confirms early stage creep cavitation in weld.

3 circ weld indications. Manual UT Minor indications.

Ghent 1 HRH link piping

4 elbows, formed. In-seam indications. One in-seam indication in horiz. segment past elbow.

Manual UT Recordable UT indications in all five areas with manual UT. HRH link elbows match pattern, severity predicted by AE. All repaired. East lead elbows repaired through-wall show 95% reduction in AE activity. Material sample shows no evidence of cracking.

Ghent 1 HRH 4 elbows, one other near hanger on horizontal segment. In-seam indications.

Automated UT (P-Scan), later TOFD UT

No indications with P-Scan. Two years later TOFD confirms indications near hanger on horizontal segment and in clamshell elbow.

Valmy 2 HRH Several hanger positions, and temperature sensitive indication below hanger on vertical segment

Manual UT, TOFD and FATS UT

Double blind UT on lower half of line. 2nd AE test (Dec 97) duplicated findings of first test (Apr 97). Inclusions, cracking in repair weld below hanger on vertical segment during 2nd UT inspection. Additional AE tests June 98, Jan 99, July 99. No indications of creep damage


Station and Unit

AE Indications

Verified by Comments

Brown 3 HRH, SH Links

2 main steam leads, in-seam indications.

Automated UT (phased array)

Material removed during Spring 98 outage. One section to EPRI for investigation. No findings with EPRI multi-angle UT procedure. UT phased array finds indications in SH seam weld matching AE locations. Cryo-cracking analysis by M&M Engrg (Austin, TX) shows early stage creep cavitation in mid-weld fusion zone.

Lon C. Hill 4 HRH

Lower level seam weld indications at turbine deck, and batwing hanger indications.

Manual UT, MT Inspection during 3/98. UT confirmed multiple discontinous laminar indications in seam weld at turbine deck location. Batwing hanger indication during startup and cooldown only. Multiple OD-cracks to 7.6 cm (3 in) found in batwing hanger welds (MT).

Horseshoe Lake 7 HRH

5 Elbows, seam weld sources. Manual UT Midwall indications on three elbows to 2.5 cm (1in) long. No material investigation to date.

Batwing hanger attachment welds. Indication shows pressure sensitivity.

MT OD cracks on both hanger and pipe wall sides. AE during online and startup, but not on cooldown.

Navajo 2 Clamshell elbows. Pressure sensitive sources indicating creep damage.

TOFD and FATS UT Prior inspection verified creep damage with UT, metallography. Elbows monitored with AE among lowest ranking damage with UT.

Baldwin 1 4 HRH radiused bends, bends in turbine leads, Y-block welds.

TOFD and FATS UT One clamshell inspected with TOFD and FATS ultrasonics. FATS confirmed early stage creep damage in midwall of extradose weld.

All tests performed to EPRI AE Guidelines standards (Nov 1995)


Weld Base Metal

Magnification: 500X

Figure 1. Typical high-temperature creep cracking defects occuring in seam-welded fossil piping systems constructed of P11 or P22 grade steels (top; Foulds et al 1996)). Type IV damage in the fine-grained HAZ typically occurs in subcritically annealed welds, which is more typical of thick-section SH link piping and some HRH piping. Bottom left shows cavitation damage in the cusp region of a HRH double-V weld (Munson et al 1999). Bottom right shows advanced damage in the form of microcracking from a failed long seam bend (Foulds & Carnahan 1997).


Figure 2. Virginia Power Mt. Storm Unit #1 SH link piping failure in June 1996 (top). This piping had been UT inspected just 14 months prior to the catastrophic failure. KCP&L Hawthorne #5 SH link piping failure in August 1998 (bottom). This piping was not known to be seam welded.


Figure 3. Example of an AE waveguide stud welded to a steam pipe. The waveguide serves to isolate the sensor from the hot piping surface, and acts as a sound conductor from the pipe wall to the sensor.

Figure 4. A general depiction of the process of AE detection and location on seam welded high energy piping. High frequency sensors are mounted on welded waveguides at 4.6- 6.1 m (15-20 ft) spacing on the pipe. Detection of signals at adjacent sensor positions, combined with time of arrival calculation, allow the source location of signals along the piping length. Source location cluster analysis is performed to assess the source(s) of the emission and the significance of the findings.


Figure 5. A seam-welded clamshell elbow on Illinois Power’s Baldwin #2 HRH line shows typical creep-related AE behavior under cyclic pressure loading (upper left). The mechanism shows high sensitivity to load, and shows a distribution of activity along the elbow that varies with each load cycle (lower left). Bottom right shows the amplitude density feature map of this elbow location. Distributed higher activity sources are evident, and the amplitude dynamic range is larger in the high activity area. Defect growth at this early stage is probably related to decohesion of inclusions that are being affected by the creep process.


Pressure Profile

Figure 6. Results of Joslin HRH testing, showing the suspect hanger position (upper right). An 18 cm (7 in) long series of indications were found with both manual UT and multi-angle automated UT under the hanger strap position. The combined sensor UT side-B scan is shown in the bottom center plot. Two significant clusters are identified from the midwall to ID positions, centered ~10 cm (4 in) apart. The AE location vs time results for a 76 cm (30 in) segment encompassing this position is shown at upper left. There are actually several location cluster peaks in this distance, but the two principal peaks correlate well in intensity and location spacing with the two prominent UT indications.


Figure 7a. Weld cross-section from Joslin HRH sent to M&M Engineering for metallurgical (cryo-cracking) examination (R. Munson, 1998). The lower weld pass was unusually large, and showed visible segregation. Figure 7b. SEM photo at 5000X of cryo-cracked surface from the lower weld bead (“b” in Figure 7a). There is evidence of visible creep cavitation around inclusions, with the orthogonal shaped features to the cavity associated with early stage creep damage.


“A”

“E” “B”

Figure 8. Results of AE monitoring of the Ghent #1 East HRH link piping. Configuration isshown in top right. Three AE clusters were detected (A, E, B) as shown in the linear sourcelocation plot in lower left. Upper left shows a composite graph of plant parameters and AE source location vs time for the three clusters detected. The pressure sensitivity of the sources is very obvious, marking them as creep-related sources. Bottom right shows the relative activity in events/inch-hr at different pressures for Cluster “A” in the original monitoring program. After through-weld repair of the piping, the activity drops by >90% for this area, indicating that the original sources of emissi een removed.

East HRHLead

West HRHLead

Y-BLOCK

WG1

WG2

WG3

WG4

WG5

WG6

Area "A"

Area "B" Area "E"

Area "C"

Area "D"

WG # Spacing1-2 13 ft2-3 16 ft3-4 12 ft5-6 9 ft

Area ”B”

Areas ”A” & “E”

To be presented at the ASME Pressure Vessel & Piping Confe Diego, July 2004 16 of 19
on have brence, San

Penthouse Roof 90

West SH Lead

Figure 9. Results of online AE monitoring of Brown #3 SH West lead link

piping. Top left shows configuration of 3.6 m (12 ft) seam-welded link segment and the location of AE waveguides. Top right shows the source location plot developed during Spring 1998 monitoring. Bottom right shows a composite graph of plant parameters and AE source located events detected. The sources are more active at high pressure, indicating they are creep-related sources. The AE location cluster is just below the penthouse roof.


OD ID

Creep Cavities on a grain boundary on weld centerline

f HAZ near fusion line at mid-wall of pipeg Weld near fusion line at ID root pass

c Weld center near ID and OD intersectiond HAZ near ID/OD weld cusp e HAZ near mid-wall of pipe

b Fine grained intraweld zone at mid-walla Fine grained intraweld zone 1/3 from OD

Figure 10. Results of the investigation of the AE indication on the Brown #3 SH West link segment at the EPRI NDE Center. Results of the Phased Array UT scan showed indications down the centerline of the weld in the area indicated by the AE source location results (upperleft). A 7.6 cm (3 in) core sample was taken from the suspect area (upper right) and subjected to both normal metallographic (500X) andcryocracking examination with SEM examination at 5000X (lower right). No cavitation was visible at 500X, but the cryocracking examination showed creep cavitation only at the mid-wall centerline position of the weld. This was in the location of the Phased Array UTindications.


Intradose WeldLSW4I53.5"

Extradose WeldLSW4E

110"

Girth WeldGS-11

Elbow HRF4

CS Hanger(U-bolt clamp)

Seam WeldLSW9

Spool HR9 WG#15

Spool HR8

Girth Weld GF-10

Seam Weld LSW8

WG#14 Illinois Power

Baldwin Unit #1Elev 553

Clamshell Elbow

Focused Array UTInspection

48-50"From GF-10

101-103"from GF-10

24"

16" 56"

Centerline distanceWG #14 to WG #15 = 192"

0LOCATION, 6-INCH INTERVALS

ILLINOIS POWER BALDWIN UNIT #1 HRH, WG'S 14-21, FILE BA1511A ONLINE: 3:45 PM 5/11/98 TO 2:48 AM 5/14/98

FILTERED: RDC>=25, FREQ>= 50 KHz

50

100

150

200

250

LOC

ATE

D E

VEN

TS

OD ID

C-Scan

Side B-Scan

End B-Scan

Figure 11. Results of Baldwin #1 HRH AE monitoring and follow-up UT inspectionwith TOFD and FATS UT methods. Two clamshell elbows show emission characteristic of early creep damage (upper right). The upstream elbow and a portion of the downstream piping between waveguides 14-15 was inspected 100% with TOFD, and spot checked with FATS UT in several areas (top right). No indications of creep damage were identified with the TOFD inspection. The AE source location results were used as guidance to inspect a midspan location on the extradose seam weld of the clamshell elbow. Evidence of early stage creep damage was identified with the FATS UT scan of a 5 cm (2 in) weld segment (bottom right).The C-scan image shows a fog of dots that are characteristic of cavitation clusters. The side B-scan shows these are at the mid-wall location, at the expected location of the double V weld cusp. The end B-scan image shows the cluster of indications falling along the expected fusion line of the weld.


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