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8/6/2019 The Application of NDT and Analytical Assessment Techniques to Defects in High Temperature Pressure Equipment
http://slidepdf.com/reader/full/the-application-of-ndt-and-analytical-assessment-techniques-to-defects-in-high 1/7
The Application of NDT and Analytical Assessment Techniques to Defects in
High Temperature Pressure Equipment
W. A. Spencer and D. Ross*
Connell Wagner, 116 Military Road, Neutral Bay, New South Wales 2089, Australia*Connell Wagner, Advanced Technology Centre, University of Newcastle, Callaghan, New South Wales 2089,
Australia
ABSTRACT
Routine NDT of critical pressure equipment for plant operators occasionally reveals defects in the material of the
structure. When confronted with this situation the plant owner is faced with the choice of whether to run, repair or
replace the equipment, with serious safety and cost implications.
Application of leading edge NDT techniques has detected defects that may have not have been detected previously.
Inspection and test techniques are applied including surface methods NDT to check for cracking, high-sensitivity
ultrasonic testing for detection of sub-surface defects, and metallographic surface replication to detect microstructuraldegradation, for example creep in high temperature components. Defects may be either from original fabrication and
may propagate with service, or service-induced cracks. Such service-induced cracks in the pipe weld root area of crack
sensitive steels can be difficult to detect by conventional ultrasonic methods.
This paper describes practical approaches to the application of codified methods for the assessment of such defects.
Methods for the application of fracture mechanics to defects in high temperature plant are described in codes such as
British Standard 7910:2005, Guide to methods for assessing the acceptability of flaws in metallic structures . In the
approach described in this paper component loads are calculated based on thermal transients, pressure loading, residual
stresses due to welding and system stresses from piping. Plant visual inspections are carried out ensure that no unusual
situations exacerbate the loads.
Connell Wagner has applied these NDT and analytical techniques to critical pressure equipment at power stations
within Australia. The NDT and analytical techniques provide vital information to the decision making process of theplant owner. Several of these cases will be presented in this paper. The approaches described are generally applicable to
other types of equipment operating at high temperature.
1. INTRODUCTION
High temperature pressure equipment, operating in thecreep range, such as Main Steam and Hot Reheat piping
systems at coal fired power stations, are subject to a
range of loads in both the hot and cold conditions. Flaws
arising from original fabrication, particularly those in
welds, or flaws that have originated in service may be
detected during routine inspections using sensitive Non-
Destructive Testing (NDT) techniques. Upon detectionof a flaw outside the requirements of the applicable
pressure equipment standard AS/NZS 37881
requires
assessment of the significance of the flaw with respect
to the loading to which the component is subjected. As
AS/NZS 37881
does not cover such assessments of high
temperature equipment and refers to other international
standards, one such standard is BS79102.
A plant owner faced with the situation where a defect
has been detected in their pressure equipment needs to
make decisions about whether to continue to run the
equipment with the flaw, whether to repair the flaw or to
replace the damaged equipment. Such a decision has
serious cost and safety implications and it is important
that appropriate engineering advice is brought to bear in
this situation.
Firstly this paper describes the advanced NDT
techniques which have been used to detect flaws in
pressure equipment and their advantages over traditional
techniques. Secondly the methodology, based on the
methods of BS79102, are described. Thirdly two case
studies are presented which briefly describe the
application of these NDT and analysis techniques topressure equipment in thermal Power Stations.
2. METHODOLOGY
2.1 NDT Techniques
A range of NDT techniques is routinely applied by staff
from Connell Wagner’s Advanced Technology Centre
to inspect pressure equipment. These techniques are
summarised as follows.
Magnetic Particle Test (MT) - to detect surface
cracking. Colour-contrast magnetic-flow test method to
AS 1171 requirements using hand-held AC yoke magnet
(refer Figure 1). The weld and HAZ locations must be
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carefully linished, free of grinding scratches, in order to
detect small surface cracks of ~1mm or greater.
Figure 1: Magnetic Particle Test of Pipe Bend Seam
Weld.
High-Sensitivity Ultrasonic Weld Test (UT) – to
detect sub-surface defects, especially pipe weld rootcracking that may have developed in-service or may
have been below the reporting threshold of previous
inspections. Testing is done in accordance with AS 2207
Method UMB1 with the additional high-sensitivity
requirements developed by E.ON UK’s Power
Technology Group to detect weld root cracking in low
alloy steel (Figure 2). It has been demonstrated that aservice-related oxide-filled crack initiating from the
weld root zone can go undetected when testing to level 1
sensitivity requirements of AS 2207. It is important to
note that cracking can also initiate outside of the weld
zone for example at a step on the bore. Very highsensitivity settings on digital flaw detectors are used for
defect detection then the defect needs to be accurately
sized and plotted for position relative to the weld.
Figure 2: Ultrasonic Weld Test of Header SeamWeld.
Metallographic Replication (RP) - to detect the
presence of creep damage on the surface. Acetate
replicas are taken on both heat-affected zones at 3 or 4
equi-distant locations around the weld joint (Figure 3).
The number of replicas depends on the weld size and
geometry. The surface is polished and etched, usuallywith 2% Nital to reveal the microstructure. It is
important to achieve a microstructure that is free of
deformation and contamination. An acetate replica strip
is applied to the polished area then peeled off and
mounted flat on a slide, gold coated in a sputter chamber
and examined for creep under the metallurgicalmicroscope up to 1000x magnification. The early stages
of creep appear as isolated voids, usually at grain
boundaries, and the later stages being gross cavitation
and microcracking (Figure 4).
Figure 3: Replication of Pipe to Header ReducerTerminal Weld.
Figure 4: Aligned Cavitation and Creep Micro-
Cracking at a CMV Weld HAZ
Material Identification (PMI) - to confirm materials in
case of weld repair. Portable x-ray fluorescence method
used to identify the materials (Figure 5).
Remote Visual Inspection (RVI) – may be used toconfirm the presence of a weld root crack using CCTV.Usually only done if access for internal inspection is
available.
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Figure 5: Positive Material Identification.
2.2 Analysis Techniques
Following the detection of a flaw analysis techniquesbased on the methods of BS79102 are implemented. A
flow chart of the general activities involved in such an
assessment is presented in Figure 6.
Flaw Detected and Sized by NDT
Gather Appropriate MaterialData
Calculate stresses at locationof flaw – piping system
stresses, pressure, throughwall thermal stresses, residual
stresses due to welding.
Perform level 1 assessment to
requirements of BS7910
Provide results of assessmentto plant owner
If required carry out level 2assessment and flaw growth
calculations to requirements of BS7910.
If flaw is in piping systeminspect the system to confirmsupport behaviour.
Figure 6 Flow chart of flaw detection and assessment
process
BS79102 provides for three fracture assessment
methods. The level one (simplified) and level 2 (normal
assessment) are indicated in Figure 6. Both assessment
levels are based around the failure assessment diagram
concept. The failure assessment diagram examines two
parameters, the load ratio and fracture ratio. The load
ratio considers the margin against plastic collapse, and
the fracture ratio reflects the margin against fastfracture.
The level 1 approach provides for conservative material
property inputs and treatment of stress data to yield a
simplified but conservative result. The approach
described in this paper is to use simplified assessment toprovide information to the plant owner during the
outage period to assist the decision making process.
The level 2 failure assessment diagram does not include
the inherent conservatism of the level 1 approach.
Connell Wagner’s methodology has been to couple thelevel 2 assessment with an assessment of the possible
defect growth with further service using the techniques
detailed in BS7910
2
.
3. CASE STUDY – DEFECT ASSESSMENT
FOR A TURBINE CASING
3.1 Introduction
An NDT examination of a high-pressure steam turbine
casing revealed a large defect in the diaphragm slot
radius as indicated in Figure 7. The plant owners
concern was whether they would be able to continue
safely operating the unit until a replacement casing
could be sourced from the turbine OEM, a period of
eighteen months.
Figure 7: High pressure turbine casing the arrow
indicates the position of the flaw.
3.2 NDT Results
Linear surface indications were detected at a routine
inspection in the upper and lower halves of the HPcasing. The major indications were located at the No. 2
diaphragm slot, pressure side radius. Subsequentlyultrasonic techniques were used to estimate the defect
depth and replicas were taken at the defect tips to
provide more information about the defect.
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Ultrasonic examination of the lower half casing defect
revealed its extent over a length of 420 mm and depth
ranging up to 85 mm on a casing thickness of 120 mm.
Examination of the major defect showed it to be oxidefilled with little if any secondary branching evident
(Figure 8). Close examination of the defect tip displayed
numerous intergranular microcracks and associated
creep cavitation (Figure 9).
Figure 8: Micrograph of defect, heavily oxidised with
no secondary branching.
Figure 9: Defect tip and associated intergranular
micro-cracking.
Replication did also indicate the presence of a weldrepair in the vicinity of the defect. Later examination of
the OEMs manufacturing data records confirmed that a
weld repair had been made in this area.
3.3 Stress Analysis and Fracture Mechanics
A relatively simple finite-element method model of the
portion of the casing in which the defect was found was
created (Figure 10).
Figure 10: FEM model of casing.
Loads were applied to the casing to simulate the
pressure and thermal loads which the casing experiences
in service. The loads considered included normal
operation and the transient operating conditions present
during startup and shutdown of the turbine. Measuredtemperature data for various types of transient operation
were provided by the plant owner.
Convective heat transfer coefficients were applied to the
inner and outer surfaces of the model. From an initial
estimate of the coefficients an iterative process was
applied to calibrate the coefficients so that the transienttemperature profiles accurately matched the measured
temperatures data from the plant. Figure 11 provides a
comparison of measured and calculated temperatures for
one of the transient operations examined.
ColdStart
0
100
200
300
400
500
600
0 10000 20000 30000 40000 50000 60000
Time(s)
T e m p ( D e g . C )
0
10
20
30
40
50
60
70
80
90
100
MeasOuter
MeasInner
InnerT
OuterT
DeltaT
deltaT1
Figure 11: Comparison of measured and calculated
temperatures and temperature differences.
Once the point at which the peak temperature difference
occurred had been calculated the stresses at this point
were determined based on the relevant temperaturedistribution. Figure 12 provides an example of one such
temperature distribution and Figure 13 provides an
example of the associated stress distribution. This was
done for each type of transient operation. Similarly
stresses were calculated for the normal operation case
including the appropriate pressure and thermal loads.
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Figure 12: Calculated temperature distribution
during a transient operation.
Figure 13: Calculated stress distribution during atransient operation.
Having calculated the stress distribution for all relevant
plant operations the stresses perpendicular to the crack
face were linearised into membrane and bending
components to facilitate fracture mechanics analysis
using the techniques of BS79102.
A table of the linearised membrane and bending stresses
was prepared for each operation type. The stress
components were split based on whether they resulted
from primary loads (pressure) or secondary loads
(temperature). Also in the case where a plant operationwent below the fracture appearance transition
temperature (FATT) for the material the maximum
stresses were used for both portions of the operation (i.e.
above and below the FATT).
Using the methods of BS7910 the load ratio and fractureratio were calculated for the range of plant operations
and appropriate material properties. Defect growth due
to both fatigue and creep were calculated for the planned
operating scenarios. A through wall defect was assessed
and found to be tolerable up to a much longer length
than the defect found, indicating that the defect should
leak before burst.
3.4 Results of Assessment
Based on the results of the metallurgical investigation it
was postulated that the defect might have originated
from reheat cracking due to the weld repair in that
region during original manufacture. This scenario wasreinforced by the fact that the stress analysis did not
indicate any unusually high stresses in the vicinity of the
defect during normal or transient operations.
The casing was returned to service for a period of
eighteen months until a replacement casing could beprepared and installed. A check of the defect size
following the removal of the casing from service
indicated that no significant defect growth had occurred
in this period, as predicted by the analytical results.
4. CASE STUDY – DEFECT ASSESSMENT
FOR BOILER HEADER CONNECTION
TO HOT-REHEAT PIPE
4.1 Introduction
Ultrasonic inspections of the welds between the
Reheater Outlet header and both left and right hot reheat
(812.8 mm OD) steam leads of a large coal fired thermal
unit revealed indications of possible flaws at the internal
surface of the weld. Connell Wagner was engaged to
provide an assessment of the effect of these flaws on the
integrity of the pipework for continued operation over a
two-year period until the next inspection opportunity.
To allow for a possible delay of the next inspection the
assessment was made for a period of continued future
operation of three years.
4.2 NDT Results
A condition assessment was carried out on a Hot Reheat
Pipe Terminal Weld at the Reheater Outlet Header of a
large coal fired boiler. The weld tested was a Pipe to
Header Reducer circumferential butt weld. Surface
preparation required for testing included grinding and
linishing to a 120-grit finish. The surface profile across
the weld should ideally be flat and flush across the weld
and on both sides of the weld, however in this instance
some weld cap was still in place and a taper restriction
was on one side of the weld.
A range of NDT techniques were applied to the weld,
including magnetic particle, high sensitivity ultrasonic
weld test, metallographic replication and material
identification. Observations were as follows:
1. No surface cracking was observed.
2. Accurate scanning was limited to one side of the
weld only, due to access restrictions. Ultrasonic
testing detected a 900mm length of semi-continuous
cracking from the root of the weld. The crack height
varied from 7-12mm (average approx 10mm) andappears to propagate along the reducer-side fusion
zone.
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3. No significant creep damage was observed. Minor
creep damage (isolated voids) was observed on the
identical weld at the other end of the header.
4. The PMI results confirmed 2¼Cr 1Mo low alloy
steel, as specified.
5. No significant cracking had been reported from the
previous inspection in 2001. The ultrasonic test
method used at the 2001 inspection was not the
high-sensitivity procedure, but rather carried out in
accordance with AS 2207 Level 1. It is possible thatsome shallow cracking, or original weld defects,
may have been present although not reported.
4.3 Stress Analysis and Fracture Mechanics
An existing finite element method model of the Hot
Reheat system was validated and modified whereappropriate for use in this study. The modelling
simulated the effect various factors have on the stress
state of the piping system, including; system anchormovements due to thermal expansion, thermal
expansion of pipes, variation of support loads, cold pull,
self weight, and internal pressure.
An inspection of the piping systems in the vicinity of the
Reheater Outlet Header revealed that the pipe supports
were functioning reasonably. However the support
position indicators located close to the header were
indicating that the hot reheat pipes (and possibly the
header also) are up to 45 mm lower than designed.
No other indications of possible distress to the piping
systems were observed. The effects of the observed ‘as-found’ positions of the pipe hanger indications were
implemented (by modifying spring support forces) into
the finite element model of the hot reheat piping system.
Results of analysis with the implemented effects of ‘as-
found’ conditions were compared against the results
from analysis with ‘as-designed’ conditions. The
comparison revealed a marginal increase in the pipe
loads at the flaw location (the maximum increase was
7.7% - for the resultant moment and 22% - for the
resultant force). An envelope of the ‘as-found’ and ‘as-
designed’ loads (including those from the original
design) was used for the assessment. The loads
considered included all normal operating and out of service state for the system.
For the purposes of the flaw assessment, the system
bending stresses were treated as primary stresses. This is
a conservative assumption, which means there was no
allowance taken for the fact that they may relax with
time in service due to creep deformation.
Membrane stresses due to pressure loading were
calculated based on the cross sectional area of the pipe
and the design internal pressure of the hot reheat system.
The minimum wall thickness at the weld location of the
pipe cross section was used to calculate the membraneand bending stress levels.
As the flaw was located between a plain pipe section
and an enlarger cone a detailed finite element model was
created for the local area (Figure 14). Refined stresses
were obtained by applying the loads obtained from the
global pipe model.
Figure 14: Local model of pipe-enlarger connection
loaded with internal pressure and system loads from
global model.
Residual stresses due to welding were calculated on the
basis of the advice of BS7910 Cl 7.2.4.2. This clause
allows for residual stresses, at welds subject to stress
relief, at the rate of 20% of the yield strength of the
material.
Thermal stresses due to local rapid cooling of the pipewere conservatively estimated based on an assumed
through wall temperature difference.
4.4 Results of Assessment
The margins against fast fracture and overload were
checked using the level 2 approach under clause 7 of
BS7910. All normal operating conditions were checked
and adequate margin against fast fracture was
demonstrated. Figure 15 indicates the position of the
assessment point for normal operation (plus thermal
loads due to cooling) on the failure assessment diagram.
Several points are indicated, allowing for upper bound
levels of defect growth.
Level 2 FAD
0
0.2
0.4
0.6
0.8
1
1.2
0 0.5 1 1.5
Lr
K r
Kr
Figure 15: Failure assessment diagram for as found
and projected flaw lengths.
Acceptable
region
Normal
Operation
Cold
Conditio
18mm(Oct 08)
15mm(Jun 07)
12mm(Oct 05)
18mm
12mm
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Critical Defect Size
The effect of various size flaws on the position of the
assessment point on the failure assessment diagram was
considered. A flaw depth of 32 mm was found to give a
fracture ratio (Kr) equal to the maximum allowable
fracture ratio. In other words this is the critical defectdepth based on the Level 2 failure assessment diagram.
Future Defect Growth
The effect of the operating history to date was
considered. The defect was assumed to be present from
the start of operation of the plant. The reference stressvalue for normal operation was calculated and a mean
rupture life was found. On this basis the current creep
damage fraction was calculated and indicated there is
adequate margin against creep rupture.
As for future operations, in October 2008, the predicteddefect depth is calculated as 18 mm, based on upper
bound defect growth parameters. It was calculated based
on the assumption that the unit will be in service all thetime. The reference stress value for normal operation
was re-calculated. For the revised reference stress and a
temperature of 540oC a mean bound rupture life was
calculated. The current creep damage fraction was re-
calculated and again indicated that there is an adequate
margin against creep rupture.
The creep strain accumulated to date was also
determined based on the calculated reference stress and
the operating temperature. The creep strain was found to
be εc=0.53%. The creep strain data was based uponmean strain rates (the predicted creep strain, in October
2008 would be 0.63%).
The defect growth due to creep was considered. It was
based upon the weld material upper bound defect
growth parameters. The creep defect growth rates
calculated were greater than ten times those previously
calculated due to fatigue. This validates the initial
assumption made to ignore defect growth due to fatigue.
A check of the elastic and creep strain levels determined
that stress redistribution was complete at the defect tip
for growth in the depth direction. Under the rules of
BS7910 this means that the calculated defect growth
rates do not need to be doubled. The assessmentindicated that even in 3 years time, the resulting flaw
will be less than the critical defect size by an adequate
margin.
The predicted pattern of future crack depth increase
together with the increase of the expended creep lifefraction is shown in Figure 16.
5. CONCLUSIONS
An advanced approach to the detection and assessmentof defects in high temperature pressure equipment has
been developed and applied to a number of situations.
The close coupling of advanced NDT and analytical
assessment techniques has provided useful information
to assist critical decisions about whether to run, repair or
replace plant components in which defects are found.
While the assessment inputs and approaches are not of a
high accuracy the prudent use of conservative input data
means that they may yield very useful information to
guide decision-making.
Two case studies have been presented to illustrate the
application of the methodology. The defects were sizedusing the NDT techniques described. The material
properties, loads and stresses were calculated using the
techniques described in this paper. In both cases the
results proved valuable to the plant owners in making
critical engineering and business decisions.
6. ACKNOWLEDGMENTS
The authors would like to acknowledge the contribution
of their colleagues Robert Small, Andrew Kucyper and
Peter Wilk to the body of work which forms the basis
for the techniques and case studies presented in this
paper.
5. REFERENCES
1. AS/NZS 3788: Pressure Equipment – In Service
Inspection, Standards Australia, 2001.
2. BS7910: Guide to methods for assessing the
acceptability of flaws in metallic structures, BSI,
2005.
3. Severud L K 1984 A simplified method evaluation
for piping elastic follow up Proceedings of the 5th
International Congress of Pressure VesselTechnology, ASME, San Francisco, pp. 367-387.
Crack depth increase and Creep life fraction expenditure
10
12
14
16
18
20
22
24
26
28
30
150000 155000 160000 165000 170000 175000 180000
Operational hours
C r a c k d e p t h ( m
m ) & E x p e n d e d C e e p L i f e F r a c t i o n ( % )
Crack depth
CreepLifeF
Figure 16: Future defect growth and creep life
consum tion