the application of ndt and analytical assessment techniques to defects in high temperature pressure...

8/6/2019 The Application of NDT and Analytical Assessment Techniques to Defects in High Temperature Pressure Equipment

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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



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.



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.



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.



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.



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



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

the application of ndt and analytical assessment techniques to defects in high temperature pressure...

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