I. M. DMYTRAKH and V. V. PANASYUK  Karpenko Physico-Mechanical Institute, National Academy of Sciences of Ukraine

5 Naukova Street, Lviv, 79601, UKRAINE

2nd Hungarian - Ukrainian Joint Conference“Safety, Reliability and Risk of Engineering Plants and Components”

Institute for Problems of Strength of NASUKyiv, Ukraine, 19- 21 September 2007

RELIABILITY AND FRACTURE RISK ASSESSMENTRELIABILITY AND FRACTURE RISK ASSESSMENT OF HEAT-AND-POWER-ENGINEERING PIPELINES OF HEAT-AND-POWER-ENGINEERING PIPELINES

WITH CRACK-LIKE DEFECTSWITH CRACK-LIKE DEFECTS

Problem of corrosion and corrosion fatigue damaging of feeding pipelines of heat-and-power generating units under long-term operating conditions is considered. The two main factors were taken into account: degradation of metals properties and purity of operating aqueous environment that causes by ecological pollution of natural water scoop. Corrosion fracture mechanics approach for assessment of workability and fracture risk of pipelines with crack-like defects is proposed, which based on conception of threshold and critical cracks depth and also corrosion fatigue crack growth parameters.

ABSTRACT

LECTURE OUTLINE

1. Introduction

2. Problem of Damaging of Feeding Pipelines of Heat-and-Power Generating Units under Operating Conditions

3. Fracture Mechanics Approach for Assessment of Workability and Fracture Risk of Pipelines with Crack-Like Defects

4. Determination of Corrosion Fatigue Crack Growth Resistance of Feeding Pipelines Metal

5. Forecasting of Workability and Fracture Risk of Feeding Pipelines with Crack-Like Defects from Different Power Plants

6. Conclusions

1. Introduction

Nowadays the pipelines, used for supplying water and oil or other liquids and gases, may be considered as important objects within the social and industrial infrastructure. It has become increasingly paramount to ensure their safe utilisation in order to prevent economical, social and ecological losses.

From an engineering point of view, pipelines are complicated three-dimensional structures that include straight pipes, pipe-bends, dissimilar welded joints, etc. In addition, their operating conditions can be quite severe, that is, internal pressure and cyclic loading (pulsation) combined with the influence of internal and external corrosive environments. The potential synergy of such parameters can lead to an increase in the risk of damage and unexpected fracture of these structures during their long-term exploitation.

Therefore the key challenge for this and similar engineering problems is the development of a reliability and fracture risk assessment tool based upon a scientific understanding of the failure mechanisms occurring in structures subject to corrosion and corrosion fatigue damaging.

This lecture contains the data of corrosion and corrosion fatigue damaging of the feeding pipelines of heat-and-power generating units and also proposes some approaches to expert assessment of their workability and reliability in cases of crack-like defects presence.

2. Problem of Damaging of Feeding Pipelines of Heat-and-Power Generating Units under Operating Conditions

Fig. 1. Corrosion damaging of feeding pipelines of heat-and-power-generating units under operating conditions: general corrosion of surface (a); initiating of localised corrosion (b); corrosion furrows and corrosion pits nucleation (c).

Fig. 2. Typical corrosion fatigue defects in the wall of feeding pipelines: sharp crack (a); blunted cracks (b, c); cracks branching (d).

TABLE 1. Statistic data on the exploitation regimes of power plant units

3. Fracture Mechanics Approach for Assessment of Workability and Fracture Risk of Pipelines with Crack-Like Defects

3.1. CRACK-LIKE DEFECTS MODELLING

C o rro s io n fu r ro w

C o rro s io n p it

iac

jac

C o rro s io n fu r ro w

C o rro s io n p it

iac

jac

Fig. 3. Model presentation of corrosion and corrosion fatigue defects in pipelines wall

3.2. STRESS INTENSITY FACTOR FOR TUBE WITH SEMI ELLIPTICAL CRACK UNDER INTERNAL PRESSURE

αψβ1λ62.0α4.11ββ6.04.0απ

θ2β

k

13.1

β75.01α1

α2β3βπ

θ213.0β48.012.1

π

1cπσΔKΔ

2

f

2

I

201α0;α6015.1

201α,1λ

2 332/1 α1α1ααψ

t2dpΔσΔ

(1)

a/cβ t/cα

3.3. CORROSION FATIGUE CRACK GROWTH RESISTANCE

Θ

0a2

ia2

icΔ

0c ict

0AiA

0BiB '0B '

iB

Θ

0a2

ia2

icΔ

0c ict

0AiA

0BiB '0B '

iB

Θ

0a2 0a2

ia2 ia2

icΔ icΔ

0c0c ic ict

0A0AiAiA

0B0BiBiB '0B'0B '

iB'iB

Θ

0a2

ia2

icΔ

0c ict

0AiA

0BiB '0B '

iB

Θ

0a2 0a2

ia2 ia2

icΔ icΔ

0c0c ic ict

0A0AiAiA

0B0BiBiB '0B'0B '

iB'iB

Θ

0a2 0a2

ia2 ia2

icΔ icΔ

0c0c ic ict

0A0AiAiA

0B0BiBiB '0B'0B '

iB'iB

Θ

0a2 0a2

ia2 ia2

icΔ icΔ

0c0c ic ict

0A0AiAiA

0B0BiBiB '0B'0B '

iB'iB'

0B'0B '

iB'iB'iB'iB

dNdcdNda

thK fcK

IKΔ

nKΔCdNdc

nKΔCdNda

dNdcdNda

thK fcK

IKΔ

nKΔCdNdc

nKΔCdNda

N,Cfac m

(3)

(2)

n

n

KΔCdNda

KΔCdNdc

Fig. 4. Crack growth rate diagram

3.4. THRESHOLD DEFECTS DEPTH CRITERION

3.5. CRITERION OF LIMITATION OF CORROSION FATIGUE CRACK GROWTH RATE

3.6. CRITICAL DEFECTS DEPTH CRITERION

(4)

dNdcdNdc (5)

.constdNdcunderacΦc

.constacunderKΔcc thth

(6)

(7)

(8) .constacunderKΔcc fcfc fcI KΔKΔ

3.7. DIAGRAM FOR ASSESSMENT OF WORKABILITY AND FRACTURE RISK OF PIPELINE WITH CRACK-LIKE DEFECTS

Fig. 5. Schematic view of diagram for assessment of workability and fracture risk of pipeline with crack-like defects

Z o n e o f B r i t t l e F r a c t u r e

Z o n e o f S a f e E x p l o i t a t i o n

acshapeDefects

thcc

fcc

E x p l o i t a t i o n w i t h P r e d i c t e d C r a c k G r o w t h

caFc 1th

caFc 3fc

.constdNdcundercaFc 2

Z o n e o f B r i t t l e F r a c t u r e

Z o n e o f S a f e E x p l o i t a t i o n

acshapeDefects

thcc

fcc

E x p l o i t a t i o n w i t h P r e d i c t e d C r a c k G r o w t h

caFc 1th

caFc 3fc

.constdNdcundercaFc 2

4. Determination of Corrosion Fatigue Crack Growth Resistance of Feeding Pipelines Metal

4.1. EXPERIMENTAL PROCEDURE

C Mn Si Cr Ni Cu S P As Fe 0,12 1,2 0,7 0,3 0,3 0,3 <0,04 <0,03 <0,08 Bal.

Ultimate stress MPa480σU Yield stress MPa250σY

TABLE 2. Chemical composition of steel 16HS (in weight %).

D =219

S=42

50

50

50

a)

b)

c)

Fig. 6. Element of pipe (a) and schematic cutting plan (b).

a) b)

Fig. 7. General view (a) and principal scheme of testing system (b)

b)

F

pH

E

PC

14

13

11

15

1612

105

7

1

2

3

4

6

9

8

F

pH

E

PC

14

13

11

15

1612

105

7

1

2

3

4

6

9

8

12 16

9

4

3

P

1

13 1514

8

76

5

2

11

10

17

12 16

9

4

3

P

1

13 1514

8

76

5

2

11

10

17a)

4.2. CORROSION FATIGUE CRACK GROWTH RESISTANCE DIAGRAMS OF FEEDING PIPELINES METAL

TABLE 3. Corrosion fatigue crack growth resistance data for metal of feeding pipelines

N o S y s t e m “ m a t e r i a l – e n v i r o n m e n t ”

n

C mMPa,K th

mMPa,K fc

1 N e w m e t a l – n o m i n a l

e n v i r o n m e n t 1 1 . 2 1 161071.8 6 . 3 2 2 2 . 0 5

2 N e w m e t a l – w i t h o r g a n i c a d m i x t u r e s

1 0 . 5 5 151002.3 6 . 3 6 2 3 . 7 9

3 E x p l o i t e d m e t a l f r o m P o w e r p l a n t L -

n o m i n a l e n v i r o n m e n t

3 2 . 8 7 331066.1 6 . 8 3 9 . 9 4

4 E x p l o i t e d m e t a l f r o m P o w e r p l a n t L - w i t h o r g a n i c a d m i x t u r e s

1 8 . 3 6 221036.4 6 . 8 6 1 4 . 5 7

5 E x p l o i t e d m e t a l f r o m P o w e r p l a n t V -

n o m i n a l e n v i r o n m e n t

1 4 . 0 7 181066.1 6 . 8 9 1 8 . 3 5

6 E x p l o i t e d m e t a l f r o m P o w e r p l a n t V - w i t h o r g a n i c a d m i x t u r e s

1 0 . 6 6 151024.3 6 . 1 1 2 2 . 8 7

Fig. 8. Corrosion fatigue crack growth diagrams for metal of feeding pipelines. Number of curve corresponds of the number of “material-environment” system given in TABLE 3.

1,00E-07

1,00E-06

1,00E-05

1,00E-04

1,00E-03

1,00E-02

1 10 100

K I , MPa(m) 1/2

dc/d

N, m

m/c

ycle

123456

5. Forecasting of Workability and Fracture Risk of Feeding Pipelines with Crack-Like Defects from Different Power Plants

5.1. TUBES SIZE AND OPERATING CONDITIONS

TABLE 4. Tubes size for feeding pipelines

The four dimension-types of tubes are used for feeding pipelines of heat-and-power generating units (see TABLE 3). High purity water at maximal pressure pmax= 35MPa

serves as operating environment. The possible operating pulsation of pressure is p= 10.5MPa.

Nominal external diameter D, mm, Nominal thickness of wall t, mm Material

526 50

467 45

405 40

165 16

Steel

16HS

5.2. INFLUENCE OF EXPLOITATION TERM

Fig. 9. Diagrams of workability and fracture risk assessment for new (a) and exploited on Power plant L (b) pipes with crack-like defects for nominal environment: 1 - cth; 2 –

c* (dc/dN=10-5 mm/cycle); 3 - c* (dc/dN=10-4 mm/cycle); 4 - c* (dc/dN=10-3 mm/cycle); 5 -

cfc (pipes size: D=526 mm; t=50 mm).

0

10

20

30

0 0,2 0,4 0,6 0,8

Defects shape (c/a)

Cha

ract

eris

tic d

efec

ts d

epth

c th,

c*,

cfc,

mm

1

2

3

4

5

a)

0

2

4

6

8

10

0 0,2 0,4 0,6 0,8C

hara

cter

istic

def

ects

dep

th c

th, c

*, c

fc,

mm

1

2

3

4

5

Defects shape (c/a)

b)

5.3. INFLUENCE OF LOCATION OF DEFECTS AND THEIR SHAPE

Fig. 10. Influence of cross section form of pipe (kf) on threshold cth (a) and critical

cfc (b) defects depth of different shape (new pipes: D=165 mm; t=16 mm; nominal

environment).

0,5

1

1,5

2

2,5

3

3,5

4

0,8 0,85 0,9 0,95 1k f

Thr

esho

ld d

efec

ts d

epth

cth, m

m

0,01 0,05 0,1 0,2

0,4 0,6 0,8

Defects shape ratio c/a

a)

4

6

8

10

12

14

16

0,8 0,85 0,9 0,95 1k fC

ritic

al d

efec

ts d

epth

cfc, m

m

0,01 0,05 0,1 0,2

0,4 0,6 0,8

Defects shape ratio c/a

b)

5.4. INFLUENCE OF ENVIRONMENTAL COMPOSITION AND PIPES SIZE

Fig. 11. Threshold defects depth cth versus defects shape for different size of

pipes from Power plant V: a - operating environment of nominal composition, b - with organic admixtures.

0

0,5

1

1,5

2

2,5

3

3,5

4

0 0,2 0,4 0,6 0,8

Defects shape (c/a)

Thr

esho

ld d

efec

ts d

epth

cth

, mm 526x50

467х45

405х40

165х16

a)0

0,5

1

1,5

2

2,5

3

3,5

4

0 0,2 0,4 0,6 0,8

Defects shape (c/a)T

hres

hold

def

ects

dep

th c

th, m

m 526x50

467х45

405х40

165х16

b)

Fig. 12. Critical defects depth cfc versus defects shape for different size of pipes

from Power plant V: a - operating environment of nominal composition, b - with organic admixtures.

0

5

10

15

20

25

30

35

0 0,2 0,4 0,6 0,8

Defects shape (c/a)

Cri

tical

def

ects

dep

th c

fc, m

m

526x50

467х45

405х40

165х16

a)

0

5

10

15

20

25

30

35

0 0,2 0,4 0,6 0,8

Defects shape (c/a)

Cri

tical

def

ects

dep

th c

fc, m

m

526x50

467х45

405х40

165х16

b)

CONCLUSIONS

1. Problem of corrosion and corrosion fatigue damaging of feeding pipelines of heat-and-power generating units under long-term operating conditions was considered with taken into account of metal degradation and real composition purity of operating aqueous environment.

2. Corrosion fracture mechanics approach for assessment of workability and fracture risk of pipelines with crack-like defects is proposed, which based on conception of threshold and critical cracks depth and also corrosion fatigue crack growth parameters.

3. For assessment of the detected defects in pipelines, the special diagrams are developed, which contain three zones: safe exploitation, brittle fracture risk and zone of exploitation with predicted growth of existed defects. Here, the importance of factors of exploitation term, location and shape of defects and composition of operating environment has been shown.