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Deposit Removal from Industrial Turbines

Using Blast Cleaning

Alex Raykowski

A project report submitted in conformity with the requirements

for the degree of Master of Applied Science

Graduate Department of Mechanical and Industrial Engineering

University of Toronto

O Copyright by Alex Raykowski, 2000

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Abstract

Deposit Removal from Industrial Turbines

Using Blast Cleaning

M.A.Sc. Thesis by

Alex Raykowski

Dcpartment of Mechanical and Industrial Engineering

University of Toronto

1999

Blast cleaning is used to remove deposits from surfaces of industrial turbine

components. The main goal of the thesis was to evaluate the aggressiveness of

different blast media and provide the best combination of blasting parameters

for effective cleaning while minimizing the component substrate damage. The

blasting process was characterized by the media kinetic energy and power

based on velocity and mass flow measurements. This made the experimental

results independent of equipment and test conditions. A qualitative

characterization of both individual and multiple impact sites established the

mechanism of deposit removal from turbine surfaces. An assessrnent of

deformation at the edges of turbine parts such as at blade and disc grooves was

made to identify blast conditions that minimized substrate damage.

Acknowledgments

I would like to thank my supervisor Professor Jan K. Spelt for his

guidance, support and generosity during this project. 1 would like also to thank

my colleagues Marcello Papini, Munir Ahmed, Payam Tangestanian. Yijun Tu

and Shuwen Wang for the collaboration and friendship.

This work was made possible through the support of the Rotor

Manufacturing Department of Siemens Westinghouse Inc. led by Brian C.

Maragno and coordinated by Mike Hader and Mazhar C. Khan.

I would also like to express my gratitude to Tim Callaghan and Grant

Woods who provided technical assistance in performing experiments in the

Engineering Laboratory of Siemens Westinghouse Inc.

And, of course, special thanks to my wife for al1 of the great things she

has done for me.

Table of Contents

1 . Introduction ............................................................ 1-1

. 1 . 1 Background ................................................................. 1 1

l . 2 Thesis objectives ......................................................... 1-4

1 . 3 Thesis outline ............................................................. 1-5

2 . Turbine and deposit characterization .................. ...... -2-1

2.1 Turbines .................................................................. 3-1

2.2 Blades and discs ....................................................... 2-2

..................................................... 2 . 3 Operating conditions 2-3

2.4 Deposirs ...................................................................... 2-3

2 .5 Summary ...................................................................... 2-8

.................................... . 3 Blast process characterization 3-1

3 . 1 Equipment ................................................................... 3-1

3 . 2 Media ........................................................................ 3-5

......... 3.3 Velocity measurement and kinetic energy assessrnent 3-10

..................... 3.4 Stream aggressiveness via renshape profiles 3-25

3 .5 Almen Strip test .......................................................... 3-33

.......................... 3.6 Material temperature rise during blasting 3 -3 7

.................................................................... 4 . Results 4-1

.................................................... Substrate de formation 4-1

........................................................... Deposit removal 4-17

......................................... Shooting at blades with airgun 4-68

................................................. Summary and discussion 4.83

......................................................... Dynarnic hardness 4.8 8

.............................................................. 5 . Conclusions 5-1

......................................... 5 . 1 Blast media in portable cabinet 5-1

........................................................... 5.2 Substrate damagc 5-2

............................................................ 5 .3 Deposit removal 5-4

....................... 5.4 Recommendations to optirnize blast cleaning 5-5

List of Tables

Table 2.1

Table 2.2

Table 2.3

Table 2.4

Table 2.5

Table 2.6

Table 2.7

Tables 2.8

Tables 2.9

Table 2.10

Table 2.1 1

Table 2.12

Table 2.13

Table 3.1

Table 3.2

Table 3.3

Maximum thickness of deposits on W9 1 gas turbine blades and discs .... 2-9

Maximum thickness of deposits on W 17 1 gas turbine blades and discs ... 2-1 1

Maximum thickness of deposits on W 19 1 gas turbine blades and discs ... 2- 13

Maximum thickness of deposiis on W62 gas turbine blades and discs ..... 2-15

EDX analysis of crushed deposits from discs of W9 1 gas turbine .......... 2-21

EDX analysis of cmshed deposits from discs of W 17 1 gas turbine ........ 2-22

EDX analysis of crushed deposits from discs of W 1 9 1 gas turbine ....... 2-23

EDX analysis of crushed deposits from compressor blades of W62 gas

turbine ............................................................................. 2-24

EDX analysis of crushed deposits from turbine blade of W62 gas

turbine ............................................................................. 2-26

....... EDX analysis of deposits fiom 1 st stage disc of D96 s t em turbine 2-27

..... EDX analysis of deposits from lSt stage disc of EM20 s tem turbine 2-28

....... EDX analysis of deposits fiom 1" stage disc of M25 steam turbine 2-28

...... EDX analysis of deposits fiom 1" stage disc of M32 steam turbine 2-28

Characteristics of media tested ................................................. 3-8

Hardness of work-hardened Chr-10 stainless steel shot as it is reused,

................. portable blast cabinet, 4.8 mm straight noule. 38 cm. 10 s 3-8

BT-7 glass bead average velocity measured by double- disc method.

main room. 11.1 mm Venturi node. 38 cm standsff. 10 s ............... 3-14

Tables 3.4 Average velocity measured by double-disc method, portable cabinet

with straight 4.8 mm nozzle, distance 38 cm, 10 s. . . . . . . . . . . . . . . . . . . . . . . . . . 3- 16

Tables 3.5 Media mass flow in portable cabinet, straight 4.8 mm nozzle,

38 cm, 15 s. ...................................................................... 3-18

Table 3.6 BT-7 glass bead mass flow in main room, 1 1.1 mm Venturi nozzle,

15 S. ...................................... , ........................ . ......... 3-19

Table3.7 MC-lplasticsmassflowtestsatRitcheySupplyLtd.,modelFS-

3648 portable cabinet, 6.4 mm straight nozzle, 15 s. . . . . . . . . . . . . . . . ... .. .. 3-19

Table 3.8 Wheat starch mass flow tests at The University of Toronto,

model PCN 4050 cabinet, 6.4 mrn straight nozzle, 15 s.. . . . . . . . . . . . . . . . . . . 3- 19

Table 3.9 BT-7 glass bead aggressiveness on renshape bars in the main room,

1 1.1 mm Venturi nozzle, a=90 deg., 483 kPa, 10 s. .. . . . . . . . . . . . . . . . . . . . . .. 3-25

Tables 3.1 0 Media aggressiveness on renshape bars in portable blast cabinet with

4.8 mm straight nozzle, a=90 deg., 483 kPa and 1 0 s. . . . . . . . . . . . . . . . . . . . .. 3-28

Table 3.1 1 Almen strip test, 64 cm, 1.0 min exposure. .. . . . . .. . . . . . . . . . .. . . . . . . . . . . . . . . .. 3-34

Tables 3.12 Temperature rise of blades blasted with BT-7 glass beads, 1 1.1 mm

Venturi nozzle, 345 kPa, 38 cm, a=90 deg., ambient temperature 28 OC,

measurement delay 4 s. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-38

Table 4.1 List of performed experiments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4- 1

Tables 4.2 Repetitive blasting at flat surface of W501 turbine disc with BT-7 glass

beads, main room, 1 I . I rnm Venturi nozzle, a=9O O... . . .. . . . . . . . . . . . . . . . . . . 4-3

Table 4.3 W501 disc root edges blasted with BT-7 glass beads in main room, 1 1.1

nun Venturi nozzle, a=90° ..... . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... 4-5

Table 4.4 Inspection with IOX magnification optical comparator of dental casts

fiom W 191 disc roots blasted with BT-7 glass beads in main mom,

1 1.1 mm Venturi nozzle, 483 kPa. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-7

Table 4.5 Blasting at blade roots of steam turbine in main room, BT-7 glass

beads, 1 1.1 mm Venturi nozzle, 483 kPa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4- 10

Table 4.6 Blasting at blade roots of steam turbine in portable cabinet, 4.8 rnrn

straight nozzle, 483 kPa, a=90°, P=90°.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4- 12

vii

Table 4.7

Table 4.8

Table 4.9

Table 4.10

Table 4.1 1

Table 4.12

Table 4.13

Table 4.14

Table 4.15

Table 4.16

Table 4. f 7

Repetitive blasting at turbine blade roots of W62 gas turbine in main

room, BT-7 glass beads, 1 1 .1 mm Venturi nozzle, 483 kPa

a=90° P=90°.. .................................................................... 4- 1 5

Edge of thin compressor blades from gth and 9" stages of W62 gas

turbine, BT-7 glass beads, main room, 1 1.1 mm Venturi nozzle. ......... 4-1 6

Deposit removal fiom 4-th stage cornpressor blades of W9 1 gas

turbine, main room, BT-7 glass beads, 1 1.1 mm Venturi nozzle.. ........ 4- 19

Deposit removal fiom 7-th stagc comprcssor bladcs of W 17 I gas

turbine, main room, BT-7 glass beads, 1 1.1 mm Venturi nozzie.. ........ 4-23

Deposit removal from 70th stage compressor blades of W 19 1 gas

turbine, main room, BT-7 glass beads, 1 1.1 mm Venturi noule.. ......... 4-27

Deposit removal fiom compressor 8th / 9th stage and turbine 1st stage

blades of W62 gas turbine, 38 cm offset, a=90 deg. ....................... 4-34

Impact site EDX analysis. Cornpressor blades of W62 gas turbine

blasted at 38 cm, a=90 deg. .................................................... 4-46

Impact site EDX analysis. Turbine blades of W62 gas turbine blasted ai

38 cm, a=90 deg. ................................................................ 4-48

Impact site EDX analysis. Deposits on compressor blades of W62 gas

................... turbine blasted with individual particles, airgun, a=90°. 4-72

Impact site EDX analysis. Deposits on turbine blade of W62 gas turbine

shot with individual BT-4 glass beads, airgun, 93 mis, a=90°. ........... 4-73

Velocity of a particle propelled by airgun fiom a caiibration chart for

................................................................... 0.4 m long barre1 4-73

viii

List of Figures

Figure 2.2

Figure 2.3

Figure 2.4

Figure 2.5

Figure 2.6

Figure 2.7

Figw 2.8

Figure 2.9

Figure 2.10

Figure 2.1 1

Figure 2.12

Figure 2.13

Figure 2.14

Maximum Lhickness of deposiis on cornpressor surfaces of W9 1 gas

turbine.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2- 1 O

Maximum thickness of deposits on turbine discs of W9 1 gas turbine.. .2- 10

Maximum thickness of deposits on cornpressor surfaces of W 17 1 gas

turbine. . . . . . . . . , . . . . . . . . . . . . , . . . .. . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2- 12

Maximum thickness of deposits on turbine surfaces of W 17 1 gas

turbine. . . . . . . . . . . . . . . . . . . . . . . , . . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2- 12

Maximum thickness of deposits on compressor surfaces of W 19 1 gas

turbine. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2- 14

Maximum thickness of deposits on compressor surfaces of W62 gas

turbine. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2- 16

Cross-section of deposit sample fiom 10" stage compressor blade of

W62 gas turbine. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . .. 2- 1 7

Cross-section of deposit sample fiom 1 1' stage compressor

blade of W62 gas turbine. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . 2- 1 8

Cross-section of deposit sample fiom 8" stage compressor disc

of W9 1 gas turbine. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2- 1 9

Deposit fiom EMM32 stem turbine stuck to the test tape. .. . . . . . . ... . . 2-20

Deposit fiom W171 gas turbine stuck to the test tape. . .. . . . . ... . .. .. .... 2-20

Deposit fiom W91 gas turbine stuck to the test tape. . .. . . .. .. . . . . . . ... . . 2-20

Deposit fiom W62 gas turbine stuck to the test tape. . .. . .... . ...... ... . . 2-20

Al, Fe, S weight percentage, deposits fiom compressor blades of

W62 gas turbine. . . . . . . . . . .. . . . . . . . . . . . . . . .. . . . . . .. . . . . . . . . . . .. . . . . . . . .. . . .... . . 2-27

Figure 3.1

Fi y r e 3.2

Figure 3.3

Figure 3.4

Figure 3.5

Figure 3.6

Figure 3.7

Figure 3.8

Figure 3.9

Figure 3.10

Figure 3.1 1

Figure 3.12

Figure 3.13

Figure 3.14

Figure 3.1 5

Figure 3.1 6

Nozzle and shutter in main blast room. . . . . . . . . . . . , . , . . . . . . . . . . . . . . . . . . .. . . . . 3 -2

EF2448A model blast cabinet. . . . . . . . . ... .. . . . . . . . . . . . . . . . .. . . .. . . . . . . . .. . . . . . . 3-4

Airgun setup. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 -5

Hardness of work-hardened Ch-1 O stainless steel shot, portable blast

cabinet with straight 4.8 mm nozzle, 38 cm distance, I O S. . . . . . . . . ... . ... 3-9

Double disc senip.. . . . . ... . . . . . . . . . . . . . . . . . ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 3- 1 1

Photograph of double discs. .... ......... . ... .. . . .. ........ . ... . ... .. ... ... .... 3-1 1

BT-7 glas bead average velocity mesured by double-disc method,

main room with 1 1.1 mm Venturi nozzle, 38 cm, 10 S. . . . . . . . . . . . . . . . . .. 3- 1 5

Glass bead and Chr-1 O stainless steel shot average velocity measured

by double disc method, portable cabinet with straight 4.8 mm nozzle

38cm, 10s ...................................................................... 3-17

BT-7 glass bead mass flow in main room, 1 1.1 mm Venturi noule,

15 S. . . . . . . . . . . . . . . . . . , . .. . . . . . . . . . . . . , . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 3-20

BT-7 glas bead stream power in main room, 1 1.1 mm Venturi noule,

15 S. ...................................................................... + ..... . 3-20

BT-4 and BT-7 glass bead mass flow in portable cabinet, 1 1.1 mm

Venturi nozzle, 1 5 S. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-2 1

Mass flow of BT-9 glass bead, Chr-IO and Chr-20 stainless steel shot

in portable cabinet, 1 1.1 mm Venturi noule, 15 S. . . . . . . . . . . . . . . . . . . . . . .. 3-22

Stream power of BT-4, BT-7, BT-9 glas bead and Chr-1 O stainless

steel shot in portable cabinet, 1 1.1 mm Venturi noule, 15 S. . . . . . . . . . .. 3-23

MC4 plastic media mass flow tests at Ritchey Supply Ltd.,

model FS-3648 portable cabinet6.4 mm straight noule, 15 S. . . . . . . . . .. 3-24

Wheat starch mass flow tests at The University of Toronto,

mode1 PCN 4050 cabinet, 6.4 mm straight nozzle, 15 S. . . . . . . . . . . . . . . ... 3-24

Renshape bar profiles &er blasting with BT-7 glass beads in the main

room, 1 1.1 mm Venturi noule, a=90 deg., 483 kPa , 10 S. Offset

distance: a) 25 cm, b) 38 cm. ......... .. .. ...... .... .... ... .. . .. .. . . .. . ... . .. 3-26

Figure 3.1 7

Figure 3.18

Figure 3.19

Figures 3.20

Figure 3.2 1

Figure 3.22

Figure 3.23

Figures 3.24

Figure 4.1

Figure 4.2

Figure 4.3

Figure 4.4

Figure 4.5

Figure 4.6

Figure 4.7

Crater diameter on renshape bars for two offset distances. a=90 deg.,

................................................................... 483 kPa. 10 s 3-29

Crater depth on renshape bars for two distances. a=90 deg., 483 kPa.

.............................................................................. 10 s 3-29

Ratio H I D of renshape bar craters for two distances. a=90 deg .. 483

kPa. 10 s ..................................................................... 3-30

Craters on renshape bars after blasting with BT-7 glass beads in

portable cabinet. 4.8 mm straight nozzle. a=W deg.. 483 kPa. 10 s .... 3-31

Volume of material removed from renshape bars for two offset

distances. a=90 deg.. 483 kPa. 10 s ........................................ 3-32

Almen strip test. 64 cm. 1.0 min exposure. five values of system

pressure. BT-7 glass beads in main room .................................. 3-35

Almen strip test. 64 cm. 1.0 min exposure. three a impingement

..................................... angles. BT-7 glass beads in main room 3-36

Temperature rise of W62 gas turbine blades blasted with BT-7 glass

beads. 1 1.1 mm Venturi nozzle. 345 kPa. 38 cm. a=90 deg ............ 3-39

W50 1 disc root edges. BT-7 glass beads. 1 1.1 mrn Venturi nozzle.

........................................................... 483 kPa. 38 cm. 90 s 4-5

W5O 1 disc root edges. BT-7 glass beads. 1 1.1 mm Venturi nozzle.

414 kPa. 38 cm. 90 s ........................................................... 4-5

...................... Angle a and serration plane designation of disc root 4-8

......................................... Model S-22 microfinish comparator 4-8

Root section of turbine blade . Nozzle normal to root serration plane

....................................................... when P=90° and a=90° 4-10

Steam turbine blade roots. BT-7 glass beads. main room. 1 1.1 mm

................ Venturi nozzle. 483 kPa. 25 cm. 60 s. a=90° and P=90° 4-11

Steam turbine blade roots. BT-7 glass beads. main room. 1 1.1 mm

................ Vennui nozzle. 483 kPa. 38 cm. 60 s. a=90° and P=90° 4-11

Figure 4.8

Figure 4.9

Figure 4.10

Figure 4.1 1

Figure 4.1 2

Figure 4.1 3

Figure 4.14

Figure 4.15

Figure 4.1 6

Figure 4.17

Figure 4.18

Figure 4.19

Stem turbine blade roots, BT-7 glass beads, main room, L 1.1 mm

Venturi nozzle, 483 kPa, 25 cm, 30 s, a=45O and B=45 O... ............. 4-1 1

Steam turbine blade roots, BT-7 glass beads, main room, 1 1.1 mm

Venturi nozzle, 483 kPa, 38 cm, 30 s, a=45'and P =45O ................ 4-1 1

Steam turbine blade roots, BT-4 glass beads, portable cabinet,

4.8 mm straight noule, 483 kPa, 25 cm, 60 s, a=90°, P=90°. ......... 4- 1 3

Stem turbine blade roots, BT-7 glass beads, portable cabinet.

4.8 mm straight nozzle, 483 kPa, 38 cm, 60 s, a=90°. P=90°. .......... 4- 1 3

Stem turbine blade roots, BT-9 glass beads, portable cabinet,

4.8 mm straight nozzle, 483 kPa, 38 cm, 60 s, a=90°, P=90°.. ......... 4- 13

Stem turbine blade roots, Ch-10 glass beads, portable cabinet,

4.8 mm straight n o d e , 483 kPa, 25 cm, 60 s, a=90°, P=90°. .......... 4- 13

Compressor blades of W62 gas turbine, BT-7 glass beads, 1 1.1 mm

Venturi nozzle, 345 kPa, 25 cm, 20 S. ..................................... 4- 1 7

Compressor blades of W62 gas turbine, BT-7 glass beads, 1 1. l mm

Venturi nozzle, 345 kPa, 38 cm, 20 S. ..................................... .4- 1 7

Deposit removal from 4-th stage compressor blades of W9 1 gas

turbine, main room, BT-7 glass beads, 1 1.1 mm Venturi noule,

38 cm offset, 276 kPa and 345 kPa ........................................ .4-2 1

Deposit removal from 4-th stage compressor blades of W91 gas

turbine, main room, BT-7 glass beads, 1 1.1 mm Venturi nozzle,

64 cm offset, 276 kPa and 345 Wa ......................................... 4-22

Deposit removal fiom 7-th stage compressor blades of W 17 1 gas

turbine main room, BT-7 glass beads, 1 1.1 mm Venturi nozzle,

3 8 cm offset, 276 kPa and 345 kPa ......................................... 4-25

Deposit removal h m 70th stage compressor blades of W 171 gas

turbine main room, BT-7 glass beads, 1 1.1 mm Venturi node ,

64 cm offset, 276 kPa and 345 kPa ......................................... 4-26

xii

Figure 4.20

Figure 4.21

Figure 4.22

Figure 4.23

Figure 4.24

Figure 4.25

Figure 4.26

Figure 4.27

Figure 4.28

Figure 4.29

Figure 4.30

Figure 4.3 1

Figure 4.32

Figure 4.33

Deposit removal fiom 7-th stage compressor blades of W 191 gas

turbine main room, BT-7 glass beads, 1 1 .1 mm Venturi noule, 38 cm

offset, 276 kPa and 345 kPa , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-29

Deposit removal from 7-th stage compressor blades of W 19 1 gas

turbine main room, BT-7 glass beads, 1 1 . 1 mm Venturi nozzle, 64 cm

offset, 276 kPa and 345 kPa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-30

Deposits on compressor blade of W62 turbine, main room, BT-7

g las beads 172 Wa, 0.5 S.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-38

Deposits on compressor blade of W62 turbine, main room, BT-7 glass

beads 345 kPa, 0.5 S.. . . . . . . . . . . . . . . . . . . . . . . . . . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4-38

Deposits on compressor blade of W62 turbine, main room, BT-9 glass

beads 172 kPa, 10 S... . . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4-38

Deposits on turbine blade of W62 turbine, main room, BT-7 glass

beads 345 kPa, 10 S.. . . . . . . . . . . . . . . . . . . . . . , . . . . . . . . . . . , . . . , . . . . . . . . . . . . . . . . . . . .. 4-38

Deposits on compressor blade of W62 gas turbine BT-7 glass beads,

main room, 172 kPa, 0.5 S.. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . 4-39

Deposits on compressor blade of W62 gas turbine BT-7 glass beads,

main room, 345 kPa, 0.5 S.. . .. . .. . . , . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-40

Deposits on compressor blade of W62 gas turbine BT-7 glass beads,

main room, 345 kPa, 5 s .............. ................ . .. ......... ............ 4-41

Deposits on compressor blade of W62 gas turbine BT-7 glass beads,

main room, 345 kPa, 10 S.. . . . . . . . . . . . . , . . . . , , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-42

Deposits on turbine blade of W62 gas turbine BT-7 glass beads,

main room, 1 72 kPa, 0.5 S.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-43

Deposits on turbine blade of W62 gas turbine BT-7 glass beads,

main room, 1 72 kPa, 10 S. . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-44

Deposits on turbine blade of W62 gas turbine BT-7 g l a s beads,

main room, 345 kPa, 5 S.. . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-45

Designation of impact region areas.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-47

xiii

Figure 4.34

Figure 4.35

Figure 4.36

Figure 4.37

Figure 4.38

Figure 4.39

Figure 4.40

Figure 4.4 1

Figure 4.42

Figure 4.43

Figure 4.44

Figure 4.45

Figure 4.46

Figure 4.47

Figure 4.48

Deposits on compressor blade of W62 turbine, portable cabinet,

BT-9 glass beads, 1 72 kPa, 0.5 S. ............................................ 4-5 3

Deposits on turbine blade of W62 turbine, portable cabinet,

BT-9 glass beads 345 kPa, 1 O S.. ............................................ 4-53 Deposits on compressor blade of W62 gas turbine BT-4 glass beads,

portable cabinet, 172 kPa, 5 S.. .............................................. 4-54 Deposits on turbine blade of W62 gas turbine BT-4 glass beads,

portable cabinet, 173 kPa, 10 s ............................................... 4-55

Deposits on turbine blade of W62 gas turbine BT-4 glass beads,

portable cabinet, 345 kPa, 0.5 s ............................................... 4-56

Deposits on turbine blade of W62 gas turbine BT-4 glass beads,

portable cabinet, 345 kPa, IO S.. ............................................. 4-57

Deposits on turbine blade of W62 gas turbine BT-7 glas beads.

portable cabinet, 172 kPa, 5 S.. .............................................. 4-58 Deposits on turbine blade of W62 gas turbine BT-7 glas beads,

portable cabinet, 345 kPa, 5 s ................................................ 4-59

Deposits on cornpressor blade of W62 gas turbine BT-9 glass beads.

portable cabinet, 172 kPa, 5 S. ............................................... 4-60

Deposits on turbine blade of W62 gas turbine BT-9 glass beads,

portable cabinet, 345 kPa, 5 S.. ............................................... 4-6 1

Deposits on turbine blade of W62 gas turbine BT-9 glas beads,

............................................... portable cabinet, 345 kPa, 10 s 4-62

Deposits on compressor blade of W62 gas turbine Chr-10 steel shot,

portable cabinet, 172 kPa, 0.5 S.. ........................................... 4-63

Deposits on compressor blade of W62 gas turbine Chr-10 steel shot,

.............................................. portable cabinet, 172 kPa, 5 S.. 4-64

Deposits on turbine blade of W62 gas turbine Chr-10 steel shot,

portable cabinet, 345 kPa, 5 S.. ............................................... 4-65

Deposits on turbine blade of W62 gas turbine Chr-20 steel shot,

............................................. portable cabinet, 1 72 kPa, 1 O S.. 4-66

xiv

Figure 4.49

Figure 4.49

Figure 4.5 1

Figure 4.52

Figure 4.53

Figure 4.54

Figure 4.55

Figure 4.56

Figure 4.57

Figure 4.58

Figure 4.59

Figure 4.60

Figure 4.6 1

Figure 4.62

Figure 4.63

Deposits on turbine blade of W62 gas turbine Chr-20 steel shot,

portable cabinet, 345 kPa, 10 S.. ............................................. 4-67

Deposits, compressor blade, W62 gas turbine, BT-4 glass bead,

........................................................................... 82 mis 4-74

Deposits, compressor blade, W62 gas turbine, BT-4 glassi bead,

93 d s . . .......................................................................... 4-75

Coating, compressor blade, W62 gas turbine, BT-4 glass bead,

82 &S.. ......................................................................... 4-76

Coating, compressor blade, W62 gas turbine, BT-4 glass bead,

93 d s , , .........................*.......,.... 4-77

Deposits, compressor blade, W62 gas turbine, BT-7 glass bead,

93 d~.. ......................................................................... .4-78

Coating, compressor blade, W62 gas turbine, BT-7 glass bead,

93 d s . . ...................................................................... 4-79

Deposits on compressor blade of W62 gas turbine, airgun,

..................................................... BT-4 glass bead, 93 m/s. 4-80

Deposits on turbine blade of W62 gas turbine, airgun,

...................................................... BT-4 glass bead, 93 m/s. 4-80

CAD presentation of 450 pn diarneter BT-4 g l a s bead penetrated

14 pn into the deposits on compressor blade of W62 gas turbine,

.............................................................. airgun, 1 590 kPa. .4-80

Deposits on compressor blade of W62 gas turbine, airgun,

...................................................... BT-7 glass bead, 78 rn/s.. 4-8 1

Deposits on compressor blade of W62 gas turbine, airgun,

..................................................... BT-7 glass bead, 93 mk.. 4-8 1

Deposits on compressor blade of W62 gas turbine, airgun,

......................................... Chr-20 stainiess steel shot, 89 m/s.. 4-8 1

Deposits on compressor blade of W62 gas turbine, airgun,

......................................... Chr-20 stainless steel shot, 93 mis.. 4-8 1

Deposits, turbine blade, W62 gas turbine, BT-4 g l a s bead, 93 mis.. ... 4-82

List of Symbols

Incident contact radius

Nozzle impingement angle

Nozzle impingement angle

Crater diameter

Indentation depth

Crater depth

Separation of two rotating discs

Mass of incident particle

Media mass tlow

Power of media Stream

Dynamic hardness

Incident load

Particle radius

Mark distance frorn the disc center

Separation between two marks

Temperature rise

Average particle velocity

Incident velocity

Disc rotational velocity

xvi

Chapter One

Introduction

1.1 Background

Most interna1 surfaces in industrial turbines are covered with deposits

composed of either atmospheric pollutants or products of combustion. Such

deposits can reduce power output by 5 to 10 percent, according to reports of

ASME (American Society of Mechanical Engineers) International Gas Turbine

Institute, mainly due to raised operating temperatures and fuel consumption.

Therefore, there is a need for an effective way of cleaning turbine parts.

The following criteria may be applied to any potential cleaning

techniques:

- cost of application procedures, maintenance, materials

- environmental regulations

- cost of waste storage, transportation and disposa1

- deposit removal rates

- applicability to a range of turbine components and deposit

compositions

- potential o f substrate damage

- health and safety hazards.

Some conventionai methods of refurbishing involve fluids. typically

chernical solvents, surfactants or deionized water. These liquid cleansers,

however, have several disadvantages. They are ineffective against inorganic

deposits such as salts. Also solvents must be applied in large quantities which

increases disposa1 costs due to environmental regulations. Finally, most of

them are toxic which presents a hazard to working personnel.

The major alternatives to solvents are:

- water blasting

- dry-media blasting.

Of these, dry-media blasting can be considered as the most economical

solution to clean contaminated parts, such as stationary blade assemblies,

turning vanes, disc and shaft surfaces. In this method, deposit removal is

performed with a stream of abrasive material, for example glass beads,

aluminum oxide or steel shot.

Glass beads are used rnost frequently to remove surface contaminants

without appreciably affecting dimensional tolerances, to provide a smooth

finish, and to shot peen. Glass beads are manufactured from lead-free, soda-

1-2

lime-type glass and contain no free silica. This media purity prevents

contamination on soft metals. It is a non-toxic media, and so it diminishes the

hazard to operator health and reduces clean-up time as well as the cost of

disposal. The almost perfectly spherical shape of glass beads can be regarded

as an advantage for the following reasons. First, compared to angular media,

this provides better control over the erosion process. Second, the enlarged

contact area is better at crushing deposits which are often brittle in nature. The

shape also makes it an ideal media for research purposes, since impact sites

have a spherical shape that is readily identified and measured.

Anot her media which deserves special attention in turbine refurbishment

is a recently developed stainless steel shot. The hardness of this media is, in

many cases, comparable to that of the turbine substrate materials. This creates

an opportunity of finding blast conditions that remove deposits while the

substrate surface remains undamaged. However, the hardness of stainless steel

shot increases with the number of blast cycles as it becomes work-hardened.

Commonly, certain turbine surfaces are coated with corrosionlerosion

protective layers such as aluminum and ceramics. Such layers may have

significantly different properties from those of the substrate.

Another critical issue is that sharply cornered and thin sections are more

easily subject to deformation by the blast stream. This problem is complicated

by situations where deposits on the profiled surface are hard to reach by direct

particle bombardment such as the roots of turbine blades and discs.

Most of the present research was performed at the Siemens

Westinghouse Inc. turbine manufacturing and refurbishment facility in

Hamilton, Ontario. In order to have the greatest relevance, the experiments

were conducted on actual gas and steam turbine components. Most turbine

parts could not be taken from this site. Destructive tests could only be done on

the cornpressor and turbine blades of a mode1 W63 combustion turbine since

they were being scrapped.

1.2 Thesis objectives

The main objectives of the present research were to investigate

the process of deposit removal from surfaces of industrial turbines, provide

recornmendations for choosing the best blast media, and establish an optimal

set of blasting parameters to clean contaminated areas with minimum of

substrate damage.

A qualitative analysis of impact sites produced by individual particles of

different media was performed to study the mechanism by which the deposits

were removed from the turbine components. Relations between media particle

parameters as size, shape, density and the particle speed, Stream flow-rate and

pressure were also examined.

1.3 Thesis outline

Chapter 2 describes the turbine components which were used in the

current research. Furthermore, it depicts the techniques of thickness

measurement, adhesion strength assessrnent and EDX (Energy Dispersive

X-ray) analysis of the deposits.

Chapters 3 gives information about the existing main blast room, the

portable cabinet, the nozzles and the procedures of measuring Stream power

and aggressiveness. Test data are tabulated and the influence of media and

blasting parameters are discussed.

Chapters 4 describes the experiments using individual particles and

particle streams and present the experimental results. Data were obtained on

contaminated surfaces of four combustion turbines. The effectiveness of

different types of media is compared and optimal blast parameters are

suggested.

Chapter 5 gives the summary, conclusions and recommendations for

future work.

Chapter Two

Turbine and deposit characterization

2.1 Turbines

The turbines subjected to blast cleaning in the present work may be

classified in two major groups: gas and steam. They also differed in size,

operating pressure and temperature, power rating and service environment.

Structurally, turbines differ in the number o f stages - from one for steam

applications to nineteen compressor and five turbine stages in combustion

turbines.

Components from four combustion turbines, models W62, W 9 1, W 17 1

and W 191, were used in this work. These turbines operate at a turbine inlet

temperature of 790 Co and 4912 - 6000 RPM. Their discs and blades are often

made o f Discalloy.

In addition, components from four steam turbines, models D96. EM20,

M25 and M32. were investigated. They operate at various environmental

conditions. The disc materiais were alloy steels such a s ASTM A294 class 5,

while the blades were made of stainless steels such as AiSI 403.

2.2 Blades and dises

Suitable blades for experiments were available mainly €rom combustion

turbine components. Blade height ranged approximately from 5 cm to 20 cm,

depending on the stage.

Typically, in combustion turbines the blades of the turbine and

compressor are made of quenched and tempered stainless or alloy steels which

have a hardness range of 18-24 Rc. The compreswr blades often contain a

high percentage of Fe and Cr, while the turbine blades are mostly composed of

Ni, Co and Cr.

It is quite usual that compressor blades o f combustion turbines are

covered with an erosion/corrosion protective layer. For example, the

compressor blades of W62 turbine were coated with a layer which has no

specified hardness requirements and is composed of up to 62% aluminum,

38 pm - 62 Fm thick.

The coataminating deposits were mostly on the flat areas of the suction

side of each blade with much less deposited on the pressure side. The curved

leading and trailing edges were often contaminated very lightly or not at all.

In contrast, the deposits covered the discs quite uniformly.

Operating conditions

Every turbine works under unique conditions depending on the

application and its operating environment. The first may be characterized, in

the case of combustion plants, by the kind of fuel which can be oil or gas. The

environment can Vary widely, from a desert to the middle of a lake or sea.

Deposit formation is influenced by these operating conditions. Natural

gas, as a fuel, often increases the percentage of sulfur in the contaminants. A

surrounding desert may raise the level of silicon. If a turbine serves an oil

platform than salt is a contaminant.

Furthermore, hurnidity and daily environmental cycles can be another

factor affecting deposit formation. The presence of turbine air filtration may

play an essential role in contaminant buildup.

The contaminated surfaces of the combustion turbine components

usually looked quitc smooth. In contrast, the deposit appearance on the steam

turbine components varied from polished to highly rough. Thetefore, it was

often impossible to assess such deposit thickness and very difficult to obtain

steam turbine samples. However, some deposits were scraped with a scalpel

from the surfaces of EM20 and M25 steam turbines. The thickness of some of

these flakes exceeded 2 mm.

The coating thickness gage mode1 6000 (DeFelsko Corporation) was

used to non-destructively measure the thickness o f coatings on turbine a n d

compressor siirfaces. The gage first attempts a measurement using the

magnetic principle applicable to the non-magnetic aluminum coating and

ferrous substrate. For the non-conductive coating and non-ferrous substrate i t

switches automaticaily to the eddy current principle.

The location o f the maximum deposit thickness was first established on

a blade. To estimate the mean value of the maximum thickness, similar

locations on three other blades from the same turbine or compressor stage

were measured.

The most contaminated place was found by scanning a surface in the

radial direction. Upon establishing such a spot, three other areas located at

approximately the same radius and evenly displaced approximately 90' around

the disc were examined to get the average of the maximum deposit thickness.

The deposit thickness distribution by stages for W91, W 171, W 191 and

W62 gas turbine blades and discs for both the compressor and turbine stages

are presented in Tables 2.1 - 2.4. It is seen that the compressor stage surfaces

contained contaminants of non-uniform thickness characterized by the

maximum values at the mid-stages. For example, the compressor blades of the

W91 turbine were found to have thickest deposits at the 5th stage (Figure 2.1),

at the 7th stage of W171 turbine (Figure 2.3) , and at the 61h stage of both the

W191 and the W62 turbines (Figures 2.5 and 2.6). Sirnilar peaks. though less

steep, characterized the deposit thickness distribution on the compressor discs.

However, the maximums on the blades and discs did not belong strictly to the

same stage as can be seen in Figures 2.1 and 2.3. After the highest values,

typically, the deposit thickness tended to decrease and reach minimum values

close to the combustor stages. This tendency and the presence of peaks may be

explained by the different conditions at each stage such as temperature.

velocity of gases and pressure. A study of fuel-gas spray conversion into

deposits on simulated combustor surfaces [ l ] showed that the process was

highly influenced by changes in the surface temperatures. Possibly, the higher

velocity of gas Stream at the end of compressor resulted in the decreased

contaminant thickness. Usually, discs had thicker deposits than blades,

perhaps due to the flatness of the disc surface, its lower radial velocities and

the absence of an axial gas flow. These factors might improve the conditions

for deposit formation.

To verify the mcan thickness values, scanning electron rnicroscopy

(SEM) was used t o examine a number of cross-sections of deposit samples

taken from both the comprcssor and turbine discs of W9 1, W 17 1, W 19 1 gas

turbines and compressor stage blades of the W62 gas turbine. The samples

were obtained by applying drops of epoxy glue to the contaminated areas and

then peeling thcm off togethar with the loyer of deposits. However, not evcty

epoxy application resulted in deposit samples being peeled from the substrate.

This illustrated the variance in properties, composition and adhesion strength

of the deposits formed on the components of different turbines.

The results presentcd in Tables 2.1, 2.4 and the three images depicted in

Figures 2.7 - 2.9 demonstrate a good agreement between the deposit height

values obtained with the coating thickness gage, in the Tables, and the epoxy

glue method, in the Figures. For example, for the sth stage compressor disc of

the W91 gas turbine the maximum deposit thickness is 48 Fm, in Table 2.1,

while Figure 2.9 shows a range of 44 Fm - 48 Fm.

An attempt to estimate the adhesion strength of deposit layers was made

by applying adhesive tape to the contaminated surfaces in accordance with the

proced ure s of ASTM D 3359 "Test Methods for Measuring Adhesion by Test Tape" .

The deposits, however, could not be peeled from the blade or disc surfaces.

This was confirmed by thickness gage measurements that did not show any

change in thickness aftet the test tape had been removed. Usually, on ly a few

scattered deposit grains stuck to the tape (Figures 2.10 - 2.13).

Energy dispersive X-ray (EDX) analysis was performed on deposits from

both compressor and turbine blades and discs of W9 1, W 17 1, W 19 1 and W62

gas turbines. The deposit werc scraped with a scalpel and crushed before

examination. The results givcn in Tables 2.5 - 2.9 and Figure 2.14 revealrd a

high perccntage of Si, S. Fe in the compressor deposits which may be

explained by the environmental pollutants in the gas flow. Contaminants of the

turbine stages showed that the percentagc of S i and Fe remained high

indicating that these elements passed through the combustor. However, a

significant increase of Ni and Co was observed in the turbine deposits from

the W 171 gas turbine suggesting that turbine-stage blade erosion took place

since Ni and Co were the base elements of the blade alloy.

The EDX analysis was also conducted on deposits from the first stage

discs of D96, EM20. M25 and M 3 2 stram turbines. The results presented in

Tables 2.10 - 2.13 demonstrated for three out of four turbines the

overwhelming prevalence of Fe, evidently coming from the blade material and

forming oxides. The contaminants from the last D96 s t e m turbine contained a

combination of Fe and typical environmental elements such as Si, Na, S, and

Cl*

The blades of several compressor stages and the first turbine stage of

the W62 gas turbine were available for EDX analysis. The results presented by

Tables 2.8 and Figure 2.14 show a sharp drop of Fe and an increase in the Al

and S percentages as the compressor stage increased. Evidently, the Al came

from the protective layer discussed in Section 2.2. Apparently, the

temperature, pressure and gas flow conditions facilitated the creation of

oxides containing mostly Fe at the first stages and the formation of

contaminants with S at the last stages. The turbine blades of the W62 gas

turbine revealed elevated percentages of Ni and Co which were the main

elements of the blade alloy. Besides, the S and Si from the environment as

well as Al and Fe from the compressor stage blades contributed the most to the

formation of dcposits on the turbine stage blades.

Tables 2-10 - 2-13 give the results of EDX analysis of deposits €rom the

1'' stage disc of steam turbines. It is seen that Fe was the dominant element.

Its level was close or exceeded 90% in the deposit sarnples taken from the

three examined turbines. Apparently, the oxides of Fe were formed on the disc

surface in the hot-wet air inside the turbine. The other deposit material was

mostly the environmental elements such as S, Si, and Cl.

2.5 Summary

The maximum thickness values of the deposits on combustion turbine

parts tested ranged as follows:

- 10 pm - 33 pm on the cornpressor blades

- 19 pm - 48 pm on the comprcssor discs

- 18 pm - 3 1 pm on the turbine blades

- 17 pm - 26 pm on the turbine discs.

This suggests that the components of gas turbines had an essentially uniform

distribution of deposit thickness.

Table 2.1 Maximum thickness of deposits on W9 1 gas turbine blades and discs. Average values of four point mcasurements, maximum variations I 1.3 pm

Stage 1 Bladc surface 1 Disc surface 1 No. 1 [PI 1 [PI

COMPRESSOR

Stage

Figure 2.1 Maximum thickness of deposits on cornpressor surfaces of W91 gas turbine. Average values of four point measurements, maximum variations * 1.3 pim. Data of Table 2.1

3 4 Stage

Figure 2.2 Maximum thickness of deposits on turbine surfaces of W9l gas turbine. Average values of four point measunments, maximum variations I l .3 )un.

Data of Table 2.1

Tabla 2.2 Maximum thickness of deposits on blades and discs of W 17 1 gas turbine. Average values of four point measurements, maximum variations 11.3 pm

Disc surface

[w]

Stage No.

COMPRESSOR

Blade surface

[ml

Figure 2.3

Stage

Maximum thickness of deposits on compressor surfaces of W 17 1 gas turbine. Average values of four point measurements, maximum variations I1.3 p. Data of Table 2.2

3 Stage

Figure 2.4 Maximum thickness of deposits on turbine surfaces of W 17 1 gas turbine. Average values of four point measurements, maximum variations i 1 . 3 p. Data of Table 2.2

Table 2.3 Maximum thickness of deposits on W 19 1 gas turbine blades and discs. Average values of four point measurements, maximum variations + 1.4 ~IXI

Stage Blade surface Disc surface No. [PI [PI

COMPRESSOR

Figure 2.5 Maximum thickness of deposits on cornpressor surfaces of W 19 1 gas turbine. Average values of four point measurements, maximum variations k1.4 p. Data of Table 2.3

Table 2.4 Maximum thickness of deposits on W62 gas turbine blades and discs. Average values of four point measunments, maximum variations * 1.2 pm

Blade surface 1 Disc surface 1 No. 1 [PI 1 [PI

L I

COMPRESSOR

1 + on blades

1 2 3 4 5 6 7 8 9 10 1 1

Stage

Figure 2.6 Maximum thickness of deposits on compressor surfaces of W62 gas turbine. Average values of four point measurements, maximum variations * 1.2 p. Data of Table 2.4

r 2.7 Cross-section of deposit sample h m 1 O* stage cornpressor bladc of W62 ges turbine. Epoxy glue to the lcft and embedding epoxy to the right of the deposit sample

Figure 2.8 Cross-section of deposit sarnple fiom I 1' stage cornpressor blade of W62 gas turbine. Epoxy glue ta the left and embedding cpoxy to the right of the deposit sampie

Figure 2. .9 Cross-section of dcposit sample h m ah stage cornpressor disc of W9l gas turbine. Epoxy glue to the l e i and embcdding epoxy to the nght of the dcposit sarnple

Figure 2.10 Deposit from EMM32 steam turbine stuck to the test tape

Figure 2.12 Deposit from W91 gas turbine stuck to the test tape

Tables 2.5 EDX analysis of cnished deposits fiom discs of W91 gas turbine

Tables 2.6 EDX analysis of cnished deposits fiom discs of W 17 1 gas turbine

Tables 2.7 EDX analysis of cnished deposits fiom discs of W 19 1 gas turbine

Tables 2.8 EDX analysis of crushed deposits from cornpressor blades of W62 gas turbine

I Cornpressor 2"a stage 1 Element 1 Atomic % 1 Weight %

Compressor 3" stage Element

S Na Al . Zn Si Ca Fe

Cornpressor stage

K 1 1 1

Atomic %

24 3.7 15

0.8 5

2.4 43

Weight %

19 32.8

2.3 5

4.7 4.2 22 3.2 9

Element 1

S S Na Al Ca Cr Fe

Mg Ni *

1

Weight %

19 2

10 1.3

1

3.3 2.3 57

Atomic %

24 25.7 3.7 6.5 4.3

3 14

3.4 5.4

Tables 2.8 EDX analysis of crushed deposits fiom compressor blades of (continued) W62 gas turbine

Compressor 5" stage L

Element 1 Atomic % 1 Weight %

sol

I Cornpressor 7"' stage I I

Element l

S Al Si Ca Fe K

- -

Compressor 8' stage

Atomic %

54 17.5

13 8.9 3.5 1.5

Weight % I

53 ,

14.5 . 1 1 I l 6

1.8

Weight %

5 1 4

19 7

3.6 , 6.5

5

1

ELernent S Cu Al I

Si P Ca Fe

Atomic %

52 2 23

8 3.7 5.3

3

Tables 2.8 EDX analysis of crushed deposits fiom compressor blades of (continued) W62 gas turbine

Tables 2.9 EDX analysis of cnished deposits fiom turbine blade of W62 gas turbine

Cornpressor loh stage r

Element S Cu Al Si

Turbine 1'' stage

Atomic %

48 1

28 7.5

I

Element v

S Al Si Fe Ca L

Ti . Cr

1

Co Ni K

Weight % I

49 2

24 6.6

P Ca Cr Zn Fe Mg K .i

Atomic %

10.6 18 13 12

2.5 1

1.5 1.4

13.4 23.4

1.2

4 5

0.8 0.4 2.5

1 1 ,2

Weight %

9 8.5 10

20.1 2.6 1.5 2.3 2.5

15.6 25.3

1.3

4 6.5 1.3 0.9 4.3 0.7 1.5

L

2 3 4 5 7 8 10

Stage

Figure 2.14 Al, FE, S weight percentage, deposits fkom compressor blades of W62 gas turbine. Data of Table 2.8

Table 2.10 EDX analysis of deposits fiom 1" stage disc of D96 stem turbine

1 Element 1 Atomic % 1 Weight % 1

Table 2.1 1 EDX analysis of deposits from 1" stage disc of EM20 steam turbiné

Table 2.12 EDX analysis of deposits fiom lst stage disc of M25 stem turbine

Table 2.13 EDX anaiysis of deposits fiom lsl stage disc of M32 steam turbine

Element S Si Fe Cs Cr

Atomic %

1.2 0.4 97 o. 1 0.7

Weight %

0.6 I

0.2 98 0.2 0.7

Weight %

0.5 0.4

97 . 1 .S

Eiement L

S Si Fe Cr

Atomic %

1 0.7 97 1.5

Chapter Three

Blast process characterization

3.1 Equipment

Three types of blast systems were employed to test the media for deposit

removal and turbine substrate deformation. First, the main blast room (Blast

Cleaning Corporation Ltd.) where Siemens Westinghouse cleans al1 turbine

components that are being refurbished. The main room employed an 1 1.1 mm

exit diameter Venturi nozzle with sintered carbide liner. A specially designed

swinging shutter was clamped ont0 the nozzle to abruptly redirect the abrasive

Stream and provide precise control of the blast duration. The nozzle and the

shutter parts are depicted in Figure 3 . 1 . The Siemens Westinghouse operator

controlled the impact duration according to signals through the blast room

door window by swinging the frame such that either of the two steel plates

were positioned at an oblique angle to the stream path. By aligning the shutter

supports with the nozzle axis the operator let the abrasives go through the

frame between the two deflecting plates to reach the target.

Figure 3.1 Nozzle and shutter in main blast room. 1- hinge, 2- nozzle, 3- ciamp, 4- support 5- Stream redirecting steel plates

Media particles (usually BT-7 glass beads) flowed from a hopper

through a nonadjustable throttle and were driven by air a t 200 kPa - 480 kPa

to impact a target usicg a stand-off distance of 25 cm - 50 cm, depending on

the type of blast surface. The operator aimed the nozzle, with the shutter in

the redirecting position, at the component being blasted and started the

abrasive flow by pressing a button on the nozzle handle. The tlow could be

siopped by either pressing the button again or by opening the blast room door

equipped with the systern shut-off switch.

The second kind of apparatus used at Siemens Westinghouse Inc. was a

combination of the mode1 EF2448A portable blast cabinet (Empire Abrasive

Equipmcnt ) and the direct pressure pot ni th variable throttle (Lindsay

Company) capable of accommodating up to 45 kg of media (Figure 3.1). The

straight 4.8 mm exit diameter ceramic nozzle was used to accelerate the glass

bead and stainless steel particles in the same 200 kPa - 480 kPa pressure

range.

To propel an individual particle at a target, an airgun [\O] was used

(Figure 3.3). The single projectile was loaded into a cylindrical polyurethane

sabot placed in a steel barrel via a breech. The barrel breach was attached to a

solenoid valve connected to a cornpressed air cylinder. When actuated, the

solenoid valve initiated a burst of compressed air to accelerate the sabot. At

the barrel's end, a urethane ring stopped the sabot, but allowed the particle to

exit. To diminish the effect o f particle deceleration, the target was clarnped

ont0 a specimen holder d o s e to the barrel's end. I t was verified that the

projectile's velocity was independent of its size, density and shape. The chart

of velocity versus pressure provided the required settings in the airgun

experiments.

Figure 3.2 EF2448A model blast cabinet (Empire Abrasive Equipment).

Two additional blast machines were employed to test alternative types

of media. The direct pressure mode1 FS-3648 portable cabinet (Empire

Abrasive Equipment) with the straight 6.4 mm diameter nozzle was used to

test plastic media at Ritchey Supply Ltd. and the model PCN 4050 cabinet

(Clemco Iiidustries), reclaimer and dust collector was utilized to blast the

specimens with wheat starch at the University of Toronto.

Characterization of both these systems was Iirnited to just obtaining the

3-4

media mass flow-rates since the experiments soon showed the ineffectiveness

of the plastic and wheat starch media in removing deposits from the turbine

sragc blades of combustion turbines.

Air I

Cylinder -r\ -- \ --.

. Solenoid

Figure 3.3 Airgun setup [10].

3.2 Media

Blast media is usually classified in three major categories - natural, by-

product and manufactured. The chosen media has to be environmentally

acceptable and economical. The media used during the present experiments are

presented in Table 3.1. In the assessrnent of media effectiveness in removing

deposits o r deforming the substrate, several characteristics may be regarded as

the most influential.

Particle density and size were the parameters determining mass flow and

Stream power that played an important role in material erosion [!SI. The

stainless steel shot was the densest of al1 test media. The plastic particles had

the smallest density. One of the factors influencing the erosion effectiveness

was the particle size. The relations between size and indentation pressure were

studied i n [4.7]. The MC-I plastic had t h e bisgest and the Chr-10 stainless

steel shot had the smallest particles.

Shape of the abrasives should be considered a significant parameter

affecting the nature of deposit and substrate deformation. According to

research i n [9], under normal (perpendicular to surface) impact conditions,

angular particles could initiate deep cracks in the substrate, resulting in

strength reduction. In contrast. spherical particles were considered responsible

for lateral cracks spreading out in the thin subsurface layer and causing the

erosive Wear [9]. This was observed during the experiments with a ceramic

coating. When blasting occurred at oblique impingement angles, angular

projectiles tend to produce a cutting type of deformation rather than the

plowing caused by spheres [4, 51. In the present experiments only the plastic

and wheat starch media were angular.

I t was clear from the tests, that the blast media hardness was one of the

most important parameters in the process of deposit erosion. The experimental

results mentioned in Section 3.1 showed that the plastic and wheat starch

media, which had the lowest hardness, did not produce any removal of the

deposits from the turbine blades of the W62 gas turbine. In contrast, the glass

beads showed the highest aggressiveness.

It is noted that the hardness of stainless steel projectiles increased with

the number of blast cycles due to work-hardening of the austenitic material.

According to the manufacturer (Vulkan Blast Shot Technology), the hardness

of stainless steel shot raises rapidly from 30 Rc io 35 Rc and than u p to 52 Rc

rather slowly as i t was used in successive blast cycles. This increasing trend

was confirmed during the present tests as shown in Table 3.2 and Figure 3.4.

The measurement instrument was a micro-hardness tester (Schimadzu

company) using a Knoop diamond indentor with a 300 g load and a loading

time of 30 S. The Knoop hardness number (KHN) was determined by using the

following formula:

KHN = 14229 Pl(d2) (3 .3 )

where P was the load [g], d was the length of diamond's long diagonal [prn].

The Knoop values were converted into Rockwell C-scale (Rc) numbers. The same

method was employed to determine the glass bead particle hardness (Table 3.1).

Table 3.1 Characteristics of media tested

I --

IStainless steel shot: 4.71 30-48 [Rc] 1 spherical 1 medium (

Material

IMC- 1 plastics 10.72-0.96 1 4.0 [Moh] 1 angular 1 medium 1 1.19- 1.68mn l~nvirostnp wheat starch 1.451 2.8-3.0 [Moh] 1 angular 1 high 1300-600 um

Density

[@cm31

Table 3.2 Hardness of work-hardened Chr- 1 O stainless steel shot as it is reused, portable blast cabinet, 4.8 mm straight noule. 38 cm, 10 S.

Glass beads: 1.36-1.44 BT-4 1

1 Blast 1 ~ardness 1 ~ardness 1 ~ardness 1 ~ardness 1

Hardness [Moh] [Rc]

5.5 [MohIlSO [Rc]

Shape

cycle N

1

spherical medium

Friability

sarnple # l

[Rcl 20

Size

1

sample #2

[Rcl 22

125-177 um

sample #3

[Rcl 25

average

[Rcl 22.3

+ Average

1 2 3 4 5 6 7 8 9 12 15 18 Number of blasts

Figure 3.4 Hardness of work-hardened Chr-1 O stainless steel shot, portable blast cabinet with straight 4.8 mm nozzle, 38 cm distance, 10 S.

3.3 Velocity measuremen t and kinetic energy assessrnent

It has been shown [4, 51 that the particle impact velocity and its kinetic

energy play a significant role in determining the volume of material removed.

Moreover, these are basic properties that characterize the blasting process.

independent o f the particular equipment used, e.g. nozzle, system pressure etc.

In practice, a few techniques such as multiple flash photography, high

speed carneras. laser Doppler velocimetry and the rotating doub l e -d ix method

have been used to measure particle velocity. Each technique has limitations in

terms of measurement accuracy and applicability. Multiple flash photography

and high speed cameras are limited to relatively big particles. Laser Doppler

velocimetry cannot be used with blast strearns having a large number o f

projectiles per unit volume.

For the above reasons, the rotating double-disc setup was used. It

consisted of two circular alurninurn discs fixed with a known spacing, rotated

by a shaft as shown in Figure 3.5. The shaft was driven by 0.19 k W motor

(Prestolite). Rotat ional speed was monitored by tachometer ([RD Mechanalysis

Inc.).

The f i rs t d i s c had a radial slit , the second was coated with an easily

removable layer of high gloss black enamel paint (Armor Coat). When the

discs remained stationary during blasting, an image o f the sl i t was obtained an

the second disc. Once the discs were rotated, the particles passing through the

slit produced a mark shifted from the previous one (Figure 3.6).

Figure 3.5 Double disc setup I - slotted disc, 2 - painted disc, 3 - motor, 4 - nozzle, 5 - motor casing

Figure 3.6 Photograph of double discs

3-1 1

The velocity of a particle, V, is calculated using the following formula:

V = (271 r L O ) / S ( 3 . 1 )

where L is the separation between the two discs, R is the shaft rotational

speed, S is the length of the arc separating the two marks at radius r from the

disc center. This relation cornes from the fact that a particle travels the

distance L in the time L N , while the discs make (L!V)R reuolutions. In

practice, the moving slit produces a mark of finite width Sz - S I .

The average particle stream velocity is then calculated by:

where S I and S2 are closest and furthest distances from the d i t ' s image to the

mark's boundary at radius r, respectively.

The power of particle streams is obtained as follows:

where m' is a media mass tlow.

In the present setup, the discs were 2 mm thick, 450 mm in diameter

srparated by 127 mm. The motor speed was kept constant within 3350 - 3500

revlmin and was measured using a tachometer. The radial slit was 3 mm x 100

mm. Al1 setup dimensions and a 10 second exposure time were chosen as the

most suitable to the stream parameters after multiple preliminary experiments.

The following are some limitations of the method:

- mark boundaries were not absolutely clear,

- no information could be provided about projectile velocity distribution

inside the stream,

- rotating discs created air disturbance to the particle flow thus

producing a systematic error in the velocity assessment.

The estimated error of the velocity measurement by the double-disc

method was approximately 1004 [16].

For each particular blast condition, at least three double-disc blasts

were performed to obtain an average velocity. The maximum difference

between values obtained by using Eq. (3.2) did not exceed 8 % for a particular

set o f blasting parameters.

The main room velocity, mass flow and stream power, Eq. (3 .3 ) , data

are given in Tables 3.3 and 3.6 and Figures 3.3, 3.9 and 3.10. The results of

the portable cabinet tests are presented in Tables 3.4 and 3.5 and Figures 3.8

and 3.1 1 - 3.13. The data showed that the nozzle used in the main room

(Section 3.1) accelerated the BT-7 glass beads to higher velocities while

producing a bigger mass flow than those in the portable cabinet. This resulted

in much higher values of BT-7 glass bead stream power in the main blast

room.

Cornparisons of al1 media tested in the portable cabinet demonstrated

that the BT-7 glass beads gained the highest values of velocity and stream

power, though both parameters were quite similar to those of the BT-9 glass

beads and Chr-10 stainless steel media. The BT-4 glass beads, the largest

beads tested, showed the lowest rates of velocity, mass flow and power. It

should be mentioned that the Chr-20 shot was available only for the velocity

test at 483 kPa. At this pressure, its velocity and mass flow were slightly

lower than those of the Chr-10 shot. Tables 3.4 and 3.5 respectively.

Two media, the M C 4 plastic and the Envirostrip wheat starch described

in Section 3.2 were tested using different blast cabinets (Section 3.1). The

resulis givan in Tables 3.7 and 3.8 sugpest that the plastic tended to produce

lower mass flow-rates than the wheat starch.

Table 3.3 BT-7 glass bead average velocity measured by double- disc method, main room, 1 1.1 mm Venturi nozzle, 38 cm stand-off, 10 S. Mass flow-rates are in Table 3.6

Pressure [ k W

Shaft speed h m 1

Velociîy

[m/sl Average velocity

[m/sl

276 345

Pressure [kPa]

Figure 3.7 BT-7 glass bead average velocity measured by double-disc method, main room with 1 1.1 mm Ventun nozzle, 38 cm, 10 S. Data of Table 3.3.

Tables 3.4 Average velocity measured by double-disc method, portable cabinet with straight 4.8 mm node. distance 38 cm, 10 s a) BT-4 glas bead, b) BT-7 glass bead, c) BT-9 glas bead, d) Chr-IO stainless steel shot, e) Ch-20 stainless steel shot. Mass flow-rates are in Table 3.5

Pressure [ k W

207

Shaft speed h m 1

3400

Velocity WsI

98

Average velocity [m/sl I

99

345

Pressure [Wa]

Figure 3.8 Glass bead and Chr-1 O stainless steel shot average velocity measured by double disc method, portable cabinet with straight 4.8 mm nozzie, 38 cm, 10 S. Data of Table 3.4.

Tables 3.5 Media mass flow in portable cabinet, straight 4.8 mm nozzie, 38 cm, 15 S.

a) BT-4 glass bead, b) BT-7 glas bead, c ) BT-9 glass bead d) Chr- 1 O stainless steel shot, e) Chr-20 stainless steel shot

1 Pressure 1 Media velocie 1 Mass flow 1 Stream power 1

Table 3.6 BT-7 g las bead mass flow in main room, 1 1.1 mm Venturi nozzle, 15 s,

Table 3.7 M C 4 plastics mass flow tests at Ritchey Supply Ltd., model FS-3648 portable cabinet (Empire Abrasive Equiprnent), 6.4 mm straight noule, 15 S.

-~ressure

[kpal 207 345 483

Table 3.8

Media velocity [&SI

131 228 275

Pressure WaI

207 1

345 483

Wheat starch mass flow tests at The University of Toronto, model PCN 4050 cabinet (Clernco Industries), 6.4 mm straight nozzie, 15 S.

Mass flow [ k m

65 91.1

127.2

Mass flow [km1

328 515 635

Stream 6wer

[wl 782

3718 6670

Pressure r

1 03.5

Mass flow P%hI

244.8

345

Pressure [kPa]

Figure 3.9 BT-7 glass bead mass flow in main room, 1 1.1 mm Venturi nozzle, 15 S.

345

Pressure [Wa]

Figure 3.1 0 BT-7 g las bead strearn power in main room, 1 1 .1 mm Venturi nozzfe, 15 S.

345

Pressure [kPa]

Figure 3.1 1 BT-4 and EST-7 glass bead mass flow in portable cabinet, 1 1.1 mm Venturi nozzle, 15 S.

345

Pressure [kPa]

Figure 3.12 Mass flow of BT-9 glass bead, Chr-10 and Chr-20 stainless steel shot in portable cabinet, 1 1.1 mm Venturi nozzle, L 5 S.

345

Pressure [kPa]

Figure 3.13 Stream power of BT-4, BT-7, BT-9 glass bead and Chr-1 O stainless steel shot in portable cabinet, 1 1 . 1 mm Venturi nozzle, 15 S.

345 Pressure [kPa]

Figure 3.14 MC-1 plastic media mass flow tests at Ritchey Supply L id . , model FS-3648 portable cabinet (Empire Abrasive Equipment), 6.4 mm straight nozzle, 15 S.

1 03.5 Pressure [kPa] 207

Figure 3.15 Wheat starch mass flow tests at The University of Toronto, model PCN 4050 cabinet (Clemco Industries), 6.4 mm straight noule, 15 S.

3.4 Stream aggressiveness via rensbape profiles

Tests to compare the aggressiveness of different media on renshape bars

[6] 50 mm high x 25 mm wide x 400 mm long were conducted under similar

conditions in both the main room and the portable cabinet.

The renshape is an isotropic polyurcthane material used to reflect the

magnitude and distribution of the erosion potential of the impacting flow. The

material is eroded leaving a crater of size proportional to the cutting action of

the stream.

The craters were characterized by a bel1 shape with the bottom center

coinciding with the axis of the impacting stream. Cross-sections through the

crater center were measured and corresponding profiles were reproduced by

using C A D software. Then the CAD profile images were revolved solidified

and their volumes were estimated by the program. The experirnental results

and calculations for the main room are given in Table 3.9 and Figure 3.16.

Table 3.9 BT-7 glass bead aggressiveness on renshape bars in the main room, 1 1 .1 mm Venturi nozzle, a=90 deg., 483 kPa, 1 0 S. Flow-rates are in Table 3.6.

1 Crater diameter. D 1 Crater de~th. H 1 h / d l~roded volumd

1 Offset distance 25 cm 1

1 Offset distance 38 cm 1

Figure 3.16 Renshape bar profiles after blasting with BT-7 glas beads in the main room, 1 1.1 mm Venturi n o d e , a=90 deg., 483 kPa , 10 S. Flow-rates are in Table 3.6. Offset distance: a) 25 cm, b) 38 cm.

The similarity of the crater profile shapes illustrates the uniform

character of stream aggressiveness for different offset distances. The greater

nozzle stand-off resulted in a more dispersed stream thus producing a wider

crater and an increased erosion volume,

Experiments to assess the aggressiveness of various media streams on

renshape bars were continued in the portable cabinet. Again, al1 craters had

the bel1 shape depicted in Figure 3.20. This likeness confirmed the uniformity

of media stream aggressivcness at different offsets. The experimental results

i n the portable cabinet are given in Table 3.10.

It is seen from Figure 3.17 that for al1 test media and two offset

distances the crater diameter had a tendency to decrease as the particle size

decreased. Thus the BT-9 glass beads and Chr-lO stainless steel shot created

craters of the smallest diameters. A similar pattern was obtained for the crater

depth except with the largest BT-4 glass beads that made a crater of the least

depth. It was observed that the BT-7 glass beads produced the deepest craters

for both stand-off distances and removed the biggest volume for a stand-off o f

25 cm while for the 38 cm distance it was almost equal to the volume

produced by BT-4 glass beads. These results suggest that the BT-7 media was

the most aggressive.

An attempt to establish relations between crater parameters was

performed by presenting the ratio of h (crater depth) to d (crater diameter). Its

graph is depicted in Figure 3.19. From the graph it can be inferred that the

ratio was higher for the smaller media (BT-7, BT-9) implying that they were

less dispersed in the Stream causing greater removal in the Crater center.

Tables 3.10 Media aggressiveness on renshape bars in portable blast cabinet with 4.8 mm straight noule, a=90 deg., 453 kPa and 1 O S. Flow-rates are in Table 3 S. Glass beads: a) BT-4, b) BT-7, c) BT-9, stainiess steel shot: d) Chr-20, e) Chr-10

1 Crater diameter, D 1 Crater depth, H 1 h 1 d 1 Eroded volume 1

1 1

Offset distance 38 cm 1

Offset distance 25 cm 451 8.21 18.21 4590

Offset distance 38 cm 601 4.11 6.81 3460

1 Offset distance 25 cm 1

1 Offset distance 3 8 cm 1

Offset distance 25 cm 471 6.21 13.21 2210

1 Offset distance 38 cm 1

1 Offset distance 25 cm 1 401 5.91 14.81 2030 l

Offset distance 25 cm

glas glass glas stainless stainless bead bead bead steel steel

shot shot Media

Figue 3.17 Crater diameter on renshape bars for two offset distances. a=90 deg.. 483 kPa, 10 S. Flow-rates are in Table 3.5.

BT-4 BT-7 BT-9 C hr-20 C hr- 1 O glass glass glass stainiess stainless bead bead bead steel steel

Media shot shot

Figure 3.1 8 Crater depth on renshape bars for two distances, a=90 deg., 483 kPa, 10 S.

Flow-rates are in Table 3.5.

The volume of polyurethane removed from the renshape bars was

another parameter characterizing the media erosion aggressiveness.

Figure 3.21 demonstrates that such volumes were larger for the 25 cm offset

distance then for the 38 cm distance except with BT-4 glass beads.

It should be noted that the erosion volumes were least affected by the

standoff distance for the largest particles of the two media types (BT-4 glass

beads and Chr-20 stainless steel shot). This is opposite to the trend observed

i n the main blasting room (Table 3.9).

It is interesting that the h/d ratios, rather than the erosion volumes,

obtained for the media tested on the renshapes corresponded to the particle

average veiocities (Table 3.4) but did not match the Stream power (Table 3.5).

glass g las g l a s stainless stainless bead bead bead steel steel

shot shot Media

Figure 3.19 Ratio h I d of renshape bar waters for two distances, a=90 deg., 483 kPa, 10 S.

Figures 3.20 Crater~ on renshape bars der blastiiig with BT-7 glas beads in portable cabinet, 4.8 mm straight nozzie, a=90 deg., 483 kPa, 10 S. Flow-raies are in Table 3.5. a) 25 cm offset distance, b) 38 cm offset distance

BT-4 BT-7 BT-9 Ch-30 C hr- 1 O glass glass glass stainless stainless bead bead bead steel steel

shot shot Media

Figure 3.2 1 Volume of material removed from renshape bars for two offset distances, a=90 deg., 483 kPa, 10 S. Flow-rates are in Table 3.5.

The results presented in Table 3 .5 and Figure 3.21 show that an average

particle velocity correlated with a volume of removed renshape material for

the glass beads blasted at 25 cm offset. The BT-7 glass beads were

characterized by the highest velocity and volume removed while the BT-4

glass beads had lowest values of these parameters. The stream velocity, power

and removed volume varied little during tests with Chr-10 and Chr-20

stainless steel shot at this distance.

In contrast, the removed volume of renshape material corresponded to

the media particle size for both the glass beads and stainless steel shot when

blasting was conducted at 38 cm offset, i.e. the larger particles removed most

material regardless of power and velocity.

3.5 Almen strip test

One o r the methods to evaluate the impact energy of a blasting stream

uses standard shot peening Almen Strips. In this method, one face of thin fiai

steel sheets called strips is subject to a stream of media particles and the

resulting radius of curvature developed on the specimens is measured. The

extent of such bowing is dependent upon the degree of compressive stress on

the impact surface and is a measure of the stream energy.

The BT-7 glass beads in the main blast room were tested according to

the ANSIlSAE J443 standard. Almen strips 75 x 18 mm of 0.79 + 0.005 mm

thickness were placed in a strip holder and blasted from 64 cm distance for

1 min each at different impingement angles and system pressures. The heights

of the developed arcs are presented in Table 3.1 1 and Figures 3.22, 3.23.

From the graphs it can be concluded that the Stream energy tended to

increase with the system pressure, and as the impingement angle a approached

90 deg. The lower arc height at 552 kPa 1 90 deg. blast conditions could be

caused by an error in the experimental setup.

Table 3.1 1 Almen strip test, 64 cm, 1 .O min exposure

1 Pressure 1 Angle, a 1 Arc height I

75

Angle a [deg.]

Figure 3.22 Almen stnp test, 64 cm, 1 .O min exposure, five values of system pressure, BT-7 glas kads in main room.

+ 45 deg.

483

Pressure [kPa]

Figure 3.23 Almen strip test, 64 cm, 1 .O min exposure, three a impingement angles, BT-7 glas beads in main room.

3.6 Material temperature rise during blasting

It was observed that. during blasting experiments. the small, thin

compressor blades became noticeably hotter after relatively short exposure

periods. A similar phenornena occurred in the large, heavy compressor and

turbine s tage biades subject to extended blasting. It was, therefore, of interest

io measure the temperature rise to assess any possible effects on the deposit

and blade substrate properties.

A UX-40 Ultimax Infrared Thermometer with a range 50 to 1000 Co,

(Ircon Inc.), was used to measure the temperature rise o f the turbine blades

blasted a t a=90°. The t ime necessary to enter the main room after blasting,

aim and focus the thermometer was about 4 S.

The highest temperature rise, 6 2 S 0 C, was obtained when the relatively

ihin and small 71h stage compressor blade of the W62 gas turbine was blasted

at 38 cm offset distance within 60 S. The results o f these measurcments,

presented in Table 3.12 and Figure 3.24, suggest that the temperature increase

was not big enough to influence the properties of the deposits and affect the

mechanisms of deposit removal.

Tables 3.12 Temperature rise of blades blasted with BT-7 glass beads, 1 1 . 1 mm Venturi nozzle, 345 kPa, 38 cm, a=90 deg.. ambient temperature 28 O C , measurement delay 4 S.

W62 gas turbine: a) compressor and 9' stages, b) turbine 1" stage

a) r Exposure time [SI

10 30 60

Temperature nse

A T, [ O C 1 11.5

21 62.5

30 Exposure time, [s]

Figures 3.24

30 Exposure time, [s]

Temperature rise of W62 gas turbine blades blasted with BT-7 glass beads, 1 1 . 1 mm Venturi nozzle, 345 kPa, 38 cm, a=90 deg. a) cornpressor 7" stage, b) turbine 1" stage

Chapter Four

Results

The experimental results were grouped by the utilized equipment in

Table 4.1 and then designaied by objective, media and test specimens.

Table 4.1. List of perfonned experiments. SD - substrate deformation. DR - deposit removal.

Main Room (MR)

Exp. 1 Objective 1 Media 1 Specimens

MR3 1 SD, root serration blasting, dental casts 1 BT-7 glass beads 1 W 19 1 Ras turbine disc

MRI MR2

, SD, flat area blasted normally BT-7 plass beads SD. root edges blasted fiom side BT-7 glass beads

MR4 MR5

W501 Ras turbine disc W50 1 gas turbine disc

MR6

SD, root semtion blasting, optimum criteria SD, root serration normal repetitive blasting

MR7

for short exposures SD, cornpressor last stages, tests on geometry deformation

MR8 MR9

BT-7 glas beads BT-7 glass beads

DR, cornpressor stages, normal and oblique blasting to d e t e d e erosion nature

Steam turbine blades W62 gas turbine blades

1 fiom turbine stage

DR, cornpressor stages, erosion mechanism DR, nubine stage, erosion mechanism

BT-7 glass beads -

BT-7 glas beads

W62 gas turbine blades

W9 1, W 17 1, W 19 1 gas turbine blades

BT-7 glass beads BT-7 pSass beads

W62 p s turbine blades W62 gas turbine blades

Table 4.1. List of performed experiments. (continued) SD - substrate deformation. DR - deposit removd, Expt. - experiments.

Portable Cabinet (PC)

Cas Gun (GG)

Exp. PCl

PC2

PC3

4.1 Substrate deformation

Objective SD, root serration blasting, optimum criteria and media cornparison DR, cornpressor stages, erosion mec hanism DR, turbine stage, erosion mec hanism

Exp. GGl

GG2

GG3

Disc flat surface

The experiments, MRl in Table 4.1, started with a flat area of the WSOl

gas turbine disc blasted with the BT-7 glas beads in the main room at

irnpingement angle a=90° and two pressures: 345 kPa and 483 kPa. Two offset

distances, 38 cm and 51 cm, were used. Blast stream exposure varied from 2 s

to 150 S. The main purpose was to estimate the effect of distance on non-

destructive blasting and, then, to assess the exposure at which the surface

started to deform. 483 kPa was chosen as an upper pressure limit based on the

Media BT-4, BT-7, BT-9 glass beads and Chr- 1 O stainless steel shot BT-4, BT-7, BT-9 glas beads and Chr-1 O, Ch-20 stainiess steel shot BT-4. BT-7, BT-9 glass beads and Chr- 1 O, C hr-20 stainless steel shot

Objective DR, deposits on cornpressor stage blades, erosion mechanism DR, deposits on turbine stage blades, erosion mechanism SD, aluminwn layer of compressor

- stage bblades, erosion mechanism

Specimens Stem turbine blades W62 gas turbine blades W62 gas turbine blades

Media BT-4, BT-7, BT-9 glass beads and Chr- 1 O, Ch-20 stainless steel shot BT-4, BT-7, BT-9 glas beads and Chr- 1 O, Ch-20 stainless steel shot BT-4, BT-7. BT-9 glass beads and Chr-10, Ch-20 stainless steel shot

Specimens W62 gas turbine W62 gas turbine W62 gas turbine

blast cleaning experience at Siemens Westinghouse Inc. The impact sites were

inspected with Leica optical microscope at 40X overall magnification. Visual

assessment was made using a mode1 S-22 microfinish comparator (Gar

Electroforming Div.), Figure 4.4.

Tables 4.2 Repetitive blasting at flat surface of W501 turbine disc with BT-7 glas beads, main room, 1 1.1 mm Venturi nozzie, a=90° (Expt. MRI). a) 483 kPa, 51 cm; b) 483 kPa, 38 cm C) 345 kPa, 51 cm; d) 345 kPa, 38 cm

1 Exposure time 1 Comments on deformation

1 2 1 no deformation

301 no de formation 1

I 601 enlarged crater on video image 1

d) 1 i 201 no deformation

C)

# 1

I 1 sol I

60 90 90 90

no deformation

The observations presented in Tabie 4.2 suggest that the disc flat

surface could stand up to repetitive normal impacts of 90 s duration at both 38

c m and 5 1 cm offsets, 345 kPa and 483 kPa pressures without significant

erosion and visible deformation. Based on this, futther tests were conducted at

an offset distance equal or less than 38 cm and exposures not exceeding 90 S .

Disc root edges

The tests were followed by a normal blasting at the root sides of W5Ol

gas turbine disc (the Stream was parailel to the root serration P=Oo) . The

definition of terms is given in Figure 4.5. These experiments are designated as

MR2 in Table 4.1. The goal was to assess the erosion resistance of a profiled

impact surface. In contrast to the disc flat areas, the disc root edges were

significantly deformed, these surfaces became corrugated at the 276 kPa low

pressure and both 25 cm 1 60 s and 38 cm 190 s condition sets. The largest

deformation occurred at 25 cm offset distance (Table 4.3, Figures 4.1 and 4.2).

From these results i t was inferred that the disc material exhibited a ductile

type of erosion, since the peak deformation occurred at impact angles

significantly lower than 90" (91. The results also suggested conducting further

experiments at 25 cm - 38 cm offset and less than 60 s exposure.

Table 4.3 W501 disc root edges blasted with BT-7 glas beads in main room, 1 1 . 1 mm Venturi nozzle, a=90° (Fig. 4.3), (Expt. MM). a) 25 cm, 60 s; b) 38 cm, 90 s

a) 1 Pressure 1 Comments on deformation 1

1 2071 little de formation 1

[kPal 172

1 2761 defonnation 1

no defomation

345 4 14

1 2761 deformation 1

deformation and corrugaiion high deformation and comgation

b)

I 345 1 deformation and little comgation 1 4 1 4 comgation and high deformation 483 high deformation and comgation

L

Figure 4.1 WSO 1 disc mot edges, BT-7 glas beads, 1 1.1 mm Venturi

nozzle, 483 kPa, 38 cm, 90 s (Expt. MR2).

172 207

Figure 4.2 W501 disc mot edges, BT-7 glas beads, 1 1.1 mm Venturi

nople, 414 Wa, 38 cm, 90 s (Expt. MR2).

no deformation no defonnation

Disc root serration

The serration is a toothed profile on the disc and blade roots that

provides their interlock during an assembly. This set of tests was intended to

study the dependence of substrate erosion on the a and P impingement angles.

The definition of terms and angles is given in Figures 4.3 and 4.5. The

experiments are designated as MR3 in Table 4.1. Disc roots of the W191 gas

turbine were blasted with the BT-7 glass beads. First, the Stream was directed

normally (a=90° and P=90°) at the serration followed by changing the P angle

(between the nozzle and the root serration plane) to 45'. Second, the impact

became oblique with the a and P angles equal to 45'. Dental casts taken from

the blasted roots were compared to that of the non-blasted root by using an

optical comparator (Bausch & Lomb) at IOX overall magnification, Table 4.4.

Only the roots blasted at a=45O and P=4S0 conditions were eroded

beyond the toierance limits. This can be considered as another confirmation of

substrate ductile erosion characteristics. Though the tooth sides of the root

serration experienced oblique blasting when the P angle was 90°, no

significant deformation of the impact surface was observed. As an

explanation, it can be mentioned that during oblique impact conditions

(P=4S0) the particles were able to leave the root channels freely after

rebounding from the root surface. In contrast, under the normal (P=90°) blast

conditiocs a "cloud" of debris and rebounding glass beads was created. This

cloud had to be penetrated by the incoming particles. Such phenornena and the

probability of collision between incident and rebound projectiles was

4-6

investigated in study [17]. The collisions, evidently, caused the lower erosion

aggressiveness. Yet, the glass bead direction change due to multiple collisions

may be considered a fûcilitating factor for the particles to reach the parts of

blade and disc roots hidden from direct impact.

Table 4.4 Inspection with 1OX magnification optical comparator of dental casts fiom W 19 1 disc roots blasted with BT-7 g l a s beads in main roorn, 11.1 mm Venturi nozzle, 483 kPa, angle a meaning is given in Figure 5.3 (Expt. MR.3). a) nozzle axis at P=90° angle to root serration plane b) noule axis at 0 4 5 ' angle to root serration plane

Angle a [deg*l

45 45 45 45 90 90 90 38 60 90 38 90

Offset distance

km1 25 25 38 38 25 25

Exposure time

[SI 30 60 60 90 30 60

Tolerance excess [mm1

not found

4

Nozzle and stream direction

4. Q

Root serrat ion C ' plane

Figure 4.3 Angle a and serration plane designation of disc root.

Figure 4.4 Mode1 S-22 microfinish comparator (Gar Electrofirming Div.).

4-8

Blade root serrat ion

Blast experiments were performed on steam blade roots at 483 kPa

pressure and various impingement angles to verify the conditions of

nondestructive blasting in the main room, MR4 i n Table 4.1. A second

objective was to compare the aggressiveness of various media in the portable

cabinet. PCI in Table 4.1. when the nozzle was positioned on ly in a plane

normal to the plane of root serration, Figure 4.5. The impact sites were

visually assessed using the mode1 S-22 microfinish comparator.

In contrast to blasting at the disc roots, blade root erosion was observed

after impacting in the main room at a=90° and both oblique P=4S0 and normal

P=90° angles, Figures 4.6 - 4.7 and Figures 5.8 - 5.9 correspondingly. The P

angle is defined in Figure 4.5. Apparently, the "protective" cloud of rebound

particles, formed in the case of disc root blasting, was not created during

normal blasting at the blade roots. The deformation occurred i n the form of

profiled surface corrugation as in Expt. MR2. The results presented in Table

4.5 indicate that non-destructive blast conditions may be achieved in the main

room provided that the impacting is normal, the exposure lime is equal or less

than 30 s and the stand-off distance is no less than 38 cm,

Cornparison of normal blasting with various media in the portable

cabinet (Expt. PCl) , Table 4.6, showed that the BT-7 glass beads had the

highest aggressiveness, Figure 4.1 1. The BT-4 glass beads were quite

aggressive, producing a rough surface, Figure 4.10. At the sarne time, the BT-

9 glass beads had little effect and left blast surface smooth, Figure 4.12,

Root serration plane

Root side - -- -III- -- ̂ _ - ----a t3

Nonle at oblique angle to root serntion

to root serration d

Figure 4.5 Root section of turbine blade. N o d e nomd to root serration plane when p=90° and a=90°.

Table 4.5 BIasting at blade roots of stem turbine in main room, BT-7 glass beads, 1 1 .1 mm Venturi nozzie, 483 kPa (Expt. MR4). a) a=4S0, b) P=90°.

[offset distance I~xmsure time 1 Comments on deformation 1

1 25 1 30lhigh comgation 1

25 25 38 38

30 60 30 60

25 38

high comgation and edge deformation severe corrugation, high edge defocmation comgatioa and little edge deformation high comgation and edge deformation

60 30

severe comgation almost no deformation

Table 4.6 Blasting at blade roots of s t e m turbine in portable cabinet, 4.8 mm straight nozzle, 483 kPa, a=90°, P=90°, (Expt. PC1). a) BT-4 glass beads, b) BT-7 glass beads. c) BT-9 glass beads, d) C b l O stainless steel shot

)offset distance ~EXDOSW time 1 Comments on deformation

25 25 38 38

1 3 8 1 30lminor edge deformation I I 3 8 ( 6Olcomgation and edge de formation I

30 60 30 60

comgation and edge deformation high cormgation and edge deformation

25 25

minor edge deformation. rough surface edge deformation, rough surface

1

no damage, rough surface little edge deformation, very rough surface

:

30 60

1 381 301110 damage 1

25 25

381 601110 damage, veiy smooth surface I

30 60

381 601no damage, very smooth surface I

minor comgation and edge deformation little corrugation and edge deformation

no damage no damage no damage, very smooth surface

25 25 38

30 60 30

and the Chr-10 stainless steel shot was not capable of deforming the blade

roots a t ail, Figure 4.13. It should be mentioned that the 30 s exposure at 38

cm offset was the safest blast condition during experiments in the portable

cabinet.

A cornparison of aggressiveness at 38 cm offset (Table 4.6) with

velocity and power (Table 3.5) shows that the BT-7 glass beads had the

highest values of all three characteristics. However, the BT-9 glass beads

produced the least substrate deformation though their velocity and power were

close to those of the BT-7 glass beads. This suggests that the size of the media

is an important factor; i.e. smaller diameter media are less aggressive on the

substrate.

Blade root. short exposure, retieated blasting

Tests were continued with short duration, repeated blasting of the

turbine blade roots of the W62 gas turbine at 483 kPa pressure, MR5 in Table

4.1. Since the previous experiments revealed poor erosion resistance of gas

and steam turbine substrates and profiles to oblique impacts, the blade grooves

were blasted with the nozzle positioned perpendiculariy to the root serration

plane (a=P=90°). A substantial time was given for the target surface to cool

down between subsequent impacts.

The results presented in Table 4.7 showed that no deformation occurred

to the root material and profile when the number of blasts reached 6 for 25 cm

15 s and 38 cm110 s blast conditions. This is in contrast to results of blasting

at the steam blade roots, Expt. MF24 and P C l , when 25 cm130 s and 38 cm160 s

conditions resulted in substrate deformation. This means that substrate cooling

may have improved erosion resistance.

Tables 4.7 Repetitive blasting at turbine blade roots of W62 gas turbine in main room. BT-7 glass beads, 1 1.1 mm Venturi nozzle, 483 kPa, a=9O0, P =9 0". (Expt. MR5). Offset distance a) 25 cm, b) 38 cm

1 Ex~osure timel Number 1 Comments on deformation 1 oftimes 1

1 21 3Ino change

no change no change

-- - - . - -

5 5

10

1 201 3 lmaterial eroded, profile changed

--

3 6

10

1 201 6 lmaterial substantiall y eroded, profile changed

3 start of material erosion 6 material eroded and profile started to change

b)

5 5

201 3 1 start of material erosion

2 2

10 10

201 6lmaterial eroded and profile started to change

3 6

3 6

no change no change

3 6

no change no change

no change no change

Thin blade ~ r o f i l e deformation

I t was observed that the edges o f some blades were curved by the BT-7

glass bead streams in the main room. Therefore, tests on thin compressor

blades of the W62 gas turbine were performed to assess the conditions when

distortion of the blade geometry might occur, MR6 in Table 4.1. Results given

in Table 4.8 suggest that the Stream exposure time was the major factor

affecting the blade profile deformation as pressure and offset variations had

less effect. I t may be recommended that the blast tirne should be limited to

approximately 3 s for normal blasting at the 345 kPa - 483 kPa pressure and

25 cm - 38 cm offset ranges. Examples of deformed blades are given in

Figures 4.14 and 4.1 5.

Table 4.8 Edge of thin compressor blades fiom 8" and 9' stages of W62 gas turbine, BT-7 glas beads, main room, 1 1.1 mm Venturi novle (Expt. MR6).

pressure

[@al 345 345 345 345 276 276 276 276

Exposure

[SI 20 20 10 3

20 20 t O 5

0ff-set

[cm1 25 38 38 38 25 38 38 38

Comments

high deformation ,

high deformation ,

moderate de formation no deformation high deformation hi& defomation minor deformation no deformation

Figure 4.14 Cornpressor blades of W62 gas Figure 4.15 Cornpressor blades of W62 gas turbhe, BT-7 glass beads, 1 1 .1 turbine, BT-7 glas beads, 1 1.1 mm Venturi nozzle, 345 kPa, mm Venturi noule, 345 kPa, 25 cm, 20 s (Expt. MR6). 38 cm, 20 s (Expt. MR6).

4.2 Deposit removal

Normal and obliaue blastinn at cornmessor stage blades in main room

Tests of oblique (a<90°) and normal (a=90°) blasting with the BT-7

glass beads, MR7 in Table 4.1, were performed on blades of the W 9 1 , W17 1

and W 19 1 gas turbines in the main room to examine the deposit erosion

characteristics. In these experirnents p was always 90°. The impact sites were

assessed using the optical microscope and the thickness probe. The probe was

used to determine erosion rates. During the tests, each blade was subjected to

the streams for 0.5 s, 2 s and 5 s, consecutively. Examination and

measurements were conducted between blasts. Timing was achieved by using

the shutter described in Section 3.1. The blasts were performed and

measurements were taken at the identical spots on each blade.

The results obtained for blasting at various impingement angles. 38. 64

cm off-sets and 276, 345 kPa pressures are given in Tables 4.9 - 4.11 and

Figures 4.16 - 4.21. The deposit removal rates were highest at a=90°. This,

according to ref. (181, indicates the deposit Wear had a brittle nature. It is seen

from the figures that the difference in deposit thickness removed at normal

and oblique impacting increased with the blast duration for every type of

blade.

The data of cumulative thickness of deposits eroded, Figures 4.16, 4.18

and 4.20 suggest that the 38 cm off-set and 345 kPa pressure provided the

conditions of an almost complete removal while impacts at the sarne distance

and 276 kPa pressure were less effective. This was especially characteristic

for the 90" irnpingement angle condition. For example, the thickness of

deposits left on the blade surface normal blasts ranged from 0.2 pm to 1.2 Fm.

The tests at the 64 cm off-set showed a very poor removal effectiveness with

the BT-7 glass bead stream, Figures 4.17, 4.19 and 4.21. To compare the

deposit erosion resistance by the turbine type, the ratio of deposit initial

thickness to that measured after three consecutive impacts was introduced,

Tables 4.9,4.10 and Table 4.1 1. This ratio was the highest for the blades of the

W 191 gas turbine (low erosion resistance) and the lowest values for the blades

of the W91 gas turbine (high erosion resistance).

Table 4.9

a)

Deposit removal fiom 40th stage cornpressor blades of W9l gas turbine, main room, BT-7 glass beads, 1 1 .1 mm Venturi nozzle, i= 1 ,.. .,4 - blast nurnber (Expt MR7). a) offset distance 38 cm. b) offset distance 64 cm

Sequence [il

1

Pressure [WaI

276

Angle a Wg.1

75

Exposure, T [SI

O

Deposit thickness, H [VI

27

H 1 -H4 / H i -

Table 4.9 Deposit removal from 40th stage cornpressor blades of W9 1 gas turbine, (continued) main room, BT-7 glass beads, 1 1 .1 mm Venturi nozzle, i= 1 ,..., 4 - blast nurnber.

(Expt MR7). a) offset distance 38 cm, b) offset distance 64 cm

- - -

scquence [pressure /Angle a (~xposure. T 1 Deposit thickness, H ( H 1 -H4 / H4 1

Exposure time [s]

O 0.5 2

Exposure time [s]

Figure 4.16 Deposit removal from 4-th stage cornpressor blades of W91 gas turbine main room, BT-7 glas beads, 1 1 . 1 mm Venturi nozzie, 38 cm offset, 276 kPa (above) and 345 kPa (below), (Expt. MR7).

- + 90 deg.

O 0.5 2

Exposure time [s]

O 0.5 2

Exposure time [s]

Figure 4.17 Deposit removal fiom 40th stage compressor blades of W9 1 gas turbine main room, BT-7 glass beads, 1 1 . 1 mm Venturi nozzle, 64 cm offset, 276 kPa (above) and 345 kPa (below), (Expt. MR7).

Table 4.10 Deposit removal fiom 7-th stage cornpressor blades of W 17 1 gas turbine, main room, BT-7 glass beads. 1 1.1 mm Venturi nozzle, i= 1 ,. ..,4 - blast number. (Expt MR7). a) offset distance 38 cm, b) offset distance 64 cm

Sequence Pressure Angle a Exposure, T Deposit thickness, H H 1 -H4 / H4 [il [kPal [deg-1 [SI Ccrml

Table 4.10 Deposit removal from 7-th stage compressor blades of W 17 1 gas turbine, (continued) main room. BT-7 glass beads. 1 1 - 1 mm Venturi nozzle, i=1, ..., 4 - blast number.

(Expt MR7). a) offset distance 38 cm, b) offset distance 64 cm

b) I Sequence Pressure Angle a Exposure, T Deposit thickness, H H l -H4 / H4

[il [mal [de&] [SI [ p l 1

1 276 60 O 32 I

2 0.5 3 1 3 2 25 4 5 19 0.7

0.5 2

Exposure time [s]

Exposure time [s]

Figure 4.1 8 Deposit removai fiom 70th stage compressor blades of W171 gas turbine main room, BT-7 glass beads, 1 1 .1 mm Venturi nozzie, 38 cm offset, 276 kPa (above) and 345 kPa (below), (Expt. MR7).

Exposure time [s]

0.5 2

Exposure iime [s]

Figure 4.19 Deposit removal fiom 70th stage compressor blades of W 171 gas turbine main room, BT-7 glass beads, 1 1 .1 mm Venturi node, 64 cm offset, 276 kPa (above) and 345 kPa (below), (Expt MR7).

Table 4.1 1 Deposit removal fiom 70th stage cornpressor blades of W 191 gas turbine, main room, BT-7 glass beads, 1 1.1 mm Venturi noule, i=1, ..., 4 - blast nurnber. (Expt MR7). a) offset distance 38 cm, b) offset distance 64 cm

Sequence [il

1 2 3 4

Deposit thickness, H [PI

20 13

7.8 4.1

H 1 4 4 / H4

3.9

Exposure, T [SI

O 0.5

2 5

'pressure P a l

276

&le a [deg-1

45

Table 4.1 1 Deposit removal from 7-th stage cornpressor blades of W 19 1 gas turbine, (continued) main room, BT-7 glass beads, 1 1 . 1 mm Venturi nozzle, i=1, ..., 4 - blast number.

(Expt MR7). a) offset distance 38 cm, b) offset distance 64 cm

Exposure, T

[SI 1 276 45 O 2 0.5 3 2 4. 5 .

Deposit thickness, H !---,

[wl 20.5

16 13 7.7.

Hl -H4 / H4

1

I

1.7-

0.5 2

Exposure time [s]

Exposure time [s]

Fi g ure 4.2 0 Deposit removal fiom 7-th stage cornpressor blades of W 19 1 gas turbine main room, BT-7 glas beads, I 1.1 mm Venturi nozzle, 38 cm offset, 276 kPa (above) and 345 kPa (below), (Expt. MR7).

Deposit thickness

[PI Deposit thickness

Normal blastine at compressor and turbine stage blades in main room

Assuming that deposit removal was a form of brittle erosion, normal

(90') blasting with the BT-7 glass beads (MR8 in Table 4.1) was performed on

the 81h and 9th stage compressor stage blades of the W62 gas turbines in the

main room at 38 cm distance to obtain deposit removal rates and investigate

the erosion mechanism. The impact sites were examined by utilizing the

WYKO non-contact optical surface profilorneter, the scanning electron

microscope (SEM) and the thickness probe. A masking technique. consisting

of a steel hose tightly clamped around the blades, resulted in a sharp border

between blasted and protected areas. The exposures were 0.5 s, 5 s and 10 s,

each on different blades; Le. every blade experienced impact only once.

Measurernents and observations, Table 4.12, showed that 5 s and 10 s

Stream exposures at 345 kPa pressure were too aggressive on the compressor

blades, as they had a protective aluminum coating that was often removed

together with the deposits. For example, blasting for even 5 s with the BT-4 in

the portable cabinet and with the BT-7 in both the portable cabinet and main

room resulted in compete removal of the aluminum layer. It was important to

measure value of this layer thickness in order to estimate the deposit

thickness with the probe. This was done by using the WYKO on the sites

where the aluminum coating had been removed, Figures 4.28 and 4.29. Such

sites revealed sharp borders between the blade substrate and protected areas.

Height differences at this junction gave a value of 65 Fm with i 3 prn

maximum deviations based on four measurements performed separately on

each of three blades. The obtained value fell within the 51 Fm - 76 prn range

specified by Siemens Westinghouse Inc. To determine the deposit thickness on

the compressor blades, the 65 pm number was deducted from the combined

thickness of the deposit and protective layer measured with the probe. It was

assumed that the aluminum coating affected the deposit removal results.

Therefore, the compressor stage blade blasting was used mostly for the erosion

mechanism investigation rather than removal quantitative assessment. Figures

4.26, 4.27 (WYKO) and Figures 4.22 - 4.24 demonstrate that the spherical

shape impact craters had a smaller diameter than that of the particles. This

suggests that particles penetrated into the material only partly. Also, it was

observed that deep microcracks characteristic of the deposits vanished after

blasts implying that the deposits were compacted. Compaction was also

supported by a brighter appearance the impact surface after blasting. it is

thought that light reflectivity of the compacted surface increased.

Testing on the turbine blades is presented in Figures 4.30 - 4.32

(WYKO) and Figure 4.25 (SEM). It is seen that the thickness values of the

removed deposits obtained with the probe (Table 4.12) agree with the

differences in the height of the deposited and impact areas on the WYKO

images. It should be noted that blasting produced a much rougher surface

without spherical craters, in contrast to that on compressor blades. The oniy

exception was blasting at 172 kPa for 10 s which resulted in a very smooth

surface with a height difference of about 10 Pm. Cornparison of this value to

the 17 pm estimated average thickness of deposits on turbine blades suggest

that the glass beads were able to compact the surface rather than produce

significant removal.

The SEM analysis was utilized to verify the WYKO and thickness probe

measurements. The SEM results are presented in Table 4.13 (compressor stage

blades) and Table 4.14 (turbine stage blades) where the percentage of elements

contained in the impact sites was used to estimate the depth of penetration. A

spot in the center and on the edge of individual craters and a region containing

several craters were examined. Schematically, the spots and region are defined

in Figure 4.33. The SEM results showed that the BT-7 glass beads penetrated

into the alurninum layer of the compressor stage blades when blasted at 172

kPa for 5 s and IO s since there was a high percentage cf Al on the impact

surface. Blasting at 345 kPa resulted in a raised level of Fe that confirmed

total removal of the protective Al layer. On turbine stage blades, the SEM

analysis demonstrated that the combined percentage of Ni and Co remained at

the same level after blasting with BT-4, BT-7 and BT-9 glass beads.

Table 4.1 2 Deposit removal fiom compressor 8th / 9th stage and turbine 1 st stage blades of W62 gas turbine, 38 cm offset, a=90 deg. a) compressor blades, main room, BT-7 glass beads,

1 1.1 mm Venturi noule (Expt. MR8) b) turbine blades, main room, BT-7 glas beads.

1 1.1 mm Venturi nozzle (Expt. MR9) c) compressor, portable blast cabinet. BT-4 glas beads,

4.8 mm straight nozzle (Expt. PC2) d) turbine blades, portable blast cabinet, BT-4 glas beads,

4.8 mm straight nozzle (Expt. PC3) e) compressor blades, portable cabinet. BT-7 glass beads,

4.8 mm straight nozzle (Expt. PC2) t) turbine blades, portable cabinet, BT-7 glass beads,

4.8 mm straight nozzle (Expt. PC3) g) compressor blades, portable cabinet. BT-9 glas beads,

4.8 mm straight novle (Expt. PC2) h) turbine blades, portable cabinet, BT-9 glas beads,

4.8 mm straight noale (Expt. PC3) i) compressor blades, portable cabinet, Chr-10 stainless steel shot,

4.8 mm straight n o d e (Expt. PCîj j) turbine blades, portable cabinet, Chr- 1 O stainless steel shot,

4.8 mm straight noule (Expt. PC3) k) turbine blades, portable cabinet, Ch-20 stainless steel shot,

4.8 mm straight nozzle (Expt. PC3)

) Pressure 1 Time 1 Thickness of deposits removed 1

- -

345 0.5 7.3 #

345 5 coating removed 345 10 coating removed

i

Table 4.12 Deposit removal from cornpressor 8th I 9th stage and turbine 1 st stage (continued) blades of W62 gas turbine, 38 cm offset, a=90 deg.

1 pressure 1 Time 1 Thickness of denosits removed

345 345 345

0.5 5

10

4.4 172

345 345

2 coating removed coating removed

-

0.5 172 1 72

5 10

5 coating removed 101 coating removed

coating removed coating removed

Table 4.12 Deposit removai fiom compressor 8th / 9th stage and turbine 1 st stage (continued) blades of W62 gas turbine, 38 cm offset. a=90 deg.

1 Pressure 1 Time 1 Thickness of deposits r e m o v e d l

172 t 72

1

172

345 345 345

t

345 0.5 12 345 5 15.5 345 10 coating mnoved

1

0.5 5

10

172 172

I

1 72

6.9 t

1 1 1

coating removed

0.5 5

10

12 16

coating removed

0.5 5

10

6.6 11

coating removed

Table 4.12 Deposit removal frorn compressor 8th / 9th stage and turbine 1st (continued) stage blades of W62 gas turbine, 38 cm offset, a=90 deg.

Pressure 1 Time 1 Thickness of deposits removed 1

Mode: VSI - - -.- -- .- -- Mag : 10.2 X

Title: W62 C, BT-7 strearn Note: 345 kPa, 38.1 cm, 0.5 s, main room

Figure 4.27 Deposits on compressor Made of W62 gas turbine, BT-7 \ glass beads, main room. 345 kPa, 0.5 s (Expt. MW).

Mode: VSI -- .-- . Mag : 10.2 X

Title: W62 C , BT-7 stream Note: 345 kPa, 3 8.1 cm, 1 0 s, main room

Figure 4.29 Deposits on compressor blade of W62 gas turbine, BT-7 glass beads, main room, 345 kPa, 10 s (Expt. MR8).

Mode: VSI ---. -- . . ..-. Mag : 10.2 X

Size: 368 X 236

Title: W62 T, BT-7 stream Note: 172 kPa, 38.1 cm, 0.5 s, main room

Figure 4.30 Dcposits on turbine blade of W62 gas turbine, BT-7 glass bcads, main room, 172 kPa, 0.5 s (Expt. MR9).

Table 4.13 Impact site EDX anaiysis. Cornpressor blades of W62 gas turbine blasted at 38 cm, a=90 deg. a) crater center, main room, BT-7 glass beads,

1 1.1 mm Venturi n o d e , 172 kPa, 5 s (Expt. PC2) b) crater ridge, main room, BT-7 glass beads,

1 1 . 1 mm Ventun nozzle, 172 kPa, 5 s (Expt. PC2) C) bornbuded region, main room, BT-7 glas beads,

1 1 . 1 mm Venturi noule, 172 kPa, 10 s (Expt. PC2) d) crater center, portable cabinet, BT-4 glass beads,

4.8 mm straight nozzle. 345 kPa, 10 s (Expt. PC3) e) crater ridge, portable cabinet, BT-4 glas beads,

4.8 mm straight noule, 345 kPa. 10 s (Expt. PC3)

b

Element Si L

P S Al

_r

Atomic %

4.1 7

2.8 81.8

Weight %

4 7.7 3.1 77.6

Table 4.13 Impact site EDX analysis. Compressor blades of W62 (continued) gas turbine blasted at 38 cm offset, a=90 deg.

Crater ridge

Crater wall

Central spot

Impact material

Abrasive sphere

Crater diameter

Bombarded region t

Figure 4 .33 Designation of impact region areas.

Table 4.14 Impact site EDX analysis. Turbine blades of W62 gas turbine blasted at 38 cm, a=90 deg. a) crater center, main room, BT-7 glas bead,

1 1 .1 mm Venturi nozzle, 345 kPa, 10 s (Expt. PC2) b) crater ridge, main morn, BT-7 glass bead,

1 1 . 1 mm Venturi noufe. 315 kPa 10 s (Expt. PC2) C) bombarded region. portable cabinet, BT-7 glas bead,

4.8 mm straight noule. 345 kPa, 10 s (Expt. PC3) d) crater center, portable cabinet, B T 4 glass bead,

1 1 . 1 mm Venturi noule. 345 kPa, 10 s (Expt. PC3) e) crater ridge, portable cabinet, BT-4 glass bead,

1 1 . 1 mm Venturi nozzle, 345 kPa, 10 s (Expt. PC3) £) bombarded region, portable cabinet, BT-9 glas bead.

4.8 mm straight noule. 345 kPa, 5 s (Expt. PC3)

Element I

Ni I

Co r

Cr Si I

S

Atomic %

44.1

Weight %

48 27

13.9 5.1 2.7 3.6

1

29.4 1

13.4 2.6 1.6 1.8

Table 4.14 Impact site EDX analysis. Turbine blades of W62 gas (continued) turbine blasted at 38 cm. a=90 deg.

Weight %

35.8 1

26.7 1

10.8 i

5.6 2.9 7.2

Element 1 Atomic %

Ni Co Cr Si S Al

28.7 21.4 11.7 9.5 4.3

12.7

Cornpressor and turbine staae blades blasted in portable cabinet

The experiments were performed with the BT-4, BT-7, BT-9 glass beads

and Chr-10. Chr-20 stainless steel shot in the portable cabinet at 38 cm offset

(Expt. PC2 and PC3 in Table 4.1).

A typical impact site of the BT-4 glass beads at 172 kPa, 5 s is depicted

in Figure 4.36 (WYKO) showing wide craters separated by thick ridges of

elevated material. These ridges contained deep, narrow cracks characteristic to

the non-blasted deposited areas. Impacting with the BT-4 glass beads at 345

kPa pressure demonstrated high aggressiveness on the compressor blade

deposits and aluminum layer. Both were removed, 5 s and 10 s blast duration

(Table 4.12). The SEM analysis of this bombarded region (Table 4.13) showed

a high percentage of Fe, the main eiement of the blade substrate. This

indicoted that the aluminum coating had been removed completely. On the

turbine stage blades, the probe rneasurements (Table 4.12) showed only 6.5 pm

and 10 Fm thickness of the deposit eroded for 5 s and 10 s corresponding

exposures. The SEM analysis of the impact sites demonstrated quite high level

of elements like Si, S and Al non-characteristic to the turbine stage blade

substrate (Table 4.14). The WYKO images in Figures 4.37 - 4.39 revealed that

the beads produced quite rough impact surface that was characterized by the

random shape indentations and ridges. The difference in height between

impact and protected areas corresponded to the thickness probe readings.

BT-7 glass beads demonstrated very high deposit removal effectiveness

on both compressor and turbine stage blades. Results in Table 4.12 show that

the protective layer on the compressor blades was completely removed. On the

turbine stage blades, 9.5 pm and 15 pm of deposits corresponding to 5 s and

10 s blast duration. The WYKO images ( Figures 4.40 and 4.41) depict a

smoother impact surface containing indentations of a smaller size compared to

those of the BT-7 glass beads. Also, it is seen that the impact sites had fewer

microcracks than those produced by BT-4 beads. The raised percentage of Ni

and Co. the elements composing the blade substrate, revealed by the SEM

(Table 4.14) confirmed the deposit high removal rate.

The BT-9 glass beads removed the deposits and protective layer from

the compressor stage blades at both 172 kPa and 345 kPa when blasted for I O

S. The tests produced deposit removal of 7.5 pm for 5 s and 14 pm for 10 s

(Table 4.12) from the turbine stage blades. This was comparable to the results

of the BT-7 glass bead blasting. The level of the substrate characteristic

elements in the impact regions (SEM, Table 4.14) was, again, comparable to

that of the BT-7 glass beads. The WYKO images of the blast sites of the

compressor stage blades at 172 kPa for 5 s (Figure 4.42) and of the turbine

stage blades at 345 kPa for 5 s (Figure 4.43) and for 10 s (Figure 4.44)

demonstrated the highest smoothness of the impact surface of those produced

by the glass beads. Two SEM images in Figures 4.34 and 4.35 show the sharp

border between impact and intact areas. The border profile revealed the

spherical marks from the bombarding particle on the cornpressor stage blades

while on the turbine stage blades it was more random and sharp. In general,

such appearance was typical for the tests with al1 media.

The Chr- 1 O stainless steel shot removed both the deposits and aluminum

coating from the compressor stage blades at 172 and 345 kPa 10 S. The probe

measurements showed deposit removal of 6 pm after 5 s and 12 Fm after 10 s

(Table 4.12) from the turbine stage blades. This was lower than that of the BT-

7 and BT-9 glass beads. The WYKO images (Figures 4.45 - 4.47) demonstrate

that the impacts resultrd in a smooth surface on the compressor stage blades.

The blasted areas on the turbine stage blades were rougher and more ridged

than those produced by the glass beads. This could be due to the small size o f

these particles. Also, it is seen that blasting with the Chr-IO stainless steel

shot did not smooth over microcracks inherent i n the deposits.

The Chr-20 stainless steel shot produced an effective deposit removal

from the turbine stage blade areas at 345 kPa - 10 pm after 5 s and 15 prn after

10 S. The impact surface became smoother (Figures 4.48 and 4.49. W Y K O )

compared to that blasted with the Chr-10 stainless steel shot. This might be

due to the increase of this media particle diameter. It should be noted that the

average particle velocity (Table 3.4) and stream power (Table 3.5) of the Chr-

10 stainless steel shot was just slightly bigger than those of the Chr-20 shot.

Also, the hardness of the new (non-workhardened) stainless steel shot that was

used in the experiments was significantly lower than that of the glass beads.

From this i t may be inferred that the particle diarneter was an important factor

in the deposit removal effectiveness.

Figure 4.34 Deposits on compressor blade of W62 turbine, portable cabinet, BT-9

glass beads, 172 kPa, 0.5 s (Expt. PC3)

Figure 4.35 Deposits on turbine blade of W62 turbine, portable cabinet, BT-9

glass beads 345 kPa, 10 s (Expt. PC3)

Mode: VSI P. - - . . - .- .. -.. Mag : 10.2 X

Title: W62 T, BT-4 strearn Note: 1 72 kPa, 3 8.1 cm, 1 0 s, portable cabinet

Figure 4.37 Deposits on turbine blade of W62 gas turbine, BT-4 glass beads, portable cabinet, 172 kPa, 10 s (Expt. PC3).

2D Profilesx-~rofiie / 2 Pt / Radial

Title: W62 T, BT-4 stream Note: 345 kPa, 38.1 cm, 0.5 s, portable cabinet

Figure 4.38 Deposits on turbine blade of W62 gas turbine, BT-4 glass beads, portable cabinet, 345 kPa, 0.5 s (Expt. PC3).

4-56

NOTE TO USERS

Page($) not included in the original manuscript are unavailable from the author or university. The

manuscript was microfilmed as received.

4-58 figure 4.40

This reproduction is the best copy available.

UMI

v @ - -

Mode: VSI -. - . --- ----- -Mag: 5.3X

1 1 \ r 1 1 U

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1 . . 1 . 1 , 1 .!< un

Size: 368 X 236

Title: W62 T, BT-7 stream Note: 345 kPa, 38.1 cm, 5 s, portable cabinet

Figure 4.41 Deposits on turbine blade of W62 gas turbine, BT-7 glass beads, portable cabinet, 345 kPa, 5 s (Expt. PC3).

4-59

g r v ' & Mode: VSI Mag : 5.3 X

04/09/99 2D Profilesx-~rofile / 2 Pt / Radial 13:37:46

Size: 368 X 236

Title: W62 C , BT-9 streatn Note: 172 kPa, 3 8.1 cm, Ss, portable cabinet

Figure 4.42 Deposits on cornpressor blade of W62 gas turbine, BT-9 glass beads, portable cabinet, 3 172 kPa, 5 s (Expt. PC2).

dz5&z33 .---..---. .... ... . . --- . . .- Mag Mode: : 5.3 VS1 X 2D Profiles 04/09/99 IX-Profile / 2 Pt / Radial 09:57:44

Title: W62 T, BT-9 stream Note: 345 kPa, 38.1 cm, 5 s, portable cabinet

Figure 4.43 Deposits on turbine blade of W62 gas turbine, BT-9 glass beads, portable cabinet, 345 kPa, 5 s (Expt. PC3).

Mode: VSI - . -- Mag : 5.3 X 2D Profilesx-~mfile / 2 Pt / Radial

Title: W62 C , Chr-1 O stream Note: 172 kPa, 38.1 cm, 0.5 s, portable cabinet

Figure 4.45 Deposits on compressor blade of W62 gas turbine, Chr- 1 O steel shot, portable cabinet, 172 kPa, 0.5 s (Expt. PCZ).

- -.- - Mode: VSI

- -- .-. . - . . Mag : 5.3 X

Title: W62 C, Chr-10 stream Note: 1 72 kPa, 3 8.1 cm, Ss, portable cabinet

Figure 4.46 Deposits on cornpressor blade of W 6 2 gas turbine. Ciir- lO steel shot, portable cabinet. 172 kPa, 5 s (Expt. PC2).

Title: Note:

06/29/99 2D Profilesx-~rnfile 1 2 Pi 1 Radial 10:55:01

W62 T, Chr-20 strearn 1 72 kPa, 38.1 cm, 10 s, portable cabinet

Figure 4.48 Deposits on turbine biade of W62 gas turbine, Chr-20 steel shot, portable cabinet, 172 kPa, 10 s (Expt. PC3).

4.3 Shooting at blades with airgun

Bombardments with the single glass bead and stainless steel shot

particles were made by using the airgun dsscribed in Section 3.1. The tests

were performed on the compressor and turbine stage blades of the W62 gas

turbine. The gun velocity as a fincrion of pressure is given in Table 4.17. The

impact sites were assessed using the WYKO non-contact optical surface

profiler and the scanning electron microscope (SEM).

D e ~ o s i t s on cornpressor stage blades

The WYKO and SEM images as well as EDX analysis of the impact sites

produced by low velocity particles on compressor blade deposits of the W62

gas turbine (Expt. GGI in Table 4.1) suggest that the deposits were compacted

but not crushed or removed. This cornes from the observation that the

compressed area produced by the

BT-7 g l a s bead at approximately 78 mls impact velocity (Figure 4.59, SEM)

was characterized by a high percentage of S that was typical of the deposits. In

contrast to the undisturbed regions, it was observed that the impact area had a

much brighter appearance caused by a higher light reflectivity. This indicated

that the deposits were indeed compacted producing smooth Crater walls that

reflected light better.

As the velocity or diameter of the particle was increased, the impact site

EDX analysis (Table 4.1 5) depicted a marked increase in Al, the main element

4-68

of the protective coating. The WYKO images of such sites (Figures 4.50, 4.51

and 4.54) showed some elevation of material in quite a wide area surrounding

the craters. However, the microcrack structure of the surrounding deposits did

not change as could be seen on the SEM images, (Figures 4.56 and 4.60 - 4.62)

suggesting that the material may have buckled a s a film rather than forming a

narrow ridge that usualfy characterizes ductile erosion [4, 51. It is interesting

that the images in these figures were the only ones that clearly showed spots

i n the crater bottom that contained crushed residual deposits. As was

confirmed by the EDX analysis, Table 4.15. To verify the crater dimensions

obtained with the WYKO, a CAD drawing of a segment of 450 pm diameter

glass bead shown in Figure 4.58, suggests that when it penetrates 14 Fm, 156

Fm diameter crater is left. Such observations of just partial penetration of the

particle into the impact surface were characteristic of other examined sites.

The depth of penetration by the heavy, higher speed particles, like BT-4 g l a s

beads (Figure 4.5 1 , WYKO) and Chr-20 stainless steel shot, was comparable

to the 17.3 pm - 17.7 Pm range of the deposit depth, (Section 2.4), meaning

that such particles stopped near the interface of the deposit protective coating.

The lighter BT-7 glass bead produced quite a deep indentation (Figure 4.54)

which, possibly, could reach the aluminum protective layer as the EDX

analysis of the crater bottom revealed a high level of Al (Table 4.15). From

the above observations it may be inferred that the deposit compression was

followed by the deposit buckling and breakage that started when the particle

reached the deposit protective coating interface. This type of deformation was

also observed in the study of organic coating removal [10].

Protective coating on compressor staae blades

These tests are defined as Expt. GG2 in Table 4.1. The impact of

individual BT-4 and BT-7 glass beads against the clean protective coating

(free from deposits) resulted in almost circular craters (Figures 4.52. 4.53 and

4.55) surrounded by high narrow ridges. From this it may be concluded that

the aluminum coating was deformed in a ductile manner characterized by

considerable amounts of plastic straining [14]. It was also observed that the

depth of penetration of the BT-4 glass bead into the protective coating at 93

mls (Figure 4.53) was less than into the deposits at this speed (Figure 4.5 1 ) .

This suggests that the hardness of the Al was greater than that of the deposits.

Turbine blades

The impact sites of a bombardment with single particles against the

turbine stage blade deposits of the W62 gas turbine (Expt. GG3 in Table 4.1)

were characterized by a low ridge surrounding the crater. The BT-4 glass

beads were the only particles that produced distinguished individual

indentations. The image of such site (Figure 4.63, WYKO) showed that the

crater r ims were not very smooth and did not resemble the circles that were

seen on the aluminum coating of the compressor blades. From these facts, the

brittleness of the turbine blade deposits can be inferred. A lower depth of

particle penetration into such deposits compared with that of the compressor

blades could mean that the turbine stage blade contaminants had a higher

dynamic hardness. It should be mentioned that the level of Ni and Co was

increasing with the deposit depth (Tables 2.9 and 4.14). The EDX analysis of

the individual impact area (Table 4.16) demonstrated an elevated Ni and Co

percentage in cornparison to that of the crushed deposits but lower than that

near the substrate interface. Such observations imply that the deposits were

removed by the single BT-4 glass bead rather than being compacted, since this

percentage would have remained at the same level during compaction.

Discussion

Observations of the impact sites produced by individual particles

suggest that deposits on both compressor and turbine blades of the W62 gas

turbine exhibited characteristics of brittle erosion. The compressor blade

protective coating and the turbine blade substrate rnaterial were characterized

by a ductile mode of erosion.

Tables 4-15 Impact site EDX analysis. Deposits on cornpressor blades of W62 gas turbine blasted with individual particles. airgun, a=90° (Expt. GG 1 ) a) crater center, BT-7 glass bead, 78 m/s b) crater edge, BT-7 glass bead, 78 m/s c) crater central bottom spot. BT-7 glass bead. 93 mls d) wall at Crater bottom, BT-7 glass bead. 93 m/s

Elernent I

Si S

1

Al

Atomic %

5.3 61

18.4

Weight % I

4.3 56.5 14.4

Ca 1 6.2 7.1

Table 4.16 impact site EDX analysis. Deposits on turbine blade o f W62 gas turbine shot with individual BT-4 glass beads, airgun, 93 m/s, a=90° (Expt. GG2)

Table 4.17 Velocity of a particle propelled by airgun fiom a calibration chart for 0.4 m long barrei.

Element r

Ni Co Cr Si S Al P Fe

Air cylinder pressure Estimated velocity l

[WaI b f s l

Atomic %

32.4 26.6 3.4 8.1 4.5

4 12.9

5

Weight %

36 29.5

9.3 4.3 2.7

2 7.5 5.3

'Ri'

in' Q) a O

gi n 6 4

- ' " Mode: VSI aIz&zm - Mag : 10.2 X

Title: W62 C, BT-4 airgun Note: 1590 kPa, substrate

Figure 4.53 Coating, cornpressor blade, W62 gas turbine, BT-4 g l a s bead. 93 mls, (Expt. 0 0 3 ) .

Mode: VSI Mag : 20.2 * 2D Profilesx-~rorile 1 2 Pt / Radial

Title: W62 C, BT-7 airgun Note: 1590 kPa, deposits

Figure 4.54 Deposits, compressor blade, W62 gas turbine, BT-7 glass bead, 93 mls (Expt. GGI).

Mode: VSI -* -- - -_+ . - . Mag : 20.2 X

Title: W62 C , BT-7 airgun Note: 1590 kPa, substrate

Figure 4.55 Coating, cornpressor blade, W62 gas turbine, BT-7 glass bead, 93 mls (Expt. GG3).

Figure 4.56 Deposits on cornpressor blade Figure 4.57 Deposits on turbine blade of of W62 gas turbine, airgun, W62 gas turbine, airgun,

BT-4 glass bead, 93 m/s (Expt. GG1). BT-4 g las bead, 93 rn/s (Expt. GGZ).

Impact particle

Penetration depth \

Tigun 4.58 CAD presentation of 450 pm diameter BT4 glas bead penetmted 14 pn into the deposits on compressor blade of W62 gas Rubine, airgun, 1590 kPa Comsponding WYKO image in Figure 4.5 1.

Figure 4.59 Deposits on cornpressor blade Figure 4.60 Deposits on compressor blade of W62 gas turbine, airgun, of W62 gas turbine, airgun,

BT-7 glas bead, 78 m/s (Expt. GGI). BT-7 glass bead, 93 m l s (Expt. GGI).

Figure 4.61 Deposits on cornpressor blade Figure 4.62 Deposits on cornpressor blade of W62 gas turbine, airgun, of W62 gas turbine, airgun

Chr-20 steel shot, 89 mls (Expt. GGI). Ch-20 steel shot, 93 m/s (Expt GG1).

4.4 Summary and discussion

The particular mode of damage to the deposits or turbine component

substrates observed for a given impact condition reflected a complex

interaction of parameters inherent to both particleltarget interaction and blast

conditions. The former included the deposit composition and thickness; impact

particle size. mass, shape, velocity and hardness; substrate properties l ike

elastiç rnodulus, yield stress, hardness and strain to failure. The blast

parameters were mainly the nozzle specifications, supply air pressure, media

mass flow-rate, offset distance, angle of attack, and exposure time.

ï h r turbine components were made of ductile metal alloys while the

deposits consisted of both the elements of the substrate, environmental

contaminants and fuel residues forming a scale-like layer containing

microcracks. The abrasive media included the spherical glass beads of threr

sizes and the two grades of rounded stainless steel shot.

By varying blast conditions in the main room, the component substrate

and profile erosion resistance was determined. Thus, the turbine disc flat areas

could withstand extended normal impacts of at least 150 s cornbined duration

with the BT-7 glass beads at 38 cm offset and up to 483 kPa pressure. Tests of

the disc root edges with the BT-7 glass beads showed that the exposure at 38

cm offset and up to 483 kPa pressure should be much less than 90 s to avoid

substrate deformation. The experiments with the same media on the disc roots

showed high erosion resistance at 25 cm 1 60 s and 38 cm / 90 s conditions

with the nozzle positioned normally to the groove plane at 483 kPa. Much

4-83

lower resistance was demonstrated by the turbine blade roots blasted at the

same pressure. The blasting conditions for them should be limited to 30 s

exposure at 38 cm offset. The corrugating type o f deformation seen on turbine

component surfaces was characteristic of ductile substrate erosion. Such

ductile properties were also observed in forrn of craters with elevated edges

created by individual particles. According to ref. [Ml. where the erosive Wear

was investigated, the rates of erosion on ductile metals should be minimum

when the impingement angle approaches 90'. This was confirmed by the

present test results on the turbine substrate.

The experiments in the portable cabinet showed that the BT-7 glass

beads produced the greatest deformation of the blade substrate compared to

other media. This may be attributable to the fact that the BT-7 glass beads had

the highest average velocity, though their stream power was almost the same

as the BT-9 glass beads and the Chr-IO stainless steel shot. It is interesting

that the BT-7 glass beads had an average particle size between those of the

BT-4 and BT-9 glass beads. This suggests that the media of the highest

aggressiveness rnay have a certain optimum particle diameter. It should be

noted, also, that during tests in the portable cabinet the substrate experienced

high corrugation by al1 media if the nozzle axes made an oblique angle to the

root serration.

Individual particle impact and stream bombardrnents demonstrated the

brittle erosion characteristics o f the deposits with probable surface fracture

and cracking. The idea is supported by investigation on oxides carried out to

provide data for simulation and modeling the erosion processes, ref. [ 9 ] . In

this work, the brittle type of deformation in a form of chipping and cracks was

thought to be related to a fatigue loading caused by multiple bombardments.

The rate of deformation due to fatigue was determined to be dependent on the

particle impact angle and velocity. It was also proposed that the breakage of

deposit layer under cyclic loading could be increased by reducing the media

particle size. Such reduction usually results in a growth of the impact cycle

number due to increased amount of particles in a stream. According to ref.

[14], the fatigue concept assumed the nucleation and growth of a crack under

cyclic loading conditions. In study [14] the experiments on ductile metals

were performed to develop the model of erosion. It should be noted, though,

that under certain transient impact conditions, in practice, more than one

mechanism of deposit removal or substrate deformation might occur

simultaneously, therefore, more than one model may be required.

As in experiments on the turbine substrate, the BT-7 glass beads

demonstrated the highest deposit removal effectiveness from both the

cornpressor and turbine stage blades. Blasting with the BT-9 glass beads that

had a smaller diameter, also resulted in high removal rates. Both the BT-7 and

BT-9 glass beads had the highest average particle velocities. This emphasizes

the importance of velocity in determining the deposit removal effectiveness of

the media. The cornparison of stream power, however, showed that this

parameter had less influence. For example, Chr-10 stainless steel shot had

lower deposit removal rates althoug its Stream power was equal to that of the

BT-7 and BT-9 glass beads.

Dynamic hardness

The test results of the turbine blade bombardments with individual

parricies were used to estimate the target materiai dynamic hardness. The

crater size, particle characteristics and velocities were employed in these

calculations. The crater diameter was taken at a height equal to the

undisturbed target surface, as shown in Figure 4.64. The dynamic hardness

was obtained by setting the incident kinetic energy equal to the work done in

plastically deforming the coating as in the following formula:

where V i is the incident velocity, m is the mass of the incident particle; and

P(6) is the load as a function of the indentation depth 6 . If the elevation of

material adjacent to the crater edges is neglected, the indentation depth S can

be expressed in terms of the contact radius, a. Therefore, Equation (4.1) was

rearranged in the following expression for the dynamic hardness [20]:

where R is the particle radius, and a,,, is the maximum contact radius reached

at deepest penetration, Sm.,. Obviously, the dynamic hardness is a function of

the impact velocity.

In the present gas gun experiments, the BT-4 and BT-7 glass beads

produced spherical caters on the deposited surfaces and the compressor blades

OF the W62 gas turbine. It was thus possible to use the approximate Crater

diameters to estimate the dynamic hardness. It was found that the deposits on

the compressor blades had a lower dynamic hardness, (0.24 GPa), than did the

compressor blade substrate which included the protective aluminum coat ing,

(0.36 GPa), Table 4.18. The deposits on the turbine blades of the W62 gas

turbine exhibited a much higher dynamic hardness, (6.33 GPa). The greater

dynarnic hardness is consistent with the higher erosion resistance demonstrated

by the deposits on the turbine stage blades.

sklbme

Figure 4.64 Cross-section of impact site.

Table 4.1 8 Dynamic hardness, W62 gas turbine blades, airgun, glass beads.

Contact radius ha, [ml

compressor blade deposits, BT-4 glass bead

compressor blade substrate, BT-7 glus bead 8.00~-051 2.00~-041 1.18~-081 93 1 0.3

1.9SE-04 1.75E-04

compressor blade substrate, BT-4 glass bead

turbine blade deposits, BT-4 g las bead 8 SOE-051 4.508-041 1 .35~-071 93 1 6.33

Dynamic hardness 3

Pd,[GPa] Particle radius

R, [ml

1.80E-04 1 SSE-04

4.50E-04 4.50E-04

Particle mass m. [kg]

4.50E-04 4.50E-04

Velocity Vi,[m/s]

1.35E-07 1.3 SE-07

1.35E-O7 1.35E-07

93' 0.22 82

93 82

0.26

0.3 0.43

Chapter Five

Conclusions

I t is concluded that blast cleaning of industrial turbines with glass beads

and stainless steel shot is an effective method of removing the deposits from

turbine components. Complete removal rnay be achieved with a minimum of

deformation to the substrate by following certain procedures. The conclusions

are summarized below.

5.1 Blast media in portable cabinet

r The BT-7 glass beads had the highest and the BT-4 glass beads had the

lowest velocities.

a The Chr-10 stainless steel shot had the highest and the BT-7 glass beads

the lowest mass flow.

Stream power of BT-7, BT-9 glass beads and Chr-10 stainless steel shot

were highest and almost equal while the BT-4 glass bead Stream power was

much lower.

Since the BT-7 glass beads were the most aggressive with respect to

deposit erosion and substrate deformation during the tests, it is concluded

that the particle velocity and Stream power were the major factors affecting

media erosion characteristics. The role of velocity is thought to be more

important. This is consistent with the conclusions in ref. [19] regarding

starch media blasting of aircraft paint. The particle size is considered a

parameter contributing to the velocity rates. Also, the size is a factor

affecting stress concentration in the deposits and substrate.

5.2 Substrate damage

Mec hanisms

a Substrate surfaces blasted at 90" impingement angle demonstrated higher

erosion resistance than those blasted at oblique angles. This means that the

turbine component material displayed a ductile Wear behaviour.

Individual particle impacts produced general material elevation around a

the impact Crater, but not well-defined ridges [4, 51. According to ref. 1141,

this elevation is related to the plastic deformation zone bencath the

particle.

a As sharp lips were not formed, there was no material removal by

subsequent impacts knocking off such lips. This may explain why the

turbine component substrate was corrugated rather than eroded during

oblique blasting.

O ~ t i m u m media

a I t was observed from experiments in the portable cabinet that the BT-9

glass beads produced the least deformation to the turbine component

substrates. At the same time, this media produced the srnoothest substrate

surface after blasting.

Optimum blast Parameters

a 90" impingernent angle (normal blasting).

Offset distance should be greater than 38 cm.

0 480 kPa -500 kPa range was considered an upper limit of blast pressure.

a Normal impact time should be limited to 10 s for profiled surfaces. When

required, such impacts rnay be repeated for up to 5 times providing that the

material is allowed to cool between impacts. Flat surfaces resisted a single

blasting of 60 s duration at 38 cm and 480 kPa.

5.3 Deposit removal

Mec hanisms

a Deposits were removed from the compressor stage blades of three

combustion turbines by normal blasting more effectively than by oblique

impacts. This suggests that the deposit had brittle erosion characteristics.

a It was observed that the deposits on compressor stage blades of the gas

turbines were initially compressed prior to removal. Breakage and removal

of compressed deposits might occur near the interface between the deposit

and underlying substrate.

Deposits on turbine stage blades were eroded by crack propagation to the

substrate interface followed by the deposit spallation and chipping. Cracks

were initiated by the accumulation of high strains induced by impacting

particles.

O ~ t i m u m media

a Cornparisons in the portable cabinet showed that the BT-7 glass beads

exhibited the highest deposit erosion effectiveness. The effectiveness of

the BT-9 glass bead was also high and close to that of the BT-7 glass

beads,

The BT-4 glass bead were much less effective.

r Hardness of Chronital stainless steel shot measured usiag the Knoop

method increased from 22 Rc to 29 Rc within 18 blast cycles. This was

considered a disadvantage as deposit renioval by blast cleaning with this

media became hard to control.

O ~ t i m u m blast parameters

90' impingement angle (normal blasting).

8 3 8 cm offset distance was considered most effective.

a Flat area o f the gas turbine disc demonstrated good substrate erosion

resistance to normal impacts for 60 s at 345 kPa - 483 kPa pressure range.

, Root serration was blasted without substrate deformation for 10 s each

impact repeated for 6 times at 483 kPa.

It is recommended to limit normal blasting of the thin blades to

approximately 3 s at the 345 kPa - 483 kPa pressure.

280 kPa - 350 kPa pressure range and 10 s total blast time was found to be

effective to remove deposits from the turbine blades.

5.4 Recommendations to optimize blast cleaning

Given the ductile nature of the turbine substrate deformation and the

brittle character of deposit erosion, it is advised to perform blasting normal to

the surface whenever possible.

When necessary, short exposure blasts between cool down intervals are

recommended.

Based on the performance of both the BT-7 and BT-9 glass beads in

removing deposits, it is thought that blasting at turbine components can be

effectively conducted with the BT-8 glass beads. This suggestion comes From

high deposit removal effectiveness of both the BT-7 and BT-9 glass beads

while the aggressiveness of the former is lower on the turbine component

substrate.

Attention should be paid to experiments with other types of a nozzle.

The supersonic flow nozzle producing a uniform, parallel surface impact

(CAE) is considered the most interesting.

Keeping the media particle size within a small range is helpful in

controlling the blast process. Therefore, it may be advantageous to install

recycling systems designated to remove pieces of broken particles from the

blast systcm.

References

[1] A. J. Dean, J. E. Bradt, J . F. Ackerman, "Deposit formation from No. 2

distiilate at gas turbine conditions, " International Gus Turbine and

Aeroengine Congress & Exhibition, American Society of Mechanical

Engineers, 1996, pp. 1-6.

[2] 1. Finnie, "Erosion of surfaces by solid particles," Wear 7 1, 198 1, pp.

191-210.

[3] J . S. Hansen, "Relative erosion resistance of several materials, " Erosion:

Prevention and Useful Applications, ASTM STP 661, American Society

for Testing and Materials, Philadelphia (1978), 1979, pp. 148-162.

[4] 1. M. Hutchings, R. E. Winter, J . E. Field, "Solid particle erosion on

materials: the removal of surface material by spherical projectiles,"

Proceedings of the Royal Society, London, V o l . A348, 1976, pp. 379-392.

[5] 1. M. Hutchings, "Mechanisms of the erosion of metals by solid

particles," Erosion: Prevention and Useful Applications, ASTM STP 664,

American Society for Testing and Materials, Philadelphia (1 9 7 8 ) , 1979,

pp. 55-76.

[6] M. H. Johnston, "Nozzle development for dry media blasting processes,"

Abrasive Blast Cleaning News, Summer 1998, pp. 4-7.

[7] A. Korsunsky, P. D. Warren, D. A. Hills, "Impact fracture thresholds in

brittle solids," Wear 186, 1995, pp. 99-104.

[8] B. S. Mann, "Solid-particle erosion and protective layers for steam

turbine blasting," Wear 224, 1999, pp. 8-12.

[9] J . R. Nicholls, D. J. Stephenson, "Monte Car10 modelling of erosion

processes," Wear 186, 1995, pp. 64-77.

[IO] M. Papini, J. K. Spelt, "Organic coating removal by particle impact,"

Wear 213, 1997, p.p. 185-199.

[I 11 V. Ponnaganti, D. E. Stock and G. L. Sheldon, "Measurement of particle

velocities in erosion processes," Symposium on Po&yphasc Flow and

Transport Technology, San Francisco, ASME, pp. 21 7-222.

[12] A. W. Ruff and L. K. Ives, "Measurements of solid particle velocity in

erosive wear," Wear 35, 1975, pp. 195-199.

[13] P. H. Shipway, 1. M. Hutchings, "Measurement of coating durability by

solid particle erosion," Surface Coating Technology 7 1 , 1995, pp. 1-8.

[14] G. Sandararajan, "A comprehensive mode1 for the solid particle erosion of

ductile materials," The International Conference on Wear O/ Materials,

American Society for Testing and Materials, 1991, pp. 503-5 1 1.

[15] 0. Sandararajan, "The depth of plastic deformation beneath eroded

surfaces-The influence of impact angle and velocity, particle shape and

material properties," The International Conference on Wear of Moterials,

American Society for Testing and Materials, 1991, pp. 1 1 1 - 12 1.

[16] A. N . J . Stevenson, 1. M. Hutchings, "Scaling lows for particle velocity in

the gas-blast erosion test," Wear 18 1 , 1995, pp. 56-62.

[17] H . Uuemois, 1. Kleis, "A critical analysis of erosion problems which have

been little studied," Wear 3 1 , 1975, pp. 359-371.

[18] J . A. Williams, Engineering-, Oxford University Press, 1994,

pp. 190-194.

[19] B . Djurovic, "Coating removal from fibre composites using wheat starch

biast cleaning," Theses Masters, National Library of Canada, 1999. I