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THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS

345 E. 41St New York. N.Y. 10017

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from ASME

for fifteen months

atter the

meeting.

Printed

in USA

9 2 G T 3 5 0

M agn et ic Part ic le Inspe ct ion o f Turb ine B lade s in

Pow er Generat ing P lants

CLEMENT IMBERT

The University of the West Indies

St. Augustine, Trinidad

KRISHNA RAMPERSAD

Trinidad Tobago Electricity Comm ission

63 Frederick Street

Port-of-Spain, Trinidad

ABSTRACT

Modern societies expect and depend on regular, rel-

atively uninterrupted, supply of electric power. Preven-

tive maintenance is therefore vital for power generating

plants. Non-Destructive Evaluation (NDE) is a signif-

icant element of the maintenance programme of power

plants. Power plants use a wide variety of steam and

gas turbines. Turbine failure can occur without warning

and with disastrous results. Such failures are invariably

caused by cracks. Such defects are readily detected by

NDE techniques such as Magnetic Particle Inspection

(MPI) if they are on or near the surface and accessible.

This paper reports on the use of MPI in the exam-

ination of martensitic stainless steel turbine blades in

power plants in Trinidad and Tobago so as to quantify

the testing parameters and determine field strength in

relation to defect detectability. Specific recommenda-

tions are made regarding the configuration and optimum

placement of magnetizing coils for turbine blade inspec-

tion insitu and detached.

NTRO U TION

The regular supply of electric power is crucial to modern in-

dustrial societies. In most developing countries electric power

is provided by state or quasi-state companies. The Trinidad

and Tobago Electricity Commission (T&TEC), a state owned

company, is soley responsible for electric power generation and

distribution in Trinidad and Tobago. The generating capacity of

the four power stations in the T&TEC system is close to 1200

megawatts, obtained from twenty-one generating units. With

a population of about 1.2 million people, the consumption of

electric power in Trinidad and Tobago is one of the highest in

the developing world at approximately one hundred watts

for

every two persons, which works out at about half the generating

capacity. This extra capacity caters for generating units being

out of service during scheduled and unscheduled shutdowns and

maintenance overhauls

The generating units at T&TEC comprise state-of-the-

art

steam and gas turbines from several manufacturers. In the

last few years there have been several failures of turbine com-

ponents. Blading failure has been the subject of fairly extensive

study at T&TEC in the recent past [1]. These failures have been

very costly in terms of downtime, replacement parts and restora-

tion of units to service. As Armor [2] has pointed out, fractures

in the turbine system are usually catastrophic to the generating

equipment and also pose potential danger to plant personnel.

Preventive maintenance, incorporating Non-Destructive Evalu-

ation (NDE) techniques, is vital for determination of the relia-

bility of turbine components. In this regard, blading represents

one of the key areas requiring improved crack detection methods

[3] .

In the past, the Trinidad and Tobago Electricity Com-

mission has contracted the Original Equipment Manufacturers

(OEMs) for inspection services in maintenance overhauls, as is

common in many developing countries. This is very costly and

is not very convenient or expedient. The commission is therefore

taking steps to do much of its inspection inhouse and as such

inspection

proccduici, must

be followed that suit the particu-

lar

conditions :. tiier the OEMs

nor the literature provided

detailed inspection procedures. Procedures therefore had to be

formulated from available

information. In order to do this prop-

erly, some experimental work is required to determine inspection

parameters and defect detectability.

Magnetic particle testing is the most widely

used NDE

method for the inspection of ferro-magnetic turbine blades for

detection of

flaws. This paper covers some of the experimen-

tal work done on turbine blades in power plants in Trinidad

and Tobago in order to establish procedures for the detection

of discontinuities using the Magnetic Particle Inspection

(MPI)

technique. The test methodology is outlined. A subsequent pa-

per will deal with the quantitative relationships between crack

characteristics and flux density.

Presented at the International Gas Turbine and Aeroengine Congress and Exposition

Cologne Germany June 1-4 1992

Copyright © 1992 by ASME

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2 EXPERIMENTAL PROCEDURE AND RESULTS

2 1Test

Equipment.

The equipment used for the tests consisted of a Magnaflux port-

able magnetic unit (Type M-500), a Bell 610 gaussmeter model

H7B1-0608 with transverse probe, and a Magnaflux ultraviolet

light Model ZB. Magnaglo 14A magnetic particles were used,

suspended in water with magnaflux WA-2A conditioner.

The Type M-500 Magnaflux unit is designed to furnish

dialed self-regulated high amperage alternating and half-wave

direct current for inspection of medium to large machinery com-

ponents. Maximum intermittent current ratings are 4000 amps

A.C. or 4000 amps D.C. through 9m 30ft) of 95mm 2 4/0 AWG)

cable. The continuous rating is 1200 amps. Output currents are

regulated by means of a calibrated system.

The Bell 610 gaussmeter is solid state construction and

uses Hall effect magnetic field probes capable of measuring field

strength in the range of 1 to

100,000

gauss.

The Magnaflux model ZB black light is a

100 watt mer-

cury vapour arc lamp capable of delivering sufficient energy in

the 356 nanometer range well above the minimum intensity re-

quirement for inspection (800 microwatts/cm2 at the inspec-

tion surface). Actually the source used had a strength of 1800

microwatts/cm2 300mm away

2 2

General Test Parameters

In the magnetic particle inspection of

turbine blades in

situ (i.

e.

without removing them from their

working positions on the

rotor, diaphragm or cylinder) the

indications sought are cracks

transverse to the length of the blades. Because these cracks can

be very tight and also because of the complex structure of the

parts and accessibility, the wet fluorescent method was found to

to be most effective.

It is very important that the blades are clean prior to

inspecting. This is readily achieved by dust blasting but care

must be taken not to interfere with the integrity of any coating

on the blades. The blades are magnetized in the longitudinal di-

rection. Magnetic particles are then applied to the blades which

are inspected under ultraviolet (black) light. Any discontinuity

normal to the lines of flux, such as a transverse crack, will cause

the magnetic particles to form a distinct visible pattern.

Half-wave rectified single phase current was used. This

provides excellent sensitivity [4]. The residual magnetism me-

thod was tried but found to be unsatisfactory as indicated by

US military specification MIL-STD-1949 A [5]. Therefore the

continuous method was used.

2 3

Turbine Blades Tested

Several different shapes (curvatures) and sizes of blades, mount-

ed on their shafts, were tested. The blades, martensitic stainless

steel type 416, ranged in size from 65mm long by 50mm average

width to 640mm by 60mm. The results reported here are for

the largest blade.

2 4

Coil Configurations

Three sets of tests were performed which represent the methods

that can be used to magnetize the blades in the desired direction.

The methods were:

1.

Forming a coil around an individual blade which was de-

tached from the rotor. (Figure 1)

2.

Wrapping a coil around the rotor body or shaft, with the

same number of turns on one side of the row of blades as

on the other side. (Figure 2)

3. Making up a coil and placing it over a number of blades

in one row on the spindle or shaft (Figure 3).

2.4.1 Coil

formed around a

single

detached blade. Fig-

ure 1 shows the coil configuration and the magnetic field direc-

tion for the coil formed around a single detached blade. A coil

of 5 turns and 280 mm diameter was used. The blade length

was 640 mm and average cross section of 60 mm wide. The area

in which the inspection was performed was enclosed and dark-

ened. A half-wave D.C. was applied, varying from 200 Amperes

to 1200 Amperes in steps of 100 Amperes. The corresponding

field strengths along the body of the blade and at the tip were

measured and tabulated for each current increase. Flux density

at the tip of the blade was much higher than that along the

body of the blade for the same magnetizing current. The field

strength along the body (i.e. away from the tip) was not con-

stant, and as such an average value was used to plot magnetic

field strength vs magnetizing current. The average was taken

from readings at twelve points (as shown in Figure 1) on the

blade surface. After this was completed, the blade was demag-

netized using a rapidly, continuously diminishing AC current

from 1500 to zero Amperes. Figure 4 shows plots of average

magnetic field strength vs magnetizing current, and maximum

blade tip magnetic field strength vs magnetizing current for the

detached blade.

Two blades with known fatigue cracks were in turn placed

in the coil. Starting with a current of 200 Amperes half-wave

D.C., wet fluorescent particles were applied to the blade in con-

tinuous mode. As before, the current was increased in steps of

100 Amps to 1200 Amperes each time inspecting the blades. As

the flux density increased the crack indications appeared dis-

tinctly at about 50 gauss and became more distinct as the cur-

rent (and flux density) increased. At 60 gauss the indications

were very distinct increasing in intensity up to about 100 gauss

which corresponds roughly to the upper point of inflection on

the curve.

2.4.2 Coil

wrapped

around

rotor body

Figure 2 shows

the coil configuration and magnetic field direction when the coil

is wrapped around the rotor body. As can be seen, the flexible

cable, after making a coil on one side of the row of blades, is

looped over to the other side and the coil wrap is continued in

reverse direction to the first coil. This creates similar magnetic

poles on either side of the row of blades. The net effect is to

force the magnetic flux lines through the longitudinal direction

of the blades.

Figure 5 shows the plots of average magnetic field strength

vs magnetizing current along the body and at the tip of the

blades with the coil wrapped around the rotor body. As in the

case of the single detached blade the values of flux density varied

along the body of the blade for any given current and therefore

an average value was used to plot the current vs flux density

curves. As previously the average value was taken from twelve

points along the surface of the blade. This was also done for the


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600

400

300 co

i-

<

20 0

c ‘ 1

100

M XIMUM FLUX T TIP OF BL DE

VER GE FLUX LONG BODY OF BL D E

8

10 11

MAGNETIZING CURRENT AMPS) X 100

FIGURE 5 - Flux Density vs Magnetizing current

for a coil forrned around turbine

shaft rotor body) across one row of

blades

0

2

6

r z

2

70 0

60 0

50 0

4 00

300

200

BL DES

10 0

method (section 2.4.3) where a preformed coil was looped over

a number of blades in one row.

2 4 3

Preformed coil looped over a number of blades in

one row

Figure 3 shows the configuration and field direction

of the preformed coil looped over a number of blades in one row

A coil was formed comprising three turns of adequate size to

X - POINTS OF MEASURING FLUX DENSITY ALONG BODY

IS1- POINT OF MAXIMUM FLUX DENSITY AT TIP

FIGURE

I

Coil

configuration and magnetic field

direction for detached blade.

M GNETIC FLUX LINES

ROW OF TURBINE

X - POINTS OF MEASURING FLUX DENSITY ALONG BODY

- POINT OF MAXIMUM FLUX DENSITY AT

TIP

FIGURE 2 - Coil configuration and magnetic field

for coil wrapped around rotor body

across one row of blades

FLUX DIRECTION IS IN

TH E

AXIS OF THE BLADES. AS FOR

THE SINGLE BLADE IN FIGURE

1 AND THE COIL CON-

FIGURATION IN FIGURE 2 THE

MAXIMUM)FLUX DENSITY AT

THE TIP OF THE BLADE WAS

MEASURED AS WELL AS

TWELVE READINGS OF FLUX

DENSITY ALONG THE BODYOF

THE BLADE FOR EVERY VALUE

OF CURRENT.

FIGURE 3 - C oil looped over number of blades in one row

cover one third the number of blades in a row. The coil was

then placed over the blades. Figure 6 shows plots of average

magnetic field strength vs magnetizing current, for preformed

coil looped over a number of blades in one row, along the body

and at the tip of the blades.

FIGURE 4 - Flux Density vs Magnetizing current

for a single detached turbine blade

FIGURE 6 - Flux Density vs Magnetizing

current for preformed coil looped

over a numbe r of blades in one row

3


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ZERO

FIELD

RECORDED

IN THIS

REGION

TURBINE

SH FT

FIGURE

7: Coil configuration and magnetic field for

coil wrapped around rotor body b ut

looped across two rows of blades

FIGUR E 8: Coil configuration and magnetic field for

coil wrapped around rotor body b ut with

the wrong configuration across the row

of blades i.e. in the same direction).

3 DISCUSSION OF RESULTS

3.1 Response

Curves Of Turbine Blades

The Flux Density vs Magnetizing Current curves presented in

Figures 4, 5 and 6 for the largest blade tested (640m by 60mm),

are similar to results obtained for the other sizes of blades.

32

Single Detached Blade

The coil formed around the detached blade was used to establish

the characteristics of the magnetic field which is required to be

induced in order to detect cracks. The flux density at the tip

of the blade was between three to three and a half time the

flux density along the body. The standard deviation of the flux

density along the body of the blade within the 50-100 gauss

range was between 7 to 10 gauss. The cracks in the defective

blades served to establish the range of magnetization in which

defects are distinctly visible.

Because of the configuration of the blades when assem-

bled on the rotor, it is impractical and uneconomical to have

them either detached for inspection or to wrap a coil around

each individual blade on the rotor. Thus the results obtained

from the detached blade were used as a reference for the other

(insitu) methods and to determine the maximum magnetization

for optimum detectability of defects in turbine blades mounted

on the shaft.

33

Blades Assembled On Rotor Shaft

The second method of testing (with the coil wrapped around

the rotor on both sides of a row of blades insitu) yielded results

generally similar to the test on the detached blade as Figures 4

and 5 show.

In the (third) method, where a preformed coil is placed

over a number of blades as shown in Figure 3, a similar pattern of

flux was obtained as for the method of wrapping the coil around

the rotor body but with a higher magnitude of flux density for

the same current used. From the graph in Figure 6 a current of

550 Amperes induces an average magnetic flux of approximately

70 gauss along the body of the blade compared to 600 Amperes

for the same average flux by the second method, a difference of

about 10%. Also, the flux was more evenly distributed, using the

third method, along the surface of the blade as can be observed

from the results.

The standard deviation of the flux density along the body

of the blade, within the 50-100 gauss range, was about 5 gauss

for the third method, as compared to 15 gauss for the second

method, where the coil was wrapped around the rotor shaft. The

flux density at the tip however, is over four times that along the

body of the blade at the higher levels of flux density.

Using the basic method of wrapping the coil around the

rotor body, the coils were wrapped such that two rows of blades

were within the coils (Figure 7) instead of one row, as in Figure

2.When the magnetic flux was checked along the surface of the

blades, it was found that no field was detected along the inner

side of both rows of blades.

Figure 8 shows the coils wrapped around the rotor body

but with a wrong configuration, i.e. the coil continues in the

same direction on both sides of the row of blades. This did not

produce the desired field in the blades because dissimilar poles

were created across the row of blades so that the field remained

confined, more or less, to the turbine shaft.

As Figures 4, 5 and 6 clearly indicate the maximum flux,

much higher than what was measured along the body of the

blade, was obtained at the tip. This should be expected since

it is the point where the majority of flux leaves the blade in

longitudinal magnetization because of the severe geometric dis-

continuity at the tip.

As can be observed from the results in Figure 5 even at

a current as low as 300 amperes, the flux measured at the blade

tip was 60 gauss at which value in the body of the blade defects

are clearly visible. However, the average flux in the body of

the blade was only 10 gauss which is much too low for proper

determination of defects.

4 CONCLUSIONS

The single blade technique is only practical for detached blades

- new or repaired blades for example.

Of the two practical methods of inducing magnetic fields

for Magnetic Particle Inspection of turbine blades fixed on the

rotor, the method of placing a preformed coil over a number of


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blades in one row as shown in Figure 3 is preferred to the method

of wrapping the coil around the rotor body across one row as

shown in Figure 2. This is because of the greater uniformity

of the magnetic flux induced and the lower current required to

induce adequate flux density with the preformed coil. However,

both methods provide satisfactory magnetic flux and either can

be used. When using the latter method there should only be one

row of blades within the coil since only the (outer) sides of the

blades nearest to the coil would be properly magnetized. This

is illustrated in Figure 7. Also the coil must change direction

on either side of the row of blades. If the coil does not change

direction the flux effectively stays in the rotor. This is illustrated

in Figure 8.

Optimum defect detection occurs at a current well below

the point where the flux density saturates, thus eliminating any

tendency for masking of indications that tends to take place

close to and beyond the saturation point. It has been confirmed

that defect indications show up most clearly in the range of 60-

100 gauss in the body of the blade.

It is necessary to ensure that sufficient magnetization is

induced in the body of the blade since the ratio of flux density at

the tip of the blade to the body could easily be over four to one

and measuring flux density at the tip would be very misleading.

REFERENCES

1. Imbert, C. and Bhattacharya, K. Department of Mechanical

Engineering, The University of the West Indies St. Augustine,

Trinidad. Several reports for the Trinidad and Tobago Elec-

tricity Commission.

2. Armor, A. F. Turbine-Generator NDE: An EPRI Perspec-

tive in Nondestructive Evaluation of Turbines and Generators:

Proceedings of Conference and Workshop: WS-80-133. pp1.3-

1.24. 1981. Editors: R H Richman and T Rettig. California:

Aptech Engineering Service.

3 .

Reinhart, E. R. A Study of NDE Methods for Turbine

Blades and a Critical Review of Turbine Spindle Inspection

in Nondestructive Evaluation of Turbines and Generators: Pro-

ceedings of a Conference and Workshop: WS-80-133. pp3.43-

3.63. 1981. Editors: R H Richman and T Rettig. California:

Aptech Engineering Service.

4. Manual on Magnetic Particle Inspection: 48-GP-11M. Cana-

dian General Standards Board, 1981.

5. Magnetic Particle Inspection: MIL-STD-1949 A. US Military

Standard 1989. Washington: USA.

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