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THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS
345 E. 41St New York. N.Y. 10017
The Society shall not be responsible for statements or opinions advanced in papers or in dis-
cussion
at meetings of the Society or of its Divisions or Sections or printed in its publications.
Discussion is printed only if the paper is published in an ASME Journal. Papers are available
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.