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FAILURE ANALYSIS OF A BURNER COMPONENT
EI111111111,1) 111111111
Kenneth L. Saunders
Hopper and Associates
Redondo Beach, California
1
Brian P. Copley
Watson Cogeneration Company
Carson, California
ABSTRACT
The cogeneration facility of interest contains four identical
cogeneration units which produce about one half million Kg
per hour of steam for an adjacent refinery and 385 megawatts
of electricity. To supplement the production of steam, burners
are used to heat the gas turbine exhaust. These burners
incorporate shields to deflect exhaust gas flow around the
flame base.
In an effort to improve burner emissions of the units, the
burner shield design was modified. This alteration resulted in
gross deformation of the shields which interfered with
combustion. A failure analysis of these components was
conducted to ascertain the root cause of the observed behavior.
Loads were estimated based upon operational conditions and
material properties were obtained from the open literature. An
evaluation was conducted to determine the temperature
distribution first. This temperature distribution was then
coupled with mechanical loading to obtain total operational
stress levels. The stress levels at the observed temperatures
clearly placed the material in the high strain rate (creep)
region. The computed stress distribution confirmed the
observed failure configuration.
A new design was proposed to eliminate this failure
mechanism. Detailed evaluations revealed that the new design,
while a significant improvement, still operated near the creep
region for the material.
NOMENCLATURE
wing plate area, m 2
A
=
wing plate width, m
b
flow coefficient
CN
=
stress distribution coefficient (6 for
k
=
elastic distribution, 4 for plastic
distribution)
.
wing plate length, m
L
P
t
=
pressure, IcPa
wing plate thickness, m
V
p
=
=
a
=
exhaust gas velocity, m / s
exhaust gas density, Kg / m 3
bending stress, MPa
INTRODUCTION
In a never ending quest to improve the performance of
equipment, either to increase efficiency, output or some other
critical parameter, changes are made to extant systems.
Sometimes these alterations result in the desired changes with
no adverse side effects. Other times there is an unexpected
response which may be undesirable. In our particular example,
an unexpected response occurred in the mechanical behavior of
a critical component due to alterations made to improve
combustion parameters. The unexpected response was gross
deformation of a key item due to a creep phenomenon. These
deformations resulted in new combustion problems which
necessitated a second modified design to acquire the desired
original combustion improvements without creating any new
undesired responses.
A review of the original design geometry and operating
history was conducted first to identify key design and response
features. Once it was understood what the controlling
parameters of the original design were, a failure analysis was
performed for the modified design to again identify key design
and response features. In particular, what was occasioning the
undesirable response of the critical component. The second
modified design was then evaluated for all important loading
conditions and the identified critical failure mechanisms.
It was found that the second modified design greatly reduced
sustained stress levels. However, the safety margin was smaller
than typically desired for components in this industry . For
this reason key locations were identified for inspection on a
regular basis. After the second modified design was placed in
•
Presented at the International Gas Turbine and Aeroengine Congress & Exhibition
Birmingham, UK — June 10-13, 1996
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FLOW
FLOW
b. Modified
a Original
O
0 0 0 0
0 0 0 0 0
O
0 0 0 0
00.000
O
0 0 0 0
0 0 0 0 0
O
0 0 0 0
0 0 0 0 0
O
0 0 0 0
0 0 0 0 0
O
0 0 0 0
0 0 0 0 D
O
0 0 0 0
0 0 0 0 0
O
0 0 0 0
0 0 0 0 0
O
0 0 0 0
O
0 0 0 0
O
0 0 0 0
O
0 0 0 0
O
0 0 0 0
O
0 0 0 0
O
0 0 0 0
O
0 0 0 0
O
0 0 0 0
O
0 0 0 0
O
0 0 0 0
O
0 0 0 0
O
0 0 0 0
O
0 0 0 0
O
0 0 0 0
O
0 0 0 0
•
FLOW
SIDE
FRONT
C. Second Modified
FIG. 1 BURNER RUNNER CONFIGURATIONS
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was believed that this scalloping was contributing to the
observed combustion performance degradation.
service, larger thermal loads than those used for evaluation
purposes were observed in the field. A quick re-evaluation of
all key components was made for the new thermal load. It was
found that the margins were reduced significantly. The initial
performance has been very satisfactory. However, due to the
small safety margins and uncertainty in the thermal loading,
contingency plans were prepared to further refine the
geometry.
MODIFIED DESIGN FAILURE ANALYSIS
Two major configuration changes from the original design
were I) increasing the height of the wing plates and 2) the
addition of horizontal extensions at the two ends of the burner
runner wing plates (see Figure 1). Extending the wing plates
increased the bending stress oh the wing plates due to exhaust
gas flow pressure loading. The horizontal extensions
effectively eliminated the ability of the wing plate to scallop,
and hence, to resist creep deformations about the horizontal
axis perpendicular to the exhaust gas flow direction as shown
in Figure 2.
SYSTEM DESCRIPTION
Cogeneration Facility
The plant provides a reliable source of steam and electricity
to an adjacent refinery and electricity to the local utility. It
consists of four independent Gas Turbine Generators (GTG's)
exhausting to four Heat Recovery Steam Generators (HRSG's)
with supplementary duct firing. Two, dual extraction,
condensing Steam Turbines Generators (STG's) balance the
steam system.
Other major equipment includes a cooling tower, a water
treating system, a refinery waste gas treatment and
compression system and a natural gas compression system, and
a 230kV Gas Insulated Switchgear substation. Duct firing
burners are provided which can increase the HRSG inlet
temperature up to 230 degrees Celsius to help meet steam
demands. The duct firing rate cycles on a daily basis.
Burner Runners
After the combusted gases exit the turbine exhaust, they enter
the Heat Recovery Steam Generation (HRSG) unit. Initially,
the exhaust gases are heated via burner runners. Five rows of
burner runners traverse the square duct. Exhaust gases flow at
a rate of about twenty-three meters per second and at a
temperature of six hundred and fifty degrees Celsius. A side
view of the original burner runner is shown in Figure I. The
modified configuration and the second modified configuration
is also shown in Figure I. All three configurations contain a
four inch diameter pipe and burner tips which do not vary from
one design to the next. What changes is the wing plate and
gusset plate configuration. The wing and gusset plates are
made from 30955 for all three designs. Thermal and gas
pressure loading are in the significant creep region for all three
designs.
a. Original
ORIGINAL DESIGN PERFORMANCE
The original burner runner design performed sufficiently well
from a mechanical standpoint. An unexpected design feature
(which did not result in an undesirable response, but rather,
produced a very desirable one) was the ability of the wing
plates to "scallop". These deformations were noticeable as can
be seen in Figure 2, but were not catastrophic. Because the
burner runner wing plate curved about this particular axis, it
provided sufficient stiffness about the perpendicular axis so
that no additional deformations resulted. The observed
scallops were not a part of the original design, however, they
provided a very desirable response which gave this
configuration adequate mechanical capabilities. However, it
b. Modified
FIG. 2 CREEP RESPONSES
3
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Treating the wing plate as a cantilever with the fixed end at
the edge of the gusset, the bending stress was estimated from
the exhaust gas flow using the following formulas (Marks,
1978):
P= 1/2 CN pV2
cr.
BURNER RUNNER RAMS VS TIME FOR 23 /5 TF. GAS FLOW
101111
EAU
2bt2
The flow coefficient for the top wing plate was higher than
the bottom wing plate due to the orientation of the wing plate
to exhaust gas flow direction. A plastic stress distribution
coefficient was used since it was believed to better represent
the actual stress distribution in the failing parts (Mendelson,
1983). For our particular case, the variables produced a plate
pressure loading of 5.0. IcPa which resulted in a bending stress
of 12.4 MPa.
The temperature of the part at the high stress location was
estimated based upon the observed hot color of the wing plates
which ranged frdm red to orange at the hottest locations. A
review of the literature for steel color/temperature relationships
placed the hot temperature range between 820° C and 900° C.
Creep rate properties were obtained next for 30955
(International Nickel Company, 1963) and, assuming the failed
parts did so at a constant radius as shown in Figure 3, a
relationship was found for expected creep curvature as a
function of time. This relationship along with observed
conditions for all four operating units at various times is shown
in Figure 4. Good correlation was found between predicted
and observed conditions. This curve was used to assist in
operating decisions for the short term as well as to justify using
a design temperature of 900° C for the second modified design
analysis. In order to anticipate any additional mechanical
responses, a detailed evaluation of the new design was
conducted.
Eta
2
8213-C
900-C
.02
10
'WE (WEEKS)
100
FIG. 4 PREDICTED AND OBSERVED CREEP
DEFORMATION
SECOND MODIFIED DESIGN ANALYSIS
The proposed second modified design (Figure 1) was an
attempt to eliminate the undesirable large creep deformations
observed on the modified design without introducing any new
unwanted responses. The main difference between the second
modified design and the first modified design was the
extension of the gusset plate to the outer edge of the wing plate
(Figure 1). Although this design change should eliminate the
previously undesirable deformations, the burner runner wings
would still potentially be loaded in the significant creep region
and it was desired to know if any other gross deformation was
to be expected. Additionally, stresses were expected to vary
significantly from the previous design and other potential
failure mechanisms needed to be investigated. (Collins, 1981;
Boller, 1987)
Toward this end, a detailed finite element model (FEM) was
developed of the second modified design as shown in Figure 5.
A temperature distribution was estimated first for the wing and
gusset plates considering convective cooling from the
upstream turbine exhaust gases as well as convective and
radiative heating from the downstream combusting gases. Key
parameters were adjusted within rational limits to produce a
temperature distribution which matched previously observed
worst case field conditions and desired operating conditions.
Thermal stress ranges were computed using these temperature
distributions. Additionally, a stress distribution due to exhaust
gas flow pressure was determined.
Potential failure mechanisms identified for consideration
included creep deformation, creep rupture, low cycle thermal
fatigue and wear. Of particular concern was the weld
connecting the wing and gusset plates; (Comm, 1989). A
failure of this weld would result in catastrophic failures similar
to those previously observed.
A linear elastic analysis was performed for the burner runner
model only. Where stresses exceeded yield at temperature, a
strain energy technique was used to estimate plastic strains.
FIG. 3 ASSUMED CREEP FAILURE DEFORMATION
PROGRESSION
4
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• tel•Sz•C_
a11 in a
21W/it:OK:AS
ITP• mum
•USli
Plea M/10411•00CL
FIG. 5 SECOND MODIFIED DESIGN FINITE ELEMENT
MODEL
FIG. 6 SECOND MODIFIED DESIGN CREEP RESPONSE
CLOSING REMARKS
Maximum pressure induced stresses were reduced bian order
of magnitude providing a significant improvement in safety
margin, with two exceptions. The tab connecting the two wing
plates at its two outer edges was a high stress region. A safety
factor less than two was computed at this location and it was
felt that some of these items may fail during operation.
Therefore, they were removed from the model and another
analysis performed. There did not appear to be any
detrimental effect with the removal of the tabs. The second
potential concern item was the contact between the gusset and
four inch diameter pipe that is not welded. This location was
identified as a possible wear site. A connection was made in
the model between the gusset and pipe to simulate a weld and
reanalyzed. Again, significant safety factors were computed
for critical components. However, this repair was not intended
to be implemented before the parts were placed in service. It
was decided to first inspect this region carefully after some
operation to see if wear was occurring.
Although there is a certain satisfaction in identifying and
quantifying key parameters in relation to a failure analysis, it is
much preferred not to have to go through these efforts. It is
the desire of the authors that the industry benefit in some small
way by being aware of this experience. If further details of our
presentation would be useful, we would encourage inquirers to
contact the authors.
REFERENCES
Eighth
Edition, McGraw-Hill Book Company, 1978.
Mechanical and Physical Properties of the Austenistic Cr-Ni
S
"S V
•
The International
Nickel Company, 1963.
Timoshenko, S.P. and Goodier, J.N., Theory of Elasticity,
Third Edition, McGraw-Hill Book Company, 1970....
Mendelson, Plasticity: Theory and Application, RE. Krieger
Publishing Company, 1983.
I1
1:1
Collins IA.
: ft
'0 John
Wiley and Sons, 1981.
Boller, C. and Seeger, T., Matcrialapaillii2LZfdiclaadiags
Materials Science Monography 42C, Elsevier, 1987.
Corum, J.M., "Evaluation of Weldment Creep and Fatigue
Strength Reduction Factors for Elevated Temperature Design",
ASME PVP Volume 163, 1989.
RECENT OPERATIONAL HISTORY
The initial performance of the second modified design was
satisfactory. Creep deformations have occurred in the manner
anticipated, although somewhat larger than expected. Some of
the tabs failed indicating that the actual thermal loads have
exceeded those used during the evaluation (which is consistent
with the observed global creep). Wear has not been observed.
The welds between the gusset and wing plates have all
performed very satisfactorily. Minor design modifications
were subsequently made to the second modified design to
further enhance its creep response. The performance has been
satisfactory thus far analysis and remediation as can be seen in
Figure 6.
5
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