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Application of NDT Methods as Tools for Successful Reconstruction of
Sickle plate at Srisailam Left Bank Hydro Electric Project
Rama Dasu Pittala
M/s Sun-Mann Engineers & Consultants, Secunderabad-500011, India
Email : [email protected]
Keywords: Sickle Plate, Bifurcation Joint, NDT Methods, Weld Sequence, Process Control.
Abstract : For the first time in the world, Sickle Plate reconstruction work was carried out
successfully in the Penstock piping at bifurcation Joint 2 of SLBHES- Srisailam, during
May-July 2013. The Original Sickle Plate failed during 2008 due to various factors like
Design inadequacy, Operational Imbalance, Geometric / Metallurgical Stress raisers at the
inner curve of Sickle Plate, lack of side wall fusion in horizontal weld joints etc. Weld
sequence has been prepared to minimize the internal stresses due to welding. Weld Procedure
and Welders were qualified as per applicable Codes.
NDT Methods MT,UT, RT,PT and Visual have been used at various stages including Final
Inspection to ensure the integrity of construction as per Specifications.
Detailed Ultrasonic Examination and Magnetic Particle Examination of remaining portion of
failed sickle plate was carried out , prior to commencement of reconstruction.
Plates used for preparing sickle plate segments were tested for presence of laminations before
cutting. Gas-cut and ground edges were tested by Magnetic Particles for ensuring freedom
from open to surface / sub surface flaws.
During welding in addition to strict process control ( including preheat, current, speed,
cleanliness at each pass, start stop points control, post heating temperature control etc), visual
and dry magnetic particle testing carried out to ensure freedom from flaws.
Final NDT carried out using NDT methods RT,MT,UT, VT and ensured that the newly
constructed sickle plate is free from un acceptable flaws. The newly Constructed plate has
been put in to service which has enabled the Units to generate electricity to the 100% of the
rated capacity.
Introduction :
Bifurcation Joints are required to divide flow in a penstock to feed multiple units. Plan,
Elevation and Isometric view showing typical bifurcations is shown in Fig.1 At the
bifurcations Internal sickle plates and external yokes are constructed to strengthen the
structural members and counter the unbalanced forces created by the flow. Location and
construction details of Sickle Plate are given in Figs. 2 & 3..
Fig.1[2] Plan, Elevation and Isometric View of Bifurcations
Fig.2 [2] Location & Construction details of Sickle Plate - Plan
Fig.3 [2] Location & Construction details of Sickle Plate - Elevation
The Failure :
On 11th
Oct 2008 around 6.00Am, heavy vibrations were observed in Unit 2 during
service. Draft tube was emptied and on 19th
Nov 2008 and when examined inside, part of
Sickle-plate at bifurcation between Units 2 & 3 was found stuck-up in the spiral casing of
Unit 2 ( ref.Fig.4). Another broken part was found in the Penstock at Pressure shaft 1 sump .
Remaining portion of sickle plate at the bottom position is shown in Fig.5. Schematic
diagram showing fracture profile is given in Fig.6
APGENCO constituted expert committee comprising experts from CWC, IIT-Delhi, L&T +
MWH – USA, APGENCO, NML, CEA, WRI-BHEL Trichy, FCRI-Palghat to look in the
problem, examine the causes of failure of Sickle Plates and suggest remedial / rectification
measures.
Remedial measures: As suggested by the expert committee, the fractured surface of
remaining portion of buried sickle plate was smoothened by Cutting / Grinding and weld
metal deposition. The Units 2 & 3 were permitted to operate in Generation mode at a
Geometric Stress raiser at Inner Curve
ACEGI : Part of Sickle Plate found
in Spiral casing of Unit 2
BDFH : Part of Sickle Plate found
in Pressure Shaft 1 Sump
Fig.4[1] – Part of Sickle Plate found in Spiral
casing of Unit 2
Fig.5[1] –Remaining Part of Sickle Plate
at the bottom position.
Fig.6 . Failed Sickle Plate at Bifurcation between Units 2 & 3
capacity not exceeding 75% with a condition both the units must be in operation. If one unit
stops, the other unit also must be stopped.
Root cause of failure: The committee could not come to any definite conclusion regarding
the root cause of failure. Design inadequacy, Operational Imbalance, Geometric /
Metallurgical Stress raisers at the inner curve of Sickle Plate, presence of Lack of Side wall
fusion of Longitudinal Weld etc. might have contributed for the failure. However in the
opinion of the Author the most probable failure scenario may be as follows.
A) Due to presence of Geometric and metallurgical stress concentrations in the inner
curve, crack initiation has occurred in the horizontal weld joint, which has propagated
till the outer curve, aided by the presence of lack of side wall fusion in the
longitudinal weld [1] and differential pressures on the both the surfaces of sickle
plate [2].
B) Subsequently due to the differential pressures on both the surfaces the sickle plate
segments might have been subjected to repetitive bending cycles resulting in the
separation by fracture of parent metal due to fatigue.
New Design : Based on various studies and deliberations by the expert committee, M/s
MWH-USA has given a new design of sickle plate as per Fig. 7. Free End patch-plates, Patch
plates to join Sickle Plate to Ferrule were included in the new design to meet the design
requirements. The Author attended 19th
and 20th
Expert committee meetings on behalf of
APGENCo and proposed location of weld joints and sequence of welding. The weld joint
locations were chosen in such a way that there is minimum amount of welding and the Hot
rolled plates are used in proper direction with respect to the direction of rolling, and for the
patch plates there is overlap between similar welds located in the opposite faces (ref.Fig.8).
The Weld sequence was prepared to minimize residual stresses and distortion (ref.Fig.9).
Fig.7 – New Design of Sickle Plate Provided by M/s MHW - USA
Fig.8 [3] –Location of Weld Joints, Shown in Red-Vertical and Blue
Fig.9 [3]– Weld Sequence, Shown in Roman Numerals
Methodology:
Detailed Methodology has been prepared and implemented to ensure that the
Reconstruction of Sickle plate confirms to the project technical specifications / codes (
Ref. Diagram-10).
DIAGRAM.10 METHODOLOGY
Process Control: Welding Process was controlled and monitored by deploying Welding
Inspectors on 24 X 7 basis during the project execution. Ensuring deployment of Qualified
Welders for applicable Welding Position, Electrodes Control (including storage, baking,
handling and usage), Preheat, Welding Current, Electrode Travel Speed, bead width, inter-
Approval of Drawings as per MWH
recommendations, Qualification of Welders
and WPS, preparation of Final WPS,PQR and
WPQ, Approval of NDT procedures, Weld
sequence
Fit-up of part Nos 1 & 2 and welding and
post heating as per Qualified WPS /
sequence
Fit-up of part Nos 3 & 4 and welding and
post heating as per Qualified WPS /
sequence
Process Control including preheating and
post heating, In-process visual , NDT and
distortion measurement
Final visual, UT,MT &RT of new Sickle plate
welded joints, before patch-plates fit-up
UT & MT of plates to ensure freedom from
laminations at the cut and ground edges
Tracing of profile by templates for
preparation of patch plates components
UT & MT of plates to ensure freedom
from laminations at the cut and ground
edges of patch plate components
Verification of Material TCs. Electrode
TCs, Tracing of profile at the remaining
portion of failed sickle plate using
templates and marking for cutting
UT and MT on remaining portion of failed
Sickle plate at bifurcation 2.
Process Control including preheating and
post heating, In-process visual , NDT and
distortion measurement
Repair of cracks appeared on ferrule weld
joints, and ferrule to old sickle plate weld
joints, as per MWH reviewed repair
procedure
Qualification of modified WPS-4,
preparation of PQR for welding of patch-
plates
Fit-up of patch plate components, welding
and post heating as per modified /
Qualified WPS / weld sequence
Process Control including preheating and
post heating, In-process visual and NDT
Final visual, UT,MT of patch plate welded joints before painting
pass cleaning, MT/Visual examination for each layer of welding, Post heating, Humidity data
logging etc. are some of the important parameters that were monitored to ensure compliance
to the approved and Qualified WPS.
NDT Scheme followed :
1) Prior to welding, MT & UT of remaining portion of sickle plate and weld joints
performed as per approved NDT procedures.(ref. Figs .11 & 12). For MT wet visible
particles with white contrast were used in combination with Half-wave DC Yoke. For UT,
4MHz 20 Dia TR probe, 4 MHz 45° 8X9 size, and 4 MHz 60° 8X9 size probes were used.
Calibration blocks used for UT was made as per ASME Sec V Article 4, Fig.T-434.2.1 Non
Piping Calibration Block. The findings indicated that there are no un-acceptable flaws.
2) The Hot rolled plates of ASTM 517 Gr F material was tested and certified by steel
mill to SA 578 Level C, which permits lamination up to 25mm dia. Hence to rule out the
possibility of any lamination opening on the weld edge, 100mm wide scanning performed
covering both sides of cutting plane, after marking. 6mm FBH was used as reflector for
calibration, and 4MHz 25mm dia TR probe was used for scanning.
3) During welding after each layer and back grinding also MT was performed using Dry
Visible Particles in combination with Half-wave DC Yoke. Relevant Indications found
were removed before next layer is welded.
4) After 72 Hrs of completion of welding and post heating, final NDT (VT,MT UT
&RT) performed on new sickle plate weld joints. For MT wet visible particles with white
contrast were used in combination with Half-wave DC Yoke. For UT, 4MHz 20 Dia TR
probe, 4 MHz 45° 8X9 size, and 4 MHz 60° 8X9 size probes were used. Calibration
blocks used for UT was made as per ASME Sec V Article 4, Fig.T-434.2.1. Radiography
performed using Ir 192 source with single wall single image composite film viewing
technique. In UT and RT, one un-acceptable linear indication of about 6mm long was
found in Vertical weld ii at around 850mm from bottom. The same was repaired as per
qualified WPS and repeat NDT (MT,UT & RT) performed to ensure soundness of the
weld.( ref.Fig.13). UT being performed on Vertical weld V is shown in Fig.13
5) During MT, cracks were found in the weld joint between remaining old sickle plate and
ferrule, and also in weld metal of ferrule to ferrule near T joints. Apparently these cracks
appeared due to differential expansion of remaining old sickle plate and ferrule during
Fig.11- MT of Remaining Sickle Plate
before welding new Sickle Plate.
Fig.12- UT of Remaining Sickle Plate /
adjacent Old Weld Joints.
preheating / post heating. All the cracks were repaired as per MWH reviewed procedure
and NDT (MT / PT ) performed to ensure soundness.
6) Subsequently Welding of Free End patch plates and Sickle plate to Ferrule patch plates
carried out as per approved and qualified WPS. In process MT carried out as indicated in
Point-1 and ensured soundness of each layer.
7) After 72 Hrs of completion of welding and post heating, final NDT(VT,MT&PT)
performed on the new sickle plate to patch-plate weld joints. For MT wet visible particles
with white contrast were used in combination with Half-wave DC Yoke. For UT, 4MHz
20 Dia TR probe, 4 MHz 45° 8X9 size, and 4 MHz 60° 8X9 size probes were used.
Calibration blocks used for UT was made as per ASME Sec V Article 4, Fig.T-434.2.1.
No flaws were found. Weld Visual Test confirmed that the weld profile / size is
satisfactory.
Conclusions: Using NDT Methods as tools at various stages in combination with strict
Process Control during welding, reconstruction of Sickle Plate at Bifurcation Joint No.2 of
SLBHES – Srisailam has been completed meeting the Quality Requirements of the Project.
The Units have been put in to service at 100% of the rated capacity since August 2013. For
the first time in the world such a difficult and complex job of constructing Sickle Plate in situ
has been completed successfully.
Balanced operation of Units and Periodical Condition assessment during off season has been
recommended to maintain the structural integrity of the new sickle plate.
Acknowledgements: The Author acknowledge the useful discussions held with Expert
Committee Members and support of M/s APGENCO in successful completion of the project.
References [1] S.R.Singh, National Metallurgical Laboratory-Jamshedpur, Failure Analysis of Penstock Liner Sickle Plate
of SLBHES-Srisailam, report No.NML/MST/APGENCO/2009
[2] J.Chandrasekhar Iyer, Rakesh Toteja, CWC-New Delhi, Power point Presentation on Engineering Failure
Analysis to find out the Cause and Evolve a Solution - A Case of Penstock Bifurcation Sickle Plate Failure at
SLBHE- Srisailam , 2013.
[3] Rama Dasu Pittala, Sreedhar Mallipudi, Sun-Mann Engineers & Consultants,-Secunderabad, India, Report
on Reconstruction of Sickle Plate at Bifurcation Joint No.2 at SLBHES-Srisailam, 2013.
[4] ASME Boiler and Pressure Vessel Code – Section V & VIII Div-1, 2011a
[5] AWS D 1.1-2010, Structural Welding Code-Steel.
[6] Dr.Kohsuke Horikawa- Osaka University, Nomozu Watanabe- NPSICL/MHIL, Japan , Application of High
Tensile Strength Steels for Hydro Power Plants - 2009
Fig.13: UT of Vertical weld joint
of New Sickle Plate.
