POWER September 2021 - 19

FOCUS ON O&M
intensive scheme, increasing the cost of
operating a geothermal plant due to the
higher cost of carrying excess inventory.
This cost, though, can be reduced by
improvements to the plant's equipment,
including
redesigning
internals
within
the existing housing, and without any
civil modification. There are many benefits
to the redesign of internal components,
including:
■ Improvement in efficiency.
■ Reduced operational expenditure.
■ No change in the civil foundation.
■ Existing turbine housing being reused.
■ Life extension of asset.
■ Increase in span between overhauls.
■ Reduced inventory.
Case Study
A case study of the redesign of internal
equipment for a geothermal plant provides
examples of the benefits. A major
customer in the geothermal sector was
facing issues of erosion and corrosion,
which was leading to below-par performance
of the turbine. The turbine, a direct-drive
type, was designed for output
of 17 MW, with single-cylinder, singleflow
backpressure.
erations were carried out and design
changes were made during the manufacturing
of the new rotor. The existing rotor,
which had a riveted shroud design, could
not be de-bladed. That was because
the blades would have to be replaced
once de-blading activity was completed,
which would have resulted in extended
downtime for the customer. A de-blading
would have enabled Triveni's designers
to understand the existing root design.
However, the specialists understood the
customer's requirement and captured
only minimal information including the
outside geometry of the rotor and inside
envelope of the casing.
With the design team's immense experience
of working on multiple, similar
situations, and based on a study of various
root designs, the rotor was redesigned
with new and upgraded roots.
The rotor data that was captured had
to be 3D-modeled to conduct a preliminary
design review to select the suitable
root designs. Upon completion,
a detailed engineering evaluation was
carried out by a thermo-structural study,
with a goal of limiting stresses within
the allowable value. This was followed
by frequency analysis to understand the
Current technologies in use at geothermal
facilities include dry steam power stations,
flash steam power stations, and binary
cycle power stations, along with enhanced
geothermal systems.
The operator did its due diligence,
looking at possible options to source the
rotor, prior to partnering with India-based
Triveni Turbines to remanufacture the
rotors. Triveni is an original equipment
manufacturer, and among the world's
largest makers of steam turbines for
power generation.
The customer listed three major challenges
it faced at its geothermal operation.
They were:
■ Frequent erosion in the riveted tenon
of the blades.
■ Formation of cavity by erosions at
high-pressure gland areas due to impurities
in the steam.
■ A need to improve the rotor material
to extend the life of the rotor.
Based on the inputs, a number of itSeptember
2021 | POWER
www.powermag.com
dynamic stress effect on the blades. A
careful review was also done to ensure
the locking blade integrity with the new
design was sustained.
Additionally, several design modifications
were carried out. The existing riveted
shroud design was causing erosion
due to the geothermal environment. The
existing design was modified to a new,
improved design, to remove the possibility
of erosion at the riveted area.
The new rotor (Figure 3) was upgraded
to a material with higher resistance to
corrosion and erosion, allowing for better
creep resistance. To ensure that the
material of the rotor did not impact the
bearings or any other components, Triveni
carried out a coating procedure on the
journal area. This significantly improves
the life of rotors.
3. This image shows a bladed rotor ready for
packing after high-speed balancing and application
of a preservative. Upgraded materials
and coating procedures can ensure higher
resistance to corrosion and erosion, significantly
improving the life of the rotor. A turbine
redesign also can help reduce stress on the
surface and other areas of the rotor. Courtesy:
Trevini Turbines
Redesign to Reduce Stress
The redesign included a special process
to reduce the stress induced on the surface
of the rotor disc, blade root, and
transition fillet to increase fatigue life.
This action induces residual compressive
stress on the surface, which delays
the appearance of cracks and enhances
the lifecycle.
These value-added additions were
done to ensure the customer would receive
an extended lifecycle for the turbine,
while also improving the availability
and efficiency of the plant. The rotor was
subjected to high-speed balancing, a
process by which the mass distribution
about the spin axis is checked and corrected
to ensure the bearing forces and
vibrations of the shaft corresponding to
the frequency are within specified limits.
This process eliminates the vibrations
caused by imbalance and helps ensure
the safety of rotating equipment.
The new rotor was swapped with the
old one and commissioned with vibration
in an acceptable range on the first run.
This high level of performance allowed
the customer to immediately commence
commercial operations.
-Arun Mote is executive director and
CEO at Triveni Turbines.
Extending the Strength and
Life of Tie Rod Bolts
Turbines are the backbone of power generation,
so maintaining top performance
of all their moving parts is important for
continued, efficient operation. Tie rod
bolts that connect gas turbine stages
are susceptible to fretting fatigue at the
edge of the contact region between the
turbine rotors and the tie rod landings.
19
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POWER September 2021

Table of Contents for the Digital Edition of POWER September 2021

Contents
POWER September 2021 - Intro
POWER September 2021 - Cover1
POWER September 2021 - Cover2
POWER September 2021 - Contents
POWER September 2021 - 2
POWER September 2021 - 3
POWER September 2021 - 4
POWER September 2021 - 5
POWER September 2021 - 6
POWER September 2021 - 7
POWER September 2021 - 8
POWER September 2021 - 9
POWER September 2021 - 10
POWER September 2021 - 11
POWER September 2021 - 12
POWER September 2021 - 13
POWER September 2021 - 14
POWER September 2021 - 15
POWER September 2021 - 16
POWER September 2021 - 17
POWER September 2021 - 18
POWER September 2021 - 19
POWER September 2021 - 20
POWER September 2021 - 21
POWER September 2021 - 22
POWER September 2021 - 23
POWER September 2021 - 24
POWER September 2021 - 25
POWER September 2021 - 26
POWER September 2021 - 27
POWER September 2021 - 28
POWER September 2021 - 29
POWER September 2021 - 30
POWER September 2021 - 31
POWER September 2021 - 32
POWER September 2021 - 33
POWER September 2021 - 34
POWER September 2021 - 35
POWER September 2021 - 36
POWER September 2021 - 37
POWER September 2021 - 38
POWER September 2021 - 39
POWER September 2021 - 40
POWER September 2021 - Cover3
POWER September 2021 - Cover4
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