FIELD OF INVENTION
The present invention relates to a process to prevent droplet impact
erosion of Low Pressure Steam Turbine blade with adhesion of hydrophobic coating
after grit blasting. More particularly the invention relates to an addition of composite
epoxy mixture coating after through grit blasting of Low pressure steam turbine
(LPST) blade to provide the blade resistance against droplet impact erosion.
BACKGROUND OF INVENTION
In steam turbine, the pressurized steam is typically drawn-out along
several stages of the turbines as high pressure (HP), intermediate pressure (IP) and
low pressure (LP) stages of the turbines. In the axial turbines of LPST there are
radial arrangement of fixed stator vanes alternatively with radially arranged rotating
blades. In the LP stage, the steam is expanded and cooled close to the condensation
point usually results in wet steam and drained into the condenser. In a steam
turbine, the rotational energy of stator and rotor blades is exposed to high-
temperature and high-pressure steam supplied from a boiler. At the last stage of
LPST steam get wet or condensed which result in droplet impact erosion and pit of
base materials causing cracks on the blades. Flame hardening is the process that is
currently being adopted for the sub-critical LPST blades. Laser hardening method for
X5 17-4 PH steel has been recommended to overcome droplet erosion.

As described in the United States patent number 6623241, the mass
content of the condensed water in the wet steam can be around 14%. The
manifestation of liquid phase in the rotating and stationary elements of the turbine
may result in amplified dissipative losses [1]. In a LP turbine, about 12-14% of the
mass steam can be generated in the form of water. This condensation loss results
in efficiency loss of the LP turbine around 6-7%, which corresponds to a loss of 1-
2% efficiency of entire steam power plant. The extent of loss depends on condensed
water drops size. [2]. Generally, water drops contained in the steam phase may be
in the range of micrometers. As per state-of-the-art, these drops do not coalesce
into larger drops as long as they keep floating or flowing in the steam and have no
adverse effect on either the operation or on the performance of the turbine [3].
Vapor, flowing along with the steam through the guide blades and rotating blades
grow in size applies an impulse onto the blades. Small condensed droplets spread
on the blade surface and form condensed film that flows on the guide blades over
the concave or convex surfaces, subject to the effect of shearing forces of the steam
[4].
The centrifugal force induces larger drops to roll outward of the
rotating blades, towards the turbine housing. This phenomenon upsurges with the
size and mass thus the centrifugal force of the droplets rises [5]. In addition,
accumulation of water inside casing of LPST results in dissipative friction losses on

the rotating vane tips and vane covers. According to the United States patent
number 6623241 B2, drop diameter enlarges in the range of 100-200 microns and
speed in the range of more than 250 m/s, is the reason of erosion due to the impact
of the drops in blades. Under constant bombardment of the condensed water steam,
blade material erodes much faster subjected to airfoil defect. In the United States
patent number 6623241, it is revealed that the hydrophobic coating includes
amorphous carbon or a plasma polymer having thickness between 0.1-8
micrometers and hardness between 500 and 1500 Vickers [1]. In this patent top
coat has thickness 0.1-8 micrometers. In between soft gradient layer and hard
gradient layer is given. Each discrete layers is 0.1-2 micrometers. In patent number
6623241 nanoparticle used 0.1-8 micrometers thickness coating has been applied.
In between soft gradient layer and hard gradient layer, a discrete layer of 0.1-2
micrometers has been applied which might have worked as adhesive. In 6623241
patent both back and top hydrophobic coat was applied where top coat has
thickness. Which make the patent difficult for application and might have increased
the price of coating.
But in present invention the grit blast of base samples has ensured
good adhesion between coating of nano-particles and epoxy. This has reduced the
price of application. It can be applied very simply on the blade materials by dipping
the blade perpendicularly in the uniform viscous liquid of epoxy and nanomaterial.

Getting the coating dried the blade material can be made ready for application.
Thickness of our application is 0.1-0.6 microns.
Attempts have been made to provide blades of turbines with a coating
that reduces erosion thus extending their self-life. There are various coatings for
the blades of a ST that consists of a hard, wear-resistant material on a substrate.
In the United States published patent application number 2009/0298369 A1,
coatings and methods are described including the use of hydrophobic particles
incorporated into a polymeric coating material wherein the coating servers as a
matrix to bind the particles to an underlying substrate. In this vision an effort is
made in the field of hard-wearing coatings for turbine parts [6].
Compared to patent 2009/0298369 A1 we combined Al2O3 (40 nm)
into an epoxy (Fevitite) by stirring continually with glass rod to get uniform viscous
solution which can easily be applied by anyone on the blade material. And binding
between base and coating is ensured by using grit blasted blade materials. Here
coating material is not wasted to provide extra matrix layer to bind the coating with
underlying base. This has naturally reduced the coast of coating application.
It can be seen as one object of the present invention to provide epoxy
composite coating for large parts of steam turbine particularly for the blades of low
pressure turbine or stage.

OBJECTS OF THE INVENTION
Therefore, it is an object of the invention to propose a process to
prevent droplet impact erosion of Low Pressure Steam Turbine blade with adhesion
of hydrophobic coating after grit blasting, which is capable of guarding the blade
from droplet impact erosion.
Another object of the invention is to propose a process to prevent
droplet impact erosion of Low Pressure Steam Turbine blade with adhesion of
hydrophobic coating after grit blasting which includes a method of preparing nano-
composite hydrophobic coating composition.
A still another object of the invention is to propose a process to
prevent droplet impact erosion of Low Pressure Steam Turbine blade with adhesion
of hydrophobic coating after grit blasting which is cable to achieve a surface
roughness of around 7-8 micrometers on the blade material to provide a better
adhesion of coat on the blade.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig.1a - Shows Pristine blade material before coating.
Fig 1b - Shows surface wettability characterization on composite coated on
grit-blasted blade samples coated with displaying wettability.

As described in the diagram WCA is the angle which a liquid makes
with solid and air. Here we measured WCA which is 89o proving
hydrophobicity with the coating. In this Figure 1 we are able to observe
the base material when uncoated has hydrophilicity of 28o after
application of coating WCA increased to 89o.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
According to the invention, the turbine blades are grit-blasted. In this
invention it was observed roughness (7-8 micron) attained by grit-blasting has
created good adhesion of films with base material.
Grit-blasting is a cold working process generally used to clean surfaces
and remove corrosion. The stream of high-speed harsh particles hits the surface,
removes oxides. An increased roughness results in an increased surface area thus,
increased nominal or apparent surface energy. It is usually used for the local
flexible distortions in a fast, easy and durable method. Powder of 2 grit size is
used for this purpose which provided the roughness in micron range. Grit-blasted
samples display large roughness with 7.8 µm as rms roughness value. Adherence
of coating increases for which we have coated with single layer nano-composite.

The present invention is about making homogeneous mixture of nano-
composite and polymer with proper viscosity. It is moderately hydrophobic coating
(water contact angle ≈ 85°) by simple dip coating to provide erosion prevention
against droplet impact on LPST blades. A sacrificial layer of the nano-composite
epoxy is provided on the blades to substantially cover the base to denounce surface
erosion resistance and for substantially restoring the failure of overall steam turbine
constituent. Consequently, the nano-composite coating preserves the underlying
substrate of the turbine components from being pitted off.
The sample is immersed in the reservoir mixture of commercially
available epoxy “Fevitite Rapid” (Pidilite Industries Limited) is used as base material.
1 wt. % of alumina (Al2O3) nanoparticles (40nm mean diameter) are mixed in epoxy
thoroughly with a glass rod. They are then dried in air/O2 plasma to remove any
leftover organic contaminant. The cleaned grit-blasted turbine blades are dip coated
in the epoxy nano-composite mixture with 10 mm/s dipping speed, 10 second
dipping time and 10 mm/s dragging out speed. The dipped samples were left
hanging for 30 seconds for gravity drainage. Subsequently it is left for gravity
drainage above the reservoir only. Once the nano-composite coating become
completely bubble free, it is left for drying in ambient condition for overnight. Upon
complete drying, the coated blades showed contact angle of 89 degrees.

WE CLAIM
1. A process to prevent droplet impact erosion of Low Pressure Steam Turbine blade
with adhesion of hydrophobic coating after grit blasting, the said process
consisting of the following steps;
preparing a rough surface on turbine blades by grit-blasting to achieve better
adhesion of films of coating on the said blades;
preparing a nano-composite hydrophobic coating of hydrophobic epoxy matrix
and alumina nano-particles having mean diameter of 40 nm;
cleaning the said blade;
dipping the cleaned grit-blasted turbine blades in the epoxy nano-composite
mixture kept in a reservoir with 10 mm/s dipping speed, 10 second dipping time
and 10 mm/s dragging out speed wherein the blades are then taken out of the
reservoir and left hanging for 30 seconds for gravity drainage above the reservoir
till the nano-composite coating becomes completely bubble free when the blades
are left for drying in ambient condition for overnight, wherein upon complete
drying the coated blades show water contact angle of 89 degrees.
2. The process as claimed in claim 1, wherein 1 weight % of alumina (Al2O3) nano-
particles are mixed in epoxy thoroughly with a glass rod to prepare the coating.

3. The process as claimed in claim 1, wherein a surface roughness of 7.8 µm as rms
value is achieved after grit-blasting.