FIELD OF INVENTION:
This invention relates to a method of erosion resistance coating of polyurethane
added with titanium carbide powder and coated parts thereof for hydro turbine
components.
BACKGROUND OF THE INVENTION
The hydro turbine components are prone to silt erosion. These hydro turbine
components which are worn out due to silt erosion are coated by HVOF (high
velocity oxy fluel), spray process with hard carbide based powder. The hardness
of the coating is 1150 to 1200 HV. The coating is highly dense compare to other
thermal spray techniques like plasma spray. In applications, where abrasive or
erosive wear resistance is of primary importance, WC-CO (tungsten carbide with
cobalt) with or without nickel or chrome is used. WC-CO-Cr (tungsten carbide
with cobalt and chromium) powders are preferred when high corrosion resistance
is needed. The abrasive and erosive wear resistance also depends upon oxides,
pores and the phase transformation occurring during spraying. High velocity oxy
fuel sprayed coatings are commonly applied by HP/HVOF JP-5000, DS-100, Met
jet II, Diamond jet and Praxair 2000 HVAF (High velocity air fuel) systems. These
systems are based on liquid as well as gaseous fuel and oxygen in air.

The limitation of HVOF coating process are the process complications and
economics. Due to its higher cost, many users do not apply HVOF coating
instead they change the whole component with a new one. In view of this there
is need for an economical coating which can replace or give similar result like
WC-Co-Cr based HVOF coating. It has been established that polyurethane
coating can be a suitable replacement of HVOF coating and at the same time it is
economical than HVOF coating. Polyurethane coating is economical due to its
lower material cost compared to HVOF coating where the expensive tungsten
carbide (WC) based powder is used. The finish of polyurethane coating is much
better than HVOF coating. The present invention has superior erosion resistance
compared to the conventional polyurethane coating by the addition of fine hard
powders of Titanium carbide (TiC).
Erosion of hydro turbine components such as spears, needles, guide vanes,
lowering top cover, labyrinth seals is a serious problem. This is mainly due to
presence of excessive silt in water during monsoon. The silt comprising of hard
quartz (hardness in Moh scale is 7) particles of various size strikes on the
exposed surface of the hydro components. This problem is particularly acute
during rainy season when excessive land slides on the river bed cause the
number of silt particles to increase in excess of 5000 ppm. This kind of erosion
on the components is called silt erosion.
Methods used to reduce the damage caused by silt erosion is either by reducing
the particle velocity, controlling their size and concentration or by using HVOF
cermet coatings. HVOF cermet coatings of hard carbide phase (WC) embedded in
ductile matrix consisting of WC-Co, WC-Co-Cr are being used commonly.

The major disadvantages of the HVOF coating are its cost and difficulties in
coating application. Attempts have been made to bring down the cost by using
plasma nitriding and boronising. These attempts have not been successful
compared to HVOF coating. Research on this has established that polyurethane
coating is an economical alternative to HVOF coating.
The present invention uses TIC (titanium carbide) having particle size range upto
5 micron in polyurethane coating.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a method of polyurethane
coating with addition of titanium carbide powder and coated parts thereof for
erosion resistance which eliminates the disadvantages of prior art.
Another object of the invention is to propose a method of polyurethane coating
with addition of titanium carbide powder and coated parts thereof for erosion
resistance to improve the erosion resistance of the coating.
A still another object of the invention is to propose a method of polyurethane
coating with addition of titanium carbide powder and coated parts thereof for
erosion resistance to reduce the cost of coating.

SUMMARY OF INVENTION
Accordingly there, is provided an erosion resistance coating, for
hydroturbine components, consisting of equal proportion of poly-
isocynate and a mixed slurry of polyol having 2.5% to 5% Titanium
Carbide(TiC) by weight and a method of coating comprising the
steps of: (i) cleaning the whole pipeline of coating equipment with
Methyl Ethyl Ketone to prevent solidification of chemicals so added
and choking of the said pipeline; (ii) an intermediate coat of organic
base red oxide primer used for better adhesion of coating; and (iii)
applying final coat of said erosion resistance coating.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig.1- show slit erosion resistance of various polyurethane (PU)
coating 1 hour duration. Volume loss is calculated after 1
hour.
Fig.2-show comparison of different coating like HVOF, PU-2 and
PU-0 coatings. Volume loss is calculated after 1 hour.

Table 1- shows details of experimental trials. PU stands for polyurethane.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
In the present invention, extreme mix system is used for polyurethane coating.
Coating equipment is very simple in nature and can be moved to any remote site
with the help of rubber wheel. It does not require any electric supply for its
operation because it works with the help of compressed air. Hence the
equipment is very useful in any of the remote site because it is portable. This
equipment contains two hoppers where polyisocynate and a mixed slurry of TIC
powder and polyol are independently poured in. A mixed slurry of TiC powder
and polyol is made in a mechanical mixer before pouring in the hopper. Fine
hard powders of titanium carbide are added into the polyol as per proportion
using a controlled electric stirrer. There are two high pressure pumps which draw
in these chemicals into a mixing chamber to form polyurethane matrix and from
there these chemicals are sucked to the spraying gun.
Before coating and after coating the whole pipeline of the coating equipment for
delivering the coating is cleaned with Methyl Ethyl Ketone (MEK) to prevent the
chemicals to solidify and choke the pipeline. Surface to be coated is grit blasted
using 12-16 mesh alumina grit powders for making the surface rough so that
mechanical bonding is good. Before applying the top coat of polyurethane an
intermediate layer of organic base primer is applied to a thickness of 50 to 100
microns for getting a better adhesion of the coating. Four set of samples are

prepared by adding different fractions of fine powders of TiC. For better mixing
of the powder the powder is added slowly so that the powder does not get
agglomerated. The coating thickness is maintained around 2000 microns. By this
process the coating thickness can be applied up to 2500 microns. The coating
experiment has been carried out with different fractions of fine powder of TiC as
shown in Table-1.

These polyurethane coated samples are checked for hardness by shore hardness
tester (A scale); for surface roughness by perthometer, for slurry erosion
resistance by slurry erosion testing. The Ra value of polyurethane coating is 1.15
microns which indicate that the surface of polyurethane coating is very smooth
compared to other thermal spray coatings like plasma spray (8 to 10 µm), HVOF
coatings (5 to 7 µm). The surface roughness data is very important as far as
hydroturbine applications is concerned, as smoother the surface, higher is the
efficiency of turbine.

Silt erosion is a major concern for hydroturbine applications. Accordingly the
samples are tested for different experiments. In every experiments, the volume
loss of coating is calculated from the weight loss. For calculating the volume loss,
density of polyurethane coating is taken as 1.05 gm/cc and for PL) coating with
TiC based powder is taken as 1.10 gm/cc. The experimental results are shown in
Fig.1 and Fig.2.
In hydroturbine, when a component gets eroded, it looses its shape and size
which finally reduces the efficiency of turbine. Hence maintaining a geometry is
very important. Shape and size are directly proportional to volume loss. Hence
experiment results as shown in fig-1 and fig.2 correlates erosion resistance with
volume loss.
The same silt erosion study is also conducted on base metal 13-4 (Cr-Ni)
stainless steel, WC-CO-Cr based high velocity oxy fuel (HVOF) coating and other
PL) coatings with different percentage of 2.5%, 5% and 7.5% by weight of TiC
powder added.The test result is shown in Fig.l and Fig.2. It indicates clearly that
PU-2 with 5% of TiC added has the best erosion resistance compared to other
PL) coatings. It can be also inferred from Fig-2 that PU-2 with 5% addition of TiC
with PL) has improved silt erosion resistance almost 10 times compared to 13Cr-4
Ni stainless steel, a commonly used material for hydro machinery.
As shown in Fig.1 and Fig.2
PU-O is conventional Polyurethane Coating.
PU-1 is Polyurethane coating modified with 2.5% by weight of titanium carbide
powder.

PU-2 is Polyurethane coating modified with 5.0% by weight of titanium carbide
powder.
PU-3 is Polyurethane coating modified with 7.5% by weight of titanium carbide
powder.
13-4 stainless steel is 13Cr-4ni stainless steel.

WE CLAIM
1. An erosion resistance coating for hydroturbine components comprising;
polyurethane matrix comprising polyisocynate, 5.0% by weight of fine
hard powders of titanium carbide and polyol and an intermediate layer of
organic base primer for applying before application of the said polyurethane coating for better adhesion of the said coating.
2. The coating as claimed in claim 1, wherein the thickness of the
intermediate layer of primer is upto 50-100 microns.
3. The coating as claimed in claim 1, wherein the thickness of final coat of
polyurethane mixed with titanium carbide is upto 2000 to 2500 microns.
4. The coating as claimed in claim 1, wherein the said coating is applied to
obtain a minimum surface finish of Ra 1.15 microns.
5. A method of erosion resistant coating of hydroturbine components
comprising the steps of:
subjecting the hydroturbine components to surface roughening by grit
blasting the said hydroturbine componets with 12-16 mesh alumina grit
powder.

cleaning the pipeline of the coating equipment for delivering the coating
with methyl ethyl ketone to prevent the chemicals to solidity and choe the
pipeline.
characterized in that polyol mixed with titanium carbide is
poured from one hopper of the equipment and polyisocynate from the
other to mix in the mixing chamber to form polyurethane matrix sucked
by the spraying gun to spray the said coating on hydroturbihe surface
when an intermediate layer of organic base primer is applied before the
application of the polyurethane coating for getting a better adhesion of
the said coating.

ABSTRACT

TITLE: AN EROSION RESISTANT COATING FOR HYDROTURBINE
COMPONENTS AND METHOD OF COATING
The present invention relates to an erosion resistance coating, for
hydroturbine components, consisting of equal proportion of poly-isocynate
and a mixed slurry of polyol having 2.5% to 5% Titanium Carbide(TiC) by
weight and a method of coating comprising the steps of: (i) cleaning the
whole pipeline of coating equipment with Methyl Ethyl Ketone to prevent
solidification of chemicals so added and choking of the said pipeline; (ii)
an intermediate coat of organic base red oxide primer used for better
adhesion of coating; and (iii) applying final coat of said erosion resistance
coating.