FIELD OF INVENTION
The invention relates to a method of polyurethane coating over HVOF coating for
combating erosion of hydro turbine components.
BACKGROUND OF THE INVENTION
Degradation of Indian hydro turbine components such as spears / needles, guide
vanes, lower ring, top cover, labyrinth seals, etc. is a serious problem. This is
mainly due to presence of excessive slit in water during monsoon. In hydro
turbines there are basically two types of erosion. They are slit and cavitation
erosion. The first i.e. the slit comprising hard quartz (hardness on Moh scale is
7) particles of size ranging from 10 to 200 microns strike on the exposed
surfaces of the hydro components. This problem is particularly acute during
rainy season when excessive land slides on to the riverbed cause the number of
slit particles to increase in excess of 5000 ppm. This kind of erosion on the
components is called slit erosion. The second type of erosion is cavitation. If
the pressure of a fluid drops below its vapour pressure due to an increase in its
velocity or otherwise, vapour bubbles tend to form. These bubbles grow their
size at low pressure and when they come in contact with regions of high
pressure, i.e. on the runner surface or draft tube, etc. they collapse and implode.
Over a period of time, small pits are developed on the surface of the component
exposed to repetitive collapse of the bubbles thereby causing erosion of the base
material. This phenomenon is called cavitations. Attempts are being made to
reduce the damage caused by slit erosion 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;
typically coating system consisting of WC-Co, WC-Co-Cr, WC-Ni-Cr and FeCrAlY-
Cr3C2 are being used in different industries. Tungsten carbide based powders
are widely used in the HVOF spraying system. These are used to produce dense,

high hardness and excellent wear resistance coatings generally to combat the
erosion and corrosion occurring in hydro power plants and pumps. Polyurethane
is anti scratch resistant coating used for abrasion resistant specially in the mining
industry for sieving the minerals of different sizes.
The hydro turbine components which are worn out due to slit and cavitation
erosion are coated by HVOF (high velocity oxy fuel) spray process with WC
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 with and without nickel or chrome used. WC-Co-Cr 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, OSU, Diamond jet and Praxair
2000 HVAF (High velocity air fuel) systems. These systems are based on liquid
as well as gaseous fuel and oxygen / air. HVOF coating are excellent for slit
erosion however lacks cavitations erosion resistance polyurethane coating are
excellent for cavitations erosion.
OBJECT OF THE INVENTION
It is therefore an object of the invention to propose a method of polyurethane
coating over HVOF coating for combating erosion of hydro turbine components
which is novel and much improved method for cavitations erosion.

Another object of the invention is to propose a method of polyurethane coating
over HVOF coating for combating erosion of hydro turbine components which
protects the hydro turbine from being worn out due to silt and cavitations
erosion.
A further object of the invention is to propose a method of polyurethane coating
over HVOF coating for combating erosion of hydro turbine components which is
able to make a hard and dense coating.
BRIEF DESCRIPTION OF THE INVENTION
This invention relates to a method a method of polyurethane coating over HVOF
coating for combating erosion of hydro turbine components. The method
comprising coating with a layer of high velocity oxy fuel of thickness 250 to 500
microns; then
an intermediate metallic coat of thickness 100 to 120 microns by twin wire are
spray techniques.
a thin layer of organic based paint over the intermediate coat of thickness 70 to
150 microns for better adhesion;
a final coat of polyurethane of thickness 1500 to 2500 microns is applied;
According to this invention there is provided a process for coating hydro turbine
comprising the steps of:
applying HVOF coating on the surface;
applying a intermediate coating by twin wire are spray technique;
applying a thin coat of organic based primer over the intermediate coat for better
adhesion.
then applying a coat of polyurethane.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
The invention can now be described in detail with the help of the figure of the
accompanying drawing in which
Figure 1 shows a flow chart of the method.

DETAILED DESCRIPTION OF THE INVENTION:
Power generation by hydro turbine Is one of the major source In Northern India.
These Hydro turbine operates under sited water condition. Due to high silt
content, the hydro turbine components are eroded severely. This erosion leads
to break down of the hydro power house and that lead to huge loss of electrical
power. In order to improve the performance of the hydro turbine components,
high velocity oxy fuel (HVOF) coating has been applied and it has given good
result. For further improvement, poryurethane coating has been applied over
HVOF coating. In this novel technique the surface roughness of HVOF coated
surface has been improved by wire are spray costing of Ni-AI wire of 100 to 120
microns thick.
A poryurethane coating is an organic coating made from the reaction of
Isocyanate-rich and polyol-rich compounds. Polymerization Is made possible by
using di-and poly-functlonal Isocyanates and poryols. There Is no byproduct from
the reaction between isocyanates and alcohols. After few hours the poryurethane
coating get cured whereas other polymer like epoxy takes few days to get cured.
This poryurethane coating is applied by airless high pressure spray gun. Before
applying the poryurethane coating, an organic based primer is applied on the
surface to get better adhesion. In order to apply the poryurethane, first the resin
and hardener are mixed thoroughly by an electrical stirrer in a pot. The mixture is
sucked by a hydraulic system and then sprayed by the airless spray gun.
Before applying the poryurethane coating, the surface is prepared by wire are
spray coating of Ni-AI wire. Several combination of wires (stainless steel wire,
ARMACOR wire) have been studied for the purpose of Improving the surface
roughness by a conventional wire are spray machine. Experiments were

conducted using different wires such as Ni-AI, stainless steel, ARMACORE wires
and their surface roughness values were recorded. The values are given in table-
1,2,3,4,5,6 & 7.

From the experiments it is recorded that excellent surface roughness has been
achieved by using Nickel aluminium wire approximate thickness 100-120
microns. The roughness values of nickel wire is more homogenous than other
wires and their roughness values are much more even than wire flame spray
techniques as wed as grit blasting techniques.
s

AMENDED CLAIM

WE CLAIM
1. A method of polyurethane coating over high velocity oxy fuel (HVOF) coating for
combating erosion of hydroturbine components comprising:
- applying a high velocity oxy fuel coating;
- roughening the HVOF coated surface by wire are spray coating of Ni-AI wire;
- applying an organic based primer on the surface to get better adhesion;
characterized in that,
the resin and hardener mixed thoroughly to prepare polyurethane wherein the
said mixture is sucked by a hydraulic system and sprayed over the surface for
coating.
2. A method as claimed in claim 1, wherein the polyurethane coating gets cured
after few hours.
3. A method as claimed in claim 1, wherein the polyurethane coating is applied by
airless high pressure spray gun.
4. A method as claimed in claim 1, wherein improved surface roughness achieved
by wire are spray of Ni-AI wire over HVOF coating is of 100 to 120 microns thick.
5. A method as claimed in claims 1 to 4, wherein the HVOF coating is upto a
thickness of 250 to 500 microns.

5. A method as claimed in claims 1 to 4, wherein the HVOF coating is upto a
thickness of 250 to 500 microns.
6. A method as claimed in claims 1 to 5, wherein the thin layer of organic
based paint applied is of thickness 70 to 150 microns.
7. A method as claimed in claim 1, wherein the polyurethane coat comprises
of poly-isocyanate and polyol.
8. A method as claimed in claim 1, wherein the polyurethane coating applied
is of thickness 1500 to 2500 microns.
9. A method as claimed in claim 1, wherein the resin and hardener are mixed
thoroughly by an electric stirrer in a pot.
10. A method as claimed in claims 1 and 9, wherein the said mixture is sucked
by a hydraulic system before spraying by the airless spray gun.

The present invention relates to a method of polyurethane coating over HVOF
coating for combating erosion of hydroturbine components. The method
comprising the steps of applying a high velocity oxy fuel (HVOF) coating; then an
intermediate metallic coating of Ni-AI using wire are spray process which is
applied on the said HVOF coated surface to improve the surface roughness of
the said coated surface; then an organic based primer is applied on the surface
to get better adhesion followed by application of polyurethane coating layer
which is produced by mixing the resin and hardener with an appropriate ratio.