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International Journal of Mechanical Engineering and Technology (IJMET) Volume 8, Issue 7, July 2017, pp. 524–536, Article ID: IJMET_08_07_060 Available online at http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=8&IType=7 ISSN Print: 0976-6340 and ISSN Online: 0976-6359 © IAEME Publication Scopus Indexed

COATING MATERIALS AND

METHODOLOGIES FOR THE PROTECTION OF

HYDRO TURBINE RUNNER

Pragyan Senapati

Department of Mechanical Engineering, Siksha ‘O’ Anusandhan University, Bhubaneswar, India

Mamata Padhy

Department of Mechanical Engineering, Siksha ‘O’ Anusandhan University, Bhubaneswar, India

Trilochan Sahu

Department of Civil Engineering, Govt. Engineering College, Kheonjhar, India

ABSTRACT

Energy is an essential factor which gives a comfortable life to the human beings. It

has a vital contribution in all social and economical developmental activities. The

economic development of many countries is hindered due to paucity of energy. Over two

billion people in the world are still underprivileged of electricity. The conventional

sources of electrical energy are not enough to provide energy to developing world, as

energy usage has increased manifold due to population growth, industrialization,

modernization and improved lifestyle. For rural and remote electrification renewable

energy systems viz. small and medium hydro schemes can be undertaken. The other

sources of renewable energy are: biomass, wind, solar and geothermal. Countries like

India, China and Nepal are gifted with many resources of run-of- river schemes in

Himalayan region. . In India particularly in the Himalayan region huge amount of silt

is carried to downstream during the rainy seasons eroding the turbine blades to a

significant extent. Hydraulic turbine is the heart of a hydropower plant and the erosion

of turbine blades is one of the major factors which affect the efficiency of the turbine.

Since last two decades researchers are working in this field to find out solution for this

particular problem. Thus several investigators have taken up the subject and carried

out actual experiments for protection of the turbine runner against striking silt applying

spraying of different materials adopting different methods on the runner. The current

article presents an overview of the works carried out by different investigators on hard

surface coating materials and methodologies for the protection of hydro turbine runner

failure of turbine runner material and the hard surface coatings applied to minimize the

effect of erosive wear based on an extensive literature review.

Pragyan Senapati, Mamata Padhy and Trilochan Sahu


Key words- Hydro turbines, Surface Coating materials, Coating Methods.

Cite this Article: Pragyan Senapati, Mamata Padhy and Trilochan Sahu, Coating Materials and Methodologies For The Protection of Hydro Turbine Runner. International Journal of Mechanical Engineering and Technology, 8(7), 2017, pp. 524–536. http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=8&IType=7

1. INTRODUCTION

Erosion of hydro turbine runner and other components often lead to loss of generation and efficiency and in most severe case turbine is shut down for repair work/replacement of runner or other components. The failure mechanism which cause these type of severity include silt erosion, cavitation erosion, surface fatigue of underwater parts of turbine, sometime improper installation leading to vibration of mechanical components[1]. These problems can be reduced or eliminated by understanding the causes of erosion, erosion mechanism and method of prevention. Hydro electric project sites in Himalayan range and north eastern states of India get severe erosion problem due to silt in river water during monsoon. Silt erosion of turbine runner and other underwater parts caused due to the unsettled silt particles coming after passing through the desilting tank which strikes the turbine runner and other underwater components along with the flowing water. The silt contains mostly quartz having hardness 7 in Moh’s scale.[2-6].Naidu[7] reported that the silt contents of river water of Himachal Pradesh is near about 10,000 tons per day per turbine for Baira Siul Project. Nepal faces loss of $120-150 million per year or more due to silting problem [8,9]. Some other countries like China and to some extent Norway are facing silt erosion problem in turbine components. Many investigations detailed by researchers are also reported earlier [10-22]. The eroded profile due to silt erosion helps in nucleating the air bubbles leading cavitation erosion by subsequent bursting of the same near the metal surface further aggravating the problem. Whereas some investigators [23] reported that the synergetic effect of cavitation and silt erosion rather reduces the erosion problem in turbine components.

These are the very challenging problem for the power generation from the renewable energy source like small hydropower. Since last few decades researchers are working on suitable materials for hydro turbine runner and other affected components. 13Cr-4Ni steel are usually considered and used for turbine runner and slurry pumps. Though these steels have excellent mechanical properties still these materials are less resistance to erosive wear and get damaged due to excessive silt content in water with run-of-river type power plants. NiCrFeSiB alloy show good resistance to abrasive wear. Ni provides ductility and enhances corrosion resistance. Chromium has high wear and corrosion resistance property. Boron enhances wear resistance property and silicon reduces melting point and acts as flux [24]. Different thermal spray coatings such as HVOF, flame spraying, Plasma spraying etc. are used to further enhance the erosion resistance properties of different components by surface modification [25]. In the present paper an attempt has been made to discuss on various studies carried out by different investigators on erosion resistant hydro turbine material, material and methods used for surface coating of hydro turbine components.

2. EXPERIMENTAL INVESTIGATIONS

Effect of impact angle is studied by Brustein[26]. The investigation was carried out by using a slurry erosion test rig. Stainless steel was used as specimen. The erosion effect was measured by electro chemical current time transient and acoustic emission (AE) simultaneously. The sharp current rise was indicating the rupture of surface and removal of oxide film. It was observed that the erosion increases with decreasing impact angle. Inclined angles showed more

Coating Materials and Methodologies For The Protection of Hydro Turbine Runner


erosion than normal impact angle. The mean scar area makes a sinθ curve with impact angle θ as shown in Fig.1. It is observed that larger scars produced at higher impact angle. Three types of plastic deformation were observed viz. (a) indentations were observed at normal impact angle (b) cutting mode 1 observed at higher impact angle and (c) cutting mode II observed in lower impact angle.

Figure 1 Impact angle versus mean scar area [26]

Slurry erosion properties of NiCrBSi sprayed coating and SUS304 stainless steel was examined by Lin et al [27]. From the experimental study they predicted that life of needles of nozzles can be increased 3-4 times when NiCrBSi coating is sprayed on a substrate material. The comparison of erosion rate of both the materials is shown in Fig.2. The investigation was done at higher impact speed and lower impingement angle. At lower impingement angle of 30 degree furrows and ridges are observed on both SUS304 stainless steel and NiCrBSi sprayed coating as shown in Fig.3. Micro forging and extrusion were observed to be increased significantly with high impingement angle and high speed.

Figure 2 Erosion rates versus impingement angle for (a) NiCrBSi sprayed coating (b) SUS304 stainless steel at various impingement speeds. [27]



Figure 3 SEM micrograph for SUS304 stainless steel (a) at 300 (b) at 900 impingment angle and NiCrBSi sprayed coating (c) at 300 and (d) at 900 impingment angle [27]

Ceramics such as ceramic paste, ceramic paints and hard facing alloys are used in power plants as standard coating. Thapa et.al. [28] Experimented with 86 WC-10Co-4Cr and observed improvement in erosive resistance. Santa et.al [29] investigated on CA6NM ASTM A743 grade steel and AISI 431 steel. They also used two different coating material E-C 29123(WC/CO in Fe-Cr Ni matrix) and T 35 MXC (Al2O3- reinforced high carbon steel) on AISI 304 steel. E-C 29123 was coated on AISI 304 by oxyfuel power (OFP) process and T 35 MXC was coated by wire arc spray (WAS) process. The substrate surface was prepared by sand blasting for OFP coating and grinded using grinding wheel for WAS coating. For both the cases Nickel rich and pass was applied before coating.

It was observed that cermet particles removed under erosion testing, but the volume loss was not predicted due to above phenomenon. The Fig.4 Shows the erosion mechanism for all material considered for this study. All coating show plastic deformation. Good erosion resistant property was observed for E-C-29123 coating. The authors predict this coating may be suitable for hydraulic machines components exposed to silt erosion.



(e)

Figure 4 SEM micrograph. [29] (a) AISI 431 stainless steel, (b) ASTM A743 CA6NM stainless steel, (c) AISI 304 stainless steel and (d) T 35 MXC coating.(e) E-C 29123 coating.

He et.al. [30] reviewed on Nanostructured WC-Co coatings investigated by different researchers. They summarized the significant achievement of different investigators as follows:

1. Unlike conventional WC-Co powder, the nano structured WC-Co powders can be produced without non WC-Co phases.

2. Under same spraying parametric condition nano structured WC-Co powers deposite more than corresponding conventional powder, thus degrading the coating properties.

3. Under optimised controlling spraying parameters near nano structured WC-Co can be synthesized successfully.

4. Proper synthesization of the coating may give better properties viz. hardness, toughness and wear resistance.

Sahraoui et.al.[31] studied microstructural properties such as WC-12Co. They adopted HVOF process. Different phase compositions obtained depending on the spray conditions with porosity contended limited upto 1%. Substantial improvement in hardness obtained at smallest stand off distance and fuel flow rate. Proper choice of feed stock powder is also important



parameter and it helps in phase composition of coating. The powder characteristics are as follows: Nano particles contain less number of brittle phase than bigger size particles. There is less chance of phase decomposition of WC particle when nano particles are used [32,33].

Chauhan et.al.[34] selected turbine materials (satellite-6, Cr3C2-NiCr) as surface coating material to study the erosion behavior. The substrate material was hot rolled 21Cr-4Ni-N steel. Detonation Gun (D Gun) method was selected for coating operation. For their corrosion parametric condition stellite-6 coating contributed maximum damage at 30o and 90o impact angles. WC-Co-Cr coating also at 30o impact angle proved to be more erosion prone. In both the cases erosion was increasing at initial stage and then remaining constant as shown in Fig.5a. The mechanism of erosion remained same for both the cases. Materials show ductile erosion i.e. shearing and ploughing at 30o impact angle and craters are found at 90o impact angle. Fig.5b shows the histogram of erosion rate at 30o and 90o impact angle. Erosion rate seems to be more at 90o impact angle than 30o impact angle for all types of material considered.

Figure 5(a) Erosion rate (g/g) as a function of erosion time for substrate material as well as for various coatings at impingement angles (a) 30° and (b) 90°.[34]

Figure 5(b) A histogram illustrating cumulative weight loss of substrate (hot rolled 21-4-N steel) and different D-Gun sprayed coatings.[34]

Marteen et.al.[35] adopted High Velocity Oxy Fuel (HVOF) method. Duplex Co-coated near nano structured cermet power was chosen as coating material to prevent carburisation of WC particles in the coating. It showed better result towards hardness and wear resistance than WC-17Co system. Sliding wear test also showed better result for near nano powder coating and brittle fracture seemed to be avoided.

Grewal et.al[36] studied erosion behaviour of coating of WC-10Co-4Cr powder over 13/4 and 16/5 steels. They adopted D-Gun spray method for coating. 16/5 steel showed better resistance to slurry erosion than 13/4 steel both in uncoated as well as coated condition. Coated



material enhanced the resistance to slurry erosion by 3 to 4 times. SEM reveals that plastic deformation is the mechanism of erosion for uncoated steel where as interlinked surface cracks observed on the coated steel.

Goyal.et.al.[37] had taken CF8M as substrate material and HVOF as coating methodology. Coating materials are WC-10Co-4Cr and Al2O3 13TiO2. WC-10Co-4Cr has shown better resistance to slurry erosion than Al2O3 13TiO2 coating. Unmelted Al2O3 particles observed on coated surface may be due to its high melting point. These unmelted particles observed to be spalling onto of the coated surface unlike the WC-10Co-4Cr particles as shown in Fig.6&7. Mechanism of erosion revealed that substrate material showed ductile behaviour, WC-10Co-4Cr coated material showed mixed behaviour where as Al2O3 13TiO2 showed brittle behaviour towards slurry erosion.

(a) (b)

Figure 6 Micrograph of HVOF-sprayed WC–10Co–4Cr coating on CF8M steel. (a) Cross-sectional SEM micrograph, (b) Surface SEM micrograph [37]

(a) (b)

Figure 7 Micrograph of HVOF-sprayed Al2O3+13TiO2 coating on CF8M steel. (a) Cross-sectional SEM micrograph, (b) Surface SEM micrograph [37]

Grewal et.al.[38] considered Ni-Al2O3 based composite coating for protection against silt erosion on hydroturbine steel (CA6NM). High velocity flame spraying method was adopted. From Fig.8, roll of aluminium was found to be significant. 40% wt alumina exhibited best result among considered samples. Erosion rate found to be reduced by 2.2 times than the uncoated steel. A correlation for erosion resistance was developed as a function of fracture toughness and hardness is (KIC

2H) 1/3. Where KIC and H represent fracture toughness and hardness respectively. The prediction of erosion mechanism is well illustrated in Fig.9.



Figure 8 Illustrates volume loss with time for bare steel and different coatings at 900 impact angle and 16 m/s velocity [38]

Figure 9 Schematic illustration of possible erosion mechanism in Ni-Al2O3 [38]

Grewal et.al [39] also adopted friction stir method (FSP) for improving the mechanical properties of commonly used turbine steel, 13Cr4Ni steel. By this method there was refinement of microstructure reduction of grain size observed by factor of 10. Submicron and ultrafine grain structures are observed and confirmed with electron backed scattered diffraction (EBSD) method. The micro hardness also increased by 2.6 times. Resistance to cavitation erosion improved by 2.4 times which is well explained in Fig.10. The mechanism of erosion remained similar for both processed and unprocessed steel.



Figure 10 Variation in mass loss with time for the friction stir processed and unprocessed CA6NM steel subjected to cavitation erosion testing.[38]

Basak et.al [40] observed that HVOF sprayed nano sized WC-12Co coating gives better erosion resistance performance than micro sized steel particles. W2C phase can be eliminated by using nano size particles. Addition of Al in small quantity with WC-12Co coating, resistance to erosion can be improved further. Wear resistance seemed to be improved 3 times in case of nano particle coating as compared to micro particle coating of WC-12Co.

Rajkarnikar et.al.[41] attempted for modification of traditional design of a Francis turbine. In the laboratory they designed a test set up called Rotating Disc Apparatus (RDA) as shown in Fig.11. They used Francis runner blade as test specimens. They reached at a point that silt erosion behaviour of Francis turbine can be studied using this set up in laboratory. They also got the similar result from CFD analysis and observed quite significant amount of erosion in the specimen.

Figure 11 Rotating disc apparatus (RDA) [41]

Keller et.al.[42] used self healing materials for coating. They tested two types of self healing material compositions. These are polydimethylsiloxane (PDMS) and an epoxy coating with one part isocyanate. Both are microcapsule base healing materials. Erosion test was done on both the materials. The later has showed better result as compared to former. Former showed insufficient kinetic viscosity for successful healing.



3. COMPUTATIONAL WORK

Hassani et.al. [43] Developed a dynamic model based on finite elements method to study the erosion resistance properties of coating. Solid particle erosion was simulated for different coating thickness. It was observed that erosion mechanism are different depending on coating thickness (t). Fig.12 Shows the graph between volume of material removed verses thickness (W~t). It is observed that material removal is more for thin coatings (t<8 μm). When coating thickness is 8 μm or more, the volume of material removal W decreases and remains almost constant.

Figure 14 Volume removed (W) ~ coating thickness (t) [43]

The volume of material removed by a single particle impact was evaluated with respect to

the impact velocity V, particle size r, particle density ��, and the effect of coatings' properties

such as Young's modulus E, hardness H, fracture toughness KIC, and thickness t. The correlation developed is as follows.

��..��/(��.��.��)

Hassani et. al. [44] also studied the resistance of surface coating against impact test by using finite element methodology (FEM). Residual stress and Young’s modulus was applied as input parameter of coating system. The impact energy was optimised with respect to the resistance to erosion for protective coating. Stresses developed during impact can be estimated from FEM analysis. Optimal coating architecture was designed considering: tensile strength reduction, energy absorption, and multilayer configuration with specific Young’s modulus and residual stress distribution along the coating depth. It has been observed that multilayer coating with controlled Young’s modulus distribution enhances erosion resistance.

Sahraoui et. al.[31] designed tribological performance of the surface coatings based on available experimental data. Analysis of hardness, porosity results could not be justified because of the small number of experimental data. Instead friction moment -sliding distance correlation could be well predicted for all studied conditions. Optimisation of process parameters for HVOF is fully predictable with this method. Friction moment, was found varying linearly with the increase of the fuel flow rate and had parabolic variation with the stand-off distance

4. CONCLUSION

Erosion of hydro turbine runner and other components exposed to silt laden water cannot be avoided completely, but excellent materials and surface coating can be used to increase the life of the runner and other underwater parts, which can save the down time, maintenance cost and shall not reduce the power generation capacity of the hydropower plant. Many investigators have studied the erosion behaviour of different hydro turbine materials, hard surface coating



materials through experimental and analytical studies. Some of the investigators have reported from their laboratory scale experiment that there is improvement in the life of the specimen material after providing the coatings. Their cost effectiveness should be studied. Further experimental studies are also required for studying the effect of erosion in different base materials and hard surface coatings taking into consideration shape of hydro turbine components and parameters related to silt erosion and operating conditions of hydro power plants.

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