innovative approach to minimize silt...

ICHPSD-2015

158

INNOVATIVE APPROACH TO MINIMIZE SILT EROSION IN HYDRO TURBINES

Mukesh Mangla Additional General Manager, Centre of Excellence – Hydro Machines,

Bharat Heavy Electricals Limited Bhopal, INDIA Telephone-91 0755 2505058 email- [email protected]

ABSTRACT The perennial Himalayan Rivers, which contribute to a large portion of Hydro Potential available in India, have very high quartz rich silt content (>10000 ppm in monsoon). Erosion of hydro turbine components due to silt content in water is a huge problem encountered by hydro power plants situated in Himalayan region, where the quartz content in water is very high. High quartz content in Himalayan Rivers is attributed to a variety of reasons like steep gradient of the Rivers, large scale deforestation, and extreme weather conditions combined with the fact that Himalayas are geologically young mountains with sedimentary rocks. Due to such high quartz rich silt content the turbine parts get eroded very fast leading to a loss in efficiency & thereby output, inciting cavitation, pressure pulsations, vibrations, mechanical failures and frequent shut downs. The present paper elaborates innovative use of CFD tool, model testing and the strategy adopted by BHEL to mitigate the erosion due to silt. Keywords: Silt Erosion, HVOF Coating, Erosion Model, CFD, Model Testing INTRODUCTION Bharat Heavy Electricals Limited (BHEL) supplied Hydro sets account for 26800 MW (nearly 60%) of the total installed capacity of 44602 MW in the country. BHEL is engaged in the design, engineering, manufacturing, construction, testing, commissioning and servicing of a wide range of products and services for the core sectors of the economy viz. Power, Transmission, Industry, Transportation, Renewable Energy, Oil & Gas and Defense. The power generation sector comprises thermal, gas, hydro and nuclear power plant business. BHEL has been playing a pivotal role in the design & development activities and successfully designing and manufacturing all types of hydro turbines including Francis, Kaplan and Pelton turbines as well as Pumps, Pump turbines and Bulb turbines. For Research and development of hydro product, company has established a state-of-the-art Laboratory named as "Centre of Excellence - Hydro Machines (COE – HM)" in 1989 at its Bhopal plant. COE - HM is equipped with latest instrumentation and possesses the capability to perform various tests on any type of hydro turbine model, along with the latest tools, where in-depth mathematical modeling and CFD analysis of complete water-path components is performed. COE – HM has developed expertise in Ansys CFX 15.0 by adding customized macros for analysis of hydro machines.

International Conference on Hydropower for Sustainable Development Feb 05-07, 2015, Dehradun

159

There are several examples of hydro turbines located in Himalayan region, which are facing severe silt erosion like Nathpa Jhakri, Maneri-Bhali, Baira-Siul, Salal, Dehar etc. (see Fig. 1 & 2). Therefore COE-HM is genuinely involved in developing solutions to curb the menace of silt erosion on hydraulic machinery.

Fig. 1: Eroded Runner of Dehar HEP

Fig. 2: Eroded Guide Vanes of Nathpa Jhakri HEP

This paper showcases capabilities of BHEL to use the computational fluid dynamics tool and model testing for developing silt-friendly runner profile and strategy adopted to tackle silt erosion problem. The successful experimental model test proves the strength of BHEL to address threat due to silt, whereas maintaining almost the same efficiency and other required performance parameters. BACKGROUND Factors to be considered with regard to silt erosion Quality of water silt (quartz) concentration (>2000 ppm in monsoon) Petrographic analysis of silt (quartz) particle

Silt Erosion Basics Metal removal by erosion/abrasion of components is due to shearing-off of the metal by hard & sharp silt particles or damage due to impact of silt particles. Of the two, removal of metal by shearing-off is the major cause of erosion in ductile materials. The trailing edge of profiled components like runner, guide vanes and liner plates, seals etc. are mostly affected by the shearing-off phenomenon whereas damage on the runner inlet edge is mostly due to impact of silt particles. Silt particles carried by water cause abrasive wear and the rate of abrasion depends on the hardness of parent material, concentration of the silt particles, their grain size, shape and their hardness, velocity of silt particles and their angle of impact. Erosion rate coefficient ∝ { }PS

n HHSdcVf ,,,,,, θ

ICHPSD-2015

160

Where

‘V’ is the velocity of silt particles, ‘c’ is the concentration of silt particles, ‘d’ is the size of silt particles, ‘S’ is the shape factor of silt particles, ‘θ’ is the impact angle, ‘Hs’ is the hardness of silt particles, ‘Hp’ is the hardness of parent metal,

Value of exponent ‘n’ for ductile material can be chosen as 2.1. For a particular Hydro Power Plant site and for steady state calculations, the concentration, size, shape, hardness of silt particles entering the turbine and hardness of parent material are hydraulic profile independent factors and may be safely considered as constant. Thus the mathematical model can be greatly simplified by omitting these factors and thus the erosion rate for a given hydro site would be the function of velocity of silt particles and the angle of impact. Erosion rate coefficient = k*Velocityn * f(θ) Where ‘k’ is a constant for a given site and ‘f(θ)’ is a function relating wear to impact angle θ The shearing of metal can be reduced to a large extent if the particles hit the components with lower velocity and at lower angle of attack. Since experimental model testing by silt injected water is not feasible, modern analytical tools along with high speed computing facilities can be used effectively to determine extent of silt erosion. Modern powerful CFD tools are effectively used to determine the trajectories of the silt particles of various sizes and concentration to determine the velocities and angle of attack. The geometry is suitably modified to reduce erosion by reducing angle of attack or shifting the location of attack to zones of lesser velocities. APPROACH To address the silt problem, a five pronged approach is adopted: Provision of de-silting chamber to eliminate large size silt particles. Selection of machine with a lower specific speed, with comparatively lower velocities

& reduced erosion. Design of runner profile to make it silt friendly using advanced CFD simulation. Model testing to ensure almost same efficiency as without consideration of silt. Application of coating like HVOF coating over the silt prone zones of the profile, if

required.


161

PROVISION OF DE-SILTING CHAMBER Erosion rate is dependent on the size of silt particles hitting the turbine components. Large size of silt particles increases the metal removal rate with tremendous increase in area affected by silt. The trajectories of heavier silt particles are much different than the lighter silt particles and they hit the components in distinct zones. This can be achieved by preventive measures like proper catchment area design and effective de-silting to restrict large size silt particles from entering the turbine. Various de-silting arrangements are available, but such facilities are costly. Smaller the size of particle to be removed higher is the cost. After studying the effect of various size of silt particles and keeping overall cost consideration it is recommended that all particles above 75 microns size must be removed from the water before entering the turbine. Also a structured plan of periodic flushing of de-silting bays is recommended. SELECTION OF LOWER SPECIFIC SPEED MACHINE Erosion rate is strongly dependent on relative velocity to mitigate silt erosion it is recommended for a given hydraulic data, machine with a lower specific speed should be selected as it would result in lower absolute and relative velocities and thereby reduction in erosion. For example, in a project of rated head of 120 m and 72 MW output, the rotational speed of 214.3 rpm or 200 rpm can be selected. 214.3 rpm corresponds to specific speed of 145 m-kW, whereas 200 rpm corresponds to specific speed of 135 m-kW. To reduce effect of silt erosion, 200 rpm is recommended. SILT FRIENDLY HYDRAULIC DESIGN The amount of erosion damage of different components in a hydro turbine depends strongly on the head /specific speed of the machine. High and medium head Francis machines have maximum erosion damage at exit of guide vanes and inlet of runner blades. Occasionally there can be breaking and dislodging of material due to impact of silt particles at the inlet of runner blade and guide vane. In low head Francis turbines maximum erosion damage occurs on the runner blades towards the outlet edge near the skirt side. Maximum damage occurs on the pressure side of the runner blade. While hydraulic designing of turbine for silt prone zones, utmost discretion is required for designing of individual components keeping the head and specific speed in mind. NUMERICAL TOOL In modern time Numerical analysis of hydro turbines has come up a long way and forms the backbone of hydraulic design. Using the latest CFD techniques, the designer can estimate losses in various components, thereby fine tune them to get better performance and enable a better understanding of the mechanisms leading to silt erosion.

ICHPSD-2015

162

The flow calculations in the present work are based on full 3-D Navier-Stokes equations (incompressible, viscous & rotational) and the turbulence effects are taken into account by using standard ‘κε’ turbulence models with standard logarithmic wall functions using commercially available CFD package. Lagrangian particle tracking module is used to simulate silt, whereby required number of particles of a various sizes and concentration are injected into the flow and their trajectories studied. The module takes into account the influence of silt particles on fluid and vice-versa, to calculate velocity of particles and the angles at which they hit the blade surface. Resultant data and the mass of particles are used to calculate the local erosion rate (Non-dimensional) using a suitable erosion rate mathematical model. In the present work "Finnie’s model for erosion wear" is applied, which relates the rate of wear to the kinetic energy of impact of particles on the surface, using n=2.1 and the function relating wear to impact angle is defined as:

( ) θθ 2cos31

=f 3

1tan >θif

( ) θθθ 2sin3)2sin( −=f 31tan ≤θif

HYDRAULIC DESIGN OF TURBINE COMPONENTS AND RUNNER PROFILE Hydraulic design of turbine components including runner profile is carried out conventionally to satisfy the hydraulic performance criteria like optimum speed, optimum discharge, desired efficiency, output & setting. In the next stage a detailed study of the effect of silt on various components is taken up using CFD tool. For example a case study with following inputs (as per petrographic analysis): Particle Size

65% particle size below 0.075 mm 25% particle size in the range of 0.075 mm to 0.15 mm. 7% particle size in the range of 0.15 mm to 0.20 mm. 3% particle size in the range of 0.20 mm to 0.25 mm.

Silt particle concentration = 2000 PPM Particle shape factor = 1.5 Hardness of silt particles < 7 mohs Density of silt particles = 2600 kg/m3 Erosion Model : Finnie Drag force model : Schiller Naumann


163

Figure 3 shows the domain used for CFD analysis which includes spiral casing, stay vanes, guide vanes, runner blades and draft tube. Figure 4 shows the mid span of runner blade profile with nomenclature used. The erosion pattern obtained on the conventional runner blade from CFD analysis is given in Fig. 5 and Fig. 6.

Fig. 3: Domain used for CFD analysis

Fig. 4: Mid span of runner blade profile

Fig. 5: Runner Erosion pattern due to silt

of particle size 50 mic. Fig. 6: Runner Erosion pattern due to silt of

particle size 300 mic.

It can be seen from the fringe plot (Fig. 5 & Fig. 6) of the erosion pattern on conventional blade, there is a lot of erosion towards the skirt side and outlet edge of the blade, indicated by light patches on the surface of blade. Also the erosion on suction side and at inlet edge of the blade is negligible in comparison to that of pressure side at optimum point of operation. Rest of the turbine components including guide vane profile are experiencing far less amount of erosion in comparison to runner. For the root cause of above erosion pattern, micro level analysis is carried out with the help of contour plots and vector plots obtained from CFD analysis. Figure 7 shows the normalized relative flow velocity (Vnr) w.r.t. stream wise location along runner blade at mid span. Vnr is defined as the ratio of the actual relative velocity with spiral inlet velocity. It is clear from the curve that the velocity in the zone near outlet edge is around 2 to 2.5 times higher in comparison with the velocity near inlet and mid part of the blade.

ICHPSD-2015

164

Fig. 7: Flow velocity along runner blade Fig. 8 shows the velocity vector plot at the mid span of the runner and Fig. 9 elaborates the vector directions near outlet edge of the blade. On careful examination of Fig. 9, it is evident that the distribution of blade angle β is not matching with flow angle on the pressure side of the blade, causing a comparatively greater impact angle whereas on the suction side, blade angle β and the flow angle are almost collinear. Thus large impact angle in conjunction with higher velocities near outlet edge of the pressure side of blade is the main reason of severe erosion pattern.

Fig. 8: Velocity vector plot at mid

span of conventional runner Fig. 9: Vector directions near outlet

edge of the conventional runner To minimize the erosion due to silt, the modification of blade angle along streamline direction is done. This is an iterative process and a number of design variants analyzed before finalizing


165

the most optimized variant. The final runner variant profile at mid span and the comparison with conventional runner blade profile at mid span is illustrated in Fig.10

Fig. 10: Comparison between conventional and final (modified) runner blade profiles In the final runner blade profile the blade angle near outlet edge is adjusted such that it is almost matching with the flow angle with a much lower impact angle and thereby reduction in the erosion. To achieve the desired performance parameter like runner efficiency, output etc, the blade angle is further modified near inlet edge and mid part of the blade. Since the velocity values near the inlet and mid part of the blade are comparatively less, so the modification of the blade angle in this zone does not adversely affect the erosion pattern. Velocity vector plot over the final blade profile at mid span can be seen in Fig.11 and Fig.12.

Fig. 11: Velocity vector plot at mid span of final (modified) runner

Fig. 12: Vector directions near outlet edge of the final (modified) runner

ICHPSD-2015

166

Erosion pattern obtained on the final blade profile is presented in Fig.14 and Fig.16. For comparison erosion pattern on conventional blade is shown in Fig.13 and Fig.15.

Fig. 13: Erosion pattern on conventional runner

Fig. 14: Erosion pattern on final (modified) runner

Fig. 15: Erosion pattern on conventional runner

Fig. 16: Erosion pattern on final (modified) runner

Comparing above figures, it is evident that with modification of runner blade profile a vast reduction in the area of runner being eroded is visible. Experimental validation of numerical simulation results Numerical simulation alone does not have much acceptance unless until it is backed by experimental results. COE-HM, BHEL, Bhopal is equipped by a sophisticated Test set-up where experiments on a small scale model are conducted to evaluate the various performance parameters of hydraulic turbine. The test bed is accredited by National Accreditation Board for Laboratories as per International standard ISO/IEC17025:2005. All the designs fine-tuned by Numerical simulation are validated by manufacturing a scaled model and verifying the improvements by experimental model testing.


167

Figure 17 shows the test rig for experimental model testing at COE-HM, BHEL Bhopal

Fig. 17: Experimental test set-up Experimental Model testing of the two runners showed a very small reduction (0.3%) in peak efficiency, whereas reduction in numerical erosion rate coefficient is of the order of 50%.

Fig. 18: Hill chart of conventional runner

ICHPSD-2015

168

Fig. 19: Hill chart of final modified runner With model test it was noticed that there is a slight improvement in cavitation characteristics of modified runner. This can be seen in Fig. 20.

Fig. 20: Comparative cavitation characteristics of convention and final modified runner HVOF COATING BHEL has been working since a long time for development of hard coatings suitable for Hydro turbine components and study of these coatings to resist silt erosion. Elaborate laboratory tests have been conducted to study the various coatings. It has been observed that Tungsten Carbide powder with High velocity Oxy-fuel (HVOF-WC) spraying has been quite


169

successful for Hydro turbine components in terms of its bonding, chemical stability and erosion resistance. HVOF-WC coatings in steel improve erosion resistance by 30-50 times in comparison to plasma nitrided steel or ordinary steel. Also these coatings can withstand silt particles with velocities up to 70m/sec with low angle of incidence. In Fig. 21 and Fig. 22 BHEL facility for HVOF is shown. In very high silt prone rivers it is recommended to have HVOF coating of minimum 500 microns thickness near the outlet edge and other silt prone zones of the final modified runner to ensure greater safety.

Fig. 21: Manual HVOF Coating Fig. 22: HVOF Coating by robotic arm CONCLUSION BHEL has successfully developed techniques which can be used to reduce the ill-effects of silt by hydraulic redesigning of Francis turbine runner blades without losing much in efficiency. Although the numerical analysis provides a very detailed understanding of the phenomenon of silt erosion and excellent qualitative assessment of damage, the mathematical model is quite simplified to predict physical values of erosion. Since experimental turbine model testing with silt injected water is not feasible, to accurately predict physical quantity of metal removal, the mathematical model needs to be closely calibrated with actual erosion data from site . REFERENCES 1) Naidu B.S.K., “Addressing the problems of silt erosion at hydro plants”, Hydro Power

and Dams, 1997. 2) Jacoby G. et al., “Silt erosion in hydraulic machinery”. Hydropower into the next

century, The International journal on Hydropower and Dams, 1995. 3) Finnie I., “Erosion of surfaces by solid particles”, Wear, 1960. 4) Ansys-CFX " User Manual & Theory"

ICHPSD-2015

170

5) Pande V.K., Shrinivas Rao V., “A new design approach for combating silt erosion”, Hydro -2003, Hydro Power and Dams.

6) Mann B.S., Arya Vivek , “ Abrasive and erosive wear characteristics of plasma nitriding and HVOF coatings: their application in hydro turbines”, Wear 2001

7) Mukesh Mangla, Shrinivas Rao V., “Design Considerations for Hydro turbines situated in silty areas: Exploring new technological avenues” Powergen India 2012

innovative approach to minimize silt...

Documents