Design of Hydromechanical component for Sustainability of Hydropower
Structures: A Case Study of Bifurcation for Daraudi Khola Hydropower
Project, Nepal
Ravi Koirala
1*, Sailesh Chitrakar
1, Hari Prasad Neopane
1, Balendra Chettri
21
Turbine Testing Lab, Kathmandu University, Dhulikhel, Nepal
2Nirvana Tech Pvt. Ltd., Kathmandu, Nepal
* ravikoirala@ku.edu.np
Abstract
The worldwide demand for production of clean energy through hydro power has been a major issue of concern. Most of the hydropower resources from the developing countries of Asia like Nepal are yet to be harnessed. The technology regarding design of hydromechanical components in Nepal is still in the prenatal phase where the traditional design attempts and processes are being followed. These trends in the design process lack the virtual analysis part which has been causing the modification of design after construction, failure of the system etc. Implementation of Computer Aided Simulations with penstock pipes and similar other civil structures can be an effective approach for pr-estimation. A hydraulically optimized and structurally stable design ascertains sustainability of plants. More advanced design attempts with incorporation of Computational Simulations increases the reliability along with cost efficiency by optimizing the factor of safety. A case study of Daraudi Khola Hydropower of 6 MW, an under construction power plant in Nepal is used, in which hydraulic and mechanical design for a bifurcation has been performed using Computational analysis. The design is currently under the fabrication stage in the site.
More over water during the dividing phenomena shows irrelevant behavior hence special care and attention is required during the hydraulic and mechanical design of bifurcation. Determining the angle of bifurcation and sectional dimensions for the minimum head loss and identification of structural members for the rigidity are the major issue of concern. Implementation of computational methods for more reliable design process with visual aids to estimate the phenomena is the prime need in these kinds of complex cases. Primarily traditional analytical methods were used to estimate the basic dimensions and CFD simulations were performed for analysis and optimization. Analytical design with computational analysis is an essential design methodology for critical structures.
Keywords: Bifurcation, Computational Analysis, Design
1. Introduction
Hydropower stands as the green solution to global energy demand. Most of the countries in Asian continent like Nepal have recently entered into the pool of investment in hydropower sector. In fact hydropower development requires long construction period, large investment and intense Research and Development. In case of compromise in any of the variables may result into inefficient power development system. Moreover, here in this paper we are focused on need of proper design and design analysis of Hydromechanical components whose failure may result into catastrophic disasters like failure of 9/10 units at Sayano – Shushenskaya Power Station, Russia in 2009, penstock rupture in 2000 at Bieudron Power Station, Switzerland because of inability to resist the internal pressure, Penstock rupture at Lapino Power Plant, Poland due to mechanical failure under water hammer, etc. (1).
Figure 1 Penstock rupture at Lapino Power Plant, Poland
Design attempts at current state are more focused on analytical approach, where calculation processes has been simplified for results. The hydraulic behavior of water at different phases of operation shows irrelevant behaviors hence examination with finite element analysis and identification of flow nature with finite volume analysis is an important perspective to be included.
This paper forwards a design attempt of bifurcation implemented for an under construction hydropower which implements synergy of both the analytical and computational approaches. Bifurcation refers to the penstock segment with for flow division to two plant units as shown in figure
Figure 2 Underconstruction bifurcation
2. Need of Bifurcation in Penstock
Particularly, there can be many important reasons behind the need and use of bifurcation in the hydropower plant (2) (3). These reasons are discussed below:
i. Flow variation
There can be significant amount of flow variation in the water bodies in an Annual Seasonal Cycle [ASC]. Since the part flow and part load operation of turbine results into efficiency deterioration along with the maintenance issues on continuous operation in off design condition, a number of units are installed. Hence, instead of operating a turbine in 20% flow with single unit, one of the two units will be closed and more efficiency can be attained.
ii. Economical perspective of penstock
An alternative for installation of two units without bifurcation is using two penstocks but this doubles the cost of penstock installation, hence bifurcation is preferred.
iii. Maintenance perspective
If bifurcation is included in a plant, then cyclic operation of plant on the basis of priority can be done without completely shutting down the entire system for maintenance activities.
iv. Geographical perspective
The topography of the area and the gradient in the site may not allow the installation of larger penstock or multiple penstocks hence a bifurcation would be required to install multiple units.
v. Technical specifications
Although it is rare but in some cases, the capacity of the turbine and the potentiality of the site may be different in such case installation of multiple units may be assisted by it. In other cases there may be effect of the size of penstock or material used in the penstock to select the branching in them.
3. Daraudi Khola Hydropower Project
Daraudi Khola Hydropower Project is a run-of-river type hydropower project developed over Daraudi river situated in Gorkha district of Western Nepal. This project is currently under construction phase.
Table 1 Specification of Daraudi Khola Project Specification Description Unit
Head 63.8 m Main Flow 11.32 m3/s Each Flow 5.66 m3/s Penstock diameter 2.1 m Length 298.77 + 16 m
4. Hydraulic design of Bifurcation
Wyes branches must be designed for smooth hydraulic flow to avoid excessive head loss, vibration, and cavitation. They must be geometrically detailed for proportional flow distribution, eliminate acceleration or deceleration of flow in the adjoining branches, and thus minimize head loss. Head loss in the penstock, including losses in wyes and branches, contribute to inefficiencies in the power generation system and may result in lost generating revenue or in the case of pumped storage projects, additional pumping costs.
Angle of bifurcation, ratio of cross sectional area, type and shape of bifurcation, flow, velocity and reynold’s number are some of the major factors governing head losses. Approximation of these parameters using set of equations at two dimension, may not be relevant to determine the effectiveness.
So far the practices are concerned, often hydraulic design (angle) of bifurcation are prepared based on the flow ratio referencing the graphs resulted from various researches. In some cases the graph may give a valid bifurcation angle (but it’s rare the cases match) but many others were designed on larger hydraulic losses. Based upon the series of experiments, graphical representation of the losses has been prepared.
Graphical representation from the Miller experiments and Munich test are some of the major representations. (4) (5) (6).
Figure 3 Miller's Plot for Head Loss Coefficient in Symmetric Bifurcation (5) Geometry development and Computational analysis attempts
The branching sections were only considered during the modeling and the mesh was prepared. Primarily, Mesh Independent Test with the predefined convergence criteria of 1% on the Design was performed and on that size i.e. 2,018,417 nodes, rest of the analysis was done. An important consideration during the analysis should be maintained during the selection of the length for computational analysis to obtain steady developed flow.
After the predetermination of the size of computational model, mesh on the geometry of each angle from 45o to 70o was performed to determine the point of minimum head loss with the domain definition, boundary conditions and solver parameters defined in Table 2. (7)
Table 2 Boundary conditions and fluid features for CFD analysis Domain Definition
Fluid Water
Density 1000 kg /m3
Morphology Continuous fluid
Domain motion Stationary
Turbulence Model Shear Stress Transport Boundary Conditions
Inlet
Pressure 624000.366 Pa
Outlet1
Mass flow rate 5682.9 kg/sec Outlet2
Mass flow rate 5682.9 kg/sec Wall Features Mass & Momentum No slip wall Wall roughness Smooth wall
Analysis type Steady State Analysis Convergence control
Min Iteration 1
Convergence Criteria
Residual type RMS
Residual target 10e-5
The head loss was determined by the Equation 1and described graphically as in the Figure 2. ℎ𝑙𝑖 = 𝑃𝑖𝑛 𝜌∗𝑔+ 𝑉𝑖𝑛 2∗𝑔 − 𝑃𝑜𝑢𝑡 𝑖 𝜌 ∗𝑔 + 𝑉𝑜𝑢𝑡 𝑖 2∗𝑔 [Equation 1]
Figure 4 Head Loss in the bifurcation
From figure 2, it has been found that the angle of bifurcation for the minimum head loss is at 51o.
Figure 5 Pressure and velocity distribution in the flow domain 26 27 28 29 30 31 32 33 34 35 45 47 49 51 53 55 57 59 61 63 65 67 69 H ea d L o ss Va lue [ m m ]
Bifurcation Angle [Degrees]
Figure 6 Pressure distributin in the central plane and the point of maximum pressure
5. Mechanical Design of Bifurcation
5.1. Identification of basic mechanical features
The backgrounds like material properties and fluid properties are identified prior to the analysis. The Indian Standard guide lines for the selection of the material, its allowable stress and corrosion allowance has been allocated. Table 3 shows the background for the analysis.
Table 3 Structural Properties for analysis
Particular Description
Dimensions ID 2100 mm
Angle 51o
Hydraulic Parameters Design Head 63.8 m
Flow 11.32 m3/s
Units 2
Internal Pressure Max. Static Head 63.8 m
Max. Surge Head 26.2 m
Total Head 90 m
External Pressure Buried Pressure [8 m buried] 435.8465 N/mm Material Properties
[IS Standard]
Mild steel of the standard MS IS-2062
Density 7850 kg/m3
Young’s Modulus 210 GPa
Poisson’s Ratio 0.3
Ultimate Tensile Strength 410 MPa
Yield Strength 250 MPa
Allowable Stress (8) σallowable 102.5 MPa Thickness IS code 2825: 1969 (Clause 3.2.2)
Minimum shell thickness 18.03 mm Corrosion Allowance 1.5 mm Minimum standard thickness 20 mm
5.2. Design of structural members
The structural members are designed incorporating both the analytical method and the finite element method. Analytical calculations are used for pre-estimation while the finite element was used for optimized solution. 5.2.1. Simplified Curves Beam Method (9)
A penstock wye or branch connection usually has several stiffening beams to resist the loads applied by the shell of the pipe. The method incorporated rib shortening, shear deformation of the stiffener beams, and variable flange width. Although valid, this systematic design process is not efficient considering available modern computing methods. However, a spreadsheet can be used to drastically cut down the time involved with this design method. To analyze the wye and branch connections using beams, many simplifications and approximations are used (10). The localized effect of structural discontinuities, restraints of the stiffening beams, foundation support, and dead load of the water filled pipe are neglected. End load effects and conicity of the outlet pipes are also neglected and considered to be small in comparison to the vertical load on the beams. Here a finite certain sections are considered and computation is prepared based on this approach. This analysis also includes this approach of approximation. The prime design processes include the deflection of the members AO and BO at point O is equal from figure 5. This computation was performed considering the uniformly distributed load on symmetric section as shown in figure 6.
Figure 8 Simplified beam method for computation of the stiffener (10) 5.2.2. Finite Element Analysis
The finite element analysis design method provides a more complete representation of the penstock shell – stiffener system. It is becoming the standard design tool for penstock wye design in the hydropower industry (2) (3). The main aim in this analysis will be fulfilled by this method. Where the detail analysis will be explained and performed based on this process.
The minimum shell thickness was determined using the analytical methodology for pre-setting the computational limit. based upon which 3D CAD was developed to perform the unstiffened structural analysis for locating the critical locations with maximum stress and deflection.
The primary dimensions are then modified to counter act on the resultant deflections and stresses. The result was obtained with iterative computation of the stiffer and girdle rings. The main constraint for the selection of the size of the support is the site consideration.
Figure 7 shows the result of the finite element analysis. After an iterative computation for the sizes, 20 mm thickness at the point of division, 26 mm girdle thickness and 16 mm pipe thickness in the branched segments has been identified. Both the manufacturability and the performance were considered during the analysis.
Figure 9 Stress distribution in the geometry
6. Conclusion
The paper discusses over the design of bifurcation for a hydropower plant. The hydraulic design of the bifurcation was performed considering the standard design guidelines and further computational analysis regarding selection of angle.
This selected angle was applied with mechanical features and iterative analysis for thickness of pipe and stiffeners was performed. All the analysis was focused on developing optimum geometry of the bifurcation with minimum head losses for reliability.
Based upon this design experience, it can be seen that the hydraulic and mechanical structures are either operating in risk or in high safety factor. A proper engineered structure should be hydraulically, structurally and economically optimum for sustainability of the plant and it is the recommendation to uplift the existing standard with modern days design and developmental tools.
7. Bibliography
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5. Miller, D.S. Internal Flow Systems. 2. Cranfield : BHRA, 1990. 6. Institute, American Iron and Steel. Buried Steel Penstocks. 1998.
7. Dobler, W., Knoblauch, H. and Zenz, G. Hydraulic investigation of a Y-bifurcator . University of Garz. 1995. 8. Bureau of Indian Standards. IS 11639 - 2 India, 1995.
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10. Reclamation, Bureau of. Stress Analysis of Wye Branches . s.l. : United States Department of the Interior , 1964.
