A Review on Sediment Erosion Challenges in Hydraulic Turbines

Sediment constitutes several mineral compositions depending upon the geological formation and geography. In many of the rivers in Himalayas and Andes, Quartz is found as a main constituent (more than 50%), along with feldspar and other hard minerals. These particles have hardness more than 5 Moh’s scale, which is capable to erode turbine components. In hydraulic turbines, flow is highly turbulent and unsteady, which can aggravate the erosion problems. Depending upon the nature of the flow, different components of turbines are eroded with different mechanisms. This chapter will provide a review on how various flow phenomena is responsible for particular types of erosion in turbines and their potential consequences. Some examples of the effect in existing power plants will be shown. This chapter will also discuss about some preventive measures that have been proposed and implemented to reduce the impact of the sediment particles in hydraulic machineries

Neonatal Septicemia in Nepal: Early-Onset versus Late-Onset

Introduction. Neonatal septicemia is defined as infection in the first 28 days of life. Early-onset neonatal septicemia and late-onset neonatal septicemia are defined as illnesses appearing from birth to three days and from four to twenty-eight days postnatally, respectively. Methods. In this cross-sectional study, blood samples from the suspected infants were collected and processed in the bacteriology laboratory. The growth was identified by standard microbiological protocol and the antibiotic sensitivity testing was carried out by modified Kirby-Bauer disk diffusion method. Results. Among total suspected cases, the septicemia was confirmed in 116 (12.6%) neonates. Early-onset septicemia (EOS) was observed in 82 infants and late-onset septicemia (LOS) in 34 infants. Coagulase-negative staphylococcus (CoNS) (46.6%) was the predominant Gram-positive organism isolated from EOS as well as from LOS cases followed by Staphylococcus aureus (14.6%). Acinetobacter species (9.5%) was the predominant Gram-negative organism followed by Klebsiella pneumoniae (7.7%). Conclusions. The result of our study reveals that the CoNS, Staphylococcus aureus, Acinetobacter spp., and Klebsiella pneumoniae are the most common etiological agents of neonatal septicemia. In particular, since rate of CoNS causing sepsis is alarming, prompting concern to curb the excess burden of CoNS infection is necessary

Childhood Septicemia in Nepal: Documenting the Bacterial Etiology and Its Susceptibility to Antibiotics

Introduction. Children are among the most vulnerable population groups to contract illnesses. The varying microbiological pattern of septicemia warrants the need for an ongoing review of the causative organisms and their antimicrobial susceptibility pattern. Therefore, the objective of this study was to document the bacterial etiology of childhood septicemia and its antibiotic susceptibility profile. Methods. Cross-sectional type of study in 1630 suspected patients was conducted at CMCTH from January 2012 to December 2013. Blood samples were collected aseptically for culture. The organisms grown were identified by standard microbiological methods recommended by American Society for Microbiology (ASM) and subjected to antibiotic susceptibility testing by modified Kirby-Bauer disk diffusion method. Methicillin resistance was confirmed using cefoxitin and oxacillin disks methods. Results. Septicemia was detected in 172 (10.6%) cases. Among Gram-positive organisms, coagulase negative staphylococci (CoNS) were leading pathogen and Acinetobacter spp. were leading pathogen among Gram-negative isolates. Vancomycin, teicoplanin, and clindamycin were the most effective antibiotics against Gram-positive isolates while amikacin was effective against Gram-positive as well as Gram-negative isolates. Methicillin resistance was detected in 44.4% of Staphylococcus aureus. Conclusions. This study has highlighted the burden of bacterial etiology for septicemia among children in a tertiary care center of central Nepal

Sediment Erosion in Hydro turbines

Sediment erosion is caused by the dynamic action of sediment flowing along with water impacting against a solid surface. Hydraulic turbine components operating in sediment-laden water are subject to abrasive and erosive wear. This wear not only reduces the efficiency and the life of the turbine but also causes problems in operation and maintenance, which ultimately leads to economic losses. This is a global operation and maintenance problem of hydropower plants. The high sediment concentration combined with high percentage of quartz content in water causes severe damage to hydraulic turbine components. Withdrawal of clean water from the river for power production is expensive due to design, construction and operation of sediment settling basins. Even with the settling basins, 100 % removal of fine sediments is impossible and uneconomical. A number of factors can influence the process of sediment erosion damage in hydro turbine components. The erosion intensity depends on the sediment type and its characteristics (shape, size, hardness, concentration etc.), hydraulic design and operating conditions of turbine (flow rate, head, rotational speed, velocity, acceleration, turbulence, impingement angle etc.), and material used for the turbine components. All these factors are needed to be considered for predicting the erosion. Therefore, dealing with sediment erosion problems requires a multidisciplinary approach. More research and development is needed to investigate the relationship between the particle movement and erosion inside a turbine and to establish the operating strategy for the turbine operating in sediment-laden water. In order to achieve the main objective of this PhD study, the overall research methodology adopted for this work ‘sediment erosion in hydro turbines’ include; experimental studies, numerical simulation, and field studies. This research work is based on result from laboratory experiment, and numerical simulation. A previously made test rig (Thapa, 2004), was reviewed and modified to create a strong swirl flow in curved path. This flow was found similar to the flow between the guide vane outlet and the runner inlet of a Francis turbine. The flow in the guide vane cascade was simulated in order to verify the particle separation process and to investigate the relation of the velocity and the drag coefficient with different shape and size of the particle. There was a provision to introduce particles, with sizes ranging from 1 to 10 mm, and to observe the motion of the particles from Plexiglas windows located on the cover of the tank using a high-speed digital camera. When a particle is flowing in swirl flow, drag force and centrifugal force are two major forces influencing the particle equilibrium. The equilibrium of these two forces provides a critical diameter of the particle. While, a particle larger than the critical diameter move away from the centre and hit the wall, a particle smaller than the critical diameter flows along with the water, and ultimately sinks. For critical diameter, the particle continues to rotate in the turbine. Different shapes and sizes of particles were tested with the same operating conditions and found that triangularly shaped particles were more likely to hit the suction side of the guide vane cascade. Furthermore, this study supports the concept of separation of particles from streamlines inside the test rig, which led to the development of an operating strategy for a Francis turbine processing sediment-laden water. This study also permitted experimental verification of the size and the shape of a particle as it orbits in the turbine, until either the velocity components are changed or the particle became smaller. The steady state numerical simulations were carried out on the Cahua power plant Francis turbine design, mainly at two operating conditions with varying particle size, shape, and concentration using ANSYS CFX. The predictions of erosion, based on the Lagrangian calculation of particle paths in a viscous flow, are described for stay vanes, guide vanes, and runner vanes of a Francis turbine, for which the results of the field tests have been available for verification. The flow simulation was obtained through use of a commercially available computational fluids dynamics (CFD) code, namely ANSYS CFX.  The code utilizes a finite-volume, multi-block approach to solve the governing equations of fluid motion numerically on a user-defined computational grid. The flow solution procedure first generates the computational grid. A pre-processor is available in the software to perform this task. Second, the solution option such as inlet and boundary conditions, turbulence model, and discretization scheme, are specified. The final step is running the flow solver to generate the actual flow field simulation. Sediment erosion analysis of a Francis turbine gives an indication of relative erosion intensity and critical zones of erosion damage of the turbine components. The most realistic numerical prediction of erosion is found on a turbine blade. The highest velocities and accelerations occurred at outlet of the runner blade and more erosion was predicted especially at the pressure side of the blade outlet and at the lower cover. Furthermore, unexpected sediment erosion was found at the suction side of the guide vane where concept of critical diameter can be utilized. It has been concluded that if the particle size in the water is more than critical particle sizes, the turbine should not be operated at low guide vane opening. The numerically obtained erosion pattern and the field test observation and inspection at Cahua Francis turbine components are in good qualitative agreement. The encouraging agreement shows that, for this application, numerical simulation really can be used in a predictive manner. This information may serve as an input in an early stage of turbine design process to identify the regions where special surface treatment is necessary in order to increase the lifetime of the components for new hydropower projects involving risks of sediment erosion. The size of a particle is inversely proportional to the velocity of the particle, and it was determined that spherically shaped particles had higher settling velocities than particles with other shapes. However, non-spherical shape of the particles will tend to have lower settling velocities because both decreases in spheroid and increases in angularity tend to decrease velocities. Moreover, larger cross-sectional areas tend to be directed perpendicular to the transport path.  As a result, higher coefficient of drag, higher rotational motion and more separation of flow are likely to occur and hence more erosion rate was predicted.  The roles played by the shape of the particle significantly affect erosion rate prediction inside the Francis turbine components. Furthermore, it has been found that the erosion process is strongly dependent on the particle size, shape, concentration, and operating conditions of the turbine. The reduction of the erosion is not only linked to the reduction of particle velocity but also is linked to the reduction of separation of flow, which further depends on shape, size, and concentration of the particle. The significant reduction of erosion rate can be achieved by operating turbine at best efficiency point. The full load operation reduced efficiency, increased turbulence, and increased relative velocity of flow at outlet of the blades. The present knowledge and findings, although may not be enough to deal with this problem completely, can be utilised to achieve one major step forward in sediment erosion prediction and prevention.      

Numerical investigation of the effect of leakage flow through erosion-induced clearance gaps of guide vanes on the performance of Francis turbines

Abrasive wear in the clearance gap of guide vanes (GVs) increases the gap size, which deteriorates the flow and causes loss of efficiency. This paper investigates the performance of a Francis turbine including erosion-induced clearance gaps on the GVs. The effect of the gap on the performance of the turbine is studied numerically, by using the GV and runner blade passages. The results are compared with an experiment conducted in a single GV rig, developed for the same model. Simulations are performed for GVs with NACA0012, NACA2412 and NACA4412 profiles with each at 11 operating conditions. It is found that the clearance gap induces a leakage flow due to the pressure difference between adjacent sides. The leakage flow mixes with the main flow, forming a vortex filament, which is driven inside the runner. By using an example of a power plant in Nepal affected by sediment erosion, it is found that these vortices containing sediment particles erode the inlet of the runner blade towards hub and shroud. Comparison between the three NACA profiles shows that NACA0012, which is the current shape of GV in the plant, causes a maximum loss due to the leakage flow. The asymmetrical profiles contrarily are found to increase the efficiency of the turbine at all operating conditions. Such profiles are also inferred to have the minimum influence of erosion and pressure pulsations problems at runner inlet. In short, this paper gives an overview of the potential effect of the eroded GV on the turbine’s performance and compares different GV profiles to minimize such effects

Effect of Guide Vane Clearance Gap on Francis Turbine Performance

Francis turbine guide vanes have pivoted support with external control mechanism, for conversion of pressure to kinetic energy and to direct them to runner vanes. This movement along the support is dependent on variation of load and flow (operating conditions). Small clearance gaps between facing plates and the upper and lower guide vane tips are available to aid this movement, through which leakage flow occurs. This secondary flow disturbs the main flow stream, resulting performance loss. Additionally, these increased horseshoe vortex, in presence of sand, when crosses through the gaps, both the surfaces are eroded. This causes further serious effect on performance and structural property by increasing gaps. This paper discusses the observation of the severity in hydropower plants and effect of clearance gaps on general performance of the Francis turbine through computational methods. It also relates the primary result with the empirical relation for leakage flow prediction. Additionally, a possible method to computationally estimate thickness depletion has also been presented. With increasing clearance gap, leakage increases, which lowers energy conversion and turbine efficiency along with larger secondary vortex

The numerical and experimental investigation of erosion induced leakage flow through guide vanes of Francis turbine

In Guide Vanes (GV) of Francis turbines, a portion of the pressure head of water converts into velocity head. This causes high acceleration of the flow in GV before reaching the runner. Furthermore, GVs are accompanied with a small clearance gap at both ends to adjust the opening angle based on various operating conditions. In the case of sediment affected power plants, the hard fine particles mixed in water erode the connecting ends due to horse-shoe vortices. This erosion together with the head cover deflection due to water pressure increases the size of the gap. Due to the adjacent pressure and suction sides in GV, the flow passes through the gap from high pressure side to low pressure or suction side. This leakage flow disturbs the main flow in the suction side, which can be observed in the form of a vortex filament. Depending upon the GV profile and opening angle, the vortex can have different characteristics. This study uses numerical and experimental techniques to study the potential effects of the leakage flow in overall performances of the turbine. The experiment is done to measure the velocity field around GV using Particle Image Velocimetry (PIV) technique on a GV cascade rig. The GV in this rig corresponds to 1:1 scale model of 4.1 MW Francis turbine, with the chord length of 142 mm and span height of 97 mm. Similarly, 14 pressure senssors are placed around the GV cover plate to measure the GV loading. The velocity and pressure field are compared with with the results from CFD. In the study, two GV-profiles and 7 GV angels are studied. Results show that at Best Efficiency Point (BEP) and small opening or closing, the pressure difference between the adjacent sides of GV and consequently, the leakage flow and the intensity of the vortex filament in NACA4412 is less than in NACA0012. However, at high opening angle or during full load, the direction of the leakage flow in NACA4412 is in opposite direction due to small or negative GV loading compared to BEP. It is shown how these vortices affect the runner performances and how the particles erode the runner inlet as a consequence of these vortices

Numerical investigation of the effet of leakage flow through erosion-induced clearance gaps of guide vanes on the performance of Francis turbines

Abrasive wear in the clearance gap of guide vanes (GVs) increases the gap size, which deteriorates the flow and causes loss of efficiency. This paper investigates the performance of a Francis turbine including erosion-induced clearance gaps on the GVs. The effect of the gap on the performance of the turbine is studied numerically, by using the GV and runner blade passages. The results are compared with an experiment conducted in a single GV rig, developed for the same model. Simulations are performed for GVs with NACA0012, NACA2412 and NACA4412 profiles with each at 11 operating conditions. It is found that the clearance gap induces a leakage flow due to the pressure difference between adjacent sides. The leakage flow mixes with the main flow, forming a vortex filament, which is driven inside the runner. By using an example of a power plant in Nepal affected by sediment erosion, it is found that these vortices containing sediment particles erode the inlet of the runner blade towards hub and shroud. Comparison between the three NACA profiles shows that NACA0012, which is the current shape of GV in the plant, causes a maximum loss due to the leakage flow. The asymmetrical profiles contrarily are found to increase the efficiency of the turbine at all operating conditions. Such profiles are also inferred to have the minimum influence of erosion and pressure pulsations problems at runner inlet. In short, this paper gives an overview of the potential effect of the eroded GV on the turbine’s performance and compares different GV profiles to minimize such effects

Selection of Optimal Number of Francis Runner Blades for a Sediment Laden Micro Hydropower Plant in Nepal

The present study is conducted to identify a better design and optimal number of Francis runner blades for sediment laden high head micro hydropower site, Tara Khola in the Baglung district of Nepal. The runner is designed with in-house code and Computational Fluid Dynamics (CFD) analysis is performed to evaluate the performance with three configurations; 11, 13 and 17 numbers of runner blades. The three sets of runners were also investigated for the sediment erosion tendency. The runner with 13 blades shows better performance at design as well as in variable discharge conditions. 96.2% efficiency is obtained from the runner with 13 blades at the design point, and the runners with 17 and 11 blades have 88.25% and 76.63% efficiencies respectively. Further, the runner with 13 blades has better manufacturability than the runner with 17 blades as it has long and highly curved blade with small gaps between the blades, but it comes with 65% more erosion tendency than in the runner with 17 blades