Sediment Erosion in Hydro Turbines

Transcription

1 Sediment Erosion in Hydro Turbines Hari Prasad Neopane, PhD Associate Professor & Head Department of Mechanical Engineering Kathmandu University Nepal

2 Outline of Presentation Status of Electricity Generation of Nepal Introduction to Sediment Erosion Factor affecting Sediment Erosion CCD approach to minimize Sediment Erosion IEC Guidelines for Turbine design Ongoing Studies at TTL Financial Aspects of Sediment Erosion Conclusions and Recommendation

3 Status of Electricity Generation Hydropower Potetial: 83,290 MW (Theoretical) 42,130 MW (Practical) Total Power Supplied: 972 MW Supply Contributed by NEA hydro: 478 MW Annual Peak Demand: 1444 MW Demand Contributed by NEA thermal: 53 MW Contributed by IPP hydro: 441 MW Total Power Shed:472 MW

4 Sediment Erosion Impact of sediment against solid surface Dynamic action of sediment passing through turbine Mechanical wear of components Abrasive and erosive wear Theoretically, erosion increases with (velocity) 3

5 Sediment Erosion Contd.. Both run-of-river and storage hydropower projects suffer from sediment erosion Severe damage to hydro turbine components in runof-river reduces efficiency life of the turbines problem in operation and maintenance leads to economic losses Global operation and maintenance problem e.g. Pelton and high head Francis turbines

6 Erosion in Pelton turbines Inlet system: manifold & valve Nozzle system: nozzle ring & needle Turbine runner: splitter bucket tip bucket surface Rangjung HPP, Bhutan 1) MEL HPP, Norway 1) 1) Dahlhaug 2004, NTNU and KU Andhi Khola HPP, Nepal 1)

7 Erosion in Francis turbines Inlet system: spiral casing and stay vanes Guide vanes system Runner vanes Labyrinth seals Draft tubes and shaft seals Jhimruk Facing plate 1) Jhimruk GV, Nepal 1) 1) Dahlhaug 2004, NTNU and KU Jhimruk Runner, Nepal 1)

8 Factors Responsible for Erosion Sediment type and its characteristics: shape size quantity Hydraulic design and operating condition of turbines: flow rate, head, rotational speed, velocity, acceleration, impingement angle, etc. Material used for turbine components: hardness, chemical composition, elastic properties, surface morphology, etc.

9 CCD approach to minimize Erosion C: Controlling sediment C: Coating existing turbines and their components D: Design new turbine

10 Design Water Conveyance System Valve Material and abrasion resistant coating selection Stainless steel overlays Protection (closing) of the gap between housing and trunnion Stops located outside the valve Proper capacity of inlet valve operator Increase bypass size to allow higher guide vane leakage Bypass system design Turbine Hydraulic Design Mechanical Design Operation Spares and regular inspections Particle sampling and monitoring

11 Guidelines for Turbine design Thicker runner blades may result in decreased efficiency and increased risk of vibrations from von Karman vortices. Fewer runner blades (in order to improve the access to the blade surfaces for thermal spray surface treatment) may result in reduced cavitation performance. Erosion resistant coatings may initially result in increased surface roughness, which may reduce the efficiency. Reduced runner blade overhang may result in reduced cavitation performance, which in turn may reduce the output that can be achieved for a turbine upgrade. Many erosion resistance design features will increase the total cost of the power plant

12 Recent Optimized Design in terms of Erosion Different shapes of the blade angle distribution for the parameter study Efficiency corresponding to runner blade design and operating conditions; SST Turbulence model Erosion on runner blade corresponding to runner blade design and operating conditions: SST model Erosion on the runner at a) BEP b) Full flow condition c) Part flow condition

13 Recent Studies Contd.. Background: Problem In the case of Guide Vanes of Francis turbines A small clearance gap is present for changing the angle according to operating conditions Flow containing sediment particles passes through this gap causing erosion Reduction in efficiency Disturbances in the main flow The gap enforces leakage flow due to pressure difference Increases the size of the gap Increase in erosion

14 Most Recent Studies Contd.. Results from turbine s simulation In cascade CFD in turbine Vortices hitting the runner blade at LE edges

15 Most Recent Studies Contd.. Results from turbine s simulation at all operations At part load and BEP, NACA4412 shows least leakage flow and its consequent effects. However, some negative leakage flow was observed at full load conditions.

16 Issues on Turbine Selection Maintenance /replacement issues for turbine selection (3 days Vs 3 weeks) For the same plant lower specific speed machines are normally bigger and have lower water velocities in the runner outlet For Pelton turbines the water velocity does not depend on the specific speed. However, a lower number of jets is beneficial for a Pelton turbine since the buckets will be larger which in turn gives less water acceleration in the buckets and thus less erosion damage

17 Financial Aspects of Sediment Erosion

18 Previous 1 mm thickness increase of splitter of Pelton turbine means 1 % efficiency drop 20 % increase of outer diameter of Francis turbine reduce 60 % erosion 1 % efficiency loss caused about 1 million of US$/year lost out of 309 MW installed Cent /kwh energy price The significant reduction of erosion rate can be achieved by operating the turbine at best efficiency point

19 Conclusions Sediment content of water can no more be overlooked in any phase of hydropower project implementation One solution in order to decrease the sediment erosion is to increase the size of the turbine, thereby increases the hydraulic radius of curvature, and thus decreases the accelerations. This result in a higher price of the turbine, but it will reduce the maintenance costs during its lifetime. Furthermore, while it is possible to design a Francis turbine to be more resistant against sediment erosion, this may adversely, affect other aspects of the turbine. It should be understood that every hydraulic turbine is a compromise between several requirements.

20 Recommendation Sediment erosion cannot be completely controlled, nor avoided, however be managed It requires multidisciplinary approaches Should consider strong turbine design and selection parameter Variable speed turbine without guide vanes Technical, managerial and economical aspect need to explore further

21 Thank You.