Experimental investigation and mitigation of pressure pulsations in Francis turbines

Hydraulic turbomachines, such as Francis turbines, have been utilized for more than a century to generate renewable energy. Research and development has led to an outstanding level of efficiency and reliability, with the Francis turbine at the top end with almost 96% efficiency. To achieve such levels of efficiency, the turbine has to be design for a specific operating point. The development of a continental transmission grid and introduction of intermittent renewables such as wind and solar has led to a fundamental change in the energy market. The hydraulic turbomachines in hydro power has now become one of the stabilizing force of an energy market demanding flexibility. Amongst other reasons, this has led to operation of hydraulic turbines outside the design point. Operating turbines outside the design point may lead to heavy vibrations and potentially mechanical failure. In addition, modern design is focused on increasing efficiency, possibly at the expense of the runner characteristics outside the design load. The vibrations are a consequence of the turbine, which generates pressure pulsations. The pressure pulsations propagates into the water conduit. In case of resonance, the pressure pulsations will increase in amplitude and induce further vibrations. However, they can be reduced. The main objective of this thesis has been to investigate and possibly reduced the pressure pulsations occurring at part load operation. An experimental investigation of different methods to reduce pressure pulsation in Francis turbines has been carried out. Air injection was investigated at La Higuera Hydro Power Plant (HPP) in Chile. A free rotating runner cone extension (FRUCE) was developed and investigated at the Waterpower Laboratory and in Leirfossene HPP. The air injection through the runner cone and draft tube wall was tested in La Higuera HPP. Both options gave a significant dampening of pressure pulsations at part load by reducing the maximum value of the peak-to-peak values by 60%. However, an increase in the vortex rope frequency was observed when air was injected. At the Waterpower Laboratory, the free rotating runner cone extension was tested with different lengths. The developed FRUCE is an extension of the runner cone where the outer shell is mounted on bearings allowing it to rotate independently from the turbine. It was concluded that the FRUCE had some dampening effect, but that the diameter was too small to achieve a significant dampening effect of the pressure pulsations. Three FRUCEs with different length and diameter were designed and tested at Leirfossene HPP. A dampening in pressure pulsations was achieved at part load operation. However, different FRUCEs worked best at different loads. The FRUCE also reduced pressure pulsations at full load, but the drawback is reduced efficiency at full load. An adjustment of the FRUCE length and diameter is necessary to achieve the maximum dampening. The FRUCE had development potential and may be possible solution to reduce pressure pulsations in Francis turbines. However, it will require numeric simulations, further prototype testing and possibly implementation of technology to actively control the FRUCE length and possibly the rotational speed

Hydraulic design of Francis turbine exposed to sediment erosion

High concentrations of sediments is a serious problem for hydropower stations in the Himalayas and the Andes Mountains. For run-of-river power plants sediment causes heavy erosion even with settling basins. This leads to reduced operating hours and high maintenance cost. In addition, the original design experienced problem with heavy cavitation.The objective of this master thesis is to carry out new hydraulic design of the runner and guide vanes of the existing Francis turbines in La Higuera Power Plant with reduced velocity components. To achieve this the cause of the heavy cavitation, which made the turbine fail, has to be established.Results from numerical simulations indicates a low pressure zone causing heavy leading edge cavitation is the reason for the turbine failure. The off-design operation has made the cavitation even worse.To carry out a new design, the in-house design software Khoj was used. Some new parameters, like blade leaning, were included in the program. Blade leaning is an important tool for pressure balancing the runner blade. Further, a parameter study was carried out to investigate the effect of blade leaning, blade angle distribution and blade length. The numerical simulation indicates proper pressure balancing could have avoided the cavitation problems and a new design should have an X-blade shape. Because the power plant is already built, the number of variables is limited. The rotational speed, inlet and outlet diameter remained constant. This made it impossible to significantly reduce the relative velocities. Therefore, coating of all wet surfaces is proposed to reduce the effect of erosion.The main objective for this thesis has been to identify the cause of the turbine failure and develop a new design to fit in the existing power plant. Complete 3D-drawings of the design, including runner and guide vanes, has not been made due to lack of time

Evaluation of runner cone extension to dampen pressure pulsations in a Francis model turbine

Today's energy market has a high demand of flexibility due to introduction of other intermittent renewables as wind and solar. To ensure a steady power supply, hydro turbines are often forced to operate more at part load conditions. Originally, turbines were built for steady operation around the best efficiency point. The demand of flexibility, combined with old designs has showed an increase in turbines having problems with hydrodynamic instabilities such as pressure pulsations. Different methods have been investigated to mitigate pressure pulsations. Air injection shows a significant reduction of pressure pulsation amplitudes. However, installation of air injection requires extra piping and a compressor. Investigation of other methods such as shaft extension shows promising results for some operational points, but may significantly reduce the efficiency of the turbine at other operational points. The installation of an extension of the runner cone has been investigated at NTNU by Vekve in 2004. This has resulted in a cylindrical extension at Litjfossen Power Plant in Norway, where the bolt suffered mechanical failure. This indicates high amplitude pressure pulsations in the draft tube centre. The high pressure pulsation amplitudes are believed to be related to high tangential velocity in the draft tube. The mentioned runner cone extension has further been developed to a freely rotating extension. The objective is to reduce the tangential velocity in the draft tube and thereby the pressure pulsation amplitudes

Investigation of the unsteady pressure pulsations in the prototype Francis turbines during load variation and startup

This work investigates the unsteady pressure fluctuations in two prototype Francis turbines during load variation and start-up. Although hydraulic turbines are expected to experience such events over their lifetime, the resulting pressure amplitudes are so significant that they take a toll on a machine's operating life. The interest of the present study is to experimentally measure and numerically characterize time-dependent pressure pulsations. Specific focus is on (1) how pressure pulsations of both synchronous and asynchronous types in vertical- and horizontal-axis turbines change when the load of a turbine changes from steady conditions, (2) what the pressure amplitudes during load change are, and (3) how quickly pressure amplitudes vary when a generator is synchronized to the power grid (load) during start-up. To this end, four pressure sensors were integrated in the draft tube cone. The results are quite interesting, especially during transition from the steady state to the transient load change. In the vertical-axis turbine, amplitudes of asynchronous pressure pulsations are 20 times larger than those of the synchronous component; whereas, in the horizontal-axis turbine, amplitudes of asynchronous pressure pulsations are two times smaller than those of the synchronous component. During a load change, amplitudes of synchronous pressure pulsations are nearly double compared with the asynchronous component. For the turbine startup, only synchronous-type pressure pulsations are found and the flow was asymmetrical over the draft tube circumference

Investigation of the Unsteady Pressure Pulsations in the Prototype Francis Turbines -Part 1: Steady State Operating Conditions

Hydropower is one of the most reliable renewable sources of electricity generation. With high efficiency and good regulating capacity, hydropower has the ability to meet rapid changes in power demand. Large investments in intermittent renewable energy resources have increased the demand for balancing power. This demand has pushed hydraulic turbines to generate electricity over the operating range from part load to full load. High-amplitude pressure pulsations are developed at off-design conditions, which cause moderate damage to the turbine components. The pressure pulsations may be either synchronous- (axial)-type, asynchronous- (rotating)-type or both. In this study, pressure measurements on low specific-speed prototype Francis turbines were performed; one of them was vertical axis and another was horizontal axis type. Four pressure sensors were mounted on the surface of the draft tube cone. Pressure measurements were performed at five operating points. The investigations showed that, in the vertical axis turbine, amplitudes of asynchronous pressure pulsations were 20 times larger than those of the synchronous component; whereas, in the horizontal axis turbine, amplitudes of asynchronous pressure pulsations were two times smaller than those of the synchronous component. For part 2 of the paper, please read Trivedi, C., Gogstad, P. J., and Dahlhaug, O. G., 2017, “Investigation of Unsteady Pressure Pulsations in the Prototype Francis Turbines during Load Variation and Startup,” Journal of Renewable Sustainable Energy, 9(6), p. 064502. https://doi.org/10.1063/1.4994884