Abstract
Hydro-abrasive erosion on hydraulic turbines is economically and energetically important. To better understand the causes and consequences of turbine erosion and to further develop mitigation measures, the suspended sediment concentration (SSC) and size distribution of mineral particles in the turbine water of the 64 MW high-head run-of-river hydropower plant (HPP) Fieschertal, Switzerland, have been continuously measured since 2012. An innovative combination of measuring techniques is used: turbidimeters, single-frequency acoustics, densimetry, laser diffraction and gravimetric analysis of water samples. Automatic warnings have been defined in the HPP’s control system to allow for systematic HPP shutdowns when the SSC in the valve chamber exceeds the threshold value of 10 g/l for at least 15 minutes. During the flood of July 29 and 30, 2017 with an SSC peak of 110 g/l, the intake was closed and the HPP was shut down for half a day. This prevented about 8000 tons of fine sediment from entering the HPP, which corresponds to 9 % of the turbines’ annual suspended sediment load (SSL) in 2017, or 12 % of the SSL in an average year without a significant flood. If the intake had been closed one hour earlier, the SSL could have been further reduced by about 3000 tons. For an earlier closing of the intake in case of floods, the turbidimeter at the intake shall be complemented by an additional instrument capable of measuring also medium to high SSC. Apart from the described flood event, SSC peaks in the range of 5 to 20 g/l occurring typically in late summer were attributed to re-suspension events in the storage tunnel due to HPP operation.
Highlights
Wear of turbine parts due to abrasive particles is an important issue at medium- to high-head hydropower plants (HPPs)
The causes for temporarily elevated suspended sediment concentration (SSC) in the penstock, i.e. sediment transport events leading to higher than average turbine erosion rates for some time, can be classified as follows: i) Operation of the HPP, affecting the settling, transport and re-suspension of sediment particles in the HPP system, mainly in the storage tunnel, and ii) Natural events, i.e. mainly intense or abundant rain leading to increased river discharge, or glacier-related processes
The temporary closing of the intake kept about 8000 tons of fine sediment out of the HPP, which corresponds to 9 % of the annual suspended sediment loads (SSL) in 2017, or 12 % of the average annual SSL in years without a significant flood
Summary
Wear of turbine parts due to abrasive particles is an important issue at medium- to high-head hydropower plants (HPPs). Hydro-abrasive erosion reduces turbine efficiency, HPP availability, electricity generation and revenues, and increases costs for repairs and replacements. Turbine erosion is of interest to HPP owners, operators, consultants, turbine manufacturers and authorities for mainly technical and economic reasons. The suspended sediment mass concentration (SSC) and the particle size distribution (PSD) in the turbine water may vary quickly and considerably over time, leading to a large variation in the instantaneous turbine erosion rate and induced actual costs. Temporary HPP shutdowns (or “switch-offs”) in periods of exceptionally high erosion potential are an economically interesting option for operational HPP optimization [2] [4] [5]. Real-time suspended sediment monitoring (SSM) with sufficient reliability is required [6]
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