Floating debris
The flotsam rake is a hydraulic structure to catch flotsam that could damage a subsequent system.
Flotsam rakes are mostly located in power station or pumping station inlet channels. A distinction is made between coarse screens with clear bar spacings of 100 mm or more and fine screens, for which bar spacings between 10 and 20 mm are currently selected for new systems. In many cases the fine screen is preceded by a coarse screen. To protect fish from swimming in, a fine screen is necessary.
A floating debris rake consists of a large number of parallel metal rods of different cross-sections. The rectangular cross-sections that used to be common are increasingly being replaced by profiles that are more favorable in terms of hydraulics and cleaning technology. The slide bars can be arranged either vertically or horizontally to the river bed, the arrangement depending on the fish protection and fish descent concept.
Areas of application
In order to prevent damage to the sensitive turbine systems in hydropower plants by floating debris, a rake is installed in front of the turbine inlet, which in the classic design extends from the bottom of the channel to above the surface of the water. All foreign bodies that exceed the size specified by the distance between the slide bars get stuck there and must be combed out regularly with a trash rack cleaner. A rake provides for the water flowing through a flow resistance is that in the subsequent hydroelectric power plant to a power loss by a reduction in the fall height leads. If the rake is blocked by floating debris, the flow resistance and thus the loss of the height of fall increase significantly. In addition, the back pressure can mechanically damage an occupied rake. In modern hydropower plants, automatic trash rack cleaning machines are used, possibly with flotsam cranes . These are switched on when the water level difference on the screen exceeds a certain adjustable value.
Since the maximum flow rate of the hydropower plant ( absorption capacity ) is a fixed variable, in the event of flooding the outflows exceeding the absorption capacity are diverted via weirs into the old channel of the river or channel. This prevents the river from overflowing above the weir. Since the absorption capacity of the hydropower plant is negligible compared to the runoff of larger floods, even turning off the turbines can hardly noticeably affect the level of the flood. Since the underwater levels rise during floods, the height of fall decreases so much that the output of the turbines is significantly reduced. Since the amount of debris (a lot of driftwood) increases with the flood, the turbines are often shut down during major floods.
Flotsam rakes are also used in sewage treatment plants for wastewater treatment, see rake (sewage treatment plant) .
Examples
- Gasteiger rake , wood rake of the Enns, Styria, from 1570
