In view of growing worldwide environmental awareness and reinforced environmental laws, the protection of fish is gaining in significance as an influencing factor in the hydropower licensing process.

In the EU, for example, hydropower construction work is subject to the guidelines of the European Water Framework Directive, which applies to new constructions and refurbishment projects as well as to the renewal of water rights. As a result, current technical developments are coming into focus, including “fish-friendly turbines” and modified guard grating systems that provide a barrier in the intake area to keep aquatic animals away from the turbine. Where screen cleaning is concerned, Bavarian industry specialist “Erhard Muhr GmbH” already has a string of innovative, high-quality reference projects under its belt.

So it is no surprise, that particularly intake screens have seen a marked increase in demand for horizontal designs in recent years. This is because horizontally aligned bars are inherently more efficient than vertical designs in keeping fish away. As a result, horizontal screens are usually accepted by fish protection authorities with a wider bar spacing than vertical systems. For hydropower operators, this has the added advantage of higher flow rates and reduced cleaning expenses. „Of course, horizontal screens, like vertical ones, require proper equipment for efficient cleaning. Due to the different orientation of the cleaning motion there are ­distinct differences in terms of the technical ­demands on the cleaning systems and their ­resulting design,” explains Florian Kufner, a ­trade journalist with specialist expertise in Muhr technology. It is obvious that horizontal cleaning systems have no commons with the traditional cable operated systems for vertical screens. But there are also significant differences compared to hydraulically operated vertical cleaning systems. According to Kufner, especially the drive system and the hydraulic control require different solutions, whereas many mechanical parts are quite similar.

More than thirty years’ experience
Erhard Muhr GmbH has been designing, manufacturing and installing hydraulic screen cleaners for hydropower stations since 1983. The company, which is headquartered in Brannenburg, Bavaria, delivered its first horizontally aligned screen cleaner in 1989. “A lot of our previous experience in hydraulic vertical cleaning went into the development of our horizontal cleaners. In this ­particular design, the hydraulic controls ensure that the grab rake has an accurate, even contact pressure. This is a huge advantage in terms of operational stability,” says Kufner. Whereas standardised machine configurations can be used in small-sized arrangements or facilities with relatively shallow cleaning depths of up to 3 m, larger-sized facilities and sites with greater cleaning depths require the equipment to be adjusted individually to constructional and locational conditions. Where debris filtering is concerned, there are various options. In practice, the choice depends on the answer to one central question: is it permissible to leave the floating debris in the river and simply dispose of it through a flushing gate, or are there reasons against leaving it in the watercourse?
“Muhr offers a variety of solutions, from simple arrangements with all-electric drives to large-scale systems with grab rakes and movable machines with slewing gears. Of course, Muhr also provides the ­required screens, valves and other hydro steel structures. This ensures that all components are perfectly compatible, and customers benefit from having to deal only with a single-source provider – this makes things much easier in terms of warranty and servicing, for example,” says Kufner.

Horizontal screen cleaning systems “made by MUHR”
Static systems are the most basic type of horizontal screen cleaners. The name is not quite accurate, actually, as the rake does move across the entire length of the screen. But since the entire unit is firmly attached to the building, it is, by definition, a statically installed screen cleaning device. A reference product of this type, the HYDRONIC H-3500, is in use at hydropower plant Holenstein in Switzerland. As the name says, this particular machine moves horizontally to clean, at a cleaning depth of 2.3 m, a horizontal rack that measures approximately 21 m in length and 2 m in height.

Cleaning on the right track
“An alternative to static systems are movable ones. Equipped with a travel carriage, these machines move along the screen on rail tracks. This type of system is suitable primarily for long screens and/or greater cleaning depths. In some cases, gear racks are used, depending on the forces that the system has to withstand,” says Kufner, referring to the Muhr HYDRONIC H-6500 V system at hydropower plant Raguhn in Eastern Germany as an example: “The machine there cleans a 28 m screen using a 4 1/2 m wide rack at a depth of 5 1/2 m. Considering the high flow velocity of 7 1/2 m/s, this arrangement required a really sturdy and powerful design. In this case, the screen cleaner is also fitted with a hydraulic gantry that has an orange-peel grab for removing larger-sized floating debris.”

Multi-purpose screen cleaners
Multi-purpose screen cleaners for horizontal screen cleaning are built by equipping a movable rig with slewing gears, a grab rake and controls. In addition to cleaning screens, systems of this type can also be used for grabbing the collected debris and depositing it at a designated location for disposal. A multi-purpose machine of this type, a HYDRONIC H-9500 BDV, is used at hydropower plant Keselstraße in Kempten in Baden-Württemberg, Germany. Equipped with a driver cabin, this machine is designed for the automated cleaning of a horizontal screen that measures around 23 m in length. The grab rake has a cleaning depth of around 6 m and a cleaning width of 3.3 m.

An intelligent alternative: Screen drum systems
With its RO-TEC screen drum units, Muhr offers a state-of-the-art alternative to the combination of horizontal racks and screen cleaners. A constant revolving motion generated by an integrated electric motor enables the drums to virtually ‘self-clean’, thus ensuring consistently high flow rates. Screen drums are suitable for intakes of any width. Solutions range from single-drum arrangements for small overflow or intake channels to arrangements consisting of any number of drums in a row that can reach across even the widest of intake structures. However, as Kufner points out, there is one condition that intakes must meet to enable the use of screen drums: to ensure that the collected debris can be properly flushed down the watercourse, the intake must run parallel – or, at the most, at a slight angle – to the direction of the water flow.

Screen drums have many benefits
The true highlights of this innovative ­guard grating system, says Kufner, are its unique ease of installation and maintenance. Each screen drum is completely pre-­assembled at the factory. On-site installation consists merely of placing the drum onto the guide rails and connecting it to its power supply. The guide rails are the same as the ones that are used for stop logs. This provides the system’s second big advantage: Individual drum modules can be removed for inspection purposes and replaced by stop logs that can be placed on the guide rails. “This way, the removed modules can be inspected, cleaned and maintained properly on dry land. Normally this is necessary only in individual cases, as the drum units are designed to be maintenance-free and wear-resistant,” explains Kufner.“
As far as fish protection is concerned, the screen drums are considered to be the current benchmark. The specific flow conditions around the drums have a naturally ‘repellent’ effect on aquatic animals. Even in case a fish happens to be bold enough to approach the drum screens, there is no real danger, as the drums revolve very slowly, and there are no sharp edges or other harmful elements on the system.” Last but not least, the innovative technology is also easy on the eye, as the installed drums are almost invisible.