Trashrack cleaning the German way10 March 1998
In the past, trashrack cleaning systems have not been viewed as one of the most important pieces of equipment at hydropower plants. Using the experience of German hydro stations to state his case, Roland Muhr* explains why the situation is changing.
The improved efficiency of turbines will always be a priority for hydropower plant operators and, in comparison, automatic trashrack cleaning is often seen as a necessary evil. However, recognition must be given to the fact that an appropriately designed trashrack cleaning system guarantees maximum efficiency, as well as operational and staff cost-savings. Furthermore, performance and revenue losses can be avoided at low water flows where savings can amount to over 10%.
As in all fields of technical science, advances have been made in the development of trashrack cleaning systems over the last few decades. In the past, large rack areas were cleaned by hand. This led to increasing staff costs and resulted in a desire for automation and the situation today where mechanical cleaning is now used for even the smallest rack areas.
The first generation of automatic trashrack cleaners were of a chain or cable variety but, over a decade ago, a second generation of hydraulic trashrack cleaning systems was developed. Their advantages of improved operational safety, maintenance and flexibility eventually led to the displacement of conventional trashrack cleaners.
Hydraulic trashrack cleaning systems exhibit the following special features:
•At a standstill all elements are situated above the water thus increasing durability of the plant. Maintenance is a relatively quick and easy process. Assembling the plant is also much easier and, if necessary, can take place during full flow of water.
•The rake has unrestricted movement and reaches well in front of the rack, following large objects like trees, which can then be grabbed and removed. During the whole trashrack cleaning process the rake is pressed onto the rack by the hydraulic cylinder, guaranteeing thorough cleaning of the rack. The pressure is individually adjustable.
•Relatively simple construction including usage of hydraulic techniques ensures an almost wear- and maintenance-free operation.
•Due to the hydraulic drive the trashrack cleaning system operates quietly and is therefore well accepted in noise-sensitive areas, such as residential districts.
German operating examples
At the Krämermühle hydropower plant in Gräfelfing a telescope trashrack cleaner, with a 3.60m wide rack and a 2.60m cleaning depth, is used.
The telescope version is mainly used for smaller rack areas up to 25m2. This version is equipped with a swivelling telescope arm which is fixed at a base. The base is seated in the middle of the sealing of the intake construction - if there is no sealing or the sealing is not strong enough the base can be substituted by a steel construction which transfers the forces to the side walls of the intake. The rake is fixed at the telescope arm.
When starting the cleaning process, the swivelling cylinder lifts the telescope arm from its rest position and the hydraulic cylinder, integrated in the telescope tube, rises. The most extensive grabbing in front of the trashrack is achieved when the telescope arm has completely risen (the grabbing depends on the dimension of the plant and differs between 1m and 3m). When the telescope arm has risen to its full height, the swivelling cylinder drives the rake against the trashrack. Afterwards, the telescope cylinder retracts and lifts the rake. During the whole cleaning process the pressure and power of the rake’s stroke are infinitely adjustable.
The trashrack cleaning process can be started by hand, time control or by measuring the difference in water levels. The control unit and hydraulic aggregate can be situated inside the power house as well as in the base of the trashrack cleaner (in this case, the aggregate is in the base and the control unit inside the power house). Operation during winter does not cause any problems as the plant is equipped with oil heating. Furthermore, the trashrack cleaner can be equipped with an ice programme.
As the intake of the hydropower plant Krämermühle is situated just in front of a guest-house, the quiet operation of the hydraulic trashrack cleaner is a distinct advantage.
At the Bäckermühle hydropower plant in München the trashrack cleaner has an angular extension arm which enables stationary machines to clean rack areas up to 100m2 in one step. The cleaner has a 10.50m wide rack and a cleaning depth of 3.50m. Construction and working operation are similar to a hydraulic excavator.
When starting the cleaning process, the rake is lifted by swivelling the cylinder. The rake moves in a circular motion towards the bottom of the trashrack and the angular extension arm system performs the most extensive grabbing in front of the trashrack when the telescope arm is at full height. The grabbing depends on the dimension of the plant but it can reach over 4m.
One of the latest developments concerning trashrack cleaning technology is the EMRA-3000 system manufactured by muhr of Germany. This is described as a trashrack cleaning and raking transportation system rolled into one. Equipped with a grab rake and an operator cabin it can clean to depths of 15m. Due to the product’s multifunctional construction drifting objects can be grabbed and loaded. Rack areas and dam beams can also be tackled.
Automatic operation on the EMRA-3000 is used for rack cleaning and disposal of rakings. When the water level controller registers clogging at parts of the trashrack, the cleaner moves out of the neutral position and stops at the registered point of the trashrack. According to its width the respective trashrack section will be cleaned in one or more cleaning steps. After each cleaning step rakings are taken by the grab rake, transported and dropped into a container.
Manual operation is used for grabbing and loading drifting objects, surface skimming or for handling rack sections or dam beams. All single functions of the trashrack cleaner can be conveniently controlled by two operating handles positioned in the operator’s cabin.
The trashrack cleaner can be used in a similar way to an excavator; allowing the equipment to be positioned accurately to pick out drifting objects.
Hydropower plants which are utilising the EMRA 3000 include the Schönmühl plant, which previously used a cable cleaner. The trashracks of the two different intakes are now cleaned depending on the clogging of each trashrack. At each cleaning step the rakings are taken from the trashrack by the grab rake. Afterwards the trashrack cleaner drives to the disposal box, turns 120° and drops the rakings directly into the box. Bulky rakings lying in front of the trashrack are taken by the grab rake and transported to the disposal pit.
At the Klosterbuch plant the EMRA 3000 trashrack cleaner is used manually for taking the rakings out of the disposal pit. The equipment was also used for mounting the new trashrack.
Due to their efficient operation, an increasing number of hydropower plant operators are replacing conventional cable and chain trashrack cleaners with hydraulic cleaning systems. All the while, further advances are being made to increase the systems’ cleaning depths and their ability to prevent objects from drifting dangerously near turbines. Such developments are improving the value of trashrack cleaning systems throughout the hydropower industry and one of the greatest success stories is the Isar-Amperwerke plant in Haag, which operates the largest hydraulic trashrack cleaner in the world.