HRW-Hydro Review Worldwide

Choosing Trashrakes for Three Hydro Plants

For two hydro plants in Morocco and one in India, the project owners considered unique site conditions when choosing trashrack cleaning systems. The goal of this selection was to minimize losses and maximize electricity generation.

By Helmut Erdmannsdoerfer, Alfred F. Patzig and Jose Costa Simoes

Selection of a trashrack cleaning system is an important consideration during development of a hydroelectric facility. The proper system is needed to deal with unique site conditions and configurations while ensuring minimal loss of water flow through the intake screen. This article discusses the considerations that went into selecting trashrack cleaning systems for three projects, two in Morocco and one in India.

Understanding trashrakes

Trashrack cleaning machines (also called trashrakes or rack rakers) are used to remove debris that has collected on trashracks or intake screens. This can include organic debris, such as branches and vegetation, as well as industrial trash, such as tires and bicycles. This debris will block the free flow of water to the turbine(s).

All hydro stations require some sort of screen at the intake to protect the turbine blades from damage caused by solids in the flow passing through the turbine. The importance of keeping the intake screen unobstructed through the use of trashrack cleaners is widely acknowledged. Therefore, the inclusion of a trashrake is considered a standard requirement with any new hydropower station.

A number of companies manufacture trashrack cleaning machines for hydroelectric applications. Ossberger has been building automatic trashrack cleaners for about 50 years. The company offers stationary and moveable (traversing) trashrack cleaning machines that can be used at run-of-river hydro facilities, irrigation canals and intake structures on natural lakes, rivers and reservoirs.

Requirements are different for each intake and depend mainly on the type of debris accumulating and the size of the trashrack itself. Hydraulically operated rakes with articulating arms typically are used for intake sizes up to 8 meters wide and 10 meters deep. Chain-driven rakes are stationary machines for larger intakes, maximum size 12 meters wide and 12 meters deep. For extremely deep intakes, cable-operated rakes are used, which can be stationary or traversing, offering rake heads up to 4 meters wide for cleaning depth to 25 meters.

Development of the Ossberger trashrake designs began with chain-driven machines that consist of two support stands mounted on top of the operating deck, with a horizontal shaft and one arm on each side of the intake, equipped with sprocket wheels that guide the endless heavy-duty chain. An electric gear motor turns the shaft, which pulls the chains and the connected horizontal rake head down the two arms and back up, thereby cleaning the trashrack area. Because these chains require periodic maintenance, hydraulically powered rakes came more into use. A single deck-mounted center stand holds the pivoting main arm. The rake head is attached to the second articulating arm, which extends to the bottom of the intake.

However, the geometry limits its application range and cleaning depth. Rake type RRH 3 uses non-telescoping arms, which permits cleaning to 7 meters deep. At deeper intakes, a telescopic arm extension is added to permit cleaning depths up to 10 meters. In theory, deeper reaches could be realized. However, the machine would have to be considerably larger and thus more costly. Therefore, a solution applying a cable drive has been found that permits cleaning of deeper intake structures. The rake head descends to the bottom by gravity and is powered up by a cable winch system.

Both the hydraulic and cable-driven machines are offered as stationary and traversing units.

Application in Morocco

Morocco’s Office National de l’Electricite (ONE) invested about €85 million (US$123.3 million) in the development of the 18 MW Tanafnit and 22 MW El Borj hydro stations, on the Oum Er Rbia River in the Middle Atlas Mountains. These stations are about 40 km northeast of Khenifra, a city with a population of 80,000 in central Morocco. Two-thirds of the cost to develop these stations, which were completed in 2008, was financed using funds from the KfW Bankengruppe in Germany.

 
The trashrack cleaning system installed at the 18.6 MW Tanafnit project in Morocco was tailored to deal with specific site conditions. It features a twin cable winch that pulls the cart along the intake screen panels and a trashrake head that travels downward by gravitational force.

The stations are considered one of the major prestige projects undertaken in central Morocco. Accordingly, high emphasis was placed on selecting reliable mechanical equipment. The main contract for the electromechanical portion of the power stations was awarded in 2006 to an international consortium comprised of Andritz Hydro Austria (formerly VA Tech Escher Wyss GmbH) as the turbine supplier, Cegelec of France to coordinate electromechanical construction, and Koncar GIM of Croatia to supply the generators.

For the Tanafnit and El Borj plants, ONE chose Ossberger to supply the trashrack cleaners. It made this decision in part because the company was involved in various other projects in Portugal.

The trashracks at the intakes of the Tanafnit plant have an inclination of 82 degrees, with three screen panels 2.7 meters wide by 15 meters high. The depth of this intake suggests the use of a cable-operated trashrack cleaner. Ossberger offers this type of cleaner as one of its standard designs.

Because three intake sections, side by side, had to be cleaned, the most practical solution was a moveable trashrack cleaner that traverses horizontally to reach individual sections of the three screens. Alternatively, three stationary units would have been necessary, at a higher investment cost and greater difficulty to dispose of each rake’s individual trash retrieval and accumulation. To keep installation time as short as possible, ONE requested a minimum number of sub-assembled components to be prepared for shipment. For this situation, Ossberger designed and manufactured a tailored unit.

The trashrack cleaner designed for the Tanafnit plant features a twin cable winch that pulls the rake head up and down the intake screen panels. The drive unit for the winch is equipped with torque-dependent overload protection. The rake head is 2.8 meters wide and traverses a distance of 13 meters. It travels downward by gravitational force, and engagement of the trashrake head is accomplished electromechanically at the bottom of the screen before it travels upward on its cleaning cycle. As the trashrake head reaches the top position, the container apron (actuated by a hydraulic servomotor) swivels out to permit discharge of the debris into the trash container. The container, which travels along with the rake, offers a volume of 5.5 m³.

The trashracks at the intakes of the El Borj plant have an inclination of 65 degrees, with a screen panel 6.4 meters wide and 13.2 meters long. The trashrack cleaner designed for this plant is chain-driven.

Due to its heavy duty design and simplicity, the Ossberger trashrake offers long life expectancy. All components subject to wear and tear are designed for a lifespan of 100,000 operating hours, which relates to 20 years of actual service life, with little maintenance required. All electric drive units are maintenance-free. At regular monthly intervals, the flange bearings of the cable winch drums and the clevis pins of the hydraulic cylinders need regreasing. Once a year, the hydraulic power unit requires an oil change.

This trashrake provides fully automatic operation but also can be run using a manual control. ONE will operate the units primarily in automatic mode by level differential control, comparing the water levels before and behind the trashrack. In case of exceeding a preset level difference, the unit will start for one cleaning cycle. When full, the container will be emptied by manual intervention.

Each of the powerhouses contains two turbine-generator units. Annual generation from the two plants will be about 220 TWh. The electricity produced is being transmitted to Khenifra via a 60 kV transmission line.

Application in India

Another application of the Ossberger trashrake machine is at the 330 MW Shrinagar plant in India. This run-of-river station, on the Atnanga River (also called Alaknanda) in the province of Uttaranchal, is owned by Alaknanda Hydro Power Co. Ltd., a subsidiary of GVK Group. The project is expected to cost INR20.69 billion ($460 million).

Bharat Heavy Electricals Ltd. received the electromechanical works contract, and SMEC International of Australia and SMEC India are providing engineering services. The powerhouse will contain four 82.5 MW turbine-generator units.

 
At the 330 MW Shrinagar hydro station in India, a trashrack cleaning system manufactured by Ossberger will be installed in the spring of 2012.

Ossberger is supplying a chain-driven stationary trashrack cleaner 4.4 meters wide to clean a screen panel almost 10 meters long, at an inclination of 76 degrees. This system was chosen because of the depth of the intake. This unit will be furnished with a container with a capacity of 7 m3 and an operator’s cabin, which is considered a standard requirement in India. The unit can operate in multiple modes: automatic, semi-automatic (manual initiated start with automatic stop), and manual (complete operator control).

The Shrinagar plant is under construction. Installation of the Ossberger trashrake is scheduled for spring 2012.

Conclusion

Different solutions for trashrack cleaning must be considered, primarily dependent on the layout and site-specific requirements of each intake structure, as well as the screen size. First and foremost, the rakes must have a heavy duty structure and reliable design. The return on investment can be short as a result of the increase in power generation. The owner of a station in Peru similar in size to the Tanafnit and El Borj plants reported this investment was returned within a year of operation.

Operators of hydro stations appreciate quick and easy installation of trashrake units, as well as reliable operation. In addition, it is expected that the drive system requires little energy to complete an effective cleaning cycle

Helmut Erdmannsdoerfer is general manager at the Ossberger manufacturing plant in Germany. Alfred Patzig, Eng., president of Hydropower Turbine Systems Inc. (HTS-INC), is Ossberger’s representative abroad. Jose Costa Simoes, with COSIM S.A. in Portugal, is a technical advisor for the El Borj station.

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