Formerly Addl. Director
Central Water and Power Research Station, Pune, India
E mail: firstname.lastname@example.org
Trash in rivers is a continuing problem for hydropower plants. Flowing water picks up solid matter like industrial waste, plastic, textiles, rubber and also trees, branches, leaves etc. This matter collects near the intake and enters into the turbines, if not prevented by suitable device like trash rack. However, damages to the trash rack structures of several hydro plants all over world are of concern for designers, plant owners and operators alike.
Trash racks may be fixed structures embedded in the concrete around the inlet or may be removable, operated by a crane using a grappling. A typical trash rack panel is made up of vertical bars of rectangular section (say 15 cm x 1.2 cm) and horizontal bars of circular section (say 3.75 cm Ø). Vertical bars are closely spaced, typically 10-12 cm apart, whereas horizontal bars may be 100 cm apart. Although, trash rack cleaning machines are provided on top of dams, occasional surges of increased trash materials collide with trash rack bars and accumulate there, forming almost impenetrable wall. In addition to imposing excessive drag on trash rack bars, this also restricts smooth flow in to the intake and creates head loss resulting in loss of power generation. More seriously, this blockage induces change in the direction of approach flow towards the intake, resulting in vortices and vibrations. Besides, variable pattern of power generation with instantaneous change in intake discharge, common at hydro plants, adds to the problem of vortex shedding with a wide range of frequency. This is believed to be responsible for damage.
Several research studies are currently under way to understand the mechanism of damage and evolve remedial measures for averting such damage. Observations have shown that the most critical location for the origin of damage is the junction of the vertical rectangular bars with horizontal circular bars, where three pairs of vortices are generated. Varying degree of blockages and changes in discharge generate vortices of different patterns and frequencies.
Both, physical and numerical modeling has been resorted to understand the flow structure responsible for damage. It is found that the origin of damage was fluid-structure interaction. Out of a wide range of frequencies of vortex shedding, caused by blockages of various degrees and sudden changes of discharge, some frequency lie in the neighborhood of natural frequency of the trash rack screen. This creates a resonant condition responsible for damage.
Efforts for eliminating condition of resonance include evolving shape of trash rack bars that would reduce/prevent formation of vortices. Instead of vertical bars of rectangular section, bars with wedge-shaped pointed end at the upstream followed by tapering on the downstream have been examined. Also, the circular horizontal rods were positioned nearer to the pointed end of vertical bars, rather than in the centre. This arrangement has indicated promising improvement in the flow conditions.
While the research efforts towards improvements continue, there are at least three issues that need to be addressed.
- Trash rack cleaning devices: Trash rack cleaning machines operating from top of dam, pick up material from the rack bars. Is it possible that some device sucks up the trash from the water surface, lifts it up, separates it out at the top and returns the clean water in the reservoir? This is something in line with the sediment sucking from the reservoir.
- Floating barriers: At some plants, floating barriers are placed at some distance away from the intake. Such barriers prevent trash from approaching towards intake. Boats or barges then collect the trash from the barriers. It is observed that some material that is in suspension with flow, escape from below the barriers and enter in to the intake. Some improvement in the design and operation of floating barriers is needed.
- Though the wedge-shaped bars described above are effective in improving flow conditions near the trash rack screens, a cost effective design must be evolved, because such shapes for the bars are very expensive to manufacture and handle.
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