Long ago I had the privilege of internally inspecting the volute of one of the power turbines in a major power dam across the St. Lawrence River. There is no 'pipe' upstream of the turbines.
The upper face of the dam is almost entirely covered with 'trash racks', large coarse screens. By large, I mean maybe fifty or sixty feet square, maybe bigger. Each extends for nearly the full depth of the dam and the full inter- turbine pitch distance. The air gap in the generators measures about an inch radially, and 12 feet in diameter. Each generator produces ~1200 amps at 1200 volts, 60hz. My friend Newt, a farm boy, opined that it would 'make a right good welder'.
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Contrary to what's been expressed in the popular press, trash racks have nothing to do with keeping fish out of turbines. Fish are exquisitely sensitive to horizontal currents, which is why a school of fish changes direction all at once, and why any water intake with horizontal flow is not going to catch fish; they just swim away from it, if it's large enough to keep the velocity low enough for them to swim against. Intakes with a vertical flow are guaranteed to catch fish. Trash racks are for trash, like telephone poles and errant boats.
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Inboard/downstream of the trash racks, the interior of the dam comprises a gently tapered spiral of gargantuan proportions. The smallest part of the volute is still big enough to stand upright in.
There is a tubular lumen downstream of the turbines, extending down well under the tailrace level to an elbow and then an expander. That lumen is large enough to drive an SUV through.
Every once in a while, they replace the trash racks with solid plates and drain the volute so they can hardface the turbine runners, which is why that particular volute was accessible.
The adjacent turbines were still in service, and despite the huge mass of concrete, flow noise was pretty much everywhere, audible without amplification. It was loudest at the thick steel access hatches, and punctuated by irregular events that sounded like a big heavy rock rolling and bouncing down the volute and through the turbine. We were assured that the noise was not from rocks, but from cavitation, which did more damage than rocks could, hence the hardfacing.
That particular dam works with a relatively low head, less than a hundred feet. Taller dams exist with turbines at the downstream end of long penstocks. E.g. Hoover Dam, covered occasionally on the Discovery Channel. It appears to me, a layman in these matters, that iron penstocks are by far the most flexible part of any such installation. I assume that they typically include elbows just so they can flex enough to not self- destruct from temperature changes and settling of the massive structures.
I remember seeing much smaller penstocks associated with much smaller dams, made of iron and wood (with closely spaced iron bands). They seemed to be braced with heavy saddles and abutments and such. I've never noticed anything I would identify as a vibration damper, of the sort you see on aerial wires. I guess they rely on inertia, mostly. They do make noise that's clearly audible from a distance. I've never seen a new penstock; I assume that flow- induced vibrations, surely they occur, cause early failure, and I've just seen the reinforced replacements.
Mike Halloran
Pembroke Pines, FL, USA
