Dear Rupert....this idea sounds great...but I have a 2 meter head unit, though it's only about 125 ltrs/scnd at low water, maybe 300 at high. I'm wondering if this could work with my turbine, and if so, if you could supply more info. Also, I don't quite understand your "sucker" option....Thanks.
Jeffe [EMAIL PROTECTED] ----- Original Message ----- From: Evans Engineering <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Monday, April 26, 2004 3:29 AM Subject: RE: [microhydro] New (???) trash rack design > Dear Marc de Piolenc, > I have been designing and making drum screeners for many years and would > like to summarise the main design decisions for the benefit of those > interested. Mechanically scraped screens have to be strong (especially > if they are inward flow ie. The pressure is trying to collapse the > screen) because even small twigs and debris can exert a considerable > force on the screen it they get jammed in the scraper. Scrapers tend to > shred the debris an send quite a lot through the turbine (which may or > may not matter depending on the type of turbine, even small amounts of > material going through a fixed geometry propeller turbine can reduce the > output considerably, because it can build up on the leading edges of the > runner blades, causing the flow to become turbulent. If you have a lot > of leaf material, a backwashing screen is a much nicer solution because > the material simply floats off the screen and so it can be made of much > lighter material such as thin stainless steel. They do use water if run > continuously. This may or may not be an important factor. The drum > either rotates or the scraper or back washing 'sucker' rotates and the > drum stays still. Screeners are expensive if well made and reliable but > they can be made to work well with a little thought. I will provide some > pictures if others are interested. The real problem is with large low > head projects below 3 metres of fall, where 'fishery interests' can > require screen spacing of 10mm or less. 'Fish Friendly' turbines are > perfectly capable of passing quite large fish without damaging them, but > convincing fisherman they are 'Quite safe for them to kill'.....! (is > another story) > Getting back to your vertical screen, it is a bit of a problem with a > spiral scraper because it will have to cut any sticks that poke through > the screen unless the pitch is very fine or the screen mesh is very > fine. From my experience it is always a bit of a compromise between > cost, maintenance and the frequency of jamming. One golden rule is that > the screen must be able to withstand the hydrostatic head with total > blockage or there must be a totally failsafe system to shutdown the > plant(I have seen many plants with collapsed or damaged screens, caused > by blockage) > Regards > Rupert > www.microhydro.com > > > -----Original Message----- > From: Marc de Piolenc [mailto:[EMAIL PROTECTED] > Sent: 23 April 2004 05:54 > To: Microhydro List > Subject: [microhydro] New (???) trash rack design > > A couple of weeks ago I was privileged to visit a medium hydro project > near Baongon, Bukidnon, owned by one of the independent (i.e. > non-NAPOCOR) power producers now emerging here in the Philippines. This > was the first hydro project by this company, and was attended with > serious troubles which were overcome mainly by grit and perseverance. > Their original hydro expert consultant made some poor design decisions > which continue to cause operational problems, and a contractor chosen by > him managed to drop a generator set weighing 12 tons twenty meters to > the turbine-house floor, with predictable material damage and one life > lost. The company, instead of abandoning the project, simply fired the > consultant, assembled their best people and set them to work acquiring > the necessary expertise in-house. The plant has now operated profitably > for a little over three years, and the company is considering further > projects. > > There are still problems, however - leaky sluice-gates cost them some > power generating capacity, especially during periods of seasonal low > water flow, and the trash rack, while adequate most of the year, gets > clogged with debris during peak flows, limiting plant capacity at just > the time when the plant should be delivering maximum output...and > maximum profit. > > The power company's renewables manager, the plant manager, the civil > works contractor and Yours Truly were kicking over possible > modifications to the trash rack, since a solution to that problem would > produce a very large gain. I brought up the moving trash rack design I > had seen somewhere - you know, the one that looks like those toasters > you see in restaurants - a series of linked sections driven and guided > by sprocket wheels. Then we started counting up moving and wearing parts > and got discouraged. The renewables guy came up with a simpler mechanism > - a horizontal-axis cylinder. I think they're now looking into that as a > retrofit to the plant. > > The reason I'm writing to the list is that I kept doodling variations of > this scheme on my way home and afterward, and came up with something > that looks promising to me that I would like to offer for critique. This > is not a possible retrofit to the existing plant in Baongon, however, > because it requires the penstock to upen upward into the forebay tank > instead of the horizonatally oriented opening that now exists. If it is > any good, however, I would like to propose it to them for future > projects. > > The idea is to have a vertical axis cylinder whose axis coincides with > the centerline of the first, vertical segment of the penstock. The idea > here is that water can enter through the full perimeter of the cylinder, > making the full surface area usable (the earlier scheme makes only the > projected area usable). Of course a scraper has to be provided to raise > and remove the debris, and this is provide by a spiral fixture that > makes one full turn from the base of the screen cylinder to the top, > where a conveyor belt or some other arrangement takes over to dump the > debris downstream. I can send a sketch to anybody who is interested and > needs something better than my verbal description. > > Anyway - while I spent some time congratulating myself on my cleverness, > in retrospect this seems like a fairly obvious solution, which leads me > to suspect that it has flaws that I have so far failed to perceive. > Comments by more experienced list members would be welcome! > > Marc de Piolenc > Iligan City, Philippines > > > > Does your company feature in the microhydro business directory at > http://microhydropower.net/directory ? If not, please register free of > charge and be exposed to the microhydro community world wide! > > NOTE: The advertisements in this email are added by Yahoogroups who > provides us with free email group services. The microhydro-group does > not endorse products or support the advertisements in any way. > > More information on micro hydropower at http://microhydropower.net > > To unsubscribe: send empty message to > [EMAIL PROTECTED] > Yahoo! Groups Links > > > > > > > > > > > Does your company feature in the microhydro business directory at http://microhydropower.net/directory ? If not, please register free of charge and be exposed to the microhydro community world wide! > > NOTE: The advertisements in this email are added by Yahoogroups who provides us with free email group services. The microhydro-group does not endorse products or support the advertisements in any way. > > More information on micro hydropower at http://microhydropower.net > > To unsubscribe: send empty message to [EMAIL PROTECTED] > Yahoo! Groups Links > > > > > ------------------------ Yahoo! Groups Sponsor ---------------------~--> Make a clean sweep of pop-up ads. Yahoo! Companion Toolbar. Now with Pop-Up Blocker. Get it for free! http://us.click.yahoo.com/L5YrjA/eSIIAA/yQLSAA/FGYolB/TM ---------------------------------------------------------------------~-> Does your company feature in the microhydro business directory at http://microhydropower.net/directory ? If not, please register free of charge and be exposed to the microhydro community world wide! NOTE: The advertisements in this email are added by Yahoogroups who provides us with free email group services. The microhydro-group does not endorse products or support the advertisements in any way. More information on micro hydropower at http://microhydropower.net To unsubscribe: send empty message to [EMAIL PROTECTED] Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/microhydro/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
