Rupert I would be interested to see the pictures you offered to show. I have a 1 m diameter mild-steel pipe feeding an 18 inch Francis. The (21 m long) pipe is fed from a lake with 55,000 m3 of water which should be enough to keep it running a full tilt for approx. 1 day.
The pipe is expected to be under water almost all the time so I would hope to have less problems from flotsum. Objects up to 1.5 inches across should squeeze through the turbine blades. We thought about using two simple interchangeable racks one of which could be cleaned while the other worked... Regards, Martin ----- 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 > > > > > 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 <*> 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/
