Dear Rupert: Thanks for all the detial. My "water race" overflows occasionally during high water, where a few inches of water passes over the side, but usually at low water the pool level is lower than the sides of the race, thus all the water that comes in to the race goes through the turbine. Would either the back washing or the rotating drum system work? You can see my system in the June issue of Home Power magazine for reference. Is there a picture of these two cleaning systems on the web I could look at? Jeffe
----- Original Message ----- From: Evans Engineering <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Thursday, May 13, 2004 6:24 AM Subject: RE: [microhydro] New (???) trash rack design > Dear Jeffe, > The design of your screen will depend on the amount and type of the > debris, how much room you have at the intake, amount of spare water (if > any) and the type of turbine. If you have to exclude everything, then a > backwashing screen is best because a scraper will cut up some of the > rubbish and force it through the screen. Any fixed geometry turbine will > tend to collect the small pieces of grass and leaf on the leading edges > of the guide vanes, spear supports etc. Just shutting of the plant or > changing the flow, may be enough to dislodge this build-up. Other people > have probable come across the situation where power falls steadily but > goes back to full power as soon as you introduce some turbulence. > > A back washing system can be a flushing tank system which operates only > when required (uses very little water as it will operate every few > minutes, hours or days as required). A rotating drum system will remove > vast amounts of rubbish which would block a conventional screen in five > or ten minutes. They are expensive to make well but will deal with a > wide range of rubbish from leaves to plastic bags. > > The 'sucker system' uses a fixed screen (flat or curved) and a vacuum > cleaner head that tracks across the screen and removes the leaves. It is > a bit cheaper than the drum screen and cannot cope with very large > objects, as they can get jammed under the rubber edge of the cleaner, > but the do work well and should not get jammed up more frequently than > once a month or so (it's almost impossible to design a totally > 'fool-proof' system, there will always be one small 'bloody minded > stick' that will find its way through or across some vital part! > > The coanda type screen is fine for the smaller intakes on high head > sites where some head loss is insignificant. Using the right bar shape > will make a significant difference to the amount of water entering and > how well rubbish such as grass is coped with. > > I regret that there isn't one simple solution and it still drives me to > distraction sometimes, but I urge any prospective builder not to under > estimate the difficulty in getting a screener to work reliably on your > site. > Regards > Rupert > > > -----Original Message----- > From: Jeffe [mailto:[EMAIL PROTECTED] > Sent: 07 May 2004 23:18 > To: [EMAIL PROTECTED] > Subject: Re: [microhydro] New (???) trash rack design > > 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. 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