Loading ...
Sorry, an error occurred while loading the content.

RE: [microhydro] Re: New (???) trash rack design

Expand Messages
  • Evans Engineering
    Regarding the use of wedge-wire screens. They are only self cleaning if a significant portion of the flow continues past the screened intake because if the
    Message 1 of 12 , May 1 4:07 AM
    • 0 Attachment
      Regarding the use of wedge-wire screens. They are only self cleaning if
      a significant portion of the flow continues past the screened intake
      because if the bulk of the flow is through the screen it must become
      blocked because there I nowhere for the debris to go.

      Regarding drum screens. Most of the problem with screening is when river
      flows are high and there is plenty of water but as you rightly say,
      there is a problem when flows are low. This can be overcome by running
      the screener on demand (ie. It runs only when the screen needs cleaning)
      This is achieved by closing off the 'drive water' or by using a 'back
      flushing' design, that only operates when the head-drop across the
      screen in more than about 50mm.

      It is impossible to design one screener which will fit all heads, flows
      and debris, so one has to compromise. Many site suffer from a wide range
      of material from leaves and grass, right up to trees, plastic containers
      and boats! Usually a 'guard screen' or skimmer channel' will remove the
      big or floating objects, leaving the automated screener to cope with
      just the fine stuff.

      If you are screening less than about 70mm, it is often better to screen
      very fine (less than 25mm) if there are sticks that can get lodged in
      the bars ( even a 25mm stick will take a lot of power to shear it) you
      will have to make it all very strong or have a slip clutch (as soon as
      the screener stops the plant will probably have to shutdown, which is
      why I have always favoured 'back-washing'. The system will still
      function even it you have a number of sticks tangled in the actual
      screen.

      Sorry to go on a bit, but screening is one of the most overlooked areas
      and one of the most common reasons for hydro schemes being abandoned.
      Scrapers, in my experience only work if they are simple, and in line
      with the screen bars.

      Regards
      Rupert

      -----Original Message-----
      From: Marc de Piolenc [mailto:piolenc@...]
      Sent: 25 April 2004 08:14
      To: microhydro@yahoogroups.com
      Subject: [microhydro] Re: New (???) trash rack design

      rkweir@... wrote:

      >
      > Anyone who is thinking outside the box, so to speak, is to be
      congratulated.
      > It just might work. Have you ever considered using a tilted wire wedge
      wire
      > screen? There are no moving parts, minimal maintenance and for all
      practical
      > purposes are totally self cleaning. There is usually a way to either
      utilize an
      > existing structure or easily build something that will hold the
      screens. They
      > will remove all debris larger than .25mm and the material of
      construction is
      > 304 stainless steel. When the value of time and lost generation are
      considered
      > they are well worth the money.

      Dear Robert,

      I've visited your Web site (difficult, because Netscape 4.7 can't
      display it and I had to install and - gulp! - open Internet Exploiter to
      see it). As far as I can figure out from the pictures - there doesn't
      seem to be any verbal explanation of how your various products work -
      these screens need a continuous flow of water over them, bypassing the
      turbine, to be self-cleaning. The plant I am concerned with (and this
      will be true also of all future projects) cannot afford to divert water
      year-'round, as surplus flow only exists during peak flow periods, which
      are rather short. So IF I have understood your products' MO correctly,
      they are not applicable. Too bad, because I must say they are very
      elegant.

      Best,
      Marc




      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
      microhydro-unsubscribe@...
      Yahoo! Groups Links
    • Jeffe
      Dear Bob: This screen you speak of, I believe the coanda, sounds great...and has sounded great for a couple of years. If Marc makes a request for information
      Message 2 of 12 , May 7 3:13 PM
      • 0 Attachment
        Dear Bob:

        This screen you speak of, I believe the coanda, sounds great...and has
        sounded great for a couple of years. If Marc makes a request for information
        from you, I hope you could send a copy of your response to me as well, as
        I've sent you 3 requests over the past couple of years, with no response,
        about how to view specs and prices on them.

        Thanks,
        Jeffe

        ----- Original Message -----
        From: <rkweir@...>
        To: <microhydro@yahoogroups.com>
        Sent: Friday, April 23, 2004 12:47 PM
        Subject: Re: [microhydro] New (???) trash rack design


        > Marc,
        >
        > Anyone who is thinking outside the box, so to speak, is to be
        congratulated.
        > It just might work. Have you ever considered using a tilted wire wedge
        wire
        > screen? There are no moving parts, minimal maintenance and for all
        practical
        > purposes are totally self cleaning. There is usually a way to either
        utilize an
        > existing structure or easily build something that will hold the screens.
        They
        > will remove all debris larger than .25mm and the material of construction
        is
        > 304 stainless steel. When the value of time and lost generation are
        considered
        > they are well worth the money.
        >
        > Sincerely,
        > Bob
        >
        > Robert K. Weir P.E.
        > Hydroscreen CO
        > e-mail rkweir@...
        > www.hydroscreen.com
        >
        >
        > [Non-text portions of this message have been removed]
        >
        >
        >
        >
        >
        > 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
        microhydro-unsubscribe@...
        > Yahoo! Groups Links
        >
        >
        >
        >
        >
      • Jeffe
        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
        Message 3 of 12 , May 7 3:18 PM
        • 0 Attachment
          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
          jeffe@...


          ----- Original Message -----
          From: Evans Engineering <re@...>
          To: <microhydro@yahoogroups.com>
          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:piolenc@...]
          > 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
          > microhydro-unsubscribe@...
          > 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
          microhydro-unsubscribe@...
          > Yahoo! Groups Links
          >
          >
          >
          >
          >
        • Martin.Leahy
          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
          Message 4 of 12 , May 10 6:53 AM
          • 0 Attachment
            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 <re@...>
            To: <microhydro@yahoogroups.com>
            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:piolenc@...]
            > 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
            > microhydro-unsubscribe@...
            > 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
            microhydro-unsubscribe@...
            > 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 microhydro-unsubscribe@...

            Yahoo! Groups Links
          • Evans Engineering
            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)
            Message 5 of 12 , May 13 6:24 AM
            • 0 Attachment
              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:jeffe@...]
              Sent: 07 May 2004 23:18
              To: microhydro@yahoogroups.com
              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
              jeffe@...


              ----- Original Message -----
              From: Evans Engineering <re@...>
              To: <microhydro@yahoogroups.com>
              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:piolenc@...]
              > 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
              > microhydro-unsubscribe@...
              > 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
              microhydro-unsubscribe@...
              > 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
              microhydro-unsubscribe@...
              Yahoo! Groups Links
            • Jeffe
              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
              Message 6 of 12 , May 14 3:20 PM
              • 0 Attachment
                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 <re@...>
                To: <microhydro@yahoogroups.com>
                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:jeffe@...]
                > Sent: 07 May 2004 23:18
                > To: microhydro@yahoogroups.com
                > 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
                > jeffe@...
                >
                >
                > ----- Original Message -----
                > From: Evans Engineering <re@...>
                > To: <microhydro@yahoogroups.com>
                > 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:piolenc@...]
                > > 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
                > > microhydro-unsubscribe@...
                > > 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
                > microhydro-unsubscribe@...
                > > 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
                > microhydro-unsubscribe@...
                > 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
                microhydro-unsubscribe@...
                > Yahoo! Groups Links
                >
                >
                >
                >
                >
              Your message has been successfully submitted and would be delivered to recipients shortly.