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m : 4 October 2011 • 2:11AM -0400

[microhydro] Low head self cleaning intake screen.
by Darragh O Callaghan


Dear Group

I am close on the completion of a high head intake using Coanda
Screens. This project was a redesign and upgrade of an existing side intake
that was prone to blockage due to a poor initial design. I am very impressed
with the operation of the screens. This site has 210m head and a turbine
design flow of 500l/s.

I also operate a low head station with 15m head and twin diagonal Kaplin
turbines with a design flow of 6m3/sec. This cleaning system works ok but
because of its design the plant has to shut down while in cleaning mode.
Basic intake operation is as follows,

Water is abstracted from a large weired river pool through two river gates
which are what we call hydraulic 'guillotine gates' into an intake pool. In
this pool there is a submerged intake with a series of intake screens on
which a strain gauge is connected. When trash (which is mostly leaves)
blockage gets to a certain amount it adds pressure to the strain gauge and
the turbines switch to cleaning mode.There is a second set of guillotine
gates behind the intake screens which we call 'trash gates' which open to
create large flow over the screens. this flow carries the trash back to the
river downstream if the weir pool.

Cleaning mode operation is as follows,

-River gates close
-Turbine shutdown
-Trash gates open

When the trash gates open all trash is flushed out. The operation then runs
in reverse to start up again. The whole operation lasts about 6 minutes.
This I am not happy about and feel I'm loosing out on allot of production
annually because of this stopping and starting.

So with my recent appreciation of the Coanda type intake my question is, is
there a similar system as the Coanda screen available but designed for high
capacity with minimal head losses?

I am very aware that there are hydraulic trash rack cleaners and that they
are the standard cleaning devices for low head commercial stations but this
is not what I am looking for.

The current intake screens spacing is set at 15mm and cannot be enlarged. I
am not aware of a spacing larger that 2mm available on a Coanda screen.

Any thoughts on this would be greatly appreciated.


Darragh O'Callaghan


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