Friday, July 23, 2010

Dealing with sand, gravel, ice, leaves just to mention a few things in the water.

PVC moving belt trash rack

Frazil ice, leaves and gravel have been my biggest maintenance headaches. But I'm hoping those headaches are behind me now that I've designed and built a PVC moving belt trash rack.

Freezing up of the intake screen after 3 to 4 nights of subzero temperatures is a common problem in cold climates. As the water splashes down over rocks it ends up supercooled (30°F) and slushy. The instant supercooled water hits any cold material, especially metal, it sticks and builds up, shutting off the largest of openings, even with trash rack / screens totally removed.

In a case here I have a 48" wide X 8" high opening at the inlet end of a 30 ft near horizontal covered flume that leads water to a 4' X4'X 5' deep box with a plastic, moving conveyor screen installed near the top that dumps debris and slush (sometimes referred to as frazile ice) over the downstream edge of the box. The penstock pipe leaves the bottom of the box and the whole thing is covered with PT 2X.

Now I thought I had my freezing intake screen problems solved, using a covered, PVC, moving screen, that runs mostly underwater except for the 6 inches or so that sticks out over the edge of the baffle to dump the leaves, gravel and ice. This screening arrangement allowed me to remove the metal trash rack at the 48"X8" entrance to the flume. ' Thought that big hole would never be able to freeze up with water running into it. Wrong!  The water gets so thick with slush that it dams up the stream itself, then naturally, the ice dam overflows and the overflow freezes on the lip of the ice dam. And so, once the slush freezes solid, you get these interesting ice stairs, ice dam with level ice behind it, then another ice dam with level area... and so on up stream. Usually after a few days of ice stair formation, and as daytime temps rise, the water and slush disappear from the surface and water will continue to run underneath the now frozen hard ice cover. So long as the ice cover remains it will keep the water from supercooling and forming slush. You will have no problem running your micro hydro as the water runs under the ice cover. But how to deal with those few days of slushy ice damming water?

A deep (6') collection reservoir will go a long way toward solving this problem, but my stream flows over bedrock before falling off a cliff. A small dam would probably work but for various reasons was ruled out for now. I thought about heating tapes. They might ease the slush formation in the immediate area of the intake. They would have to be put inside antifreeze filled iron pipes to protect them from the rocks, logs and other storm debris. But thinking along these lines I remembered that we have a spring on a somewhat distant higher hillside. I had previously run a 1 1/2" PVC pipe from this source of 50°F spring water to our house for use as a domestic water supply. There is plenty of water especially in winter.  So I ran another 500' of 1/2 inch PVC downhill to the 48X8 problem intake flume and squirted the warm? water out upstream from the flume. This did the trick, at least this last winter, which was plenty cold. I think this could also work with pumped well water which is also relatively warm. The water would only have to run for the few days before the ice covers the stream. The pipe would retain more heat if it were buried and/or insulated, it should be pitched so it drains out completely when shut off.

You can purchase plastic conveyor belt with a large % of open space for the water. Order it to size or get it used (cheap) on Ebay and reconfigure it to the size you need. I am anxious to try this stuff in really cold weather. I have had to remove my old metal screen whenever the temps went below Zero °F. I also covered the whole intake flume, box and screen to keep it from freezing up. So far it has worked very well to keep all kinds of debris out of my penstock. The excess water that does not go down the pipe rinses the leaves off as they invert over the drive roller. The 18 RPM Bodine gear motor is directly coupled to the 3" PVC pipe that drives the screen. A slew of 3/16 holes drilled into the PVC pipe in exact locations matching the screen holes, and fitted with plastic pins, gives positive traction to the screen. Even though the screen moves slowly at about 6"/second, I still put a timer on the drive motor so it is on for 3 minutes then off for 6. This saves energy and wear and tear.

The PVC belt is made up of small sections hinge pinned together with 3/16" plastic (welding) rod to make any size screen. The short section of belt wrapped around the PVC pipe allows the accurate drilling of holes to accept short (1/2") pieces of 3/16 rod that act as sprocket teeth. I had to make the outside diameter of the standard 3" PVC pipe bigger by 1/4 inch to get it to accept the pitch of the plastic belt. If you are going to build this send me an email and I'll send you more specific 'how to' info.

Here is how I got my belt, and I turned the square hole sprockets on the lathe to just fit inside the 3" PVC pipe so it could be driven with a square tube drive shaft.

The basic drawing of the whole thing. Double (or Right?) click on the image to see it enlarged.

The 'Gravel Baffle Box' with the covers removed from the butterfly valve drive (left) and the screen drive motor box on the right. (looking up stream, the dam and falls are behind the viewer)

Monday, July 19, 2010