Caddo Creek Dam – Pt III

Dam Intake
With the damn basically finished I had to work on the intake. This is where the dammed water exits the dam and travels down the PenStock/flue to the turbine. In ‘my’ damn book I found this illustration of the mechanism which is more then enough to create something similar for my dam.

The following is taken directly from the book about the illustration.

In the left background at A is the dam. In the foreground at B is the forebay. C is the trash rack and D the gate hoist to control the flow of water into the penstock E. When this gate is closed the penstock E is emptied by opening the gates of the wheel, in which case it is frequently an advantage to have the little air inlet valve F in the penstock to let air into the penstock automatically, thus relieving the penstock of outside air pressure, as the draining of the penstock naturally creates a vacuum within the penstock. Such a vacuum would be a heavy strain on any type of construction, but in this case the simple little air inlet valve relieves the pressure immediately. This picture shows an ideal arrangement for the home or small town power plant. It has every practical convenience and refinement that the huge water power plants have.

With the illustration from the book showing the intake etc. I could go ahead and create something similar for my dam. The ‘water’ wasn’t going to be very deep at all .. just enough to look like it went deeper.

Note: This might be considered an ‘exercise in futility’ .. or some other appropriate term as the dam sits about 18″ back from the edge of the layout. Oh well. Having fun.

Finishing up paint
Got the intake bits and pieces in and a ‘bottom’ for the epoxy that will come later. This pic shows it with the first coat of primer. Did they usual .. some sanding and putty .. but didn’t get too crazy as .. well .. it’s supposed to be rough concrete after all.
With all the sanding finished I dabbed on a mixture of Modge Podge and sanded grout to add some texture/spalling etc. I also taped off the are that will be ‘water’

On the front glued on a couple of strips of paper (12″ planks) to indicate a little movement in the lagging of the form

A coat of Rust-Oleum ‘Fine Textured Finish’ in Sandstone. I like this as a base color for concrete.
This next bit is a kind of ‘magic’. Sprayed on some dark AI wash. This not only brings out the shadows but changes that sandstone to something darn near like concrete.
Dry brushing with some FolkArt ‘Linen’ .. on-purpose left vertical streaking.

I sat this aside for a while. I needed to add the PenStock .. rust will come off of that of course! .. prob some geenish-ish for mold – I needed to look at some proto pics first. I also needed to pick up some Envirotex at Michaels .. that 50% off coupon makes a big differnce.


Caddo Creek Dam – Pt III — 3 Comments

  1. Hi. Sorry. Me again. Professional interest.

    PenStock (Note spelling) is too small and too high.

    Pressure drop is in accordance with Bernouli. h=V^2/2g.
    Penstock looks about 15″ dia. So area c 1ft^2. So velocity 126ft/sec. h~250ft. This is about 8 atmospheres so would have extreme cavitation except that as the penstock only has about 2ft submersion there would be pronounced vortices and a vast amount of air entrainment. Watch the water flowing out of your bath. Either way the turbine would be hopelessly inefficient and suffer damage. I would expect to see an axial flow turbine of at least 3ft diameter. This would reduce the head drop by the ratio (15/36)^4, say 3%. 7ft head drop would mean that if the turbine was at least 7ft below the reservoir level the pressure in the turbine wouldn’t drop below atmospheric. The penstock should have 7ft to top of pipe and be 3ft dia.

    Having said all that – I like the model.


  2. PenStock vrs penstock – that (penstock) was taken directly from that book pub in 1920. That being said .. I like your’s better and will use it instead.
    PenStock dia. You were close .. it’s 1/2-in Evergreen tubing or 24-in full-size.
    In the discussion on the turbine, NE_Brownstone said “I’d make the intake almost the same size as the pressure housing. If not, the pen-stock will be a bottleneck.
    Which I repled – “I had seen that the intake was about the same size as the housing but I had thought possibly that was ‘perhaps’ that they used different size turbine wheels and that maximizing the intake let them do that. My first design did that .. made the intake about same as the housing but I was partially limited by the 1/2″ Evergreen tubing I had. That size tubing (ok .. pipe 🙂 .. would supply water with a 16ft head to power a 33in wheel.
    mmmmm. I have to admit that this was one bit of my take on the turbine that has been bugging me. Did the intake pipe stay the same size from the dam to the turbine? That would be fine with me .. except that would require a 1-in tube. Looked .. Plastruct has 1″ Butyrate tubing.
    NE_Brownstone then commented later on in the thread “After looking at some of my pictures I did notice that a few turbines did have smaller intakes. Most were of the single runner turbine variety.” .. which I took as an ‘out’ by immediately declaring that mine was a single runner turbine.
    We then got into a discussion re: the draft tube. NE_Brtownstone – “The draft tube needs to be submerged to create the siphoning effect, or negative pressure. You want a solid slug of water flowing through the pen-stock and turbine at all times when it’s in operation. Draft tubes are tapered a bit which helps drag the water through the turbine. That’s why I pointed out the diameter increase in the elbow casting in the Bradbury turbine. Not that you have to have it in your model, but if you are looking for fidelity this is how they would and still do it.
    About the placement of the PenStock on the dam. NE_Brownstone pointed out to me that – “Ed, you may want to put the collar down a few feet more just so you don’t start sucking in air if the river is low.” – to which I replied .. “.. too late to move the collar as there is a hole to socket it to the dam. I can always say that there’s an elbow at the collar level. That would get me a foot under the spillway top.“. This doesn’t address the problem you pointed with cavatation of course
  3. Penstock is correct. The problem was that you’d had a stutter on your s and written Pentock on several occasions.
    Back to engineering. The penstock is often parallel for simplicity, there will be a trumpet shaped intake to reduce entry losses. However a very long penstock could be tapered because the wall of the pipe needs to get thicker as pressure increases. Mind you a very long penstock (say over a mile) also requires surge towers or surge vessels to prevent instability or even structural damage during rapid shutdown.
    Back to your plant. I would run a pipe from the foot of the dam straight into the back of the turbine house, the diameter would be about 3ft or more as I stated. The best turbine would be Kaplan as in because this maintains high efficiency with varying output because the intake blades convert the head into velocity even at low flows. The outlet should be as low as possible in order to use the the full head available.
    As I said the result is good and not everyone is as pedantic as I.

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