A look at those leaking penstocks
March 15, 2014
In the main section, we showed photography of and provided information about the Grandfather Falls Recreation Area and hydroelectric facilities north of Merrill in Lincoln County.
The purpose of this sub-section is to talk about the penstocks serving the hydro facilities. This is a photo of those at Grandfather Falls. They are carrying the water from a canal offshoot from the Wisconsin River to the hydro plant. This is an amazing sight, largely because there is quite a lot of water leaking from them.
First, some quick background from the previous section.
This is an overview of the whole area.
This is a north-south aerial photo of the Grandfather Falls area, courtesy of the US Geological Survey and presented by TerraServer USA. We are interested in the area bounded by the two red arrows, both sides of the water, the water showing up black in this photography. The Wisconsin River is flowing from the top (north) of the photo to the bottom (south). The top arrow points to the Grandfather Falls dam, referred to in some documents as the Upper Grandfather Falls dam. Most of the time, most of the river's water is diverted to the canal to the right, while the flow of water is carefully regulated that goes through the dam and into the riverbed on the left. You can see even with this photo that the canal is very full and placid, while the riverbed is not so full and appears to have some rapids caused by rocks on the riverbed. We'll show you all this closer in a bit.
Note that the canal terminates to the south. You can barely see what is a very large set of tubes known as penstocks that carry that water overland and to a hydroelectric plant located at the terminus of those penstocks, where they meet the river again.
Mark Lutz of Minneapolis, Minnesota, saw the main article and expressed a great interest in the penstocks, and provided us with some great background. While I was on my visit, I was mystified by the penstocks, especially how they leaked. Lutz tipped me off to some good information on how these are built and why they leak. This is good stuff.
Let’s go to the southern end of the canal.
In the lower right you can see an outlet. That takes water back to the river should it reach high levels in the canal and be too much for the hydro plant to handle. Then just right of center, there is a another gate that moves the water from the canal through two long penstocks and into the hydroelectric plant.
Let's now take a closer look at the outlet back to the river.
You are now looking down the outlet shown in the previous photo and if you look closely, you can see the river below in the upper left quadrant of the photo. You can also see that not much water is flowing down to the river, which means they have achieved a nice balance between what is in the canal and what they are using. Should the canal rise, then this outlet can get pretty busy with water and it would be quite a sight to see the water hit those rocks below.
These are the two large penstocks, and the main subject of this section. They are carrying the water from the canal to the hydro plant. This is an amazing sight, largely because there is quite a lot of water leaking from them. You can see the leaks fairly well here, but let's zoom in on a few.
So, the question is, should these leaks be there or is this poor maintenance or a poor design? Actually, they are expected.
The penstocks are now entering the backside of the hydroelectric plant. You can see how they are on a more steep downward grade to get the water inside moving along at a nice clip.
Okay, so with that as background, let’s learn something about penstocks.
The first thing to know is that this penstock, and most of them, are a made out of wood. Frankly I was astonished to learn that. The penstocks at Grandfather Falls are about 13 feet in diameter, and they are made of wood. The original penstocks at Grandfather Falls were made of redwood. In 1975 those were replaced by ones made of yellow pine.
This is a photo of redwood penstocks that existed at the Thomson Hydroelectric Station in Carlton, Minnesota, in eastern Minnesota just southwest of Superior, Wisconsin. Note the redwood pieces, which are called staves, lie horizontally side by side. This is the way barrels were made. These particular redwood staves were banded together by 0.75 inch steel rods. They were replaced here in 1970, and the redwood was still strong and pliable. In most cases, the penstocks were built on location, in the trenches already dug. Now if you look carefully along the sides of the penstocks, toward the bottom halves, you will see protrusions. Let’s take a look at a penstock seen in New England.
The penstocks at Grandfather Falls are 11 ft. and 13 ft. in diameter, now made of yellow pine. It turned out the yellow pine was considered as good as the redwood and much less expensive. The penstocks run about 0.25 miles. These protrusions you saw in he other photo are clamps used to tighten the straps.
On December 13, 2012, the Federal Regulatory Commission held a public meeting in Tomahawk and discussed these penstocks and the overall Grandfather Falls project. One in the audience asked about the leaking, and whether the penstocks should be replaced with some other material than wood. The response was, “Right now they still have adequate integrity, but eventually they will have to be replaced.”
Why wood? The companies involved in manufacturing these say they are more environmentally friendly. This photo was taken from the Goodfellow Inc., Canbar Division web site on Woodstave Pipes. There is also an excellent article in No Tech magazine, “Wooden Stave Pipes,” with great diagrams and old time photos. Both highlight the advantages of wooden stave pipes.
First and foremost, keep the wood thoroughly wet and it will not rot. By itself that surprised me. If there is an issue, it has to do with the quality of the metal bands. Expansion joints are not required as the wood absorbs the water and expands. Steel restraining bands are used and the wood will expand against those. The metal bands are used only to provide strength. Even when they corrode and lose their strength, the wood will hold together and the bands can be easily replaced. The carrying capacity exceeds that of metal pipe, in large part because the interior walls remain smooth and do not form tubercles. The wood components are easily transported to the sites, which can be remote. No massive hoisting apparatus is needed. They do not require concrete foundations, but “float” on the gravel. The wood is easy to bend, so the contractors can follow a more natural contour; for example, bending around curves. There is no need to cover them. The wood has natural insulation. They can last for 40-50 years. Simple carpentry can be used for repairs. Assembly is easy.
This is a very fine shot of a wooden penstock at the Pepperell Hydro station in East Pepperell, Massachusetts. Note the wood and straps, and the leakage we saw at Grandfather Falls.
Now, why do we see so any leaks?
Leaks do occur at the end of a stave, at what is called the butt-joint, most often when combined with a breakdown or severing of a steel band at that point. In addition, steel plates are sometimes placed in the slots at each stave end, and these steel plates can corrode. Also, some erosion can occur at the end of a stave, and develop into a hole. In this instance, the steel band in that area might corrode and sever, and the pressure of the water inside might break off a section of the stave, however small. Metal corrosion also sets up a mild acidic condition. The acid can degrade the wood. There can be a breakdown in the staves when the water pressure inside varies a lot. You will seldom see wooden penstocks for example in positions where turbines can vary the water pressure output in large degrees. This creates what is known as the hammer effect which can beat up a wooden penstock quickly. It’s best to try to keep the inside water pressure as even as possible. This said, small leaks can self repair as the wood expands. Even large breakdowns in the staves can be repaired.
In most instances, the leaks are tracked closely and there is very little risk of a catastrophic failure. Once the loss of water gets to be unbearable, then replacement action is usually in order. Since the penstocks at Grandfather Falls were installed in 1975, they are 39 years old as of 2014.
Now, it must be said that when the leakage does get unbearable, then the penstock might well be replaced using different materials. Fiberglass is being used in several places for example. Steel is now also being employed, as is PVC.
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