Preventing hydropower turbine failure

May 26, 2015, Norwegian University of Science and Technology
Preventing hydropower turbine failure
A drawing showing the essential elements of a Francis turbine, one of the most common hydropower turbines in Norway. Credit: Kværner

The Francis turbine is the most common type of water turbine used in Norwegian hydropower plants, and has been for many years. About half of the world's Francis turbines are found in Norwegian plants.

But for the past few years, more and more of these turbines have been fracturing. The problem has been particularly pronounced in high-pressure turbines, which run at head heights of 200-300 metres.

"At least five of the main hydropower plants in Norway have had turbine problems in the past few years, among them Svartisen," says Professor Ole Gunnar Dahlhaug of the Department of Energy and Process Engineering at the Norwegian University of Science and Technology. "Svartisen had to stop production for six months, and the company lost huge amounts of money."

"We think we know what's causing the problem, but we aren't able to make the calculations that would enable us to better understand it" says Dahlhaug. "It has to do with resonance, and an interaction between pressure from the water and the steel in the turbine blades."

Explained simply: a changing water current, such as one caused when production starts and stops a lot, will result in an oscillating pressure on the steel turbine blades. If the frequency of this oscillation approaches the fundamental frequency of the material, it causes a resonance that can weaken the steel. It's essentially the same as the apocryphyl crystal glass breaking when an opera singer hits the right note.

Dahlhaug was previously head of the Norwegian Hydropower Centre (NKVS), which is a collaboration between several universities, research institutions, the hydropower industry and the Norwegian government.

New power regime

Hydropower production can be turned on and off on short notice, and can therefore be adjusted to changes in market prices and power needs. This flexibility means that it is a good option in combination with newer , such as wind and solar energy. Norwegian hydropower plants can work as a sort of green battery to deliver sufficient power on cloudy or still days.

But it turns out that the Francis turbines aren't able to handle this flexible running schedule.

The problem seems to have started as a result of the new power production regime implemented in the 1990s. Power production was no longer controlled by grid system operation, but rather by the power market. Individual power companies could choose when they ran their turbines, mostly decided by changing prices and demand in the power market.

"The turbines installed in the 1960s and 1970s were designed to run more or less all the time, with a constant amount of pressure. Now they can be started and stopped as many as ten times a day. The turbines can't handle it. In some cases, the actual steel in the blades has cracked," Dahlhaug says.

Newer turbines have also had problems—a turbine in Driva failed after only ten days. A new turbine also failed at the Svartisen power plant.

Industry collaboration

To solve this problem, researchers, power companies and turbine manufacturers have all come together to collaborate. Competing businesses are sharing and comparing data in several different research projects, all working towards the same goal.

"This is the first time that I know of the whole industry coming together as such a team on something that really is basic research," Dahlhaug says. "But it is a technical problem that everyone has in common. The knowledge isn't something that we want to compete about— it's not something that anyone wants to miss out on."

The most important job is to make a simulation tool to see what happens inside a turbine under different conditions. Or to simply find a method to calculate what the turbines can handle under different amounts of strain.

Like a crystal glass breaking

"We think we know the cause of the problems, but we aren't able to calculate it," says Dahlhaug.

It has to do with resonance, and the interaction between the pressure of the running water and the steel in the turbine blades. Explained simply: a changing water current, such as one caused when production starts and stops a lot, will apply an oscillating pressure to the steel turbine blades. If the frequency of this oscillation approaches the fundamental frequency of the material, it causes a resonance that can weaken the steel. It's essentially the same as a crystal glass breaking when an opera singer hits the right note.

Unfortunately, finding the fundamental frequency of the steel isn't as simple as you'd think. Factors such as submersion in water and rotation can cause it to change.

"There are a lot of variables that we can't control here, and that we aren't able to calculate yet. Running simulations of conditions that change so much requires immense amounts of computing power," Dahlhaug explains.

Difficult to calculate

Statkraft has many high-pressure Francis turbines. The first incidence of major damage to turbines was in Leirdøla kraftverk in 1978, says Kjell-Tore Fjærvold, Section Manager of Mechanical Technology at Statkraft.

"Since then, there have been a number of similar incidents at Statkraft as well as other power companies in Norway and in Europe, with similar turbines. We've also had problems with several of the high-pressure Francis turbines more recently. The challenge is that providers aren't able to calculate the appropriate amounts of strain and stress due to varying water currents during construction of the turbines. This means that we as customers can't be sure if the parts we buy will have problems when we use them," Fjærvold explains.

Need for basic research

Project Francis 99 started last year. Different groups are all comparing calculation methods and data based on a common reference model. The first project summit was held last year, with representatives from 16 universities, as well as from all of the main turbine providers in the sector.

This project will soon be followed by High Head Francis, a new four-year research project. It is being initiated by Statkraft through NVKS, but will be run by NTNU. This is also a collaboration with power companies and providers, Dahlhaug explains.

"Usually, businesses are interested in more practical research. But in this case, the industry is taking part in basic research. Statkraft has a representative working 200-300 hours a year, and other businesses are contributing with technology researchers," he says.

The third step is an EU project that will be started next year, assuming that it is funded.

"Hydropower plays a key role in providing energy, along with other sustainable energy sources. But first, we need to understand what is happening, and be able to calculate the requirements for safe, resilient turbines," Dahlhaug concludes.

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37 comments

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gkam
1 / 5 (4) May 26, 2015
They may be in a regime where Pelton runners can be used.
Eikka
5 / 5 (2) May 26, 2015
They may be in a regime where Pelton runners can be used.


Pelton turbines cant be made as large, and Francis turbines are reversible. The generator can act as a pump to fill the reservoir.

A Pelton wheel must also be placed above the water outlet, which reduces the head height of the dam.
gkam
1 / 5 (4) May 26, 2015
No, it doesn't. The Pelton wheels which I saw in the bay itself were vertical shaft, with six nozzles holding back what was then the highest hydro head in the world in a two-unit powerhouse owned by So Cal Ed.

The heads of Pelton-configured systems is so high, those few feet would not make any difference. Perhaps you are confused with Kaplan runners.
Eikka
5 / 5 (2) May 26, 2015
No, it doesn't. The Pelton wheels which I saw in the bay itself were vertical shaft


Yes it does. You still need to have the outlet above the water. It can't swim in it like a Francis turbine does, which means the maximum water level at the lower reservoir has to be lower than the turbine.

The Francis turbine's outlet is normally well below the water level even if the turbine itself isn't, because the tube in between is full of water. Technically, the turbine could be all the way up the dam, powered by the mass of water pulling through the outlet tube, although that would create cavitation.

So the Francis turbine head height is the difference between the upper and lower reservoir water levels, whereas the Pelton wheel is between the upper level and turbine outlet.
Eikka
5 / 5 (1) May 26, 2015
those few feet would not make any difference


The difference is in the single percentage points in cases where both could be used the same, but in a large dam project, that couple percentage points means millions of dollars of money.

And in large dams, you can have fewer Francis turbines than Pelton wheels anyways, so again, cost advantage.
Eikka
5 / 5 (2) May 26, 2015
The heads of Pelton-configured systems is so high


That's what it's best for. High-head and low-flow. The opposite case of low-head and high-flow is dealt with the Kaplan turbine, while the Francis turbine is best in the region between.

The Santa Ana river plant I suppose you're referring to has a head height of 735 feet or about 250 meters. The majority of dams in Norway seem to be between 140-180 meters or up to 550 ft, although some get up as high as 1000 feet.

The thing is that Norway is looking to build a lot of pumped hydroelectric storage to buy and sell power to/from central Europe, so they need the Francis turbines. Otherwise they'd need to build one turbine for power and another for the pump.

gkam
1 / 5 (4) May 26, 2015
"The difference is in the single percentage points in cases where both could be used the same, but in a large dam project, that couple percentage points means millions of dollars of money."
-----------------------------------------------

Nope. The head I had referred to was over 2,000 psi, as I recall. I'd have to look it up for you. And most modern Pelton wheels are horizontal, with vertical shafts, like I said.
Eikka
5 / 5 (2) May 27, 2015
And most modern Pelton wheels are horizontal, with vertical shafts, like I said.


That's irrelevant. The Pelton wheel is an impulse turbine. Instead of the water dragging the impellers along, it hits the buckets and transfers momentum to the wheel, then drops off. You could be shooting bullets or throwing rocks at the rotating scoops - it's the same principle.

If the buckets are submerged in water it stops working as an impulse turbine and starts working similiar to the Francis turbine without the guiding vanes. The water has to drop down from the scoops freely for proper operation. That's why the turbine has to be above the outlet water level.

It makes no difference which way you put the wheel as long as it isn't upside down.

Nope. The head I had referred to was over 2,000 psi, as I recall.


So it's the Big Creek then? That's got 6,200 ft of head.
gkam
1 / 5 (4) May 27, 2015
No, Eikka, they do not work like that. We had all kinds of horizontal and vertical axis Pelton wheels in California, where we have high-head water. Since you have probably not seen any up close, it is understandable. Go through the California gold country and you can still see them.

Big Creek? I am not sure. The powerhouse I was in had only two units, and was not the grand thing we see in the picture. It was in 1970 that I was Unit Two, which was shut down for maintenance, and we went into the pelton wheel chamber on temporary grids which dropped down from the walls for maintenance. We huddled around the wheel itself between it and the the six dripping nozzles, covered by slide gates.

Go take a class and learn about these conversion devices. Our classes went to almost every hydro site in Central California, stood in the firebox of Unit Two supercritical boiler at Moss Landing (around 1,000 Megawatts), and other stuff. Be ready to explain all the steps in the systems.
gkam
1 / 5 (4) May 27, 2015
Water does not "drop freely" from the Pelton wheels, with that high head it sprays all over. Did you notice the split in the buckets? That is to keep it from spraying straight back and hitting the stream itself. It splits in two and goes around the streams. The bottoms are open for the water to go down the raceway.
MR166
1 / 5 (2) May 27, 2015
"The problem seems to have started as a result of the new power production regime implemented in the 1990s. Power production was no longer controlled by grid system operation, but rather by the power market. Individual power companies could choose when they ran their turbines, mostly decided by changing prices and demand in the power market."

In other words they are yet another victim of intermittent solar and wind power. Before power companies could predict power demand and adjust output slowly. Now they must change the output quickly to compensate for variations in renewable outputs.
MR166
1 / 5 (3) May 27, 2015
I love how carefully the article was worded so as not to cast solar and wind in a negative light. This shows the vast power and influence of Big Green Machine.
gkam
1 / 5 (3) May 27, 2015
MR 166 apparently does not understand that is how hydro has been used for almost a hundred years now. The power companies will leave one unit running up to speed motoring, until it is needed for load following or peaking. Then, they adjust the exciter and the wicket gates, bring it to synchronicity, and put it online.

I found it interesting that they ran the generator as a motor electrically, instead of running water through the turbine for standby.
MR166
3.7 / 5 (3) May 27, 2015
"MR 166 apparently does not understand that is how hydro has been used for almost a hundred years now"

Read the article dolt. The problem is of recient occurrence.
gkam
1.8 / 5 (5) May 27, 2015
"Power production was no longer controlled by grid system operation, but rather by the power market. Individual power companies could choose when they ran their turbines, mostly decided by changing prices and demand in the power market."
--------------------------------------------

Absolute ignorance. As soon as multiple units were available,that is how they were ALWAYS operated. Who wrote that?
MR166
3 / 5 (2) May 27, 2015
Gkam it just amazing how you know so much, and I am not being sarcastic here you really do, but when it comes to negative points about solar and wind the simplest comment is beyond your grasp.
gkam
1 / 5 (3) May 27, 2015
Mr166, we understand all of that. We have been using this stuff here for decades. Those are all factors we faced and are overcoming. Please understand we have been in this fix in the West for a generation now. We understand the problems, and will work them out.

Sorry for the crabbiness, I have to admit it is PTS from the service. I have no patience for some stuff, and it is one of the reasons I had so many professions. As soon as I would be asked to do something I thought was not ethical or legal, I would make a fuss and leave. But for the most part, it was because every job becomes a job after a while. I do not want to learn to do the same thing everyday. Some folk find comfort in it, but not me.
Eikka
5 / 5 (3) May 30, 2015
No, Eikka, they do not work like that.


Yes, gkam, they do work like that.

I'm perfectly aware of the theory of operation.

The Pelton wheel's head height ends where the nozzle opens, and where the water ends up after that can only impair the function of the wheel.

That's the lowest point of the system as far as the potential energy of the water is concerned, whereas with a Francis turbine, since the whole turbine is immersed in water by design, the head continues to the end of the exhaust tube or to the level of the lower reservoir whichever is higher, because the water column inside the tube exerts a pull on the water upstream towards the turbine. This does not happen with a Pelton wheel.

http://www.learne...ine.html

Pelton turbine gains mechanical energy purely due to change in kinetic energy of jet, not due to pressure energy change. Which means Pelton turbine is a pure impulse machine.
Eikka
5 / 5 (3) May 30, 2015
Water does not "drop freely" from the Pelton wheels, with that high head it sprays all over.


With that comment you demonstrate that you know absolutely nothing about what you're talking about.

The point of the impulse machine is that the jet of water has a certain kinetic energy, which is extracted upon impact to the scoops or buckets on the wheels. At maximum efficiency, the water would be left with no kinetic energy whatsoever, therefore no velocity one way or the other, and would drop down freely under gravity.

The offspray from the buckets is simply due to the fact that the system is never perfect. If the Pelton wheel was swimming in its own exhaust water like a Francis turbine does, a part of the impulse energy would be spent pushing the scoops through the water for no reason and the efficiency would be lower. That's why the Pelton wheel must always remain well above its outlet water surface.
gkam
1 / 5 (4) May 30, 2015
You are making this stuff up from your own imagination. Go look at a real Pelton wheel. I have been inside the chambers themselves.

The spray from the buckets is a reversal of the water stream, which imparts more impulse to the wheel. I suggest you go look at one. The stream is split to avoid the jets, hits the walls from which the nozzles project, and drops down into the raceway.
gkam
1 / 5 (4) May 30, 2015
I think you are looking at the drawings and not real units themselves.
Estevan57
5 / 5 (2) May 31, 2015
Good explanation and source, Eikka. A source is worth a thousand "I've seen it myself".
Eikka
5 / 5 (2) May 31, 2015
You are making this stuff up from your own imagination. Go look at a real Pelton wheel. I have been inside the chambers themselves.


I would say you are. You've seen the thing up close, but you have no idea how it actually works so you're interpreting it differently.

The spray from the buckets is a reversal of the water stream, which imparts more impulse to the wheel.


Theres no more impulse to be had than what kinetic energy there is in the jet. The reversal of the stream is just how the bucket extracts the energy. The point is to change the direction of the jet, which imparts a force, like a motorcyclist driving the wall of death. It goes around the bend and pushes on the scoop, and in doing so loses kinetic energy into the motion of the scoop.

If it could do that with perfect efficiency the water would not spray anywhere - it would just drop down off the bucket. The backspray is all the kinetic energy that was not collected.
Eikka
5 / 5 (2) May 31, 2015
The stream is split to avoid the jets, hits the walls from which the nozzles project


Actually, the stream is split in half because it imparts equal force on the wheel. Otherwise the water would impact more on one side or the other and impart torque perpendicular to the axle, which would put undue wear on the bearings and possibly induce wobble which would destroy the wheel.

So they use a scoop with a splitting ridge in the middle to balance it out. If the wheel tilts or flexes one way, then the ridge will move slightly with the wheel and split the jet more the other way, returning it to center.

There's many ways to build the wheel - some use two jets and two half-scoops on either side of the wheel, both projecting the offspray to the middle where the two jets hit head on so there's no backspray.
gkam
1 / 5 (4) May 31, 2015
Eikka, wiki will not bail you out on this one. You can rephrase all you want, but you screwed up. Pelton buckets are split so the stream is not quenched by the reflected water. You hit a metal curved plate with 2,000 psi water, and you think it just stops there and drops down?

gkam
1 / 5 (4) May 31, 2015
" If the Pelton wheel was swimming in its own exhaust water like a Francis turbine does, . . "

The Francis runner is in the flow of the water, not in it's "own exhaust", which drops from the center into the tailrace. And no Pelton wheel is even near the level of the tailrace. The one which I was in had several feet between us at wheel level and the tailrace level.
gkam
1 / 5 (3) May 31, 2015
I just looked up Pelton wheels and found why Eikka thinks the way he does. They show a horizontal axis unit, which is what we used a hundred years ago, instead of the vertical axis units driving a large generator on the floor above, like you see with the Francis turbines in Hoover Dam and Shasta. The quotation is "Most Pelton wheels are mounted on a horizontal axis, although newer vertical-axis units have been developed."

The only horizontal ones I have seen were in museums or no longer in service, still on the floors of some of our really old powerhouses.
Estevan57
1 / 5 (1) Jun 01, 2015
Hey gkam
Perhaps wiki will "bail Eikka out". Electrical-Engineering Portal also, to keep you from getting a rash.

"Typically two buckets are mounted side-by-side on the wheel, which permits splitting the water jet into two equal streams (see photo). This balances the side-load forces on the wheel and helps to ensure smooth, efficient transfer of momentum of the fluid jet of water to the turbine wheel."

http://en.wikiped...on_wheel

"...This also caused an end thrust to one bearing. To eliminate the new problem, Pelton then alternated the buckets as shown by his sketch Figure 12 the next step was obvious, the buckets were joined and centered to split the steam as shown by his sketch 13.
The tests of the joined buckets were so surprising that Pelton took steps to obtain his patent."

http://electrical-engineering-portal.com/lester-allan-pelton-father-hydroelectric-power

Ya gotta look stuff up. It shows that Eikka does.

Eikka
5 / 5 (2) Jun 01, 2015
You hit a metal curved plate with 2,000 psi water, and you think it just stops there and drops down?


If the metal plate is moving at half the speed of the water jet in the same direction - yes! The impact transfers the kinetic energy of the jet to the wheel and the water itself stops moving.

That's how the turbine is ideally supposed to work. I suggest YOU actually read a bit of basic theory in dynamics, or ask someone who's designed such turbines; they'll only confirm what I just said.

Ya gotta look stuff up. It shows that Eikka does.


Gkam doesn't look stuff up, because "wiki goobers" don't know how things really work, since it's impossible to learn real knowledge from books.
Eikka
5 / 5 (2) Jun 01, 2015
Notice that the buckets in a Pelton wheel aren't subjected to a continuous jet, but each bucket gets a squirt of water as they move in front of the jet and then out of it.

So, the problem of catching a 2000 psi water jet is actually the exact same problem as catching a chicken egg thrown at you. If you just hold your hand open in the air, the egg goes splat and sprays everywhere - but if you move your hand backwards with the flying egg as you catch it, there's a smooth transfer of energy from the egg to your hand and you can safely bring it to a halt - extracting all the kinetic energy that was in the throw.

The rotation of the wheel is the key point. The shape of the scoop is secondary. A wheel with flat scoops or simple flat bottom buckets will still work as a Pelton wheel, although it is less efficient because the transfer of momentum isn't as smooth. The round scoop stops the water more gradually.

Eikka
5 / 5 (2) Jun 01, 2015
The Francis runner is in the flow of the water, not in it's "own exhaust", which drops from the center into the tailrace.


The Pelton wheel, if the buckets were underwater, would be running in its own "exhaust", which is what I said. You're just mincing words here.

And no Pelton wheel is even near the level of the tailrace. The one which I was in had several feet between us at wheel level and the tailrace level.


That's what I've been repeatedly telling you.

The Francis turbine's tailrace tube is full of water as it is directly connected to the turbine outlet with no drop in between, and the flow of the water in the tube still exerts a pressure differential across the turbine, which means the turbine is still extracting power out of the water that has already passed through.

A Pelton wheel does not do that, so it loses part of the head height between the upper and lower reservoirs, which makes it less efficient when both types could be used.
gkam
1 / 5 (4) Jun 01, 2015
Eikka, you are relying on wiki and they do not tell you everything. You are also thinking Peltons are used with horizontal shafts, when the ones I have been "IN" have vertical shafts, split buckets with the water splitting to it does now quench the main stream. I guess I will have to look it up for you.
gkam
1 / 5 (4) Jun 01, 2015
Hey Estavan, your words do not contradict mine. You folk seem to have the Reagan Syndrome, where you are unaware of the amount of ignorance ina particular field, and assume we are all guessing, . . like you and Eikka.

Cut and paste all you want: You are incorrect. I told you both how the Big Boys do it for a living. If you want to argue, go ahead. Leave me out. You are just playing ego games now.
Eikka
not rated yet Jun 05, 2015
You are also thinking Peltons are used with horizontal shafts,


No. I keep saying it doesn't matter which way you have them - it doesn't change how the wheel works. I don't care if you have them at 45 degrees.

It's like you don't even read the comments.

You folk seem to have the Reagan Syndrome, where you are unaware of the amount of ignorance ina particular field, and assume we are all guessing, .


It's you who is ignorant and operating on some sort of home-cooked theory on how things work based on having once visited a hydroelectric station, or a nuclear station, or having seen someone install a HVAC system, or whatever it is you're claiming authority on that particular day.

YOU are SO ignorant you don't even recognize your own mistakes when they are pointed out, because you don't understand the right answer.

Every source I can find on Pelton wheels, and the basic physics I have been taught all agree. It's you alone who's in disagreement here.
Eikka
not rated yet Jun 05, 2015
Eikka, you are relying on wiki and they do not tell you everything.


If I dig up an engineering textbook on Pelton wheel design and it says exactly the same thing that the wiki page does, what then?

Do you claim that the textbook is wrong, that there's some "secret sauce" they're still not telling you because you have to be there to know? That it's patently impossible - with all the engineers sitting around their drawing desks - to design a working Pelton wheel based on the theoretical knowledge of physics as learned from a textbook?

Estevan57
5 / 5 (1) Jun 05, 2015
It doesn't matter, Eikka, gkam only really wants to strut around an get attention for his massive, gigantic wealth of alleged experience.

He doesn't want to be corrected when challenged for his mistakes.
Even with good valid sources he still insists he is right because he is the only person in the room with experience. It also comes from being too old to realize that there is a wealth of information online.

This forum is text based, cut and past is how people transfer information and links to outside sources. Including real engineers.
Estevan57
5 / 5 (1) Jun 05, 2015
I find it amusing to see gkam call himself a "Big Boy" , but if you argue with him it's an " ego game".

At any rate Eikka, you're right about the Pelton wheel bearing load and other points.

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