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Loss of material in penstock.

Loss of material in penstock.

Loss of material in penstock.

(OP)
Hello everybody:

In a penstock of a hydroelectric power plant, the problem of corrosion of the outer face of the pipe arises, just as it is no longer embedded in the concrete at the entrance of the spiral case.

The pipe is UNI 7070-72 S355J EN10025. The diameter of the pipe is 3,250 mm and the wall thickness is 18 mm. The damaged part presents a loss of the order of 6 mm (deep) along about 500 mm at one point and 600 mm at another one. The head on the turbine is 85 m.

To repair this loss of material I think of at least two possibilities: one is to replace the lost material with an epoxy product, or to recharge with welding.

I would like to know your opinions about it.

Thanks in advance.

El que no puede andar, se sienta.

RE: Loss of material in penstock.

The problem is at the lowest point where any moisture between the steel and concrete will flow, and also settle because the interface there is horizontal. I suggest you send a inspector inside (safety permitting) and perform UT thickness measurements.

The corrosion will extend far beyond where you can see, and probably well beyond where epoxy can reach. Some removal of concrete might be necessary, and hopefully the corrosion is not so severe that steel needs to be replaced. Although now I think about it, a layer of steel could be welded to the inside surface.

In hindsight this part of the penstock maybe should have been coated before concrete pour. How old is the station?

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

RE: Loss of material in penstock.

(OP)
Hello everybody:

Thanks ironic metallurgist for your comments.

The hydroelectric plant has been in operation for 7 years. In the section of penstock next to the spiral case, voids have been detected between the concrete and the pipe. Civil Works Maintenance plans to perform contact injections to close those gaps.

El que no puede andar, se sienta.

RE: Loss of material in penstock.

I would argue against the use of a doubler inside of the pipe of the corrosion is coming from the outside. A double simply delays the inevitable and the the swelling due to corrosion underneath could accelerate the failure of the doubler.

RE: Loss of material in penstock.

Having seen the photographs, this looks like a demolishing job. For the concrete at least.
Cladding will not be evident, because of the vertical position of the pipe. An epoxy repair (after sandblasting) will depend on the mechanical requirements, although if it is correct that you only have 6 mm material loss (which I would double- and triple check), is possible. But that wouldn't mitigate further problems.
Cut, replace and foresee the necessary measures to prevent this in the future, would be a third option.

To quote IM, a repair from the inside would be quite an undertaking from the safety department's point of view: You've got height (and a giant turbine waiting for you down below), enclosed space, welding, probably not easy to ventilate / evacuate fumes, risk for electrocution, you will be welding on steel with concrete on the other side (only 12-18 mm away), and probably some more ... Those are a lot of red flags!

How "clean" does the water needs to be when it arrives at the turbine? I imagine some spatter probably won't do much damage, but what if the repair fails for some reason (this ain't shop welding!), and a steel sheet falls in the turbine?

RE: Loss of material in penstock.

Is the pipe with the corrosion really only 7 years old? It looks like the corrosion was painted over. What is the cause of this corrosion? Does it look the same all around the pipe, or is it worse on the top? Was there ever a leak above? Adding a doubler to the inside could potentially cause cavitation pitting on the inside. Addressing the cause, chipping the concrete to sound steel, filling the pits with weld or replacing a section of the pipe seems in order.

RE: Loss of material in penstock.

21121956

I think this would have been better in the corrosion engineering forum but from the limited amount of detail, I think you're at serious risk of catastrophic failure. Is that lower area actually weeping water? If not where is the water coming form?

These are long areas of corrosion and normally this limits the loss of wall thickness to around 20% or 3.6mm, you have nearly double that.

I suggest you consider whether to shut this pipe down, get a UT scan from the inside of the pipe across the concrete area and look at removal of some of the concrete to allow you to reinforce this section. Epoxy on its own will not do much for you without being able to reinforce the corrosion area in a circumferential manner.

You haven't said what role you are in or what other inspections are planned.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Loss of material in penstock.

(OP)
Hello everybody:

Thank you all for your valuable comments.

In relation to your questions, I tell you the following:

The water that goes to the turbine is water from a reservoir, fed by a river, it is therefore clean water since the solids that it could drag are deposited at the bottom of the reservoir.

Upon closer inspection, it was determined that the corrosion is in the form of flakes (laminations), giving the impression that, when painting during assembly, the paint was applied to a surface not properly cleaned.

When removing the flakes, it was found that the thickness loss is not uniform, and it is of the order of 1 - 2 mm.

Corrosion is more concentrated in the lower area of the pipe. The water that comes out there comes from the gap between the pipe and the concrete. For this reason, it is planned to carry out contact injections between the pipe and the concrete.

From the UT tests, it was found that the average thickness of the pipe in the diameter of the damage zone is 17.72 mm.

12 readings (measurements) were taken, the least being 17.05 mm and the greatest being 17.95 mm.

Once again, thank you for your input.

El que no puede andar, se sienta.

RE: Loss of material in penstock.

The reservoir water may be clean, but the concrete is not, in terms of ions it will contribute. The flaking appearance does not surprise me.

You have enough data to establish a safe Remaining Life based on FFS Code. With a limited number of spot measurements, you will have missed the very thinnest locations, so take that into account.

You are in for a long term mitigation effort, trying to postpone major surgery for as long as possible.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

RE: Loss of material in penstock.

1-2mm loss of wall is a lot different to the 6mm you mentioned earlier in the OP.

That's a big difference.

I still don't like where this water is coming form and simply sealing the gap between concrete and pipe is nothing more than a sticking plaster and still doesn't say where the water is coming from.

Do you have a drawing of this pipe concrete entry point in section view? Simply having a pipe with a nominal paint coating encased in concrete doesn't sound like a great plan. What was supposed to happen?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

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