Hydroelectric Water Tower Corrosion 1

[nicknikolov]

I'll be inspecting and determining the integrity of a very old (installed 1965) surge tank used in a hydroelectric project.

The latest drawings on the project are all dated to 1965, and the tank is stated to have a corrosion allowance of 3/32".

Since this tank is not under pressure, I believe this corrosion allowance was arbitrarily picked. Also, since it is an old tank there will definitely be significant corrosion. Not sure if I should just stick to what was picked by the engineer that originally stamped the drawings, or if I should make a more specialized recommendation.

Are there any detailed reports or studies that have dealt with these non-code, non-pressurized tanks? If I see a single point with corrosion greater than 3/32" is that enough to condemn the tank, or at least require a new coating in the area?

 

[Dhurjati Sen]

You can always weld build up at such locations and then flush them.

Anyway, check API 510 for guidance.

DHURJATI SEN
Kolkata, India

 

[0707]

Make a random UT inspection to avaliate the remaining thickness.Then decide if the tank supports some patches and a new anticorrosion coating or simply condemn the tank.

luis

 

[EdStainless]

There is actually one step in between. Do a UT survey, concentrate near the bottom and near the top water level. If the results look highly suspect then the tank needs to be drained, cleaned, and inspected in more detail, then decide if repair is feasible, if it is repair, blast and coat; if not then start designing the replacement.

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P.E. Metallurgy, consulting work welcomed

 

[nicknikolov]

The UT crew is worried they can't get good UT measurements from the inside of the tank due to the rust build up. And operations does not want to remove any insulation from the outside of the tank. I'm working on this communication in the mean time, but for now an extensive UT survey of welds and wall thickness' is not possible.

The UT crew is proposing to use a pencil probe that gives data in a very small area and can get around the rust.

 

[TugboatEng]

Marine industry allows 15%-20% wastage on structures as a rule of thumb.

https://rules.dnvgl.com/docs/pdf/dnvgl/cg/2016-02/dnvgl-cg-0182.pdf

 

[0707]

nicknikolov without eggs one can't make omelettes.

luis

 

[EdStainless]

They will have to strip sections of insulation or at least create inspection ports (which should be there anyway). They only other options is to drain and blast the ID, and that really should be a later step.

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P.E. Metallurgy, consulting work welcomed

 

[LittleInch]

"And operations does not want to remove any insulation from the outside of the tank."

Hard luck. Tell them they need to remove the insulation if they want a report that's actually going to be useful.

Operations need to get their head out of their behind and do things which make the inspection possible, not be obstructive.

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

 

[nicknikolov]

I agree about the insulation, I'll keep pushing on that part.

The dam will be shutdown and the tank will be drained anyway. So if the corrosion is not as bad as expected, UT from the inside should not be an issue.

Did not know tanks would have inspection ports, once on site I'll look around for these. The tank is quite large (40' diameter x 100' height).

Mostly wondering about the corrosion allowance on non-code tanks. The 15-20% rule is similar to the initially stated corrosion allowance on the 1965 drawing so I might just go with that.

 

[TugboatEng]

As an operator... You'll want to gauge from the "good" side under the insulation. This will prevent the gauger from targeting pits and give you an average thickness. Individual deep pits result in leakage can be dealt with but you need to know average wastage to prevent structural failure.