Hydroelectric Cathodic Protection

[Truemanator]

Hey guys, first post. Seeing as how I'm electrical I figured it would be best to ask electricals who may have encountered this problem.

I'm working on a hydroelectric run-of-river generating facility. We've been asked to determine whether cathodic protection is required, in light of the ~1m diameter steel penstock pipe entering the facility and the copper ground grid being close by.

The grid is 1m from the foundation and 1.2m below the penstock at the closest point. The ground is considered to be frozen 140 days per year and has a worst case, 125 ohm-m resistivity. Electrical connection length between penstock pipe and grounding grid is on the order of about 10m, so resistance of that path is negligible in comparison to soil resistance. Differential between copper and steel (iron) is +0.78V. I have estimated a total loss of iron to be ~165 grams over the desired 40 year lifespan of the pipe (after a few assumptions and conservative estimates)

Any suggestions if I am going wrong or missing something? Is this a potential for severe failure with the copper and steel?

Thanks.

 

[itsmoked]

Nicely worded question Trumanator.

Hopefully someone who knows corrosion will pipe up.

I would suggest you also post to thread378-229027 asking that the discussion be carried out here.

Copy the whole: "[ignore]thread238-229180[/ignore]" above, for the link back here.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[dpc]

This may be of some interest:

http://www.elkeng.com/pdfs/5-Conflict Between Copper GroundingCP82002.pdf

Some type of sacrificial anodes or impressed current systems sometimes combined with pipe coating systems are typically used for these situations, in my experience, but I'm not a corrosion guy.

You may want to seek some advice from a corrosion engineering specialist.

 

[ScottyUK]

Pretty sure Keith meant to post a link to forum338, not to that particular thread.


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If we learn from our mistakes I'm getting a great education!

 

[Kiribanda]

Trumanator,

From your description, I feel that "unintentional bonding" between the steel penstock and the existing copper ground grid is unavoidable. The result would be that one day you will loose the penstock due to corrosion.

Therefore, from electrical point of view, my advice is,

1) Isolate the penstock from other metal housings using insulating flanges (widely used in CP systems)as much as possible, so that any possible electrolytic corrosion current is blocked.

OR

2) Knowing the resistivity of the soil, connect few Magnesium sacrificial anodes to the steel penstock.

OR

3) IF POSSIBLE, re-design and replace the copper ground grid with a zinc coated steel ground grid. Then no need of insulating flanges. No worries about possible "unintentional bonding".

Hope this helps.

 

[itsmoked]

Thanks Scotty! Where did I get that link?!!?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[fdac]

Is the penstock buried? If so, then you have a galvanic couple with the copper earthing which will accelerate corrosion of the steel penstock.

The metal loss may be only 165 grams of iron, but this will be concentrated at coating defects in the form of pits. What % wall thickness loss is acceptable to the pipe designers? what is the impact of a sudden leak? etc. etc.

"A few magnesium anodes" will not necessarily work, particularly if you're connected to the copper earthing grid. A coated pipeline may need a few hundred millamps of CP current, but trying to protect an entire earthing grid may need a few hundred amps.

Isolate the penstock from the earthing for starters, but get a CP specialist on board to sort it out.