Cathodic Protection procedures, please help!!!

[thien2002]

I have 54'dia. x 35 crt chilled water storage tanks for thermal storage. Our tank contractor provide a submittals for cathodic protection and since I don't have any technical knowledge on cathodic protection, could anyone please point me to the right direction for cathodic protection specs approval?

Here is what I am going to have:
Anotec Type: 2660
Nominal Dimensions Inches: D (2.6), L (60), A (3.0)
Nominal Weight lbs c/w Anchor: 50
Nominal Area sqft: 3.5
Nominal Discharge at .75-1.0 Amps per Ft2: 2.6-3.5

I have some sketches for the installation. Could anyone give me a good start? Thank you so much in advance!!!

 

[Metalguy]

Assuming steel tanks, are they going to be coated internally? If you use a decent coating, you can use the CP to protect the coating defect areas, and your anode life will be FAR longer.

 

[thien2002]

Your guess is right, this is going to be a steel tanks, but you didn't answer my question. I need a little more guides as far as identifying the right CP material and components. I don't know how to explain it but I need some guides to check if the vendors are providing me with proper CP packages. Thank you.

 

[JoeTank]

Forgive for saying what may sound rude, but if you have to ask these types of questions about CP, why are you the one reviewing and approving the CP vendor's submittals?

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com

 

[JoeTank]

Further to my previous post... If yo decide to seek the review assistance of corrosion engineer with a strong CP system design background, I can suggest one for you. Said specialist is not a coworker.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com

 

[thien2002]

Steve,

Your concern is precised, I really need a specialist to review on my Cathodic Protection package, but since we are working for the Air Force, there are 7 CP submittal reviews distributed among all the responsible parties. I am one of the 7 since I want to contribute a few comments that is why I am coming to this Forum. Steve, you are NOT rude but very exact, however, we are operating a little different than the private industries. I am learning please help me. Thank you.

 

[saldanha]

Thien2002

Anodes for this type of corrosion protection are usualy made of Zn or Mg. I will not be able to tell you the exact spec for your material but if you can consult the ASM metal handbook or some ASTM spec recarding cathodic protection you will have your unswer.
Regards

 

[JoeTank]

thien2002,

I've sat down a few times to scratch out some pointer's, but I keep coming back to the point that I'm no CP guy. This may be a case of blind leading blind. First of all, I am a structural guy that more in tune with the structural integrity issues of tanks, but I have significant experience witrh coating's related issues too. Coatings knowledge is not the chemistry side, but more along the lines of proper detailing for long coatings life, constructability, etc.

Here'e my checklist that I'm making up as I go along.

Potable water tanks typically have impressed current CP with automated electrical systems to keep the current flow at the right range to protect the tank.

Most CP systems, if speced in a minimal way are usually designed to protect only a few percent of the tank wall in the event of a coatings failure. We write specs that require a ten year life assuming a 25% coatings failure. With that in mind I would go thru the calcs to look for indications of design life, percent failure assumed in the design, etc.

CP is a belts-and-suspenders thing for me in that it protects in event of coating failure only. So we often spec out a CP system be installed and available IF the lining begins to fail. Once coatings start to fail, the CP system will be energized and will minimize the tank damage thereafter. Hopefully it's never required.

Great emphasis should be placed upon the coating system being selected properly and then even graeter effort put in to seeing that it's applied correctly. Contractors almost never get on on right unless it is inspected FULL TIME. Don't let anyone ever convince you that coatings can be properly inspected by inspecting at critical phases... it's ALL ctritical phase!

Make sure that the CP installation details are properly considered. They should be installed in a manner that includes all of the hardware clips, suppoprts, etc being properly welded into the tank BEFORE the coatings phase begins. I have seen numerous tanks coatings damaged by CP installations that had the misc hardware supports and brackets after the coatings were complete.

As you can see my comments tend to be about construction, sequence, details, etc.

Hope it helps.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com

 

[saldanha]

The primary protection of your structure against corrosion is your coating. There is no doubt about that. Of course here we are assuming that it is a perfect coating. However we all know there is no such thing as perfect. Your coating will always have defect. And corrosion will start on those small area where your coating is defective and disbond your coating and spread.
So you need cp to be installed from the moment your tank is put in operation.
I do not know much about your tank but generally water tanks use sacrificial anodes for cp protection. If your cp contractor tells you about impressed current, he must tell you why. The decision about impressed current vs. sacrificial anode cp is based on current requirement. This is because sacrificial anode can produce only a limited amount of current and the current requirement can be calculated. I can recommend NACE books or design document for further information.
Ciao

 

[thien2002]

Thank you so much Mr. Braune and Mr. Saldanha.

 

[quadswift]

I would have thought that you as the purchaser would have developed (or had developed on your behalf) some strategy for your requirements. It is usual to consider design aspects of corrosion and material selection and provide some specifications and data sheets to issue to your potential bidders.Or you ask them to provide bids broken down showing the difference between the options.
I assume from what you say it is design, procure, pre-fabricate, construct and commission the tanks.

If you just tell someone you want a car, then you get a wide range of responses form a 'Smart' car to a Rolls-Royce and they are all still cars.

It appears that you are leaving it to the bidders to tell you what kind of system they think you need and you are now trying to decide from the responses which system you need / suits your requirements . The difference in the amount of work in the installation, operating and monitoring costs between the various sytems is significant.

YOU would normally be making judgements / specifying how long you expect the tank to last and the level of additional operational costs etc you are prepared to add to your maintenance budget.
Your decisions would be made on all kinds of influencing factors including product treatment & specification / coating requirements / external environmental conditions / material selection etc.
Leaving it to the bidders to decide is a bit like putting the fox in charge of the hen-house.
You need the involvement of an expert.
Regards,
Quadswift