Steel bolted tank above concrete raft on piles - Is cathodic protection mandatory? Where exactly? 3

[T2ioTD]

I'm filling some technical papers for a bid, and my boss, who is a mechanical engineer, insists no cathodic protection is required for a steel water tank above concrete pad (I designed the pad myself, and the water is for firefighting purposes). Another mechanical engineer said only the tank bottom needs to be protected in this case, but upon reading the client technical documents, which otherwise dodge this issue completely and concentrate more on pipes, I got the impression that it is the inside of the tank in full (not just bottom) that needs to be protected, since it is contact with water.
We already have added 2 mm to the tank for corrosion allowance.

The client is European, and besides the EN 12954 that the clients refers to (pipes), i don't know what EN document is applicable for my case.
I consulted also API 651, which deals with petroleum storage tanks, and is vague when the foundation is concrete.

Any comment may be helpful.

 

[TugboatEng]

Inside or outside of the tank? Cathodic protection can only protect submerged surfaces. That means it doesn't work on the outside of the tank unless it is sitting in a pool of water. Designing drainage for the pad would be a more effective option.

Inside the tank, cathodic protection does work but only from the waterline down. Small fluctuations in level can help spread out the wear that occurs at the waterline.

 

[T2ioTD]

The pad is 50 cm, 30 cm buried in sand (water table at like 2 m below ground) and 20 cm above. Climate is dry but humidity is high (near sea). Based on TugboatEng reply, I think I should mention only internal protection, just for the sake of filling the papers.

 

[Dhurjati Sen]

Simply mentioning internal coating of the tank would do?
Or do you need to specify the painting system and the quality assurance and control part of it also?
A poor internal coating is far worse than a bare tank.

DHURJATI SEN
Kolkata, India

 

[SJones]

NFPA 22 would call for internal coating if, by chance, that was a referenced standard. EN ISO 28765 would be applicable to vitreous enamel coated bolted tanks for water. EN 12499 would be applicable to internal cathodic protection. Externally, a watertight seal needs to be ensured at the tank bottom - foundation interface to keep water out and prevent condensation.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.

 

[MJCronin]

It has been my experience that cathodic protection is NOT required for most steel water tanks ....

Cathodic protection, is very maintenance expensive and IMHO should only be considered as a last resort. Electric power consumption (24/7/365) is a substantial issue .... Maintenance of an ever-failing CP system will eventually cause it to be replaced !!!

A much more reasonable and common solution, IMHO, is to PAY FOR A PREMIUM COATING SYSTEM upfront (your MBA supervisor will scream his objections) .... HOLIDAY TEST THE COATINGS !!!, KEEP GOOD RECORDS !!! ...

Then periodically examine the tank.... in accordance with the laws/regulations of the installation

MJCronin
Sr. Process Engineer

 

[FacEngrPE]

Slightly different opinion than M J
An expensive high quality paint system on a tank is actually cheep insurance.
Passive cathodic protection of a painted tank will extend the coating system life for the submerged coating, and do nothing for the portion above the water line.

Active cathodic protection systems are highly dependent on IMHO personnel that understand the system. Will corporate memory ensure this knowledge and commitment be available for the service life of the asset being protected? If not active cathodic protection system is a waste of effort.

If this is a potable water tank, your coating and if used, anodes must be approved for potable water use. NSF61 coatings and anodes are available.

 

[LittleInch]

Underside of the tank base should be coated using some sort of bitumous coating of its above the water table and above ground level. CP would be ineffective.

Internal surface should preferably be coated/ painted, with CP (sacrificial anode usually) optional.

For fire water use the corrosion is usually concentrated at the water air interface where waves and small changes in level plus well oxygenated waste can eat bare steel more than 2mm and CP is ineffective in the splash zone.

If your fire water is in any way brackish then your in a different (bad) place.

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

 

[dik]

Careful, some bituminous coatings are carcinogenic.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[TugboatEng]

There is a new type of self-healing coating consisting of gelled calcium sulfonate. This is possibly a replacement for bituminous coatings.

 

[EdStainless]

One advantage of an internal cathodic protection system using sacrificial anodes is that when you test (lift the leads and take electrical measurements) you are getting a reading of the coating condition.
If the current draw goes up it almost always means that you have a breach in the coating.
And as long as the coating is good your anodes will last a very long time.
The external bottoms of tanks are often coated, and if bedded in sand they also have CP installed.
The outside of the bottom can never be inspected so it must be protected to the maximum extent possible.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[bimr]

Very rarely have I seen a water tank with cathodic protection. Water tanks typically have coatings that will last 15-20 years.

You can also delete the tank bottom with a ring wall foundation.

During installation of above-ground storage tanks, there are some advantages to concrete foundations for tanks when it comes to corrosion—the high pH of the concrete acts to passivate the steel, unless you have an above ground storage tank (AST) liner pad or something that is between the concrete and the tank bottom. If you can effectively seal the chime from the ingress of water and oxygen, the corrosion rates are generally quite small. Unfortunately concrete foundations for larger diameter tanks are not typically practical and can be quite expensive to properly install. Concrete foundations with appropriate AST liners are best for smaller diameter tanks.

Matcor