Cathodic Protection of Underground Vessels

[Electriman]

Dear Friends,

I have read contradictory documents about CP vessels and I would like to get your idea about that. Please help with supporting your idea with code and regulatory statements.

1- Do underground CP vessels need to be electrically isolated from attached above ground vessels and piping?
2- Do underground CP vessels need to be grounded to the same ground that the rest of the vessels are? In other words do I need to bond the ground grid of the CP vessels and un CP vessels and structures?

 

[tom baker]

What is a CP?
And perhaps an example

 

[ron]

What is a CP?
And perhaps an example

From Wikipedia
Cathodic Protection (CP) is a technique used to control the corrosion of a metal surface by making it the cathode of an electrochemical cell.^[1] A simple method of protection connects the metal to be protected to a more easily corroded "sacrificial metal" to act as the anode. The sacrificial metal then corrodes instead of the protected metal. For structures such as long pipelines, where passive galvanic cathodic protection is not adequate, an external DC electrical power source is used to provide sufficient current.
Cathodic protection systems protect a wide range of metallic structures in various environments. Common applications are: steel water or fuel pipelines and steel storage tanks such as home water heaters; steel pier piles; ship and boat hulls; offshore oil platforms and onshore oil well casings; and metal reinforcement bars in concrete buildings and structures. Another common application is in galvanized steel, in which a sacrificial coating of zinc on steel parts protects them from rust.

 

[Electriman]

From Wikipedia
Cathodic Protection (CP) is a technique used to control the corrosion of a metal surface by making it the cathode of an electrochemical cell.^[1] A simple method of protection connects the metal to be protected to a more easily corroded "sacrificial metal" to act as the anode. The sacrificial metal then corrodes instead of the protected metal. For structures such as long pipelines, where passive galvanic cathodic protection is not adequate, an external DC electrical power source is used to provide sufficient current.
Cathodic protection systems protect a wide range of metallic structures in various environments. Common applications are: steel water or fuel pipelines and steel storage tanks such as home water heaters; steel pier piles; ship and boat hulls; offshore oil platforms and onshore oil well casings; and metal reinforcement bars in concrete buildings and structures. Another common application is in galvanized steel, in which a sacrificial coating of zinc on steel parts protects them from rust.

Yes. That is what it is. CP stands for cathodic protection. Any idea on my concerns?

 

[luckylerado]

When Galvanic Cathodic Protection is required to be installed on metal water line, in my experience, an isolation kit must be installed on the first fitting after the pipe emerges from below grade. This gets tricky when you are cathodically protecting a water main that you must also use as a grounding electrode or as part of a grounding electrode system. I understand that there are products that essentially work similar to the way a diode works that can be installed to control current flow from the CP components back in to the grounding system but I have never installed any.

On impressed current type systems however, isolation from earth ground not possible and thus requires a rectifier and when you say vessel, I assume this is the type of system that you are talking about.

Bottom line is you should lean on someone that is NACE certified to design the system.

Frankly it is my opinion that depending on soil resistivity and PH and assuming that the coatings of the structure being protected remain intact during backfill, CP is often a waste of money. At least in the commercial construction environment that I am used to. I suppose it is a good additional layer of protection for hazardous wastes storage or similar industry where leaks due to corrosion can be catastrophic.