cathodic protection design procedure

[vafaei]

I want to know how should I choose one of the methods for CP (Sacrificial Anode or Ipressed Current)?
Also what is the procedure of design for each one?

 

[unclesyd]

You are asking a lot for one sentence.
Please let the forum know a little more information about your intent. It is most important that the CP system be addressed in the design stage of the underground as cost get exponential in a hurry.

The first order of business would be to get some of the NACE publications or books on Cathodic Protection (CP) to if nothing else become familiar with the nomenclature used.
You should become a member of NACE if you are going to handling this aspect of your site.

Questions like this are the starters and go on from there.
Is this a start from scratch or an add-on system?
What is the magnitude of the project?
What are you trying to protect?
It is information like this you need to post to be guided in the right direction.

You can do something as simple as asking a neighbor, if friendly, how they are they are accomplishing the CP on their site. If there is any hint that their system is complex it’s time to hire the consultant, fast. If you hire a consultant and as stated before know a little about what’s going on the system soon becomes a maintenance item that you can handle.

Here is a source for the NACE book and several others.

http://www.asminternational.org/Tem...=Ecommerce/ProductDisplay.cfm&ProductID=11849

This is the NACE website

http://www.nace.org/nace/index.asp

 

[CathodicGuy]

There are two basic methods of applying cathodic protection, although there are many variations on installing cathodic protection. These two methods are known as sacrificial (galvanic) anodes and impressed current (rectifier) cathodic protection.

Sacrificial anodes are usually made of an alloy of magnesium, zinc, or aluminum. Different alloys are available for various electrolytes such as seawater, soil, or concrete. Each type of sacrificial anode will provide a source of cathodic protection current due to the anode’s higher electrical potential energy than the structure intended for protection. The Practical Galvanic Series provides guidance on the selection of anodes that can be used to provide cathodic protection for various metals. The selection of an alloy for a sacrificial anode must be made with consideration for the electrolyte in which the anode and the structure are placed. Additional factors are the size and shape of the anode, the structure’s coating, and the intended design life of the system.

An impressed current cathodic protection system uses inert anodes that are powered by an external source of DC current. Anodes can be materials such as graphite, cast iron, mixed-metal oxide-coated titanium, and platinum-niobium-clad metals. Many sizes and shapes of impressed current anodes are available including wire, rods, tubes, sticks, plates, and disks. The composition of the electrolyte is very important when selecting an impressed current anode. The DC current source is usually a rectifier, although other sources of DC current are available, such as solar voltaic energy systems. In this type of system, the anodes are installed within the structure’s electrolyte and are connected to the positive output terminal of the rectifier. The structure is connected to the rectifier’s negative output terminal. Current flow is from the anodes through the electrolyte and onto the structure. The metallic connection from the structure to the rectifier completes the DC circuit. Impressed current systems can have many anode configurations that are dependent upon the electrolyte and the structure to be protected.

Hope this helps!

 

[Metalguy]

"Current flow is from the anodes through the electrolyte and onto the structure."

Since "current flow" is actually electron flow (not ion flow), the current flows from the rectifier to the structure, and then to the anodes. Another way of looking at the problem of structure corrosion is that either the sac. anodes or IC anodes supplies the electrons to the structure that end up in the elecrolyte, instead of having the structure lose both electrons AND metal ions.

 

[CathodicGuy]

Unfortunately, Ben Franklin defined current flow back in the 1700's when he was referring to positive (ion) current flow (opposite to electron flow). CP folks still use this terminology and it often confuses new engineers who are used to "electron flow."

Now to answer the original post:

Cathodic protection can mitigate corrosion provided that a sufficient amount of DC current is available to polarize the open circuit potential of the active (corroding) areas along the pipeline. However, the selection of an anode system and its configuration for each site is dependent upon knowledge of several factors:

Soil Resistivity – Required to select anode material.
Location of areas to install anodes – Required to optimize current distribution.
Coating effectiveness – Required to estimate current required to meet protective criteria. (Rectifier vs. sacrificial)
Piping system drawings – Required for anode placement/electrical isolation details.

 

[18071972]

THERE ARE A LONG BUT NOT COMPLICATED PROCEDURE TO MAKE THE DESIGN OF CP.