You hire a consultant trained in designing these systems.
It requires knowledge of the soils (and how conditions are likely to change) as well as the characteristics of different anodes and installation methods.
You can go to class on this subject, and in a few years you will know how to do it.

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P.E. Metallurgy, consulting work welcomed

Hi zjam,

In order to calculate the number of rectifiers and groundbeds required for an underground pipe installations there is much information required before the calculations can be done. This information would include where is the pipe going to be installed (I.E. desert, temperate, coastal etc.) Carrying out a pipeline survey along the pipe route, to include soil resistivity, ground acidity, and bacterial presence as a minimum. What size of pipe to be used. What length of pipe. What type of coating. Are there any existing pipes or services along the pipe route? Will the pipe cross any existing services. Are there any high voltage cable routes adjacent (within 1000m)or crossing the pipe?. Are there any other Direct Currents sources that may interact or interfere with the protection of your pipe?

There are many considerations to be considered when using Cathodic Protection for pipeline corrosion protection. Care must be taken to ensure not only getting the right number of rectifier and groundbeds, but to ensure other issues are mitigated such as AC and DC interference. I agree with Ed, retain the services of a competant Cathodic Protection Engineer to guide you through the process.

Hope this is of some help.

link:
http://www.electrochemsci.org/papers/vol16/210547....

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

Agree with all the above.

Depending on the soil, the design, the resistivity, the coating, other pipelines etc etc you could have a T/R and ground bed every 10km up to every 100km or anywhere in between.

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

NACE Peabody's spreadsheet is one to start with.

* Finding a solution is great * Knowing how to implement it is fantastic * Believing it is the only one and best is naive ?

Thanks to all for your answer
I am aware of some above recommendations and how to make the designs but still have some confusion about how often a CP transformer rectifier have to be installed along the pipeline I’m looking for the Mathematical formulation.
thanks
jm

Your problem is that the formula requires inputs that you don't have.

CP is one of pipelines "dark arts" and it is not wise to try and sort cut this.

Employ an experienced CP engineer and do some surveys.

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