Cathodic Protection & Earthing system 1

[zaza123]

Happy New Year !

I have some Questions regarding Cathodic Protection & Earthing system, i am referring to Pipeline structures

1. Is both overground & underground pipelines are required to be protected by CP system or only pipelines below ground need CP protection.
2. Is earthing required for all the pipelines ,is it true that earthing nullifies the effect of CP system
3. Does earthing of pipeline is proper protection for Lightining,i.e if the pipeline is not earthed & lightining occurs on the pipeline it will destroy the Pipeline.
4. I had checked the Earthing resistance of the Pipeline by Digital Earth Leakage Test Meter ,value is 19.5 ohm, ,Is the earthing readings OK

The pipeline i am talking is Oxygen Pipeline.

 

[Kiribanda]

zaza123,

1)Nnly the underground portion is supposed to be protected by a CP system because only the underground portion is subjected to corrosion which is also not visible from outside.Therefore,when the line comes out of ground insulating flanges are installed to stop draining the CP current to the overground portion.Normally overground portion can be protected by painting etc.

2)It is always better to ground any metal surface.Pipe lines carrying petroleum products are grounded to avoid static charge buildup.But grounding of the line will not nullify the CP system installed.

3)If the subject pipe line is struck with a direct lightning stroke then most probably it might get destroyed eventhough it is properly grounded.If it is hit by an indirect stroke then the difference in current will be easlily conducted by the line and there cannot be a damage.But in my opinion the line profile and installation details are required to provide a better answer.

4)What do you understand by the grounding resistance of a long underground pipe line?I cannot understand how you obtained a very high value of resistance when any pipe line is always bonded to ground.Also I donot think the usual testing techniques (Fall of Potential method,Slope method etc)used for measuring ground electrode resistance are valid for underground pipelines.

 

[zaza123]

Thanks Kiribanda,

1. Will the Underground Portion which is protected by CP system will also be protected by grounding.
4.We did not measure at the Electrode, we measured with Earth resistance meter connecting the Earth point to Pipeline.There are three leads in Earth resistance meter. Current Spike, Voltage Spike & Earth.We connected the Earth Point of Meter to Pipeline structure after removing small pained coating.The value showed 19 Ohms.The result suggested that the Pipeline is grounded but not properly as it has high resistance.

 

[Kiribanda]

zaza123,

1)I donot understand what do you mean by "protected by grounding"?.The underground line is definitely mechanically bonded to various other grounded structures like tanks, vesels etc.

4)The method that you used for earth resistance measurement is the Fall of Potential method.But as far as I know this Fall of Potential method cannot be applied for such long underground lines.

4.1)Does the user guide of your tester specify how to measure the resistance of a long underground pipe using Fall of Potential Method?

4.2)If yes,can you please upload that page with the make & model of your tester?

4.3)Why are you trying to measure the ground resistance of this already grounded metallic pipeline?

 

[zaza123]

Dear Kiribanda,
1) The Underground Pipeline is not mechanically bonded to any structure.Only a small portion of the pipeline is underground rest is overground.
4.1) The user guide specifies the method of Testing, but it does not specify whether the pipeline grounding can be measured by it.It is basically used for measuring the resistance of Earth electrodes.
4.3) We had an incident in which there was explosion in the Piepline due to Lightining, i am measuring the Earth resistance of the pipeline to check whether the pipeline is groinded or not.

 

[Kiribanda]

zaza123,

If the line is not mechanically connected,then what are connected to its either ends?

Pipe line ground resistance cannot measure using Fall of Potential method.It is true only for a single electrode or for a small grounding system consisting several electrodes.

From your description it seems that the line had been struck by a direct stroke.Therefore, I donot understand why you are worried about ensuring a low grounding resistanec for the line.

Could you please upload (pdf)a simple hand drawn sketch showing the line,its elevation,end connections and the point where the lightning struck?