cathodic protection 1

[zaza123]

hi!
To protect pipelines in our system cathodic protection is done, i have observed that there is no earthing of these pipelines, i have read somewhere in this forum that grounding will hamper the cathodic protection of the pipeline.Is it true.
Also the CP system does not need external power supply.is that possible.
I am new to cathodic protection.

 

[waross]

Cathodic protection may be passive or active.
A passive uses sacrificial electrodes attached to the material being protected. The electrodes form a rudimentary battery. The potential developed tends to offset galvanic corrosion. The electrodes must be replaced regularly. In severe installations the sacrificial electrodes must be replaced several ties a year or more often.
An active cathodic protection system uses electronics to develop a protection voltage between the material being protected and a buried electrode. An active cathodic protection system has the facility to provide the optimum voltage for complete protection and minimum electrode erosion. An active cathodic protection panel may also have alarms to indicate loss of protection from a number of causes.

Bill
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"Why not the best?"
Jimmy Carter

 

[zaza123]

Hi Waross,
So ours is a Passive Cathodic Protection,the Anodes are also placed near the Pipelines.What are the values which suggest that the Anode should be replaced.

 

[waross]

I don't know. The passive systems that I am familiar with are used for ships, and jetties. The "Zincs" are cast onto a steel strap which is welded to the structure to be protected. The zincs are inspected visually and more are added when the existing zincs are partially consumed.
We have some pipeline people here who will be better able to answer your question.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Kiribanda]

zaza123,

The criterion for adequate protection of a cathodically protected underground pipeline is that when you measure the potential of the line with respect to a copper-copper sulphate half cell it should not be less than -0.85 Volts dc.

So if the reading is lower than the above (which will be given by the designer who had designed the CP system)

1)then you have to replace the active magnesium anodes where the potentials are low.

2)increase the voltage output of the transformer rectifiers to reach the adequate protective level 0.85 Volts.

Hope this helps

 

[ScottyUK]

For future corrosion-related discussion forum338 is probably a better place. But don't re-start this one in another forum, there are enough answers here.


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If we learn from our mistakes I'm getting a great education!

 

[zaza123]

Thanks Waross & Kiribanda,
What we are using here is magnesium Anodes. There is no external supply i.e you can not increase or decrease the protection voltage level. Due to this reason i am referring it as a passive Cathodic protection system.

 

[ScottyUK]

zaza123,

If you can measure the current flowing from the mag anode and know the starting mass then you can predict the length of time for the anode to be consumed. I'd suggest replacement after about 75% of this time. Sorry I don't have the formula to hand - it's at work and I'm at home (it's half past six in the morning).


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[Kiribanda]

zaza123,

As explined the criterion is the measured protection level along the length of the pipe line. Replace the anodes where the protection levels are lower than -0.85 V dc with respect to a copper-copper sulphate half cell. Also you can install new anodes at positions where the levels are low.

While doing that you should also ensure that the protection level will not go beyond -1.1 V dc to protect the coating of the pipe line.

Hope this helps

 

[fdac]

There is a fair bit of misinformation around these forums on CP. To answer some of the OP's questions:

1) Directly bonding copper earthing to the pipe will stop the CP working, yes. If external earth connections are required then DC decoupling devices should be used.

2) The power is provided by the natural voltage difference between magnesium and steel. Current is provided by the dissolution of the anode metal.

3) Don't muck around trying to predict anode massess and lifetimes for replacement intervals. Follow the many existing CP standards, undertake normal routine monitoring, this will tell you if and when any replacement anodes might be needed. Kiribanda is on the right track.

I would suggest engaging a suitably experienced CP consultant or contractor to guide you. AS 2832.1, NACE SP 0169, etc. etc. are there for reference.