Cathodic Protection System for Gas Pipeline

Excessive half cell voltages indicate the possibility of excessive hydrogen formation which can lead to damage to protective coatings. Monitor your half cell voltages carefully and adjust the current to any anodes that are too high. The manufacturers should be able to supply a complete maintenance manual and procedure.

Nguyen,

I don't know exactly what system you have, so I'll describe the system I'm used to. The system I worked on protected a wharf structure and used several transformers at intervals. Internally they had a 3 phase variac (autotransformer) with motorised adjustment and a 3 phase isolation / stepdown transformer and 3 phase full wave bridge rectifier. The anodes were platinum / iridium rods around 20mm diameter and any damage to the XLPE cable or encapsulant will cause anode failure.

The bulk current was adjusted by stepping the variac up or down. If one anode was drawing too much current or the half cell voltage in that region was too high a resistance was inserted in that leg (nichrome resistance wire or at a pinch a stainless welding rod). Lengths and thicknesses were found by trial and error.

An area which is high can also be caused by failure of adjacent anodes. You need to measure all anode currents and half cell voltages (once a month). Also make sure there is no AC present in the anode current (shot through diodes). The system should have been balanced as new and any abberation most likely is caused by a fault, so find and fix any faults before making any adjustments. A Hall effect tong tester is essential (it will measure DC and AC and help you know the difference), as is the correct half cell for voltage measurement. The half cell needs to be looked after and maintained.

Get as much information as possible before making any change, a corrosion engineer would be the man to talk to.

Mr Emjay,

We use galvanic anode system, anodes are Zince material. it it not impress current system.

Lenght total of 18 inch onshore pipeline is 30 kilometers. Monolothic joint was intalled to isolate currents from equipments.

Can we insert resistance into curcuit to reduce current?

Regard

Nuygen,

This is an intirely different puppy. I would make sure all the anodes are working as designed, all producing current and it is the correct polarity (damaged anodes may have exposed copper undoing all the good work. Use of actual half cells (Zinc / soil electrolyte) is supposed to be self regulating. High voltages could be result of a couple of things. 1. Some anodes not working so the remainder doing all the work. 2. Materials other than Zinc. 3. Particularly aggressive soils. 4. Incorrect measurement techniques.

Due to its inherantly self regulating nature I wouldn't try fiddling with resistance apart from servicing all the connections to eliminate any unplanned resistance. Impressed current is different in this regard.