Most of the underground pipeline nowadays installed with Cathodic Protection System. Either Impressed Current System or Sacrificial Anode System together with the existing coating system.
A basic Impressed Current Cathodic Protection for pipeline will require the following materials.
1. Anodes (made of inert metals)
3. Test Post
4. Transformer Rectifier
Diagram of a basic Impressed Current Cathodic Protection for pipeline shall be as the folowing:-
Design of an Impressed Current Cathodic Protection for pipeline is determined base on the following parameters:-
1. Length of the Pipeline
2. Pipeline Coating Type
3. Soil Resistivity
4. Groundbed location
5. Consideration of nearby existing ICCP system
6. Metal crossing
7. High Voltage AC transmission line
8. River crossing
The output size of the ideal transformer rectifier to be used is base on the requirement of cathodic protection current and groundbed soil resistivity range . In order to determine groundbed resistivity, site survey has to be carried out . Soil coring and 4 pin method are usually used, however soil coring method does provide more precise result. Deep well groundbed normally used soil coring method, on the other hand 4 pin method used for shallow type groundbed.
If the groundbed have high soil resistivity range, usually if more than 5 ohm, other suitable location of groundbed have to be determined. Low resistivity location usually near the wet area. Low soil resistivity range area is required to minimized the voltage output from transformer rectifier since
V = IR (as per Ohm's law)
The installation of test post depends on the clients requirements, normally test post have to be installed at every 2km interval of pipeline (depends on the total length of pipeline), pipeline crossing, river crossing, road crossing, drain point, and end of pipeline.
Usually , transformer rectifier will be installed near to the AC power source and near to the groundbed it self . This is normally done to safe the cable usage which can minimize the cost. A junction box sometime installed at the drain point to control current supply to the anode , this junction box can also be used as the pipeline potential measurement point which also considered as a test post.
A very simple technique to know whether the whole stretch of pipeline protected (more than -950mV wrt Cu/CuSO4 reference electrode) is by measuring the potential at the end point of pipeline. Adjustment shall be done at Transformer Rectifier output if results indicates under-protection or over-protection reading.