Cathodic Protection Study 3

THE 'IR DROP IN THE SOIL'

Cathodic protection specialists have now recognised that there is an error in the 'pipe to soil potential' and have described this as 'the IR drop in the soil', as it is caused by the current (I) passing from the cathodic protection anode to the pipeline metal, through the resistance (R) of the soil. The term 'IR drop' comes from the formula E=IR and the value is commonly called volts.

It is believed that the corrosion status is retained for a short period before the effects of the cathodic protection start to decay. It is now recommended that the voltage measurement is made immediately after the cathodic protection current is switched off, thus removing the error caused by the 'IR drop'.

THE MODEL

A simple model of this feature can be examined on CPSTUDY3.EXL but a certain amount of 'license' has been taken in creating this model. It is assumed that the cathodic protection current is flowing to an infinite remote earth and will therefore follow the inverse square laws of radiation.

The model shows the groundbed (GB) in cell A4 (at the top left hand side of this exert) and applies the inverse square law to the neighbouring cells progressing outward from A4. The results are colour coded and show the amount of current passing through each cell.

In the model, the potential of the pipeline is used as a reference against which to measure the potential of the electrode, plus the errors caused by the IR drop in the soil. In reality the pipeline has very little electrical resistance and the voltage across a span of pipeline would represent much less than 0.01% of the total reading.

In the model the potential of the soil, at the points where the electrode touches, is a result of the current (I) passing through the resistance (R). When the output current from the anode, shown as the value of cell A4, is reduced to zero the error is removed.

USING THE MODEL

The user can adjust the value of the resistance of the soil along the pipeline route to give the readings shown in the sample, but of course, it is not suggested that the resistance of the soil is the sole cause of the variation of the errors along a pipeline route.

The user can insert values into the 'pipe-to-soil potential' column, from their own records and balance them to the resistances using the "goal seek" facility within EXCEL.

This model can be used to help students to understand potentials in the soil and potential gradients which are manifest in the voltages measured in cathodic protection field work.

This model indicates that it much more likely that the so-called 'attenuation curve' can be attributed to the geographical relationship to the groundbed than to the amount of protection afforded to the pipeline.

This concept can be demonstrated, at cathodic protection groundbeds, by plotting the ground voltage with respect to a fixed electrode in a remote position, or in relation to the pipeline metal. The resulting voltages can be plotted as equipotential contours and reflect the degree of variation suggested in model CPSTUDY3.EXL.