As mentioned before, tram- and railways are one of the most important sources of earth corrosion currents. Trains positioned on a rail are fed by a power station through an overhead wire and return their current back to the power station via the rails. When these re-turn rails are poorly insulated from earth, considerable amounts of DC currents leak into the soil and can be picked up by metal structures such as protected pipelines. This type of interference is referred to as DC traction interference.
Consider the practical situation of Figure 13. A large gas transport pipeline with a total length of about 60 km is protected by two vertically placed anode beds indicated with CS1 and CS2. These anode beds, composed with 15 anodes, are placed perpendicular to the pipe at a distance of 80 m. On the left of that pipe, at a certain varying distance, lies a railroad that crosses the pipeline. The power stations, indicated with PS1 to PS4, are at a distance of about 15 km. As in fact this pipeline is composed of two parallel lines and since this example aims only to demonstrate stray current effects, the two pipes are modelled by one pipe having an equivalent outer and inner diameter of 1.9658, 1.9486 m and an axial resistivity of 1.2 10-7 W m.
The soil resistivity is taken to be 100 W m. For the coating an overvoltage-current density relation h = 3800.J - 0.85 V (where h is the overvoltage and J is the current density in A/m2 ) has been applied. The electrochemical characteristics of the groundbed are represented by a similar relation: h = 5.J + 0.92 V. Groundbeds CS1 and CS2 deliver respectively 1.43 A and 1.75 A at 6.15 V and 6.5 V.
In Figure 14, the overpotential (or soil to pipe potential) is plotted along the developed pipe length for the case when both anode beds are operational. The whole pipeline is protected using only two anode beds. A plot of the axial current along the pipeline is presented in Figure 15. This current is defined as being positive if it does flow in the direction from lower left [point 1] to upper right [point 13] on Figure 13. Negative values in Figure 15 there-fore represent axial currents in the opposite direction. Remark the two "current jumps" at the position of the groundbeds where respectively 1.43 and 1.75 A are "extracted" from the pipe.
In what follows trains are placed at two different positions (TR1,TR2) as indicated in Figure 13. The resistivity of all feeders and rails is respectively 1.75 10-8 W.m and 1.6 10-7 W.m and the transition resistance between track and soil is 10.000 W.m. The power stations deliver a voltage of 1.5 kV and the load resistance representing the traction current is 1 W.
Consider first train 1 at position 1. The train takes 1216 A from the current feeder. This current is delivered by the power stations PS1 (201 A), PS2 (337 A) and PS4 (678 A). The current returns to the power stations mainly via the rails. However, looking at the overpotential along the pipeline of Figure 16 and the current density profile of Figure 17, it appears that an important part of the current enters the pipe at the crossing with the rail. This effect induces locally a cathodic protection of the pipe that is even more important than the anode influences. This stray current leaves the pipe along important parts. Between anode-bed CS1 and the crossing, the current leaves the pipe and flows via the rails back to power stations PS1 and (in less amount) PS4. To the North, the stray current leaves the pipe, enters the rail and flows back to power station PS2. The rail between PS2 and PS3 plays an important role and it can be observed that near km 56, where one has the shortest distance between rail and pipe, the net current density becomes positive (h > -0.85) such that corrosion occurs. Fortunately, but not by chance, the position of anode bed CS2 considerably reduces the effect.
Next consider train 2 at position TR2. The train takes 1162 A from the current feeder. This current is delivered by power stations PS2 (531 A) and PS3 (631 A) and returns mainly to the power stations via the rails. The corresponding overpotential along the pipeline is given in Figure 18. An important part of the traction current leaves the pipe at the crossing with the rail and returns via the rail back to the power station PS2. As a result the part of the pipe near to the crossing is made anodic and severe local corrosion occurs. The stray current enters the pipeline in the neighbourhood of anode bed CS2 where the distance between rail and pipe is small. The effect of the stray current is added to the effect of the anode bed CS2. Overprotection might take place. The region near to anode bed CS1 is scarcely influenced during this situation.
To eliminate the influence of traction stray currents, a current drainage is highly advised and in practice one is also present. Results of CatPro simulations with a unidirectional drainage (i.e. a diode in series with a resistance R) at the crossing are given in Figure 19. With R respectively equal to 10.0, 5.0 and 1.0 W, a drain current of 5.2, 10.1 and 38.7 A is found. From Figure 19 (R = 1.0 W) it can clearly be seen that the low resistance drainage is working very well, ensuring a protection of the pipeline along the whole length.
For the latter situation, the potential distribution at surface level has also been calculated and plotted in Figure 20. The highest potential level is found at the position of the train where the DC traction currents leaks into the soil. Due to the low resistivity of the ground, the maximum soil potential difference in the region of the pipeline-railway system is only 2.4 Volt.
These calculations of practical situations show clearly how stray current can have important influences at a long distance from their place of creation. In fact this example was chosen for a better understanding (a posteriori) of the problems and difficulties that take place in practice, in order to realise an adequate protection. Fortunately, the calculated examples are only pictures at a given time. Trains are travelling continuously and due to their combined effects, quite important potential fluctuations are measured at a given position. For that reason - although time simulations are possible by combining the results of several calculations, each with a different train position - the model is to be applied as a worst case design tool.
On the object of stray current corrosion see also: DC traction, Cathodic protection, Coating, Contour plots, Definition, Detection, Examples, External currents, Historical perspective, Impressed current, Interference, Mechanisms, Modeling. Pipeline, Potential distribution, Prevention, Stray fields and leakage, Transit systems
See also: Anode interference, Cathode Interference, Combined Interference, DC Traction Interference, FPSO ICCP , Induced Interference
Study and Evaluation of Stray Current Influences on Cathodic Protection Systems of Buried Pipelines, L. Bortels, ELSYCA - Kranenberg 6 - 1731 - BELGIUM, ELSYCA