The size of the damaged area of the coating and its position on the pipeline impacted the cathodic protection potential, and there was a damaged area of the greatest impact value. When damaged area was 300 mm2, the IR drop was the largest, and this situation could easily lead to inadequate protection; when the parallel spacing between pipeline and interference source was unchanged, the measured value curves of cathodic protection potential presented “U” shaped trend with the increasing stray current interference intensity. Under certain parallel spacing between pipeline and interference source, high alternating stray current intensity would cause serious negative offsets, so that the overprotection of the pipeline occurred, and make the coating crack; there was a parallel threshold length. When less than the threshold, the pipe-ground potential increases rapidly with the parallel length increasing. In order to judge whether a pipeline was interference by AC stray current and the risk of stray current corrosion, we should make a comprehensive analysis of the cathodic protection energizing potential, the switch-off potential, AC pipe-soil potential, IR drops, and so on.
With the continuous development of energy, electric power, and railway transportation, oil or gas pipelines may run parallel with overhead AC high voltage power lines and AC-powered electric railway or sometimes even cross them in developed areas or geographically special corridor regions [
By controlling variables to reduce interference effect on experiment result and improve its reliability, we build an uncovered box, whose external material is wooden and the internal material is PVC. Loess was used for its good moisture retention. During and after the experiment, soil surface was covered with nylon type to prevent soil electrical resistance changing. Before each experiment, trace tested solid electrical resistance. It would not begin until soil resistance values were the same.
AS shown in Figure
The schematic diagram of experimental facility.
Buried pipeline was protected by the double-show potentiometer, DJS-292, which provides constant potential cathodic protection. Wasted steel was used as auxiliary anode of the cathode protection system.
The current source whose number is JJ10DD23KT was used as stray current interference AC power. Copper strikes were used as discharge electrodes and reflow electrode, which has low resistance and good contact. To better simulate the stray current leak in the way of point power on spot, electrodes were bound with waterproof insulation tape, leaving its ends to 20~30 mm to remain in contact with soil.
Under the circumstance of different damage areas at the same point on the pipeline coating, we researched the interference influence that stray current had on the cathodic protection system. At damage point 1, cut of the specimen was, respectively, set as 50, 100, 200, 300, 200, and 625 mm2. The potentiometer output protection potential was controlled constantly to be −1.000 V, interference intensity of AC stray current was 0.1 A, and tube-rail parallel spacing was 0.6 m.
By analyzing the cathodic protection potential, when the pipeline reached 0.1 A AC stray current interference, the real protection voltage [
The potential value statistics of different damage areas of the coating were shown in Table
The pipe-soil potential under different coating defect areas.
The defect area |
The maximum value of the moment interference |
The measuring protection potential without interference |
The real protection potential without interference |
The measuring protection potential with interference |
The real protection potential with interference |
---|---|---|---|---|---|
50 | −1.523 | −1.000 | −0.948 | −1.034 | −0.857 |
100 | −1.282 | −1.000 | −0.955 | −1.031 | −0.842 |
200 | −1.354 | −0.998 | −0.951 | −1.030 | −0.847 |
300 | −1.302 | −0.997 | −0.956 | −1.034 | −0.795 |
400 | −1.455 | −0.992 | −0.946 | −1.020 | −0.812 |
625 | −1.298 | −0.988 | −0.952 | −1.020 | −0.834 |
It could be seen that different pipeline anticorrosive coating damage areas have little impact on cathodic protection potential of the pipeline; with the increase of damaged area, cathodic protection potential decreased slightly but overall did not change that much. The stable measured value was about −0.99 V and the stable real value was around −0.95 V. IR drop was basically less than 0.05 V. Therefore, within the scope of certain length, different damaged area of the anticorrosive coating would have a little impact on the pipeline cathodic protection potential. However, it could be predicted that the decrease trend of cathodic protection potential would be significant with the increase of the pipe length and damaged area. Finally, it may reduce pipeline cathodic protection potential to the point of losing protection.
There existed stray current interference; with the increase of the damaged area on the pipeline coating, pipeline cathodic protection potential measurements had a tendency of decrease, and the change of protection potential real value was more complex. The measured protection potential value with AC interference was greater than that without interference. However, the real protection potential value with inference was far less than the value without interference.
We drew the protective potential of measured value and true value difference along with the change of coating damage area under the same coordinate, as is shown in Figure
The potential difference curves of the measured value and true value along with defect areas.
By Figure
It could also be observed that the moment stray current interference happened, there would be a strong potential signal, and the size of the potential maximum
We set the parallel length to 3.0 m without change, parallel spacing from 0.2 m to 1.0 m, AC stray current interference intensity to 0.1 A, and output voltage of cathodic protection system potentiometer to −1.0 V. We researched on influence on the cathodic protection system caused by AC stray current interference in the conditions of different length and spacing between pipeline and the parallel tracks.
During experiment, it was observed that potential at different rail spacing was changed periodically during the interference existing, and the period was about 40 s. The time AC inference came out, pipe-to-soil potential abruptly changed, which remained about 3 s. The potential value statics before and after interference were shown in Figure
The potential curves under different parallel distance.
From Figure
In order to further analyze the change relationship of the measured and real value in the condition of the stray current interference, we, respectively, drew the two curves, which is shown in Figure
The measured and true potential curves under different parallel distance.
In Figure
IR drop equals the real values from measured values with inference subtraction. Drawing of the curve which reflected the relationship of IR drop and tube-rail distance is shown in Figure
The IR drops curves along with parallel distance.
It could be observed that IR drop with no interference was small, basically 0.05 V. When confronted with inference, the IR drop curve was the quadratic polynomial, and fitting degree was quite high. With the increase of tube track spacing, the IR drop constantly decreased and the rate of decreasing progressively grew smaller. That is, the greater the spacing, the smaller the IR drop caused by AC stray current, and the real potential value was more close to the measured value. At the same time, the IR drop caused by the stray current interference was greater than the IR drop without inference; thus pipeline protection would be declined from stray current. The further the pipeline and stray current interference sources were, the smaller the inference degree of pipeline was affected, and so was the IR drop.
Set AC stray current interference intensity to 0.1 A, output protection voltage of the cathodic protection potentiometer to −1.0 V, tube spacing
In the experimental process, the protection potential under the stray current interference was changed periodically, and the change period was about 40 s. At the instant of stray current interference, the protective potential of different parallel length has a mutation value, and the duration was about 3 s, but the potential curve of the parallel length, 0.5 m, did not have a clear mutation. When the stray current interference disappeared, the tube ground potential was restored to the natural corrosion potential. The protective potential values of the stray current interference were shown in Table
The relation between the parallel length and the potential.
The parallel length (m) |
|
|
|
|
|
---|---|---|---|---|---|
0.5 | −0.968 | −0.934 | −0.983 | −0.893 | −0.998 |
1.0 | −0.966 | −0.933 | −0.996 | −0.869 | −1.300 |
1.5 | −0.967 | −0.934 | −1.017 | −0.859 | −1.315 |
2.0 | −0.947 | −0.917 | −1.005 | −0.810 | −1.893 |
2.5 | −0.952 | −0.922 | −1.003 | −0.810 | −1.365 |
3.0 | −0.961 | −0.931 | −1.027 | −0.810 | −2.052 |
From Table
Draw the protective potential with the variation of the parallel length as shown in Figure
The potential curves along with the different parallel length.
In Figure
As could be seen from Figure
The IR drops curves before and after the AC interference.
In the experiment, the tube-rail parallel spacing was 0.2 m, parallel length was 3.0 m, and other external conditions were fixed, measuring pipeline cathodic protection potential when the stray current intensity varied from 0.1 A to 0.5 A, getting the real protection potential by means of coupons.
During the experimental process, it was found that protection potential over time into periodic volatility changes by stray current interference, and the amplitude of the fluctuation increased with the increase of the intensity of the interference current, even lower than the natural corrosion potential of pipeline, variation period was 40 s, and under different AC interference intensity curve shapes were also different. Taking the average potential value of a period as the measured potential interferenced by AC stray current, it can be seen that the potential would gradually fall back close to the natural corrosion potential when suddenly disconnecting the cathodic protection, but the potential drop would be under the natural corrosion potential and even be positive potential with the increasing interference current intensity. When the interference current was cut off, the potential of the tube was gradually restored to the neighbourhood of natural corrosion potential. The potential values of different disturbance intensities were shown in Figure
The potential value curves along with the AC stray current interference.
The electric potential measurement values and the true values of the stray current interference were parallel to each other and were maintained close to −0.90 V and −0.95 V, respectively. This was because various conditions of the cathodic protection did not change, and many times of measuring results do not change too much. The presence of stray current interference, as could be seen in the measuring potential value, first increased and then decreased with the increase of intensity of disturbance; measurement values under 0.4 A and 0.5 A AC interference intensity were even lower than the potential measurements before interference. The real potential value with the increase of the interference decreased; in 0.4 A and 0.5 A AC interference intensity of real value was even lower than the natural corrosion potential of pipeline, illustrating that pipeline corrosion rate will strengthen and within a short period of time can cause severe corrosion effect.
The experiment concentrated on the impact caused by stray current interference intensity in the spacing between the tube rails of 0.2 m; for comparative analysis, experiments were conducted to the impact caused by stray current interference intensity in the condition of spacing between the tube rails of 0.4, 0.6, and 0.8 m; contrasted results were shown in Figure
The potential curves along with AC interference intensity under different parallel distance.
From the curve in Figure
Set the output voltage of potentiometer from −0.85 V to −1.50 V; the stray current interference intensity was 0.1 A and the parallel distance and length were 3.0 m and 0.6 m, respectively. Plot curve of potential with the output voltage is shown in Figure
The potential curve along with different output CP voltage.
From Figure
At the same time, through analysing IR drop before and after the interference, it could be found that the IR drop before the interference increases linearly with the increase in the output value of the protection voltage, and the IR drop is the maximum between −1.35 V and −1.15 V.
Under the condition of the anticorrosion layer damaged area was 100 mm2, studying stray current interference effects at the condition of different locations along the tube length direction which appeared damaged. In the experimental study, the effects of 1-point single damage, 1-point and 2-point damage at the same time, 3-point and 2-point damage at the same time, and 4-point and 2-point damage at the same time on the cathodic protection were studied. The results were shown in Table
The potential value under different defects connection.
|
|
|
|
|
---|---|---|---|---|
Only 1 point damage | −1.000 | −0.955 | −1.031 | −0.842 |
Point 1 and point 2 damage | −0.935 | −0.906 | −0.955 | −0.814 |
Point 3 and point 2 damage | −0.935 | −0.904 | −0.944 | −0.828 |
Point 4 and point 2 damage | −0.942 | −0.912 | −0.952 | −0.839 |
From Table
Through curves of potential, with time obtained from various experiments, the protection potential under the stray current interference was changed periodically, and the change period was about 40 s. Meanwhile, when the intensity of interference was low, the protective potential had a mutation value at the instant of stray current interference, and the duration was about 3 s.
With alternate stray current interference, when the damaged area was 300 mm2, real value of protected voltage was minimum. When the damage area of pipeline anticorrosion layer was less than 300 mm2, the effect by pipeline cathodic protection increased with damaged area of anticorrosion layer increasing; real value of protected voltage was small more and more; when the damage area of anticorrosion layer was more than 300 mm2, which was in the range of damaged area verified by experiment, true value protected voltage gradually increased, but the increasing rate was slow.
With the increase of the distance of parallel, IR drop caused by stray current interference decreased. Overall trend seemed to be that the longer the parallel length was, the bigger the influence on cathodic protection was; after parallel length increased to more than 2.0 m, the true value basically maintained at a constant value, the effect on the cathodic protection was essentially the same with the parallel length increasing.
With the increase of the stray current interference, the measurement value curve of protective potential first increased and then decreased, and the “U” shaped trend was presented. Real potential value curves are basically coincident and, under different tube track spacing, decreased gradually with increase of the intensity of interference, but the speed of decreasing was slow; real values under 0.4 A and 0.5 A AC interference intensity were even lower than the natural corrosion potential of pipeline, illustrating that pipeline corrosion rate will strengthen and within a short period of time can cause severe corrosion effect.
With the increase of the output voltage of the cathode protection, the measured value of the protective potential before and after the interference and the real value before the interference both presented a linear change. The true value after interference showed S-shaped increasing trend, in the output voltage of −1.15 V to −1.35 V range; before and after the interference potential real value difference was very big and he real potential value was gradually approaching before and after the interference when the output voltage was less than −1.15 V or greater than −1.35 V.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The research work was supported by the Central College Foundation of CAUC under Grant no. 3122014D027, the Experiment Technology Innovation Foundation of CAUC under Grant no. 01-14-01, and the College Students’ Innovative Entrepreneurial Training Program (201510059069).