CC BY
25UDC 620.197.5:621.316.991
S.S. Gus'kov1, e-mail: [email protected]; R.V. Aginey1, e-mail: [email protected]; E.V. Isupova2, e-mail: [email protected]
1 Giprogazcentr JSC (Nizhny Novgorod, Russia).
2 Federal State Budgetary Education Institution of Higher Education "Ukhta State Technical University" (Ukhta, Russia).
Theoretical Assessment of the Electrical Contact Effect of the Pipeline with the Groundings of Electrical Equipment on the Cathodic Current Distribution in the Underground Pipeline
Sometimes, the electrical contact between underground pipelines and grounding system of electrical equipment occurs on the territory of industrial sites (compressor stations, gas distribution stations, etc.). The shielding effect of the cathodic protection current by groundings is arising. It leads to a decrease in the protective efficiency of underground pipelines against corrosion, accelerated destruction of the anodes, an increase in the current consumption of the cathodic protection, and creates additional obstacles for its equal distribution. It is necessary to investigate the effects of groundings on the current distribution in the underground pipeline as well as on the potential difference between the pipeline and the ground to develop recommendations for minimizing the negative effect of shielding the current of cathodic protection by groundings. This article presents the results of software development that allows calculating the current distribution in the pipeline and the potential difference between the pipeline and the ground considering the fact of galvanic connection groundings to the pipeline. The examples of calculating the current distribution in the underground pipeline and the potential difference between the pipeline and the ground in the presence of a cathodic protection station and electrical contact of the pipeline with a grounding were analyzed. The effect of the mutual influence of the cathodic protection station and grounding on the distribution along the pipeline of the cathodic protection current and the potential difference between the pipeline and the ground was studied. The influence of the characteristics of the cathodic protection station, groundings and ground on the features of the shielding effect of the cathodic protection current by groundings was analysed.
Keywords: industrial pipeline, grounding system, electrochemical corrosion protection, mathematical modeling, software
Results of electrometric surveys of underground pipelines on the territory of industrial sites shows the decline of effectiveness of electrochemical protection associated with shielding the current of cathodic protection by grounding system in the presence of galvanic contact between protected pipelines and metal structures of the industrial site [1-5]. In the standard operating mode of the cathodic protection station, the cathodic current is fully used to protect the pipeline from corrosion. However, in the presence of galvanic connection between the cathode-shielded pipeline and grounded equipment, a huge part of the cathodic current flows on the groundings, that becomes a reason of appearance
of unprotected pipeline zones and, as a result, the irrational consumption of electricity and anodic metal. The obvious need is to investigate the effects of groundings on the current distribution in the underground pipeline subject to electrochemical protection, and on the potential difference between the pipeline and the ground to develop recommendations for reducing the negative effect of shielding the current of cathodic protection by groundings [6]. The results of these studies can be used both in the design of electrochemical protection systems for newly constructed industrial sites, and in exploitation of the current site facilities of gas pipeline transport systems to eliminate the negative effect of groundings.
SOFTWEAR FOR MODELING
The mathematical modeling method of calculating the current distribution in the underground pipeline and the potential difference between the pipeline and the ground in the presence of electrical contact of the pipeline with the grounding system of the electrical equipment is used.
To perform a numerical simulation of the shielding effect of the cathodic protection current of an underground pipeline with groundings having electrical contact with the pipeline, the specific software was developed (Fig. 1). The software makes possible studying the regularities of the effect of groundings having electrical contact with the pipeline on the distribution the cathodic
For citation (ссылка для цитирования):
Gus'kov S.S., Aginey R.V., Isupova E.V. Theoretical Assessment of the Electrical Contact Effect of the Pipeline with the Groundings of Electrical Equipment on the Cathodic Current Distribution in the Underground Pipeline. Territorija "NEFTEGAS" = Oil and Gas Territory, 2017, No. 12, P. 54-58. Гуськов С.С., Агиней Р.В., Исупова Е.В. Теоретическая оценка влияния электрического контакта трубопровода с защитными заземлениями электроустановок на распределение тока катодной защиты в подземном трубопроводе // Территория «НЕФТЕГАЗ». 2017. № 12. С. 54-58. (In English)
ANTICORROSIVE PROTECTION
Fig. 1. Main window of the software for modeling the shielding effect of the cathodic protection current by groundings
current and the potential difference between the pipeline and the ground along the pipeline for different values of the following parameters:
• longitudinal resistance of the pipeline Z, Q/m;
• conductivity of the insulation coating Y, S/m;
• length of the considered pipeline section L, m;
• linear coordinate of the point of connection to the pipeline of the cathodic protection station xCK3, m;
• distance between the pipeline and anode grounding yCK3, m;
• linear coordinate of the point of connection to the grounding x33, m;
• distance between the pipeline and grounding y33, m;
• soil resistivity pr, Q.m;
• current of cathordic protection station
ICK3, A;
• resistance current spreading of grounding R33, Q.
A rectilinear pipeline section is considered. It is assumed that the linear coordinate of the anode grounding of the cathodic protection station coincides with the linear coordinate of the point of connection of the pipeline and the cathodic protection station, and the linear coordinate of the grounding coincides with the linear coordinate of the point of connection to the grounding. The software allows to make calculation and visualization the dependencies of
the linear coordinate of the current in the pipeline I(x) and the "pipe-to-soil" potential U(x). This software makes it possible to save the initial data and the results of the calculations performed, as well as ensures the ability to work with previously stored data.
RESEARCH AND PRACTICAL APPLICATION
To assess the effect of grounding on the current distribution along the pipeline and the potential difference between the pipeline and the ground, let's consider a section of the underground pipeline with the following parameters: Z = 12 |Q/m (this longitudinal resistance is in the pipeline with an external
diameter of 530 mm and a wall thickness of 8 mm, the procedure for calculating Z is given in [7]);
Y = 100 |S/m (this conductivity value of the insulation coating, according to [8], is allowed for pipelines with tape polymerbitumen or mastic insulated coating after 10 years of operation with the outer pipe diameter of 530 mm); L = 1 km.
Suppose that cathodic protection station is connected to the pipeline at a point with a linear coordinate x33 = 0,40 km, the anode grounding is located at a distance yCK3 = 0,10 km from the pipeline, the soil resistivity is prp = 100 Q.m. The results of calculating the dependences I(x) and U(x) for the current of the cathodic protection station ICK3 = 0,034 A are shown in Fig. 2 (line number 2).
Here and below, it should be keep in mind that during calculations the values of the potential difference between the pipeline and the ground are determined, due to the presence of cathodic current in the pipeline. In other words, the stationary potential of the metal of the pipeline is not considered. Values of the pipeline potential relative to the copper-sulfate reference electrode may be obtain by adding the value Ust = -0.55 V [9]. Assume that the grounding is connected to the pipeline at the point with a linear coordinate x33 = 0,60 km. The distance between the pipeline and grounding y33 = 0,05 km. Resistance current spreading of grounding R33 = 3 Q. In this case, for the same value of I , the I(x) and U(x)
Fig. 2. Dependences of I(x) (a) and U(x) (b) for xCK3 = 0,40 km, yCK3 = 0.10 km, x33 = 0.60 km, y33 = 0.05 km ICK3 = 0.14 A, R33 = 3 Q (1), ICK3 = 0.034 A, without grounding R33 = ~ (2), ICK3 = 0.034 A, R33 = 3 Q (3)
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Fig. 3. Dependences of I(x) (a) and U(x) (b) for xCK3 = 0.40 km,yCK3 = 0.10 km,y33 = 0.05 km, R33 = 3 Q,
ICK3 = 0,14 A, prp = 100 Q.m
x33 = 0.60 km (1), x33 = 0.50 km (2), x33 = 0.70 km (3)
Fig. 4. Dependences of I(x) (a) and U(x) (b) for x33 = 0.60 km, yCK3 = 0.10 km, y33 = 0.05 km, R33 = 3 Q, ICK3 = 0.14 A, prp = 100 Q.m
xCK3 = 0.40 km (1), xCK3 = 0.30 km (2), xCK3 = 0.50 km (3)
Fig. 5. Dependences of I(x) (a) and U(x) (b) for x33 = 0.60 km, xCK3 = 0.40 km, yCK3 = 0.10 km, R33 = 3 Q,
ICK3 = 0.14 A, prp = 100 Q.m
y33 = 0.05 km (1), y33 = 0.02 km (2), y33 = 0.01 km (3)
dependences will significantly change (line number 3 in Fig. 2). The values of potential on the pipeline section are reduced (in absolute value) by approximately 0,26 V. To return the value of potential to the previous level, it is necessary to increase the cathodic current ICK3 to 0,14 A (line number 1 in Fig. 2). At the same time, a local decrease (in absolute value) of potential values is observed near the connection point of grounding. Thus, an electrical contact between the pipeline and the grounding leads to a change in the distribution of the cathodic protection current in the pipeline, which is accompanied by a decrease (in absolute value) of the "pipe-to-soil" potential. This fact especially pronounced near the point of connection grounding to the pipeline.
We need to evaluate the effects of the mutual location of the cathodic protection station and the grounding on the current and the "pipe-to-soil" potential distribution along the pipeline. There are considered the results of calculating the dependences I(x) and U(x) for the same pipeline section for different cases of the mutual location of the cathodic protection station and grounding, which has electrical contact with the pipeline (Fig. 3-6). The following parameters were used in calculations: Z = 12 |Q/m; Y = 100 |S/m; L = 1 km. If the linear coordinate of the grounding connection to the pipeline changes, the value of "pipe-to-soil" potential will be also changed (Fig. 3). It was determined that the increase of the distance between the pipeline and the grounding leads to more negative values of potential (except for the section between the point of connection to the pipeline of the cathodic protection station and grounding) (Fig. 4). If the distance between the grounding and the pipeline decreases, the current leakage through the grounding also becomes smaller. As a result, the values of potential become more negative with a constant cathodic current (Fig. 5). If the distance between the anode grounding and the pipeline increases, it will be the reason of increase of the potential difference between the pipeline and the ground near the point of connection of the cathodic
ANTICORROSIVE PROTECTION
Fig. 6. Dependences of I(x) (a) and U(x) (b) for x33 = 0.60 km, xCK3 = 0.40 km,y33 = 0.05 km, R33 = 3 Q, ICK3 = 0.14 A, prp = 100 Q.m
yCK3 = 0.10 km (1), yCK3 = 0.05 km (2), yCK3 = 0.15 km (3)
Fig. 7. Dependences of I(x) (a) and U(x) (b) for x33 = 0.60 km, xCK3 = 0.40 km, yCK3 = 0.10 km, y33 = 0.05 km, ICK3 = 0.14 A, prp = 100 Q.m R33 = 3 Q (1), R33 = 5 Q (2), R33 = 10 Q (3)
Fig. 8. Dependences of I(x) (a) and U(x) (b) for x33 = 0.60 km, xCK
y33 = 0.05 km, R33 = 3 Q, prp = 100 Q.m.
Im = 0.14 A (1), I = 0.10 A (2), Im = 0.18 A (3)
= 0.40 km, yCK3 = 0.10 km,
protection station to the pipeline. In addition, we can observe a decrease in the absolute magnitude of the potential difference between the pipeline and the ground far from the point of connection of the cathodic protection station to the pipeline (Fig. 6).
Based on the results of the I(x) and U(x) dependency calculations, the influence of the cathodic protection station characteristics, grounding and soil on the cathodic current and the "pipe-to-soil" potential distribution along the pipeline can be analyzed.
Consider the same section of the pipeline for different values of the resistance current spreading of grounding R33, the current of the cathodic protection station ICK3 and the soil resistivity prp (Fig. 7-9). The following parameters were used in the calculations: Z = 12 |Q/m; Y = 100 |S/m; L = 1 km. The simulation results allow concluding that with increasing resistance current spreading of grounding, the leakage current through the grounding is reduced, as a result, the potential difference between the pipeline and the ground with the same cathodic current increases in absolute value (Fig. 7). The increase in the current strength of the cathodic protection station causes the increase in the absolute magnitude of the potential difference between the pipeline and the ground (Fig. 8).
It can be concluded that if the resistivity of the soil decreases, the absolute value of the potential difference between the pipeline and the ground decreases over the greater part of the pipeline section. At the same time, the characteristic extremum of the U(x) dependence near the point of connection of the cathodic protection station is smoothed out. The values of U(x) near the point of connection to the grounding do not depend on the value of soil resistivity (Fig. 9). Thus, the most significant effect on the values of I(x) and U(x) is provided by parameters such as resistance current spreading of grounding and the distance between the grounding and the pipeline.
Discussion of the present findings has confirmed the data of previous research. As a result of studies of the screening
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Fig. 9. Dependences of I(x) (a) and U(x) (b) for x33 = 0.60 km, xCK3 = 0.40 km, yCK
y33 = 0.05 km, R33 = 3 Q, ICK3 = 0.14 A
pr = 100 Q.m (1), pr = 50 Q.m (2), pr = 20 Q.m (3)
0.10 km,
effect from groundings influencing on the effectiveness of cathodic protection some practical conclusions were made. A simple mathematical model can be used for development of specialized software allowing to calculate the distribution of current and potential along the length of the pipeline in the presence of galvanic connection with the grounding. The proposed software helps in choosing the optimal parameters of cathodic protection systems and points of disposition of groundings at the industrial sites, as well as for actions which may reduce the negative effect of groundings, including the determining of parameters and locations of devices for
galvanic isolation between the pipeline and the grounding.
CONCLUSIONS
1. The software for calculation of the current and the "pipe-to-soil" potential distribution in the pipeline, considering the cathodic protection stations connected to the pipeline and electrical contact between the pipeline and the groundings of the electrical installations, was developed.
2. The article presents an example of calculation of the influence of grounding on the distribution along the pipeline of the cathodic protection current and the potential difference between the
pipeline and the ground. It is shown that connection the grounding for a pipeline section with an outer diameter of 530 mm and a wall thickness of 8 mm leads to decrease of the absolute value of the potential by 0.26 V. To restore the potential difference to the previous level, it is necessary to increase the current of the cathodic protection station by more than 4 times (from 0.034 to 0.14 A).
3. Basing on the modelling results, the following conclusion was made: there is an influence of the mutual arrangement of the cathodic protection station and grounding on the distribution of the cathodic current and the potential difference between the pipeline and the ground along the pipeline. The obtained data allows concluding that when the distance between the grounding and the pipeline decreases from 50 to 10 m, the value of "pipe-to-soil" potential, far from the grounding connection point, increases in absolute value by 0.12 V.
4. Research of the influence of the characteristics of the cathodic protection station, grounding and soil on the distribution of the cathodic current and the potential difference between the pipeline and the ground along the pipeline was made. Increase of the resistance of grounding to current spreading from 3 to 10 Q (with the current strength of the cathodic protection station unchanged) is a reason of increasing the potential in absolute value by 0.37 V.
References (translation)
1. Glotov I.V., Aginei R.V., Yushmanov V.N. Experimental Determination of Mathematical Models for Optimization of Underground Oil and Gas Pipelines Protection by Several Cathodic Stations. Zashchita okruzhayushchey sredy v neftegazovom komplekse = Environmental Protection in Oil and Gas Complex, 2009, No. 8, P. 18-22. (In Russian)
2. Korotyaev A.G. Influence of Grounding Loops on the Protection Level and the Resource of the Electrochemical Protection System of the Area Objects. Korroziya "Territorii "NEFTEGAS" = Corrosion of the Oil and Gas Territory, 2016, No. 3 (35), P. 60-62. (In Russian)
3. Prokhorov A.A, Radchenko V.V., Zhukov R.A. Experience in Designing Corrosion Protection of Underground Pipelines at ORS Sites. Nauka i tekhnologii truboprovodnogo transporta nefti i nefteproduktov = Science and Technologies of Pipeline Transport of Oil and Oil Products, 2017, Vol. 7, No. 2, P. 82-86. (In Russian)
4. Selina L.A., Teleten I.G. Designing of Electrochemical Protection Means at the Bovanenkovo-Ukhta Main Pipelines System. Results of Construction and Commissioning Work. Korroziya "Territorii "NEFTEGAS" = Corrosion of the Oil and Gas Territory, 2016, No. 3 (25), P. 76-79. (In Russian)
5. Teleten I.G., Patryshev N.Y. Features of Construction of the Electrochemical Protection System in the Presence of Earthed Structures. Korroziya "Territorii "NEFTEGAS" = Corrosion of the Oil and Gas Territory, 2014, No. 1 (27), P. 76-77. (In Russian)
6. Aginei R.V, Isupova E.V Research of the Influence of Groundings of Electrical Installations on the Efficiency of Electrochemical Protection of Underground Pipelines at the Territory of Industrial Sites. Truboprovodnyy transport: teoriya i praktika = Pipeline Transport: Theory and Practice, 2017, No. 3 (61), P. 16-20. (In Russian)
7. Zubkov A.A., Gus'kov S.S., Aginey R.V. Mathematical Model of the Geomagnetic-Induced Current Formation in an Extended Isolated Pipeline. Truboprovodnyy transport: teoriya i praktika = Pipeline Transport: Theory and Practice, 2015, No. 3, P. 16-19. (In Russian)
8. State Standard GOST R 51164-98 Steel Main Pipelines. General Requirements for Corrosion Protection. Moscow, Gosstandart of Russia, 1998, 46 p. (In Russian)
9. Tkachenko V.N. Electrochemical Protection of Pipeline Networks. Moscow, Stroyizdat, 2004, 320 p. (In Russian)
