Experimental Study of Surface Detection of Gas Pipeline Buried in Soil

Journal of Shanghai Jiaotong University(Science), Vol E-10, No 3, 2005, 307-310Article ID:1007-1172(2005)03-0307-04Experimental Study of Surface Detection ofGas Pipeline buried in SoilLI Yan-hng2(李艳红),HEJi-min(何吉民),LI Zhi-peng2(李志鹏), ZHANG Yong-gang2(张永刚), FENG Bao-he2(冯宝鹤)(1. School of Mechanical Eng, Shanghai Jiaotong Univ, Shanghai 200030, China;2. North Shanghai Gas Co. Ltd, Shanghai 200001)Abstract: Pipeline is a key segment in the transportation of city gas and its safety affects the safety of industrialand domestic application. The characteristics of Shi Dongkou east gas steel pipeline buried in soil were discussedand its parameters related to safety were measured, including the state of anticorrosive layer, the soil resistivitythe natural potential and the protective potential of gas pipeline. The experimental results were confirmed byvating, which are of value to the knowledge of the gas pipeline buried in soil in Shanghai. The experimental datawere analyzed which provide the scientific basis for the assurance of the gas pipeline safety and the reparation of aticorrosivelayerKey words: steel pipeline cathodic protection; experimental studyDocument code: AIntroductionthe pipelines inevitably come up against miscellaneous corrosion currents produced by stray allGas pipelines buried in soil are necessary in sorts of electrical currents and high-voltage electri-transport process of city gas. Whether it is safe or cal field All of these factors affect safe operationnot is related to the safety of tens of thousands do- of underground pipeline seriously, so it is neces-mestic customers in life and the industrial cus- sary to have the good understanding about operatomers in worktional situation of the pipelineBecause of the high-speed development of theThe results of the experiment in this papergas business in Shanghai, the gas users are ebring us further understanding about the circum-panded from hundreds of thousands in several mil- stance of gas steel-pipeline buried in soil in Shangions, and the gas pipeline is also developed from hai region, and provide scientific warrant to repairingot tube into steel pipe. The underground steel- the erosion resistant coating of pipeline and ensurepipelines will bring into all kinds of corrosions in safe operation of the pipelineoperation. Because of different locations, groundtemperature and oxygen contents, the surface of 1 Choice of Experimental pipelinepipelines willappear various degrees corrosionThe east pipeline leaving the factory of Shin Shanghai, the underground soil salinity, the Dongkou Gas Production Ltd. lay along Wenchuanhigh underground water level the high tempera- Road and pass Sheng Qiaomen Station, Yangxingture when leaving factory in gas transport processStation and Gong Fumen Station to Miao Xingmenand etc, will result in the electrochemical corrosion Station, totally 16 km long (see Fig. 1, and mainin macro and the chemical corrosion in microscopic ly is used for supplying gas to the residents whoview. In addition, in the long distance transpordiffic中国煤化工 Most pipeline aremanaged, becauseReceived date: 2004-03-25CNMH G Changjiang riverE-mail:liyh@sjtu.edu.cnwhere covered with lakes and rivers densely. Once308 LI Yan-hong(李艳红),HEJ-min(何吉民),ealthe pipeline has any fault, it will result in the gas into two groups one tests from the beginningshortage of the north of the city directly 2 Con- power-on point, the other tests from the middleidering the purpose of this experiment and the power-on point And every power-on point mainopinion of North Shanghai Cities Gas Sale Ltd, ac- tains definite distance in order to avoid interfecording to the characteristics of the pipeline, weencechoose the pipeline shown in Fig. 1 as the experiDuring the detection, firstly, we adopt a rackmental segment of our researchof“A” type of the RD-PCM to carry on examination of locating position of damaged point. Second-nation we use the SL-6 to repeat the test. LastWenchuan roadaccording to the result of examination we finallyYueluo roadchoose two damaged points to excavate. The onepoint is 180 m away from 7 testing pile and theExperimentalother is 845 m away from 7# testing pileThe whole situation of anticorrosive coating ofexperimental segment is common, but the coatingsof individual pipeline segments thin down badlyand are damagedApplied Cathodic Protection Station2. 2 Measurement of Soil Electrical ResistivityWe measure the soil electrical resistivity bysoil electrical resistivity instrument. And the prin-8]Shi Dongkou Gas Production Ltd.ciple of measurement is four-electrode methodFig. 1 Position of measuring pipelinee test the soil electrical resistivity at the ex-cavating point: 845 m away from 7# testing pile2 Measure of the Testing Parame- and the result is comparatively low. It is 49.93tersn.m. According to the standard of syoo07-1999the corrosion degree of the pipeline is under the2.1 Anticorrosive Coating DetectionWe adopt the RD-PCM and Haian SL-6 anti- middle2. 3 Measurement of Natural Potential and Cacorrosive coating of underground piping detectionthodic Protection Potential of the Pipelineinstrument to detect the anticorrosive coating syn-nder solthetically. First, we use rd-PCM to fix the pipeWe measure natural potential and cathodicposition and measure the depth of burial. Then welocate the damaged point and evaluate the degree of protection potential of the pipeline under soil byCse detection instrument which is widely used inthe pipeline coating synthetically. Last we use the foreign countries. The results are listed in Tab. 1Haian SL-6 anticorrosive coating of undergroundpiping detection instrument to repeat the test in orWhen the piping is buried totally, the averagevalue of the natural potential of the pipeline underder to improve the accuracy of measurementsoil is -0.62 VConsidering the depth of the undergroundThe data of experiment indicates the effect ofpipeline and whether the pipeling outstray current on the cathodic protection is compareasily, two faulty pipeline segments are chosen tolocate the damaged point. The beginning point ofthe measurement is 7# testing pile (as 0 m), and 2.4 Excavation and Check Resultsthe end point is 4#(3 910 m). Every testing point中国煤化工845 m away fromis arranged at intervals of 50 m, and there are total 7# teCNMHG poInt 845 m awayfive power-on points. The experiment is dividedfromwent pipe from southExperimental Study of Surface Detection ofTab 1 Data of cathodic protection potentialTesting pile No.Yangxing StationNatural potential/v0.574athodic protection potential0.7290.7700.8131.885Potential difference of before0.155protection and after protection/V0.1300.1430.436Protection degreePartly protected Partly protected Partly protectedProtectedProtectedto northeast, and excavation line is 2 m long. The of RD-PCM (as shown in Fig. 2), we can find thepipeline itself corrodes not very seriously. The ex- whole situation of this pipe segment coating is genternal appearance of the whole coating is good, and eral but some parts of the coating are worsethe angle fitting presents deep brown and the two among which the point 718 m away from 7#testides of it are black (It indicates the iron itself is pile (i. e. 50 m away from south of Meipu bridge)corroded partly ). Because this part is the field of is slightly bad I level, according to SY5918-94operation and repaired section, the coating appears 'Standard of Heavy Repair Technology of Bitumenobvious hurt trace. After getting rid of the coating Anticorrosive Coating of Steel-pipeline Buried inabout 100 mm X50 mm size, we find that the cor- Soil,'),and the anticorrosive coatings of pipe 2 235rosion of the pipe is not very serious, and there are m away from 7# test pile(i. e. 70 m south andhanical pitting and a little of iron rinorth away from Wanshi bridge ) are very bad(Nv level), and the situation of anticorrosive coat-(2) Excavating the point of 180 m away fromthe coating quality of middle section of pipelines kings of other pipelines ar7# testing pile. The excavating point 180 m away better than the two ends. The damping factor offrom 7 #t testing pile is circumferentialseam repaired section, and the excavation line is one meter current flow is very big at the two ends, and thelong. The quality of the coating is bad, and thequality of coatings needs to be researched morecoating of repaired section is deep brown and thetwo sides of it are black (It indicates the iron itselfis corroded partly ) Just under thee pipe there is adamaged coating. The binding power of coating isbad, and some parts of coating slough. And therere pits on the naked pipe. Getting rid of the coatng 100 mm 50 mm size, we find no corrosionthe pipe According to the situation of the excavation, the cathodic protection is effective becausethere is no obvious corrosion found on the steepipe itself.Distance/kmTherefore, safety supervision of this pipelinFig 2 Insulation resistance of anticorrosive coating ofshould be strengthen, especially the supervision toWenchuan road 7# 4# testing pilethe safe performance of the piping of the key sec- 3.2 Cathodic Protection Analysistion. The main contents of the supervision are theCathodic protection potential (as shown insituation of pipeline coating and the validity of ca- Fig 3)analysis: According to the Standard of Chithodic protectionna SY/T5919-94, the protective ratio of cathodic3 Result analysisprote中国煤化工about 62.5%Ld is -0.85 V3. 1 Anticorrosive Coating AnalysisterionHCNMHGmay be differentAfter software analysis of the detection result from this role. Although the cathodic protection310 Ll Yan-hong(李艳红),HEJi-min(何吉民),etalpotential of the pipe between 6# and 4# testing coatings damaged. But the pipeline itself is notpile are obviously lower than the natural potential, corroded seriously because of the effective cathodic0.85V, we think thiprotection. The two excavation points are both lopipeline segment is partly protected. The attenua- cated between 6# and 7# testing pile, so the ex-pipeline between 7# and 6# testing pile is 354 the cathodic protection potential ination result oftion of the cathodic protection potential of the cavation also validates the examination result ofmV. Compared to other pipeline segments, the po-tential attenuation is very big, so there may be a4 Conclusioncorrosion pit on this pipeline segmentThis experiment verifies the possibility andpracticability to detect gas pipeline buried in soilNatural potentialwithout excavation. The data achieved in the experiment by RD-PCM reflect the situation ofpipeline anticorrosive coating truly, which does aStandard potentialvery good attempt for further research. The experotective potentialiment also approves that the cathodic protectiongas pippecially in Shanghai where covers many lakes andrivers, the importance of the cathodic protection ismuch more prominent. At the same time, t.periment still discovers that spray current haTesting pile Nohe cathodic protection, so the buriedFig. 3 Pipeline natural potential & protective potentialpipelines should be supervised strictly and inspected seriously where there are many spray currentsThe natural potential is higher, which is relatsuch as surrounding areas of the power station anded to the effect of the stray current where the the metro traffic of city. And the contents of supipeline is laying under soil. However, as a whole, pervisory include the situation of anticorrosivewe find that the effect of stray current and other coating and effectivity of cathodic protection tofactors can been eliminated in a certain extent by avoid the pipeline accidentsthe software and the protection potential analysisof RD-PCMReferences3. 3 Excavating Point Analysis[I Feng Bao-he, Xu Hu-kang, Niu Yue-hu. AdoptingOne excavating point is a bend pipe (845mimpressed-current technology to reform old piping ofaway from 7# testing pile ) the other is a jointcathodic protection[J]. Technology and equipment of(180 m away from 7#t testing pile), both of whichPipeline,2002,50(4):33~37are the area of repaired section. By measuring the[2] Feng Bao-he, Zhu Wen-yue. Examattonthickness of coating of excavation point it can besis of the sacrificial anodic protection system forfound that the qualities of repaired section are nonburied steel pipeline in the Bao Steel Area in ShanghaiDJ]. Corrosion &Protection, 1999,20(12):45-48homogeneous. Contemporary, the groundwater [33 CJJ61-2003, Technical specification for detecting andlevel is high and the soil is backfill soil or architec-surveying underground pipelines and cables in cityture rubbish etc, which makes the anticorrosive中国煤化工CNMHG