Van Cheng-long1, Xu Huoli1,2,Chen Guopei1,Fu Xibi1,2,Tang Bin Kun3
(1. Xiamen Special Equipment Inspection and Testing Institute; 2. Fujian University, College of Mechanical Engineering and Automation; 3. Xiamen China Resources Gas Co., Ltd.)
Polyethylene (PE) pipes are widely used in urban gas and water supply applications due to their corrosion resistance, high flexibility, and ease of installation. According to statistics, more than 90% of new urban gas buried pipelines in China use PE pipe materials. In addition to damage caused by third-party construction, plant roots are also a factor affecting the safe operation of PE pipes. On the one hand, the entanglement of deep-rooted plant roots can exert pressure on the pipes. On the other hand, when strong winds topple trees, the pipes can be pulled up along with the roots, causing cracks at weak points in the pipe connections, resulting in pipeline leakage and safety accidents.
During regular inspections of underground natural gas polyethylene pipelines in urban areas, excavation and inspection were carried out around deep-rooted plants or plants with close spacing. This revealed that the roots of the plants were wrapped around the PE80 pipe, resulting in extensive fine black fibrous deposits on the pipe surface. However, due to the appearance, it was difficult to determine whether the root exudates would corrode the pipe itself. Therefore, to ensure safety, the pipeline users could only replace the affected sections of the pipe.
To better guide subsequent regular inspection work, it is necessary to perform material performance testing on this section of the pipeline. The author first takes samples from the replaced pipeline to measure the wall thickness, verifying any changes in the pipeline wall thickness. Then, penetration testing is performed on the affected pipe sections to verify the density of the pipe material. Finally, infrared spectroscopy testing is conducted on different locations on the inner and outer surfaces of the sampled pipe to verify changes in the pipe material composition, and to determine if it has aged or corroded.
1 Overview of Regular Inspection for Polyethylene Gas Pipelines
During the period from June to August 2022, while conducting routine inspections of the public gas pipelines in Haicang District, Xiamen City, it was found that the distance between the gas pipelines and the trees was generally not in compliance with the requirements specified in GB50028-2020 "Urban Gas Design Code," which states that "the horizontal distance from the pipeline to the center of the tree must be greater than 0.75m." Upon investigation, it was determined that the pipeline installation occurred before the urban greening and planting. In accordance with the inspection requirements of TSG D7004-2010 "Rules for Regular Inspection of Pressure Pipelines—Public Pipelines," an excavation verification inspection was conducted on the PE pipelines.
1、Equipment Overview
The inspected equipment consists of pressure pipelines in Haicang District, Xiamen, with unit names of Haicang Avenue Gas Project and Langqin Bay Gas Project. The installation date for the Haicang Avenue Gas Project pipeline is December 20, 2004, and the installation date for the Langqin Bay Gas Project pipeline is August 31, 2004. The pipeline design pressure is 0.4 MPa, and the operating pressure is 0.18 MPa. The specifications are D250 and D6, with polyethylene as the pipe material. Regular inspections revealed that the excavation points for the affected pipelines are shown in Figure 1.
Figure 1: Location of excavation points for the pipeline.
2、 Issues identified during inspection
Plant root systems can damage steel pipes that have early-applied asphalt-based anti-corrosion coatings, primarily causing crushing and piercing damage. Currently, there are no known cases of plant root systems damaging pipes with 3PE anti-corrosion coatings. The secretions from plant roots are corrosive and accelerate the corrosion of the pipes, with the most significant impact being the penetration of plant roots into the anti-corrosion layer of steel pipes. Polyethylene (PE) materials have strong resistance to acids and bases, are insoluble in solvents, and are difficult for plant root secretions to swell. Furthermore, PE materials also have good resistance to biological degradation in the soil environment. However, during the excavation and inspection of the PE pipes, small black streaks were observed on the surface of the pipes, as shown in Figure 2. These black streaks were found in large areas at five locations along the excavated pipe, and they resembled small cracks, which could be mistaken for signs of pipe aging. However, there were no other signs of aging or degradation on the pipe surface. The black streaks are the result of plant root secretions penetrating the pipe surface and their impact needs to be further verified through physical and chemical testing.
Figure 2: Appearance of pipes at excavation site
2 Polyethylene Pipe Performance Testing
Upon discovering the leak in the excavated pipe, initial on-site leak detection was performed using a laser methane detector and bubble testing. The results indicated that there was no leakage. To understand the impact of the black streaks on the pipe, the following tests were conducted on pipes with visible black streaks:
1、Wall thickness measurement
Measuring pipe wall thickness can not only verify whether the pipe material thickness meets the design requirements, but also provide information about the pipe wall thickness in areas affected by root secretions. The measurement is performed using a specialized ultrasonic thickness gauge for polyethylene pipes, with verification using a caliper. The process is illustrated in Figure 3.
Figure 3: Measurement of wall thickness in the area affected by root exudates.
The ultrasonic thickness gauge model is Tritex Multiguage 5300, with a sound speed set to 2300 m/s, and a 1 MHz low-frequency single crystal probe is used. Before measurement, a sample of the same material (polyethylene) is selected for calibration. During measurement, the probe face should maintain good contact with the pipe surface. A caliper is used to measure the pipe ends, and the tip of the caliper should be parallel and aligned with the pipe axis during measurement.
2、Penetration testing experiment
According to NB/T 4701.5-2015 "Part 5: Penetrant Testing of Pressure Vessels," surface penetrant testing was performed on polyethylene pipes affected by root exudates, as shown in Figure 4. The testing process also included sensitivity test block detection, using the defects in the sensitivity test block as the sensitivity requirement for defect detection.
Figure 4: Detection of penetration of root exudates in the affected area.
3、Infrared Spectroscopy Testing
First, the pipe sample was divided into 8 equal segments along the bottom circle, with each segment marked from 1 to 8, as shown in Figure 5(a). Then, the sample was divided into 3 segments along the axis, with each segment marked a, b, and c, as shown in Figure 5(b). A total of 24 samples were taken, as shown in Figure 5(c). Three samples were taken along the axis from the inside of the polyethylene pipe (marked as 9), and were labeled as 9a, 9b, and 9c. Fourier transform infrared spectroscopy tests were performed on each of the marked sample positions, resulting in 27 spectra.
Figure 5: Schematic illustration of infrared spectroscopy sampling.
3. Results and Discussion
1、Wall thickness measurement results
Measure the wall thickness of the affected area where root exudates are present, and obtain ultrasonic thickness measurement results and measurements using a caliper, as shown in Table 1.
Table 1: Measurement results for pipe wall thickness
As shown in Table 1, the ultrasonic thickness gauge measured a maximum wall thickness of 6.60mm and a minimum wall thickness of 6.35mm. The normal pipe wall thickness is 5.80mm, with deviations of 0.80mm and 0.55mm respectively from the maximum and minimum measured values. The caliper measured a maximum wall thickness of 6.40mm and a minimum wall thickness of 6.08mm, with deviations of 0.60mm and 0.28mm respectively from the normal wall thickness. Considering the manufacturing tolerances of the material, the calibration errors of the ultrasonic thickness gauge, and the measurement errors of the caliper, the above deviations are still within the acceptable range. However, based on the results of both measurement methods, the deviation between the ultrasonic thickness gauge and the caliper measurement is small, with a maximum deviation of 0.28mm. Therefore, the use of ultrasonic thickness gauge for wall thickness measurement is fully feasible on site.
2、Penetration test results
Conduct penetration tests on the affected area due to root secretions, while also verifying the sensitivity of the monitoring method. The results are shown in Figure 6.
Figure 6: Permeation test results
As shown in Figure 6, the third point of the three-point test sample exhibits clear radial cracks, indicating that the sensitivity of the penetration detection system meets the requirements. The penetration detected on the pipe surface is a false indication, and the root secretion area (fine black silk traces on the pipe surface) shows no penetration. Therefore, it can be concluded that the black silk traces are not defects such as gaps or pores, and the pipe surface is intact.
3、Infrared spectroscopy test results
Infrared spectroscopy tests were performed on samples from different parts of the sampling pipe, and the results showed that the spectra of all 27 samples were essentially consistent. The test results for samples from positions 6 and 9 are shown in Figures 7 and 8. As shown in Figures 7 and 8, the absorbance of the samples from positions 6 and 9 is consistent at different wavenumbers, and does not exceed 1. The infrared spectra of the samples from different positions are essentially the same, indicating that the properties of the material in different parts are not significantly different.
Figure 7, sample at position 6, infrared spectrum
Figure 8: Infrared spectrum of the sample at position 9
Compare the infrared spectrum obtained from the test with the polyethylene standard spectrum. The results show that the test spectrum is consistent with the polyethylene standard spectrum, indicating that the test sample is polyethylene. The infrared spectrum of the polyethylene pipe surface and interior is consistent, indicating that the pipe material properties have not changed.
Inspection revealed insufficient space between the pipe and the street tree, with the plant roots wrapping around the pipe. The plant roots' secretions have penetrated the shallow surface of the pipe, forming fine, dense crack-like patterns. However, based on the above inspection and testing analysis, it has been confirmed that the pipe material has not undergone aging or degradation, and the secretions from the plant roots have not directly affected the safety of the pipe.
4. Conclusion
(1) The surface of the PE80 pipes in the early stage exhibited large areas of fine black fibrous marks due to the wrapping of plant roots. However, it is difficult to determine from the appearance whether the root exudates have caused corrosion to the pipe body.
(2) Leak detection revealed no issues with pipe leaks. Wall thickness measurements, penetration tests, and infrared spectroscopy testing also confirmed that the pipe materials had not undergone aging or degradation, and their performance remained unchanged.
(3) Although the secretions from plant roots have affected the color of the pipe, the thickness, density, and composition of the pipe wall have not changed. The secretions from plant roots do not directly affect the safety of the pipe. However, further inspections are needed to pay more attention to the pipe stretching and deformation caused by the roots.
(4) It is recommended to strengthen the supervision of greening construction, and eliminate the planting of deep-rooted plants at the source to eliminate the risks associated with greening construction affecting pipeline operation.
Source: Corrosion & Protection, Vol. 25, Issue 2, 2025