Author: Carlos Álvarez, November 3, 2023
Introduction
High-density polyethylene ( HDPE ) pipes are widely used in the construction and infrastructure industry due to their durability and corrosion resistance. However, like any other infrastructure component, HDPE pipes are subject to degradation over time. Regular inspection is necessary to ensure its integrity and proper operation.
The evaluation of the structural integrity of thermoplastic pipes has become an interesting area of research due to the increase in their use. Ultrasonic guided wave testing has gained increased industry attention for the inspection of elongated structures due to the reduction in inspection time and cost compared to conventional non-destructive testing techniques, e.g., ultrasonic testing, radiography and visual inspection.
In this article, we will explore an advanced inspection method: guided wave testing and how it is applied to high-density polyethylene pipe.
Introduction to High Density Polyethylene pipes
High-density polyethylene (HDPE) is a thermoplastic polymer widely used in pipe manufacturing due to its corrosion resistance, durability, and low maintenance cost. These characteristics make high-density polyethylene pipes ideal for a wide range of applications, such as supplying drinking water, natural gas, chemicals, among others. Despite its advantages, HDPE pipes are prone to degradation, making regular inspection essential to ensure long-term safety and performance.
Importance of HDPE Pipe Inspection
Inspection of high-density polyethylene pipes is essential to early detect any signs of damage such as cracks. If these issues are not addressed in time, they can lead to leaks, water contamination, damage to surrounding infrastructure, and ultimately, costly repair costs. Proper and timely inspection can help prevent these problems and ensure continued pipe functionality.
The Guided Wave Test Method
Guided wave testing is a non-destructive testing method used to evaluate the integrity of pipes and other structural components. Guided waves refer to mechanical or elastic waves that can propagate in a medium in a frequency range of 10 KHz to 2 MH, guided and confined to its geometry. The medium where the wave travels has a strong influence on its behavior.
Unlike conventional ultrasound, where the inspection is localized, in the guided wave system the inspection is carried out using a ring of transducers to emit low-frequency ultrasonic waves that travel towards one or both sides of the component, thus achieving inspection. over long distances from a single point of application
Advantages of Guided Wave Testing in HDPE Pipe Inspection
1. Non-destructive inspection : Guided wave ultrasound is a non-destructive technique, which means that it is not necessary to disassemble or cut the pipes to carry out the inspection. This minimizes disruption to operations and reduces maintenance costs.
2. Early defect detection: Guided wave ultrasound can detect incipient defects, such as cracks and corrosion, before they become critical. This allows for planning of repairs or replacements before significant damage occurs.
3. Inspection of pipe sections through road passes and retaining walls: Guided wave ultrasound can be used on high-density polyethylene pipes in road passes, such as pipe racks and culverts, as well as as through retaining walls, and also through buried pipes, although the latter with limitations on the wave’s path through the subsoil.
4. Inspection in support areas: One of the great applications in pipes that has been developed in recent times is in the evaluation of those areas where the pipe is supported, which are difficult to evaluate using conventional methods and techniques. To do this, a derivation of the test method known as short-range guided waves is used.
5. Continuous monitoring: Guided wave ultrasound can also be used for continuous monitoring of pipelines, allowing changes in pipeline integrity to be detected over time and preventative measures to be taken.
Testing for inspection of HDPE pipes using GWT
Inspection of thermoplastic pipes using GWT is an emerging area of interest. Various tests using the piezoelectric effect as a means to excite GWT have been applied to detect cracks in plastic pipes and at different distances, both in pipes without cracks and pipes and with different numbers of this type of failure. Each pipe sample has been tested with different distances and at different excitation frequencies.
The results generally show differences in the frequency response between cracked and uncracked pipes and also between the number of cracks, demonstrating that GWUT can be used to detect cracks in plastic pipes, on the other hand, the throw and catch technique (pitch & catch) has proven to be more effective in detecting cracks. However, it is observed that the amplitude monitored in these tests in the frequency range decreases in all samples due to the attenuation in the medium as the distance from the source to the receiver increased and with a higher dead zone.
Some findings have revealed that cracks with lengths equal to or greater than the interaction mode wavelength were more easily detectable, while smaller cracks required greater crack depths for detection. The findings of these studies have shown that the potential of GWT for monitoring high-density polyethylene pipes is evident.
Challenges in HDPE Pipe Inspection
The research tests have been aimed at defining the optimal frequency range for the inspection, the design of the arrangement and the duration of the inspection, in order to face and solve the following challenges.
Coupling difficulties: HDPE is a lighter and less rigid material than steel, which makes it difficult to dock ultrasound probes. Specific coupling techniques are required to ensure adequate transfer of ultrasonic energy between the probe and the pipe.
Attenuation and Dispersion: Ultrasonic waves can experience greater attenuation and dispersion in HDPE compared to other materials. This can affect signal quality and the ability to detect defects.
Anisotropy: HDPE is an anisotropic material, meaning its properties can vary in different directions. This can complicate the interpretation of ultrasonic data and the detection of defects, since the speed of wave propagation can vary depending on the direction.
Inadequate training and experience: Effective GWT inspection for HDPE pipe requires highly trained technicians with specialized knowledge. Insufficient training and experience can lead to misinterpretation of inspection results and undetected defects.
Conclusions
Inspection of HDPE pipes using GWUT guided wave ultrasound promises to be a valuable tool for evaluating the integrity of these components. Despite the aforementioned challenges, this technology promises to be effective in detecting cracks that develop during service, focusing efforts on defining the optimal frequency range for inspection, the design of the probe array, and the duration of the inspection.
Inspection of these pipelines is essential to ensure the safety and efficient operation of pipeline systems in critical applications such as gas transmission networks. Early detection of these types of defects can prevent costly outages and, more importantly, ensure the integrity of infrastructure and environmental protection.
In conclusion, guided wave ultrasound inspection of HDPE pipes is an important tool that, despite its challenges, can provide significant benefits in terms of safety and operational efficiency. As technology continues to evolve, inspection capabilities are likely to be further enhanced, strengthening their role in infrastructure preservation and environmental protection.
Bibliographic references
PREMESH SHEHAN LOWE, HABIBA LAIS, VEENA PARUCHURI, TAT-HEAN GAN. Application of Ultrasonic Guided Waves for Inspection of High Density Polyethylene Pipe Systems; Consulted on October 27, 2023; https://ouci.dntb.gov.ua/en/works/9GQQJd
JAY SHAH, SAID EL-HAWWAT, HAO WANG. Guided Wave Ultrasonic Testing for Crack Detection in Polyethylene Pipes: Laboratory Experiments and Numerical Modeling; Consulted on October 28, 2023; https://www.mdpi.com/1424-8220/23/11/5131
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