Polyethylene Pipe Material Research Articles

Defect detection for polyethylene pipelines based on ultrasonic-guided waves

Abstract The identification of small non-penetrating defects in polyethylene (PE) pipes, utilizing ultrasonic-guided waves, serves as the cornerstone for ensuring the safe operation of these pipes. However, owing to the PE pipe material characteristics, the guided wave has high attenuation in PE pipe, which seriously limits the detection range and accuracy of the guided wave. To address this problem, the dispersion and dissipation characteristics of ultrasonic-guided waves in PE pipes were derived, and the results indicated that the excitation frequency was the important parameter affecting the propagation distance. Then, an experimental platform for PE pipe testing was built using macro fiber composites. The acoustic attenuation coefficient and dispersion were calculated. After considering the effects of dissipation and dispersion on the guided wave, the optimal excitation frequency was selected to extend the guided wave detection distance to 4 m. Finally, an experimental study on ultrasonic-guided wave detection of defects in PE pipes was conducted. The experimental results showed that non-penetrating small defects with a section loss rate of 8% could be effectively identified and located using ultrasonic-guided waves.

Investigation on the methane emissions from permeation of urban gas polyethylene pipes under the background of hydrogen-mixed natural gas transportation

Methane, a more potent greenhouse gas than carbon dioxide, aggravates the greenhouse effect of ecological environment through its emissions. When transporting hydrogen-mixed natural gas through urban polyethylene pipes, methane will be emitted due to the permeation characteristic of polyethylene pipe material. Therefore, it is crucial to pay attention to the methane emissions from permeation of polyethylene pipes for protecting the environment and climate. In this paper, the solubility and diffusion coefficients of methane within hydrogen-mixed natural gas in polyethylene pipe materials are calculated through the Monte Carlo method and molecular dynamics simulation to determine the permeation coefficient of methane. Simultaneously, the methane emissions and equivalent carbon dioxide emissions of the hydrogen-mixed natural gas in polyethylene pipes are investigated. Results indicate the methane solubility coefficient decreases with increasing temperature and hydrogen mixing ratio. The methane diffusion and permeation coefficients increase with the rising temperature and pressure, and decreasing hydrogen mixing ratio. The pressure exerts minor influence while the temperature and hydrogen mixing ratio have more significant impact. That is, the lower the temperature and the higher the hydrogen mixing ratio, the less methane emissions caused by permeation. Therefore, the effective reduction of methane emissions can be achieved by adding the insulation layer to mitigate the impact of high temperature and increasing the hydrogen mixing ratio. For example, for the polyethylene pipe with the operating pressure of 4 bar and the size specification of SDR11, when the temperature drops from 300 K to 260 K, the methane emissions of hydrogen-mixed natural gas with hydrogen mixing ratios of 0 vol%, 20 vol%, 50 vol% and 80 vol% will be drastically reduced by 73.3%, 74.4%, 77.2% and 78.5% respectively, and when 20 vol%∼80 vol% hydrogen are mixed in the natural gas at 260 K, 300 K and 310 K, the methane emissions will be effectively reduced by 26.7%–85.6%, 23.3%–82.1% and 23.9%–82.4% respectively. The findings of this study can serve as guidance for reducing the methane emissions from permeation of polyethylene pipes transporting hydrogen-mixed natural gas.

Biofouling effect of different landfill leachates on high-density polyethylene pipe material

Pipelines used for collecting and transporting landfill leachates may be affected by biofouling, subsequently posing environmental risks. This study investigates the surface biofouling on high-density polyethylene (HDPE) pipes immersed in leachates acquired from operating (MSW-1) and closed (MSW-2) landfills and from a landfill containing incinerated bottom ash and municipal waste (MSW-3). Water quality monitoring, material characterization, and microbial high-throughput sequencing analysis were conducted to analyze the biofouling effects of different landfill leachates on HPDE pipes. The total fouling masses in the MSW-1, MSW-2, and MSW-3 leachates were 29.2 %, 35.2 %, and 24.5 %, respectively; MSW-2 depicted severe biofouling. The biofouling mass was dominated by living mycobacterial cells (69.93 % of the total biovolume). Proteobacteria was the dominant phylum (30.43 %) and the key bacteria group to cause biofouling. The leachates had a significant impact on the microbial community structure, with Castellaniella and Luteibacter being the endemic genera in MSW-3 and norank_f__Rhodothermaceae being the endemic genera in MSW-1 and MSW-2. Furthermore, Ca2+ was the dominant water-quality parameter to affect the distribution of microbial communities. This study provides valuable insights for pipeline disinfection and sterilization to mitigate bio-clogging in HDPE pipes.

Ultrasonic defect detection of high-density polyethylene pipe materials using FIR filtering and block-wise singular value decomposition

Condition monitoring of high-density polyethylene (HDPE) pipes used for fluid and gas transfer is important for the safety of energy conservation and the environment. Ultrasonic phased array imaging methods provide a solution to detect and assess defects in HDPE pipes. However, ultrasonic bulk waves propagating in these viscoelastic media are strongly attenuated, resulting in reduced signal amplitude. In this study, a linear-phase Finite Impulse Response (FIR) filter is used to remove unwanted frequency components from the measured ultrasonic signals to improve the signal-to-noise ratio before applying the imaging algorithm of the total focusing method (TFM). Building upon this, a block-wise singular value decomposition (SVD) technique, which can adaptively determine the singular value cutoff threshold based on each block in the whole TFM image, is used to enhance the obtained TFM image quality. The performance of the combination of FIR filtering and block-wise SVD technique is validated by experimental data of HDPE pipe materials. Results demonstrate that the proposed approach generates good images to provide the detection and characterization of side-drilled holes in HDPE pipe materials.

Detection of defects in highly attenuating materials using ultrasonic least-squares reverse time migration with preconditioned stochastic gradient descent

Accurate detection and characterization of defects in high-density polyethylene (HDPE) pipe materials are very important in assessing the structural integrity of critical structures in the nuclear industry. One specific challenge here is the presence of the viscoelastic attenuation of this material, which can lead to resolution degradation and loss of detail in ultrasound imaging. In this work, an effective ultrasonic imaging technique using the least-squares reverse time migration with preconditioned stochastic gradient descent (LSRTM-PSGD) is developed to improve image quality. Compared with standard ultrasonic imaging methods which only consider the direct ray path of ultrasound, least-squares reverse time migration (LSRTM) is a powerful wave-equation-based approach and it has the ability to account for rapid spatial velocity variations and to utilize all wavefield information. The LSRTM is an inversion method, which iteratively updates the reflectivity model by minimizing the modeled data and measured data. The proposed LSRTM-PSGD combines the advantages of stochastic gradient descent (SGD) and adaptive learning rate. The SGD updates the parameter on each transmitter and the fluctuation of SGD can enable it to reach a better minimum, thus improving the imaging quality. Compared with the conventional LSRTM algorithm using a fixed step size, the proposed LSRTM-PSGD algorithm can use the adaptive moment estimation to calculate the adaptive learning rate for the parameter, thereby updating the parameter appropriately. The performance of the LSRTM-PSGD algorithm is tested with experimental data. The results show high-quality reconstructed images with good resolution for defect identification in HDPE pipe materials, especially for deep defects.

Research on slow crack growth resistance of polyethylene pipe based on strain hardening modulus

AbstractSlow crack growth (SCG) is the principal factor for determining the operating lifetime of polyethylene (PE) pipes. Exploring the relationship between the molecular structure and the strain hardening (SH) modulus is helpful to evaluate the SCG resistance of PE pipe materials accurately. This paper uses an efficient SH test method to study the SH modulus for four kinds of PE pipes. Meanwhile, the correlation between SH modulus and sample thickness, material crystallinity (Xc), comonomer type and content, weight‐average molecular weight, and molecular weight distribution has been established and investigated respectively. The results represent that the comonomer type and the Xc of PE materials played significant roles in the SH modulus. The SH modulus of ethylene‐hexene copolymer is 5–7 MPa higher than that of ethylene‐butene copolymer. The amount in the range of 1 × 103 to 1 × 105 of the molecular weight distribution of PE materials has a significant contribution to the establishment of the tie molecular network. In addition, the higher Xc and the thinner thickness were beneficial to obtain the higher measured SH modulus.

Fabrication of anti-scaling HDPE/fluorinated acrylate polymer/nano-silica composite for landfill leachate piping system

Clogging generally happens to the leachate piping system, which poses a risk to the environment. A low surface energy nanocomposite is prepared to mitigate the cloggings, by adding the fluorinated acrylate polymer and hydrophobically modified nano-silica into high-density polyethylene (HDPE) substrate. The best addition of the fluorinated acrylate polymer and the nano-silica is given as 15% and 5%, to produce the composite with a low surface energy of 29.4 mJ/m2. Through the characterization of contact angle (CA), electrochemical corrosion, scanning electron microscopy (SEM) with energy dispersive X-ray spectrometer (EDS), atomic force microscope (AFM) and thermogravimetry (TG), the composite shows low wettability, good corrosion resistance and thermal stability. The surface hydrophobic property of the composite remains unchanged after being immersed in an acidic (pH = 2) and an alkaline (pH = 12) solution, indicating that the prepared composite has strong adaptability to the extreme environments. In addition, the composite shows better anti-scaling performance than that of the commercial high-density polyethylene (HDPE) and polyvinyl chloride (PVC) pipe materials by application of a dispensing leachate immersion test. The results provide insights into engineering practice for the design and manufacture of pipe materials for leachate collection and transport.

Research on Pressure bearing capacity of Defective Urban Gas Polyethylene Pipes

Polyethylene pipes have been widely used in the urban gas industry. However, there are few researches on the safety performance evaluation of polyethylene pipes in use at home and abroad, which makes it impossible to carry out Fitness-For-Service evaluation of defects found in the inspection, and thus cannot accurately judge the safety status of defective polyethylene pipes. In this paper, two typical polyethylene pipe materials, PE80 and PE100 are selected. Based on the tensile performance test, the constitutive model is established. Using the method of finite element simulation, the ultimate pressure bearing capacity of 9 defect models was analyzed and studied. And the relationship between defect size and pressure bearing capacity was initially determined. Provides a foundation for subsequent engineering applications.

Random propagation times for ultrasonics through polyethyilene

Low power ultrasonics are used for testing high density polyethylene pipe material. Attenuation and velocity give valuable information on the material in situ and without damages. In this paper we revisit recent data in the frequency band (4,10) megahertz. We prove that propagation is equivalent to random delays following stable probability laws. Moreover, the emergence of a companion noise non-detectable by devices is compliant with the law of conservation of energy.

A method of fracture toughness JIC measurement based on digital image correlation and acoustic emission technique

The accurate measurement of fracture toughness depends on the accurate determination of crack initiation. Our previous study demonstrates a relationship between the acoustic features and microstructure transition of polyethylene pipe material using acoustic emission (AE) technique. In this study, acoustic criterion for crack initiation of single-edge notched specimen is proposed, with digital image correlation (DIC) observation, crack initiation time is determined. A combination method of DIC and AE technique is proposed to measure the fracture toughness JIC of polyethylene pipe material. DIC technique is used to obtain the deformation on specimen surface to calculate J-integral according to the rigorous definition, and AE technique is used to monitor the internal damage and capture the moment of crack initiation synchronously. Numerical simulation by ABAQUS as well as literature comparisons is conducted to verify the testing results. It's shown that fracture toughness is independent of tensile speed within range of 0.01 mm/min ~ 1.00 mm/min, JIC obtained by DIC measurement and numerical simulation is about 19.88 kJ/m2 and 19.58 kJ/m2 respectively, the average relative error is about 5.70%. The research results prove the validity and accuracy of the proposed method and can provide a reference for the fracture toughness measurement for other engineering materials.

A lab study of mineral scale buildup on line and traditional PE water pipes for acid mine drainage

Plastic, especially polyethylene (PE), pipe material is increasingly used in mining applications due to its inert nature, flexibility, low density, and low cost. Though resistant to chemical corrosion, it is susceptible to abrasion. To combat this problem, an abrasion-resistant liner is in development. However, it is not yet known how the liner will perform with regards to other common problems that affect pipe systems, such as mineral scale buildup. In mining applications, scale buildup occurs due to the very high contents of suspended and dissolved solids in water or slurry. For example, in systems transporting raw or treated acid mine drainage (AMD), scale can form on pipe surfaces due to sedimentation or the diffusion of particles onto the surface, or precipitation of solids directly onto the surface. In this study, pipe-loop experiments were conducted in the laboratory under three idealized AMD treatment scenarios (i.e., untreated, passively treated and actively treated) to compare mineral scale buildup on traditional versus lined PE pipe materials.

Long-term mechanical performance of polyethylene pipe materials in presence of carbon black masterbatch with different carriers

In this paper, two types of carbon black (CB) masterbatch with different carriers, i.e. HDPE and LDPE, are used to produce black compounds using three PE100 resins with various short chain branching distributions. Due to difference in short chain branch (SCB) distribution, the used polyethylene resins behave differently in microstructure development and long-term creep behavior. The microstructural analysis using different DSC techniques and rheological measurements revealed more sensitivity of the polyethylene resin with uniform comonomer distribution to the carbon black aggregates and their polymeric carriers. The Full Notched Creep Test (FNCT) was performed to determine the long-term creep performance of the black compounds; it is shown that the sample having more uniform comonomer distribution is more resistant compared to other samples. On the other hand, by addition of carbon black masterbatch, resistance to slow crack growth in samples decreases since carbon black aggregates can act as stress concentration spots in the structure. However, with addition of the masterbatch with LDPE carrier polymer, the reduction of this value in samples is lower compared to one with HDPE carrier. The reason for this observation is that long branches of LDPE polymer enter the structure of lamellae in the PE100 resins, making them more coherent and increasing the number of tie molecules. The samples that are blended with LDPE polymer have a rougher surface, which means linkage between two sides of crack was stronger due to higher entanglement density in these samples. The impact test confirms the same trend as FNCT test, with the sample containing LDPE carrier having higher impact strength.

A Time Dependent Process Zone Model for Slow Crack Growth of Polyethylene Pipe Material

Polyethylene (PE) pipe is widely used in water supply and gas transportation engineering. Its fracture behavior is very important for the safe operation of the pipeline. The objective of this work is to simulate the slow crack growth (SCG) process of PE pipe material via a time dependent crack-tip process zone model. To achieve this end, the crazing damage caused by stress concentration at crack tip of PE material is taken into account, the stress on the bulk/craze interface layer is supposed to be uniformly distributed due to the micro necking mechanism of the layer, and a linear crack growth kinetics is used. In order to describe the delay characteristics of the fracture process of PE materials, the fracture surface energy is considered as an exponential decay function of time, which is attributed to the creep damage of the craze fibrils. A simulation algorithm of the proposed process zone model is suggested using iterative method. The model is verified by comparing the simulation with the discontinuous SCG behavior reported in the literature.

Simplified Equations and Ansys Simulation of Head Loss on Nonlinear (Sliced) Bend for Piping Network

The head loss in the nonlinear bend caused by the friction of the wall along the bend and the direction change of the flow due to suddenly angle change. The essential elements include velocity (U) number of slices (n), length of the average slices wall (Li), angle change (α), friction coefficient (f), acceleration of gravity (g), and slope of the pipe base (S). Based on equation 12, with fixed discharge, the bigger diameter, and more number of slices in used, then the smaller head loss. Conversely, if the diameter used is getting smaller and less the number of slices, then the head loss bigger. This analysis is expected to provide optimal benefits for activities related design to piping networks, especially main networks that use large diameter pipes. Especially piping networks with the use of steel pipe or High-Density Polyethylene pipe materials. Such as oil pipes, gas pipes, and freshwater distribution pipes. The head loss that occurs will more measurable, fast, easy, and economical in implementation.

Correlation between isothermal crystallization properties and slow crack growth resistance of polyethylene pipe materials

In this study, three PE100 polyethylene materials with fairly different short chain branch distributions were used to characterize the relation between thermal properties and creep test failure time. The samples were thermally characterized using isothermal and non-isothermal differential scanning calorimetry (DSC). The three resins showed different behavior after fitting on the Avrami equation. N-100J2 sample, in which short chain branches (SCBs) are located on longer molecule chains, has a lower chain mobility and higher crystallization time, while N-100J1 has the opposite crystallization properties. Also, these samples had different Avrami index, n. Microstructural evaluation of these samples revealed that the Avrami index had a direct relationship with the morphology of samples and the higher n correlates to formation of more spherulite structures. The creep test result showed that the sample with a lower crystallization rate and lower Avrami index was more resistant against slow crack growth (SCG) because of its crystalline morphology with the obstacles in crack growth. There is a meaningful relationship between lamellae arrangement and failure time; when a crystalline segment in resin has more spherulite structures, higher Avrami index, the boundary of these spherulites may act as weak regions in front of crack and consequently lead to easier crack growth in such material.

Accelerated aging of polyethylene pipe grades in aqueous chlorine dioxide at constant concentration

The impact of disinfected water on the degradation of polyethylene (PE) was investigated with immersion tests of two PE pipe materials in 10 and 5 ppm chlorine dioxide (ClO2) medium at 60 °C. Aging experiments in 1 ppm ClO2 at 60, 50 and 40 °C were also carried out to study the effect of different aging temperatures. During conditioning, the pH was kept at 6.8. A specific exposure device with continuous concentration control and adjustment has been applied in order to generate reliable and reproducible aging conditions. Sample characterization applying scanning electron microscopy (SEM), tensile test, dynamic oxidation tests as well as FTIR-spectroscopy indicated accelerated antioxidant consumption and polymer degradation. Material aging at 50 °C and above was found to be much faster than at 40 °C applying 1 ppm ClO2 concentration. An optimized testing condition for fast material characterization in case of 1 mm thick specimens was found to be a concentration of 1 ppm ClO2 at 50 °C. The simultaneously increasing material embrittlement and the consumption of active antioxidant molecules imply an apparent unselective reaction of ClO2 with the polymer and the stabilizers molecules. Moreover, the radical nature and the high reactivity of ClO2 led to the formation of carbon-chlorine species, which are assumed to originate from degraded antioxidant molecules.

Viscoelastic and Damage Model of Polyethylene Pipe Material for Slow Crack Growth Analysis

Polyethylene (PE) pipe is widely used for oil and gas transportation. Slow crack growth (SCG) is the main failure mechanism of PE pipes. Current SCG resistance testing methods for PE pipes have significant drawbacks, including high cost, time-consuming, and uncertain reliability. Alternative method is in need to reduce the testing time and cost. In this paper, a numerical model is proposed by taking the viscoelastic and damage effect of PE material into account. The material behavior is described on the basis of linear viscoelastic integral constitutive model, along with the damage effect in effective configuration concept. A three-dimensional (3D) incremental form of a viscoelastic and damage model is derived and implemented by abaqus UMAT. It is found that the curve of tensile displacement versus time, as well as the curve of crack opening displacement (COD) versus time from numerical results fit well with those from the standard Pennsylvania Notch Test (PENT; ASTM 1473). Based on the proposed model, SCG failure process is analyzed, and the effects of damage parameters on SCG process are furtherly studied and discussed.

Correlation of molecular parameters, strain hardening modulus and cyclic fatigue test performances of polyethylene materials for pressure pipe applications

Currently, several testing methods are under development to understand the resistance of polyethylene pipe materials to slow crack growth over comparably short time periods without using aggressive chemicals to accelerate the time to brittle failure. Strain hardening and crack round bar tests have recently been developed and published as ISO testing methods. However, a better understanding of these testing methods is still required with respect to the molecular parameters of the materials. Comparative studies with existing slow crack growth testing methods such as the notched pipe test are of significant interest to the industry. This study discusses correlations of molecular weight, molecular weight distribution, short chain branching and rheological properties of different polyethylene materials with their slow rack growth resistances obtained from the strain hardening and crack round bar tests and their correlations with notched pipe tests.

Reactive antioxidants for peroxide crosslinked polyethylene

Three synthesised reactive (graftable) antioxidants, r-AO, with hindered phenol and hindered amine antioxidant functions, were examined for their grafting efficiency in polyethylene and their retention and stabilising performance in peroxide-crosslinked polyethylene pipe material. Their level of grafting in un-crosslinked high density polyethylene, HDPE, and the extent of their retention in highly peroxide-crosslinked HDPE (PEXa) material (both laboratory prepared samples and PEXa pipes produced by commercial processes) were determined after stringent solvent extraction methodologies. The uniformity of distribution of the r-AOs in the PEXa pipes (across the length and thickness of the pipes) were examined using infrared-microscopy and compared with similarly produced PEXa pipes containing commercial antioxidants having the same antioxidant functions. The extent of interference of the graftable hindered phenol and hindered amine antioxidants with the peroxide crosslinking process in the PEXa materials was assessed by comparing the level of crosslinking achieved against analogously prepared samples containing a conventional hindered phenol antioxidant (Irganox 1076).The results obtained showed that the presence of the graftable (r-AOs) antioxidants did not affect the level of crosslinking of the PEXa pipes which remained high (>85%) and that they were retained to a very high extent in the PEXa material after solvent extraction, and are very uniformly distributed across the length and thickness of PEXa pipes. The long-term stabilising performance of the graftable r-AOs in the PEXa material (in both laboratory prepared samples and in peroxide crosslinked pipes produced by commercial processes) was also examined using complimentary methods: oxidative induction time (OIT) before and after solvent extraction, physical embrittlement following oven ageing at 125 °C, as well as by a hydrostatic pressure tests conducted (with water inside and air outside) at 115 °C and 2.5 MPa pressure. The stabilising efficacy of the r-AOs was compared with that of conventionally stabilised PEXa material containing analogous antioxidant functions produced in the same way. The level of retention (in the pipes) of the graftable antioxidants and their long-term stabilising performance was shown to be significantly higher than that of the conventional AOs under all the test conditions used here.

Machining of tough polyethylene pipe material: surface roughness and cutting temperature optimization

Manufacturing of high-density polyethylene (HDPE) pipes is usually achieved by extrusion processes. However, various joining or fitted parts and mechanical testing samples are prepared by material removal methods. This study focuses on the machinability of the HDPE tough resin used for piping and fittings to make standard test specimens. The experimental plan has been conducted using a Taguchi (L27) orthogonal array. Input machining parameters are cutting speed (V c), feed rate (f), and depth of cut (a p) while output effects are represented by conventional surface roughness criteria (Ra, Rt, and Rz) and the measured cutting temperature (t°). As a result, a second-order model was established between input and output parameters via the response surface methodology (RSM). The development of predictive models is essential for machining of extruded HDPE since few experimental trends and data are available in literature as compared to metals. Optimum cutting conditions are determined using the desirability function approach. It is revealed that the feed rate (f) is the main contributing factor when minimizing surface roughness of HDPE material. The analysis of variance (ANOVA) results showed that the contributions for surface roughness criteria (Ra, Rt, and Rz) are 96.11, 86.92, and 92.22% respectively. On the other hand, cutting temperature is mainly influenced by cutting speed (V c), depth of cut (a p), and the three interactions (V c × f), (V c × a p), and (f × a p). Finally, optimized cutting temperature is discussed for the three main cases, which are lower, targeted, and upper values, i.e., t° minimum, t° targeted = 30 °C, and t° maximum allowable = 32 °C.