High Density Polyethylene Pipe Research Articles

Improvement of butt-fusion welding procedure and performance evaluation method for thick-walled bimodal polyethylene pipes

High-density polyethylene (HDPE) pipes have been continuously improved by the progress made by the resin industry and pipe manufacturers. Together with the advanced technology of welding equipment in terms of structure reliability and automations, HDPE pipes have been gradually applied to higher pressure level, bigger diameter, and thicker walls in operations which have become the preferred piping system for nuclear safety application in transporting cooling seawater. There are substantial differences in parameter settings when applying heat and force during different welding procedures, as well as the complicated evaluation methods for mechanical properties, which causes communication barriers between engineers and researchers. Thus, it is necessary to comprehensively study the welding procedures and evaluation methods to standardize the evaluation protocols in the industry. In this paper, based on the assumptions of temperature effect and molecular chain diffusion dynamics at the welding interface, the optimization of the welding procedure is proposed. Some well-established evaluation methods have been used for performance assessment and their feasibilities are discussed. The idea of creating crack along the weld interface is adopted as new method to evaluate the material ability of fracture resistance. Specimens with unilateral notch and weld interface crack are proposed and subjected for quasi-static axial tensile test. The fracture stress and fracture energy at the welding interface are used to differentiate the quality of the joint from different welding procedures. The fracture morphologies of the weld interface are analyzed. A complete picture is drawn for the mechanical fracture resistance of the weld interface. The method used in this paper can be corelated to the pipeline design conditions, therefore could be applied to the real-time engineering applications. Moreover, the evaluation method proposed in this paper is simple, reliable, and easy to implement.

Comparative study on the performance of butt fusion-welding processes for nuclear safety class large-diameter thick-walled PE pipes

New technologies in polymer synthesis and pipe extrusion equipment have led to the commercialization of high-performance, large-diameter, thick-wall high density polyethylene (HDPE) pipes. They have been used in the field of seawater transport and cooling to replace metal pipes, due to their advantages of high corrosion resistance and extensibility. Connection of HDPE pipe is important as it determines the safety of the entire piping system. Butt fusion welding is commonly used for HDPE pipe connection but may cause the formation of weak points in the welded joints, interfering the reliability of the pipeline system in the application of nuclear power plants. At present, there is a lack of research on evaluating the performance of welded joint for large-diameter thick-wall HDPE pipes made by butt fusion-welding. The purpose of this study is to investigate the influence of three different butt fusion-welding processes, i.e., single low pressure (SLP), single high pressure (SHP) and dual low pressure (DLP), by evaluating the performance of their welded joints, including characterizing tensile strength, extensibility, crystallinity and hardness. In specific, a thick-wall HDPE pipe with OD of 812.8 mm and wall thickness of 74 mm which is certified for nuclear safety class was used for study. Representative specimen from the outer, middle and inner part across the wall of the main pipe body and welded joints were taken for testing. Different test methods and specimens were designed to assess the feasibility of evaluating the welding performance from different welding process. The results showed that the mechanical properties of different locations of the welded joints were different, and the tensile strength and fracture energy of the middle part of the joint were lower than that of the inner and outer parts, which could be caused by the difference in the crystallinity and thickness of the melting zone influenced by welding processes, as can be seen from the analysis of DSC test and morphology observation. Hardness testing was conducted on the section of the welded joints, and it revealed that the micromechanical properties of the welded joints in the region of the heat-affected zone were enhanced significantly, which may be due to the annealing effect caused by welding process. In summary, The DLP process resulted in the best extensibility of the welded joints among three processes, suggesting that the joining pressure from welding process plays an important role in affecting the extensibility of the welded joints.

Effect of carbon black incorporation methods on the initial creep/slow crack growth resistance and properties of high-density polyethylene (HDPE) pipes

The increasing utilization of polyethylene (PE) pipes in diverse industries has driven a focused exploration into the properties and behaviors of PE materials. In response to this heightened interest, this study aims to investigate the impact of different carbon black (CB) incorporation methods on the properties of PE100 polyethylene, particularly its resistance to slow crack growth (SCG). Two distinct methods, direct incorporation and utilization of a commercial masterbatch, were employed to introduce CB into the PE100 matrix. Results revealed that direct incorporation led to superior dispersion of CB, thereby enhancing the mechanical properties of the resultant compound. Furthermore, SCG resistance exhibited improvement when CB was directly incorporated, contrasting with a degradation in SCG resistance observed when a CB masterbatch was used compared to neat PE. Comprehensive rheological and thermal analyses were conducted to elucidate these findings. The study concludes that the presence of low molecular weight substances and inadequate CB dispersion within the CB masterbatch significantly contributed to the deteriorated SCG resistance observed in PE100 material.

DEVELOPMENT OF FRICTION STIR WELDING TOOL FOR HIGH-DENSITY POLY-ETHYLENE (HDPE)–CASE STUDY: FIBERGLASS COMPOSITE MATERIAL

This study aims to develop an effective Friction Stir Welding (FSW) method for composite material of High-Density Poly-Ethylene (HDPE) Pipes. The development of welding tool, there was the addition of an external heating source on the shoulder and probe/pin to overcome the problem of lack of heat resulting from friction between the tool and the material to be welded. The case study was conducted to join the short fiberglass-HDPE composite with a type of ratio of 30% by weight of short fibre and 70% by weight of HDPE, which optimizing parameters such as rotating speed, welding speed, and preheating temperature. The FSW joining process for short fiberglass-HDPE composite sheets was carried out using a Fanuc Series 21i-M CNC milling machine as the driving tool with rotational speed (ω) varied in 3 conditions, namely 600 rpm, 800 rpm and 1000 rpm, and welding speed (v) or feeding at 5 mm/minutes and 10 mm/minutes. The temperature was controlled according to the liquid point of High-Density Poly-Ethylene, which was 130oC, and raised to 150oC and 170oC. The 12 pieces of thermocouple were used along the track on the material and jig plates at the top, middle and bottom. Then, the results of joining the sheets were made in the form of specimens with sizes according to ASTM 3039. The tensile tests of the specimens were carried out at a rate of 0.01 mm/s. The results showed the highest tensile strength was an average value of 24.52 MPa at a rotational speed of 800 rpm, the feeding of 5 mm/min and the temperature of 130oC. The lowest tensile strength was an average value of 17.54 MPa at a temperature of 170°C with a speed of 600 rpm.

Fatigue crack growth and slow crack growth of HDPE pipes under internal pressure and flat plate compression

High-Density Polyethylene (HDPE) pipes are widely used in city gas and water systems as buried pipelines for their excellent performance. However, in addition to the internal pressure, it also suffers loads from traffic vehicles, foundation settlements, and illegal buildings. In this paper, fatigue crack propagation of HDPE pipes with a longitudinal external crack was experimentally studied under different load ratios in two cases. Pulsatile internal pressure was implemented in the first case. In the other one, pulsatile flat plate compression and constant internal pressure were applied. The crack mouth opening displacements were recorded by the digital image correction technique. The crack depth was obtained with compliance calibration. It shows that the fatigue crack growth rate decreases as the load ratio increases. In both cases, the slow crack growth kinetics were gained by extrapolating the load ratio to 1.0, representing constant internal pressure or flat plate compression.

Exciting and Detecting Higher-Order Guided Lamb Wave Modes in High-Density Polyethylene Structures Using Ultrasonic Methods.

High-density polyethylene (HDPE) pipes are becoming increasingly popular, being used in various fields, such as construction, marine, petroleum, water transfer, process water, methane gas collection, oil and gas gathering, gas distribution systems, mining, acid and wet gas lines, offshore oil and gas and in nuclear power plants. Higher-order guided Lamb wave (UGW) modes can be used to detect various defects in complex structures. We will apply this methodology to one of the types of plastic-the structure of high-density polyethylene (HDPE). However, the excitation of UGW modes faces numerous challenges, especially when there is a need to identify which mode is excited. It is essential to note that, in the higher frequency range, multiple different higher-order modes can usually be excited. This can make it difficult to determine which modes have actually been excited. The objective of this research was to successfully excite and receive various higher-order UGW modes in high-density polyethylene structures using both ultrasonic single-element transducers and a phased array. Theoretical calculations were performed using a variety of methods: semi-analytical finite element (SAFE) method, 2D spatial-temporal spectrum analysis and finite element modeling (FEM). The results obtained from both measurements and simulations clearly demonstrate the possibility of efficiently exciting and receiving different Lamb wave modes possessing different phase velocities.

A study on pipe failure analysis in water distribution systems using logistic regression

Abstract This study presents a pipe failure analysis that aims to identify key factors affecting pipe failure in a pilot study area (PSA). For this purpose, pipe failures that occurred in the PSA were recorded for 2 years. Recorded pipe failures were integrated into the existing geographical information systems of the PSA and then visualized. Then, the logistic regression model that calculates failure probability with unbalanced and small- and medium-sized data was performed to analyze pipe failures. Pipe material, age, diameter, burst pressure, and average water pressure are the factors used in the study. The developed model achieved a success rate of 70.1%, suggesting that it can predict a considerable portion of pipe failures with a relatively satisfactory performance. Findings show that the failure probability of pipes with an age greater than 20 years is 1.675 times higher than that of pipes with an age less than 20 years. Moreover, polyvinyl chloride pipes are 5.265 times and high-density polyethylene pipes are 9.027 times more likely to fail than ductile pipes. Further research should delve into the impacts of the number of service connections, pipe lengths, and traffic load on pipe failures.

Comparison of the thermal and hydraulic performance of single U-tube, double U-tube and coaxial medium-to-deep borehole heat exchangers

Decarbonisation of heat is essential in curbing carbon dioxide emissions and can be achieved through the use of geothermal systems. Recently, single-well, closed-loop, deep borehole heat exchangers, using a coaxial design, have become the focus of attention, partly due to the potential to repurpose existing infrastructure (such as oil and gas wells); however, few have investigated the potential for other types of heat exchanger for middle-deep geothermal systems. Therefore, in this study, a comprehensive numerical analysis was undertaken using OpenGeoSys software to investigate the thermal and hydraulic performance of coaxial, single U-tube and double U-tube middle-deep borehole heat exchangers (MDBHEs). The purpose of this paper is to test the maximum operational depth for each type of pipe configuration as few wells have been completed to depths exceeding 500 m using single/double U-tube configurations. The best performing MDBHEs should minimise parasitic and hydraulic losses, whilst maximising thermal output. Furthermore, ground sourced heat pumps require electricity; therefore, at times where electricity prices are high (and drilling costs can be minimised) it may be more beneficial to utilise MDBHEs to encounter greater temperatures.Results indicate that coaxial MDBHEs provide the best performance in terms of specific heat extraction and lowest pressure/parasitic losses. Double U-tube MDBHEs can provide a similar thermal performance to the coaxial design, but have significantly greater hydraulic pressure losses across all simulations, which translates to greater parasitic pumping power costs. Single U-tube MDBHEs demonstrate the poorest performance in terms of heat extraction and pressure losses. At the end of the 25-year base case scenario for a 800 m MDBHE, coaxial, U-tube and double U-tube configurations, all with a fluid circulation rate of 5 L/s, provided specific heat extraction rates of 39.1 W/m, 32.8 W/m, and 36.0 W/m, respectively, with the fluid inlet temperature set as a constant of 5 °C. For these simulations, pressure losses were estimated as 85 kPa (coaxial), 1.46 MPa (single U-tube) and 423 kPa (double U-tube)—the single U-tube value being close to the nominal 16 bar (1.6 Mpa) pressure rating of SDR11 high density polyethylene pipe. Further parametric analysis was also undertaken, investigating depth, flow rate, rock thermal conductivity, pipe diameter and shank spacing.

Experimental investigation into the effects of voids on the response of buried flexible pipes subjected to incrementally increasing cyclic loading

In this study, large-scale fully instrumented laboratory tests were conducted to investigate the behaviour of buried flexible high-density polyethylene (HDPE) pipes, in sand beds with and without voids subjected to incrementally increasing cyclic loading. Voids with a predetermined size were created at one side of the spring-lines of the pipes, which were buried at variable depths, H, of 1.5, 2.0 and 2.5 times the diameter of the pipe, D. Results showed that increasing the pipe burial depth, H/D, contributed to decreasing the settlement of the footing, deformation of the pipe crown and invert, lateral displacement of the spring-line, and the stress and strain generated along the pipe crown and invert. Void presence led to a significant increase in the footing settlement, which ranged from 3 % up to 18 %, according to H/D. Furthermore, void presence led to a sharp increase in the crown, invert, and spring-line settlements, which ranged from 34 % to 52 %, 10 %–12.5 %, and 13 %–38 %, respectively. Increasing pipe burial depth was found to be highly effective in protecting buried pipes, minimising inevitable consequences of the presence of voids. However, this was combined with an increase in the pressure at the pipe spring-line that led to a positive horizontal support at the pipe's spring-lines resulting in reducing pipe deformation.

Cooling underground substations worldwide using heat pumps

The use of underground space in urban environments has increased at an accelerated rate over the past few years, particularly to satisfy transport infrastructure needs. Transport tunnels are being constructed at increasingly greater depths in the already congested subsurface. While transport tunnels have demonstrated significant potential as sustainable heat sources for heating under - and above-ground spaces, due to the large area in contact with the ground [4,5], cooling the substations that serve the tunnels, containing plants and machinery, is a task that poses a significant economic and environmental challenge. To tackle this, a new method that takes advantage of airflow in the tunnels and the thermal mass of the ground surrounding the tunnels has been recently introduced [2]. Similar to energy tunnels [1], these systems use water-filled high-density polyethylene (HDPE) pipes integrated into the tunnel space to exchange heat, however using the pipes to reject heat from the substations to the tunnels. This work adopts four cities, namely Sydney in Australia, Guangzhou in China, London in the UK, and Stuttgart in Germany, and investigates the suitability of this approach to cool substations for different meteorological and geological conditions around the world. Moreover, varying heat exchanger lengths and tunnel air temperatures are incorporated in the analysis, to assess the impact of these key parameters on system efficiency and performance.
 A detailed 3D finite element heat and mass transport model is utilised in this work, simulating the tunnel lining, surrounding soil, the airflow within the tunnel, the pipes attached to the tunnel linings (tunnel air side), and the circulating fluid within the pipes [2]. The location farfield temperature and material properties are shown in Table 1, for the soil conditions in the four locations and for the common materials (shaded in grey). A parametric analysis is undertaken to investigate the performance of these systems under varying conditions. For each of the locations, five different pipe leg length values are used: 50 m, 100 m, 150 m, 200 m, and 300 m and the air velocity is assumed as 0.5 m/s, typical of these tunnels. In addition, two different distributions for the temperature of the air entering the tunnel are used: the estimated tunnel air temperature () distributions based on available literature and measurements [3, 6, 7], as well as the surface air temperatures (), both shown in Figure 1-left. The latter is used to understand the impact of natural ventilation of tunnels on the system, which could inform the placement of the pipes with respect to the positions of ventilating shafts along the tunnel length.
 The results are shown in Figure 1-right, in terms of the cooling thermal power that a single pipe loop can provide (total pipe loop length being ), when operating constantly over 20 years – to replicate the real conditions in cooling substations. These values are obtained such that the temperature of the fluid does not reduce below 5 °C, such that the heat pump efficiencies remain high. The locations are shown based on the line colour and the air temperature distribution used for each location is represented by the line type (solid:, dashed:). The results show that the cooling provided can vary significantly based on the investigated parameters and suggest that the temperature of the air flowing in the tunnel is very influential to the system performance. The most favourable outcome is achieved for Stuttgart (green), which has the lowest tunnel air and farfield temperature, and the least favourable for Guangzhou (red), which has the higher air and farfield temperature. Comparing the two air temperature distributions used (solid vs dashed), using instead of , which is lower in all cases, increases the performance of the system. This is most obvious for the case of London, with an increase between 2.5 kW and 5 kW, likely due to the low value of the farfield temperature as opposed to the significantly high tunnel air temperatures (). The length of the pipes used in all cases increases the amount of energy, however that increase is logarithmic in nature and therefore after about 150 m to 200 m the investment might not be justified. Overall, this work shows that there is significant potential to utilise heat pump technologies to cool tunnel substations, especially in cooler climates such as central-northern Europe.

Full-scale investigation of the short-term continuous and intermit-tent operation of an energy piled wall section

The growing demand for highly efficient renewable energy technologies has positioned shallow geothermal energy as an attractive alternative. The initial high capital cost of traditional ground heat exchangers (GHEs), mainly associated with drilling, has extensively hindered their implementation. Conventional geostructures overcome this drawback, and their use as GHEs is getting more attention. Piles [4, 8, 10], pavements [7], and walls [1, 5] are some of the structures that are being used. Energy piles have been the most popular among them, and their implementation has increased steadily since the 1980's [5]. After more than two decades of intensive research, a large dataset of full-scale energy pile foundation observations is available [6], and a thermo-mechanical conceptual framework [2, 3] has been formulated.
 Piles are not limited to foundations; they are used in retaining walls. In contrast to foundations, retaining walls mainly work under lateral stress conditions instead of axial loads. In principle, energy piled retaining wall's initial state, circuits configuration and boundary conditions control their response. Nevertheless, in the context of (energy) piles, observations appear limited to a few full-scale thermo-mechanical studies to date [1, 5]. More attention to this energy geostructure from the scientific and engineering practice communities is required, and the documentation of additional case studies is needed to promote the energy piled walls implementation more broadly.
 In this work, a series of full-scale thermo-mechanical field testing has been performed in an energy piled wall section. The retaining wall system section consists of three piles equipped with pipes to work as heat exchangers. Each houses 4 U-loops in series made of high-density polyethylene (HDPE) pipe with an outer diameter of 25 mm and HDR of 11. The pipe circuits are connected in parallel through a common header manifold operated on the surface. Two of these piles have been instrumented with 10 pairs of vibrating wire rebar strainmeters with temperature sensors located on diametrically opposing sides of the pile. One is close to the excavation, whereas the other is to the ground side - see more details in [10].
 The thermal activation of the wall section occurred at 34.7 m depth and was achieved through an in-situ Thermal Response Test (TRT) unit. The unit heats a carrier fluid through constantly powered electrical heating components and pumps the fluid in the pipe loops within the GHEs. To emulate a conventional thermal load (e.g., energy piles 30-70 W/m, [9]), the entire wall section is supplied with a heating power of 5.5 kW over 5 days active period, and the thermal load is provided under either continuous or intermittent conditions (i.e., 5 thermal cycles, 12 hours on/off). Five recovery days allowed the ground to return partially to its initial thermal equilibrium. The testing period lasted 26 days of constant monitoring.
 The obtained measurements in one pile are presented in Figure 1. It compares the measured strains at different construction stages (i.e., excavation to 32.5, 36.4, and 34.7 m depths) against those observed at the end of both operation modes (i.e., 5 days, 5th cycle). It is noticed that the temperature-induced strains recorded by the available sensors (i.e., 11/20 in this case) are, in most cases, within the axial strain's envelope derived from previous excavation and construction stages. Only one sensor at 33 m indicates tensile axial strains larger than those observed at different excavation depths; nevertheless, it represents a minor deviation compared with the increment recorded at the same depth during the last excavation and anchors removal. Based on these observations and assuming a thermoelastic response, thermally induced axial stresses remain within the design ranges delimited by design envelopes.

Lead (Pb) deposition onto new and biofilm-laden potable water pipes

Heavy metals' interactions with plumbing materials are complicated due to the differential formation of biofilms within pipes that can modulate, transform, and/or sequester heavy metals. This research aims to elucidate the mechanistic role of biofilm presence on Lead (Pb) accumulation onto crosslinked polyethylene (PEX-A), high-density polyethylene (HDPE), and copper potable water pipes. For this purpose, biofilms were grown on new pipes for three months. Five-day Pb exposure experiments were conducted to examine the kinetics of Pb accumulation onto the new and biofilm-laden pipes. Additionally, the influence of Pb initial concentration on the rate of its accumulation onto the pipes was examined. The results revealed greater biofilm biomass on the PEX-A pipes compared to the copper and HDPE pipes. More negative zeta potential was found for the biofilm-laden plastic pipes compared to the new plastic pipes. After five days of Pb exposure under stagnant conditions, the biofilm-laden PEX-A (980 μg m−2) and HDPE (1170 μg m−2) pipes accumulated more than three times the Pb surface loading compared to the new PEX-A (265 μg m−2) and HDPE pipes (329 μg m−2), respectively. However, under flow conditions, Pb accumulation on biofilm-laden plastic pipes was lower than on the new pipes. Moreover, with increasing the initial Pb concentration, greater rates of Pb surface accumulation were found for the biofilm-laden pipes compared to the new pipes under stagnant conditions. First-order kinetics model best described the Pb accumulation onto both new and biofilm-laden water pipes under both stagnant and flow conditions.

Effects of water chemistry and flow on lead release from plastic pipes versus copper pipes, implications for plumbing decontamination

Despite being corrosion-resistant, plastic potable water pipes might accumulate heavy metals on their surface if they convey metal-contaminated tap water. This study examined the influence of water pH and flow conditions on lead (Pb) release from new and biofilm-laden potable water pipes to provide insights regarding decontamination. For this purpose, biofilms were grown onto new crosslinked polyethylene (PEX-A), high-density polyethylene (HDPE), and copper pipes for three months. Lead was then deposited onto the new and biofilm-laden pipes through 5 d exposure experiments under flow conditions. After that, lead release experiments were conducted by exposing the lead-accumulated pipes to lead-free synthetic tap water for 5 d, under both stagnant and water flow conditions. The metal accumulation study showed no significant difference in lead uptake by new pipes and their biofilm-laden counterparts under flow conditions. This could be attributed to the detachment of biofilms that have accumulated lead as water flows through the pipes. Water flow conditions significantly influenced the lead release from new and biofilm-laden water pipes. A lower water pH of 5.0 increased the release of lead from plastic pipes into the contact water, compared to pH 6.0 and 7.8. The greatest percentage of lead was released from biofilm-laden HDPE pipes (5.3%, 120 h) compared to biofilm-laden copper pipes (3.9%, 6 h) and PEX-A (3.7%, 120 h) and after exposure to lead-free synthetic tap water at pH 5.0, under stagnant conditions. On the other hand, under water flow conditions, the greatest lead release was found for new PEX-A pipes (4.4%, 120 h), new HDPE pipes (2.7%, 120 h), and biofilm-laden copper pipes (3.7%, 2 h).

Optimization of Thermal Conductivity and Tensile Properties of High-Density Polyethylene by Addition of Expanded Graphite and Boron Nitride.

Due to its mechanical, rheological, and chemical properties, high-density polyethylene (HDPE) is commonly used as a material for producing the pipes for transport of various media. Low thermal conductivity (0.4 W/mK) narrows down the usage of HDPE in the heat exchanger systems. The main goal of the work is to reduce the vertical depth of the HDPE pipe buried in the borehole by increasing the thermal conductivity of the material. This property can be improved by adding certain additives to the pure HDPE matrix. Composites made of HDPE with metallic and non-metallic additives show increased thermal conductivity several times compared to the thermal conductivity of pure HDPE. Those additives affect the mechanical properties too, by enhancing or degrading them. In this research, the thermal conductivity and tensile properties of composite made of HDPE matrix and two types of additives, expanded graphite (EG) and boron nitride (BN), were tested. Micro-sized particles of EG and two different sizes of BN particles, micro and nano, were used to produce composite. The objective behind utilizing composite materials featuring dual additives is twofold: firstly, to enhance thermal properties, and secondly, to improve mechanical properties when compared with the pure HDPE. As anticipated, the thermal conductivity of the composites exhibited an eightfold rise in comparison to the pure HDPE. The tensile modulus experienced augmentation across all variations of additive ratios within the composites, albeit with a marginal reduction in tensile strength. This implies that the composite retains a value similar to pure HDPE in terms of tensile strength. Apart from the enhancement observed in all the aforementioned properties, the most significant downside of these composites pertains to their strain at yield, which experienced a reduction, declining from the initial 8.5% found in pure HDPE to a range spanning from 6.6% to 1.8%, dependent upon the specific additive ratios and the size of the BN particles.

A Lifetime Prediction Method of Pressured Gas Polyethylene Pipes by Strain-Hardening Modulus and Tensile Test

In recent years, polyethylene (PE) pipes have been widely utilized for urban natural gas transmission. However, as the use of high-density polyethylene (HDPE) pipes increases, their service life and long-term performance assessment have become one of the most significant issues to be addressed. There has been a ton of studies on PE pipe life prediction techniques both domestically and internationally, but very little has been carried out on PE pipe life prediction in actual gas working environments with varying acid and alkaline levels. This experiment accelerates the aging of PE pipes using acid and alkaline corrosion immersion experiments to determine the lifespan of PE pipes. This study aims to investigate the performance changes of HDPE under strong, weak, and neutral corrosion conditions using corrosion solutions with PH values of 1, 5, and 8, to propose the impact of corrosion caused by various acids and alkalies on the HDPE aging life for natural gas, and to develop a mathematical model between the aging life of polyethylene and the PH values of acid and alkali corrosion solutions. The studies involved soaking and corroding HDPE pipes with various acidity and alkalinity chemicals to speed up the aging process, and then the tensile test was used to determine the mechanical characteristics of the aged PE pipes. Based on our findings, the empirical equation between acidity and service life of PE pipes is obtained by the mathematical fitting method, and a life prediction model of buried city gas HDPE pipes is proposed. The actual life of the aged pipes is determined by the relationship between strain-hardening (SH) modulus and aging time. The findings demonstrate that the service life of PE pipes changes with different levels of acidity and alkalinity: 1.872 days, 1060.507 days, and 1128.58 days following corrosive solution-accelerated aging with solution acidities of PH1, PH5, and PH8, respectively. The life prediction method applies to various plastic pipes in comparable environments as well as HDPE city gas pipes that are subject to acid and alkali corrosion forces.

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.

Evaluation of the seismic performance of butt-fusion joint in large diameter polyethylene pipelines by full-scale shaking table test

High-density polyethylene (HDPE) pipelines in nuclear power plants (NPPs) have to meet high requirements for seismic performance. HDPE pipes have been proved to have good seismic performance, but joints are the weak links in the pipelines, and pipeline failures usually initiate from the defects inside the joints. Limited data are available on the seismic performance of butt-fusion joints of HDPE pipelines in NPPs, especially in terms of defects changes inside the joints after earthquakes. In this paper, full-scale shaking table tests were performed on a test section of suspended HDPE pipelines in an NPP, which included straight pipes, elbows, and 10 butt-fusion joints. During the tests, the seismic load-induced strain of the joints was analyzed by strain gauges, and it was much smaller than the internal pressure and self-weight-induced strain. Before and after the shaking table tests, phased array ultrasonic testing (PA-UT) was conducted to detect defects inside the joints. The locations, numbers, and dimensions of the defects were analyzed. It was found that defects were more likely to occur in elbows joints. No new defect was observed after the shaking table tests, and the defects showed no significant growth, indicating the satisfactory seismic performance of the butt-fusion joints.

Analysis of the harmfulness of the water hammer phenomenon in high density poly-ethylene (HDPE) pipes

In this numerical study, the three-dimensional finite-element method is used to analyze the degree of harmfulness of the water hammer phenomenon in high-density polyethylene (HDPE) pipes damaged by cracking. The fracture criterion used in this study is the integral J, which was calculated in semi-elliptical cracks in four different positions. The effect of the flow rate and crack configuration on the J-integral is analyses. The pipe section was constructed simulating a commercially available model. A 3D finite-element analysis was performed in Warp Software. All materials used in the models were considered to be isotropic, homogeneous, and linearly elastic. The results obtained show that an increase in pressure caused by the water hammer phenomenon leads to a remarkable increase in the J-integral. It is also shown that an internal longitudinal crack in a pipe is the most dangerous. It can also be seen that an increase in the flow velocity leads to an increase in the J-integral.

Induced trench installation of the high-density polyethylene pipe using geofoam inclusion

The effects of induced trench configuration and stiffness of compressible inclusion on the high-density polyethylene (HDPE) pipe behavior were investigated through full-scale laboratory tests. Two pipe-compressible inclusion configurations (‘compressible inclusion over the pipe crown’ and ‘the pipe covered with compressible inclusion’) were tested and expanded polystyrene (EPS) Geofoam with 10 and 15 kg/m3 nominal density was used as compressible inclusion. To simulate geostatic stresses imposed by high embankment fill, the surcharge stress up to 200 kPa was applied on the surface of the burial medium. Comprehensive instrumentation was implemented to measure the pipe deflections, soil stresses on the pipe, and soil settlements in the pipe zone. Considering the pipe behavior and cost-efficiency together, the configuration in which one EPS geofoam panel with 10 kg/m3 nominal density is placed over the pipe crown arises as the optimal solution for the induced trench HDPE pipe. This solution provided a reduction in the vertical stress at the pipe crown of up to of 76% and in the horizontal stress at the pipe springline of up to 65%. The vertical and horizontal pipe deflections are reduced by 87% and 60%, respectively, under 200-kPa surcharge stress. – that is, overburden pressure imposed by a 10-m-high embankment fill.

Dynamic Response Analysis of Buried HDPE Pipes under Vibration Compaction Considering the Influence of Buried Depth and Filling Modulus

During pavement construction, the gravity load and vibration excitation from vibratory rollers can seriously affect the safety of underground pipelines. However, research on the dynamic response of buried pipelines under the action of large vibration rollers is rarely reported. Therefore, the dynamic response of a high-density polyethylene (HDPE) double-wall corrugated pipe under a vibratory roller was studied via field testing and a three-dimensional nonlinear finite-element model was established. This model was used to analyze the influence of filling property and vibration frequency on the dynamic response of an HDPE pipe under a vibration load from a vibratory roller. The results reveal that with the increase of compaction times, the backfill soil keeps changing between over-compaction (loose) and compaction states, and the pipe top pressure also keeps changing. Moreover, at a shallow burial, the pipe top pressure is more obvious when the compaction degree changes. The elastic modulus of filling soil within a certain range can effectively reduce the stress deformation of the pipeline under vibration compaction. However, when the elastic modulus of filling soil exceeds 10 times the initial elastic modulus, the deformation of the pipeline becomes greater than the initial value.