Pressure Polyethylene Research Articles

The influence of the structural characteristics of polyethylene on the release of gas mixtures for extrusion processing

The study of thermal and mechano-thermal effects in the inert and air environment on the evolution of gas formations from high pressure polyethylene (LDPE) was carried out on a Smart RHEO 1000 capillary rheometer with the CeastView 5.94.4D software, using capillaries 5 mm in length and 1 mm in diameter . Study of composition Gas formations during the deformation of polyethylenes of different grades through the channel of a capillary viscometer. Which were characterized by structural characteristics, were carried out at shear rates close to production in the range from 50 to 300 s-1 at temperatures of 160, 190 and 220 ° C. The objects of the study were domestic thermoplastics of the following brands: LDPE 10803-020; LDPE 15803020; LPVD F-03020-S; HDPE 293-285-D, characterized by structural characteristics. It is established that the gasification during extrusion processing is promoted by the branching of polyethylene, while the degree of destruction processes increases with increasing temperature and depends on the reaction medium of the working volume of the equipment. Emerging critical shear stresses in the absence of oxidants and impurities contribute to mechano-destruction, accompanied by the formation of free radicals, which recombine to form a more branched structure of the polymer. In turn, the high temperature promotes degradation in the side parts of the polymer to form volatile products that are released from the reaction volume. It should be noted that the gassing due to thermal exposure is promoted by the air environment, but to a lesser extent than with mechano-thermal action. A smaller measure of the impact in the inertial medium is apparently associated with a limited access of oxidants to the destruction centers.

Welding effect on crack growth behavior and lifetime assessment of PE pipes

Pressurized polyethylene (PE) pipes are widely used in many fields all over the world, in which welding process is employed frequently. The welding will probably cause unnegligible degradation of material properties. So figuring out the welding effect on PE material is essential to assure its integrity. In this work a typical commercial PE100 material was tested by a generally accepted testing method, Cracked Round Bar (CRB) test, under cyclic loading to make lifetime prediction of parent and welded PE material. Afterwards, fracture surfaces of all specimens have been examined by scanning electron microscopy(SEM) and was analyzed contrastively to illuminate the welding effect on the lifetime expectation of this kind of PE material.

Commercial Production of Bimodal Polyethylene (TypePE-100) in PJSC «Kazanorgsintez»

The experience on commercial production of low pressure polyethylene to produce polymer pipes and connecting elements for gas distribution network and cold water supply in PJSC «Kazanorgsintez» is provided. Depending on pipes material requirements increase development and expansion of catalyst base for production and technology is demonstrated.

Influence of carbon fillers on properties and structure of polyethylene-based polymer composites

This paper presents the results of investigations on modifying high pressure polyethylene with different carbon fillers: nanotubes, channel black, and technical carbon. The dependence of strength, deformation, and electrophysical properties and structure of polyethylene-based polymer composites on the type and quantity of the filler are shown.

Study on Stress Field of End Structure of Ultrahigh Pressure Tubular Reactor

Ultrahigh pressure tubular reactor is the key equipment of tube-type ultrahigh pressure polyethylene, its end is usually the shrink-fit structure composed of jacket and reaction tube. Stress field of end structure of the ultrahigh pressure polyethylene tubular reactor, in which fatigue fracture occurred, is experimentally studied by measuring electric resistance strain. And the local stress field after shrink-fit is obtained based on stress release method. In terms of experimental results, ANSYS FEM is built and the simulation results compared with experimental data. The results show that a large tensile axial stress appeared on the location at the edge of the shrink-fit and it's larger than hoop stress, and the local stress level increases with the increase of shrink-fit range; the influence of contact stiffness on simulation results is significant, and in order to improve the simulation accuracy, the combination of contact stiffness and convergence has to be optimized; the FEM simulation results is consistent well with the experimental data. The study results can provide some insight into subsequent numerical simulation and failure analysis of the end structure.

Influence of free surface energy of polymers on formation of their interphase contact with oxyethylated isononylphenol

The interconnection of the surface energy characteristics of polymers in a wide range of acid-base constituents of the free surface energy and their adhesive interaction with adducts of ethylene oxide and isononylphenol was investigated. It was shown that the content of the polar groups in the polymer surface layer and the length of the oxyethylene chain additively influence the wetting ability and the work of adhesion. The peculiarities of the adhesive interaction of high pressure polyethylene with oxyethylated compounds depending on the thermal oxidation of polymer and the acidity parameters of the surface were revealed.

Risk Based Inspection on the equipment of low density polyethylene

In this paper, the basic principles of Risk Based Inspection (RBI) are introduced and RBI method is used to the high pressure polyethylene device of a petrochemical enterprise. The results show that about 8% of the equipments and pressure pipelines undertake 90% risk of high pressure polyethylene devices. High-risk equipments are found, and the foundations are provided for inspection strategy.

Failure analysis of bursting on the inner pipe of a jacketed pipe in a tubular heat exchanger

In order to identify the causes of a bursting incident that occurred on the inner pipe of a jacketed pipe in a tubular heat exchanger for synthesis of high pressure polyethylene, series of characterization analysis were conducted. Metallurgical structure and chemical composition of the pipe’s metal matrix were inspected by metallographic microscope (MM) and photoelectric direct reading spectrometer; scanning electron microscope (SEM) and energy dispersive spectroscope (EDS) were applied to observe the microscopic morphology and micro-area composition on the ruptured surface; compositions of the coolant, i.e. the circulating cooling water were examined by inductively coupled plasma-atomic emission spectrometry (ICP-AES), ion chromatography (IC) and Fourier transform infrared spectroscopy (FTIR). In addition to these, finite element analysis (FEA) was employed to study the erosive effect of on the pipe. Analysis results revealed that interaction between corrosion and erosion both led by scaling, was the main cause that accelerated its thinning and eventually resulted in its premature failure. Finally countermeasures and suggestions were proposed.

A fracture mechanics concept for the accelerated characterization of creep crack growth in PE-HD pipe grades

For the lifetime prediction of pressurized polyethylene (PE) pipes based on methods of the linear elastic fracture mechanics the knowledge of the crack resistance and the kinetics of creep crack growth (CCG) is essential. In the present work a rather brittle nonpipe material was used to develop a methodology for an accelerated measurement of crack kinetics in fatigue tests on cracked round bar (CRB) specimens at ambient temperatures of 23 °C. A material and specimen specific compliance calibration curve was generated to detect the crack kinetics with only one single CRB test. Based on an already proposed concept the kinetics at different R-ratios (minimum/maximum load) was measured and extrapolated to the case of CCG. To demonstrate the transferability of this concept to pipe materials a PE 80 pipe grade was used. Although the necessary testing time increased considerably the concept still has the potential to reduce the overall testing time for new pipe materials to be certified significantly. With the presented procedure a characterization of CCG in modern PE pipe grades at room temperature and without the use of stress cracking liquids is possible within a few months.

A PCA Based Fault Detection Scheme for an Industrial High Pressure Polyethylene Reactor

AbstractA data‐based monitoring scheme is proposed to detect decomposition in low density polyethylene reactors by combining principal component analysis with a priori information on the heat balance equations around the reactor. During normal operating conditions, the heat balance equation should close at all times within reasonable limits. If excess heat is generated in the reactor, the heat balance closure error will exceed a user specified threshold limit to indicate the possible onset of decomposition. However, since precise information required to formulate the exact energy balance equations was not available, principal component analysis (PCA) was used as a model identification tool. Results from a number of decompositions case studies from an industrial low density polyethylene/ethylene vinyl acetate autoclave reactor indicate that the method was able to detect the onset of decomposition with reasonable lead time.magnified image

Relief device sizing for ethylene decompositions—high pressure polyethylene industry

AbstractThis paper discusses revisions to a method previously used for sizing relief devices (rupture disks) for vessels in high pressure polyethylene processes. The Re‐evaluation was forced by an unexpectedly high peak pressure in a product separator (high pressure separator, HPS) when an ethylene decomposition occurred. A method, based on revised equations for calculating the rate of pressure rise, is presented for determining relief requirements for ethylene decomposition in a confined space. The method applies to both HPSs and polyethylene autoclave reactors. Causes of decompositions and effect of comonomers are also discussed. Simulations for other events at different operating conditions, not shown, indicate a good match for peak pressures. Therefore, the described equations provide a good estimate of ethylene decomposition peak pressures given the initial conditions.Although these methods were derived specifically for ethylene decompositions, the methods could be used for other decompositions. © 2007 American Institute of Chemical Engineers Process Saf Prog 2007

Ferroelectric rods with adjustable dielectric tunability

The polarization, dielectric constant, and tunability of a radial-loaded ferroelectric rod inside a highly pressurized polyethylene tube are calculated for both macro- and nanodimensions within a thermodynamic model. All studied properties, including the Curie temperature, are found to vary substantially by the applied load. The authors’ results indicate that many properties of the ferroelectric rod, including its tunability, can be optimized in this design by adjusting the applied pressure.

Influence of nonylphenol–polyglycol–ether environments on the results of the full notch creep test

It is well known that resistance to slow crack growth (SCG) is important for the lifetime of pressurized polyethylene (PE) pipes. To investigate the SCG resistance, the full notch creep test (FNCT) is widely used, especially in Europe. It is generally accepted that FNCT times to failure correlate with results from internal pressure tests on pipes. Nevertheless, the reliability of the FNCT is regularly discussed in the relevant literature. In this research work the degradation of the surface active environment (2% wt Arkopal N110 in deionized water) was verified and it was found that the times to failure increase continuously with the age of the solutions. Furthermore, it was interesting that PE films stored in Arkopal solutions had a lower OIT value compared to samples stored in water. After these preliminary tests, a ranking of several PE pipe materials was established.

Cyclic crack growth tests with CRB specimens for the evaluation of the long-term performance of PE pipe grades

It is well known that resistance against slow crack growth is important for the lifetime of pressurized polyethylene (PE) pipes. Thus, several methods have been proposed in recent years to evaluate the long-term performance of PE using fracture mechanics. It is generally believed that this leads to results more quickly compared to internal pressure tests. In the presented research work, a method was implemented using fatigue loading of cracked round bar (CRB) specimens to characterize crack growth resistance. The method was applied to five commercially available PE pipe materials and the results were compared with the full notch creep test (FNCT). The same ranking was found with both methods, but it was obvious that fatigue crack growth (FCG) experiments were faster by up to two orders of magnitude, especially when characterizing modern (bimodal) PE types.

Toughness of high-density polyethylene in plane-strain fracture

Under uniaxial tension, high-density polyethylene (HDPE) often fractures in a ductile manner. The fracture involves extensive necking that is known to occur in the plane-stress condition. However, for large-scale HDPE products like polyethylene pipe, crack can grow rapidly in a brittle manner. This type of fracture is known to be in the plane-strain condition, which has much lower toughness than that in the plane-stress condition. In this study, we extended the concept of essential work of fracture (EWF) to the plane-strain condition and evaluated the fracture toughness of extruded HDPE plate. The results show that the specific work of fracture in the plane-strain condition is also a linear function of the specimen ligament length. Therefore, the corresponding EWF value can be determined in a similar manner, that is, through linear extrapolation to zero ligament length. For HDPE, the study found that the plane-strain toughness is about one order of magnitude smaller than the plane-stress counterpart. Interestingly, the plane-strain EWF value is very close to the estimated toughness of pressurized polyethylene pipe, which was reported in the literature, predicted using numerical simulation based on data from the standard small-scale steadystate (S4) test. POLYM. ENG. SCI., 46:1428 –1432, 2006. © 2006 Society of Plastics Engineers

Water hammer in pressurized polyethylene pipes: conceptual model and experimental analysis

This paper analyzes the dynamic effects of pipe wall viscoelasticity on hydraulic transients. These effects have been observed in transient data collected from two polyethylene (PE) pipe systems. The first is a 270 m pipeline, 50 mm diameter, at Imperial College London, and the second is the world's longest experimental PE pipeline, 1.3 km long, 110 mm diameter, buried underground at Thames Water Utilities (London, UK). A mathematical model has been developed to calculate hydraulic transients in polyethylene pipe systems based on the assumption that the viscoelastic behaviour of pipe walls is linear. An additional term has been added to the continuity equation to describe the retarded deformation of the pipe wall and the resulting governing equations are solved by the Method of Characteristics. The numerical results are compared with both the classic elastic solution and with collected transient data. Good agreement between numerical results for the viscoelastic solution and observed data was obtained by fitting the creep function J(t). Unlike classic water hammer analysis, the developed mathematical model is capable of accurately predicting transient pressures in polyethylene pipes and the circumferential strains in the pipe walls.

NMR study of chain dynamics in the hexagonal high-pressure phase of polyethylene

At high pressure (>3300 bar) polyethylene shows a hexagonal phase between the orthorhombic phase and the liquid phase. This first NMR investigation of polyethylene in its hexagonal phase shows that the mobility of the molecular chains is very high. Deuteron quadrupole echo spectra have been used to determine the character and rate of local reorientations of the molecular chains. The chains perform rapid axial reorientations, similar to those observed in the rotator phases occurring in the long normal alkanes. From proton rotating frame relaxation data we derive a high rate of translational diffusion in the chain direction. These observations contribute to the understanding of the important role of the hexagonal phase in the growth of chain-extended polyethylene crystals.

The Role of Primary Aromatic Amines in the Intensification of Adhesion Interaction in Polyethylene-steel System

The role of primary aromatic amines (PAA) in promoting the adhesion interaction between steel and coatings prepared from low pressure polyethylene synthesized by gas-phase method (LPPE1) has been investigated. PAA were demonstrated to come into acid-base interaction with steel by a donor-acceptor mechanism. The increase in coatings adhesiveness is realized through bond formation between an adhesive and a substrate with the help of PAA. Free surface energy of the contact to metal side of coatings modified with PAA increases in parallel with the adhesiveness increase. The acid-base interactions have been found to exert primary control over the polyethylene adhesion to steel.

Fatigue strength and crack propagation life of in-service high pressure tubular reactor under residual stress

In the production of a tubular reactor for a high pressure polyethylene plant the autofrettage process was used to produce a compressive residual stress at bore, which will increase the fatigue strength of the reactor. The autofrettage residual stress in the reactor will, however, gradually relax during operation. It is necessary to investigate the influence of residual stress relaxation on the fatigue strength of the reactors. In the design only fatigue crack initiation life was considered as its fatigue strength. What the fatigue crack propagation life is was not known. In this paper, fatigue crack propagation life of a reactor with residual stress was analysed using Paris' equation. Based on the results of an internal pressure fatigue test on a reactor with inner and outer surface artificial notches, the effects of residual stress and suitable SIF expression were also analysed. The autofrettage residual stress can greatly increase crack initiation life of plain reactors. However, since crack propagation life of the reactors is short, effort should be taken to prevent crack initiation on the inner or outer surface of the reactor caused by other factors.

Construction and exploitation of the high dimension cermet slide bearings in a polyethylene reactor

High abrasive wear of large dimension ball bearings on the mixer of a high pressure polyethylene reactor that appear during it's use causes serious failures that can be very dangerous for attendant and continuity of production. As a result of a complex analysis of the conditions of use of the polyethylene reactor, the authors of the paper have made prototype cermet slide bearings that are lubricated by the polymer generated in the reactor. After a year of use in the Polish petrochemical industry the bearings were investigated in terms of dimension, wear and structural stability. The high properties of the bearings were verified during use. The combination of hard ceramic particles with the relatively plastic matrix produces favourable properties: resistance to abrasive wear and ductility. These properties and considerable chemical resistance point to the perspective of their use in the construction of elements of highly loaded slide bearings. In the paper shows the main aspects of construction and results of the investigations of bearings mentioned above after one year of use, as well some examples of their wear.