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January 18, 2011
11 Min Read
Polyethylene (PE) has revolutionized low-pressure pipe system design on a global basis and is testimony to PE's unique combination of properties, which have driven the replacement of traditional pipe engineering materials during the last 50 years. PE pipes and fittings are used extensively in operating gas and water distribution systems safely, reliably and economically, and enjoy an excellent performance track record.
PE offers the pipe industry:
Economical, high volume manufacture - extrusion, injection molding;
Design flexibility - easily shaped;
Integrated design - multifunction, ready assembled components - couplers and fittings;
Low material cost;
Light-weight design - ease of transport and handling;
Flexibility - ease of transport and handling, use in conjunction with trenchless technologies and resistance to seismic activity;
Relative ease of jointing (compared to metallic pipe systems);
Corrosion and chemical resistance;
Biologically inert capabilities;
Toughness, impact resistance, abrasion resistance and long term durability - technical lifetime of >50 years;
Low temperature performance;
Leak-free fusion jointing - low maintenance costs;
Low friction bore - no scale build-up and efficient flow of transfer medium; and
Environmental benefits - recyclable
The success of PE for pipeline applications has been achieved through a long legacy of historical development, catering for pipe industry requirements. Over the past 50 years, PE materials have evolved and now provide a good balance of strength, stiffness, toughness and durability consistent with demands of long-term gas and water pressures, ground loading and the service environment.
A timeline for PE development for pipe manufacture is given in Table 1.0.
Table 1.0 Development of PE for the Pipe Industry
ICI discover PE
HDPE - 1ST generation
HDPE - 2ND generation (PE80)
HDPE bimodal (PE100) - 3RD generation
HDPE Bimodal (PE100) - sandless bedding
2011 - polymerization plants under construction
A first insight into early HDPE pipe system reliability was achieved by analysis of failure statistics and failure mechanisms. These PE systems demonstrated satisfactory performance compared with traditional pipeline materials such as iron, steel and concrete, but areas for improvement were identified such as joint quality and premature stress cracking resistance arising from point loading with stones, root systems and soil settlement. These observations resulted in stringent codes of practice for manufacturing and pipeline installation, accompanied by development of higher performance PEs and the associated test methodologies that validate their performance.
Today, the premature failure of a PE pipe system component is typically due to older generation materials whose properties offer limited resistance against severe environmental and operating conditions. The other major factor is human error, committed in the supply chain and site construction of installed pipe systems.
The general mode of field failure reported for PE pipe is brittle, slow crack growth (SCG) through the pipe wall. These cracks can initiate at microscopic stress-raising flaws, inherent in the basic pipe product or, more likely, from defects. These brittle mechanical failures are typically slit-type fractures that lie parallel to the pipe's extrusion direction. Circumferential hoop stress in the pipe wall is the driving force for crack opening. Typical slit type fractures are shown in Figure 1.0.
Circumferential cracks can also be initiated on either the outside or inside surface of pipes due to secondary stresses such as bending or impingement on the material. The premature failure of melt-fused joints is also common failure where cracking initiates at stress concentrations created by poor installation practices.
Visually, brittle cracks are typically smooth, featureless and devoid of any yielding and deformation process, as shown in Figure 2.0. They are initiated at stress concentrations within a materials structure, which may be inherent flaws, or defects such as residual stress, contaminant, inclusion or surface scratches. Figure 2.0 offers a close-up look at the surface of a brittle fracture
Early 1960's research of HDPE pipes established three major failure modes for PE pipe: ductile failure (mode I), brittle failure (mode II) and brittle/chemical failure (mode III). Ductile failure was associated with yielding and reflected a material's propensity to undergo large-scale, irreversible 'plastic' deformation when under stress, the yield stress describing the load intensity at which such effects occur. This mechanism, as shown in Figure 3.0, results from localized yielding and thinning, usually in an area of lowest wall thickness. This is followed by localized expansion of the wall section and final rupture of the deformed zone when the local stress exceeds the mechanical strength across the newly oriented polymer structure. This failure mode is known as the classic 'Parrot's beak failure'.
The design of PE pipe systems takes this failure mode into account by testing the material to determine its long-term ductile strength and then by applying a safety factor, reducing the operating pressure to a level such that ductile failure will not occur in less than 50 years. The internationally adopted classification of PE materials takes this into account, and suppliers of the resin designate their materials into two strength classes; PE80 (historically referred to as MDPE) and PE100 (historically referred to as HDPE). For PE80 and PE100 system classification, the minimum stress level must be at least 8.0MPa and 10MPa respectively for a lifetime of 50 years at 20oC.
The most important finding of this early research was an understanding that SCG could occur in areas of highest stress before burst type failure, and that pipes would fail in a brittle manner at stresses significantly lower than the material's yield stress.
Today it is common knowledge that polymers such as PE are visco-elastic materials, and respond to stress as if they were a combination of elastic solids and viscous fluids. Consequently, they exhibit a non-linear, stress-strain relationship and their properties depend on the time under load, temperature, chemical environment and the stress or strain level applied. An illustrative example of visco-elastic behaviour can be seen with Silly Putty, a class of silicone polymer marketed as a children's toy. If this material is pulled apart quickly, it breaks in a brittle manner. If, however, it is pulled slowly apart the material behaves in a ductile manner and can be stretched almost indefinitely. Decreasing the temperature of Silly Putty decreases the stretching rate at which it becomes brittle. Plastic designers are well aware that, in the short term, many polymers can endure strain levels of 300% or more. However, for long-term performance, the window for design strain is massively smaller.
All polymers on the application of stress exhibit 'creep', a time-dependant, non-reversible deformation, when exposed to a constant tensile stress. Creep rupture is the terminal event of creep and is a measure of the time that a material under a constant, applied tensile load takes to fail. Creep rupture can be accelerated by an increase in temperature, stress concentrations - such as notches, cyclic stressing and contact with specific chemical environments (liquid or vapour), commonly referred to as environmental stress cracking (ESC).
At the microscopic level, there are many inherent stress-concentrating defects within the microstructure of the pipe wall and on application of a few MPa of stress, the yield strength of the polymer will be locally exceeded. Micro-yielding blunts these defects and initially stabilizes them. The size and shape of the yielded zones are determined by the yield stress contour of the local stress field. The yield strength of polymers declines with time under stress, with an increase in rate with increasing temperature. Consequently, with time under stress, the size of the micro-yielded zones increases. Thin plate-like zones grow in area in a plane normal to the principal tensile stress direction, gradually resulting in an increase in thickness. Cavitation, due to volume expansion, eventually occurs within the zone and the defect is now termed a 'Craze'. With further growth of the zone and zone cavities, the inter-cavity material stretches to form highly orientated, load bearing fibrils. With further growth of the Craze, a point is reached when the most highly stretched fibrils will rupture.
For semi-crystalline PE, the tenacity of fibrils and their resistance to rupture will be highly dependent on molecular architecture, particularly molecular weight, molecular weight distribution, branching, crystallinity and the number of tie molecules. The tie molecules are embedded in the crystallites and transverse amorphous regions, acting as mechanical links between the crystalline domains, and play a decisive role in the resistance to fibril failure and overall mechanical properties when subjected to stress. The greater the number of tie molecules and the more entanglements, the greater the resistance to fibril elongation and final rupture.
On fibril rupture, the stress at the craze tip increases, the craze growth rate increases, and there is an extension of adjacent fibrils. This sequence is repeated and the unsupported part of the craze is termed a 'crack'. The crack advances behind the craze until it reaches a critical length, upon which it cannot be supported and results in complete fracture and failure of the pipe.
Early 1960's research showed that it was necessary to take account of the long-term failure mode of PE to ensure the safe operation of distribution networks for gas and water. Of paramount importance was a high degree of long-term strength in order to resist creep rupture for sustained low pressure loading of 50 years and more. Resistance to stress cracking was also critical in order to inhibit crack growth from notch type damage (scores, scratches and gouges) developed during transport and installation, and point loading due to rock and root impingement. Hence, an important design factor for PE pipes became commonly known as environmental stress cracking resistance (ESCR), which led to the development of MDPE. MDPE exhibited excellent SCG and stress crack resistance, and dominated PE pipe manufacture for 30 years until the development of bimodal HDPE (PE100).
Mode III failure is related to degradation and embrittlement of the plastic due to thermo-oxidation with time. Assuming Stage II failure is inhibited during service, due to correct design, installation and accurate qualification of material properties, it is Stage III which will ultimately determine the lifetime of a pipe material. Resistance to Stage III will be dependent on specialist additive packages added to PE during manufacture to protect it from thermo-oxidative attack. Once these additives have been depleted, the onset of Stage III failure will commence rapidly.
In the late 1970s, workers at British Gas R&D (subsequently Advantica and now GL Noble Denton) discovered that thick wall MDPE pipes operated at a critical pressure and below a critical temperature could fail catastrophically by sustained, axial rapid crack propagation (RCP), as shown in Figure 4.0. This failure mode, thought previously to occur only in metallic pipe, was an alarming discovery, leading to pressure limitations on large diameter PE pipes, and promoted the development of bimodal PE (PE100), providing high, long-term strength, stress crack resistance and RCP resistance.
RCP can be initiated due to either a defective butt joint, third party damage, such as a high velocity impact from excavation equipment, or a pipeline pressure pulse. Once initiated, ruptures can travel at high speed along the length of the pipe over significant distances as long as the stored energy from the contained pressure source is sufficient to drive the crack faster than the rate at which the energy is released. At a critical speed approaching the speed of sound, the crack will become unstable, branching in a sinusoidal pattern, until it slows and stops. Sometimes, the crack will bifurcate (split) into two directions as it travels along the pipe. The typical features of an RCP crack are a sinusoidal crack path along the pipe, a brittle -almost glassy - fracture surface, and hackle marks in the direction of the crack.
RCP is dependent on several factors:
Pipe diameter - as outside diameter increases, the possibility of RCP increases;
Operating pressure - as pipeline stress/pressure increases, the possibility of RCP increases;
Operating temperature - as temperature decreases, the possibility of RCP increases; and
The pipe material's resistance to impact fracture and resistance to RCP
While RCP failures in PE pipes have been very rare, their potential dire consequences are significant for gas distribution, as shown in Figure 5.0. In order to avoid catastrophic failure, pipes and applications are designed so that RCP may be avoided under worst case scenarios. In order to support this testing requirement GL Noble Denton has built a full-scale (FS) RCP test facility at its Spadeadam Test Site, which meets the requirements of ISO 13478. The FS test is considered to be the most reliable method for assessing RCP resistance. It is designed to reproduce idealized service conditions for a pipe in service. It defines the absolute standard for RCP testing and delivers a final judgment on the pressure carrying capacity. Extensive RCP testing conducted by GL Noble Denton identified safe operating pressures such that RCP has never been recorded within the UK Transco PE distribution network in the last 35 years. Figure 6.0 shows a full-scale RCP test at Spadeadam, GL Noble Denton's test center.
In summation, PE pipeline networks have established an impressive safety record over the years, exhibiting formidable long-term service endurance. Globally, PE materials are now undoubtedly the preferred material of choice in the construction of gas and water distribution networks.
However, failures can occur for an assortment of reasons inclusive of:
Incorrect material selection
Stressing beyond anticipated design stress
Point loading and stress raisers
For our customers, we routinely conduct incident reports and independent forensic failure analysis of PE pipe systems and other polymeric components. We know first hand that PE pipe system failure can result in explosion, fire and loss of life, and ultimately in costly litigation and damages. In addition, these failures can cause service interruption, safety concerns and loss of brand credibility. For these reasons, we believe that failure analysis must be standard practice for the utility market. We strive to determine an exact cause of failure, and develop remediate, corrective actions to prevent recurrence.
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