X Международная студенческая научная конференция Студенческий научный форум - 2018

The polyethylene pipeline installation methods are based on the specific physical polyethylene properties (plasticity, shock resistance and elasticity) and differed from the metallic pipeline one. In particular, the trenchless installation method based on an elasticity of a polyethylene pipe is widespread. During such method implementation, the pipe-liners and the relevant mechanisms should not create the loads excessing of the limit values and deteriorating of the installed pipeline operation properties.

INTRODUCTION. 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 60 years. PE pipes and fittings are used extensively in operating gas and water distribution systems safely, reliably and economically, and enjoy an excellent performance. PE offers the pipe industry an array of advanatges inclusive of:

Economical, high volume manufacture – extrusion, injection moulding;

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);

Squeeze-off for emergency gas flow stop;

Corrosion and good 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;

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 60 years, PE materials have evolved with advances in polymer science. Today’s highly engineered bimodal PE100 now provides exceptional balance of strength, stiffness, toughness and durability consistent with demands of long-term gas and water pressure containment, ground loading and the service environment2 . 2 Key to the success of PE pipeline systems is the ability to quickly form reliable end load resistant fusion joints with a strength equivalent to the parent pipe materials with a minimum design life of 50 years. However, there is an ever increasing awareness that the technology and reliability is being undermined by operative workmanship in the field, posing a risk to the pipe network. This is a particular concern for gas distribution where premature failure can have catastrophic effects resulting in loss of life. In order to reduce the risk to gas distribution pipelines there is a need for increased requirements for training and qualification of installers, including tooling requirements and servicing of tooling. The aim of this White Paper is to provide an overview of key parmeters which influcence butt fusion and electrofusion weld performance, typical failure modes seen in the field and preventative actions which can help mitigate the incidence of failure.

PE pipe due to its chemical inertness, non-corrosive nature and long term durability offers outstanding service life, with conservative estimates standing at 100+ years. Today PE is the material of choice for pipeline transportation of water and gas where pressure containment and structural integrity is considered critical for the lifetime of the pipeline since system failure can result in flood, explosion, fire and loss of life resulting in costly litigation and damages. In addition, this can cause service interruption, safety concerns and loss of brand credibility. Today, global operators of PE pipeline assets report that the major threat to PE pipeline integrity other than third party damage is poor fusion jointing. Joints are obviously a weak point in any engineering system. Axial or bending stresses caused by thermal expansion or contraction, or ground movement will increase the risk of failure of substandard joints.

STRENGTH. The strength of polyethylene pipe is normally classified at a point in time known as the 50-year design point. This should not be confused with lifetime of the installation; it is simply an accepted point in time at which the material strength should be classified for the purpose of designing a system. Classification is based on the hoop strength of the material, which is the measure of the material in a pipe form to contain the stresses imposed by the pressurised gas without rupture occurring. Using a process embodied in the internationally recognised standard ISO TR 9080, materials are classified according to their minimum required strength (MRS) at the 50-year design point, at a nominal operating temperature of 20°C and receive the following designation: · PE80; a material with MRS 8MPa · PE100; a material with MRS 10MPa · PE125; a material with MRS 12.5MPa.

FRACTURE RESISTANCE. The inherent flexibility of polyethylene pipes provides a very high level of fracture (resistance) toughness. As a simple measure, the flexibility of the pipe allows it to absorb high levels of impact loads associated with the construction phase, and vibration and stress caused by soil or ground movement post installation (fatigue resistance). Their flexibility and ability to deform significantly without failure through either ring or beam bending, enable PE pipes to be laid in difficult ground conditions (potentially unstable ground normally associated with mining subsidence or earthquakes), or to be cold bent on site to accommodate difficult road layouts and varying terrains.

THE FUSION PROCESS. PE pipe is usually supplied either in 6m or 12m lengths (sticks), or coils of 50m to 150m in length. Consequently, it is necessary to create joints between pipe lengths 3 and when transferring from one size of pipe to another, connecting branches or for connecting services to mains. There are 3 main types of fusion joint geometry, which include:

Butt weld

Socket joint

Saddle joint

CORROSION RESISTANCE. Polyethylene is chemically inert at the temperatures of operation normally associated with below ground gas distribution infrastructure. As such, it will not form a chemical or electrical reaction with the surrounding soil leading to corrosion of the pipe material.

PIPE JOINTING. Jointing of polyethylene distribution pipes should only be undertaken by suitably qualified operatives who have attended a competency based training course and have been assessed as suitably competent to undertake the activity. Presently the main route to competency is through the completion of an NVQ vocational qualification leading to the award of a gas network operative (GNO) qualification. Specialist courses are also available specifically in the techniques of welding and jointing of polyethylene pipes and these would be expected to lead to a City & Guilds qualification as a minimum.

References:

Plastics Pipe Institute. Handbook of Polyethylene Pipe, 2nd Edition, 2008.

Bromstrup, Heiner. PE 100 Pipe Systems, 2nd Edition, 2004.

Belofsky, Harold. Plastic Product Design and Process Engineering, Hanser, 1995.

Wright, David. Failure of Plastics and Rubber products, 1st Edition, RAPRA Technology Ltd, 2001.

Scheirs, John. Compositional and Failure Analysis of Polymers, John Wiley & Sons, 2000. 26

Ezrin, Myer. Plastics Failure Guide, Hanser, 1996.

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