Plastic Pipes Conference Association # 1995 Edinburgh
Marshall, Ingham, Brogden
When clients purchase plastics pressure pipes, they have frequently made that decision because they know that plastics do not corrode and they assume that if they do not exceed the pressure rating then a long lifetime should be assured. (Low cost has also been known to be a factor).
It is thus of concern for the plastics pipe industry to have test methods which can reproduce the same loading conditions which are obtained in service and knowledge of materials behaviour which allows for the extrapolation of short term data to predict performance long into the future. In designing tests, it is most important to learn from historical service failures of pipe systems and to model/simulate the worst case conditions which are likely to occur. It is also important to recognise that the client may frequently have a less than perfect knowledge of the loadings which will be imposed and is unlikely to employ contractors who will install the system precisely to instructions. This is particularly true in the UK where contractors seldom read detailed specifications and are driven by economic considerations to install pipe as quickly as possible with labour forces who are often poorly trained.
In such circumstances, materials and products are required which have large reserves of toughness to withstand at least three factors being contrary to expectations. Usually: 1) pipes will be damaged and defects will be introduced during installation, 2) the service loading will seldom be static pressure alone and 3) backfill, bedding and surround materials may differ from those specified and will often be compacted inefficiently.
4) With fusion welded PE systems there will be added problems where contractors will disobey welding instructions.
There are materials which will withstand all such conditions. The problem is to determine with assurance that they are chosen and that the process of converting the raw polymer into pipe does not alter properties.
History has shown that a Fracture Mechanics approach forms the best framework for considering both the design and assessment of products, since it is implicitly assumed that the worst case of brittle failure caused by the propagation of sharp cracks 1s always a possibility. The objective of all FM testing and appraisal is to assume that materials are imperfect and analyses exist which allow ductile/brittle transitions to be predicted.
It is the purpose here to review how the Fracture Mechanics approach pioneered for plastics pipes in the UK by both academics [1] and industry [2] has helped to improve the understanding of pipe behaviour and develop new methods of assessment, which address the conditions actually obtained in service rather than the ideals so often assumed.
Related papers
Author(s) : Peter Hubbard, Linqing Luo, Andrew Yeskoo, Kenichi Soga, Krista (Moita) Araica, Gus Cicala, Marshall McLeod
Seismically active faults pose a risk to buried water pipelines that can be complicated to quantify. Fault type, slip rate, pipeline geometry, and soil conditions all factor into a complex soil-pipeline interaction. For critical pipelines that cross faults, high-density polyethylene (HDPE) has become an attractive...
Author(s) : Edward Ingham, Mike Shepherd
Exova has previously demonstrated that under normal operating conditions PE 100 pipes would be expected to have a lifetime of over 200 years, but in the UK concerns have been reported about relatively high failure rates within the water utilities of fusion joints, particularly for large diameter pipes. For some time,...
Author(s) : Sonya Pemberton, David Chrystie-Lowe, Sabah Rafiq, Karolina Modelska, Edward Ingham
A room temperature tensile test using thin films taken from the outside and inside surface of polymer pipes has been developed to study the slow crack growth resistance of ageing PE pipelines via analysis of their strain hardening behaviour. A range of PE materials in service since the early 1970s and 80s are...
Author(s) : Dr. Dane Chang, Dr. David Marshall, Chris Cox
Plastic pipe applications often require polymers to have adequate abrasion resistance during usage. Although typical polyethylene resins have reasonably good resistance to abrasion and can be broadly used with a variety of chemicals, there are some demanding applications that require a resin with enhanced abrasion...
Author(s) : MacKellar, Lowe, Ingham, Ashdown
Polyethylene (PE) is the preferred material for 90% of all water mains laid in the UK. The pace of renovation of dilapidated cast iron water networks has led to the increasing demand for coils of flexible PE pipe, driven by the efficiency of using less joints and taking up considerably less space in the congested...
Author(s) : MacKellar, Headford, Ingham
This paper provides an overview of the basic properties of plastic pipes, in the range 50mm to 400mm, and how these should be used in the design of above ground process pipe systems made of PE and PVC. It covers the basic properties of plastics, examples of the determination of load cases and describes the basic types...
Author(s) : Marshall, Brogden, Shepher
Many early brittle failures in PVC water and sewerage pipelines, where repeated surge events are thought likely to induce embrittlement, were attributed to fatigue. The concerns of pipeline engineers led to the adoption of the British Standard Code of Practice CP 312 (ref. 1) by the UK water industry, which gives...
Author(s) : Marshall, Morley, Pearson
The Water Industry in England and Wales has responsibility for an approximate length of 300,000km of water main, ~nuchof which was laid over 70 years ago using cast iron. Subsequently, asbestos cement, PVCu and cement lined ductile iron have been used. The old iron pipe systems that were installed in many of the...
Author(s) : Marshall, Hepburn, Netherwood
The adoption of polyethylene as a pipe material by both the gas and water indusmes has undoubtedly led to several sigruf~cant economic and operational advantages. The development of narrow trenching techniques and insertion refurbishment methods only became viable once a complete fusion jointed polyethylene system...
Author(s) : Marshall, Birch, Morley
Many UK Water Companies are now using PE pipe systems because it 1s believed that use of an all-welded pipe will minimise the number of mechanical fittings employed and therefore will reduce potential sources of leakage. With pipes such as ductile iron and PVCu, which are joined in short lengths, it is often...
Author(s) : Lowe, Marshall
A Rotating Pipe Drum Test has been developed to assess the abrasion performance of plastics pipes to an extpnt where most test variables are understood. The procedure involves the rotation of relatively short pipe lcngths containing slurry on a conventional bail mlll. In the rest, the pipe surface repeatedly passes...
Author(s) : Marshall, Birch-Kisbenyi, Youd
A complete analysis of the Charpy impact system has been made to relate measurements of impact toughness to the geometry of samples. Using data obtained on pipe grade polyethylenes, Gc values at 20°C and 3°C have been found to correlate wel! with measurements of resistance to fast fracture in pressurised pipe...
Author(s) : Young, Thomas, Marshall
Within the Water Industry in England and Wales, there is an estimated length of 295,000 km of water main, much of which was laid in unlined cast iron prior to 1940. Subsequently, asbestos cement, UPVC and cement lined cast/ductile iron has been uscd. It is kn-that a substantial number of...
Author(s) : Marshall, Birch
It is proposed that present Specifications for PE Pressure pipes are essentially undemanding and tests do not simulate service conditions. Perfect pipes are assessed rather than pipes which have sustained handling and installation ‘damage’. Flaws and defects have caused many problems with other thermoplastics pipes...
Author(s) : Lowe, Marshall
A new test has been developed to evaluate the abrasion performance of different pipe materials. The Rotating Pipe Test simulates sliding bed flow of a slurry and gives quite excellent data reproducibility. The influence of sample geometry and slurry changing frequency and particle size have been assessed. The effects...
Author(s) : Marshall, Taylor, Dickinson
A study has been carried out which has sought to establish the influence of processing conditions on the properties of medium and high density polyethylene. Residual stress and fracture toughness measurements have been carried out on hdpe pipe, from which predications of the influence of diameterlwall thickness on...
Author(s) : Birch, Marshall
The mechanics of failure of uPVC pipe materials have been investigated using fracture mechanics test methods to induce brittle failure. It has been found that brittle failure can be quantified and test procedures have been developed which are capable of being used as quality control methods for classifying pipe. High...
Author(s) : Birch, Howarth, Marshall
The paper is a brief resum of a large project which has sought to investigate the use of fracture mechanics in evaluating the failure properties of a number of common thermoplastics pressure pipe materials. In this outline, the results of tests made on uPVC and YDPE pipes are described. Pre-notched pipes have been...
Author(s) : Marshall, Williams
During the last ten years, it has often been predicted that plastics will be used in increasing quantities as true engineering materials, Whilst there has been an increased acceptance of GRP as a positive alternative to metals in many applications, the use of thermoplastics for major load bearing structures has been...
Author(s) : Lowe, Ingham, Starkey
Members of the Association
