Part 1 - NDT of Fusion Welded Joints in PE100 Pipe

Part 1

Non-Destructive Testing of Fusion Joints of PE100 Pipes and Fittings

The integrity and performance of PE100 pipelines are significantly influenced by the quality of their joints. The most prevalent methods for joining these pipes arebutt fusion joining and electrofusion joining. Butt fusion joining is widely favored for its ability to create strong, continuous connections by heating the pipe ends and pressing them together to form a monolithic joint. Variants in butt fusion joining such as single low pressure (SLP), single high pressure (SHP), and dual low pressure (DLP) have been developed to enhance joint performance [1]. Electrofusion joining, another common technique frequently used in smaller diameter pipes, involves the use of fitting with embedded metal coils positioned around the two ends of the fitting that need to be joined. This method, along with mechanical fittings, broadens the range of tools available for ensuring mechanical integrity tailored to specific project needs. As joints are the weakest part of any piping system, it is more susceptible to failure for various reasons.

Fusion joints (both butt fusion (BF) and electrofusion (EF)) in PE100 pipes have been found to be a distinct advantage, but it is possible for these fusion joints to contain defects if the level of site competence falls below that required to produce satisfactory joints. This has led to a number of failures of fusion joints in service and considerable effort has been expended to minimise these failures with better training and site management techniques.

In the UK water industry [2, 3], failure statistics reveal that PE pipe performs better in comparison to other pipe materials. The majority of failures that do occur in PE100 pipelines have been found to be EF joints as a consequence of poor site practice. This has led to the development of better training and supervision for site personnel to overcome these issues, with a significant improvement in joint quality where these improvements have been made [4]. Site supervision has been streamlined using remote techniques that can detect issues including clamping, alignment, scraping, contamination and power supply that have proved to be highly effective [4].

It has also been reported in Germany [5] that EF joint quality can be controlled using automatic fusion equipment and ensuring that jointing was well monitored and observed. This meant that PE had the lowest failure rates in German gas piping systems in comparison to other pipe materials. However, there have been instances of handling errors have been found to lead to reduction in quality of the joints, which are not evident from visual assessment and for which reliable non-destructive testing would be beneficial rather than destructive testing.

For BF joining in the UK water industry, additional quality control is provided by testing the external bead in a bead twist test [6] also referred to as the bead bend-back test (BBT). This has been found to be a highly effective method of assessing the quality of the joint and is now available as an automated tool: Bead Diagonostic Instrument (BDI) for more comprehensive results and traceability [7].

In addition to these improvements, there has been significant activity increasing research and development of methods that can detect imperfections within the joints that are known to impair performance including: dust, grease, soil, voids and cold welds. Sources of contamination may be airborne, washed in by rain or surrounding water/mud or inadvertently added by contact with cloths, clothing, fingerprints or soiled site equipment. Whatever the nature of the flaw introduced to the joint, it cannot generally be detected by external visual inspection of the completed joint. In order to detect these types of flaw, research has been conducted around the world to develop non-destructive testing (NDT) of fusion joints over the last 50 years [3, 4, 6, 7, 8, 9, 10, 11,12].

The NDT methods developed include:-

·        Phased Array Ultrasound (PAUT)

·        Microwave Imaging (MWI)

Other techniques are available and additional information may be added for Time of Flight Diffraction (TOFD), for example, in the future.

PE100+ Association contacted two companies as follows to provide background to their development of these NDT techniques.

·        Dr Mike Troughton of The Welding Institute (TWI) for PAUT [8]

·        Robert Stakenborghs of Advanced Microwave Imaging (AMWI) for MWI [9, 10]

A summary of the response provided is given within sections 2-3 for which PE100+ Association is grateful to these contributors.

In section 4, a short review is provided of 4 studies of NDT analysis of both EF and BF joints to provide an indication of the current state of the art in this regard.

It should be noted that the illustrations and tables are numbered separately in each section.

References

1.     AI Alhatti, A., et all, Advancements in butt fusion jointing of HDPE pipes: A comprehensive review of fusion conditions and joint integrity assessment techniques,

Polymer Testing, 2025

2.     S. Mackellar, UKWIR National Mains Failure Database, Plastics Pipes XIII, 2006.

3.     Leakage from PE Pipe Systems, UKWIR Report Reference 10/WM/08/43, 2010.

4.     Achieving zero leakage by 2050: Laying leak-free new networks, UKWIR Report Reference 17/WM/08/64, 2016.

5.     Zerstörungsfreie HM-Schweißnaht-prüfung für Netze in der Gas- und Wasserverteilung (HM-NDT), DVGW Report Reference G 202206, 2024.

6.     UK Water Industry Specification (WIS) 4-32-08, Issue 4, 2016.

7.     Carey D, Bridgstock E & Mark N, Plastic Pipes Conference PPXVIII 2016, Germany.

8.     Phased Array Utrasonic Inspection of Welded Joints in PE Pipes, Private Communication: Mike Troughton, TWI, UK, 2024.

9.     Stakenborghs R , Using A Multi-Frequency Microwave Inspection Approach to Accurately Inspect and Categorize HDPE Butt Fusions, Plastic Pipes XXI, 2023.

10. Stakenborghs R, Using Synthetic Aperture Focused Microwave NDT data to Create a 3D Rendered Image of an HDPE Electro Fusion Joint to Simplify Accept/Reject Criteria, Plastic Pipes XXI, 2023.

11. Wermelinger J, Choi S, Worldwide ISO Round Robin Study on Phased Array Ultrasonic Non Destructive Test for Electrofusion Socket Joints, Plastic Pipes XX, 2021.

12. Wermelinger J, ISO/TC 138/SC 5/WG 17 N 323, PE butt fusion joint NDT round robin final report, 2021.