Jointing Technology for PE pipes: Design features for increased processing safety of Electrofusion fittings
May 30, 2006
Pipelines are investment objects. The largest part of the capital of an utility or sewage company lies in the subterranean infrastructure of its pipe system. Its safety is – naturally – an essential aspect for the economic success of this system and vital for it to carry out tasks reliably. This article deals with the aspects of product safety in application for the electrofusion procedure as jointing technology for PE pipes.
- the planning, calling for tender and selection of suitable installation procedures
- the selection of materials – pipes – fittings – valves,
- the selection of suitable, certified pipe system contractors with specially trained staff as well as the construction site supervision to be scheduled to maintain quality requirements
The fusability of polyethylene and the simple handling of fusion technology offer essential advantages over other materials. The homogenous connection of the materials through fusion provides a level of stressability in line with the pipe material and even considerably superior to it through electrofusion fittings (illustration 1). It is good to know that the "weakest link" is the plastic pipe with its high stressability.
The system of the fused pipe after installation is therefore not made up of individual components – pipe/connection/pipe…- as opposed to the application of mechanical jointing techniques, but consists of a single, non-detachable, material-homogenous connection due to the fuseability. The use of elastomeric sealing components can be limited here to a minimum, e.g. in valves.
The training content can be applied both in industrial pipe construction and in sewage technology. The guidelines of the DVS, German Association for Welding Technology, specifically apply. Application emphasis for the butt welding procedure lies with the larger dimensions (> DN 200, d 225) due to the time consuming procedure of making the connection, and more specifically the installation of larger pipe lengths on open ground .
Electrofusion fittings are used in the large pipe range up to d 710mm as problem solvers where the butt weld procedure, e.g. for integration into existing pipes, changes in direction or repairs cannot be applied sensibly. The advantages of the electrofusion technology are specifically:
- simple installation and handling
- high reliability
- speed of processing
- economic value due to effective use
- universal applicability with regards to PE pipe material and wall thickness
- suitability in practice for installation in restricted space in the trench.
|Dimension||Min. fusion zone length [mm] |
according to EN 12201-3, EN 1555-3 [10,11]
|Fusion zone length [mm] |
National (e.g. DIN 16963) and international (e.g. EN 1555, EN 12201, EN13244) standards usually make a minimum of requirements of the technical and geometrical features of electrofusion fittings compromising at the smallest common denominator. This of course fulfils the requirements made of the pipe system – providing a very high standard of processing.
Tests in laboratory conditions may naturally not meet the exact conditions of processing in practice. Here the manufacturers need to take responsibility to provide the user with a reliable and practice orientated system.
This requires experience and know-how both in plastic technology and in construction site practices. In spite of the significant extent of the effect of product safety on pipe system as investment item– with an expected useful life of up to 100 years – this aspect is often underplayed in the face of short-term advantages with a mention of an existing product authorisation. Generally processing must of course take place in line with current guidelines and installation instructions by the manufacturers .
The most important developer is still the user. It is only through his or her input regarding shortfalls that practice-related improvements will be possible and problems solved.
Basically the following aspects are taken into consideration for the development of electrofusion fittings  (observe manufacturers' indications!):
- Fusability of different pipe materials PE 80, PE 100 and PE-Xa
- Temperature range up to -15°C - +50°C
- Pipe series SDR 17,6 (17,0), SDR 11, SDR 7.4 (Standard dimensions in Germany)
- Dimensions deviating from standard SDR 41 - SDR 21 as well as SDR6 are possible with the application of suitable parameters.
The essential geometric aspect in the design of a electrofusion coupler is the insertion depth (illustration 2). This is subdivided into
- length of fusion zone, i.e. the effective area provided by the fitting for the homogenous connection of construction parts. Simplified, the following applies: "The longer the fusion zone, the greater the achievable tightness and reliability of a fusion connection in the application".
- the length of internal and external cold zones with the task
- of storing the melt occurring during fusion (build-up of melt pressure),
- safely compensating for slight bending stress caused on the construction site and
- balancing out or compensating for deviations from the ideal state caused on the construction site, e.g. ovality, conic dipping of pipe ends or pipe cuts not at right angles.
Faulty chambering of the melt pressure can lead to escape of melt, reduce joining quality and is therefore not authorised. The longer the cold zones, the better the resistance against bending stresses, e.g. as noted with the use of coiled pipes. The actual fusion zone will remain almost entirely unaffected by stress, as the pipe is guided through the cold zones and the increased coupler insertion depth.
This design feature was then consistently converted for the FRIALONG long coupler which provides user friendly problem-solving for the use of coiled pipes (illustration 3, illustration 4).
Shrink stresses which may have frozen in the fitting and which are supposed to be used for creating melt pressure, are degraded immediately and have practically no longer any effect on this parameter. That is, if these still exist in the first place: it is well known that inherent polyethylene stresses are degraded in time, the material relaxes. If the radial shrink for the build-up of joining pressure during fusion is included in the design calculation, a "best before date" must be indicated for the application period of the fitting.
For decades two fundamentally different design types (illustration 5, illustration 6) of the heating coil position in fittings have been available – and been discussed by users and manufacturers alike. Both systems – heating coil with a covering PE layer or exposed, visible in the moulded part – have been successfully tried and tested a million times over many years.
Through the thermal expansion of the PE material of the fitting when molten this annular gap is closed. Only then is a considerable amount of fusion energy transferred to the pipe. Due to this process heat transfer in the fusion zone is obstructed, however at the same time more heat is injected into the body of fitting due to the heating coil being covered.
The result is a melt ellipse asymmetrical to the fusion level (illustration 7) with its main proportion in the fitting and a reduced capacity for the bridging of the annular gap between pipe and fitting. The manufacturing related position of the heating coil and the thickness of the PE layer which naturally greatly affect heat penetration in the pipe cannot be recognised by the user other than through a destructive test or examination involving x-rays.
It is possible to demonstrate this easily with an experiment using physics: If you point your finger at a source radiating heat, e.g. a cooker ring, at 1 mm distance – without actually touching it, you will quickly experience the relativity of the statement "air is a poor heat conductor". Of course it is correct to say that PE and air are poor heat conductors compared with steel and water. However, the relatively small difference between these materials, especially between liquid PE and air (table 2) will become clear only when comparing the values.
|PE, rigid||0.23 W/mK||-|
|PE, liquid||0.16 W/mK||-|
|Air, dry||0,02454 W/mK||with PE rigid: 9.4 |
with PE liquid: 6.5
|Water||0.6 W/mK||with air: 25|
|Steel||47-58 W/mK||with air: ca. 2040|
Due to the direct melting of the pipe surface at the start of the fusion process, a thermal expansion of the pipe material occurs in the fusion area. The result is a very good bridging of the annular gap as the coupler "grows" inwards and the pipe outwards at the same time. The melting front does not run evenly but in waves, as opposed to the covered heating coil version.
Therefore the resulting fusion area is considerably larger. If an oxidic layer forms in the coupling – in spite of the instruction to take the fitting out of the packaging only immediately prior to processing – this layer is broken up by the wave shape of the melt front and has virtually no impact on the fusion result.
If the coupler was contaminated on the construction site, though this should really have been prevented by the packaging, the appropriate cleaning agent (DVGW VP 603) will remove it leaving no deposits.
Therefore it is impossible to damage or remove the exposed heating coil by inserting the pipe. This has been tried in practice: After 30 years’ experience in the rough conditions on the construction site we do not know of any complaints regarding damage to the heating coil by inserting the pipe. In contrast, the position of the heating coil can be easily recognised – and controlled – by the user direct. This is a quality aspect which should not be underestimated with regards to the PE layer thickness of covered heating coils which may not be controlled.
Long term experiments were carried out and completed successfully on FRIALEN safety fittings with exposed heating coils both in extremely acid and alkaline media. For decades the experiences in this field have been positive resulting in leading companies in the chemical industry using these moulded parts.
Labelling of the moulded part is defined by the appropriate indications in the standards. Basic information such as manufacturer, nominal pipe diameter, material description, SDR series and manufacturing batch must be permanently legible.
Indications relevant to the construction site may be done on separate labels (e.g. barcode label), for example fusion and traceability parameters, SDR processing range for pipes and the required cooling time after fusion. All information should be available on the fitting in assembly condition (illustration 8).
The fusion parameters are gathered in the barcode. This prevents basic faults such as the potential faulty manual programming of fusion time and current. This technology has been accepted worldwide as standard. To avoid mix-ups, the barcode has been sensibly applied as a label to each construction part and is therefore a proper part of the fitting which cannot get missed.
The design of the barcode is standardised and allows in addition to the automatic recognition of the fusion parameters the option of recording further data such as information on the manufacturer, processing dates etc. using fusion units with documenting facilities.
A second, subsidiary barcode contains data on construction part traceability which can be collected separately as an option and used electronically as a building block for the pipe book.The barcode types are determined internationally preventing confusion (illustration 9).
This creates roughly identical processing conditions in the connection zones covered and protected by the fitting. Almost all fitting manufacturers use the temperature compensation technology in order to exclude the negative impact of different ambient temperatures.
With the aid of barcode technology and the development of today’s fusion units it has been possible to optimise the fusion parameters. Current indications by national and international standards allow us today to use the low voltage range up to 48 volt.
A balanced definition of voltage and time parameters guarantees that the fusion process will lead to the best possible result bearing in mind varying tolerances regarding fusion gap, ambient temperature, PE material and of course the fitting. As PE has relatively poor heat conductivity, and we need to avoid introducing excessively aggressive energy, a short fusion time has its problems for reasons of physics.
It is possible that heat does not penetrate deep enough into pipe and fitting, which will reduce build-up of melt and affect the quality of the joint. In reality fusion times do not affect installation times on the construction site as they are usually a matter of mere seconds or minutes. There is only a superficial advantage here which is clearly contradicting the fundamental physics.
|Dimension (pipe)||Nominal external diameter [mm] according to EN12201-3, |
|Central external diameter [mm]||Tolerance [mm] |
|d 32||32||32.0||32.3||+ 0.3|
|d 63||63||63.0||63.4||+ 0.4|
|d 125||125||125.0||125.8||+ 0.8|
|d 225||225||225.0||226.4||+ 1.4|
|d 400||400||400.0||402.4||+ 2.4|
|d 630||630||630.0||633.8||+ 3.8|
In order to safeguard the build-up of melt during the fusion, the fitting must be prevented from expanding. This can take place for example through reinforcement as for FRIALEN large couplers (illustration 10). This corset prevents the coupler from "expanding" and ensures a safe and sufficient build-up of melt.
In large pipe technology with diameters from d 280 mm to d710 mm the so-called pre-heating procedure provides the user with additional safety. The authorised tolerances listed in the standards grow with the external diameter of the pipe (table 3). However, the fact that installation of the moulded part is user-friendly must be retained.
During this pre-heating process a temperature is created in the connection zone via the special parameter which lies below the melt temperature. This heat transfer affecting the pipe surface direct is made possible only by the open, exposed heating coils.
The procedure may not be applied with covered heat conductors. The reduced gap will increase the joint quality considerably in these particular problem cases on the construction site due to the improved build-up of melt pressure.
The electrofusion technology for the connection of gas and water supply pipes – and increasingly for sewage systems – has for decades been established as a reliable, economic and user-friendly procedure. Fundamental prerequisite for the reliability of the pipe system is to stay consistently with the safety aspect right through from planning to the approval and operating stage.
Particular significance lies with quality demands regarding the choice of products to be used. Clear and obvious quality features can be found in the geometrical and design aspects of electrofusion fittings: large insertion depths, long fusion zones and exposed heating coils. The product is suitable for the rough conditions on the construction site due to its additional breadth of processing range.
 DIN 8074: Rohre aus Polyethylen, 08/99
 DVGW-Arbeitsblatt GW 330: PE-Schweißer, Lehr- und Prüfplan, 11/00
 DVGW-Arbeitsblatt GW 331: PE-Schweißaufsicht
 Eckert, Robert: Heizwendelschweißtechnik als Mittel zur Kostensenkung im Rohrleitungsbau?, Energie Wasser Praxis, Juni 2001
 Montageanleitung: FRIALEN°-Sicherheitsfittings für Hausanschluss- und Verteilerleitungen bis d 225, FRIATEC AG
Gieck, Technische Formelsammlung, 28. Deutsche Auflage 1984, Gieck-Verlag Heilbronn, ISBN 3920379144
 DVGW-Arbeitsblatt GW 335-A2: Kunststoffrohrleitungssysteme in der Gas- und Wasserverteilung, Anforderungen und Prüfungen - Teil A2: Rohre aus PE80 und PE100
 DVS-Richtlinie 2207-1: Schweißen von thermoplastischen Kunststoffen
 Allcard, C.: Fusion Group Manufacturing, UK: The evaluation of factors governing long time performance of polyethylen electrofusion fittings, Plastics Pipes XI, Munich, Germany, 09/ 2001
 DIN EN 1555: Kunststoffrohrleitungssysteme für die Gasversorgung - Polyethylen (PE)
 DIN EN 12201: Kunststoffrohrleitungssysteme für die Wasserversorgung - Polyethylen (PE)
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