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Welded joints in polyethylene piping systems for water and gas supply. Assessment tests

8-11-2013 | news | Pipes test

Polyethylene (PE) piping systems and fittings have the advantage that they can be butt welded or joined by means of electrofusion fittings generating joints that are completely leaktight and resistant to tensile strength.

Part 5: ‘Fitness for purpose of the system’ of standards UNE EN-1555 and UNE-EN 12201 compiles the tests to be conducted on welded joints of PE piping systems for the conduction of gas fuels, water and sewerage under pressure, respectively.

CEIS laboratory has recently obtained ENAC accreditation to conduct two of the tests on welded joints mentioned in said standards: Determination of resistance to tensile strength and the type of failure in butt welded test pieces according to UNE-ISO 13953 and crushing decohesion of PE electrofused joints according to ISO 13955, and is planning to shortly accredit the peel decohesion of PE electrofused joints with an external nominal diameter equal or above 90mm according to ISO 13954.

CEIS is currently assessing butt welded joints of PE pipes with a thickness up to 75-80mm and electrofusion fittings up to DN250 SDR11 and the following are regular clients for this type of tests: manufacturers of PE manipulated fittings (LlaberíaPlàstics, S.L.), gas distribution companies (Gas Natural-Fenosa), construction companies (GrupoTragsa) and Certification Entities (AENOR).   

 Introduction

The more than 100 technical codes drafted by working Group W4 ‘Plastic Welding’ of the German Welding Society–DVS  offer the widest reference to welding techniques of plastic materials, inspection and test methods and recommended assessment criteria.

At national level, ‘Technical specifications by SEDIGAS for handling and welding of PE pipework for gas distribution’ is the document of reference for the gas sector and includes both in Part 2 on Butt Welding and in Part 3 on Electrofusion Welding, a clause on Welding Inspection and, more precisely, on standardised destructive tests to be conducted on both types of joints: tensile strength test on test pieces obtained by welding according to standard ISO 13953, and peel tests according to standard ISO 13954, crushing tests according to ISO 13955 and tear tests according to ISO 13956, the latter being applicable to electrofusion fittings of fixture type (T for tapping tee) that will not be examined in this article.

These are the tests referred to on Table 5 ‘Fitness for purpose of the system’ of product standards UNE-EN 1555-5 for PE piping systems and fittings for the supply of gaseous fuels and UNE-EN 12201-5 for PE piping systems and fittings for water supply and sewerage under pressure, the content of which is partially reproduced in Table 1.

 

Test on resistance to tensile strength on butt joints

Standard UNE ISO 13953 describes a method to assess the resistance to tensile strength and the fracture nature of the butt welded joints of PE pipes and/or fittings with a nominal diameter equal or above 90mm.

A series of mechanised test pieces with n strips regularly spaced around the butt welded joint circumference are subject to a tensile strength test at constant speed s=5mm/min. The aim of the test, conducted at 23ºC, is to concentrate the effort on the welded area and cause the fracture around the butt joint.

 The shape and dimensions of said test pieces are established in ISO 13953 for wall thickness where t<25mm (type A) and t?25mm (type B)

 

Figure 1. Test piece type A

Figue 2.Test piece type B

Figure 2. Test piece type B

 Throughout the test, the strength applied while stretching until the test piece is completely fractured is registered; at the end of the test the resistance to tensile strength and the maximum strength applied (in Nw) divided by the transversal section (in mm2) at the neck of the dumbbell measured at the beginning of the test are calculated. The fracture surface is examined identifying the type of fracture: ductile or fragile. 

Figure 3. Ductile fracture of test piece Type A

Figure 4. Ductile fracture of test piece Type B

Peel decohesion test according to ISO 13954

Standard ISO 13954 describes the method to assess the resistance to decohesion of electrofused joints of PE fittings with a nominal diameter equal or above 90mm.

 A series of 4 mechanised test pieces with a strip each that are regularly spaced around the electrofused joint circumference are subject to the test on tensile strength at a constant speed s=25mm/min. The aim of the test, conducted at 23ºC, is to concentrate the effort on the electrofused area and cause the separation at joint level. 

Figure 5. Test piece according to ISO 13954

 

The shape and dimensions of said test pieces are established in ISO 13954; geometry of the fitting at the rear end of the outlet has a deciding influence in the location of the through-hole since it requires Ø?3mm to keep the union shackle with the special tensile strength device adapted to the tensiometer bridge.

 The test is conducted until the parts are completely separated, registering the place where the fracture takes place (in the pipe, outlet, filaments or at joint level) and the type of fracture observed, ductile or fragile.  In the event that a fragile separation takes place at joint level, it shall be expressed as a percentage of the length existing between the first and last filament of the joint being tested (L), the maximum fragile type decohesion value admitted both in UNE EN 1555-5 and UNE EN 12201-5 is L/3. 

                                                                          

                      Figure 6. Peel Test in process.                                         Figure 7. Fragile type decohesion

 

 

                       Ductile type decohesion

Crushing decohesion test accoding to ISO 13955

Standard ISO 13955 describes the method to assess the resistance to decohesion of electrofusion joints of PE fittings with pipes having a nominal diameter from 16-225mm.

90º sectors (90?DN?225mm) or 180º sectors (16?DN?90mm) directly extracted from the joint to be tested are subject to a compression test at constant speed s=100mm/min between plates which are 200mm wide until the internal sides of the test piece are in contact with one another.

 

                                                                                                                      Figure 8.                                                                                                                    

Once the test has been conducted, both sides of the test piece are examined and the place where the fracture takes place is registered (in the pipe, the outlet, between the filaments or at joint level) and the type of fracture is observed, ductile or fragile. In the event that a fragile separation takes place at joint level, it shall be expressed as a percentage of the length existing between the first and last filament of the joint being tested (L), the maximum fragile type decohesion value admitted both in UNE EN 1555-5 and UNE EN 12201-5 is L/3. 

               Figure 9. Test piece sector 90º 

                Figure 9. Test piece sector 90º

Conclusions

Fitting for use of joints in the water and gas supply system made up of PE pipes and fittings welded to one another is assessed by means of tests established in Part 5 of standards UNE EN 1555 (for the supply of gaseous fuels) and UNE EN 12201 (for water supply under pressure).

Conformity assessment of the results of said tests, based on the type of fracture observed –ductile or fragile- or on the determination of the fragile fracture percentage of decohesion requires expert personnel who is familiar with butt welding and electrofusion techniques as that working at CEIS Laboratory, and that enables to provide accredited testing services to Certification entities, water and gas distribution companies, engineering companies, construction companies and other firms of professionals in the plastic welding sector

References

[1] DVS Technical Codes on Plastics Joining Technologies, English Edition, Volume 3. DVS Media GmbH, Düsseldorf. 2ndEdition, 2011.

[2] Especificaciones técnicas de SEDIGAS para la manipulación y soldadura de tuberías de PE para distribución de gas. (Technical specifications by SEDIGAS for handling and welding of PE pipework for gas distribution). Revision 13. December 2010.

[3] UNE EN 1555-5:2011 Plastic piping systems for the supply of gaseous fuels – Polyethylene (PE) – Part 5: Fitness for purpose of the system.

[4] UNE EN 12201-5:2012 Plastic piping systems for water supply, and for drainage and sewerage under pressure – Polyethylene (PE) – Part 5: Fitness for purpose of the system.

[5] UNE ISO 13953:2011 Polyethylene (PE) pipes and fittings. Determination of the tensile strength and failure mode of test pieces from a butt-fused joint 

[6] ISO 13954:1997 Plastics pipes and fittings — Peel decohesion test for polyethylene (PE) electrofusion assemblies of nominal outside diameter greater than or equal to 90 mm.

[7] ISO 13955:1997 Plastics pipes and fittings — Crushing decohesion test for polyethylene (PE) electrofusion assemblies.

[8] ISO 13956:2010 Plastics pipes and fittings — Decohesion test of polyethylene (PE) saddle fusion joints — Evaluation of ductility of fusion joint interface by tear test.tion of ductility of fusion joint interface by tear test.

 

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