PREDICTION OF LONG-TERM PERFORMANCE AND STABILITY OF HIGHER
PERFORMANCE POLYETHYLENE (HPPE) PIPE SYSTEM
A technical understanding of the performance capabilities of higher performance polyethylene (HPPE) pipe systems will promote an increased level of confidence in the use of this material in water supply applications.
To confirm the long-term performance capabilities of HPPE pipe, particularly with respect to the claimed minimum required strength (MRS) value of 10 MPa, and to identify any further operational implications on the use of HPPE pipe systems for water supply applications.
In March 1993 the results of a three-year collaborative research programme were reported on a new class of polyethylene pipe material with a claimed higher resistance to fast fracture and higher strength compared to MDPE. This of polyethylene pipe is available in the UK manufactured from Solvay Eltex(R) TUB 124 polymer and has been designated higher performance polyethylene (HPPE). The collaborative programme was carried out at WRc, funded by the Foundation for Water Research (FWR) and by the resin and pipe manufacturers, Solvay S.A., Wavin Industrial Products and Stewarts and Lloyds Plastics.. The long-term hydrostatic testing carried out by the resin manufacturer on small bore HPPE pipe had shown the material to meet the 10 MPa MRS design requirement. In order to assess the capability of larger diameter pipe product to meet this material classification performance, a programme of testing was initiated by the pipe/resin manufacturers with complementary testing at WRc. This testing programme commenced in mid 1992. The latest results of these investigations are reported here. Interim information on the stability of the HPPE pipe system in service was sought to help develop confidence in the longer-term product performance.
Hydrostatic pressure testing of HPPE pipes has confirmed that the material exceeds the MRS value of 10 MPa in pipe product form. The MRS currently achieved of 10.22 MPa is based on the use of "unfailed" data points, so actual performance will be better. A minimum MRS value of 10.38 MPa will required to formally qualify SDR 17.6 HPPE pipe as a 10 bar rated product at 20ºC with the ISO specified minimum safety factor of 1.25. If ongoing pressure test pieces have not failed at their current test pressures by the end of April 1994, the above requirement will be satisfied. A statement in Water Bulletin will be published at the appropriate time. Results of thermal stability testing of weathered HPPE pipe samples have shown a reduced stability after an extended period of weathering of exposed surfaces. Pipes exhumed after 3 years in service showed satisfactory thermal stability values. Future repairs to failed HPPE pipe using electrofusion socket couplers or mechanical fittings could prove difficult due to creep causing increases in the pipe OD after time in service.
As a result of this additional work it is confirmed that SDR 11 HPPE pipes can be used with confidence up to 20ºC at their stated maximum pressure rating of 16 bar. The conclusion of the testing is still required to confirm a 10 bar pressure rating for SDR 17.6 pipes. The actual pressure capability should be confirmed in a planned statement in Water Bulletin when further results become known (in April 1994). To maximise potential long-term stability it is recommended that HPPE pipes are installed as soon as possible after production or that they are protected from UV radiation if storage in excess of one year is anticipated. Attention should be given to the development of repair methods that can tolerate the in-service creep that occurs with polyethylene pipe materials.
This report summarises additional testing work which has been undertaken on HPPE pipes to assess the long-term performance capabilities and confirm the MRS classification value.
The contributions from the members of the project management team are greatly appreciated in the direction and practical support provided for this project. The project management team comprised Anglian Water Services, Severn Trent Water, Wavin Industrial Products, Stewarts and Lloyds Plastics, Solvay S.A. and Solvay U.K.
Copies of the Report are available from FWR, price £15.00 less 20% to FWR Members