Pressure pipes made of Polyethylene are successfully used since the 50ths of the last century. In more than 60 years the material has developed significantly in mechanical, thermal and physical characteristics. Mostly used in the pressure market is the pipe grade PE100 with a long term strength of 10 MPa (MRS 10). Furthermore you find in the market PE-types with optimized properties for example with a better resistance against slow crack growth, called PE 100 RC (crack resistance) or fiber reinforced polyethylene pipes (e.g. Krah PE-GF 200) with very high strength and stability (MRS 20).
The international mostly used standard for polyethylene pressure pipes is the ISO 4427. Even if this standard has some limitations in tolerances and dimensions, it is always a good basement for more detailed specifications.
The requirements of ISO 4427 are original made for external calibrated pipes (direct/axial-extruded) and the standard is limited until pipe diameter OD 2000 only. Nevertheless the physical and mechanical requirements can be adopted for larger dimensions easily.
ISO 4427 does not consider that more and more pressure pipes in large dimensions are produced in helical extrusion process with internal calibration. For helical extrusion the pipe tolerances must be verified, the wall thickness can be produced much more accurate because no sagging-effect happens during production. Additionally it would make sense for helical extruded pipes to specify the tolerances for inner diameter. Also the pressure classes should be defined more flexible, because for these pipes there is almost no limitation in wall thickness and every pressure class can be manufactured.
Also low quantity of pipes can be produced very economically with the helical extrusion process.
This kind of helical extruded pressure pipes are already used worldwide. The company UGPM from the Sultanate of Oman for example is using the newest pressure pipe production technology for direct extrusion and helical extrusion (Krah-pipes). For large dimensions UGPM installed a special isolated test basin with all equipment for testing pressure pipes and fittings until DN 3000 mm.
In the following, the test procedure for a helical extruded Krah pipe in dimension DN/ID 1700 is documented.
These pipes are part of the big pressure pipe project “Ghubra Desalination plant”, where the pipe dimensions ID1700 and ID 1900 are required in SDR 26 (PN6). All pipes and necessary bends, T-branches, stub-ends and flanges are produced and manufactured by UGPM Oman and will be tested acc. ISO 4427.
For the helical extruded PE100 pipes, the resistance against constant internal water pressure has to be determined at constant temperature bath and at prescribed duration and conditions according ISO 4427.
Date of Testing:
According to ISO 4427 and in agreement with the third party, UGPM has tested the pipes under two different load conditions:
Temperature = 20°
Duration = 100 h
Test Pressure = 9 Bar
Stress = 12,4 MPa
Samples = 3
Temperature = 80°C
Duration = 165 h
Test Pressure = 4,4 Bar
Stress = 5,4 MPa
Samples = 3
Specimen before, during and after testing
The helical extruded pipes DN/ID 1700, PN6 fulfills the requirements of ISO 4427 regarding hydrostatic pressure load at 20°C and at 80°C.
The pipe-specimens have not shown any damage during or after testing.
The helical extruded pipes are technical equal to axial extruded pipes. The pressure load capacity of helical extruded pipes is minimum the same like for axial extruded pipes. The tolerances for wall thickness of helical extruded pipes are less because of no-sagging-effect and rotating production procedure. Tolerances for ovality are more or less the same.
The difference in calibration should be considered for joining pipes and fittings.
An additional specification for helical extruded pressure pipes, concerted to requirements of ISO 4427, is available in the Download-section of Krah Community soon.
Eng. Mohammed Al Hashani
Dipl.-Ing. Stephan Füllgrabe