Influence of shock pressure testing on the service life of HDPE piping systems

Session 2B
1:20

Ulrich Schulte, LyondellBasell Industries

Before it is put into service, every newly installed piping system must – irrespective of the materials from which the pipes are manufactured – undergo a shock pressure test in accordance with European standard EN 805. This test serves to verify the tightness of the entire system when subjected to increased internal pressure. Piping systems for the supply of drinking water are in most cases rated for a working pressure of 10 bar. These systems are then subjected to a test pressure of 15 bar for at least 3 hours at a test temperature of 20 °C. For pipes manufactured from HDPE, the stress intensity is very high. Therefore installers and end users fear that the service life of the piping system may already be adversely affected through the acceptance test.

Contrary to such fears, and in spite of the very high short-term stress, the shock pressure test does not have any adverse effect on the service life of HDPE pipes manufactured from PE 100:
• In the case of a class SDR 17 pipe, the hoop stress to which the pipe is subjected during the test is 12 MPa. This value is below the limit for the minimum required strength of PE 100 pipes: as a reference point, a value of 12.4 MPa for a duration of 100 hours is still prescribed at present for the creep diagram (this will shortly be 12.0 MPa / 100 hours). The shock pressure test does not influence the limit value curve and therefore has no adverse effect on the service life of PE 100 pipes.
• The shock pressure test does not affect the two other failure mechanisms for HDPE pipes either, these being slow crack growth and thermo-oxidative degradation; neither does the test trigger stress cracking nor does it consume stabilizer during its application.

The above-stated facts have been confirmed by carefully conducted model tests. Comparisons were made between non-stressed PE 100 specimens and test specimens on which the stress had been simulated by means of the shock pressure test.

• Creep strength comparisons (temperature: 20 °C, hoop stress: 12.4 MPa) have so far revealed no differences (these tests are still in process).
• A comparison of the oxidation induction times (OIT) – as a measure of stabilizer consumption – showed no differences either.
• No significant differences were found when testing for susceptibility to slow crack growth or stress crack failure using the “2-NTC” method.

Based on these results it can be stated that Class SDR 17 pipes manufactured from PE 100 are thus suitable for local pipe networks that are subjected to a system pressure of 10 bar. Even the shock pressure test, which must be carried out in accordance with EN 805 before the piping system is put into service, does not adversely affect the service life of the system, though the testing temperature must not exceed the prescribed temperature of 20 °C.

Dipl.-Ing. Ulrich Schulte, Dr. Heinz Vogt, Dr. Hans-Friedrich Enderle, Dr. Dieter Lilge