Effect Of Eccentricity On The Measurement Of Hydrostatic Pressure Resistance Of Plastic Pipes
Suleyman Deveci, Birkan Eryigit, Senthil K. Kaliappan
Papers # 2018 Las-Vegas
The effect of eccentricity on the failure time of pipes under hydrostatic pressure test has a significant magnitude that it can easily mislead the evaluation of pipes according to requirement of ISO 4427-2. Therefore, authors propose a maximum relative eccentricity limit of 0.015, even lower, for pipes being tested according to ISO 1167.
Polyolefin pipes are increasingly used as the preferred product to transport water from the source to the service. Potable water networks are pressurized to create the required flow inside the pipe, which creates a circumferential or hoop stress on pipe wall. Therefore the pipeline has to be designed accordingly to withstand these hoop stresses. This is achieved by the correct selection of material and pipe dimensions.
The standard test method for measuring a material’s resistance to hoop stress is the hydrostatic pressure test (HPT) performed according to ISO 1167; this is applied at 20°C and elevated temperatures and used in conjunction with the ISO 9080 testing scheme to fully characterize the material’s performance. An important aspect of this method is the accurate measurement of pipe dimensions including (critically) the minimum wall thickness, which is used to calculate the required internal pressure to give a pre-defined hoop stress. The thinnest point in the pipe wall is the location of the highest hoop stress and is the point where the test specimen fails.
Plastic pipes are produced according to given standard dimensions as specified in international standards, e.g. ISO 4427-2, where limits on the outer diameter, wall thickness variation and out-of-roundness are prescribed. Wall thickness of the pipe defines the inner diameter of the pipe and the inner and outer circumferences of the pipe are expected to be concentric or at minimum eccentricity, although there is no specific mention of this in the standards. Therefore, one can produce pipe at a certain eccentricity but still comply with the product standards.
In this study, we have evaluated the effect of eccentricity of 32 and 110 mm diameter SDR11 pipes made from PE100 material on the HPT performance at 20°C and 80°C with the support of heat transfer and thermal analyses. It has been observed that pipe eccentricity has a significant effect in internal pressure testing. The effect of eccentricity is of technical significance even within the limited range of eccentricity allowed by wall thickness variation specified in current pipe standards. Therefore, we suggest a maximum eccentricity level of pipe specimens be introduced in ISO 1167 in order to prevent the misjudgment of material properties from pressure testing.
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