A.C. Seibi, R. J. Lawrence, R. Jepson and Z. Tarik
Papers # 2012 Barcelona
Design and fabrication of plastic pipe fittings have been governed by ISO 4427 where pressure derating factors of 0.8 and 0.5 have been used respectively for fabricated elbows and tees. However, laboratory data generally indicate that these derating factors are very conservative. Moreover, the design of distribution systems is typically based on the use of PN10 pipe (SDR17). In practice, distribution pressures are generally only 25% of the design pressure implying that derating is unnecessary. The mis-use of the derating factors from ISO 4427 is often further exacerbated by confusion among some designers and end users between instantaneous burst failure and the service life requirement built into the definition of ‘pipe rating’ as used in plastic pipes. The present paper, therefore, examines these controversies by conducting an experimental investigation and finite element modeling of elbows and tees of plastic pipes. The aim of the study is to predict burst pressures for both fittings and check i) if it is necessary to use the derating factors for fabricating elbows and tees and ii) how much the pressure rating should be if finite element results indicate that derating is necessary. The experimental results of burst pressure showed that the derating factor for tees ranges between 0.8 and 1.0 as compared to 0.5 specified in ISO 4427. The laboratory experiments also confirmed that a derating factor of 1.0 for elbows is achieved for cut angles between 7.5 and 15 degrees compared to 0.8 specified in ISO 4427. Finite element results of the two joints confirmed the laboratory experiments by showing that the induced equivalent stress at the welds is indeed lower than that of the pipe indicating that burst failure can take place on the pipe and not the welds if welding is done properly.
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