Critical Flaw Size in Butt-Fusion Joints for Service Life Prediction of HDPE Pipes

There is a significant interest in the US nuclear industry to use HDPE piping in essential service water applications at commercial power plants. The unique challenges posed by the use of HDPE in safety-related applications have led to the recent activities in the plastics pipe industry, ASME Code Committees, and US NRC. A new Code Case N-808 has been proposed under ASME Boiler and Pressure Vessel Code Committee’s Section XI for flaw acceptance in HDPE pipe joints. A technical basis for the process of fusion joining HDPE pipes and confirmation of the structural integrity of joints for safe and continuous operation for a 60-year service life is needed. Hence there is an increased focus on the butt-fusion joining process, essential control variables, and critical flaws that may arise in the fusion zone during the plastic pipe joining process.

The current study investigates the effect of presence of critical flaws in the butt-fusion zone and how they affect the stress intensity in the joint region and impact the structural integrity of the HDPE piping. This is needed for the proposed ASME Boiler and Pressure Vessel’s Section XI for developing a flaw acceptance criterion in PE pipe butt joints. The API 579-1/ASME FFS-1 standard provides stress-intensityfactor (SIF) solutions to various semi-elliptical flaws in the outer surface (OD), inner surface (ID), and other full-circumferential surface flaws. The SIF solutions from API 579-1 and FEA of selected cases (for benchmarking purposes) have been used to develop simplified SIF relations for circumferential surface flaws that can be used for various plastic pipes with diameters ranging from 4 through 36 inches and dimensional ratios (DR) from 7 through 13. Further, the SIF of embedded flaws akin to lack-of-fusion regions have been evaluated. Using the analytical solutions, a set of carefully planned laboratory–scale coupon and hydrostatic pipe experiments have been used to develop a reliable service life prediction methodology for fusion joints in HDPE piping.