Ultrasonic Phased Array: Detection of Detrimental Conditions in High-Density Polyethylene Butt-Fusion Joints

Today, commercial nuclear power plants are installing High-Density Polyethylene (HDPE) in non-safety-related and safety-related applications. While this material has numerous advantages over the carbon steel pipes that historically have been used for the same applications, developing a way to accurately inspect for joint integrity in HDPE has become increasingly important to utilities and the U.S. Nuclear Regulatory Commission (USNRC). This paper will investigate the ability to quantify the levels of detection of flaws and detrimental conditions using ultrasonic phased array, in butt-fusion joints throughout the full spectrum of applicable HDPE pipe diameters and wall-thicknesses. Perhaps the most concerning joint condition is that of “Cold Fusion”. A cold-fused joint is created when molecules along the fusion line do not fully entangle or co-crystallize. Once the fusion process is complete, during visual examination, there is the appearance of a good quality joint. However, the joint does not have the strength needed, as the required co-crystallization along the pipe faces has not occurred. Performing a visual examination of the bead, as required by the current revision of ASME Code Case N-755, does not provide adequate guarantee of joint integrity. Therefore, volumetric examination is of special concern to the USNRC to safeguard against this type of detrimental condition. Factors addressed will include pipe diameter, wall-thickness, fusing temperature, interfacial pressure, dwell (open/close) time, and destructive verification of ultrasonic data.