Technical Basis for Proposed Code Case on Evaluation of Flaws in Butt Fusion Joints in Class 3 High Density Polyethylene Piping

Abstract

Abstract There is a need for ASME B&PV Code procedures and acceptance criteria for evaluation of flaws detected by inspection of high density polyethylene (HDPE) piping items in safety Class 3 systems. A Section XI Code Case is proposed to evaluate circumferentially orientated planar flaws in butt fusion joints in PE4710 HDPE pipes with a parent material minimum PENT rating of 2,000 hours. The proposed Code Case contains flaw evaluation procedures and acceptance criteria for both surface and subsurface flaws. Two sets of acceptance criteria that are in terms of the allowable stress intensity factor and the allowable flaw size are provided in the proposed Code Case. The calculation of the allowable stress intensity factor is based on the statistical model documented in the paper PVP2020-21783. The model was developed for predicting the time to failure of a butt fusion joint containing a flaw in an HDPE pipe with a parent material PENT rating of 10,000 hours. This paper provides the technical basis for the application of the statistical model to flaws in butt fusion joints in HDPE pipes with a parent material PENT rating between 2,000 and 10,000 hours. For a piping lifetime of 50 years, simplified engineering equations to calculate the allowable stress intensity factor have been developed, and example calculations of the allowable flaw sizes at the maximum Service Level A & B Code allowable temperature of 60°C (140°F) and the stress level of 500 psi (3.45 MPa) are also provided in this paper.