Long-Term Strength and Design of Polyethylene Compounds for Pressure Pipe Applications

The interest in using polyethylene pipe in Class 3 safety water systems in nuclear power plants has grown tremendously in the last few years. PE pipe brings a host of benefits to the application in the form of long-term performance and reliability due to not being prone to corrosion and tuberculation. As the work continues through various ASME committees to develop the appropriate code language for the design and use of PE pipe, it is clear that plastics are not evaluated the same way metallic components would be in similar applications. However, the nature of the failure (i.e. ductile or brittle) is important for both. This paper will give an overview of the methodology used to establish the long-term hydrostatic strength of polyethylene compounds, and how that strength is used for engineering design in a safe a reliable manner. The strength of a polyethylene compound, being a thermoplastic, cannot be determined from a short-term tensile strength test, as with most metals. As such, testing and evaluation methodologies have been developed which take into account the viscoelastic creep response of thermoplastics, as well as potential changes in failure mode, in order to forecast the long-term hydrostatic strength of these materials so they can be safely used in a pressure pipe application. Since PE was first used in a piping application in the late 1950s, PE has continued to evolve as have the methodologies used to evaluate its strength against stresses induced by hydrostatic pressure. The common method for evaluation relies on putting specimens under multiple continuous, steady-state stress levels until failure. These data points are then used in a log-log linear regression evaluation. This regression equation is then extrapolated to a point sufficiently further out in time to where a long-term strength can be established. It has been clearly established that over a temperature range that the stress rupture behavior of PE follows an Arrhenius, or rate process, relationship between temperature and strength. By testing at elevated temperatures it can be “validated” that the extrapolation remains linear and ductile beyond the actual test data. This and other criteria established by ASTM D 2837 and the Plastics Pipe Institute’s Hydrostatic Stress Board allow for establishing an appropriate maximum working stress that will assure a very long design life.