Rate Process Method Applied To Service Life Forecast Of PE Molded Fittings

This article reviews several applications for the Rate Process Method (RPM) such as validation of the PE material hydrostatic design basis per ASTM D 2837 and service life forecast of older-generation PE pipe still in service and correlation with actual field failures.

We also investigate a recent application in which the RPM was used to determine the service life forecast of a molded PE heat fusion fitting that the manufacturer had flagged during a routine post-production quality check. Because several of these fittings had already been installed for gas distribution, the manufacturer wanted to know what the projected service life would be.

Here is some background. During a routine quality check, a fitting manufacturer noted a problem with one of its fittings - a ¾-inch IPS SDR 11 x ½-inch CTS 0.090-inch butt fusion reducer made from a PE 2406 material. The manufacturer noted that these fittings were failing prematurely in the knit line. They also determined that this phenomenon was limited to these butt-fusion reducers produced during a defined timeframe.

Some of these butt-fusion reducers had already been shipped to a gas utility company. The manufacturer immediately contacted the utility to alert it about these fittings. The gas company responded that some of the fittings had already been installed.

The manufacturer then contacted the authors to determine what testing could be done to determine the projected life of these butt-fusion reducer fittings that had already been installed. We recommended the Rate Process Method (RPM) as being the best test method available to determine the projected life of these PE 2406 butt-fusion reducer fittings.

The RPM uses a three-coefficient equation that relates time, temperature and pressure (or hoop stress). It is based on two well-known scientific principles:

Principle 1. There is a linear relationship between the log of stress and the log of time.

Principle 2. There is a linear relationship between the log of time and the inverse of temperature (absolute temperature in degrees Kelvin). This is the principle of Arrhenius.

Both principles are true for all thermoplastics piping materials. By combining these two relationships, the Rate Process Method three-coefficient equation results:

where:
time = failure time (hours)
T = temperature (K)
P = internal pressure (psig) or internal hoop stress (psi)
A, B, C = three coefficients

Dr. Chester Bragaw originally described the concept and mathematical basis for using the RPM for polyethylene (PE) pipe and fitting service projections. (1,2)

The Plastics Pipe Institute (PPI) Hydrostatic Stress Board (HSB) conducted an extensive evaluation of this and other methods for forecasting the effective long-term performance of PE thermoplastic piping materials. Basically, all of these methods require elevated temperature, sustained pressure testing of pipe that results in brittle-type failures as would be observed in the field. The failure data at the elevated temperatures are used to predict the performance of a material at end-use/service temperature and pressure conditions. Details of these evaluations and conclusions are reviewed and discussed in “Rate Process Concepts Applied to Hydrostatically Rating Polyethylene.”(3)