Reliability of Polyethylene pipe in water distribution systems
Polyethylene pipes have been used for many years for the supply of drinking water and gas. In Europe , High Density Polyethylene (HDPE) was already being used at the beginning of the 1960s. Since then, the materials have undergone continual improvements. The so-called “second generation” HDPE materials were produced in 1980. They were characterised by the integration of comonomers to fulfil the demands for higher resistance against environmental stress cracking (ESCR) and a better compromise between flexibility and long-term strength. HDPE materials have been used for almost 50 years and the time has come to assess their actual efficiency.
Fortunately in 2002, the German water distribution company GELSENWASSER urged SABIC to analyse a defective pipe sample. This led to the development of an analytical project for assessing the state of other pipe samples. The oldest pipe we found was from 1966, without any doubt a 1st generation product. Others came from the 1970s. As a representative of the 2nd generation, we received a pipe that had been buried in 1982.
A number of investigations were carried out on all samples. Apart from typical PE characteristics such as MFR and density, the remaining protection against oxidation and a possible degradation during extrusion was investigated. Moreover the development of ESCR and, of course, the residual resistance to hydraulic pressure was of particular interest.
What were our findings then? Could we transfer the findings towards current products? The MFR and density of the products have not changed much over the years. An important discovery was that even the oldest pipe contained enough antioxidants to still render it sufficiently protected against ageing. The elongation at break of the oldest pipe decreased to 150 %, reflecting a degree of ageing, which of course all organic material undergoes. ok. Since strains under service conditions rise up to maximum 2%, the strength of the pipe still is sufficient to prevent it from breaking.
One of our major findings was that over the years the materials have pre-eminently improved in ESCR (NCTL according to ASTM 5397) from 8 h to 20 h for the 1982 pipe. Recent materials, such as PE 100, render to have a resistance of over 1000 h, proving the outstanding quality of these materials.
The most interesting issue was the residual resistance to hydraulic pressure. For this reason, intact pipe pieces were again subjected to hydraulic pressure under the conditions that prevailed at the time when they had been buried.
The results were surprising. All the pipes that had been correctly produced, met the prevailing batch release criteria. The oldest pipe reached a time to failure of 92 h, by far exceeding the required time of 48 h as can be seen in the graph below. If one were not informed about the history of this pipe, it would be approved for another 50 years of service. Given the fact that this pipe had already consumed 36 years of its predicted lifetime this result must be regarded as very good. Other pipes, e.g. from1969 and 1974 even exceeded the regression line of PE-HD, despite having been in operation for a long time.
Resistance to hydraulic pressure of 1st generation pipe after 36 years in use
The most promising result was gained from the 1982 pipe. This pipe, as can be seen in the graph below, can even cope with the requirements for a recent PE 80 material. Despite having been in operation for 22 years the material again reached a time to failure of 2717 h. This result indeed justifies the confidence end users have had in the use of polyethylene pipes.
Resistance to hydraulic pressure of 2nd generation pipe after 22 years in use
If the old pipes originating from the very beginning of PE production already meet 50 years lifetime, what can we expect form new materials like bimodal PE 80 and PE 100? At the stress levels at which we examined the old pipes, PE 100 has already exceeded 32,000 h testing time and tests are still running in SABIC’s laboratory.
This simple comparison reveals PE 100’s high inherent potential. More modern statistical methods, as laid down in ISO 9080, provide more security concerning the lifetime prediction. PE 100 materials are currently classified for a lifetime of 50 years but actual product curves, e.g. those of SABIC Vestolen A 6060 R, still reveal the materials to fulfill the PE 100 requirement after 100 years. Hence it can be assumed that pipes made out of PE 100 can remain in operation for 100 years.
But the compound is only one part of the chain. Pipe production, pipeline construction, jointing and embedding in sand also contribute to the lifetime of a pipeline. The objective to reach 100 years lifetime can indeed be achieved if all participants in the value chain take due responsibility.
Value chain waterpipeline