Polyethylene pipeline systems have been used by our customers for drinking water supply since their introduction in the 1950s. The plastics industry has taken great responsibility in ensuring that the products used do not adversely affect water quality.
Plastic pipes are made from visco-elastic material which means the tensile or hoop stress capacity varies with time. Long term strength is determined by the results of long-term pressure testing and performing regression analysis on the test data. This gives the design stress of the material for the service life, and hence maximum operating pressure. This method takes into account the creep of the material over its design life.
To obtain the regression curve, laboratory tests are normally carried out on pipe samples subjected to a range of internal pressures, and hence pipe stresses, at 20AC, 60AC and 80AC. The test regime is designed to obtain burst test results at intervals from one hour to more than 10,000 hours (approximately 14 months). Regression analysis is carried out on the test results to predict the long term strength at 50 years and 20AC.
PE100 material has a 50 year Minimum Required Strength (MRS) of 10 MPa. In practice however the actual strength is greater than the design strength and hence the expected resulting service lifetimes are greatly in excess of the nominal 50-year requirement when the pipe is operating within its design envelope.