Design & Testing
Plastic Pipes Conference Association # 1998 Gothenburg
Vanspeybroeck
Plastics pipes and fittings are used in all types of applications in the water, gas, sewer and other industrial applications, where long life (minimum 50 years) is needed.
Early applications have already been reported since the year 1935, but the imp0rtar.t growth was after the years 1970.
A steady stream of new applications is now evolving; materials are being improved, new systems developed, and pipe and fitting joints are being much improved.
Tremendous work has been done on national and international levels with regard to standardization and quality requirements (IS0 and CEN normalization).
Each country now has its own set of standards, which are linked together through standards by the International Organization for Standardization (ISO) and the European Committee for Standardization (CEN).
An important factor in the design of plastics pipes and especially in pressure application is the allowable hoop stress or design stress, which is the leading factor for the wall thickness calculation of the pressure pipes. The methods to establish the design stress and to control the quality of the pipe in this aspect are defined in numerous standards, such as NBN T42-106, I S 0 4427, I S 0 4437, ASTM, DIN and CEN standards.
The main emphasis in pipe testing in Europe has been to find the time to burst at a sustained internal hydraulic pressure at various temperatures, under the assumption that at elevated temperatures the time to failure under pressure will be shortened (time-temperature interchangeability).
Then by "hand" extrapolation techniques, a stress was established which could be endured during at least 50 years without risk of failure. A safety coefficient or design factor is then applied on this stress value to arrive at the allowable hoop stress, from which wall thicknesses of pressure pipe can be calculated.
In the USA a statistical method has been used to extrapolate sustained internal hydraulic pressure test data at service temperature to establish the expected burst stress at 100,000 hours. A design factor or safety coefficient was applied to this value to arrive at an allowable hoop stress.=
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