Pipes extrapolation limits

In terms of time, for high temperature steels it is conventional to restrict extrapolation to a time factor of 3, although years of practical experience are allowing this to be extended as far as 10. ISO 2578 [4] recommends no more than a factor of 4, from 5,000 to 20,000 h. A maximum value of 100 (two orders of magnitude) is permitted for pipes with the restriction that the temperature difference is limited to 60 °C for polyolefins and to 25 °C for PVC. General advice is no more than a factor of 10. Figures 9.1 and 9.2 show instances of somewhat extreme prediction far into the future of the human race. 

Two different approaches for lifetime prediction are presented. The underlying lifetime limiting processes have been identified in two cases. Mathematical expressions of chemical/physical relevance were used for the lifetime predictions for PE hot-water pipes and cables insulated with plasticized PVC. Accelerated testing, extrapolation and validation of the extrapolation by assessment of the remaining lifetime of objects aged during service conditions for 25 years were successfully applied to cables insulated with chlorosulfonated polyethylene. Polyolefin pipes exposed to chlorinated water showed a very complex deterioration scenario and it was only possible to find a method suitable for predicting the time for the depletion of the stabilizer system. 

Since experimental creep rupture times rarely exceed 10" h, it is necessary to extrapolate the data, using a straight line extension of the ductile rupture line on the log-log graph. The British Gas Specification for polyethylene pipe required the 50 year creep rupture stress cr o > 10 MPa. The International Standard ISO 9080 classifies polyethylene as PE80 if the lower confidence limit of the 50 year creep rupture strength lies between 8.0 and 9.9 MPa, and as PEIOO if it lies between 10.0 and 11.9 MPa. 

Equation (4.155) shows that the smaller of the two coefficients (fc , or k ) limits the corrosion rate. Generally, when a solid film (carbonates, oxides, sulfates, etc.) forms on a metal surface, the corrosion rate progressively decreases with time. While this is a fortunate phenomenon it also complicates efforts to develop accelerated corrosion tests. Indeed, in Figure 4.40, if one would extrapolate the results obtained after two or three days only, one would grossly overestimate the rate of corrosion that determines the lifetime of the steel pipe. 

Sufficient data for HDPE geomembrane materials are not usually available from pipe pressure tests which would enable a direct extrapolation of the stress-rupture curve at 23 °C or 40 °C according to the instmctions of ISO 9080 for small stresses and longer times. The emphasis is on the brittle branch of the stress-rupture curve which results from brittle failures and determines the failure behaviour at small stresses and extremely long service lifetimes. However, one can use the experience accumulated with HDPE materials over many decades (Schulte 1997 Krishnaswamy 2005) and fall back on the extrapolation factors of DIN 16887 or ISO 9080 which were checked by measured stress-rupture data of polyethylene pipes. These factors Ke give the time limits of a permissible extrapolation of the stress-rupture curves in the following sense. Let us assume that hoop stress versus times-to-failure data are measured at several higher test tempera-... 

Therefore, in the case of turbulent flow, the drag reduction effect depends by the individual macromolecules stretching degree [1086]. By photographing J.W. Hoyt succeeded to visualise the elongated particles [1087]. The possible extrapolations and the limits of this effect have been defined by Hoyt starting from the friction velocity, u, caused by the friction to the pipe wall, which is expressed as ... 

Estimates based on limited data or devised by extrapolation. All other ratings based on operating experience or field tests pipe and linings Trademark of Union Carbide Corporation. 

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