Plastic Pipes Conference Association # 1995 Edinburgh
Once a grade of polymer has been specified for pipe production, the properties of the finished pipe depend on the processing conditions. By properties we mean here those attributes that will influence the in-service performance, and these can be grouped under three headings:
- Pipe geometry. To meet specifications and avoid wastage of material, pipe wall thickness must be sufficiently uniform. This beaimes increasingly di&cult to achieve as pipe size and wall thickness increase, due to grmity induced flow (or sag) of polymer which remains molten for long periods near the bore, whilst the pipe is wold from the outside.
- Thermal stresses. Density changes accompanying cooling induce thamal messes - compressive on the outer and tensile on the inner surface. Axial and hoop stress components are of similar magnitude - around a few MPa. Hoop stresses can be significant compared with in-service values resulting from intend pressure. Axial messes are responsible for the turn-in at a cut end, which can be inconvenient in jointing. Tensile suesses gendy can result in crack propagation - Wi & (11 state that for flaws of order 10 4 propagation can occur for stresses of 1 MPa
- Crystallinity and morphology. For a given polymer gade, the details of the cooling process determine (for semi-crystalline materials) the crystallinity and mim-mcture through the pipe wall, and these in turn are related to mechanical properties. Whilst the qualitative relationships between cooling rate and, say, spherulitc size are clean, qmtitatkve pdctions are not, at present, possible.
Author(s) : Isaac, Eccott, Pittman, Farah
This study has sought to characterise the variations in mechanical properties and residual stresses that develop as a result of the differential cooling rates that occur through the thickness of commercially produced polyethylene pipes. The mechanical behaviour has been studied through measurement of tensile...
Author(s) : Isaac, Eccott, Perridge, Pittman
A detailed investigation has been carried out on the microstructure of various polyethylenes recently developed for applications in the pipe industry. Of particular interest have been the PElOO formulations, and samples of these have been studied both from commercial piping and from laboratory produced plaques....
Author(s) : Pittman, Farah
Heat transfer coefficients for splay cooling in the manufacture of blue, yellow and black PE-80 and PE100 MDPE pipes are determined by measuring pipe surface tempemhtres in the annealing zones between splay tanks, and then fitting a computer simulation of the cooling process by adjustment of heat wander coefficients....
Author(s) : Pittman, Whitman, Gwynn, Beech
Comprehensive operating data from a large-scale pipe cooling lime are reported, including in-situ measurements of temperatures within the pipe wall. A finite element simulation of cooling, coupled with gravity induced flow, or slump, in the pipe wall is described. Computed temperatures within the wall are compared...