Anisotropy in Polyurethane Pre-Insulated Pipes

Abstract

The polyurethane foam in district heating pre-insulated pipes has a critical role to play both as thermal insulation and as load bearing element, as it serves as bond between the medium pipe and the casing. Hence, knowledge on how the foam behaves under multiaxial stresses is of great importance for the design as well as for aging predictions of the network. It is known that cell shape anisotropy in polymeric foams leads to anisotropy in its mechanical properties. In this study, we evaluate and quantify the microstructural anisotropy of PU foam from pre-insulated pipes as well as its mechanical behaviour under compression in the three orthogonal directions. We cover rigid and flexible PU foam, batch and continuous manufacturing, and different pipe diameters. The results were compared with those predicted by available rectangular and Kelvin cell shape models. We have found that PU from pre-insulated pipes is orthotropic and present stronger anisotropy than that typically found in PU slabs. The traditional bonded pipes under consideration behaved in a similar way. However, when comparing the two flexible pipes in this study, despite no significant differences in cell shape anisotropy were found, a significantly different behaviour for the E modulus ratio was observed. This shows that while the manufacturing process exerts the main influence on cell shape anisotropy, to explain the difference in stiffness behaviour other factors need to be taken into consideration, such as cell size and cell size variability.

Effects of thermal and mechanical cyclic loads on polyurethane pre‐insulated pipes

“…The foam is cyclopentane‐blown closed‐cell PU with an average density of 76.2 kg/m 3 . Details on the microstructural characterization have been presented in Doyle et al 23 The ends of the pipe samples were not sealed and in direct contact with air. However, previous research from accelerated ageing of PU pre‐insulated pipes under equivalent high ambient temperature have shown that changes in shear strength do not depend on the thermo‐oxidation of PU at the temperatures under consideration 24 and therefore this should not have a relevant impact on the obtained results.…”

“…22 In a previous work, we have found that PU in pre-insulated pipes presents a much higher anisotropy in both the microstructure and the mechanical properties under compression than that previously reported for PU slabs, arising from the different geometry of the mould during the foaming process. 23 This, together with the different geometry of the sample, an annular pipe, and the effect of high temperature, highlights the need for specific evaluation of this element.…”

Calefacción distrital en Chile y avances en la investigación de interacción entre suelo y tuberías en redes de calefacción distrital

“…El suelo si bien contrarresta en parte los movimientos repentinos, se pueden generar condiciones de esfuerzos en las tuberías que alcancen niveles de fluencia. El comportamiento del aislante PUR bajo las condiciones extremas encontradas en tuberías de calefacción distrital está recién empezándose a investigar para conocer entre otros aspectos su respuesta termomecánica (Doyle et al, 2019;Banushi et al, 2021).…”