Shear strength distribution of district heating pipe under accelerated aging condition with respect to PU foam density

Abstract

The European standard EN253 for bonded pipe systems for District Heating(DH) using polyurethane foam thermal insulation applied to bond to a steel service pipe and a polyethylene casing has been proposed to validate the residual life and soundness of an aged DH pipe by simultaneously presenting a shear strength ratio greater than 45% relative to the initial shear strength and the absolute values of the shear strength larger than 0.12 MPa. However, it is difficult to represent a reliable distribution of the shear strength ratio owing to the non-uniformity of the PU foam density along the length of the pipe. A representing method is proposed for the distribution of the shear strength ratio of the aged DH pipe by correlating the distribution of the shear strength with the PU foam density. Besides demonstrating reliability in the distribution of the shear strength ratio with small variation, the proposed method provides excellent prediction in the shear strength ratio when the initial shear strength has not been maintained since it was buried long ago by extrapolating degrading characteristics of the aged DH pipe.