Polyurethane foam insulation: Causes of failure in district heating systems

This study investigates the mechanisms of defect formation in polyurethane foam (PUF) insulation used in pipelines for energy and central heating applications. The research focuses on PUF insulation placed between a steel pipe and a polyethylene casing. A numerical thermomechanical analysis was performed using ANSYS software on a U-shaped section of an insulated pipe to simulate the operational conditions with a heat-transfer fluid temperature of 130°C and ambient temperatures varying from –20°C to +20°C in 5°C increments. Various causes of insulation defects were examined, including manufacturing factors (e.g., uneven application, incorrect foaming temperature, and moisture contamination), mechanical loads (impact and vibration), and thermal stress. The PUF failure process, which involves condensation, corrosion, and chemical degradation, is described. The investigation established that significant stress concentrations occur at the bends of pipes covered with PUF insulation. The maximum von Mises stress was determined to be 0.45678 MPa at an ambient temperature of –20°C with a temperature differential of 150°C between the fluid and environment. This value approaches the ultimate strength of the polyurethane foam, indicating that cyclic compression and expansion processes can initiate defects and lead to subsequent degradation of the insulating layer. Thus, the study demonstrates that thermal loads, along with manufacturing defects and mechanical impacts, are the primary factors in the formation of defects in PUF, such as cracks, delamination, and fatigue, which compromise the structural integrity and thermal performance of insulated pipes.