Optimization of heating sequences for joint installation of district heating pipeline with heat-shrinkable PEX casing

초록

This study develops a physics-based heating sequence optimization strategy for the installation of heatshrinkable PEX casings in district heating pipeline joints by explicitly treating the heating sequence, defined by the order and timing of circumferential heating zones, as a design variable governing circumferential thermal uniformity and joint reliability. A bottom-to-top heating sequence was designed based on thermal buoyancy considerations to suppress circumferential temperature non-uniformity during the shrinkage activation process, enabling uniform circumferential shrinkage of the casing. A qualitative comparison of radiation models confirms that the surface-to-surface (S2S) formulation provides more stable and physically consistent temperature predictions than the discrete ordinates (DO) model for the enclosed, surface-dominated heating configuration considered in this study. Process-level validation using standardized DVS 2203-6 shear testing demonstrated that the optimized heating sequence reduced shear strength variability by 73% while increasing the average shear strength and eliminating low-strength outliers, even when manual heating was performed by operators with more than ten years of field experience. Assembly-level reliability was further evaluated using an EN 489-based soil stress and water-tightness testing framework. Joints produced using the optimized heating sequence successfully completed both evaluations without sand penetration and water ingress, whereas manual heating trials exhibited limited reproducibility, with failures observed during the soil stress phase. These results indicate that a process-based approach that systematically controls the heating sequence plays a critical role in reducing the risk of installation failure and in achieving reproducible installation quality and reliable sealing performance of heatshrinkable PEX casing joints in district heating pipeline systems.

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