Thermal insulation calculation method for district heating pipelines with casing installed by horizontal directional drilling

   The development of Horizontal Directional Drilling (HDD) technology has led to a new trenchless method for installing district heating networks using casing pipes. The regulatory framework for the designing and constructing such HDD pipelines is evolving but still contains several gaps. One notable gap is the absence of a standardized methodology for thermal insulation calculation. A methodology for calculating thermal insulation of district heating pipelines with casing installed by HDD has been developed. It is based on the algorithm for calculating linear heat losses in direct buried pipelines, as specified in the Code of Practice SP 61.13330.2012 "Thermal Insulation of Equipment and Pipelines". The list of components contributing to total thermal resistance has been refined, and calculation formulas have been proposed to account for the specific features of trenchless casing installation, as well as the products and materials used. For calculating the thermal resistance of soil, an approximate formula is justified; its error is negligible within the typical ranges of burial depth and pipeline diameter for trenchless installations. New practical dependencies have been established for the thermal resistance of polymer pipes as a function of material thermal conductivity and either the Standard Dimension Ratio (SDR) or pipe series (S). It is shown that polymer pipes and casing tubes with identical SDR or S values, made of the same material, have the same thermal resistance regardless of diameter, which reduces computational effort. A straightforward approach is proposed for determining the linear heat flux, taking into account the influence of the thermal field from multiple adjacent pipelines. An example is provided for the thermal insulation calculation for a four-pipe district heating network installed using the trenchless casing method. The developed methodology can serve as a reference supplement to the Code of Practice SP 61.13330.2012 "Thermal Insulation of Equipment and Pipelines".

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