The article presents the development of an analytical hydraulic model of the side branch pipe of a three-way fire branch DN80. The relevance of the work is due to the need for accurate hydraulic calculation of fire water supply systems, the effectiveness of which directly depends on the correct assessment of pressure loss in the fittings. The model is based on the method of element-by-element calculation, which takes into account local hydraulic resistances in areas with a sharp change in the flow geometry. The model includes losses at four bends (three at 45° and one at 90°), a straight-flow valve, and sudden expansion and contraction sections. The contribution of linear head losses, similar to the central branch pipe, was considered insignificant, accounting for only about 6% of the local losses. This confirms the key role of local resistances in determining the overall head loss in this element. The model was verified by comparing the calculated data with the results of experimental studies. It has been established that the discrepancy between the theoretically calculated value of the head loss (6.86 m) and the experimental value (6.97 m) is minimal, with a difference of only 1,6 %, indicating the high accuracy and adequacy of the developed model. The key practical result of the study is the calculation of the total coefficient of local resistances for the lateral branch pipe of the fire branch, which was found to be ζ = 3.4. This value can be directly used for simplified and accurate hydraulic calculations of pump-hose systems with three-way branches.

Keywords: Model, fire three-way junction, hydraulic resistance, head, pressure, flow rate