The difference in physical parameters between hydrogen and natural gas will lead to changes in the operating conditions of the pipeline system after hydrogen blending. At present, most related studies only focused on specific boundary conditions and pipeline network structure. Therefore, the research results among them are not universal or consistent. To address this issue, this paper proposed a flow simulation model for mixed gas within the pipe network system. This model enables an in- depth investigation into the theoretical changes of various operating parameters. Specifically, parameters such as pressure, flow, heat flow, hydraulic gradient along the pipeline, inlet and outlet pressure of the compressor, and pressure ratio are analysed under different combinations of boundaries. The boundary conditions of the pipeline are classified into four types: pressure, volume flow rate, mass flow rate, and heat flow rate at the pipeline nodes. Meanwhile, the boundary conditions of the compressor consist of three control conditions: pressure, rotational speed, and pressure ratio. The computational results obtained from the Shaanxi- Beijing Natural Gas Pipeline and the Se-Ning-Lan Natural Gas Pipeline clearly demonstrate that the changes in pipeline operation conditions after hydrogen doping are closely related to the boundary conditions. In cases where the compressor employs a fixed outlet pressure or fixed pressure ratio control method, a definite relationship can be observed between the operating parameters of the pipe network and the hydrogen blending ratio. For instance, as the hydrogen blending ratio increases, the pipeline pressure decreases following a specific function relationship. However, when the compressor adopts a fixed rotational speed control method, the variations in the operating parameters of the pipe network become more complex. As an example, with an increase in the hydrogen blending ratio, the pipeline pressure may increase in certain scenarios while decreasing in others, thereby necessitating analysis based on specific situations. This article comprehensively summarizes the universal influence of hydrogen blending on the operating conditions of the natural gas pipe network under diverse boundary conditions. The research conclusions hold true for any branched pipe network and possess significant value in providing theoretical guidance for the operation and control of the pipe network system after hydrogen blending. This is of great importance for ensuring the efficient and stable operation of the pipeline system in the context of hydrogen blending. It allows for a more accurate prediction and management of the system's performance, taking into account the various factors and boundary conditions that come into play.
