The Partial Blockage Detection Theory And Method For The Natural Gas Pipeline

Natural gas is a clean energy source with increasing rates of utilization in both the total and primary energy consumption sector of China. According to the 13th Five-Year Plan for energy development, natural gas will be used for 10% of China's primary energy consumption by 2020. This substantial growth in consumption rates consequently necessitates new requirements for the storage and transportation of natural gas. For long-distance transmission of natural gas, the pipeline is the major route. As with any airtight transmission system, malfunctions in any location of the pipeline may contribute to overall failure of function the entire structure. Water, gas condensate, sand sediment, products of corrosion and other impurities may be contained in the natural gas. Unless it is separated and removed from the gas before being added to the pipelines, accumulated material within the structure may lead to transportation system inefficiency. As the blockage aggregates in severity, catastrophic system failure becomes an increasing threat. Therefore, it is inarguable that this blockage has the potential to cause extreme harm and is a significant threat to the continued safety and stable operation of pipelines.This thesis is concerned with the hazard caused by pipeline blockage, specifically, the study of unsteady flows in a pipeline with a finite number of extended partial blockage as well as blockage detection methods. The methods are based on the hydrodynamics, inverse problems of mathematical physics, optimization methods and numerical calculation theory. The blockage influence on the pipeline unsteady flow parameters was studied by combining theoretical analysis and numerical simulations. The unsteady flow in pipelines with blockage physical and mathematical models were developed and solved through analytical and numerical methods. The extended partial blockage detection method for natural gas pipelines under the running conditions was as well developed based on the analytical and numerical solutions.The specific research contents and achievements are as follows:(1) An extended partial blockage was regarded as a detected object and the physical model of gas transmission pipeline was established based on the analysis of the blockage characterization. The basic governing equations of an unsteady flow in natural gas pipelines with blockage and their linear form are included in this thesis. The validity of the linearization and general mathematical modeling of the unsteady flow was established and the used methods were discussed. The calculation formulas for parameters under steady flow conditions were selected from the requirement of unsteady flow solution.(2) The linearization of the mathematical model was necessary to analytically solve the corresponding nonlinear partial differential equations of the unsteady flow. The boundary conditions were obtained using the homogenization method, first, by introducing an auxiliary function, and then by using the extended separation variable method to solve the corresponding Sturm-Liouville eigenvalue problem. Finally an analytical solution was derived. The accuracy of analytical solution was verified through comparison with the calculations obtained using the TGNET software. The maximum relative error between analytical solution and TGNET results is -0.41% in pressure and 4.3% in mass flow rate for multiple sets of numerical experiments.(3) Based on the linearization model and its analytical solution, this thesis took the model parameter identification problem solution for reference, and proposed a new detection method which consists in reducing the pipeline blockage detection problem to an inverse problem of parameters identification. The corresponding optimization problem could be solved to acquire blockage parameters. As this is an ill-posed problem due to measuring errors, the Tikhonov regularization method was applied to obtain an optimization objective function. Depending on the characteristics of the objective function, an improved genetic algorithm was applied to solve the parameter identification inverse problem. The blockage detection method which was based on the analytical solution was verified through numerical experiments. The results showed that the maximum relative error between blockage parameter identification results and theoretical value is 9.49% for multiple blockage distributions and parameters level.(4) Taking into account the shortcomings of the linearization of the mathematical model and its analytical solution, a numerical method based on MacCormack finite-difference scheme for solving partial blocked gas pipeline unsteady flow was proposed based on the discussion about common numerical methods. This thesis investigates the mesh generation, equation discrete format, boundary conditions treatment under different schemes. It includes as well the numerical calculation of pressure and mass flow rate of an unsteady flow in a partially blocked gas pipeline. The numerical method was verified through numerical experiments, the maximum absolute deviation between the results of the numerical method based on MacCormack format and TGNET is 21 kPa in pressure and 0.5 kg/s in mass flow rate, while as the maximum relative error is 0.32% in pressure and -0.13% in mass flow rate.(5) Considering the limited applications of the existing blockage detection methods based on the linearization of the analytical solution, this thesis proposes a new detection method, where the partially blocked pipeline is divided into multiple sections of the same length. Blockage parameters are then acquired by determining section's diameter of every pipe. The advantages and disadvantages of this new method were discussed based on the detection principle. For the aforementioned ill-posedness caused by measuring errors, the Tikhonov regularization method and an improved genetic algorithm were combined to develop a blockage detection method based on the numerical solution. The accuracy of the method was verified through numerical experiments, relative errors between the proposed method and the theoretical values are within 10% and the maximum relative error is 9.83%.The methods involved in the models and solutions presented in this thesis could enrich and improve the blockage detection theory and techniques. Furthermore, it could provide a reference to the partial blockage detection of natural gas pipeline under running condition.