Optimization Analysis for Preheating Temperature and Pipe Wall Thickness of Large-Diameter Directly Buried Heating Pipes

To prevent the problems from occurring in the preheating installation of directly buried heating pipes with large diameter, high temperature and pressure, the present paper analyses constraints of the least-cost for preheating installation of large-diameter directly buried pipes by applying the nonlinear programming theory and establishing the objective function with the aim of pursuing the preheating installation's least-cost. Taking medium steel pipe wall thickness and preheating temperature as variables, this objective function works out the preheating temperature and wall thickness of the least-cost preheating installation by using Matlab Optimization Toolbox. The results of which show that the higher the pressure is, the thicker the wall thickness and the lower the preheating temperature is, that the best preheating temperature is equal to the lowest preheating temperature limit. In addition, the highest preheating temperature limit is allowed to rise with the increasing of pipe wall thickness, but the increment of temperature is much lower than the decreasing amount of the lowest preheating temperature limit. Research also indicates that increasing the pipe wall thickness is effective in avoiding local buckling, whereas, decreasing the preheating installation temperature is more effective than it in preventing the cooling abruption. Such conclusions are of important guidance significance in several aspects such as reducing installation cost of large-diameter heating pipes, saving preheating energy consumption, shortening construction period, improving pipeline security and so on.