Electrostatic potential is a critical factor to evaluate the severity of electrostatic hazard during filling oil tanks. A novel model is proposed for calculating electrostatic potential in tanks during filling process. In the model, the nonuniform electrostatic charge distribution is solved by considering the diffusion, convection, conduction, and dissipation forces acting on the electrostatic charge. Additionally, an experiment was carried out for measuring electrostatic potential, and the maximum surface potential from the experiment coincided with that of calculation well. By applying the model, the distribution of electrostatic potential is explored, and the influences of relaxation time and some other factors on the electrostatic potential are analyzed. The results show that the peak value of the maximum surface potential calculated by the proposed model occurs much later in the filling than the existed model with the assumption of uniform electrostatic charge distribution. The relaxation time is a key factor to influence the electrostatic potential, and it also affects the filling fraction at which surface potential reaches the peak value. The maximum surface potential increases with the increase of the diffusion coefficient of the ion, the dynamic viscosity of the oil and the filling rate, respectively.
To study the aging characteristic of the polyethylene (PE) 100 pipe under constant and cyclic internal pressure, a new accelerated thermal-oxidative aging test equipment was developed. Thermal-oxidative aging behaviors of PE pipe under constant pressure and cyclic pressure were studied in various temperatures (80, 95, and 110 C) by accelerated aging tests. The tensile test result shows that the fracture strength of PE pipe decreases as the aging time prolongs. Meanwhile, the pipe aged in the higher temperature behaves faster decreasing rate. A faster decreasing rate is also observed under the condition of cyclic pressure, compared to that under constant pressure. The thermal stability of the pipe gradually reduces and the reducing rate under cyclic pressure is higher than that under constant pressure. The result of infrared spectroscopy test suggests that oxygen-containing groups (i.e., C O) are formed on the surface of PE pipes, indicating that oxidative degradation phenomenon of PE pipes occurs during the aging process. Furthermore, based on the measured result of fracture strength, lifetime prediction models of PE 100 pipe under constant pressure and cyclic pressure are proposed. The result shows that the lifetime of PE 100 pipes obviously decreased under cyclic pressure, and it is 27.96% shorter than that under constant pressure.
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