Study of the stress-strain state of flange connections

Abstract

Pipe fittings are an important component of all piping systems covering housing and communal services, oil production and refining, chemical production, water supply to thermal and nuclear power plants. Particular attention is paid to the strength and reliability of flanged and threaded connections, as flanges provide strong, tight, and detachable pipe connections. Historically, the calculation of stresses in flange connections began as early as 1927, and modern research uses the finite element method (FEM), which allows detailed modeling of their stress-strain state, predicting durability, wear, etc. Bolted flange connections with gaskets are widely used, but the problem of tightness remains relevant due to the influence of combined loads, materials, and operating conditions. Many studies highlight problematic issues related to flange connections, in particular, a nonlinear analysis using FEM that takes into account the thickness of the flange, various methods of fastening bolts, their number and preload, which allows optimizing the design of the flange connection. An algorithm has been developed to determine the maximum preliminary bolt tension that can be applied to various types of flange connections. It takes into account the plasticity of materials and flange-gasket contacts. This work focuses on flange connections used in the oil and gas industry to connect various equipment located both in the ground and on the surface. Seasonal temperature changes will affect the properties of the soil and, as a result, the stress-strain state of the connections under study. To assess the stress-strain state of a flange connection, a three-dimensional model was developed, consisting of a flange connection with one part of the pipe located in the ground and the other on the surface. The simulation results showed that under axial loading, the flange connection experiences a maximum equivalent stress of 141 MPa, and the contact pressure on the gasket is 9.3 MPa. When taking into account soil stiffness similar to winter conditions, stresses increase to 241 MPa, while contact pressure on the gasket and flanges becomes uneven, leading to a loss of joint tightness. Given the prevalence of such conditions, further research should focus on the influence of soil properties, friction coefficients between the soil and the pipe, and the influence of vibrations caused by pressure pulsations in pipes on the strength and tightness of flange connections. This will improve the reliability of pipeline systems in difficult operating conditions.