Analysis of strength and eigenfrequencies of a steel vertical cylindrical tank without liquid, reinforced by a plain composite thread

Abstract

A study of the effect of a plain composite wrapping on the stress-strain state of the wall of a steel cylindrical tank is proposed, without taking into account the liquid. The stress assessment in the tank wall was carried out based on finite element modeling of the structure’s three-dimensional model deformation. Two tank wall variants were modeled: with constant and variable thickness, as well as three wrapping variants: with one, double and triple intervals, where the mechanical characteristics of the tank thread and material of different types were considered, taking into account the oil filling level, the wrapping pitch and the thread’s mechanical characteristics. It turned out that the maximum equivalent stresses of an empty traditional tank of continuous thickness are 35 % higher than the stresses of a tank with variable thickness. At that, the equivalent stress index of a prestressed tank with variable wall thickness is 13 % higher than of a tank with constant wall thickness in all three cases of winding a plain composite thread. It should be noted that with an increase in the winding pitch, the equivalent stresses of the prestressed tank decrease by 1.5 times, in the case of winding 1:1 and 1:2. When considering the case of 1:1 and 1:3, the stresses decrease by 2 times, in the case of 1:2 and 1:3, the stresses decrease by 1.35 times. In the modal analysis of the tank with continuous and variable wall thickness without liquid, the first 60 eigenfrequencies were obtained. An analysis of the spectrum of eigenfrequencies and their corresponding eigenvibrations for two models of the tank without liquid showed that the eigenfrequencies of the traditional empty tank are 2–7 % less than the tank with prestressing. However, it was determined that a decrease in the tank wall thickness at the upper free edge from 5 mm to 4 mm does not result in changes in the number of nodes in the circumferential direction. The results obtained will allow to effectively use the prestressing in order to improve the strength and dynamic characteristics to avoid the resonance effect in the structures under study, taking into account the wrapping of a plain composite thread.