Research on Perforation Failure Mechanism of Sewage Tank in a Gas Field

Abstract

A perforated leakage occurred in a gas field sewage storage tank in service. In order to analyze the cause of the perforation and to clarify the critical wall thicknesses of the common perforated points of the sewage storage tank, the micro hardness tester, universal testing machine, metallurgical microscope, scanning electron microscope (SEM), energy spectrometer (EDS) and X - ray diffraction (XRD) were used to analyze the physical and chemical properties of the storage tank, perforation characteristics, the morphology and composition of corrosion products. Results showed that the physical and chemical properties of the material met the requirements; the corrosion products were loose, mainly containing Fe2O3 and having a small amount of FeS. The corrosion pit developed from the inner wall of the storage tank to the outer wall, and the depth of the corrosion pit was about 2.6 mm. Therefore, the perforation mechanism and process of the storage tank could be destribed as follow: the water and corrosive medium penetrated between the layer and the inner wall of the storage tank, leading to an oxygen corrosion (the main corrosion) and a SRB corrosion (the auxiliary corrosion), and as a result, local corrosion pits formed on the inner wall of the storage tank. When the depth of the corrosion pit reached 65% of the initial wall thickness of the storage tank, the stress level at the bottom of the pit exceeded the internal pressure resistance of the storage tank, resulting in a perforation leakage of the storage tank.

Keywords

• sewage storage tank|perforation|oxygen corrosion|stress concentration