Analysis of the remaining life of API 5L grade B gas pipeline in the flare gas recovery unit

摘要

Carbon steel pipe material API 5L grade B is commonly used for distribution in oil and gas industry. Internal corrosion is one of the biggest corrosion problems in carbon steel material that could impact production and cause economic losses. In this work, corrosion rate, remaining life, fluid composition, and corroded sample of API 5L grade B are analysed and calculated by development of microstructure, mechanical denaturing, and change of chemical composition by comparing with new pipe sample. The results show that the thickness of the pipe API 5L grade B applied as distribution line significantly decreased from 8.4 mm to 7.1 mm in only one year with a high corrosion rate of 1.32 mm/year based on ultrasonic testing inspection during the years of 2000 - 2016. It was found that the deterioration of pipe material was caused by gas composition contain mostly gas hydrocarbon with CO2 10.1% mole and water composition containing 57.3 mg/L Cl~- ion. This chemical composition decreased the pH of liquid to 4.5, creating a good corrosion environment for this pipe. It caused the active corrosion rate significantly increase in just one year. Based on calculations, the remaining life of this pipe line was found to be just 3.6 years because of internal corrosion. XRF, XRD, and SEM analyses were done to determine the composition of the corrosion product. Based on those analysis, the main causes of the corrosion are found to be CO2 and O2. It is concluded that, the mechanical properties of this corroded pipeline still satisfy the standard for API 5L grade B with a tensile strength of 441 MPa and hardness of 74.