The Research On Monitoring Technique And Field Application For Wet H₂S Corrosion In Oil Refining Equipment

The equipment corrosion is an important problem of influencing production equipment normal operation at all times in the oil refining industry. Because of special work condition in the oil refining processes, the equipment is attacked by corrosive medium in high temperature and pressure, especially, in the environment of the crude oil with high sulphur and high acidity used in recent years. In the process of refining crude oil containing sulphur, Sulphur element is turned into deleterious hydrogen sulphide along with processing the crude oil. The atomic hydrogen is produced by wet H₂S corrosion, adsorbed on the steel surface and then permeates into the steel interior. With being accumulated over a long period, it will lead to hydrogen induced cracking, hydrogen brittleness and hydrogen attack. The safety production will be badly destroyed.According to the principle of electrochemical hydrogen permeation, the corrosion monitoring system is designed and developed in the present dissertation. The hardware system is composed of the hydrogen sensor with Pd₇₅Ag₂₅ alloy film, a Model CP5 potentiostat, a computer and a string of three-core shielding wire. The software is empoldered with C-language and there is the communion interface of human-computer. Main menu includes five items of "file", "monitoring", "option", "tool" and "help".The pervasion of atomic hydrogen in the steel obeys the Fick second law. The hydrogen damages mechanism are described from three aspects of hydrogen brittleness, hydrogen pitting and hydrogen attack. And the electrochemical estimation method of steel hydrogen damages is discussed in the current paper. Through electrochemical hydrogen permeation experiment at low temperature, hydrogen diffusion coefficients of A3, 08F, 16MnR and 20G are mensurated. Diffusion coefficients at any temperature can be calculated by the Arrhenius equations of steel diffusion coefficients. The equations between wet H₂S corrosion rate and steady permeation current density are established by weight loss testing in H₂S solution and detection of permeation current in different H₂S concentration. Wet H₂S corrosion rate is calculated by the equations in the interior wall of equipment.If 0.5 mm² shielding wire is adopted to detect permeation current of equipment within 400 meter, Field tests show that the signal intensity of hydrogen permeation will not be weakened. Therefore, it is completely feasible that the computer and Model CP5 potentiostat are emplaced in the centeral control room 180 m from the monitored equipment.The field testing was carried out in Maoming Corporation of Petroleum Refinery, Chinese Bloc of Petroleum Refinery. The field monitoring results show that the signal of developed JMH-1 hydrogen sensor is steady, credible. It can conduct the on-line monitoring of the permeation density and display wet H₂S corrosion rate and distribution of atomic hydrogen concentration in the wall of equipment. Based on the hydrogen permeation rate in the wall of equipment, the monitoring data are analyzed by the software. The system software can judge whether the equipment operation is safe or not. Then relating measures are adopted, process parameters are adjusted. Then accident befell is prevented and Fatal casualties and tremendous economy expense are avoided. Hence, the monitoring system can be used to foreshow the rate of wet H₂S corrosion and to evaluate the safety status of equipment.