A Study of Top of the Line Corrosion Under Dropwise Condensation

Top of the Line Corrosion (TLC) is a phenomenon encountered in wet gas transportation when problems of corrosion appear inside the pipe due to the condensation of water containing dissolved corrosive gases at the top of the line. Theoretically, TLC can be seen as consisting of three major processes: condensation, chemical reactions in the condensed water, and corrosion at the steel surface. In this study, the theories behind dropwise condensation, corrosion, and droplet transport are investigated. Based on the mechanisms, mathematic models are developed to predict the condensation rate, the corrosion rate, and the possibility of effective droplet transport. In the dropwise condensation model, the heat and mass conservation equations are established to account for the effect of all important heat and mass transfer resistances. A numerical method is proposed to solve the non-linear equation system and predict the condensation rate. Meanwhile, through force analysis on a single droplet, the maximum droplet size and the condensation regime can be determined. With the inputs of the condensation rate and the droplet growth scenario from the dropwise condensation model, an electrochemical mechanism from Nesic et al. (1996) is adapted to model the corrosion process at the top of the line. The breakdown of species concentrations in the droplet is established through the main thermodynamic and chemical equilibria. The general corrosion rate is predicted using the kinetics of the electrochemical reactions at the steel surface and by taking into account the mass transfer and chemical reactions occurring inside the droplet. In order to verify the mechanistic model, long term experiments are conducted in large scale flow loops equipped with an in situ camera. In comparison with the experimental results, the model was able to predict reasonably well the condensation rate, the corrosion rate, and the condensation regime. Being a standard method for corrosion control in the oil and gas industry, corrosion inhibitors are not useful for TLC prevention since traditional inhibitors are liquid and flow at the bottom of the line and cannot easily reach the top of the line where the aggressive condensed water is formed. However, it is believed that under certain circumstances inhibitors may be transported to the top of the line as entrained droplets are deposited there. In order to theoretically describe how and when a droplet is entrained from the liquid at the bottom and then deposited at the top of the line, several inception and transition criteria have been defined. Firstly, two mechanisms (undercutting and tearing) are determined for the onset of droplet formation. Secondly, according to the wave-mixing and entrainment-deposition mechanism, two criteria for the transition from stratified to non-stratified flow are established, respectively. Finally, an effective zone with well defined boundaries is introduced to provide operational guidance for the utilizing of corrosion inhibitors in TLC prevention.