A model is presented for the prediction of the corrosion rate by CO2 under dewing conditions. A mechanistic approach is developed that takes into consideration the hydrodynamics, thermodynamics, heat and mass transfer, chemistry, and electrochemistry during the phenomenon of Top-Of-The Line corrosion. This model is validated by experimental data. It offers a better insight on the role played by primary parameters such as the temperature, the total pressure, the partial pressure of CO2, the gas velocity, and the condensation rate.
In the case where a pipeline cannot be protected from internal corrosion by the injection of inhibitors of corrosion, a reliable control device and some accurate predictive tools of the corrosion rate are required. In the specific case of Top-of-the-Line Corrosion (TLC), the stratified or stratified-wavy flow regime does not provide a good wettability by inhibitors(1) of the upper part of the internal pipewall.
Non-inhibited water is present at the top of the line due to the condensation of water vapor transported along the pipe with other gas. As important heat exchanges take place between the pipe and its surrounding, a certain amount of water will condense at the pipewall leading to a corrosive environment(2). In order to qualitatively and quantitatively describe the phenomena of corrosion occurring at the top of the line, a deep insight on the combined effect of the chemistry, hydrodynamics, thermodynamics and heat and mass transfer in the condensed water is needed(3).
In this paper, the corrosion rate in the presence of carbon dioxide is experimentally studied along with the condensation rate of water in a horizontal pipeline in the presence of a noncondensable gas. Particular attention is given to the following parameters: temperature of the gas bulk, temperature of the pipewall, total pressure in the system, partial pressure of carbon dioxide, gas velocity, and condensation rate.
A mechanistic model is developed that gives a better insight on the role played by the aforementioned parameters. The model is tuned on a large set of experimental data and offers a convenient predictive tool for the risk of TLC by CO2. This model can directly be used to optimize the design of pipelines since it allows the determination of the level of thermal insulation required to avoid the condensing conditions leading to an unacceptable rate of corrosion.
EXPERIMENTAL SETUP AND MEASUREMENT TECHNIQUES
Experimental setup and procedure
In order to reproduce the conditions encountered in the field during the production of wet gas, a full-scale flow loop was built in our laboratories. The schematic of the loop is given in Figure 1. The experimental procedure is as follow: carbon dioxide is first injected in the flow loop up to a specific pressure. In a 1 m3 stainless steel tank, water is then heated by electrical resistances up to the temperature of interest. The pump is then started and the mixture flows in the 4-inch stainless-steel loop until the thermodynamic equilibrium between the gas phase, the pipe, and the surrounding is reached. A countercurrent double-pipe heat exchanger is used to cool the gas mixture and control the condensation rate. The condensed water can be collected in a pressure vessel. This configuration allows sampling without disturbing the flow and the pressure in the loop.