Numerical simulation of mixing in process deadlegs in order to model microbiologically influenced corrosion and tuberculation at these locations

Operating experience of carbon steel process water systems has demonstrated that corrosion and tuberculation in process deadlegs can be more extensive than the corrosion and tuberculation in the flow leg of the system. The reason why corrosion is seen at these locations is unknown since deadleg lines are also considered to be stagnant. However, deadlegs can have two zones, a mixing zone and a non-mixing or stagnant zone in the deadleg. The mixing zone is created by flow in the main pipe separating at the deadleg opening and creating a vortex. The vortex created means that some mixing in the deadleg is possible having a mixing depth and flow velocity profile dependent on the flow velocity in the main pipe flow leg. This phenomenon, therefore, is referred to as turbulence penetration. The resulting mixing length defines a region of the system having a variable length where microbiologically influence corrosion (MIC) and tuberculation can occur even if the flow velocity in the main pipe flow leg is high enough to prevent corrosion product accumulation. An equation relating mixing length in a process deadleg to Reynolds number is available. It was hoped this relationship may provide a means for modelling corrosion in the different mixing zones and provide a rationale for inspection locations. Experimental tests on corrosion of carbon steel in process deadlegs, however, revealed that corrosion was still seen at locations in the deadleg beyond the expected mixing length that would be created by turbulence penetration alone. Modelling of deadlegs was therefore reviewed and computational fluid dynamics (CFD) calculations were then performed to better understand the effects of turbulence penetration and the resulting mixing length on corrosion in process deadlegs. Understanding gained from these calculations, therefore, also provide a rationale to explore additional environmental conditions resulting in materials transfer into deadlegs that could explain corrosion seen at locations where it would not be expected. (author)