Using CFD to design erosion-resistant expanders

Turbomachinery flows that contain solid particles represent an undesirable yet often inescapable condition because of practical operating considerations. Dust, sand, fly ash, iron oxide, process-originated materials, and debris from abradable seals or blade rubbing are examples of the varied composition of these solid particles.

Computational Fluid Dynamics (CFD) analysis of two Fluid Catalytic Cracking (FCC) gas expander designs demonstrates that a practical treatment of erosion due to solid particles in the primary flowpath could be implemented.  Turbomachinery flowpaths can be redesigned to tailor erosion characteristics for improved blade longevity.  Therefore, it is feasible for engineers to consider erosion during design and analysis of commercial and military turbomachinery flowpaths. Since the extent of erosion damage caused by solid particles entrained in the primary flowpath can be mitigated, the useful operating life of turbomachines can be extended significantly. 

FCC hot-gas expanders form a subset of industrial turbomachinery that consistently sustains significant erosion damage. Through the application of powdered catalysts in conjunction with elevated temperature, the FCC process converts high-molecular-weight petroleum hydrocarbons into more valuable petroleum products, including gasoline. The FCC process is often operated continuously in petroleum refineries for periods of up to several months.

Flue gas is an abundant byproduct of the FCC process.  After the flue gas stream has traversed a separator, which removes up to 90% of the catalyst particles, it is directed through the FCC hot gas expander. A specialized type of turbomachine, FCC expanders are used to recover, as mechanical energy, a sizeable portion of the pressure and thermal energy remaining within flue gas. This considerable amount of recovered waste energy drives process equipment or generates electricity for other applications within the refinery. 

Beyond having elevated inlet temperatures, single-stage FCC expanders are characterized by a large pressure ratio, typically three to one. The turbine blades of the FCC expander are simultaneously subjected to substantial aerodynamic and thermal stresses that intensify erosion damage, which is caused by residual solid catalyst particles within the flue gas stream.  To improve the safety, reliability, and efficiency of FCC expanders, the rate of erosion of the blading must be understood and addressed effectively during the design process.

Commercially available computational fluid dynamics (CFD) codes have been used to optimize the design of turbomachinery flowpaths for many years.  Over the past decade, the capabilities of these CFD codes have been extended significantly by the addition of erosion models. One commercially available code with this feature is ANSYS^® CFX^TM. This technology includes the means to track solid particles in the fluid domain, as well as the means to predict solid particle erosion, through the application of theoretical models developed by Professor Widen Tabakoff and team at the University of Cincinnati. Machinery flowpath designers can take advantage of these models to quantify potential damage to passage walls and blades due to entrained particulates. Engineers also can judge the efficacy of proposed design alterations with respect to erosion.