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E2G|The Equity Engineering Group, Inc. has been providing significant value to customers using advanced Computational Fluid Dynamics (CFD) modeling techniques in solving complicated flow-related problems. CFD is particularly useful when design correlations are not available; however, CFD can also be used effectively to remove conservatism associated with the existing design correlations. Cost savings can be significant when CFD is added to Fitness-For-Service analyses or new component designs.
Typical applications of CFD include:
CFD can be effectively used to understand flow behaviors in equipment and is useful in optimizing flow patterns, flow rates, pressure drops, etc. In this example, flow behavior in a coke drum with dual inlet nozzles is simulated. For slower fill rates, CFD simulation predicts symmetric circulation zones in the plane of the nozzle and no circulation zones in the plane 90° to the plane of the nozzles. For faster fill rates , the flow becomes unstable. Small perturbations in the inlet velocities can switch the circulation zone pattern from one side to the other.
Simulations also show circulation zones in the plane 90° to the plane of the nozzle. The unstable circulation zone pattern can result in temperature gradients more than 100°F around the circumference of the bottom cone.
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In this example, CFD analysis is utilized to design a quill that minimizes the risk of thermal-fatigue cracking due to turbulent thermal mixing and/or global operating fluctuations. The CFD analysis of the mixing tee under consideration predicts that cold fluid of the branch pipe does not hit inner surface of the run pipe for a long distance away from the junction, which eventually results in significant reduction in temperature gradients at the tee junction. The reduced temperature gradients significantly reduce the risk of thermal-fatigue cracking at the junction.
Even though the quills reduce the risk of thermal-fatigue cracking, they are more prone to flow-induced vibration related failures. The geometry of the quill is chosen such that the frequencies of vortex shedding around the quill are not close to natural frequencies to avoid flow-induced vibrations of the quill.
E2G performed a comprehensive study of mixing tee failures utilizing Large Eddy Simulations (LES), standard two-parameter CFD turbulence models, finite element stress analysis, fatigue life estimation, and fatigue crack growth approaches. The developed methodology was used to assess an industrial mixing tee that experienced cracks after 15 years of service.
The results obtained from the assessment are benchmarked against in-service failure data. The study suggests that a combination of advanced analyses such as LES-based CFD, FEA, fatigue life estimation, and fatigue crack growth approaches are necessary to analyze the effect of turbulent thermal mixing and operating fluctuations for an evaluation of a mixing tee failure.
Computational fluid dynamics (CFD) analysis provides valuable insight into the sources and locations of flow-induced turbulence and allows for cost-effective design iterations to mitigate vibrations. Additionally, a CFD is a valuable tool to support root cause analysis. The simulation results below highlight the expected reduction in pressure fluctuations caused by the recommended design change.
The simulation below was developed to understand the source of the vibrations and to help prevent future failures of downstream expansion joints.
This example shows how CFD can be used to identify the root cause of flow instabilities in an inlet header. One of the four outlets of the inlet header under consideration was experiencing significant vibrations which was causing the unit to trip more often. E2G was asked to identify the root cause of the vibrations and provide recommendations to minimize vibrations and hence the frequency of unit trips. Single and multiphase CFD models were utilized to predict flow abnormalities in different outlets of the header.
It was determined that the presence of solid particles larger than a certain size can cause the imbalance in the mass flow rates which can cause one of the outlets to vibrate more than the others. Particles size adjustments were recommended to balance the flows and avoid instabilities.
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