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Static Mixer Fatigue Life Assessment with ANSYS Mechanical
The Norsea Gas Terminal in Northern Germany plays a vital role in the connection of pipeline systems coming from the North Sea to the natural gas distribution network throughout Germany. Built in 1977, the terminal was set to be replaced in 2015 by the GASSCO Emden terminal being built by Linde Engineering to ensure the continued safe and reliable delivery of natural gas to the region. The replacement terminal design called for a static mixing device to be inserted into the system where two pipelines meet to ensure uniformity of the gas mixture downstream of the terminal. Westfall Manufacturing Company, who specialize in custom-engineered pipeline system components, proposed using their 3050 series static mixer, shown in Figure 1, above, for the project.
With the mixer to be located in an ecologically sensitive coastal area, Linde stipulated that the mixer design must meet stringent fatigue life requirements as the device will be unreachable for service once installed. The mixer proposal represented an opportunity for Westfall to win business in a new industry and with a new customer, but with limited analysis capabilities they needed help verifying the fatigue life requirements. Westfall turned to CAE Associates to perform a finite element analysis of their design to determine if it would meet the required 30 year fatigue life criteria.
Operating under a tight deadline, CAE Associates worked closely with Westfall, Linde, and GASSCO to develop an analysis plan to evaluate the mixer design for load changes caused by variations in the system flow rate, as well as load changes due to oscillations in the fluid flow field. The primary region of interest for the fatigue life calculations was the welded joints between the mixer fins and the pipeline itself. The efficient analysis methodology crafted by CAEA included mapping external CFD pressures into a mode-superposition transient analysis and the development of automated fatigue analysis post-processing tools to assess the weld fatigue life in accordance with EN 13445 for pressure vessel design.
Figure 2 - Maximum Principal Stresses in the Mixer Weld Regions
The analyses performed by CAE Associates predicted a fatigue life for the static mixer of over 42 years, safely exceeding the 30 year design criteria. At the completion of the bidding process, Linde and GASSCO opted to purchase mixers from both Westfall and one of their competitors. Ultimately, GASSCO chose to install the Westfall design over their competitors due in part to the "more professional FEA support" by CAE Associates as well as the superior mechanical performance of the design. "While the CFD showed the mixer will excel at blending the gases with minimal head loss," writes Westfall's President, Bob Glanville, "there's no question CAEA's FEA work and comprehensive documentation proved the mixer will have the long-term durability this customer required."
Figure 3 - Installation of the Westfall Mixer
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