Two identical spherical tanks, designed and commissioned in the 1960s for the storage of pressurized liquid butane, require re-evaluation for a new purpose. Each tank has a diameter of 15 meters and is supported by nine vertical 30-inch pipes welded to the spherical structure. The tanks are now intended to store a different liquid medium with a density over 50% higher than the original butane. The design pressure for future use will be reduced compared to the original design pressure. Dynaflow Research Group (DRG) has conducted a re-rating of these tanks to ensure their suitability for the new application.
Analysis
The legs and their attachments to the spherical tank were assessed according to the ASME Boiler and Pressure Vessel Code Section VIII Division 2. The analysis took into consideration the historical use of the tanks, accounting for any past cumulative fatigue damage. Structural components on the spherical legs, including tie-rod brackets and gussets between the tank code and the leg, were also included in the evaluation. The tie-rod brackets were assessed using ASME Code Case N-318-5, which pertains to lugs attached to a cylindrical structure.
Results
Due to the increased weight of the new content, the storage tanks do not pass the stress assessment without reinforcement. The analysis revealed that the fillet welds used for the structural items act as stress raisers, resulting in high peak stresses. DRG has determined the necessary modifications to achieve code compliance:
- Replacing the current tie-rod brackets with newly designed ones that direct the line of action of the tie-rod force through the center of the bracket-to-leg weld.
- Removing the gussets by cutting away their mid-sections.
- Replacing part of the column-to-sphere fillet weld with a new fillet weld featuring a larger leg size.
These reinforcing measures are designed to minimize costs while ensuring compliance with the code. The new tie-rod brackets are engineered to reduce the moment acting on the weld, thereby minimizing stress. The analysis indicated that the gussets are not beneficial, and their removal is a straightforward solution to prevent high stresses in the sphere and leg material. The precise length of the column-to-sphere weld that needs replacement has been determined through analysis to keep re-welding costs low.
By implementing these modifications, the spherical storage tanks can be safely repurposed for the storage of the new, denser liquid medium.
