API 653 requires compliance with the API 650 weld spacing requirements, including the spacing between the nozzle reinforcing plate or insert plate attachment welds and the tank bottom. This project includes a review of the present weld spacing requirements in various national and international above ground storage tank and pressure vessel codes and standards, as well as the past experience with lesser weld spacing requirements than those permitted by API 650. This review indicates that that there is satisfactory experience with the 4 inch (100 mm) minimum weld spacing permitted in British Standard 2654 between the toe of the nozzle attachment weld or insert plate weld and the toe of the bottom corner weld.
Additional studies were performed as part of this project to provide justification for reducing the weld spacing requirements for storage tanks repaired or altered in accordance with API 653. The stress analysis performed shows that for all the tanks considered in this report, the shell stresses from liquid pressure at 4 inches above the bottom are usually lower than at 10 inches above the bottom. The overlap of welding residual stresses in adjacent welded joints is evaluated. The circumferential tensile residual stresses change into circumferential compressive stresses within about 2 inches from the weld. The radial residual welding stresses generally are lower than the circumferential stresses, but dampen out more gradually.
Fracture mechanics calculations were performed to evaluate critical flaw sizes in the weld metal of the pad plate or insert plate attachment welds, at the toe of the reinforcing plate fillet welds and in the shell plates. The calculated critical flaw sizes generally are larger at 4 inches than at 10 inches above the tank bottom in the tanks selected for this study due to lower stresses at the 4" location.
Fatigue is considered at nozzle attachment welds, assuming a fatigue strength reduction factor of 4 and a shell membrane stress of 17500 psi. This results in about 1,400 allowable fatigue cycles, regardless of closeness of the nozzle reinforcing plate attachment weld to the tank bottom.
Based on these studies and the satisfactory past experience with the 4" (100 mm) minimum weld spacing requirement in BS 2654, it is feasible to reduce the present weld spacing requirement in API 653 to no more than 4 inches (100 mm) between the toe of the nozzle reinforcing plate or insert plate attachment weld and the toe of the bottom fillet weld for shell plates not exceeding 1½ inches in thickness and materials not exceeding 50 ksi specified minimum yield strength, providing that certain limitations and additional requirements are met. The recommendations in this report apply strictly to the 4 inch weld spacing strictly for API Standard 653. This work recommends that the current spacing used in API 650, while conservative, be left unchanged to provide for retrofitting tanks with double bottoms without moving every nozzle and fitting.
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| Number of Pages : | 35 |
| Published : | 2000 |
