Stiffness Coefficients for Nozzles in API 650 Tanks

The analysis of tank nozzles for API Standard 650 [1] tanks is a complex problem. Appendix P of API 650 provides a method for determining the allowable external loads on tank shell openings. The method in Appendix P is based on two papers, one by Billimoria and Hagstrom [2] and the other by Billimoria and Tam [3]. Although Appendix P is optional, industry has used it for a number of years for large diameter tanks. For tanks less than 120 feet (33.6 m) in diameter, Appendix P is not applicable. In previously published papers [4–10], the authors used finite element analysis (FEA) to verify the experimental results reported by Billimoria and Tam for shell nozzles. The analysis showed the variance between stiffness coefficients and stresses obtained by FEA and API 650 methods for tanks. In this follow-up paper, the authors present stiffness coefficients for tank nozzles located away from a structural discontinuity. Factors to establish spring rates for nozzles varying from 6 to 48 inches and tank diameters from 30 feet to 300 feet and for nozzles at different elevations on the shell are provided. Mathematical equations are provided together with graphs for the stiffness coefficient factors.