API-650 App. R

In API-650 11th Edition Appendix R, there is a factor of 0.40Pe (external pressure) or 0.40Pi (internal pressure) added to various loading conditions. This same factor is used when determining anchor bolt loads on anchored tanks due to wind or seismic. However, there is also a note that says that if the operating pressure divided by the design pressure is greater than 0.40, the purchaser should consider using a higher value than the 0.40. How many times do we see a operating pressure but no specified design pressure? Many times the purchaser only knows the pressure he wants for operating and specifies that as the design as well. If you just plug these values into the formulas given then you will be designing a tank for less than what the operating conditions will be. We probably do not want to increase the design pressure to 2 1/2 times the operating pressure if the purchaser has not specified this condition as this will greatly increase the thicknesses or cause a requirement for anchorage or stiffeners. However, care then needs to be taken to address these other conditions to make sure we are providing an adequate design. Towards this end, our ITSdesign software now incorporates a check for the ratio of the operating to the design pressure, flags this condition, and provides the option to increase the factor beyond the 0.40 value as needed.