According to API-650 Section 5.9.7.1, the design wind pressure is based on a velocity
of 120 mph at an elevation of 30 feet. To account for an additional elevation or a
gust factor, this velocity is increased by 10%. An additional 5 pounds per square
foot is added to account for the inward drag associated with open-top tanks or the
internal vacuum associated with closed-top tanks.
This option can be used to disable the 10% + 5 psi increase, which may be necessary
to comply with vendor requirements or other calculation procedures. The wind pressure
computed is based on the user specified value of wind velocity, and then modified as indicated by this directive.