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API Tanks-Dixie Southern


What You Need to Know about API Tanks: API-650 vs. AWWA D-100

Sep 01, 2016

In preparation for the 2016 API Tanks, Valves, and Piping Conference & Expo in Las Vegas, today we will explore the differences between AWWA D-100 and API-650, two standard varieties of welded tank. We’ll compare and contrast the two in an analytical way, although the two codes may also differ in terms of maintenance, inspection, materials, and fabrication.
API Tanks: Specified Limits and Minimums
The AWWA D-100 specifies a minimum roof live load of 15 psf (pounds per square foot), depending on temperatures, as opposed to the 25 psf specified by API-650 codes. Both adhere to the standard wind velocity factor of (V/100)2, but AWWA D-100 only requires it for windspeed conditions over 100 miles per hour.

Similarly, the minimum shell thickness of both codes are identical in number. However, the two tanks differ in where the thickness is actually measured. AWWA D-100 states it is at the bottom of the course; in API-653 it is one foot higher.

The maximum stress allowable upon any unit, under AWWA D-1000, is capped at 15000 psi. API-650 considers this factor to be a varying function that depends on the unit’s construction and material.
API Tanks: Models Covered
API-650 allows only for one type of tank, ground-supported flat-bottomed models. AWWA D-100 provides for two additional tank classes: pedestal-elevated models (supported by one solid column) and elevated cross-braced tanks (supported by a beam structure).
API Tanks: Seismic Design
API-650’s 8th edition and AWWA D-100 have largely similar provisions for seismic design in ground-supported tanks. Small differences appear in the details: the former uses the structure coefficient in the equation for overturning moment, while the latter uses the importance factor. API-650 also uses a set value of 0.24 in the structural weight terms as opposed to AWWA D-100’s multiplier.

Tank stability limits are slightly higher in API-650, and the code also includes a set limit on annular ring width after which the unit must be anchored. This figure is not specified in AWWA D-100. The two codes also differ in their approaches to resolving stress with uplift.
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