Corrosion Assessment of API 650 Tank Annular Plates

By Doug Stelling

The annular plate-to-shell junction is one of the most critical areas in an API 650 Tank, and is one of the most difficult areas in which to assess corrosion. The annular plate typically rests on a crushed stone or concrete ring wall, and joins the highly stressed shell of the tank to the bottom plates. The bottom plates are for the most part supported by the tank pad or foundation, and are lowly stressed.

While some may think that the annular plate is lowly stressed like the bottom plates, the annular plate-to-shell junction is actually subjected to high stresses. This is because the annular plate tends to restrain the radial growth of the tank shell due to both hydrostatic pressure loads and temperature. Since the annular plate rests on a somewhat rigid foundation, the rotation of the shell is restrained and the annular plate and junction weld is therefore subjected to high bending stresses. The rigidity of the foundation determines, for the most part, the restraint to rotation at the junction. Analysis of the annular plate-to-shell junction is also complicated by the effect of flexure of the plate due to the applied loads. These loads cause the plate to bear down on the foundation outside of the shell and lift off the foundation inboard of the shell. The analysis of the junction can also be complicated by differential settlement of the tank pad and ring wall, and differential settlement of the ring wall around the tank periphery.

While relatively simple equations are used to determine the minimum thickness of the tank shell, little analytical guidance is presented in API 650 regarding the design of the annular plate or annular plate-to-shell junction. Minimum annular plate thicknesses (exclusive of corrosion allowance) are given in tables based on shell thickness and hydrostatic test stress in the shell. The minimum width of the annular plate inside of the shell is determined as a function of annular plate thickness, and the minimum weld size (exclusive of corrosion allowance) is also specified. These criteria are also fairly recent, and there are many older tanks which do not meet these criteria or for which no corrosion allowance was ever specified for the tank bottom.

Similarly in API 653, equations are provided for assessing corrosion of the shell and bottom, but there is little guidance provided on assessing localized corrosion of the annular plate or shell near the junction. Therefore when corrosion occurs in the vicinity of this junction, there is little that a typical engineer can do to assess its severity and determine if remedial action is required, other than to determine if the measured thickness exceeds specified minimum limits. However, with the aid of non-linear Finite Element Analysis, an assessment of the stresses in the junction can readily be made and required remedial actions determined on a sound engineering basis.

The next in this series of articles will focus on analysis of a corroded annular plate-to-shell junction due to just weight and pressure loads.