The term "shell" refers to the circumferential surface spanning from the tank bottom to the roof. API Standard 653 defines shell settlement as the occurrence of one or more of the following three scenarios: uniform settlement, rigid body tilt or differential settlement.
Shell settlement survey procedure carried out to determine type and extend of tank settlement, and decide if it can affect tank integrity.
The settlement shall be done before water filling and after filling at 25%,50%,75%,100% and after removing the water.
Measurements of tank settlement will be performed by personnel experienced in the types of measurement procedures being performed, using equipment capable of sufficient accuracy to distinguish settlement differences. The principle types of tank settlement consist of settlements that relate to the tank shell and bottom plate. These
settlements can be recorded by taking elevation measurements around the tank circumference and across the tank diameter.
Data obtained from such measurements should be used to evaluate the tank structure. Additional settlement readings may be required to better define local bottom depressions or edge settlements, to refine shell settlement measurements in areas suspected to have local out-of-plane settlements, or to otherwise improve bottom or shell settlement evaluation.
The term "shell" refers to the circumferential surface spanning from the tank bottom to the roof. API Standard 653 defines shell settlement as the occurrence of one or more of the following three scenarios: uniform settlement, rigid body tilt or differential settlement.
Shell settlement survey procedure carried out to determine type and extend of tank settlement, and decide if it can affect tank integrity.
The settlement shall be done before water filling and after filling at 25%,50%,75%,100% and after removing the water.
Edge settlement occurs when the tank shell settles sharply around the periphery, resulting in deformation of the bottom plate near the shell-to-bottom corner junction.
Bottom settlement is a localized deformation of the bottom plate near or away from the tank shell. The impact of this settlement type on tank integrity is usually dependent on several factors such as localized stresses and strains in the bottom plate, quality of welds, and gaps below the bottom plate. Bottom settlements can be in the form of localized bulges or depressions. Similar to edge settlement, API Standard 653 describes a method to determine the acceptability of bottom settlement.
In each stage of the construction & in final stage, it is required to check levelness and elevation as it is most important consideration in the choice of a site for a tank.
The determination of the roundness of the tank allows us to know if there are increases or decreases in the different levels. For each shell course, reading shall be taken at each horizontal location and around the circumference. It is necessary to check how vertical the shell is relative to the foundation (which has its own tolerances for levelness), and is not intended to measure foundation tilt. In each stage of construction roundness and plumbness will be evaluated.
