By Carlos Molina

Hi. This is a short post, written to teach you how to calculate the number of points for settlement measurement around a tank. If you want to become a certified API 653 inspector, this is what the body of knowledge for the 2015 API 653 certification exam asks from you

The inspector should be able to calculate the number of survey points for determining tank settlement.

At least one question on this matter will show up in the exam. And it is such an easy thing….

DETERMINING THE NUMBER OF POINTS FOR SETTLEMENT MEASUREMENT

In a tank, the number of settlement points around the periphery for external settlement  measurement is given by the following formula

where:

N is the minimum required number of settlement measurement points.

D is the tank diameter, in feet (ft).

And the following rules should apply:

1) The maximum spacing between settlement measurement points shall be 32 ft.

2) No less than eight measurement points

Before any hidrostatic test, elevation measurements should be taken inside the tank, as stated in 7.3.6.8 of API 650

Internal bottom elevation measurements shall be made before and after hydrostatic testing. Measurements shall be made at maximum intervals of 3 m (10 ft) measured on diametrical lines across the tank. The diametrical lines shall be spaced at equal angles, with a maximum separation measured at the tank circumference of 10 m (32 ft). A minimum of four diametrical lines shall be used.

There is not much to say about this issue, apart that just having a look at Figure 1 for easiness.

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IMPORTANCE

Settlement measurement stations are to be used during hydrostatic test and operation of the tank; settlement measurements should be taken at a planned frequency, based on an assessment of soil settlement predictions (See Annex B of API 653)

Of utmost importance is the maximum allowable differential settlement between 2 consecutive stations that may have several consequences: (a) Out-ofplane displacements are induced in the shell in the form of buckling under a displacement-controlled mechanism; (b) High stresses develop at the base of the shell and in the region of the settlement; and (c) High stresses develop in the tank bottom.

Further explanation of tank settlement is found in Annex B, Api 653. The purpose of this article is make you aware that you will be asked about this subject in your exam.

Thanks for reading