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ISO 7507-1 Strapping Method

First an onsite reference circumferential strapping is done only on 1st and 2nd Shell with calibrated strapping tapes and dynamometer with a proper tension and repeated 3 times and a mean value taken. This circumference is taken roughly at apposition of one-fourth from upper or lower weld of the strake. A circumference measure at 1st or lower 2nd course is chosen because there is minimum distortion or loss of circularity at this position because the strake is welded to the annular bottom plates. An external diameter (and radius) is calculated from this circumference after applying necessary correction like temperature, step over (vertical weld). We call this reference radius or diameter. Other than this plate thickness measure with ultrasonic thickness gauge, dip reference height measures, tank and course height is taken. These are done ISO 7507-1 (same as IS 2007 or API MPMS 2.2A).

There are Two Methods That Can Be Used to Calibrate The Volume Below The Dip-plate in A Vertical Tank

The tank floor profile can be surveyed physically, using one of the following tools:

  • An engineer’s level or theodolite and staff.
  • A laser plane and survey staff.
  •  A water tube or hydrostatic level tool.

The tank bottom is calibrated by filling with measured quantities of a non-volatile liquid, preferably clean water, as specified to a minimum level that covers the bottom completely, immersing the dip-plate & eliminating the effect of bottom deformations or, alternatively, calibration by physical survey using a reference plane to determine the shape of the bottom.
From data obtained the volume can be calculated mathematically. The tank floor can be calibrated volumetrically, using a meter or volumetric prover and water.