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ISO 7507-1 Strapping Method

First an on site 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).

ISO 7507-3 OTM (Optical Triangulation Method)

In combination with circumferential strapping of a reference shell as per ISO 7507-1( API MPMS 2.2A), we follow “Optical Triangulation method” for calibration of vertical cylindrical tanks as per API MPMS 2.2C (ISO 7507 – 3) International standard. Here we will see how we use a Digital electronic theodolite combined with computer software to get diameters for all the shells up to the top.

The main aim here is to find unknown radii of tank strakes from top to bottom. A calibrated surveying digital electronic theodolite or Total station with very high angular accuracy range of +-2 seconds is used. The theodolite needs to be positioned arbitrarily at different positions around the circumference of the tank at an optimum distance from tank where the shells up to the top are visible through the telescopic viewfinder. On each position, firstly, the theodolite is placed on a tripod on firm ground and auto leveled both horizontally and vertically. The viewfinder is focused on the edge the tank strakes tangentially ( tangential line of sight) at approximately one-fourth from top and bottom welds. From left edge of the tank to the right edge the theodolite is swept horizontally which gives us an angular reading in degrees, minutes and seconds accurate to 1/3600 of a degree. This process is done all the shells at 2 positions per shell from one particular theodolite/station position. Similarly it is repeated around the circumference at various positions. The minimum number of positions around the circumference is mentioned as per rule (like 16 positions for a tank of 300 mtr circumference), but we take more where possible so that the number of radii per level achieved from triangulation ( as described below) is more, which gives a greater accurate mean radius for a particular tank strake.

For each angular measurement, tangential lines of sight on a horizontal plane on the 2 sides of the tanks from the theodolite and the geometric central line from the theodolite to the center of the tank create a facility for mathematical triangulation in every horizontal plane, which enables us to measure required unknown radius of the other shells from the known reference radius ( measured by strapping described above), If we make X positions around the circumference, we get X no. of radii for one particular shell position. Since there are 2 points of measurements per shell ( top and bottom from horizontal welds), we get 2 number of radius per shell, which gives us a mean value of radius for a shell of excellent accuracy, more so because this process can also take care of tank deformations (the radii gets reduced or increased in those deformed positions) as the station positions are being rotated. This process also gives data for tank circularity and tilt.