Application of the most appropriate method * and a thorough understanding of the characteristic and geometry of a particular tank is absolutely essential. Here is where long years of experience, engineering and mathematical background is indispensable
A combination of traditional strapping and physical calibration to the latest laser distance ranging and optical triangulation technology may be required to acquire all the necessary field data of any tank. Every tank may require different methods and approaches to achieve accurate data. *
In India, we follow rules laid down by theDirectorate of Legal Metrology (click here to go to website)(Weights & Measures department) for vertical cyl. tanks for petroleum products - also called the "strapping method" :
Internationally and for any tank being approved by CPWD in India, we may also follow BIS 2007, BIS 2009, BIS 2166 and international standards like ISO 7507 - part 1 to 4, ISO 12917, API (American Petroleum Institute) 2550, API 2551, API 2552, API MPMS 2.8.A, from which the national standards and rules are derived. The ISO rules have also been recommended by OIML (International Organization of Legal Metrology - click here to download pdf) of which every country including India is a member.
Internal survey is carried out in tanks where it is safe to enter enabling us to measure datum plates, internal deadwood & tank bottom floor / shape. Underground horizontal tanks require internal measurements. Bottom plate unevenness is profiled and physical calibration with water is done upto flush point as required. Safe Laser beam measurements are also used for internal measurements for horton spheres and underground cylindrical tanks.
All field data are fed into our computer with customized software for specific corrections like tape temperature corrections, thickness corrections, deadwood corrections, etc. and shell wise calculations as per International ISO methods and Rules laid down by the Weights and Measures Dept. Complex mathematical integration, formulae and calculations are involved in case of horizontal tanks where ends are torispherical, tank is tilted as volume increment is non-linear.
Calibration charts are made in SI or British units as per requirement. In India, for spirit tanks in distilleries/breweries under excise, gauging tables are also computed as per excise rules. Calibration charts can also be given in weight. Tank calibration data can be provided on softcopy data for uploading to inventory monitoring software.
International method: ISO 12917 - part 1, API 2551. BIS: IS-2009. Internal Dia. (vertical & horizontal) of all shells, length of horizontal tanks, datum plate height. Inclination & spherical dish-end measure of horizontal tanks Calibration of Horton Spheres (API 2552 method) and horizontal bullets: determination of internal vertical & horizontal diameters, position of auto gauge with respect to tank floor. Accurate Distance Lasers with maximum / minimum length and slope determination can be used.
International API MPMS 2.8A method is followed. Measurements are done for deadwoods, corrugated bulkheads, deadrises, bilge radii, etc. Correction for trim, list and temperature is done.
For uneven, inclined tank floors, spherical or conical bottoms, determination of flush point of bottom of tanks by measured water through flow meter or prover. Floatation of roof of floating roof tanks by water is done to determine volume displacement by the weight of the roof. This displacement is to be applied for density corrections of liquid stored. Wet calibration at intervals is required for deformed, inclined horizontal tanks or irregular bulging rectangular tanks.
API 653-B method is followed. Out of Roundness, tank tilt, settlement survey