For any tank, there are two broad options. It can either be calibrated physically, i.e. by physical measurement of the tank shell and calculation of the volume or by volumetric method, i.e. adding or withdrawing measured volumes of liquid. In some cases it is clear which method is appropriate and in others either method can be used. On some occasions a combination of both methods may be required.
The most common method used for the physical (measurement) calibration of the shell of vertical tanks is “strapping”. Strapping is the process of measuring a tank by physically measuring the tank strakes with a calibrated steel tape at several levels on each course of plating.
Strapping is a good method for tanks that are scaffolded or where there is good access to the shell of the tank at all levels. In case of tanks which are large and where scaffolding is not possible, a combination of physical strapping, laser distance ranging and optical triangulation technology is required to acquire all the necessary field data of any tank. Every tank may require different methods and approaches to achieve accurate data.
There are two methods used to calibrate the volume below the dip plate in a vertical tank. The tank floor profile can be surveyed physically, using laser level instrument and from data obtained the volume can be calculated mathematically. The other method is to calibrate the tank floor volumetrically, using a meter or volumetric prover and water. Physical survey is faster and cleaner but for irregular tank bottoms, or for tanks with extreme tilts, a volumetric calibration can be more accurate.
Where safe entry is possible, internal survey is carried out in tanks to measure datum plates, deadwood & tank floor profile. Underground horizontal tanks require internal measurements for profiling of bottom plate irregularity and tilt. Laser level and distance instruments are also used for internal measurements for horton spheres and underground cylindrical tanks.
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The applicable standards are as follows:
VERTICAL TANKS
- ISO 7507-1 - Strapping method
- ISO 7507-3 - Optical Triangulation Method
- API MPMS 2.2 a,c and d - Vertical Cylindrical Tanks
- API 2555 - Liquid Calibration of Tanks
HORIZONTAL TANKS
- ISO 12917-1 - Horizontal Cylindrical Tanks
- API 2551 - Horizontal Tanks
- API 2555 - Liquid Calibration of Tanks
OTHER TANKS & MISC STANDARDS
- API 2552 - Calibration of Spheres and Spheroids
- API 2553 - Calibration of Barges
- Section 4 - Calibration of Spheres and Spheroids
- Section 5 - Calibration of Ship and Barge tanks
- ISO 7507-6 - Recommendations for checking & verification of tank calibration tables
- API 653-B - Tank Settlement Survey
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