TANK CALIBRATION PROCEDURE (ORLM)
ISO 7507-1 Strapping Method
First 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)
Optical Reference Line Method (ORLM)
Reference Standard: API Chapter 2.2B
This method establishes diameters of the courses by optical method
The method can be applied internally or externally (external easier)Procedure (Figure )
Tank divided into horizontal and vertical stations
– Number of stations horizontally vary from 8 to 36 depending on diameter
Magnetic trolley with graduated scale moved vertically
Reference circumference of bottom course by manual method (API Chapter 2.2A)
Reference offset is measured optically at the same height where the reference circumference is measured
At each horizontal station, course offsets are measured (Two per course) optically
Deviations in course offsets from the reference offset are averaged for each course
– Using the reference circumference and deviations the course diameters are Established
Important Considerations
Optical device stability is critical
Device must be in level in all directions
The optical ray must be vertical throughout the height of the tank (within limits)
At each station reference offset is rechecked b after the full vertical traverse
The optical device is checked randomly at three locations for perpendicularity by rotating the device 360 deg
In extreme windy condition, when it is difficult to maintain the trolley in contact with the shell, calibration should not undertaken
Other Measurements
Identical to manual method API Chapter 2.2 A
Development of the Capacity table
Per API Chapter 2.2A
Advantages
Much easier, no scaffolding and reference circumference is easier to control being at the base
Internal Measurements (for empty tanks)
Datum plate height, deadwood (manhole, pipes, beams, coils, etc.) roof structure, roof leg pin spaces are recorded. Floating roof weight and ladder weight for floating roof tanks are taken from existing reference to calculate volume deduction factors in density correction tables. Bottom calibration up to datum level and subsequently up to flush point is done with water flow meter.
