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MODERN OPTICAL METHOD :
 
Tank Calibration Methods :
 

The following methods are used more often for tank calibration.

  • Upright Cylindrical Tanks Calibration: Manual Strap ping. (API 2550/ ISO 7507-1:2003).
  • Optical Reference Line Method (ISO 7507-2:2005).
  • OTM, Optical Triangulation method (Laser) (ISO 7507 -3:2006)
  • EODR, Internal Electro-Optical distance ranging method (Laser) (ISO 7507-4:2010)
  • EODR, External electro-optical distance-ranging method (Laser) (ISO 7507-5:2010)
  • Calibration of Horizontal Tanks. (API 2551)
  • Calibration of Spherical Tanks. (API 2552)
 
ISO 7507-3:2006
 
Petroleum and liquid petroleum products -- Calibrat ion of vertical cylindrical tanks -- Part 3: Optical- triangulation method.

ISO 7507-3:2006 specifies a calibration procedure for application to tanks above 8 m in diameter with cylindrical courses that are substantially vertical. It provides a method for determining the volumetric quantity contained within a tank at gauged liquid levels. The measurements required to determine the radius are made either internally or externally. The external method is applicable only to tanks that are free of insulation. ISO 7507-3:2006 is suitable for tanks tilted up to a 3 % deviation from the vertical, provided that a correction is applied for the measured tilt as described in ISO 7507-1.
 
Optical Triangulation Method :
 
France has developed and been using an Optical Triangulation Method (OTM) which uses the measurement of tank angles to determine the tank diameter. This method again provides a means for calibrating vertical cylindrical tanks using external measurement of angles with a theodolite or a laser theodolite. The method also requires a measured reference circumference (same as ORLM), determined by manual strapping at its bottom course. The theodolite being used must have an angular graduation and inaccuracy equal to or less than 0.022 degrees to ensure the required accuracy of measurement is achieved.
 
Preparation :
 
As in the ORLM, the number of horizontal stations should be selected according to tank diameter. The minimum number of stations as shown in the table should be used, but the number of maximum stations is left to the choice of the person doing the work. Obviously, the more stations used the better accuracy of the calibration.
 
Number of Horizontal Stations :
 
Tank Circumference (cm.)
Min. Number of Stations
 

Upto 50 cm.
Above 50 upto 100
Above 100 upto 150
Above 150 upto 200
Above 200 upto 250
Above 250 upto 300
Above 300

4
6
8
10
13
15
18
 
The horizontal stations should be approximately spaced, at equal distances, along a circle concentric to the tank. The point of tangency sighting line to the tank should not be closer than 12" to any vertical weld seam. The weld seam will introduce errors in measurement because it does not react to tank movement in the same way as the tank shell moves.

The number of vertical stations are the same as in the ORLM Method (two per ring) and are established at 20% of the distance from the upper and lower horizontal weld seams for each tank course.
 
Tolerance on Reference Circumference
 
The reference circumference measurements taken before and after the optical readings shall not differ by more than the tolerances given in the following table.
 
Distances (m.)
Tolerances (mm.)
 

Upto 25 cm.
Above 25 upto 50
Above 50 upto 100
Above 100 upto 200
Above 200

2
3
5
6
8
 
 
Procedure
 
The base length is determined by manually strapping the bottom course circumference, 20% below the horizontal weld seam. This measurement is the reference circumference.

Reference circumference by strapping :
First, an onsite reference circumferential strapping is done only on 1st or 2nd shell with calibrated strapping tapes and dynamometer with a tension of 5 kg and repeated 3 times and a mean value taken. This circumference is taken roughly at a position of one fourth from the upper or lower weld of the strake. A circumference measure at the 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 corrections like temperature, step over (vertical welds). We call this the reference radius or diameter. Other than this, plate thickness measure with ultrasonic thickness gauge,dip reference height measure, tank and course height is taken.These are done as per ISO 7507 part 1.
 
 
The tank is then sighted from the first horizontal station using a theodolite. Two sightings must be made tangentially to the tank, on the left and right from each station, recording the angle subtended between the two sightings.

The first vertical sighting should be made at the same height as the reference circumference was taken. This measurement will determine the reference angle. The theodolite is then angled upwards to sight at the next vertical station. In order to prevent any correction for tilt in the tank, the vertical angle for each pair of sightings should not be changed during the measurement.

After the angle between each pair of sightings has been recorded for all vertical stations at the first horizontal station, the theodolite is relocated to the next predetermined horizontal station. All measurements and procedures are then repeated, beginning at the first vertical station.
 
Calculation :
 
The distance between the vertical centerline of the tank and the vertical line of any horizontal station is constant to the height of the tank. The course radii are calculated as follows:

Let T be the horizontal station site of the theodolite. The sighting T --> B and T --> B' at the exact location of the manual strapping determines the reference horizontal angle Φ.

 
 
The arithmetic mean of all the radii (r') for a given vertical station will determine the tank radius at that vertical station. As there will be two average radii per ring, the mean value of the two will be the average radius for that course.
 
Internal Measurements (for empty tanks)
 
Datum plate height, deadwood (manholes, pipes, beams, coils, etc.), roof structures, roof leg pin spaces are recorded. Floating roof weight and ladder weight for floating roof tanks are taken from existing references 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.

Volumetric Analysis and corrections on data, computation, layout of the Calibration Chart as per rules is carried out subsequently after field data consistency and quality checks. Volumetric data is also provided electronically for SAP or any other uploading.
 
Calibration of Tank Bottom Volumes
 

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 formations 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.

 
 
 
 
 
The Advantages of the Optical Method are :
 
Optical measurement is safer than strapping. There is no need to access the outside of the tank, which for physical strapping is normally carried out from a bosuns chair or with a “cherry picker” type personnel hoist. The optical plummet operator sits at the base of the tank whilst an assistant manoeuvres a magnetic trolley on the tank shell from behind the hand rail on top of the tank. The magnetic trolley is fitted with a scale that can be read through the optical plummet to a resolution of +/-1mm.

Optical calibration is faster than strapping in most cases, with less impact on other operations in the area. A typical 20-metre diameter tank could have the shell measured in only a few hours.

There is better traceability of the field data from the calibration. The data from which the tank is calibrated is recorded for later calculation and verification, as are calibration records of the optical plummet. With strapping there is the problem of the tape sagging during measurement and the real difficulty of repeatable measurements whilst the operator is suspended off the side of the tank, trying to maintain 5 kg of tension on the strapping tape.

As a side benefit of the optical method, the data from which the circumferences are calculated will also yield information on the shape of the tank shell. It is a simple matter to process the data to determine the verticality of the tank shell and to determine the “roundness’ of the tank. This is essential information to the engineer who wants to fit floating blankets or roofs to vertical tanks. It is also information that can be useful in determination of any settlement or subsidence of the tank in service.

It is simple to optically calibrate a lagged tank. After measuring tank diameters internally, the plummet can be used inside the shell.

It is probably possible to calibrate a tank in stronger wind conditions than is safe for strapping although in very strong winds the magnetic crawler used to position the scale on the side of the tank may be blown off the tank.

We say only probably here because we do not strap tanks where it is necessary to use a bosun’s chair because of concerns over operator safety.