Method Statement for Oil Storage Tanks Calibration

Tank Calibration is the process of determining the volumetric content of a tank per unit height of liquid. This procedure shall be intended for calibration of Oil Storage Tanks. The calibration shall be carried out by Optical Reference Line Method (ORLM) in accordance with API Standards i.e.  API 2550, Section 2B and ISO 7507, Part 2.

Condition for Measurements

All details about tank including drawings shall be provided to the Calibration agency before carrying out calibration.

Calibration of Shell and bottom up to datum point shall be carried out only after hydrostatic test of the tank. The tank shall be completely empty and clean.

The measurement works shall be carried out after ensuring the tank is totally available for measuring work in safety condition.

Calibration Equipment

Following equipment’s provided by calibration agency will be used for measuring.

• Strapping Tape 100 M Ritcher Metri, Germany
• Dip Tape with Dip Wt. 30 M Ritcher Metri, Germany
• Measuring Tape 3 M Freemans
• Spring Balance 10 Kg Salter
• Theodolite/Tripod Lieca, Germany/ Kolida, China
• Water Tube 50 M

Tank Calibration Method

Vertical Calibration

For use of Optical calibration a reference circumference is required. This is obtained by taking the average circumference of the bottom plate course, which is measured by strapping either continuously or for larger tanks in series of areas by the use of calibrated master tape. Reference strapping is taken at 80% from the bottom of the course.

On the reference circumference stations are marked at equal intervals around the tank wall, the number of optical perpendicular stations. This is obtained by the following formula.

Stations = Circumference in meter/6

(Rounded the nearest even number of stations with the minimum of 8 stations and maximum of 24 stations).

The horizontal stations shall be chosen to insure that the vertical transverse of the trolley along the shell at any given station shall be at least 300mm from the vertical weld seam and their should be no interference in its line of travel.
For any given horizontal station, a minimum of two vertical stations is established for each course at 20% and 80%.

At the first station the theodolite in set up with its standing axis at a known distance from the tank wall. The movable scale assembly is placed against the tank wall and pulled upwards until the graduated rule is at the same level as eyepiece of the theodolite a reading on the graduated rule is taken. Subsequently reading is then taken at all required levels. Two reading per course minimum.

Bottom Calibration

Similar number of horizontal stations marked inside the tank at equal distance around the circumference. Vertical stations are taken from the center of the tank to the inside of the wall, 5M apart with the first station at 0.5M from the wall.

By placing a water tube at the center of the tank and taking reading with a water tube at each vertical station, the profile of the tank bottom can be obtained. From this profile the volume can be calculated using the correct formula for the geometrical shape for the floor with irregular configuration.

Deadwood Measurements

The entire deadwood is accurately accounted for as to the volume and location in order to permit adequate allowances for volumes of liquid displaced by various objects and allocation of these effects at various elevations within the tanks.

Computation of Capacity Table

Once the field measurements have been carried out the accumulated data is input onto our computer program to obtain the tank table on recommended format of non-tearable tracing paper showing the capacity in liters at every 1 cm depth and fractions in mm to interpolate dip readings and get the Calibration Chart duly certified and Approved by Competent Government Authority. A Soft copy of EXCEL file of Tank Capacity table shall also be submitted to load tank capacity in the tank gauging system.