Measurements of Vertical Storage Tank

The normal tape used is made of steel, 1/8" to 1/4" width , and of enough length so that it may cover the entire circumference. If the tape is not long enough, the measurements may be made in sections, sum of which will give the measurement of the circumference.
The tape should run parallel to the seam of the tank. Loops and cords should be used to ensure this. A tension of 4.5 ±0.5 kg should be applied to the tape through a spring balance.
Three measurements, at the bottom, middle and top of each course shall be taken.

Measurement of Step-over

During the measurement of circumference obstructions such as projections, deformities, fitting or joints may be found on the surface of the tank course. Obstructions force the tape to deviate from its true circular path and giving erroneous measurements. Step-over is used to correct the measurement of circumference.

• The strapping tape should be stretched over the tank surface including the obstruction.
• Now place the legs of the step-over across the obstruction on the tape where it is in contact with the surface and note the reading between two legs of the step-over on the tape. Let the distance be 5 mm.
• Lift the step-over and place it again on the tape where it is in full contact with the surface, on the left of the obstruction. Let the distance on the tape between the legs be 4.1 mm. Then place the step-over on the tape on the right side of the obstruction. Now, the distance is 3.9 mm. The average of two readings is 4 mm.
• The step-over correction will be 5-4 = 1 mm.

Measuring Shell Plate Thickness

• Now a days, plate thickness is measured by using ultrasonic or electronic thickness gauges. But, before using, the gauge should be tested with a pre-verified thickness.
• If any direct measurement is not possible, the course-wise thickness data supplied by the manufacturer may be used.

Vertical Measurement

• a tape should be suspended internally along the wall of the shell from the top curve angle to the bottom course and the height of course is taken in mm.
• The difference in height between the datum point and the bottom course (the thickness of the datum plate) is calculated.
• Let the difference be 15 mm
• This difference should be applied to the height of a course as correction

It may not be possible to measure course height internally. In such cases, external measurement may be taken and allowance may be made to neutralize the effect of horizontal seam overlapping.

Dead Wood Measurement

Table 2/5

• Deadwood should be measured internally. If not feasible, dimensions given in the manufacturer's diagram may be accepted.
• Measurements should show the lowest and highest level of deadwood from the tank bottom adjacent to the cell.
• The total volume of each piece of deadwood should be calculated.
• Measurements should be made in increments which permit allowance to be given to tank capacity at various heights.
• Work sheet on which details of deadwood are sketched dimensions and located, should be clearly mentioned in the strapping record.
• For variable deadwood such as nozzles and manholes, located at the bottom of a tank, an average deadwood correction shall be made.

Tank Bottom Measurement

• The method most employed is "Liquid Calibration". The procedure is to fill into the tank quantities of known volume of water until the datum plate is just covered and the total quantity recorded. (say 28230 litres)
• Measure the height of the water level from the bottom when dip at datum plate was 0.0 cm. (say 16.4 cm)
• Measure the height of the bottom course. (say 150 cm)
• Effective height of the bottom course will be the difference between these two heights.
  150.0 cm - 16.4 cm = 133.6 cm

Tilt Measurement

• This is done by suspending a plump line from the top angle and measuring the offset at the bottom angle. Let the angle be Ø
• The tilt may be ignored if it is less than 1 in 50, this representing a maximum error 0f 0.2 per cent.
• Open capacity of the tank in litre per cm will have to be multiplied by sec Ø