The document discusses methods for measuring the plumbness and roundness of storage tank shells during construction and maintenance. Plumbness is measured using a total station to determine the verticality of each shell course, while roundness is measured to check how circular each course is. Measurements are taken at regular intervals and positions around the tank circumference and shell height. The document provides acceptance criteria from API standards, specifying maximum allowable deviations for plumbness and roundness based on tank size.

2. • The plumbness is a method to find out the verticality of each shell during the construction of storage
tanks. During the maintenance period,Plumbness needs to be checked to conform the integrity of the
tank.
• Sometime this method is also used to find out the distortion of Roof leg supports of floating roof tanks.
• Verticality can be checked by using different methods. Different equipment's are now available for
that. Normally a plumb is used to check the verticality of the shells. Nowadays Total Station
equipment is used.
• During maintenance period of large tanks, Verticality is checked by using Total Station equipment
instead of plumb.
PLUMBNESS

3. Verticality Measurement:
1. The verticality of each shell course shall be measured by optical method using Total Station equipment.
The equipment can measure accurately the distance horizontally and vertically in any direction.
2. Once the reference level and circumference are established, the Laser device (total station) shall be kept
as close as possible to the tank wall in each designated location (horizontal station). All the readings
shall be plotted in a graphical manner for each station location
3. Set the bench mark elevation & location from previous data (to be provided by Owner/operator) Mark the
geographical direction (north/ south etc.) or mark location in degrees to identify each measured location.
4. Mark positions around circumference at 32 feet intervals outside/inside of tank. The minimum number of
horizontal stations shall be as per table – 1 (As per ISO 7507-2)
5. Set the theodolite at the tank center and align it by horizontally by adjusting the screws on the stand.

4. 6. Take the measurement all along the circumference at this level. Put the total station at each designated
location clockwise as close as possible from the tank wall. Measure the distance from the wall. The
difference between this distance and the previously measured radius from center will constitute the ‘x’
reference which will be constant for all readings along the height of tank at that particular station.
7. Take all subsequent readings at the same station without any movement of the total station along the
height of the tank and tabulate all the readings.
8. For each shell course, at least two readings shall be taken (at 25% & 80% of shell height) at each
horizontal location. Additionally, one reading shall be taken at the Maximum design Liquid level.
9. Move around the circumference clockwise inside the tank and take the measurements at each designated
location and note down each reading.

6. ACCEPTANCE CRITERIA AS PER API 650
• The maximum out-of-plumbness of the top of the shell relative to the bottom of the shell shall not exceed
1/200 of the total tank height. The out-of-plumbness in one shell course shall not exceed the permissible
variations for flatness and waviness as specified in ASTM A6M/A6, ASTM A20M/A20, or ASTM
A480M/A480, whichever is applicable.
• The maximum out-of-plumbness of roof columns, guide poles, or other vertical internal components shall
not exceed 1/200 of the total height. The 1/200 criteria shall also apply to fixed roof columns. For tanks with
internal floating roofs, apply the criteria of this section or Annex H, whichever is more stringent.

7. ACCEPTANCE CRITERIA AS PER API 653
• The maximum out-of-plumbness of the top of the shell relative to the bottom of the shell shall not exceed
1/100 of the total tank height, with a maximum of 5 in. The 1/100 criteria, with a maximum of 5 in., shall also
apply to fixed roof columns. For tanks with internal floating roofs, apply the criteria of this section or API
650, Section 7.5.2 and Annex H, Section H.4.1.1, whichever is more stringent.
• The out-of-plumbness in one shell course shall not exceed the values specified for mill tolerances in
ASTM A6 or ASTM A20, whichever is applicable.

8. ROUNDNESS
The roundness of each shell course shall be measured by optical method by the use of Total Station
equipment. The equipment is capable of measuring accurately the distance horizontally and vertically in
any direction.
The roundness survey is based on accurate measurement of a reference circumference using a calibrated
measuring tape or by Total station at one level on an accessible, non – obstructed shell course, preferably
at the bottom of the tank. Measurements shall be repeated to agree with the specified tolerance of the tank
out of roundness.
Once the reference level and circumference is established, the Laser device (total station) shall be kept as
close as possible to the tank wall in each designated location (horizontal station). Readings shall be taken
at 20% & 80% level of each shell course in each horizontal location. All the readings shall be plotted in a
graphical manner for each station location.

9. 1. Set the bench mark elevation & location from previous data (to be provided by Owner/operator)
2. Mark the geographical direction (north/ south etc.) or mark location in degrees to identify each measured
location.
3. Mark positions around circumference at 32 feet intervals outside/inside of tank. The minimum number of
horizontal stations shall be as per table – 1 (As per ISO 7507-2)
4. Set the theodolite at the tank center and align it by horizontally by adjusting the screws on the stand.
5. Take the measurement all along the circumference at this level. Put the total station at each designated
location clockwise as close as possible from the tank wall. Measure the distance from the wall. The
difference between this distance and the previously measured radius from center will constitute the ‘x’
reference which will be constant for all readings along the height of tank at that particular station.
ROUNDNESS MEASUREMENTS

10. 6. Take all subsequent readings at the same station without any movement of the total station along the
height of the tank and tabulate all the readings.
7. For each shell course, at least two readings shall be taken (at 20% & 80% of shell height) at each
horizontal location.
8. Move around the circumference clockwise inside the tank and take the measurements at each designated
location and note down each reading.

11. Tank Diameter
m (ft)
RadiusTolerance
mm (in.)
< 12 (40) ± 13 (1/2)
From 12 (40) to < 45 (150) ± 19 (3/4)
From 45 (150) to < 75 (250) ± 25 (1)
≥ 75 (250) ± 32 (11/4)
ACCEPTANCE CRITERIA AS PER API 650

12. Tank Diameter
m (ft)
RadiusTolerance
mm (in.)
< 12 (40) ± 13 (1/2)
From 12 (40) to < 45 (150) ± 19 (3/4)
From 45 (150) to < 75 (250) ± 25 (1)
≥ 75 (250) ± 32 (11/4)
ACCEPTANCE CRITERIA AS PER API 653