Do I Need My Tank Calibrated?

The inspection and assessment procedures for determining whether a tank requires recalibration are addressed by the ISO Technical Report ISO/TR 7507-6: 1997(E); titled “Petroleum and liquid petroleum products; Calibration of vertical cylindrical tanks; Part 6: Recommendations for monitoring, checking and verification of tank calibration and capacity table”. 

The International Organisation for Standardisation Technical Committee ISO/TC 28, envisages the recommendations of the Technical Report will be included in ISO 7507-1 (Petroleum and liquid petroleum products – Calibration of vertical cylindrical tanks; Part 1: Strapping method) when it is reviewed.

The introduction of the Report states the following:

“Data currently available indicates that tanks are subject to a primary settlement which generally occurs during the first 5 to 10 years of service. Secondary settlement can also occur but appears to spread over the next 10 years to 20 years of the tank’s life. Tanks can undergo gradual changes in diameter, tank plate thickness and tilt throughout their service life. These factors affect the calibration of the tank and consequently the accuracy of any quantity assessments made using the tank capacity tables”.

The Report lists the following dimensions or characteristics that may influence the need for recalibration:

It is recommended that measurement of the bottom course diameter, plate thickness and tank tilt should be carried out every 5 years to decide whether recalibration is required. It is also recommended that a recalibration be undertaken every 15 years as a matter of routine, regardless of the findings of the 5 yearly checks.

In addition, the Report states that although tilt alteration will have relatively minor affect to the apparent tank capacity, the change in tilt is considered more important than the capacity effect. It states that:

“A significant degree of tilt can be an indication of serious structural problems in the tank’s foundations and should be investigated”.

For this reason, as well as assessing tank diameter, plate thickness and tilt as outlined in the Report, the performance of a tank bottom settlement survey for evaluation against the API Standard 653 Appendix B – “Evaluation of Tank Bottom Settlement” criteria may need to be considered.