American Petroleum Institute (API) Standard 653 is the Industry Standard most commonly used for tank inspection repair, alteration, and reconstruction. The in-service inspection, sometimes described as external inspection, is conducted by an authorized inspector and includes a visual inspection of the tanks exterior surface. This inspection is usually conducted while the tank is in operation or when the interior is not assessable or available for inspection. During the in-service inspection, InterSpec inspects the tank systems for evidence of leaks, shell distortion, signs of settlement, corrosion, and the condition of coatings, insulation, and appurtenances.
The frequency of an in-service inspection is typically every 5 years. Tanks that are in more corrosive service or accelerated corrosive conditions are inspected at shorter intervals, which frequency is determined by rate of deterioration and remaining metal thickness. External ultrasonic thickness evaluations, determine the rate of uniform general corrosion while the tank is in operation. By employing a variety of digital thickness gauges and scanners–both manual and robotic—InterSpec is able to obtain the required data for evaluating the condition of the tank and frequency requirements for inspections. The frequency of thickness examination is usually every 15 years; but a shorter inspection frequency may be required according to API Standard 653, which is determined by the difference of the measured thickness and the minimum required thickness. Maximum allowable frequencies for in-service thickness examination for new and older tanks with unknown corrosion is 5 years. This inspection is included in the out-of-service tank inspection.
The reason for tank inspection in accordance with API 653 is generally to determine the mechanical integrity of the tank and its rate of corrosion. After reviewing the inspection results, steps are taken to reduce the potential for failure and release of stored product. Maintaining a safe operating condition, along with determining any deterioration, is another reason for tank inspection.