Inspection of Storage Tanks

The inspection of storage tanks is an integrated examination process carried out to determine the structural integrity, tightness and fitness for service of atmospheric and low-pressure tanks in which liquids such as fuel, crude oil, chemical products and water are stored. These tanks are thin-walled steel structures consisting of the bottom, the shell, a fixed or floating roof and the weld seams joining them; over time they are exposed to corrosion caused by the stored product on the internal surface and by atmospheric conditions on the external surface, and to soil-side corrosion and settlement beneath the bottom. The inspection aims to quantify the rate and depth of these degradation mechanisms in order to determine the remaining wall thickness of the tank, the corrosion rate and the period for which it can operate safely until the next inspection (remaining life). The approach is based on referring construction quality to API 650 (welded steel tanks) and API 620 (large, low-pressure tanks), and in-service periodic assessment to the API 653 standard.

The inspection is applied in two basic formats. In an out-of-service inspection the tank is emptied, cleaned and made gas-free so that it is suitable for human entry; this is the most comprehensive inspection, particularly as it requires the bottom plates to be measured from the internal surface. In an in-service inspection, the external shell, foundation, earthing, drainage and piping connections are evaluated visually and ultrasonically while the tank remains in operation. In both cases the process begins with a comprehensive visual examination; corrosion, deformation, weld cracking, traces of leakage, the roof structure and anchorage connections are scanned visually and the findings are mapped for measurement-based examination.

The principal techniques applied in the inspection are Magnetic Flux Leakage (MFL) scanning, used to scan the bottom plates from end to end; Ultrasonic Thickness Measurement (UT), by which shell and bottom thickness is measured point by point; the vacuum box test, by which the tightness of the bottom weld seams is checked; and, where required, magnetic particle (MT) or liquid penetrant (PT) testing. The MFL method rapidly detects top and bottom surface corrosion by sensing the leakage created in the magnetic field by the metal loss in the magnetised bottom plate; the suspect areas flagged by MFL are then confirmed by UT to measure the true remaining thickness. In addition, planar and out-of-plane settlement occurring in the tank foundation is evaluated by the levelling measurements defined in API 653 Annex B; excessive differential settlement is examined separately, since it directly affects the shell geometry and the bottom stresses.

The depth of the application is based on the degradation data generated over the service life of the tank. For the bottom, the minimum acceptable thickness, the corrosion rate and the remaining bottom life are calculated; for the shell plates, the minimum thickness required for the hydrostatic load (t-min) is determined for each course and compared with the measured value. On floating roofs, the sealing elements (seals), the drainage system and pontoon integrity are evaluated; on fixed roofs, the pressure/vacuum valves and corrosion thinning. Cathodic protection, internal lining and earthing systems are also within the scope. These data are brought together to reach a decision on whether the tank requires repair, reinforcement or an operating limitation.

The inspection results are documented in a traceable report together with the measurement points, the MFL scan maps, the minimum thickness calculations, the corrosion rate, the calculated remaining life and the next internal/external inspection interval. Acceptance criteria are applied in accordance with the relevant sections of API 653; areas falling below the minimum thickness are addressed by repair, plate replacement or limitation of the operating pressure/level. This systematic approach reduces the risk of product loss and environmental contamination while making the safe service life of the tank predictable. TÜV AUSTRIA SILA KALİTE performs the inspection of storage tanks as an accredited Type A inspection body under TS EN ISO/IEC 17020, within the framework of the principles of impartiality and traceability.