Thickness testing on atmospheric storage tanks is governed by API 653 “Tank Inspection, Repair, Alteration, and Reconstruction”, the international reference for managing the integrity of these critical assets. Thickness mapping of tank shells and bottoms calls for instruments with a large data logger, fast scan mode and the ability to work on coated surfaces. The Dakota NDT instruments of the MX, CMX and CMX10-DL series are suited to campaigns of this kind.
What API 653 requires
API 653 defines the inspection regime for welded steel atmospheric storage tanks and covers tanks built to API 650 (the design and construction standard) or to the older API 12C. The components inspected are:
- Shell: the vertical cylinder of the tank, made up of one or more courses welded together
- Bottom plates: the base plates, welded to one another and to the first shell course
- Roof: fixed, floating or geodesic
- Welds: vertical, horizontal and circumferential
- Appurtenances (nozzles, connections, ladders, walkways)
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API 653 provides for three types of inspection: periodic external (without shutdown), formal internal (with emptying and a full shutdown) and in-service bottom inspection (special techniques). For each type the standard sets the methods, the intervals and the acceptance criteria.
Thickness measurement on the shell
The shell is the area where corrosion is most predictable and therefore the most structured to inspect. The usual approach is a grid mapping with readings every 0.5–1 metre vertically and horizontally, following a procedure defined by the API 653 Authorized Inspector. On shells 10-20 metres high with a circumference of tens of metres, that means hundreds or thousands of readings per tank. Hence the need for instruments with:
- A large data logger: 4 GB of memory to store every reading plus the waveforms
- Fast scan mode: up to 250 readings/sec on the Dakota MX and CMX, to map large surfaces in a reasonable time
- Data organised in structured grids, reconstructible in DakMaster™
- Echo-Echo ThruPaint™ mode: shells are normally painted for weather protection and the paint must not be removed
- Battery endurance: a campaign on a large tank can run for several hours at a stretch
Bottom inspection
The bottom is the most corrosion-critical area, both from the inside (contact with the stored fluids) and from the outside (water ingress, the bearing pad, soil moisture). A full bottom inspection is possible only with the tank empty and cleaned. The usual NDT techniques are:
- MFL (Magnetic Flux Leakage) scanner: automated scanning for the rapid detection of suspect areas
- UT follow-up: quantification of the remaining thickness in the areas found by MFL, typically with Dakota CMX or MX gauges
- Pitting mapping: B-Scan to display the corrosion profile (MX1-DL / MX2-DL)
Dakota CMX10-DL: the top of the range for API 653
The Dakota CMX10-DL is the top-of-the-range instrument for intensive tank and pipeline inspections. It offers the complete set of CMX measurement modes (PE, EE, PETP, CT, PECT), a high-resolution display, a structured data logger and advanced waveform display functions (full wave capture). It is the preferred choice for API 653 campaigns on large tanks and for intensive company-wide inspection plans.
Data management and reporting
For API 653 inspections the report is an integral part of the inspection. The DakMaster™ software supplied by Dakota NDT automatically generates:
- Complete measurement grids of shell and bottom, with warning colours for readings below the threshold
- Histograms and statistics by course, by sector and by quadrant
- Comparison with earlier inspections for the corrosion trend
- Calculation of the remaining life and a recommendation for the next inspection date
- Export to Excel, CSV and PDF for integration with company asset management systems
Typical applications
- Oil terminals and storage depots
- Refineries (feed and finished-product tanks)
- Chemical and pharmaceutical plants
- Industrial and fire water storage
- Food product handling terminals
For advice on the instruments best suited to Your API 653 procedures, contact our technical department.