A Review of Non-destructive Testing Methods for Aboveground Storage Tank Floor Inspection

Aboveground storage tank floor inspection is required to assess the current and future conditions of  tank floor plate top- and bottom-side corrosion in accordance with API-653, STI-001 and regional regulatory regulations [1-5].  There are a number of non-destructive testing options for steel tank floors including acoustic emission testing (AE), ultrasonic testing (UT), phased array ultrasonic testing (PAUT), and magnetic flux leakage (MFL) testing.  This article provided and introductory review of these non-destructive testing methods for carbon steel aboveground storage tank floor plates.  

Review of Tank Floor Damage Mechanisms

Above ground storage (AST) tank floor corrosion occurs on the bottom- and top-side of the floor plates.   Bottom-side, or soil-side, refers to the deterioration or degradation of the tank's external surface that is in direct contact with the soil or ground.  This type of corrosion occurs due to interactions between the steel surface and the surrounding soil environment, and it can significantly impact the structural integrity and lifespan of the tank.  Soil side corrosion in steel ASTs can be attributed to soil composition, moisture content, pH levels, and chemical constituents can influence corrosion rates. Highly acidic or alkaline soils, as well as those with high chloride or sulfate concentrations, are particularly corrosive.

Figure 1: Example of aboveground storage tank top-side corrosion.

Top side corrosion is driven by other phenomena related to the product stored and possible moisture, organic, and non-organic contaminants.  Several types of corrosion can occur on the top side of ASTs, including general corrosion, pitting corrosion, localized corrosion characterized by small pits or holes on the surface, and crevice corrosion alongside joints or welds. 

Acoustic Emission Tank Floor Inspection

Acoustic emission testing of steel tank bottoms for corrosion dates to the early 1990s [6,7] and has been adopted as a screening technique that correlates the amount of acoustic emission activity to corrosion product brittle fracture.  Corrosion product is broadly categorized as rust, scale and an oxide layer formation.  The rate and intensity at which acoustic emission is detected from tank floor corrosion product infers the current state of the tank floor and likelihood, or risk, for generalized or pitting corrosion to occur.    AE tank floor technology continues to advance with a primary focus using AE features like energy, duration, and frequency to discriminate between generalized corrosion, pitting corrosion, H2 bubbles associate with the corrosion process, and leaks. Figure 2 shows an example of acoustic emission instrumentation accurately locating  gas bubble activation [8].  

Figure 2:  Acoustic emission detection of tank floor corrosion bubble activation.

Ultrasonic and Phased Array Testing of Aboveground Storage Tank Floors

Manual (UT) and automated ultrasonic testing (AUT) are well established non-destructive testing methods for metal loss.  Manual UT may be used to spot check tank floor plates or to proof up thin locations detected by magnetic flux leakage testing (MFL).  In some special cases, AUT or C-scan ultrasonic testing may be used to perform high resolution scanning to search for underside pitting.  Automated ultrasonic C-scans are typically with 0.5” or 0.25” resolution.   The latter, however, is sometimes impractical where high resolution tank floor soil-side assessment is required due to the weight and size of the equipment in addition to an in-tank constant water supply in the 50-100 PSI range.  Phased array ultrasonic testing (PAUT) using a dual element linear probe with a large aperture is a practical alternative to conventional UT.  

High resolution PAUT scanning  of tank floor soil side corrosion may be obtained by using a PAUT dual element array (DLA) probe with a standard magnetic wheel encoder.  Resolution is superior to the conventional UT depending on the aperture used.  An encoded scan across a suspected tank bottom corrosion area is shown in Figure 2. 

Figure 3.  Phased array ultrasonic testing of tank floor corrosion using phased array dual element probe.

Magnetic Flux Leakage of Tank Floor Bottom for Corrosion

Magnetic flux leakage (MFL) testing of tank bottoms is the  industry standard for screening for corrosion areas [9-10].  The ability of the MFL technology to quantify floor loss and discriminate between top- and bottom-side corrosion continues to improve [11].   The MFL technique is applied across wire rope, carbon steel pipelines, and tank floors on a routine basis and the magnetizing, flux leakage and detection principles are similar.  The tested structure is magnetized via an electro- or permanent magnet. In the presence of a local fault or loss of metallic area of pronounced corrosion the induced magnetic field leaks outward of the magnetized section.   The resulting magnetic field leakage consists of three perpendicular field vectors but traditional MFL instrumentation are designed to detect the MFL leakage in the vertical direction, or perpendicular to the plate surface, using a horizontally aligned Hall sensor located approximately 3-6 mm from the plate surface.  The Hall sensor output is proportional to the MFL leakage in contrast to MFL inductive sensors.

The typical MFL scan width is 12” with some equipment containing 64 Hall sensors across this width for finer resolution.  Encoded MFL scans create C-scans with a maximum length of up to 50’ depending on the instrumentation used.  The recommended scan speed is 1-3 feet per second.  Inspection through coatings 1/4” on 1/4” plate and 5/32” on 3/8” plates are possible.  The recommended maximum floor plate thickness inspected using MFL testing is  0.50”. The maximum sensitivity is cited as 20% material loss under floor and top surface

Figure 4.  Vertical and axial magnetic flux leakage vectors and example output from a Hall sensor.

Summary

While magnetic flux leakage testing is the primary non-destructive testing method to screen tank floors for top- and bottom-side corrosion there other supporting techniques including acoustic emission and phased array testing that may assist inspection depending on desired inspection outcome.   

References

1. API 653 : 2020 TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION

2. STI-001: Standard for the Inspection of Aboveground Storage Tanks

3. 6 NYCRR Part 598:  Chemical Bulk Storage (CBS) Handling and Storage of Hazardous Substances – Section 598.7 Aboveground tank systems – inspection,

4. N.J.A.C. 7:1E, “Discharges of Petroleum and Other Hazardous Substances” (DPHS)

5. PADEP Chapter 245: Administration of the Storage Tan and Spill Prevention Program

6. Van de, Loo PJ. How reliable is acoustic emission (AE) tank testing? The quantified results of an AE user group correlation study. 7th European Conference on Nondestructive Testing. Copenhagen: 1998. 

7. Ronnie K M. Tank-bottom leak detection in above-ground storage tanks by using acoustic emission. Physical Acoustics Ltd, 1993, 823~829.

8. Jirarungsatian, C., Prateepasen, A., “Pitting and uniform corrosion source recognition using acoustic emission parameters, Corrosion ScienceThis link is disabled., 2010, 52(1), pp. 187–197

9. N. B. Cameron. Recommended practice for magnetic flux leakage inspection of atmospheric storage tank floors. Health and safety executive UK, 2006. 

10. P. C. Charlton. A theoretical and experimental study of the magnetic flux leakage method for the analysis of corrosion defects in carbon steel plate. PhD Thesis, Swansea institute of higher education, 1995.

11. Ramirez, A.R., Mason, J.S.D., and Pearson, N., Experimental study to differentiate between top and bottom defects for MFL tank floor inspections, NDT & E Int., 2009, vol. 42, no. 1, pp. 16–21.