Water Tank Non-Destructive Testing

In-Service Water Tank Inspection

Aboveground steel water storage tanks require internal inspection in accordance with the relevant American Water Works Association (AWWA) [1,2] , National Fire Protection Association [3], or local/state/federal regulatory requirements. Aboveground water storage tanks are most commonly used for potable water storage, wastewater treatment, and fire protection. The cost to remove the tank from service can often exceed the cost of the internal inspection. As a result, there is an ongoing effort to innovate existing, and develop new, water tank inspection technologies to accurately assess the internal tank floor, shell and other accessories remotely.

The Cost to Tank Water Tank Out-of-Service

The total cost to take a water tank out-of-service is dependent on many factors including product transfer, internal tank surface cleaning, health and safety worker protection, lost revenue, and others. An out-of-service inspection of the internal tank floor and shell is always superior to a robotic, remotely operated vehicle or a technology that can obtain internal tank metrics from the tank outside. However, if historical data from previous inspections infers the tank is in good condition, the remote inspection options become an attractive option due minimization of downtime, reduced cost, and manageable risk associated with opting for the remote internal inspection with known performance limitations versus the more comprehensive internal inspection.

Figure 1: Aboveground water storage tank drone inspection.

Non-destructive Testing Options for Water Tanks

In-service non-destructive testing options for water tanks can be broadly classified into external access methods and remotely operated vehicles or robotic systems for internal inspections. The former are conventional non-destructive testing techniques like ultrasonic thickness testing, shear wave or phased array ultrasonic testing and acoustic emission tank bottom condition assessment. The latter include tethered remotely operated underwater drones and intrinsically safe robotic and hybrid-robotic ultrasonic thickness testing technology. Disinfection of all equipment inserted into the tank must be performed with 200 ppm+ chlorine solution to prevent contamination of the water supply. The video above shows typical water tank inspection video from a tethered underwater drone. After disinfection, the drone is inserted from the tank roof manway and remotely controlled. High intensity lights delivering 1000 lumens are industry standard. During descent, the interior tank shell coating is in generally good condition with some intermittent areas of coating breakdown. The water tank coating breakdown areas may be evaluated in accordance with [4,5] ASTM D610-08 Standard Practice for Evaluating Degree of Rusting on Painted Steel Surfaces. The amount of rusting beneath or through a paint film is a significant factor in determining whether a coating system should be repaired or replaced.

This practice provides a standardized means for quantifying the amount and distribution of visible surface rust. The degree of rusting is evaluated using a zero to ten scale based on the percentage of visible surface rust. The distribution of the rust is classified as spot rust, general rust, pinpoint rust or hybrid rust.

The water tank floor also be assessed with acoustic emission testing.Tank bottom acoustic emission screens the tank floor for leaks and corrosion activity. Low frequency acoustic emission sensors mounted on the outside of the tank shell to monitor for coherent acoustic sources from water tank floor corrosion activity and leaks. Storage tank acoustic emission is a passive threshold based data acquisition technique. The sensor operates passively until an acoustic emission signal from corrosion or leak activity exceeds the instrumentation threshold level. The threshold concept is shown below for acoustic emission water tank floor data. A threshold crossing, or acoustic emission hit, occurs when an internal leak source or corrosion activity is generated with sufficient intensity to travel through the water to the acoustic emission sensor(s) mounted on the tank shell, noting that significant attenuation may occur as it travels through the water.

Figure 2: Example acoustic emission hit from water tank floor leak and/or corrosion activity.

Acoustic emission hit features including amplitude (dB), duration (us), and energy (eu) are analyzed to infer the likelihood that the tank floor AE source is leak or corrosion related [6,7]. Tank floor corrosion generated acoustic emission from the following sources: hydrogen bubbles, metal damage, passive film rupture, and gas bubble activation [8]. Hydrogen bubbles may form when steel is exposed to an acidic environment and release acoustic emission. As bubbles increase in size they become instable and eventually fracture emitting acoustic emission. During pitting corrosion, gas bubble fracture, passive film rupture, and pit propagation may emit acoustic emission. During uniform corrosion, acoustic emission is generated by gas bubbles and general corrosion. Acoustic emission waveform features may be used to differentiate between the different corrosion stages and sources. For example, bubble fracture typically occurs at high frequencies compared to passive film rupture, pitting corrosion, and uniform corrosion. The duration of acoustic emission signals generated by passive film rupture is significantly longer than that from bubble breakage and pitting corrosion. Similarly tank bottom leak sources may be discriminated from corrosion sources using acoustic emission waveform features.

Aboveground storage tank corrosion and leak activity may be located using standard 2-D source location calculation provided the longitudinal wave speed in the material is known. Tank floor acoustic emission sources that reach 3 or more sensors may be located accurately. The video below shows a 12 sensor acoustic emission layout around a tank shell and two different locations at which AE events were detected.

Figure 3: Tank floor acoustic emission corrosion or leak source location.

Summary

The nation’s potable water infrastructure system is made up approximately 2.2 million miles of underground pipes and thousands of aboveground storage tanks. The drinking water infrastructure is aging and underfunded and it is estimated that 6 billion gallons of treated water lost each day in the U.S. Innovative ROV and non-destructive testing technologies allow drinking water asset owners inspect tank and pipelines at reduced cost and are an essential service.

References

1. AWWA MANUAL M42 M42 STEEL WATER-STORAGE TANKS, REVISED EDITION

2. AWWA D100-21 WELDED CARBON STEEL TANKS FOR WATER STORAGE

3. NFPA 25 Standard for Inspection, Testing, and Maintenance of Water Based Fire Protection Systems

4. ASTM International - ASTM D610-08(2019) Standard Practice for Evaluating Degree of Rusting on Painted Steel Surfaces

5. Society for Protective Coatings (SSPC) (The) SSPC VIS 2 Standard Method of Evaluating Degree of Rusting on Painted Steel Surfaces

6. Monitoring Uniform Corrosion of Storage Tank Bottom Steel by Acoustic Emission technique. H. Bi, D. Hu, Z. Li, Q. Niu, I. Toku-Gyamerah, J. Chen. Int. J. Electrochem. Sci., 10 (2015): 6946–6958.

7. Acoustic Emission for Tank Bottom Monitoring. G. Martin. Key Engineering Materials (Conference Paper) DOI: 10.4028/www.scientific.net/KEM.558.445 (2012)

8. C. Jirarungsatian,A. Prateepasen, Pitting and uniform corrosion source recognition using acoustic emission parameters, Corrosion Science, Volume 52, Issue 1, January 2010, Pages 187-197.