Ultrasonic Tank Testing
Introduction to Testing Methods, Reporting, Applications, and Costs
Ultrasonic testing, or UT as it is commonly called, is the procedure of introducing a high frequency sound wave into the exterior side of a metal surface, and reflecting the sound wave from its interior surface to produce a measurement of wall thickness. The two way duration of travel, divided by the known sound velocity through that particular metal, provides a thickness measurement equally accurate to a micrometer or caliper reading.
Since ultrasound allows the precise measurement of wall thickness from the outside surface, and provides a measurement of remaining wall thickness over a wide sampling of individual points, it produces a very thorough corrosion evaluation within a short time and at reasonable cost.
Few Test Methods Available
Test methods such as corrosion coupon monitoring, x-ray, spool pieces, and selective metallurgical analysis, often used at piping systems, are not applicable to investigate tank integrity. Robotic video inspection is another useful tool rarely applied and of little benefit. For virtually all facilities, corrosion monitoring for water storage tanks is simply nonexistent.
For most tanks, both the internal and external tank surfaces are coated or protected against corrosion in some way, and to varying effectiveness. This is very common for domestic cold water storage tanks known to contain a higher level of corrosion threat. Where the coating holds, wall thickness may be still at new specifications. Where it fails, however, severe pitting often results.
This is well illustrated at a domestic cold water storage tank where random deterioration of the internal coating has allowed aggressive corrosion of the underlying tank wall. Ultrasonic testing of the below tank example had shown the plate steel at new 0.400 in. thickness specifications and virtually untouched where the internal coating had held, shown here by the smoother inside surface. Ultrasound also identified very deep and severe pitting to below 0.100 in. in some areas; those areas later found perfectly corresponding to the internal rust deposits shown by draining and visually inspecting the tank’s interior.
Highest Wall Loss Below Random Interior Deposits
To offer a better illustration to the random and hidden threat which often exists for tanks, we show below the wall loss present beneath each of the above areas of internal rust. Approximately 18 times a greater volume of iron oxide rust product is created from the steel tank wall – leading to much larger deposits being produced. Each rust deposit grew directly outward from its small base of deep wall loss.
Ultrasonic testing at this side wall had identified substantially less overall deterioration in comparison to the other walls shown above. In one specific area, however, and later shown to be opposite this internal tuberculation deposit, we identified widespread pitting to a lowest wall thickness of near 0.085 in. Additional examples of tank corrosion are provided in our Tank Photo Gallery.
Extended Rust Product
Underlying Severe Pitting
Ultrasound The Best Overall Testing Method
Ultrasound excels for evaluating metal water storage tanks since it allows taking thousands of wall thickness measurements during a normal working day. Such high level of coverage can then be analyzed to provide a thorough and comprehensive tank condition assessment. Our primary focus is defining wall thickness of the tank; with other physical aspects such as bottom I beam supports, cradles, U channel wall reinforcement, inlet and outlet threadolets, and other structural aspects of the tank referred to a structural engineer.
Ultrasound is most often employed as the first step in an investigation – to be then followed with an internal visual inspection to confirm the severity and extant of any wall loss. For potable domestic water storage, microbiological contamination is a very frequent concern.
Taking wall thickness measurements in a standard X by Y grid every 1 sq. ft., or every 6 sq. in., will produce 1,000 or more thickness measurements for a typical domestic water or fire reserve tank. There is no need to drain the tank or take if off-line.
Cutting a sample from the tank requires shut-down, welding, and laboratory analysis at far greater cost – yet relating to only one or a few areas of the tank.
Ultrasound is a proven technology well recognized as providing a high level of accuracy if skillfully performed. It is commonly relied upon as an inspection and diagnostic tool in aviation, aerospace, power generation, nuclear, and other highly critical applications.
While a thorough analysis of ultrasonic test data may take time, wall thickness measurements are instantaneous, and will provide on the spot confirmation of a suspected wall loss problem.
As a nondestructive testing procedure, ultrasound neither damages nor alters the material inspected. Piping systems can be in operation, and either filled or empty. There are no safety concerns for either operator or building staff, as exist with x-ray.
The ability to cost effectively inspect each tank wall, bottom, or head through hundreds of thickness measurements, ensures greater testing coverage, and greatly increases the reliability of any remaining system life assessment. In comparison, making a system wide judgement based upon the metallurgical testing of one or two cut out samples is extremely risky.
For 24/7 and critical systems, or generally for larger active water storage tanks, cutting out a sample section for metallurgical testing is virtually impossible. In such cases, ultrasound provides the only option available for determining tank wall thickness.
Ultrasonic testing’s only weakness lies not with the technology itself, but with incredibly poor quality work and investigative ability provided by some “authorities” – resulting in nothing more than a 1 or 2 page spreadsheet of inaccurate and worthless wall thickness data.
If performed properly, ultrasonic testing offers multiple benefits by providing extremely accurate data across a wide sampling of test points. Combined with statistical analysis, UT can define the extent of a corrosion problem or provide excellent advance prediction of useful service life. While commonly applied to tanks, ultrasound is also a useful tool in determining the condition of any vessel, shell, pan bottom, or other material suspected of suffering wall loss.
Ultrasonic testing is commonly employed for:
- Domestic cold water storage tanks
- Pressurized fire reserve tanks
- Steam condensate tanks
- HVAC expansion tanks
Other useful applications are:
- Refrigeration machine shells
- Shell and tube heat exchangers
- Cooling tower pans
- Compressed air storage
Ultrasonic testing does hold some disadvantages, however, by not being able to provide or identify the following:
- View of interior deposits
- Identify precise corrosion mechanism
- Tank failure mechanism
- Deposit composition
- Microbiological type and volume
- Suggestion for remedial actions
Valuable Information Derived
Unlike most other forms of pipe analysis, ultrasonic testing is nondestructive, and does not require a system shutdown or any special preparations. From an initial set of wall thickness measurements and review of the building or plant history, it is possible to derive valuable information regarding the present state of each piece of equipment tested. Information such as:
- Pipe metal loss
- Corrosion rate
- Percentage of allowable loss to minimum standards
- Remaining pipe service life
- Estimated retirement date
- Pass or fail recommendation for acceptable condition
- Overall tank condition
In addition, 1,000 or more wall thickness measurements allow generation of a detailed three-dimensional graph showing a virtual image of the internal tank wall. Examples of such are provided in our Interactive Tank Report.
Clear Presentation Of Results
Key to any form of investigation is the need to be thorough, accurate, conclusive, and clear and organized in the final presentation of results. Due to the number of individuals likely involved in the information chain, and having an interest in any tank or pressure vessel problem, the report must be understandable to the professional engineer, management company, building owner, plant operator, and layman alike. In other words, a spreadsheet full of raw test data for the client to interpret should never be considered acceptable.
Nationwide Testing Service
We have supplied ultrasonic testing services to clients throughout the United States since 1992, providing the highest level of investigation from Anchorage, Alaska to St Thomas, U.S. Virgin Islands. The statistical details of all such investigations are maintained in our database whereby we can easily extract summary information or search for items of importance.
statistics_resume_140Recently, we adapted this database to automatically produce a single page “live” statistical resume of our work and experience in the ultrasonic testing field. With each additional project completed, new data is automatically added and the resume updated.
We offer this statistical resume at left, and also provide further background information, client listings, and business references upon request. First hand references from some of our past clients are available upon request.
Fees are very reasonable, and range depending upon the physical size of the tank and location.