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.
The Leading Cause Of Pipe Failure At Condenser Water Systems
Despite various corrective measure, advanced failures at condenser water systems are on the rise. Many problems are engineered into the system from the start due to the failure to recognize the impact rust deposits and particulates have on producing higher secondary corrosion levels. In addition, most corrective measure, if they are attempted, fail to provide a solution. Here is why.
Undersized Steel Pipe
A Simple Dial Caliper Measurement Of New Steel Pipe May Reveal Surprising Results
In addition to the many corrosion influences negatively impacting piping systems, many new building properties are constructed using carbon steel pipe which is below factory specification. To the surprise of many, an FM or UL approval, like its ASTM stamp, does not define that the pipe actually meets ASTM thickness standards.
The Decline In The Quality Of Today’s Piping Products Means Greater Corrosion Problems
A large number of negative influences have comined to produce a higher frequency of corrosion problems – often in spite of all precautionary measures taken. Of those, lower quality pipe, undersized pipe, more complex piping layouts, and generally less effective chemical treatment options have produced a “Perfect Storm” contributing to more piping failures.
When Pipe Corrosion At A Fire Protection System Can Cost Lives
The time to learn of a fire pipe corrosion problem is not during an actual fire emergency. Internal rust deposits can, and have, totally blocked water flow through the sprinkler heads – resulting in the loss of human life. More common at dry systems, internal deposits are a serious threat to all fire protection systems.
Fire System Failures
Major Misconceptions Within The Fire Protection Industry
Ignoring the obvious does have serious consequences when it comes to fire protection systems. From the use of thin wall schedule 10 & 7 pipe, to lower quality pipe products, to frequent flow testing which brings in new fresh water, clear and well documented reasons exist to explain the higher corrosion activity found at today’s fire protection systems.
Fire System Corrosion
The Threat Of A High Corrosion Condition To A Fire Sprinkler Line
Often viewed only in terms of water damage in the case of a corrosion induced pipe failure, far more serious concerns exist, although rarely considered. Unlike HVAC piping systems, corrosion activity at fire related piping can impede and in some cases totally block water flow – a potentially life threatening condition during any fire emergency
The Benefits of Ultrasonic Testing in Determining Corrosion Rate and Service Life
Ultrasonic testing provides the most comprehensive, accurate, and cost-effective tool to assess the condition and remaining service life of any piping system. Planned and performed properly, ultrasound offers the first step toward identifying a potential corrosion problem, or for certifying a piping system as fit for service.
Why Not All Pipe Failures Are The Fault Of Your Chemical Water Treatment Provider
Various design elements to any piping system can have dramatic impact upon its corrosion activity. Pipe origin, schedule used, physical layout, and many other unknown factors can produce a pipe failure. And yet they are completely beyond the realm of protection offered by chemical water treatment.
Corrosion By Design
Pipe Corrosion Problems No Water Treatment Program Can Protect Against
Various changes have occurred to mechanical piping designs over the past few decades, with virtually all HVAC, plumbing, and fire protection systems having been affected in some way. Many changes relate to the materials themselves. Major changes in piping design, however, have introduced new corrosion problems no chemical treatment program can stop.
The Benefits and Limitations of Corrosion Coupons
Relied upon for decades as an indicator of corrosion activity within piping systems, corrosion coupons are highly unreliable in most examples, and totally worthless in others. Many of the most damaging corrosion failures have occurred while at the same time corrosion coupons produced excellent results. Here is why.
If Corrosion Activity Is Only 0.4 MPY, What Is Wrong With The Above Picture?
Corrosion coupons reported a 0.4 MPY corrosion rate for 6 years where the actual rate exceeded 25 MPY. Believed implicitly in contrast to multiple leaks and failures, the slow but total destruction of the entire condenser water piping system was the net result. A case history illustrating the threat from relying exclusively upon this highly flawed testing method.
Dry Fire Sprinkler
Fire Protection Contractor – Antifreeze: The Fine Line Between Hero and Defendant
Antifreeze used in dry fire sprinkler systems may solve one problem, but has also proven deadly. Rated a Class 1 flammable liquid, antifreeze can accelerate a fire, create a fireball, and even cause an explosion. Although now restricted to lower concentrations, antifreeze still adds heat value to any fire and introduces new and unknown liability to any such system.
American Welding Society – Understanding Pipe Corrosion Problems
A piping system that satisfies service life demands, requires the recognition of piping design vulnerabilities, effective corrosion monitoring, and the adoption of corrective measurements. With corrosion related failures on the rise, and with generally lower quality pipe being installed, advanced planning and an awareness of potential threats becomes more important.
World Pipelines – Investigation vs. Procedure
Substantially different findings are likely where ultrasonic pipe testing is approached as a forensic investigation based upon known system problems and vulnerabilities, rather than simply a linear based measurement procedure. A critical importance is understanding the inherent corrosion related problems to various piping systems. An adapting investigation will also produce a more definitive answer to any piping problem.
Fire Pipe Corrosion
Fire Protection Contractor – When Pipe Corrosion In a Fire Protection System Can Cost Lives
The time to first learn of a pipe corrosion problem is not during a true fire emergency when lives are in jeopardy. Thinner pipe, more corrosive steel, lower quality galvanizing, foreign pipe, dry systems, MIC – all such negative factors are driving toward higher internal corrosion deposits to render your fire protection system worthless.
Fluid Handling Systems – Finding The Remaining Service Life
Ultrasonic testing is, by far, the most informative diagnostic method available for determining pipe status, as well as extremely cost effective. An effective piping analysis is much more than a spreadsheet of a few wall thickness measurements – requiring careful statistical analysis and practical interpretation of the data.
World Pipelines – Multiple Metering And Monitoring Needs
With no single form of corrosion monitoring capable of proving full coverage to the many different forms of pipe corrosion possible, multiple testing methods are always advised. These should include ultrasonic testing, spool pieces, LPR, regular internal inspection, and a close observance to the often obvious but missed signs of a problem.
A 30+ Year Knowledge Base
Unfortunately, the above is a very common progression of events for many of our clients. Often, greater attention to chemical corrosion control and corrosion monitoring could have saved the system and avoided the problem. In others, a decades prior design flaw or poor choice of pipe supplier may be traced back as the primary fault.
During the 20 years that we have been involved in the field of ultrasonic pipe testing / corrosion monitoring, we have authored various Technical Bulletins for the benefit of our clients. These Technical Bulletins address frequent problem issues to any building owner or operator, and offer both insight as well as reasonable and proven solutions.
We offer below the various categories available, and continue to add new bulletins as time permits.
Interior Rust Deposits, Common Threats, Corrosion Types, Winter Lay-Up, MIC, Corrosion Monitoring and Testing, CUI, Corrosion Coupon Failures, Rust Removal, Reducing Corrosion Threats, Roof Level Corrosion, Drained Pipe, Corrosion Trends, Fire Sprinkler Corrosion, Corrosion At “Free Cooling” Systems
The Impact Of Flow Rate To Higher Corrosion, Inadequate Water Filtration, Piping Layout Design, “Green” Piping Designs
Corrosion Threats, Design Misconceptions, Interior Rust Deposit Threat, Dry Fire System Corrosion, Schedule 10 Pipe, Premature Failures, Clogged Fire Systems, Chemical Control Options, Remediation Choices
Condition Assessment, Due Diligence, Preparation Prior To Renovation, System Evaluation, Expert Witness
Heat Exchangers, Benefits of UT Testing, High Pressure Water Jet Cleaning, Filtration Errors, Chemical Treatment, Condenser Tube Coating, Mold Concerns, Chromate Removal, Growing Threat of Corrosion, Heat Exchanger Tub Coating, Nondestructive Testing
Schedule 40 Limitations, Piping Trends, Hidden Corrosion Threats, Dielectric Insulators, Clamped Grooved Piping, Piping Schedules, Pipe Testing Specification, Roof Pipe Draining, Low Corrosion Guidelines, Dual Temperature Piping Failure
Cold Water Threats, External Corrosion Issues, Fire Reserve Tanks, Interior Pitting, Protective Coatings, Rehabilitation
Improving Heat Transfer Efficiency, Improving Filtering Efficiency, Filter Placement, Poor Performance Causes, Filter Selection Considerations
Chemical Treatment Challenges, Limitations to Water Treatment, Corrosion Coupon Reliance