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 pipe, 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 reading.
Since ultrasound allows the precise measurement of the pipe wall dimension 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.
Test methods such as corrosion coupon monitoring, x-ray, spool pieces, and selective metallurgical analysis, will provide some information related to the corrosion rate over a fixed period of time, and for a specific location. Robotic video inspection is another useful tool. They fail, however, to provide sufficient information upon which a sound and reliable overall piping evaluation can be based.
For many facilities, corrosion monitoring is generally inadequate or ignored, often in error, or simply nonexistent. Given the many different forms of corrosion which can coexist within the same piping system, a thorough and accurate evaluation means is therefore required.
Corrosion coupons, the most commonly used testing method in use, often fail to present an adequate view of piping status. They provide instead a general indication of the corrosivity of the water, an estimate of corrosion activity at very specific test loop locations, and short term results only over a very narrow test period. They do not, however, indicate the most important information desired - which is the wall loss occurring at the piping interior itself.
Ultrasound excels for evaluating piping systems of wall loss and corrosion activity, and differs from corrosion coupons, the most common form of corrosion monitoring, in that it summarizes the cumulative effects of all forms of corrosion over the lifetime of the pipe.
Ultrasound is most often employed as the first step in an investigation - to be then followed with a metallurgical analysis of a removed pipe sample or a remote video inspection. The benefits of ultrasonic testing vs. metallurgical analysis include:
An evaluation of as many as 60 or more different areas of pipe are possible for the same cost as removing one small diameter section of pipe and performing a metallurgical analysis. Typical costs for metallurgical testing are $2,000 per single sample tested, plus the cost to cut out and replace the sample pipe - an additional $1,000 at least. UT provides significantly more information for about $75 per sample tested.
Due to its lower cost, a greater number of pipe sections can be tested using ultrasound. In addition, testing is performed in areas most likely to exhibit corrosion problems, rather than only those where it is convenient.
The greater number of test points possible through ultrasound greatly improves the reliability of any conclusion over that made based upon just one test result.
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.
Given a strict adherence to certain procedures, ultrasonic testing allows the repetitive measurement of the same section of pipe to produce the most accurate corrosion rate information possible. In effect, the pipe itself becomes the ultimate corrosion coupon.A baseline measurement of 0.289 in., followed one year later by a measurement of 0.281 in., means an undisputable 8 MPY loss.
The ability to cost effectively inspect multiple piping areas, and therefore ensure greater testing coverage, 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 12 in. piping and larger, cutting out a sample section for metallurgical testing is virtually impossible. In such cases, ultrasound provides the only option available for determining pipe wall thickness.
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.
Other closed systems
Storage and holding tanks
Fan coil condensate
Gas and fuel oil
View of interior deposits
Precise corrosion mechanism
Pipe failure mechanism
Microbiological type and volume
Suggestion for remedial actions
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 location tested. Information such as:
Pipe metal loss
Percentage of allowable loss to minimum standards
Remaining pipe service life
Estimated retirement date
Pass or fail recommendation for acceptable condition
Overall piping condition
In addition, the information from all test locations can be combined into extremely informative summary graphs - each of which will provide a powerful tool in understanding the corrosion activity and trends within any piping system. Given sufficient wall thickness testing, it is possible to create graphs allowing the comparison of:
Original pipe thickness vs. measured values
Estimated remaining pipe life
Percentage of allowable pipe loss
Actual pipe loss
Overall pipe status
Corrosion rate vs. pipe size
Corrosion rate vs. physical location
Differences in supply or return service
Corrosion rate vs. pipe orientation
Differences in new and older piping
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 interest in any piping problem, the report should 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.
The below graphic, taken directly from a typical ultrasonic testing report performed by CVI, illustrates the level of detail provided for each and every piping area investigated. An investigation addressing 50 areas of condenser water pipe would therefore result in the presentation of 50 such data pages.
A graphical summary of an ultrasonic piping evaluation provides a tremendous volume of useful information - often tying the entire investigation together in one clear picture. The below graph shows the relationship between the average and minimum measured wall thicknesses as compared to original pipe wall thickness over 25 different locations tested. Trending of test results is possible only given sufficient data points to plot.
In this condenser water evaluation of a New York City office property (based upon a separate data set from the above example), various diameter pipe sizes between 10" and 3" were tested - thus the rear descending grey base line of original wall thickness as pipe size reduces. In this investigation, ultrasonic testing showed a relatively even corrosion rate throughout all pipe sizes at the locations tested; with mild pitting present as evidenced by the lower profile of the red minimum pipe thickness value.
Actual wall loss is shown by this graph as very consistent throughout the condenser system, and suggests that other points, not tested, will likely exist having similar losses. As a general rule, the more consistent the results over different test points, the greater the reliability of a report to identify the corrosion conditions which exist. Piping systems showing a wide variance in result, always evident by such a graph, suggest less probability of fully quantifying the pipe condition.
In many cases, metallurgical testing is an integral part of identifying and correcting a corrosion problem. Typically, a thorough ultrasound inspection will provide a clear recommendation to remove the most appropriate sections of pipe for metallurgical testing - thereby improving the benefit of such service.
Although limited in some respects, ultrasonic testing continues to offer the best all around piping valuation service if performed properly, and if reported using the appropriate data analysis.
CorrView International, LLC provides ultrasonic testing in our local New York area, as well as throughout the United States. We have performed service from Miami, FL to Anchorage, AK; Long Island, NY to San Francisco, CA. Our fees for out of state work are extremely reasonable in comparison to metallurgical testing and most other testing services. With sufficient preparation , we can address corrosion issues outside the United States as well.
Fees are in the range of $100-$125 per each general piping location tested, with any necessary travel expenses additional. Data from each location is analyzed and presented in a full color report as detailed in our pipe testing specifications, and illustrated above. Most ultrasonic testing evaluations involve between 30 and 75 test locations, and provide the highest degree of information and analysis, not found elsewhere.
Typical UT testing costs will range between $2,500 and $6,500 - the equivalent of cutting out and metallurgically testing 1-2 pipe samples. We recommend dedicating at least 20 test areas at any one piping system, at exclusively the condenser water system, for example, in order to provide a representative and reliable final report. Typically, 20-30 individual locations can be tested during a normal work day - with our progress generally determined by building and piping layout, insulation, pipe surface quality, and on-site assistance.
Where budget is an issue or where the pipe has already been removed and an assessment desired, we can provide the same level of statistical analysis for pipe samples sent to our office. Full details are provided on our Pipe Sample Submission page.
© Copyright CorrView International, LLC
P.O. Box 8513Landing, NJ 07850 www.corrview.com Ph: 973-770-7764 Fax: 973-770-6576
For More Information
New content is regularly added to the site, when time allows.
For the latest updates and other advisories you are invited to join our mailing list.