Technical Bulletin # P-12


The Pros And Cons Of Destructive vs.
Nondestructive Pipe Testing



THE PROBLEM:

     By far, most investigations into the status of a piping system are prompted by the sudden appearance of some operating problem. This may be in the form of a major failure, a minor leak, a restricted flow condition, or the accumulation of rust deposits in the tower pan or strainers.

     Identifying the problem, determining its extent within the piping system, and assessing the level of threat to building operations then falls upon choosing the correct method of investigation. Resolving the problem typically requires far more effort and expense.

     Finding those answers typically begins as a choice between using nondestructive (NDT) instrument based testing or destructive metallurgical analysis - with each offering information and insight not available in the other. For many corrosion related problems, both forms of testing may be required. In others, one method will offer substantial benefits in either information gained, time and effort, investigative coverage, or money saved.


THE SOLUTION:

     Choosing the best method of investigation must first begin with a careful review of the problem itself.

     Generally, the more defined the problem appears to be, such as at a localized joint, seam split, pinhole, or weld failure, the more appropriate an intensive metallurgical review. Problems which have led to the removal or repair of a failed component automatically present themselves for a metallurgical review as a first step.

     Where the exact source of a problem is not obvious, such as the appearance of rust and chip scale in the tower pans, nondestructive ultrasonic testing (UT) offers the best option by providing a thorough assessment of the entire piping system for remaining wall thickness and corrosion rate. With many corrosion problems being related to pipe location, direction of flow, and physical orientation, ultrasonic testing generally provides the best initial look at the different forms of corrosion activity which can often coexist within the same piping system.

     The major advantage of UT testing is cost, followed by the high degree of coverage possible and, of course, convenience. A quality laboratory analysis of pipe, necessary if a reliable report is desired, typically ranges between $1,500 and $2,000 per sample. Add to that fee the labor cost to shut down, drain the pipe, cut out and replace the spool piece with new stock, retreat the system, etc., and your total cost to metallurgically test an individual sample can easily exceed $5,000. For largest diameter pipes, critical 24/7 operations, or difficult to access locations, cutting out pipe is not even a remote possibility.

     It is primarily due to the difficulty and overall cost of metallurgical testing that so few pipe samples are typically submitted for lab analysis. This encourages the very dangerous practice of generalizing the condition of an entire HVAC or other piping system based upon the metallurgical analysis of one or two random samples.

     Ultrasonic testing, on the other hand, can be easily performed throughout an entire building property to provide an almost comprehensive wall loss, corrosion rate, and remaining service life assessment. For the same cost of removing and metallurgically testing one 12 in. pipe sample, ultrasound can provide detailed information relating to 50 or more individual locations from building top to bottom. With multiple tests performed at each location, a typical UT piping investigation may involve 3,000 or more individual wall thickness measurements.

     Ultrasonic testing does, however, have its limitations.

     While providing precise wall thickness dimensions and some indication of the inside pipe surface profile, ultrasonic testing cannot offer a view of the actual interior. Such information is critically important when surface deposits exist, or where microbiologically influenced corrosion (MIC) is suspected. In some examples of very random forms of corrosion, such as often exist in galvanized or copper piping systems, UT may be totally incapable of detecting such problems.

     Ultrasonic testing excels at providing a preliminary assessment of a piping system where no information was previously available. It is frequently used as a means to identify the most ideal or representative piping locations to be later removed for metallurgical testing. Where a specific pipe failure has already occurred, metallurgical testing can be used to first determine the exact cause - followed by a system wide ultrasonic evaluation to identify the extent of similar corrosion conditions.

     The advantages of each methods of testing are:


Nondestructive - Ultrasound


Nondestructive - Ultrasound


     A far more detailed explanation of our testing and reporting procedures can be found under the heading Testing Services - Ultrasonic Pipe Testing at the Testing Services menu. See Technical Bulletin # P-06 about the limitations of corrosion coupons.




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