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Testing Methods

Except in critical nuclear, computer, military, or financial facilities, etc., regularly scheduled pipe testing is rarely performed.  By far, most investigations into the status of a piping system are prompted by the sudden appearance of some corrosion related operating problem.  This may be in the form of a major piping failure, a minor leak, a restricted flow condition, or the accumulation of rust deposits in the tower pan, heat exchangers, or strainers.  Less frequently, the testing of critical piping services may be performed prior to a property acquisition.

Identifying a 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 greater effort and expense; with the costs generally dependent upon its severity at the time it is discovered.

• Identifying The Appropriate Test Methods

Finding those answers typically begins as a choice between using nondestructive instrument based ultrasonic testing (UT) 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 the level of information gained, investigative coverage, convenience, or money saved.

Other methods such as corrosion coupon analysis, X-ray, and spool pieces will not provide the necessary wall loss information.  Even if established in response to a problem, such methods will only show the corrosion activity from that time forward – and not what has already taken place.  Choosing the best method of investigation, therefore, must first begin with a careful review of the problem itself.

• Corrosion Problem Determines Test Method

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 investigation.  Problems which have led to the removal or repair of a failed piping section or 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 or strainers, nondestructive ultrasonic testing offers the best option by providing a thorough assessment of the entire piping system for remaining wall thickness, corrosion rate, and remaining service life.  With many corrosion problems being related to pipe location, direction of flow, location, and physical orientation, ultrasonic testing will, in most cases, provide the best overall assessment of its severity and extent throughout a piping system.

The major advantage of ultrasonic 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 from 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, chemically retreat the system, etc. – and the total cost to metallurgically test an individual pipe sample can easily exceed $5,000.  A large metallurgical testing project can easily exceed $250,000.  For largest diameter pipes, critical 24/7 operations, or difficult to access locations, cutting out pipe is not even a remote possibility.

• Multiple Test Points Required

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 process plant piping system based upon the metallurgical analysis of one or two random samples.  One possible result is the premature replacement of good pipe – the other a piping failure.

Without other solid information upon which to select a test location, the removal of pipe for metallurgical analysis is often based upon convenience.  Great expense is then somewhat wasted by producing information not necessarily related to the actual problem.

Ironically, as corrosion conditions worsen inside a piping system, they also produce pitting and areas of deep and random wall loss – therefore reducing the level of probability and uniformity from area to area.  The worse the corrosion activity to have prompted an investigation, the more unlikely any individual removed section of pipe will truly represent the rest of the system, and therefore the need for even more intensive testing.

Ultrasonic inspection, on the other hand, can be easily performed throughout an entire building property or plant operation to provide an almost comprehensive wall loss, corrosion rate, and remaining service life assessment.

For nearly the same cost of removing and metallurgically testing one 12 in. pipe sample, ultrasound can provide detailed, and almost identical information relating to 50 or more individual locations – from system top to bottom; main risers to smallest runout piping.  With multiple tests performed at each location, a typical UT piping investigation may involve 3,000 or more individual wall thickness measurements.

• Certain Limitations

Ultrasonic testing does, however, have its limitations.  While providing precise wall thickness dimensions and some reasonable indication of the inside pipe surface profile, ultrasonic testing cannot show an actual view of the pipe 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, as may often exist in galvanized or copper piping systems, UT may be totally incapable of detecting such problems.  Dezincification at older brass pipe is another issue not well addressed by ultrasound, and more of a metallurgical question.

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, or the worst areas of concern for further investigation.  The opposite investigative procedure also exists.

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 as indicated by a similar wall thickness profile.

The advantages of each method of testing are:

CorrView International, LLC does not provide laboratory and metallurgical services.  We do, however, work closely with an outstanding investigative group of metallurgists we have found capable of answering any piping or corrosion related question.

© Copyright 2023 – William P. Duncan, CorrView International, LLC