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Testing Alternatives

For most property owners, managers, or plant engineers, a corrosion monitoring program exists for the primary purpose of verifying the effectiveness of the chemical water treatment contractor.  Although a performance standard is rarely established or held to, corrosion coupon results are generally used in deciding if any changes should be made to the treatment program, and of whether a corrosion threat exists.

Since corrosion coupons typically return favorable results given their isolation from the main piping system, they more often provide steady reassurance that conditions are acceptable, and that there is no need for further concern.  Rarely do corrosion coupons show true system conditions, or provide a critical look at corrosion control effectiveness.  Often secondary in importance is the interest to track the accumulated metal loss from the piping interior, gauge its remaining service life, check for deposit build-up, and judge whether interior pipe conditions are improving, declining, or remaining static.

• Different Test Methods Available

Corrosion monitoring is fulfilled to varying degrees of success through the use of different methods and procedures – each of which offers benefits and advantages over the others.  For the greatest accuracy, reliability and safeguard against unexpected operating problems, more than one corrosion monitoring method should always be employed – this following a thorough evaluation of where potential problems might exist.  Some available methods of testing for corrosion and piping condition are:

• Corrosion Coupons

Installed in an isolated loop, corrosion coupons never suffer the same environmental effects as the pipe itself, and rarely provide accurate test results.  Hardened deposits, electrical activity, under deposit corrosion, microbiological buildup, flow effects, and other common environmental factors do not exist for corrosion coupons and are therefore not measured.

A flow requirement, by definition, prevents their installation in precisely those locations traditionally showing the highest corrosion threat.

In addition, installing corrosion coupon racks at multiple points throughout a circulating system is not practical and is almost never performed – thereby forcing the questionable assumption that the test results shown for one specific location are representative over the entire piping system.

• Metallurgical Analysis

Periodically cutting out samples of pipe for metallurgical analysis is extremely expensive, usually requires a system shutdown, and is rarely carried out for large diameter piping.  For any critical or 24/7 operation, it is virtually impossible to perform.  Combined maintenance and metallurgical costs can easily exceed $4,000 per sample.

Metallurgical analysis does offer valuable information unavailable through any other means, and in the case of an MIC or under deposit problem, is critically important to pursue if a solution is desired.  Its use as a random diagnostic tool is generally limited, however, due to its cost and inconvenience.

• Spool Pieces

Spool pieces provide valuable information regarding the actual net effect of corrosion activity against the pipe surface, but again are only applicable for smaller diameter piping separate from the main lines.

Properly installed, spool pieces offer a true inside look at deposits, surface pitting, inhibitor and cleanout effectiveness, as well as provide samples for microbiological cultures.  Like corrosion coupon racks, however, they are rarely installed throughout a piping system, and therefore enjoy limited use.

Installed incorrectly, spool pieces, like corrosion coupons, can provide significantly misleading information.

• Ultrasonic Testing

Ultrasonic wall thickness testing provides the greatest volume of reliable data, and will typically produce a thorough corrosion evaluation as long as a sufficient number of test points are taken over a wide enough range, and the work performed competently.  The method of analysis and presentation of the test data is equally as important as the wall thickness measurements themselves.

Ultrasound is often used as a prerequisite to other testing methods due to its low cost and wide coverage, or as a confirmation that wall thickness conditions known to exist in one area do or do not exist elsewhere within the piping system.

It is most often used as a tool to identify the extent of an already recognized leak or rusting problem, and long term corrosion monitoring using ultrasound requires establishing specialized testing procedures.

• Remote Video Inspection

The insertion of a remotely controlled camera into the pipe offers a valuable thought very limited inspection option.  Its use requires the system to be out of service and drained, and is greatly limited by access into the piping system.  Pipe size, physical configuration, internal conditions and length of travel offer further restrictions in its use.

Remote Video Inspection (RVI) cannot provide any wall thickness data, but can quickly locate those internal indications that wall loss has occurred – such as tubercular deposits or suspected MIC growths.

Combined with ultrasonic or metallurgical testing, RVI can quickly document whether similar conditions exist in other areas of a piping system.

• Spool Pieces An Under Utilized Tool

A spool piece is a section of the same pipe, integral to the operating system, which has either been installed or modified so that it can be periodically isolated and removed for visual inspection and/or analysis.  A spool piece often provides the only means capable of revealing the actual corrosion conditions at the inside of a piping system – in effect providing a temporary window.

Spool pieces are an excellent though less often used corrosion monitoring tool.  They are typically established once a problem condition has been realized, and even then, are used ineffectively in many examples.

Spool pieces provide valuable information on many aspects of pipe wall loss due to most common forms of corrosion, and can add greatly to the understanding of interior pipe conditions.  How much information they provide, however, is greatly dependent upon the way they are installed and monitored.

• Correct Installation is Important

Similar to any testing method, improper use and application can produce useless information and even worse – misleading information.  Essentially, every piping system is constructed of endless spool pieces except that most are not removable.

Pipe leading to A/C units provide one of the most ideal examples of useful spool pieces; requiring only the installation of isolation valves to allow removal.  A small piping section can then easily be removed for inspection during off hours without shutting down the system.

A spool piece should be installed in a normal flow configuration and not installed side stream to normal flow activity.  As the photo at left illustrates for a spool piece installed side stream to the main flow, sediment settled within the spool piece to produce high under deposit corrosion and random low bottom wall thickness – a condition not present at the main line under full flow conditions.

• An Excellent Evaluation Tool

Measuring the effectiveness of a chemical cleaning operation will often utilize similar testing procedures.  And since a chemical treatment program by definition involves the inhibition of new corrosion activity, any testing program will provide information on that new corrosion activity as well.

For any older and deposit fouled system, two separate objectives exist.

• Remove Deposits

A spool piece will allow photo documentation to internal pipe conditions before chemical cleaning, and then enable an evaluation of cleaning effectiveness during the process.

The first step to reducing corrosion is to remove any built-up deposits so that chemical inhibitors can better protect the base metal.  Surface deposits substantially increase overall corrosion rate and pitting, and exist as a far greater threat than simply the problems caused to flow rate and heat transfer.

As deposits are removed, an increase in heat transfer efficiency is often noted first.  Lower pressure drops across the equipment is also a frequent finding.  Lower energy costs are guaranteed from any cleanout operation, although difficult to observe and quantify in most cases.

It is important to remember that even a low to mild corrosion rate will produce thousands of pounds of internal deposits over decades of operation.  Therefore, any cleanout effort will take substantial effort over an extended period of time.

• Reduce Corrosion

A spool piece can also provide visual confirmation to whether corrosion control is effective.

A second objective of this testing method, therefore, is to monitor chemical corrosion rate at both those areas of pipe which have been cleaned of deposits, as well as continue to prevent the formation of new iron oxide deposits throughout the system overall.

Depending on the cleanout method used, the interior pipe surface may be quite vulnerable to new and higher corrosion rates.  Therefore, special emphasis is needed to ensure that an even worse pipe corrosion condition is not created.

A spool piece installed of new pipe provides further useful information.

• Spool Piece Installation

Fundamentally, the spool piece must exist within the normal flow path, and must be representative of the overall piping system in flow rate, hours of operation, and usage.  A spool piece installed in a by-pass configuration to the main recirculating loop is often nothing more than a larger version of a standard corrosion coupon – providing the same limited information of questionable accuracy.

Any spool piece should be 2 in. or larger in order to best allow a view into its interior – a 4 in. or 6 in. diameter is ideal.  The spool piece should be installed where the flow can be periodically shut down and drained, a set of clamped type fittings or standard unions, and a second temporary replacement pipe, is all that is required – often a simple in-house piping job. For more critical 24/7 operations, valves and tees must be installed with a by-pass loop around the spool piece location.  Of course, the more spool pieces installed, the more representative and reliable the information gained.

Where the interest exists to evaluate the effectiveness of an on-line deposit cleaning program, the actual spool piece should be cut away from an existing section of fouled pipe, fitted with valves and quick connections, and returned back into service in the same location and with the same flow and spatial orientation.  In this way, the current condition of the pipe can be viewed, photographed, ultrasonically measured, returned into service, and then compared at later dates for its improvement or further deterioration.

Since the fundamental goal of any chemical water treatment program is to reduce wall loss and prevent deposits from even forming, CVI also recommends the installation of similar spool pieces having new sections of pipe.  This provides information regarding the effectiveness of the overall corrosion inhibitor program to protect any new pipe, and will also indicate if the deposit cleaner alone is possibly too aggressive, not sufficiently inhibited, or otherwise contributing to unnecessary wall loss at the pipe.

An easy argument can be made that the most effective iron deposit removal program is worthless if the chemical treatment alone cannot prevent such deposits from reforming.  Were the original chemical treatment program effective, there would, in fact, be no need for a cleanout program.

• Chemical Treatment The Key

As stated above, the primary function of a good corrosion control program is to reduce wall loss to an acceptable or tolerable level.  Second is its ability to prevent iron oxide, microbiological growths, and other foreign materials from depositing within the system.  As shown elsewhere on this site, a substantial volume of interior deposits can be created even under reasonably low corrosion rates, and such deposits can produce significant secondary corrosion and flow problems if allowed to accumulate.

Under normal operating conditions, a good chemical water treatment program will minimize both corrosion and surface deposits at the pipe.  Although a light layer of rust product might be expected, moderate to heavy deposits signal that not only is the corrosion control ineffective, but that a downward spiral of greater corrosion activity, deposits and wall loss exists as well.  Unlike ultrasound or most other testing methods which cannot provide a view into the pipe interior, a spool piece will visually show the result of any chemical treatment protection (or lack of it) over time, and provide the visual confirmation often desired.

• Cleanout Options

Methods used in the cleaning of interior pipe deposits are primarily chemical.  While the composition of the deposits themselves greatly influence the decision of the chemical agents used, it is often the speed at which removal is planned that determines the chemical cleanout procedure.  Chemical agents range from highly aggressive and quick acting acids, to more selective chelating agents, to low level and slow acting dispersants.

The fact is that a deposit fouled piping system also defines a physically weakened piping system.  Even though a corrosion failure in progress will take place eventually, a multitude of piping failures occurring throughout a building property, and within minutes of adding an acid or aggressive cleaner to a piping system, is something to avoid by all means.  A so-called “Controlled Disaster,” termed by some chemical cleanout companies to define an expected and limited pipe failure threat, is still a disaster by any definition once the water begins pouring down.

For that reason, any cleaning effort must be preceded by a thorough piping evaluation with the purpose of identifying all weakened and failure prone areas.  Ultrasonic testing is the most ideal and cost effective diagnostic tool for this purpose, followed by metallurgical analysis.

Where an aggressive cleaner is used, the piping system is often drained and flushed repeatedly to remove the loosened or dissolved deposits.  For milder on-line cleanings performed over an extended period of time, good filtration is an absolute must in order to remove the high volume of loosened deposits that should be expected.  A 10 mil per year corrosion rate, left to occur over a number of years at a typical high rise office building, will produce literally thousands of pounds of iron oxide debris to be removed.

One excellent alternative to the more aggressive acid type cleaner is the use of high pressure water jet.  Although greatly limited by the physical layout of the piping, available entry points, and requiring shutdown and other special preparations, high pressure water jet does offer the benefit of thoroughly stripping off all deposits in one quick step, and without the use of any dangerous chemicals.

Overall, spool pieces provide invaluable information to any property owner or process plant operator.  For their relatively low cost to install and monitor, spool pieces will provide years of advance notice of an internal corrosion problem, and help avoid the enormous inconvenience and remediation costs involved with cleaning up a deposit fouled piping system.

© Copyright 2024 – William P. Duncan, CorrView International, LLC