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 due to various reasons, but mostly 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 they show true system conditions, or provide a critical look at corrosion control effectiveness. Read more about the limitations of corrosion coupons. 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 equal.

     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. Review a summary of common corrosion areas for various piping systems. Some available methods of testing for corrosion and piping condition are:

     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 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 dependant upon the way they are installed and monitored.

      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.

     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. Where the flow can be periodically shut down and drained, a set of Victaulic 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 return 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 its own effort to the 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.

     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 it's 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. See Technical Bulletin C-4 about the problems associated with interior deposits.

     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.

     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 "Controlled Disaster," termed by some chemical cleanout companies to define a 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. See Technical Bulletin M-3 for more about high pressure water jet for deposit removal.

     Overall, spool pieces provide invaluable information to any property owner or process plant operator. For their relatively low cost, 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.

Review our disclaimer on any technical information contained within this Internet site.

©  Copyright