Corrosion problems can be relatively limited to a certain small areas such as the threaded run-out fittings, or may be extensive and severe enough to destroy an entire piping system. Its cause may be due to poor chemical water treatment, an engineering design error, the selection of improper piping materials or pipe schedules, defective piping materials, a welding weakness, improper start-up, or various other factors in its design and installation.
For every corrosion problem, a thorough and accurate ultrasonic evaluation is the mandatory first step necessary. Rather than pursue unsupported conjecture and speculation regarding the cause of the piping failure or corrosion conditions, a thorough ultrasonic investigation, if performed properly, accurately, and assuming that a statistical analysis of the data has been provided, will produce the clear and unquestionable direction to pursue.
Speculation to a problem, and recommendations often having more of a profit motive, often result in the wrong corrective actions. Often, an identification to a problem and recommendations are provided by those entrusted to prevent corrosion from occurring in the first place..
With the severity and extent of a corrosion problem accurately identified, various forms of remediation are possible for those systems still with acceptable wall thickness, and where total pipe replacement is not the only option. Some commonly used investigation and repair measures are offered below:
Correctly identifying the condition of the piping system is unquestionably the very first step to solving any corrosion problem. And to do this, ultrasonic testing is ideally suited. No other diagnostic tool is capable of so thoroughly identifying the severity and extent of a corrosion problem - if performed correctly. Please see our explanation to the benefits of Ultrasonic Testing for more information.
Without a thorough analysis clearly defining the corrosion problem, decisions to address the problem can only be made based upon unsupported speculation and conjecture; decisions which are often just not wrong, but a total waste of time and money as well.
For most corrosion problems, physical weakness within the piping system will limit the choice of corrective actions. In those cases where a chemical cleaning is required, for example, it is the weakness at its threaded fittings or smallest diameter piping which prevents that action until replacement of all weakened pipe has been completed.
Only worse than the original corrosion problem is to waste months and years pursuing the wrong corrective actions, or to take steps resulting in further piping failures.
The first line of defense against corrosion for any HVAC piping system is an effective and well maintained water treatment program. Although most chemical treatment providers have the foremost interest to provide a high level of protection for their clients, their best efforts are sometimes negated by factors beyond their control - such as the use of thin wall foreign produced seamed pipe, poor quality welding, dead end and low flow areas, or inherent engineering design flaws resulting in areas of high sediment buildup.
A critical review of the chemical treatment program by an outside expert is always an important step to understanding what may have allowed a corrosion problem to occur or probagate. Frequent changes in chemical treatment supplier, or their selection based upon lowest bid, is always a red flag for concern.
For relatively new piping systems, reviewing start-up records to see what was done to clean and passivate the new pipe is important.
Given the inherent error in corrosion coupons to generally produce far more favorable results than what actually exist, low corrosion coupon rate estimates should be discounted or ignored. Where heavily rusted pipe and piping failures are contradicted by extremely low corrosion coupon reports - believe the rusted and failed pipe!
Ultrasonic testing is ideally suited for defining the severity and extent of a corrosion problem. As a nondestructive testing tool, no pipe is cut, removed, modified, or in any way tampered with. No system shut down or pipe draining is required.
Metallurgical testing, on the other hand, is best used to specifically identify what caused a failure, or to define the underlying mechanism to a problem - such as microbiological influence, poor welding, mis-cut threads, sub standard pipe, poor quality galvanizing, dezincification, etc.
Metallurgical testing is best suited as a first step in a problem diagnosis where a failed pipe section has already been removed. It is a poor choice, however, where no specific area of failure is known, and where only random luck will decide if the pipe section to cut out and metallurgically tested has any diagnostic merit or benefit to a more thorough understanding of the problem.
We have strongly maintained that no corrosion problem can be resolved as long as heavy internal rust deposits exist; common to some degree for most HVAC systems under most corrosion conditions. Those rust deposits limit or completely stop any benefit from the chemical water treatment program even though manufacturers claim otherwise. (See category 5 in our Corrosion Photo Gallery)
Removing such deposits requires an aggressive chemical cleaning, often impossible due to other weakness within the system. High pressure water jet cleaning is safer and more immediately effective, but far more difficult to accomplish.
Water filtration can capture moveable and suspended rust particles only - removing visually noticeable micron level tubidity, but leaving the majority of the problem in the form of massive rust tuberculation still attached to the internal pipe walls.
Aggressive chemicals to remove the rust from the pipe and resuspend it for capture is often required, but rarely provided - resulting in clean water that has everyone cheering to their success, but which in reality has accomplished little to nothing to resolve the corrosion problem.
Unless rust deposits are removed to allow chemical corrosion inhibitors to reach the underlying base steel, no improvement in conditions can occur.
In order to ensure the most effective cleaning possible, we always recommend bench testing any proposed cleaning chemicals against an actual rust fouled sample of pipe in order to judge its safety and effectiveness.
Installing a spool piece within the corroded piping system, and of the original pipe, will provide further visual indication of cleaning effectiveness.
Effective water filtration is critically important for every piping system - whether or not suffering from a corrosion condition. Significant iron oxide rust deposits are created even under low corrosion conditions of 1 MPY. We recommend side stream basket style filtration to all closed circulating HVAC piping systems.
No form of automatic backwashing filter should be considered for a closed circulating system.
It is unfortunate that most filtration units are installed incorrectly; with the inlet to the filter from a point perpendicular to system flow. Even if installed at the bottom, velocity and inertia will move any larger size particle past the capture point to ultimately settle elsewhere within the system; never passing the filter again.
Effective water filtration is as dependent upon the installation location of the filtration unit as the quality and efficiency of the unit itself. Installing a sub-micron sand filter with its intake capture line not likely to ever see any particulate matter, again typically perpendicular to the flow, is an utter waste of money.
Most piping systems remain fouled due to the combination of improper filter installation, combined with the failure to add chemical cleaners and dispersants to remove hardened rust deposits back into the system for capture and removal.
Pipe re-lining is a well proven and useful option depending upon remaining wall thickness and most importantly the configuration of the pipe. Modern day coatings are strong and very effective, but require complete rust removal and preparation of the pipe interior to SP1- "white metal" clean finish. This requires sand blasting of the pipe and extensive planning.
Properly performed, an internal pipe lining will stop wall loss from progressing and extend service life for at least another 20 years. While many benefits exist, it usefulness and application is highly dependent upon piping configuration and requires substantial preparation and planning.
For severe corrosion problems at cooling tower systems that are not likely to be corrected through chemical or physical cleaning or better chemical treatment, one proven option is to isolate the system from the open atmosphere by the addition of a plate and frame heat exchanger.
This immediately reduces much of the outside environment influence and eliminates blowdown from the system thereby allowing much higher concentrations of corrosion control chemicals. Filtration then only has to deal with the existing corrosion product, and not a constant input from the atmosphere.
Although requiring a 2nd set of pumps and substantial capitol cost, this option has proven helpful toward resolving corrosion problems having no other possible solution.
For any open cooling tower or heat pump system comprised of multiple small diameter risers to the various units, a plate and frame heat exchanger is almost mandatory if long service life is desired.
Many paths to correct a corrosion problem exist. Unfortunately, the corrective actions recommended are often related to the product or service provided by those consulted. Chemical companies will recommend more chemical without ever addressing the rust deposits preventing their chemicals from doing any good. Filtration companies will recommend an automatic blowdown unit to a closed glycol loop. Corrosion consultants recommend additional corrosion coupon racks to often prove there is no problem and that their oversight is effective.
With sufficient knowledge about the problem, however, the solution to a corrosion problem is typically quite obvious.
© Copyright CorrView International, LLC
P.O. Box 8513Landing, NJ 07850 www.corrview.com Ph: 973-770-7764 Fax: 973-770-6576
For More Information