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Closed Piping System Corrosion

Of the many corrosion problems that plague a typical building property, corrosion at a closed HVAC piping system should never occur.

Fundamentally, corrosion impacts different piping systems in different forms and at widely varying rates.  Other than protective coatings or pipe lining, little can be done to control corrosion activity at plumbing, sanitary waste, and domestic water systems.  Some options are available for fire sprinkler lines but rarely employed. Dry fire sprinkler systems experience far greater and more advanced failures than do wet systems.  Steam systems are less impacted with very uniform wall loss; while steam condensate lines always suffer greater corrosion threat due to deep pitting.  Condenser water service typically requires the closest attention and highest chemical cost expenditure, with results widely dependent upon many different factors.

For many piping systems, layout, design, corrosion protection ,filtration, and especially materials of installation will play a significant role in its future service.

For a closed piping system such as hot water heating, secondary, closed condenser water, chill water, dual temperature, reheat, or other service, internal corrosion should be a non-issue.  Without constant aeration, the capture of airborne particulates, microbiological input, blowdown requirements, and the many other environmental issues of an open condenser water or process water system, maintaining a closed loop piping system becomes a relatively simple task.

Why then are corrosion problems common to many building properties?

• A Hidden Problem

First and foremost is the issue of maintenance.  The absence of rust and other tell-tale visual evidence of a corrosion problem, as would exist for condenser or process water, often means a lack of needed attention.  One very common misconception is that a closed piping system doesn’t corrode and therefore requires no chemical water treatment – although this theory has been overwhelmingly discredited.

While very old piping systems have been documented to exhibit little wall loss even after 60 years of no chemical treatment, it is the superior quality and higher natural corrosion resistance of older pipe that is the primary cause for such rare examples, rather than the non-existence of such a problem.  Such high quality and superior corrosion resistance no longer exists, thereby making any such observation and arguments irrelevant.  The same lack of chemical treatment protection to an equivalent closed piping system installed today would produce substantial wall loss and accelerated failure.

• A Second Corrosion Threat

For cold piping systems, a second corrosion attack exists at its outer pipe surface, and can produce substantially greater wall loss compared to that occurring internally.  Mostly avoidable, (if control over original construction and its installation is possible), outer surface corrosion is simply due to inadequate insulation.  Although the insulation has sufficient wall thickness to prevent a temperature loss, most insulation applied to chill water or cold temperature pipe is entirely too thin to prevent the migration of moisture to the cold pipe surface.  Commonly called CUI, or corrosion under insulation, it is more appropriately CUPI, or corrosion under poor insulation.

Contrary to the claims of insulation manufacturers, paper or foil backed fiberglass insulation provides an inadequate moisture barrier.  The result is for varying levels of deterioration to any cold steel pipe dependent upon temperature, area humidity conditions, and time.  Dual temperature piping systems are notoriously vulnerable given greatly inadequate insulation to perimeter run piping risers subject to higher moisture content through the building envelope.  After 40 years, and often less, the pipe is destroyed from the outside in; with no remediation options available other than pipe replacement.  It is a massive nationwide problem unanticipated in the original building design, and one totally caused due to poor insulation.

Once the building is constructed, there is virtually nothing that can be done to avert the problem.  External corrosion damage can be slightly reduced by increasing cold water temperatures which in turn reduces condensation.  For older building properties in which internal corrosion deposits have settled into the coils, heat transfer is reduced – thereby often resulting in even lower cold water supply temperatures to compensate.  Ironically, most outer surface corrosion activity accelerates over time due to this downward spiral of events

• Solutions Available

Applying a high quality protective and waterproof coating to any cold operating pipe is strongly recommended prior to insulating – although it is never performed given that benefits are not realized until decades later.  We have never seen a piping design specification calling for a protective coating for cold water pipe although it would nearly eliminate the problem.

Even under low corrosion conditions, substantial iron oxide rust is created internally during each year of operation.  Unlike open systems which provide telltale indication to rust build-up, such conditions are rarely exhibited at closed systems, and never looked for.  As internal rust is generated it settles in the lowest flow areas to reduce heat transfer efficiency as well as initiate under deposit corrosion.  The simple installation of side stream basket style filtration to a closed system, again not often provided, removes such deposits to not only improve heat transfer efficiency, but to also remove the rust product responsible for higher corrosion loss.

A final consideration to long service life of any closed piping system is to maintain higher concentrations of corrosion inhibitor.  Unlike open systems which require constant blowdown and therefore the majority of the yearly chemical contract cost, closed systems lose little if any water.

Chemical concentrations recommended by most treatment providers are not to the optimal concentration to maintain corrosion at its lowest level, but to provide a contract price the client will approve.  Chemical levels, therefore, are a compromise between cost and a tolerable corrosion loss.  Maintaining higher chemical levels offers exceptionally low cost insurance toward an extended service life to any closed piping system.

• General Recommendations

To all closed HVAC piping systems the following recommendations apply:

• Maintain higher chemical inhibitor concentrations.
• Install side stream basket style bag filtration into an effective location at the piping system.  Filters installed undersized and / or incorrectly provide virtually no benefit.  Do not install sand filtration at closed piping systems.
• Chemically clean and flush all closed piping systems every 5-8 years.
• Monitor for microbiological activity and sterilize the system if necessary.
• Maintain pump seals to reduce water and chemical inhibitor losses.
• For any cold water service such as chilled water or dual temperature, paint the pipe with an effective waterproof coating prior to insulation
• Install a minimum of 2 in. fiberglass insulation to any cold piping system.  For high humidity environments such as exist for Miami or Houston, and especially in areas near salt water, consider installing hard shell foamglass sealed at each joint and between sections, rather than fiberglass.
• For any cold water service, apply an effective final outer moisture barrier over the insulation.  This should be in the form of a self sealing tape or multiple coats of high solids waterproof paint.  Vinyl and metal outer jacketing, unless sealed at each overlap with silicone or other mastic material and taped, provides only an aestetic benefit to the piping system.
• Greatest vulnerability always exists at the smaller run-out piping due to its inherently lesser wall thickness.  This in turn requires special emphasis toward effective insulation.

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