Technical Bulletin # P-10

Avoiding Some Obvious But Surprisingly Common
Roof Level Condenser Piping Failures

THE PROBLEM:

     Most building operators see the first indication that a corrosion problem may exist at the failure of the roof level condenser water piping. Such problems may be as minor as a pinhole leak; others as major as the loss of a large section of the riser itself.

     In many instances, suspected wall loss finally becomes obvious by the presence of iron oxide deposits and chip scale in the cooling tower pans. Rust and corrosion at the pipe exterior may also be noticed in areas directly exposed to the elements, or more frequently, may exist hidden under the pipe insulation and hard outer metal covering.

THE SOLUTION:

     Roof level pipe typically suffers a two front corrosion attack. If it is drained down partially for winterization, higher corrosion rates by as much as SIX TIMES or more above those existing at the lower floors may be realized. This often creates unexpected piping failures which are decades earlier than would normally occur at the lower floors. See Technical Bulletin # C-05 about increased roof level corrosion activity during winter drain down or lay-up.

     Ineffective paint or corrosion protection of any pipe which is exposed to the elements initiates a second corrosion mechanism which can quickly produce severe wall loss and pitting - unnecessary corrosion losses which often well exceed those taking place at the pipe interior. The simple lack of regular maintenance is often the obvious cause.

     Some of the most severe examples of roof level corrosion are typically found under hard shell exterior insulation. This is due to the easy penetration of water, rain, and other moisture through the seams and joints of most insulation jobs, or to physical damage caused during maintenance work and everyday traffic. The difficulty in removing the metal covering usually deters a preventative inspection of the pipe. Water eventually saturates the fiberglass material to provide an untreated water condition at the exterior pipe surface - leaving the degree of pipe deterioration solely dependant upon the natural corrosion resistance of the pipe.

     Hidden from view over many years, such corrosion activity is often very advanced in its surface attack by the time it is detected. The degree of severity can usually be judged by the volume of rust product between the fiberglass and pipe, as well as the depth of pitting at the surface. In the most severe cases, measurable pit penetrations exceeding 0.150 in. into the pipe are possible, and ultrasonic testing may be impossible without first grinding the pipe to produce a flat test surface.

     The potential for Corrosion Under Insulation (CUI) to develop can be minimized or eliminated given certain precautions when first installed. Few options exist, however, once the insulation is installed and water has saturated the fiberglass. See Technical Bulletin # P-09 about the dangers which may exist under insulated pipe, as well as recommended insulation procedures.

Moderate Corrosion	Waterlogged Insulation

     Corrosion under insulation generally becomes more severe as the pipe temperature decreases, and the potential to condense and hold moisture increases. Therefore, the colder supply side piping will typically show the greatest wall loss. Chill water pipe is especially vulnerable to CUI, as is any pipe which is constantly drenched by cooling tower overspray. In some examples, corrosion under the insulation will deteriorate the steam or electric based heat tracing to the point of failure.

     Surprisingly, the failure to simply provide proper maintenance of exposed pipe is a major cause of all such failures. Scraping the pipe clean of all rust products and re-coating it with high solids paint or epoxy is critically important as soon as the problem is recognized. Special attention to less noticed areas, or those in close proximity to other building fixtures and pipe hangers and supports, is also important in order to ensure that the entire pipe surface is protected. A thorough visual inspection once per year is often all that is necessary to identify such areas. Given the almost 100% certainty of water infiltration through outdoor insulation, we recommend applying a strong water protective coating to any new pipe prior to installation.

     Another common problem for roof level piping is the galvanic reaction which can occur between the condenser pipe itself and the metal pipe hangers or supports in the presence of water. This, we believe, is due to the small microvolt difference in ground potential which may exist between the condenser pipe and the building steel. Whatever the precise cause, high wall loss is commonly found at pipe hanger locations or at wall or roof level penetrations. Steps to insulate the pipe and building metals from direct contact are therefore recommended.

Dissolved Pipe & Hanger	Corrosion Product
Weakened Pipe Support	Pipe & Hanger Corrosion

     A summary of general recommendations to minimize corrosion losses at exposed roof level piping areas are:

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