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Dual Temperature Piping

Of the various piping systems subject to corrosion under insulation, CUI, dual temperature heating and cooling systems have consistently shown the greatest vulnerability. In contrast to many other colder temperature piping systems, a combination of multiple factors exist. Below we itemize the top 11 issues contributing to their higher failure potential:

1. First and foremost is the generally smaller diameter pipe installed at multiple risers. Existing at 4 in. and smaller, this pipe is typically threaded in its construction and therefore suffers an immediate near 50% wall loss at the thread cut.

2. The design of dual temperature piping systems is fairly typical throughout the HVAC industry and across the United States. Almost all such systems are installed with risers at its perimeter support columns in order to feed the below window fan units – placing the pipe into a higher humidity environment. Moisture can and will penetrate the outer building envelop to ultimately reach pipe which is poorly protected from its effects.

3. Most dual temperature piping systems are reduced reverse flow, meaning that the pipe reduces in size along it travel as well as supplies and returns from opposite points. Supply pipe may begin at the 1st floor at 3 in. and end at the 27th floor at 1 in., while at the same time the return pipe at the 1st floor at 1 in. and increase as it picks up more return water to its 3 in. diameter at the 27th floor. It then returns through a large express return main.

4. Schedule 40 mild carbon steel pipe is the most common material used in the construction of dual temperature piping systems. While copper pipe may be found in some limited applications, it is typically at much smaller properties having pipe of 1 in. diameter or less. Larger diameter copper pipe at dual temperature risers is extremely rare. Where it exists, however, no outer surface corrosion problem exists.

5. Pipe wall thickness of the same ASTM schedule reduces as diameter decreases, and therefore in comparison to the 8 in. main dual temperature pipe exiting the mechanical room having a wall thickness of 0.322 in., its 1-1/2 in. riser piping has a wall thickness of 0.145 in. The same corrosion acting against thinner steel pipe means the first failures will typically occur at the smallest diameter pipe, and in areas of higher pressure.

6. Small diameter pipe is typically threaded. Reducing the pipe’s already limited wall thickness further by 0.071 in. to account for pipe wall cut away at the threads, and the true vulnerability of the risers becomes quite apparent given a remaining thickness at the exposed threads of only 0.074. For a typical 1-1/2 in. steel dual temperature riser, its weakest point and always first point of failure is at the threads.

7. Generally inadequate insulation was typically specified and installed to protect pipe at the most vulnerable environments in any building property. Insulation thickness of nominal 1 in. and even as thin as 1/2 in, was typically installed – entirely too thin to provide any moisture protection. While the foil backed paper jacket may be considered a moisture retardant, it is in no way a moisture barrier.

8. The quality of the insulation installation itself has been often found the worst in the areas typically hidden from view. A run-out tee at every floor to the window units was typically not well insulated, and in many examples we have seen, not insulated at all. Ironically, the dreaded asbestos insulation installed at many older building properties provided the best moisture protection due to its greater density, its hard shell covering, and generally more thorough and better quality installation.

9. As always, the steel pipe was never coated or even painted to protect its outer surface prior to insulation. Nor was the insulation provided any further coating or outer surface moisture protection. The net result being aggressive outer pipe surface corrosion.

10. The steel pipe further transitions to copper tube to the window units, with dielectric insulators never installed. Although generally a non-issues unless corrosion activity is high, this deficiency does pose a potential threat to any piping system. Having water soaked insulation further propagates any potential galvanic condition.

11. All dual temperature systems have a corresponding condensate return line. Where constructed of steel or galvanized steel, internal deterioration is common, and can result in either pinhole failures or rust constricted interiors having the same need for pipe replacement. The failure of either dual temperature or condensate piping is often an indication to investigate the condition of the other.

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